Summer Circulation and Water Masses along the West ...Figure 4.1: Location map of the research area including the positions of CTD transect lines performed during voyages SS09/2003

Summer Circulation and Water Masses along the

West Australian Coast

Lai Mun Woo, B.Eng. (Hons.)

This thesis is presented in fulfilment of the requirements for the degree of

Doctor of Philosophy

at the University of Western Australia, School of Water Research

Submitted June, 2005

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All merit in my research is dedicated to my beloved family: Doris, Chong Wah, Lai

Yee, Lynn, Ben and Ernest; to my dharma teacher and true friend, Jue Ru; and to the

hundreds of thousands who lost their lives to the Indian Ocean this past summer.

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“As I wend to the shores I know not,

As I list to the dirge, the voices of men and women wreck’d,

As I inhale the impalpable breezes that set in upon me,

As the ocean so mysterious rolls toward me closer and closer,

I too but signify at the utmost a little wash’d-up drift,

A few sands and dead leaves to gather,

Gather, and merge myself as part of the sands and drift.”

WALT WHITMAN, AS I EBB’D WITH THE OCEAN OF LIFE (1860)

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Contents

LIST OF FIGURES ...........................................................................................11

LIST OF TABLES.............................................................................................17

ACKNOWLEDGEMENTS ................................................................................18

PREFACE.........................................................................................................21

ABSTRACT ......................................................................................................23

CHAPTER ONE: INTRODUCTION................................................................25

1.1 The Gascoyne- a Significant Marine Environment..............................................27

1.2 Motivation................................................................................................................29

1.3 Objective ..................................................................................................................30

CHAPTER TWO: LITERATURE REVIEW.....................................................31

2.1 Local Setting of the Gascoyne ................................................................................31

2.1.1 Climate ...............................................................................................................31

2.1.2 Bathymetry.........................................................................................................32

2.2 Prominent Features of Ocean Circulation............................................................35

2.2.1 Conventional Eastern Ocean Boundary Currents ..............................................35

2.2.2 Leeuwin Current.................................................................................................37

2.2.2.1 Observational Studies .................................................................................37

2.2.2.2 Modelling Studies .......................................................................................39

2.2.2.3 Overview.....................................................................................................40

2.2.3 Leeuwin Undercurrent .......................................................................................42

2.2.4 Coastal Equatorward Current.............................................................................42

2.3 Hydrographical Structure......................................................................................45

2.4 Conclusion................................................................................................................49

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CHAPTER THREE: SUMMER SURFACE CIRCULATION ALONG THE GASCOYNE CONTINENTAL SHELF, WESTERN AUSTRALIA ....................51

Abstract.......................................................................................................................... 51

3.1 Introduction............................................................................................................. 53

3.2 Methodology ............................................................................................................ 56

3.3 Results and Discussion............................................................................................ 59

3.3.1 Topographic Controls ........................................................................................ 59

3.3.2 The Leeuwin Current ......................................................................................... 62

3.3.3 The Capes Current ............................................................................................. 71

3.3.4 The Ningaloo Current ........................................................................................ 73

3.3.5 Shark Bay Outflow............................................................................................. 77

3.4 Conclusions .............................................................................................................. 85

CHAPTER FOUR: HYDROGRAPHY AND WATER MASSES OFF THE WEST AUSTRALIAN COAST.....................................................................................87

Abstract.......................................................................................................................... 87

4.1 Introduction............................................................................................................. 89

4.2 Data Collection ........................................................................................................ 90

4.3 Results and Discussion............................................................................................ 91

4.3.1 Water Masses ..................................................................................................... 95

4.3.1.1 Tropical Surface Water (TSW) – Salinity Minimum.................................. 95

4.3.1.2 South Indian Central Water (SICW)—Salinity Maximum......................... 95

4.3.1.3 Subantarctic Mode Water (SAMW)—Oxygen Maximum ......................... 96

4.3.1.4 Antarctic Intermediate Water (AAIW)—Salinity Minimum.................... 100

4.3.1.5 Northwest Indian Intermediate (NWII) Water—Oxygen Minimum ....... 100

4.3.1.6 Shallow Oxygen Minimum...................................................................... 101

4.3.2 Surface and Sub-surface Current Systems ....................................................... 103

4.4 Conclusions ............................................................................................................ 112

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CHAPTER FIVE: DYNAMICS OF THE NINGALOO CURRENT OFF POINT CLOATES, WESTERN AUSTRALIA .............................................................115

Abstract........................................................................................................................115

5.1 Introduction...........................................................................................................117

5.2 Methodology ..........................................................................................................121

5.2.1 Numerical Model .............................................................................................121

5.2.2 Field Data .........................................................................................................124

5.3 Results and Discussion..........................................................................................125

5.4 Conclusions ............................................................................................................136

CHAPTER SIX: CONCLUSIONS AND RECOMMENDATIONS ....................139

6.1 Field Study .............................................................................................................139

6.2 Numerical Modelling Study .................................................................................142

6.3 Recommendations for Future Work ...................................................................142

REFERENCES ...............................................................................................145

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List of figures

Figure 1: Location map of the study area flanking the Gascoyne, Western

Australia 28

Figure 2.1: Generalised profile across the continental margin showing the

relationships between the provinces (adapted from Anikouchine and

Sternberg, 1981). 33

Figure 2.2: Bathymetric sections showing the shape of the seabed off the Gascoyne

coast. 34

Figure 2.3: Generalised current patterns in a typical ocean basin showing the

major circulation cells and the influencing wind systems (after Davis Jr.,

1986). 35

Figure 2.4: A common map of the surface circulation of the world (after Apel,

1987). The eastern boundary current off the Australian continent is frequently

erroneously depicted as flowing equatorward. 36

Figure 2.5: Annual average temperatures show downwelling along (a) Western

Australia, in contrast to typical coastal upwelling along (b) California and (c)

the west coast of South Africa (after Godfrey and Ridgway, 1985). 37

Figure 2.6: Schematic chart of large-scale circulation in the Indian Ocean (after

Pearce and Cresswell, 1985). 41

Figure 2.7: SeaWifs chlorophyll concentration image for 1st Nov 1997, showing an

inshore (northward) current swinging anti-clockwise at Point Cloates (Pearce,

1998). 44

Figure 2.8: T/S diagram showing a vertical profile of water masses found at a

station (24.580S, 111.650E) offshore Shark Bay. (Depths along T/S curve are in

metres. Isopleths of constant density are in σt. Cruise S05/86 station data

retrieved from CSIRO on-line database.) 45

Figure 2.9: T/S diagram envelopes showing different water masses in the surface

40m of coastal Gascoyne waters. (Isopleths of constant density are in σt.

Cruise FR01/96 station data retrieved from CSIRO on-line database.) 48

Figure 3.1: Location map of the study area including the RV Franklin cruise track

through the Gascoyne continental shelf. Location of the CTD stations are

shown as unfilled circles. 52

Figure 3.2: Bathymetry off the Gascoyne continental shelf and offshore regions.

Changes in the continental shelf and slopes of selected transects (B, E, G, I

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and J – see Figure 3.1) are shown to a maximum depth of 1000m with the

location of the 200m isobath. 57

Figure 3.3: Wind data (arrows point with direction of wind flow) collected on

board the vessel and together wind-roses from land based stations:

Learmonth, Shark Bay and Abrolhos Island for November 2000. 58

Figure 3.4: T/S diagram for 100m surface layer of water, from the coast to the

1000m isobath, including all 11 transects. Larger filled circles indicate LC at

its strongest poleward flow on each transect. 60

Figure 3.5: Surface sea temperature (SST) image together with surface current

vectors measured from the ship-borne ADCP. 61

Figure 3.6: Surface salinity distribution obtained from thermosalinograph data. 63

Figure 3.7: Transect D: cross-sections (to 150m) of (a) salinity, (b) temperature,

and (c) alongshore velocities. Shaded region indicates poleward flow. 64

Figure 3.8: Transect J cross-sections (to 150m) of (a) salinity, (b) temperature, and

(c) alongshore velocities. Shaded region indicates poleward flow. 65

Figure 3.9: (a) Alongshore velocities along the line of maximum LC poleward flow.

Shaded region indicates poleward flow >0.3 ms-1. (b) Surface alongshore

surface velocity across individual transects. 67

Figure 3.10: (a) Salinity and (b) temperature properties along the length of the

Leeuwin Current; and (c) geostrophic flow relative to 300db across the 1000m

isobath. Unshaded areas indicate flow towards the Leeuwin Current (ie

eastward flow). 68

Figure 3.11: (a) Anticyclonic eddy at Transect D, transporting warm, low saline,

tropical water. (b) Clockwise eddy at Transect I, anti-clockwise eddy at

Transect J; both transporting cooler, saline water from offshore. 70

Figure 3.12: Transect A cross-sections (to 150m) of (a) salinity, (b) temperature,


Figure 3.13 (a) Schematic diagram depicting detail of Ningaloo Current flow near

Point Cloates. (b) Shipboard ADCP data showing surface current flow. 75

Figure 3.14: Transect E cross-sections (to 150m) of (a) salinity, (b) temperature,


Figure 3.15: Transect F cross-sections (to 150m) of (a) salinity, (b) temperature,


Figure 3.16: Transect G cross-sections (to 150m) of (a) salinity, (b) temperature,


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Figure 3.17: TS-diagram for surface waters at Transect G, showing the presence of

a pronounced 35.2 water as well as Leeuwin Current water. 80

Figure 3.18 (a) T/S diagram at Transect H shows the presence of 35.2 water at 6

coastal stations. (b) Salinity profiles show the position of the 35.2 water to be

throughout the water column at the shallowest coastal station, and at the

deeper parts of 5 subsequent stations offshore. 82

Figure 3.19: Transect I cross-sections (to 150m) of (a) salinity, (b) temperature,


Figure 3.20: Schematic of the general surface circulation pattern of the major

currents along the Gascoyne continental shelf. Isobaths drawn at 100m

increments. 84

Figure 4.1: Location map of the research area including the positions of CTD

transect lines performed during voyages SS09/2003 and FR10/00, and CTD

stations (over 1000m-isobath) taken from voyages FR87/03 and FR87/04. 88

Figure 4.2: Temperature-salinity (with σT contours) and temperature-oxygen

diagrams exhibit interleaving positions of property extrema. 92

Figure 4.3: Three-dimensional ‘blocks’ of ocean depicting the cross-shelf (across

Transect J, the southernmost transect made by FR10/00) and along-shelf

(along 1000 m-isobath) distribution of (a) salinity extrema, and (b) dissolved

oxygen extrema. 93

Figure 4.4: Major water masses observed at the 1000 m-isobath along the Western

Australian shelf. Asterisks on the surface indicate CTD stations positions.

This chart combines data from voyage FR10/00 (21.3°–27.9°S) and voyage

SS09/2003 (28.1°–35.2°S). 94

Figure 4.5: Sparse CTD data indicate the major water masses observed at the 1000

m-isobath in 1987. SAMW was less ventilated in 1987 than in 2000/3 (Figure

4.4). Asterisks on the surface indicate CTD stations positions. 98

Figure 4.6: Differences between observations from 1987 (Figure 4.5) and 2000/3

(Figure 4.4) of (a) dissolved oxygen, and (b) salinity. Shaded areas indicate

values were greater in 1987 than in 2000/3. 99

Figure 4.7: 1000 m-isobath cross-sections of (a) dissolved oxygen, and (b)

chlorophyll. The dashed line in both charts traces the core of the band of

shallow oxygen minimum water. 102

Figure 4.8: Schematic diagram illustrating the general flow patterns at the

continental margin. 104

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Figure 4.9: Transect I cross-section of ADCP alongshore velocities (ms-1) shows

equatorward LU and coastal current, and a poleward LC. The trace lines

show the positions of the LC and LU as numerically modelled by Meuleners et

al. (2005). 105

Figure 4.10: Transect I cross-section of geostrophic flow (ms-1) relative to the

surface shows the LU flowing equatorward at a depth of 400 m. 106

Figure 4.11: Cross-section of dissolved oxygen levels for Transect I shows the

presence of a > 252 microM/L core at 400 m depth. 106

Figure 4.12: Transect I cross-sections of (a) salinity, and (b) temperature. Circles

along the surface indicate CTD stations positions. 108

Figure 4.13: Geopotential anomaly in m2s-2 plotted versus latitude from CTD

stations recorded along the 1000 m-isobath with straight-line fits. (a)

Calculated between depths 6 and 300 m, and (b) between depths 300 and 996

m. 109

Figure 4.14: Geopotential anomaly in m2s-2 plotted versus longitude from CTD

stations recorded along Transect E with straight-line fits. (a) Calculated

between depths 6 and 300 m, and (b) between depths 300 and 730 m. 110

Figure 4.15: Relationship among density, geostrophic velocity, and the slope of the

interface between layers, as given by Margule’s equation. (Adapted from

Knauss, 1997.) 111

Figure 5.1: Locality map showing the position of the research area that was used as

the numerical-modelling domain in this study. 116

Figure 5.2: Arrows indicate the general surface circulation pattern observed on a

Sea-Surface Temperature (SST) satellite image from Ningaloo, 18th November

2004. Isobaths are 200m and 1000m. 119

Figure 5.3: A 3-dimensional bathymetry chart detailing the shelf structure in the

vicinity of the Point Cloates 120

Figure 5.4a: CZCS satellite image from March 1980 showing no evidence of a NC

recirculation event in surface chlorophyll patterns south of the promontory at

Point Cloates. Surface wind speeds were low. 125

Figure 5.4b: (i) CZCS image from September 1980, and (ii) SeaWiFS image from

November 1997. Both show an anticlockwise re-circulation feature in the

surface chlorophyll south of Point Cloates, but no coastal current proceeding

north along the peninsula’s edge. 126

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Figure 5.4c: (i) CZCS image from May 1980 showing surface chlorophyll levels,

and (ii) AVHRR image from January 1991 showing sea-surface temperature.

Both display anticlockwise re-circulation features south of Point Cloates, as

well as the NC along the coast on both sides of the promontory. 126

Figure 5.5a: Model run R2. Simulated (depth mean) flow velocities resulting from

2 days’ forcing by a constant 2m/s southerly wind and a surface elevation

gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and

200m (see Figure 5-3). 128

Figure 5.5b: Model run R3. Simulated (depth mean) flow velocities resulting from



200m (see Figure 5-3). 129

Figure 5.5c: Model run R4. Simulated (depth mean) flow velocities resulting from



200m (see Figure 5-3). 130

Figure 5.5d: Model run R5. Simulated (depth mean) flow velocities resulting from



200m (see Figure 5-3). 131

Figure 5.6: Location of four transect lines across the numerical modelling domain.

Volume transports were calculated for the region coastward of the 70m

isobath. 132

Figure 5.7: Relationship between northward NC volume transport and wind-

forcing velocity, at four locations, i.e. 22.350S (a-a’), 22.60S (b-b’), 22.90S (c-c’)

and 23.1750S (d-d’). The NC volume transport recorded in field data taken

close to the location of c-c’ is also shown. 132

Figure 5.8: The fate of NC water travelling northward through the model domain.

133

Figure 5.9: NC distribution chart showing the percentage volume retained or lost

through the re-circulation event alone, under different southerly wind

velocities. 135

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List of tables

Table 1: Evidence of coastal equatorward currents along the West Australian coast

in summer. 43

Table 4: The different characteristics of each of the water masses found in the

1km-deep water column defined. This table combines data from voyage

FR10/00 (21.3 -27.9 S) and voyage SS09/2003 (28.1 -35 S).0 0 0 0 91

Table 5.1: Summary of general constants used in HAMSOM modelling work. 123

Table 5.2: Forcing combinations for each of the five runs. 124

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Acknowledgements

The following people are gratefully acknowledged for their contributions toward the

successful completion of this work:

My supervisor, Professor Charitha Pattiaratchi, for his continued support and guidance.

It has been a privilege working with, and learning from this gracious and learned

gentleman.

Dr William Schroeder, for his constructive comments and reviews, and for the

enthusiasm that he injected into the research each time that he came to visit.

The Captain, crew and scientific support staff of the RV Franklin and FRV Southern

Surveyor, for the successful execution of the voyages FR10/00 and SS 09/2003.

The RV Franklin shipboard scientific party: Christine Hanson, Tony Koslow, Elisabeth

Nahas, Peter Thompson, Anya Waite, for their assistance and constructive discussions.

David Griffin (CSIRO Marine Research) for supplying real-time satellite imagery for

use in Chapters Three and Five.

Michael Meuleners for generously making output-data from his numerical modelling

available for use in Chapter Four, and for helping with data retrieval from Geoscience

Australia.

CSIRO data centre for providing historical shipboard data used in Chapter Four.

British Ocean Data Centre, National Geophysical Data Center, and the Australian

Geological Survey Organisation for having ocean bathymetry data available.

Fellow residents of CWR room 2.14: Guy, Nicola, Matt, Kathy, Alexis, Claire and Ralf,

for providing stimulating conversation and contributing to a supportive, productive

working environment.

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My parents, Chong Wah and Doris, for their personal sacrifices in support of my

academic success, and for helping me get through difficult times of injury and poverty.

My sisters, Lynn and Lai Yee, for their love and constancy.

Jue Ru Shifu, Jue Ying Shifu and fellow postulants at the Australia Buddhist Bliss

Culture Mission, for shouldering some of my monastic duties so I had the chance to

finish my last paper (Chapter Five).

Nel, for accompanying me to the office in the small hours of morning to complete my

numerical modelling for Chapter Five.

This work could not have been undertaken without the support of the University of

Western Australia through a University Postgraduate Scholarship, Centre for Water

Research Adhoc Scholarship, and a Higher Education Contribution Scheme Exemption

Scholarship. The University of Western Australia and the Department of Environmental

Engineering provided financial assistance for my trip to San Diego, California, USA, to

present my scientific findings at the Oceans’03 conference.

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Preface

The body of scientific research in this thesis is presented as three separate chapters, all

of which are self-contained works. Chapter Three has been submitted to Continental

Shelf Research under the title of “Summer surface circulation along the Gascoyne

Continental Shelf, Western Australia” (Centre for Water Research reference ED 1898

MW). Chapter Four is to be submitted to Ocean Dynamics under the title of

“Hydrography and water masses off the West Australian Coast” (Centre for Water

Research reference ED 1899 MW). Chapter Five has been submitted to Marine and

Freshwater Research under the title of “Dynamics of the Ningaloo Current off Point

Cloates, Western Australia” (Centre for Water Research reference ED 1900 MW). To

maintain completeness of each chapter, a small amount of repetition in the background

and description of the study site has been unavoidable.

The content of this thesis is the author’s own work. Specific acknowledgements have

been made in the front of this thesis. Prof. Charitha Pattiaratchi is listed as the joint

author of each of the scientific journal papers produced from this study, in recognition

of the useful reviews and discussions that have come of the collaborative student-

supervisor relationship.

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Abstract

The Gascoyne continental shelf is located along the north-central coastline of Western

Australia between latitudes 21o and 28oS. This study presents CTD and ADCP data

together with concurrent wind and satellite imagery, to provide a description of the

summer surface circulation pattern along the continental margin, and the hydrography

present in the upper 1km of ocean, between latitudes 21° and 35°S. It also discusses the

outcome of a numerical modelling study that examined the physical factors contributing

to a bifurcation event persistently observed in satellite imagery at Point Cloates.

The region comprises a complex system of four surface water types and current

systems. The Leeuwin Current dominated the surface flow, transporting lower salinity,

warmer water poleward along the shelf-break, and causing downwelling. Its signature

‘aged’ from a warm (24.7oC), lower salinity (34.6) water in the north to a cooler

(21.9oC), more saline (35.2) water in the south, as a result of 2-4Sv geostrophic inflow

of offshore waters. The structure and strength of the current altered with changing

bottom topographies. The Ningaloo Current flowed along the northernmost inner coast

of the Gascoyne shelf, carrying upwelled water and re-circulated Leeuwin Current water

from the south. Bifurcation of the Ningaloo Current was seen south of the coastal

promontory at Point Cloates. Numerical modelling demonstrated a combination of

southerly winds and coastal and bottom topography off Point Cloates to be responsible

for the recirculation, and indicated that the strength of southerly winds affect

recirculation. Hypersaline Shark Bay outflow influenced shelf waters at the Bay’s

mouth and to the south of the Bay. The Capes Current, a wind-driven current from south

of the study region was identified as a cooler, more saline water mass flowing

northward. Results of the hydrography study show five different water masses present

in the upper-ocean. Their orientations were affected by the geopotential gradient driven

Leeuwin Current/Undercurrent system at the continental margin. The Leeuwin

Undercurrent was found at the shelf-slope, carrying (>252 μM/L) Subantarctic Mode

Water at a depth of 400m.

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Chapter One: Introduction

Out in space, if you gazed upon our world, you would see a glistening blue planet. But

how much do we know of ourselves, of our own world, this Planet Oceanus? With the

sheer vastness of the oceans making up most of our planet, the task of oceanographic

exploration and research is a massive undertaking, seemingly unending, stretching back

several millennia to a time when ancient mariners built crude vessels and bravely

ventured out into the great unknown of the seas. Today, with the aid of modern

technologies, such as satellites, acoustic profilers, computerised water-property sensors,

and a host of other scientific instruments aboard ships and in research stations on land

as well as in space, the tradition continues, as oceanographers carry forward the quest to

unravel the mysteries of the still largely unstudied oceans.

After all, in many ways the ocean is not unlike our mind. Its surface is filled with

constant activity, waves of ups and downs, and various influences from a diversity of

external phenomena. But in its deepest recesses, it is quite still and old; carrying with it

relics and imprints from encounters long ago. And all of it is a continuous and

connected whole, which in turn exerts its effects on the external world.

“How do we know anything, if we do not even look within and know our own minds?”

The same goes for our Planet Oceanus.

This study carries on in the spirit of the ancient mariners. Hopefully, looking within our

oceans, we may develop knowledge and understanding, to replace myth and ignorance,

so that we may be able to take care of our world, our lives and each other with greater

wisdom.

The goriest tale of Australian maritime history illustrates to us the need to understand

our coastal seas. During this tragedy which took place in 1629, the Batavia was

shipwrecked near the isolated Abrolhos shoals, off the Western Australian coast

(Saville-Kent, 1897). In desperation, the Captain and some of his officers set off in a

skiff to fetch supplies from the mainland. However, to their dismay, they were

overwhelmed by an unexpected coastal current that pulled them so far northeast that

they finally decided to proceed to Indonesia to find a vessel that would bring them back.

In the three months that followed, unspeakable atrocities ensued among the abandoned

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shipwrecked sailors. There were murders, madness, cannibalism and a bloodthirsty

mutiny, which left almost everyone dead.

Since those early days, the Western Australian population has rapidly grown into a

modern society of a couple of million people, most of whom live their lives on the

fringes of the sea. But nonetheless, much of the West Australian coast remains largely

unknown and unstudied. Consequently, as the population continues to uncover more

and more uses for the ocean (e.g. transport, aquaculture, marine parks), and to discover

valuable resources to be extracted from it (e.g. oil and gas mining, fish harvests), the

lack of appreciation for the governing marine processes has put both marine as well as

coastal inhabitants into a needlessly precarious position. Thus this study is an effort to

address this problem, and to develop a good understanding of the oceanic processes of

the Western Australian coastal region.

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1.1 The Gascoyne- a Significant Marine Environment

Along the central coastline of Western Australia is the Gascoyne region (Figure 1) – a

region possessing marine environments of remarkable ecological, scientific and

commercial significance. Ningaloo Reef stretches 260km along Gascoyne’s

northernmost coast (Figure 1). It is the only extensive fringing coral reef on an eastern

ocean boundary (Taylor and Pearce, 1999). At only 1-6 km from shore, it is also the

only extensive reef found so close to a continental landmass (Hearn et al., 1986). The

reef hosts a profusion of 250 species of coral, 520 species of tropical fish, and

significant populations of dugongs, humpback whales, shore birds and turtles (Preen et

al., 1997; WATC, 1998). One week after the full moon during March and April each

year, mass spawning of the corals occurs in a spectacular three-day event (CALM,

1998). This is followed soon after by the arrival of Whale Sharks (Rhiniodon typus)

(Taylor, 1996). From mid-March to mid-May each year, visitors from all over the world

converge at Ningaloo Reef to swim alongside these majestic creatures. Recent years

have seen the development of a healthy tourist and recreational industry in the Ningaloo

area. In 1987, under the management of the Western Australian Department of

Conservation and Land Management, the Ningaloo Marine Park was established

(CALM, 1998).

Situated south of the Ningaloo Marine Park (Figure 1) is a large (14,000km2), semi-

enclosed hypersaline coastal embayment called Shark Bay (Burling, 1998). In

recognition of its abundance of unique flora and fauna, Shark Bay was gazetted as

World Heritage in 1991 (GTA, 2000). An outstanding feature of the Bay is its

scientifically important seagrass banks. Shark Bay has the largest area of seagrass, and

the largest number of seagrass-species (12 species; 9 per m2 in some places) recorded to

date (CALM, 1998). A myriad of marine life exists in Shark Bay: there are green and

loggerhead turtles, manta rays, whales, several shark species, a secure herd of 16,000

dugongs, as well as the Monkey Mia dolphins that have become internationally known

for their penchant for interacting with humans (Preen et al., 1997; WATC, 1998). On

the western reaches of Shark Bay is Hamelin Pool, where underwater towers of rock-

like Stromatolites provide evidence of the earliest life forms that colonised the earth

some 3.5 billion years ago (Playford, 1979).

27

In addition to its natural beauty and rich diversity of marine life, the Gascoyne marine

region also exhibits considerable commercial potential. Already, there are nurseries for

penaeid prawns (Penaeus esculentus), saucer scallops (Amusium balloti), western rock

lobsters (Panulirus cygnus), pink snapper (Pagrus auratus), spot-tail and blacktip

sharks (Carcharhinus sorrah and C. tilstoni), anchovies, as well as aquacultures of pearl

oysters (based primarily on P. maxima) and freshwater aquarium fish. Moreover, the

aquaculture industry is set to expand further, with a Gascoyne Region Aquaculture

Development Plan having been put forward, and pilot projects (e.g. edible oysters, giant

clams, beta-carotene production) having been trialled successfully (Fisheries WA,

2000).

Figure 1: Location map of the study area flanking the Gascoyne, Western Australia

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1.2 Motivation

Considering the commercial, scientific and ecological importance inherent in the

Gascoyne marine environment (as discussed in section 1.1), it is imperative that the

dynamics of the ocean in which it exists be fully understood, as this knowledge would

lead: 1) to a better understanding of the marine ecosystems1, 2) to better management of

the wild fisheries (e.g. Phillips et al., 1978, Lenanton et al., 1991), 3) to an

understanding of the factors influencing rainfall (Weaver, 1990), and 4) to defining the

ocean circulation for sea safety, environmental protection and aquaculture. Furthermore,

in view of the oil industry’s interest in Cape Range Peninsula, an understanding of

current mechanisms at Ningaloo would be essential in any oil spill contingency

planning (Taylor and Pearce, 1999).

However, due to complexity of the summer ocean dynamics (section 2.2) and to lack of

data, the understanding of oceanic processes in the region remains extremely vague.

This project is an attempt to rectify this lack of knowledge.

1 Ocean conditions have been implicated to influence: coral reefs (Hatcher, 1991), seagrass beds (Walker,

1991), tropical organisms (Hutchins, 1991), and seabird distributions (Dunlop and Wooller, 1986;

Wooller et al., 1991); as well as the presence of migratory filter feeders e.g. whale sharks (Taylor and

Pearce, 1999).

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1.3 Objective

The principal objective of this study is to investigate the physical processes on the

Gascoyne continental shelf.

Specifically, the objectives are to quantify the following:

1. The summer circulation along the Gascoyne continental shelf.

In particular, to determine:

• shelf surface currents and their driving forces,

• the absence/presence of coastal upwelling, and

• the structure of the currents over varying continental shelf widths.

2. The hydrography of the upper ocean.

In particular, to describe:

• water masses,

• currents in deeper waters, and their driving forces,

• the effect of ocean currents on water masses at the continental margin.

3. The interaction between the northward coastal current and the southward Leeuwin

Current at the coastal promontory at Point Cloates.

In particular, to investigate:

• the anti-cyclonic re-circulation pattern identified from satellite imagery

immediately south of Point Cloates, and

• physical processes (eg. wind speed and direction) that contribute to

development of the re-circulation pattern.

30

Chapter Two: Literature Review

2.1 Local Setting of the Gascoyne

2.1.1 Climate

The Gascoyne region is situated on the Tropic of Capricorn, in the north west of

Western Australia. The region encompasses both tropical and temperate climatic

features: the northern part is arid and tropical, while the southern part tends towards a

more temperate, Mediterranean climate.

Climatic conditions in the Exmouth region (Figure 1) are dominated by tropical

cyclones, most of which occur during the summer months between January and March.

The climate is characterised by hot temperatures and low rainfall from November to

March. The majority of the rainfall occurs as a result of cyclonic activity. Rainfall is

highly variable but averages 278 mm per year. The mean daily maximum temperatures

are highest in January (380C) and lowest in July (240C).

South of Exmouth, the Carnarvon region (Figure 1) has a more moderate climate. Mean

daily maximum temperatures are at their highest in February and lowest in July, ranging

320C – 220C. In contrast to the northern part of the region, rainfall occurs mainly in

winter and averages 226 mm per year.

The Shark Bay area (Figure 1) has a dry, warm Mediterranean climate characterised by

hot, dry summers and mild winters. The mean daily maximum temperatures here are

similar to those of Carnarvon.

South-easterly winds are dominant over the Gascoyne coastal region through much of

the year. During winter (July), moderate southerly winds (3ms-1) occur near the shelf

edge. These winds strengthen from July through November, and remain strong through

summer (January-March), often blowing for several consecutive days at over 7 ms-1.

Then in May, a weaker, more variable winter wind pattern is again re-established

(Godfrey and Ridgway, 1985; Hearn et al., 1986; Taylor and Pearce, 1999).

31

2.1.2 Bathymetry

Profiles across continental margins are commonly categorised into different provinces

(Davis, 1986). The continental shelf may be seen as a shallow, gently sloping section

extending immediately seaward from the coast. This gentle province comes into contact

with a steep one (i.e. continental slope), followed by a gentler gradient at the continental

rise (Figure 2.1). Guided by this definition, the continental margin that flanks the

Gascoyne shall be examined2 and described.

The bathymetry off the Gascoyne coastline exhibits a range of continental shelf shapes.

As seen in Figure 2.2, the continental shelf3 north of Point Cloates is extremely narrow

(17km at Cape Range, 6km at Point Billie). It descends in a cliff-like manner into the

5km-deep Cuvier Abyssal Plain, without any visible slope-steepening at the 200m-

isobath to indicate the shelf break. South of Point Cloates (Figure 2.2), the coastline

veers sharply eastward away from the 200m-contour, making space for a gentler, much

wider shelf (e.g. 38km at Coral Bay). Southwards, the shelf remains wide (i.e. 62km at

Gnaraloo, 139km at Carnarvon, 84km at Dirk Hartog), whilst the shelf break becomes

an increasingly pronounced feature. As seen from Figure 2.2, the shelf break at Coral

Bay and Gnaraloo is distinguishable by a noticeable change in gradient, past the 200m-

isobath. And further south, at Carnarvon and Dirk Hartog, the break exists very

distinctly as the edge of a step-like structure.

Shark Bay, a large (14,000km2) semi-enclosed embayment, is located at the southern

section of Gascoyne (Figure 1). The Bay is shallow (average depth 10m) and

hypersaline, with a bottom that declines seaward. The coastline at Shark Bay is broken

at three places (Figure 2.2), namely at: 1) Geographe Channel- 35km wide, 35m deep,

2) Naturaliste Channel- 25km wide, 40m deep, and 3) South Passage- 2km wide, 6m

deep. These channels (especially the former larger two) serve as the only flux paths

between the Bay and the coastal shelf waters (Burling, 1998).

2 The author has processed shelf shapes through interpolation of a 5x5-minute gridded matrix of regional

bathymetry retrieved from http://www.ngdc.noaa.gov/mgg/global/seltopo.html. 3 Continental shelf at Gascoyne is taken to be the seabed shoreward of the 200m-isobath.

32

http://www.ngdc.noaa.gov/mgg/global/seltopo.html

Figure 2.1: Generalised profile across the continental margin showing the relationships between the provinces (adapted from Anikouchine and Sternberg, 1981).

33

Figu

re 2

.2: B

athy

met

ric

sect

ions

show

ing

the

shap

e of

the

seab

ed o

ff th

e G

asco

yne

coas

t.

Figure 2.2: Bathymetric sections showing the shape of the seabed off the Gascoyne coast.

34

2.2 Prominent Features of Ocean Circulation

2.2.1 Conventional Eastern Ocean Boundary Currents

Off the subtropical west coasts of continents in the Atlantic and Pacific Oceans, the

dominant currents (e.g. Canary, Benguela, Peru and California Currents) are typically

equatorward currents forming the eastern limb of subtropical gyres (Figure 2.3) (Church

et al., 1989; Cresswell and Peterson, 1993; Pearce, 1991; Smith et al., 1991). Generally,

these eastern boundary currents are recognised as steady surface flows of slow

(<10cms-1), broad (~1000km), cool waters, driven by equatorward wind drifts from

subtropical anti-cyclonic wind fields (Andrews, 1977; Allen, 1980; Huyer, 1990).

Figure 2.3: Generalised current patterns in a typical ocean basin showing the major circulation cells and the influencing wind systems (after Davis Jr., 1986).

In addition to being the dominant driving force of the eastern boundary currents, the

prevailing equatorward winds at the Atlantic and Pacific Oceans also drive offshore

surface drift, forcing persistent upwelling of cold nutrient-rich water to the surface at the

coast (Allen, 1980; Huyer, 1990). Consequently, a concomitant high rate of primary

production results there (Lenanton et al., 1991; Pearce et al., 1996).

35

Our study-area off the coast of Western Australia is located on the eastern boundary of

the Indian Ocean. The geography, topography as well as predominantly equatorward

direction of local winds in the region make it appear analogous to the other eastern

boundary current regions. Accordingly, the surface circulation off Western Australia is

routinely depicted to be a cool equatorward-flowing West Australian Current, forming

the eastern limb of an anti-cyclonic gyre similar to those of the other subtropical oceans

(Figure 2.4). However, in reality this is a misleading representation. Not only has there

been no evidence of a regular equatorward current within 1000km of the Western

Australian coast (Wyrtki, 1962; Hamon, 1965, 1972), the surface current that actually

flows there has been found to be warm and flowing in the opposite direction (Cresswell

and Golding, 1980), creating conditions more favourable to downwelling than

upwelling (Figure 2.5). This current has been named the Leeuwin Current in honour of

Leeuwin - the first Batavia-bound Dutch vessel to explore the waters off the

southwestern coast of Australia (Cresswell and Golding, 1980).

Figure 2.4: A common map of the surface circulation of the world (after Apel, 1987). The eastern boundary current off the Australian continent is frequently erroneously depicted as flowing equatorward.

36

Figure 2.5: Annual average temperatures show downwelling along (a) Western Australia, in contrast to typical coastal upwelling along (b) California and (c) the west coast of South Africa (after Godfrey and Ridgway, 1985).

2.2.2 Leeuwin Current

The Leeuwin Current has been the subject of extensive observational and modelling

studies. This section provides a comprehensive overview of these, followed by a

summary of the nature of the Leeuwin Current, as learnt from these studies.

2.2.2.1 Observational Studies

The earliest documentation of a poleward flowing current along the West Australian

coast was made by Saville-Kent (1897), who whilst studying marine fauna of the

Abrolhos Islands (28.50S), discovered an anomalously warm current of water

transporting tropical species to the region. Dakin (1919) analysed temperature data from

the same area, and noticed that the current was warm and most defined in winter. From

drift bottles and salinity measurements, Rochford (1969b) ascertained that the current

consisted of low salinity water that extended south of Rottnest Island (320S) in winter.

In summer, flow reversal occurred. Holloway and Nye (1985) found a similar seasonal

pattern, with maximum flows occurring along the southern portion of the Northwest

Shelf (220S) in February-June. Kitani (1977) observed transportation of low salinity

water to 320S in November 1975, thus showing that although the current appeared to

have a seasonal nature (being most pronounced in winter), its occurrence was not

confined to that period.

37

Through the course of time, further evidence of the Leeuwin Current was uncovered

through a host of different observations: e.g. ship drift observations (Nederlandsch

Meteologisch Institut, 1949), time series water property data (Rochford, 1969b),

historical bathythermograph data (Gentilli, 1972), research vessel surveys (Kitani, 1977;

Godfrey and Ridgway, 1985) and biological data sets (Wood, 1954; Colborn, 1975;

Krey and Babenerd, 1976; Markina, 1976). These established the current to be a surface

flow of warm, low salinity, nutrient depleted tropical water, beginning as a broad

(400km) and shallow (50m) stream at North West Cape, tapering (<100km wide) and

deepening (<300m) as it moved poleward along the continental slope (Church et al.,

1989; Smith et al., 1991; Pattiaratchi et al., 1998). The current transported 7 Sv of water

in midwinter (Smith et al., 1991), and 1.4 Sv in summer (Pearce, 1991). This seasonal

change in intensity was attributed to regional wind stress variability, i.e. the current

flows weakly against maximum southerly (opposing) winds in October-March, and

strongly against weaker southerly winds in April-March (Godfrey and Ridgway, 1985).

The current was also often associated with coastal downwelling.

Deployment of satellite-tracked drifting buoys in 1975-1977 added a new dimension to

data collected of circulation patterns off the Western Australian coast. In charting buoy

tracks, Cresswell and Golding (1979) observed the existence of mesoscale eddies on the

western side of the Leeuwin Current. Also, buoys from the eddies accelerated on entry

into the current and decelerated on exit; thus providing evidence of a high-speed core

current (clocking 170cms-1 from buoy positions) that was clearly defined on the

continental shelf break.

The mid 70s also saw the introduction of both the infrared and Advanced Very High

Resolution Radiometer (AVHRR) imagery. The high spatial resolution (1km) and

temperature discrimination (<0.10C) provided by these satellite techniques showed a

large wedge of warm water in Northeast Indian Ocean funnel into a narrow current near

North West Cape, and then move south along the shelf and slope (Pearce and Cresswell,

1985). Past Cape Leeuwin, it turned eastward to spread into the Great Australian Bight

(Legeckis and Cresswell, 1981). The eastward continuation into the Great Australian

Bight had previously been inferred from temperature data (Colburn, 1975) and plankton

data (Markina, 1976).

38

Between September 1986 and August 1987, very extensive current meter measurements

of the Leeuwin Current were made as part of the Leeuwin Current Interdisciplinary

Experiment (LUCIE). These measurements revealed that the current was strongest in

February–August and weakest in September–February (Boland et al., 1988). Smith et

al. (1991) reasoned that this seasonal variation in current strength resulted from

variations in wind-stress rather than in alongshore pressure gradient, since the latter had

little seasonal dependence. The LUCIE measurements also reflected a poleward

acceleration of the Leeuwin Current, as well as the presence of an equatorward

undercurrent (section 2.2.3) beneath it (Boland et al., 1988).

2.2.2.2 Modelling Studies

Thompson and Veronis (1983) were the first to model the Leeuwin Current. Their work

suggested that winter winds on the Northwest Shelf could generate a poleward current.

This theory was debunked by current meter observations (Holloway and Nye, 1985),

and also rejected by Thompson (1984). Thompson (1987) proposed instead that an

alongshore steric height gradient was the primary forcing mechanism, with winter

deepening of the mixed layer offsetting the effects of equatorward wind stress. Godfrey

and Ridgway (1985), who quantified contributions of alongshore pressure gradient and

equatorward wind-stress, supported this.

Godfrey and Ridgway (1985) also hypothesised that the large steric height gradient was

the result of flow from Pacific Ocean through Indonesian Archipelago into the

Northeast Indian Ocean. This agreed well with Gentilli’s (1972) previous suggestion

that a winter throughflow isolated in Northeast Indian Ocean during summer could be a

source for the Leeuwin Current. Later, support for this theory was revealed in satellite

imagery showing a large wedge-shaped mass of warm water off Northwest Australia

funnelling into a poleward current (Pearce and Cresswell, 1985).

The idea of an Indonesian throughflow causing the steric height gradient (Godfrey and

Ridgway, 1985) was rejected by McCreary et al. (1986), who postulated that the cause

was in fact a thermohaline gradient. Their model showed poleward surface flow as well

as an equatorward undercurrent comparable in strength to observations. Subsequently,

Kundu and McCreary (1986) modelled the throughflow alone. Production of a weak

39

poleward flow led them to conclude that the throughflow was a secondary forcing

mechanism.

Weaver and Middleton (1989) investigated contributions from both an alongshore

density gradient and warmer fresher waters from the Northwest Shelf. Although lacking

in mesoscale variability, their model presented a realistic Leeuwin Current. They thus

concluded that the Leeuwin Current was driven by an alongshore density gradient and

strengthened by the Northwest Shelf waters.

2.2.2.3 Overview

In a summary, the observational and modelling studies have shown the Leeuwin Current

(Figure 2.6) to:

• be produced by a pressure gradient that overwhelms the opposing equatorward

wind stress (this is generally agreed upon, despite dispute over the generation

mechanism of the pressure gradient itself);

• have a warm, low salinity, tropical source on the Northwest Shelf, possibly

originating in the Pacific Ocean;

• begin broad (400km) and shallow (50m) at North West Cape, narrowing

(<100km), deepening (<300m) and accelerating (to 1-1.5ms-1) poleward, whilst

being augmented by geostrophic inflow from the west;

• flow poleward along the continental shelf break, down the western coast,

pivoting at Cape Leeuwin to continue eastward into the Great Australian Bight;

• transport 7 Sv in midwinter and 1.4 Sv in summer;

• flow weakly against maximum southerly (opposing) winds in October-March

(summer), and strongly against weaker southerly winds in April-September

(winter);

• be associated with downwelling on the coastward side, cyclonic and anti-

cyclonic eddies on the seaward side and a cooler, more saline equatorward

undercurrent (Leeuwin Undercurrent) beneath it.

40

Figure 2.6: Schematic chart of large-scale circulation in the Indian Ocean (after Pearce and Cresswell, 1985).

41

2.2.3 Leeuwin Undercurrent

Very little is understood of the Leeuwin Undercurrent (LU). Off North West Cape and

Shark Bay, Thompson (1984) reported the existence of an undercurrent beneath the

Leeuwin Current at depths of 200-400m. He observed the undercurrent to transport 5 Sv

of high salinity (>35.8), oxygen-rich, nutrient-depleted South Indian Central Water

(SICW) at a rate of 32-40cms-1 northward, and then offshore. An equatorward

undercurrent was also apparent in steric height charts at 500db/3000db (Wyrtki, 1971)

and at 450db/1300db (Godfrey and Ridgway, 1985), as well as in current meter data (at

250–450m) from the LUCIE experiment (Smith et al., 1991).

Thompson and Cresswell (1983) reasoned that the source of the undercurrent was likely

to be cool, high salinity, high oxygen water from the surface of the South Indian Ocean.

Driven by an equatorward geopotential gradient located at the depth of the undercurrent

(Thompson, 1984), the water advected northward and downward underneath the

Leeuwin Current (Thompson and Cresswell, 1983).

2.2.4 Coastal Equatorward Current

Saville-Kent (1897) provided the earliest recorded observation of a distinct northward

coastal current up the western coast of Australia by recounting how sailors from the

shipwrecked Batavia (near the Abrolhos shoals) were unable to get to the mainland

because of prevailing currents that carried them too far northeast. Since then, other

studies have provided evidence of high-salinity currents, driven northward along much

of the Western Australian coastline by local summer southerly winds (Table 1).

Of particular interest to our study, is the northward coastal current indicated at Ningaloo

during summer, i.e. the Ningaloo Current (Taylor and Pearce, 1999). Evidence of the

Ningaloo Current first came about from aerial whale shark surveys made during 1990-

1992 and from current plume observations made from boats made during 1987-1992

(Taylor and Pearce, 1999). When the northward current was present along reef front, a

definite line became evident on the water, separating the calmer coastal waters from the

rougher Leeuwin Current pushing southward (against prevailing southerly winds) some

2km offshore. These preliminary observations showed the northward cool coastal

42

current to be present at Ningaloo between March and early April each year. Subsequent

confirmatory data were derived from satellite imagery (1991-1996) showing the

Ningaloo Current to be predominant from September through April (Taylor and Pearce,

1999).

Table 1: Evidence of coastal equatorward currents along the West Australian coast in summer.

Location Latitude Data Type Reference

Cape Leeuwin –

Cape Naturaliste

340S Satellite imagery,

current meter,

hydrological data

(Gersbach et al., 1999; Pearce

and Pattiaratchi, 1999)

Perth 320S Current meter (Cresswell and Golding, 1980)

Rottnest Island 320S Drift bottle movements,

hydrological data

(Rochford, 1969b)

Fremantle –

Abrolhos Islands

320-290S Drifting buoys (Cresswell and Golding, 1980)

Abrolhos Islands 290S Current meter (Cresswell et al., 1989; Pearce,

1997)

Geraldton 280S Current meter (Cresswell et al., 1985)

Carnarvon 250S Current meter (Smith et al., 1991)

Ningaloo Reef 230S Aerial surveys, boat

observations, satellite

imagery

(Taylor and Pearce, 1999)

Ningaloo Reef 230S Current meter (Smith et al., 1991)

In the southern reef where the shelf is wider (Figure 2.2), the Ningaloo Current flows

broadly (up to 3km wide), and is easily seen in satellite images. However, north of Point

Cloates where the seabed is cliff-like, the current is often less than 2km wide – too

narrow at times to be discernible in satellite imagery (Taylor and Pearce, 1999).

The Ningaloo Current is thought to be driven by southerly winds (Taylor and Pearce,

1999), much like the more southern coastal currents (e.g. Pearce and Pattiaratchi, 1999).

Considering records of cold water anomalies at Ningaloo coast (Simpson and Masini,

1986), Taylor and Pearce (1999) postulated that coastal upwelling might occur at

Ningaloo. However, convincing evidence for this has yet to be found (Taylor and

Pearce, 1999).

43

A recurring feature revealed by satellite imagery is an anti-cyclonic circulation pattern

located immediately south of Point Cloates (Figure 2.7). Although the actual dynamics

of this feature is still poorly understood, a Platform Transmitter Terminal (PTT) (which

fortuitously detached from a whale shark, effectively becoming a current drogue) has

indicated it to be a region of some degree of cross-shelf exchange/re-circulation (Taylor

and Pearce, 1999). Taylor and Pearce (1999) speculated that the presence of the

Ningaloo Current and the circulatory movement of water could have major implications

for the regional ecosystem. It had previously been accepted that mass coral spawning at

Ningaloo during March and April each year brought about a significant export of

protein out of the reef via the Leeuwin Current (Simpson, 1985). But Taylor and Pearce

(1999) hypothesised that if a circulatory movement retained the planktonic biomass

within the Ningaloo ecosystem, it would play an important role in the survival of the

reef. Also it might explain the presence of a very active food chain (possibly linked to

the concurrent appearance of filter-feeding whale sharks) at that time of year.

Figure 2.7: SeaWifs chlorophyll concentration image for 1st Nov 1997, showing an inshore (northward) current swinging anti-clockwise at Point Cloates (Pearce, 1998).

44

2.3 Hydrographical Structure

Generally, a 50-100m thick mixed layer lies on the surface of the Indian Ocean. Beneath

this, the ocean temperature falls rapidly through a thermocline, dropping to 50C at

1000m depth. Water properties within this topmost kilometre of ocean (commonly

known as the ‘upper-ocean’) are intricately layered so that a number of water masses

make up the upper Indian Ocean off the Gascoyne. These water masses can be

identified through examination of relationships on T/S diagrams, which are graphs

showing the relationship between temperature and salinity as observed together at, for

example, (a) various depths in a water column, or (b) various sampling stations across a

stretch of ocean.

Figure 2.8: T/S diagram showing a vertical profile of water masses found at a station (24.580S, 111.650E) offshore Shark Bay. (Depths along T/S curve are in metres. Isopleths of constant density are in σt. Cruise S05/86 station data retrieved from CSIRO on-line database.)

45

Offshore the Gascoyne, where water depth exceeds 1 km, the vertical water column T/S

diagram (e.g. Figure 2.8) typically shows the presence of i) surface tropical water, ii)

subtropical salinity maximum core, iii) South Indian Ocean Central Water (SICW) and

iv) Antarctic Intermediate Water (AAIW).

i) Surface tropical water - low salinity (<35.00), high temperature (22-250C) surface

water. Generally, the low salinity character of these surface waters derive from an influx

of low salinity Pacific Ocean water through the Indonesian Archipelago, as well as from

an excess of rainfall over evaporation in the NE quadrant of the Indian Ocean. This

water mass is also associated with low nutrient (near zero) and high dissolved oxygen

(near saturation 4.8-5.0 ml/L) concentrations.

ii) SICW (South Indian Ocean Central Water) - high salinity (>35.60), 170-190C waters,

typically found drifting northwards on 26.00 σt. This high salinity layer has been called

‘southern subtropical surface water’ (Muromtsev, 1959), ‘tropical surface waters’

(Ivanenkov and Gubin, 1960) and ‘subtropical surface water’ (Wyrtki, 1973). Depth of

the salinity maximum core decreases with latitude. Rochford (1969a) observed it at a

depth of 300m around 100S, and at less than 100m at 330S. And in-between those

latitudes (at 180S), Warren (1981) found the core at an intermediate depth of 250m. The

salinity maximum can be traced back to the sea surface at latitudes 250-350S where high

salinity water is found across the breath of the Indian Ocean (Wyrtki, 1971). On the sea

surface at these latitudes, an excess of evaporation over precipitation forms the high

salinity water (Baumgartner and Reichel, 1975). This then extends northward below the

surface water until 120-160S (Church et al., 1989), where it abuts against the low

salinity water flowing westward from the Indonesian Archipelago in the South

Equatorial Current (Sharma, 1972).

iii) SAMW (Subantarctic Mode Water) - low salinity (<35.00), 80-140C water, drifting

northwards on 26.80 σt surface. This water is associated with high dissolved oxygen

concentrations (>5.0ml/L) and is believed to originate by Subtropical Convergence at

latitudes 350-400S (Sverdrup, Johnson and Fleming, 1942; Muromtsev, 1959).

Reviewing names used for this water, Warren (1981) found that by ‘subtropical

subsurface water’ (a 120-150C layer), Muromtsev (1959) had indicated the upper part of

the same water mass. Later, in identifying the high oxygen properties of this mass,

46

Ivanenkov and Gubin (1960) extended the name ‘subtropical subsurface water’ to

include a layer virtually synonymous with the ‘Indian Ocean Central Water’.

Wyrtki (1973) adumbrated a different explanation for the formation of the water mass,

suggesting that the oxygen maximum was in fact a relic of deep vertical convection

rather than Subtropical Convergence. Corroborating with this theory, Colborn (1975)

and McCarthy (1977) provided evidence of well-mixed layers extending 400-500 m

from the sea surface at 400-500S latitudes in late winter, and subsequently named the

layer ‘Subantarctic Mode Water’. Retaining this name, Toole and Warren (1993) also

explained that a shallow (400-600 m) oxygen maximum in their 180S section formed

during winter cooling, by deep convective overturning in the zone between the

Subtropical Convergence and the Subantarctic Front.

iv) AAIW (Antarctic Intermediate Water) - low salinity (<34.6), 50-70C water, drifting

northward on 27.20 σt surface. This layer extends northward from the Antarctic Polar

Front to latitudes of 100-150S. It is associated with an oxygen minimum, and is thought

to flow more slowly than the overlying oxygen maximum layer (Warren, 1981).

v) NWII (North West Indian Intermediate) – high salinity (34.6), 3-90C water within

27.20-24.50 σt . Although the core of this water mass was beyond the depth range of the

station shown in Figure 2.9, influence by this water type can nonetheless be seen. A

water mass of similar description has been reported by Rochford (1961), Newell (1974),

Webster et al. (1979), Warren (1981) and Toole and Warren (1993) in other parts of the

Indian Ocean. It originates from the Red Sea (Rochford, 1969a).

47

Figure 2.9: T/S diagram envelopes showing different water masses in the surface 40m of coastal Gascoyne waters. (Isopleths of constant density are in σt. Cruise FR01/96 station data retrieved from CSIRO on-line database.)

The T/S diagram of a 40m-deep section of surface waters found along a CTD transect at

Ningaloo, shows the T/S signatures of three different water masses (Figure 2.9). The

Leeuwin Current (described in Section 2.2.2), a comparatively warm, low salinity water

mass, separates the cool, coastal Ningaloo Current (described in Section 2.2.4) from the

offshore afore-described “surface tropical water”.

48

2.4 Conclusion

This chapter has outlined some important aspects of the Gascoyne marine region,

including the regional climate, bathymetry and general hydrological structure. However,

it can be seen that there is a good deal of uncertainty with regard to (i) the summer

coastal circulation, eg. the Ningaloo Current and Leeuwin Undercurrent, (ii) details of

the three-dimensional water masses along the West Australian continental margin, and

(iii) the re-circulation pattern south of Point Cloates.

What is understood of these has either been inferred by comparisons with seemingly

analogous regions along the West Australian coast, or derived from speculations on

very limited data. In order to establish a real understanding of the oceanic system in the

region, direct studies have been made to address the following questions:

Summer surface circulation along the continental shelf

• What shelf currents are present?

• What are the driving forces of the shelf currents?

• Is the Ningaloo Current wind driven?

• Is there upwelling associated with the Ningaloo Current?

• How does the structure of the currents change as they progress along the shelf?

Subsurface circulation and hydrography

• Which are the major water masses along the West Australian coastline?

• What are their attributes and alongshore structures?

• Is the Leeuwin Undercurrent present?

• What is its driving force?

• How do processes at the coastal margin affect the cross-shore distribution of

water masses?

Re-circulation pattern south of Point Cloates identified from satellite imagery

• What physical processes contribute to the development of the pattern there?

• Can this pattern result from a combination of wind, local topography and the

Leeuwin Current? If so, what combination?

49

Consequently, three scientific papers have resulted. These are presented sequentially in

the following chapters. The papers are written as self-contained works, each focusing on

the study of a different domain of the Gascoyne coastal ocean. Firstly, a detailed

examination of the complex circulation of surface waters (top 300m) is presented. This

is followed by a description of the hydrography and subsurface circulation of the deeper

ocean, down to a depth of 1km. Finally, a numerical modelling study to further explain

the recurring re-circulation pattern observed in the surface waters at Point Cloates is

discussed.

50

Chapter Three: Summer Surface Circulation along the

Gascoyne Continental Shelf, Western Australia

Abstract


Australia between latitudes 21o and 28oS. This paper presents CTD and ADCP data

collected in November 2000 together with concurrent wind and satellite imagery, to

provide a description of the summer surface circulation pattern along the Gascoyne

continental shelf and slope. It is shown that the region comprises of a complex system

of currents that are influenced by offshore eddies, wind stress, varying shelf-widths,

coastal topography and outflow from the hypersaline Shark Bay. Four different water

types and current systems were identified from the field measurements.

The Leeuwin Current is the major current flowing through the region. It transports

lower salinity, warmer water along the 200 m isobath, poleward. The signature of the

Leeuwin Current gradually transformed from a warm (24.7oC), lower salinity (34.6)

water in the north to a cooler (21.9oC), more saline (35.2) water in the south resulting

from geostrophic inflow of offshore waters. The width and depth of the current also

changed continuously responding to the changing bottom topography and the

orientation of the coastline: in the northern section under the influence of the narrow

shelf and steep slope, the current was strong (~0.75 ms-1) and extended deeper into the

water column. In contrast, the current decelerated (to ~0.2–0.4 ms-1) when flowing past

the wider continental shelf offshore of Shark Bay and then accelerated along the

southern section along the steep continental slope. Downwelling events were

persistently associated with the current. The Ningaloo Current was confined to the

northern Gascoyne shelf within 35 km of the coast. Although upwelling was detected

along the northern section of the study region, adjacent to the Ningaloo coral reef, water

properties suggest a re-circulation of Leeuwin Current water from the south. Changes in

the shelf width at Point Cloates have a significant influence on the Ningaloo Current

resulting in bifurcation of the northward current. The higher salinity outflow from Shark

Bay influences the continental shelf region immediately offshore of the main entrances

to the Bay through the mixing of the higher salinity outflow water with the shelf waters.

51

The Capes Current, a wind-driven current originating to the south of the study region

was identified as a cooler, more saline water mass flowing northward.

Figure 3.1: Location map of the study area including the RV Franklin cruise track through the Gascoyne continental shelf. Location of the CTD stations are shown as unfilled circles.

52

3.1 Introduction

Eastern boundary current systems generally consist of cooler (associated with coastal

upwelling) equatorward currents. However, the exception to this general pattern occurs

along the eastern boundary of the Indian Ocean, where the Leeuwin Current (LC) flows

poleward along the Western Australian coastline (Church et al., 1989). Observational

and modelling studies undertaken over the past two decades have shown the LC to be

generated by a meridional pressure gradient that overwhelms the opposing equatorward

wind stress (Thompson, 1984, 1987; Godfrey and Ridgway, 1985; Weaver and

Middleton, 1989; Batteen and Rutherford, 1990; Pattiaratchi and Buchan, 1991). The

LC transports warmer, lower salinity, low nutrient water southwards, from a tropical

source off northwestern Australia. It flows broad (400km) and shallow (50m) at the

northern section of the study region, gradually narrowing (~100 km) and deepening

(~300m) as it accelerates (1-1.5 ms-1) poleward along the shelf break, whilst being

augmented by geostrophic inflow from the west (Hamon, 1965; Andrews, 1977; Church

et al., 1989; Smith et al., 1991). The strength of the current varies seasonally with a

volume transport of 5-7 Sv in winter (Smith et al., 1991; Feng et al., 2003) and 1.4 Sv

in summer (Pearce, 1991). This seasonal change in intensity has been attributed to

regional wind stress variability, i.e. the current flows weakly against maximum

southerly (opposing) winds in October-March, and is stronger from April to September

in the absence of strong prevailing winds (Godfrey and Ridgway, 1985). As a result of

onshore flow resulting from geostrophic balance, the current is also associated with

coastal downwelling (Smith et al., 1991).


Australia between latitudes 21o and 28o

S (Figure 3.1). The region encompasses both

tropical and temperate climatic features; the northern part is arid and tropical, while the

southern part tends towards a more temperate, Mediterranean climate. Southeasterly

winds prevail over the coastal region throughout much of the year. During winter (July),

moderate southerly winds (mean monthly mean of 3 ms-1) occur near the shelf edge.

These winds strengthen from July through November, and remain strong during summer

(January-March), often blowing for several consecutive days in excess of 7 ms-1. In the

summer months, the stronger winds result from a combination of the synoptic situation

and strong sea breezes (Pattiaratchi et al., 1997) In May, a weaker, more variable winter

wind pattern is re-established (Godfrey and Ridgway, 1985; Taylor and Pearce, 1999).

53

Inshore of the LC, evidence of higher salinity, wind-driven currents has been recorded

at various locations along the Western Australian coastline (Rochford, 1969; Cresswell

and Golding, 1980; Cresswell et al., 1989; Smith et al., 1991; Pearce, 1997; Gersbach et

al., 1999; Pearce and Pattiaratchi, 1999). The Capes Current (CC) along part of the

coastline north of 34oS (Figure 3.1) is well established around November when winds in

the region become predominantly southerly due to the strong sea breezes (Pattiaratchi et

al., 1997) and continue until about March when the sea breezes weaken. The dynamics

of the CC have been described by Gersbach et al. (1999). Here, the southerly wind

stress overcomes the alongshore pressure gradient which results in the surface layers

moving offshore, colder water upwelling onto the continental shelf, and the LC to

migrate offshore. The Capes Current is generally located inshore of the 50 m contour.

Gersbach et al. (1999) have demonstrated that inshore of 50 m contour, the wind stress

dominates over the alongshore pressure gradient (see also Thomson, 1987).

Of particular interest to the present study is the northward flowing Ningaloo Current

(NC) (Taylor and Pearce, 1999) postulated to be driven by southerly winds, similar to

the Capes Current to the south (e.g. Pearce and Pattiaratchi, 1999; Gersbach et al.,

1999). Considering records of cold water anomalies along the Ningaloo coast (Simpson

and Masini, 1986; Taylor and Pearce, 1999; Wilson et al., 2002), it has been suggested

that coastal upwelling occurs offshore of the Ningaloo coral reef. However, detailed

observational data to confirm this upwelling are unavailable.

A recurring feature of interest, revealed by satellite imagery, was an anti-cyclonic

circulation pattern located immediately to the south of Point Cloates (Figure 3.1).

Although the actual dynamics of this feature are still not clearly understood, a Platform

Transmitter Terminal (PTT) (which inadvertently detached from a whale shark and

became a current drogue) indicated it to be a region of some degree of cross-shelf

exchange/re-circulation (Taylor and Pearce, 1999). Through analysis of satellite

imagery and numerical modeling results, strength of the re-circulation has been related

to the local wind speed (see Chapter Five). It has been speculated (Taylor and Pearce,

1999) that the presence of the NC and the circulatory movement of water may have

major implications for the regional ecosystem.

54

Shark Bay, a large (14,000km2) semi-enclosed embayment, is located along the

southern section of the Gascoyne coastline (Figure 3.1). The Bay is shallow (average

depth 10m) and hypersaline, with a bottom that declines seaward. The Bay is open to

the ocean at three locations: (1) Geographe Channel - 35 km wide, 35 m deep; (2)

Naturaliste Channel - 25 km wide, 40 m deep; and (3) South Passage - 2 km wide, 6 m

deep. These channels, particularly the two largest, serve as the only flux paths between

the Bay and the continental shelf waters (Burling et al., 2003; Nahas et al., 2005). Shark

Bay receives minimal terrestrial runoff and experiences higher levels of evaporation

than rainfall. Thus, the salinities experienced inside Shark Bay are consistently above

oceanic levels and the innermost reaches regularly exceed 60 (Logan and Cebulski,

1970)

In this paper, we use CTD and ADCP data, collected in early austral summer

(November, 2000), together with meteorological data and satellite imagery to describe

the structure of the upper ocean current system, to a depth of 300 m, between Shark Bay

and North West Cape (Figure 3.1). For a description of the deeper waters, the reader is

referred to Chapter Four. The extensive data set revealed the dynamics of the different

surface current systems (the Leeuwin, Capes and Ningaloo Currents) on the Gascoyne

continental shelf during the summer. The physical oceanographic controls on primary

productivity of the region are presented in Hanson et al. (2005), while numerical

simulation of the mean flow properties of the LC system is presented in Meuleners et al.

(2005).

55

3.2 Methodology

Data were collected aboard the RV Franklin, a 55 m Australian National Research

Facility vessel, between 13 and 27 November 2000, at the beginning of the Austral

summer. Instrumentation used on board included a Neil-Brown Conductivity-

Temperature-Depth (CTD) profiler with 24x 5L-bottle Niskin rosette for calibration and

water sampling; a 150-kHz RDI Acoustic Doppler Current Profiler (ADCP) linked to

the Global Positioning System (GPS); meteorological sensors and a near surface

thermosalinograph.

Eleven standard transects were conducted across the continental shelf and upper slope

between 210S and 280S (see Figure 3.1). At each transect, 5-13 CTD stations were

occupied depending on the shelf width. The transects generally extended from the coast

(30 m isobath) to the 1000 m contour, with stations positioned over the isobaths of 30

m, 100 m, 150 m, 200 m, 250 m, 300 m, 500 m, 750 m, and 1000 m, and at intermediate

intervals between isobaths if they were widely spaced, especially along the continental

shelf region < 100 m depth. Surface salinity, temperature and water depth were

monitored continuously. In this thesis, salinity is expressed according to the practical

salinity scale and thus has no units.

Wind data during the voyage were recorded using the underway meteorological station

on board. In addition, half-hourly wind data were obtained from three land-based

meteorological stations at Learmonth, Shark Bay and the Abrolhos (North) Island

(Figure 3.3). During the cruise, concurrent Advanced Very High Resolution Radiometer

(AVHRR) and Sea-viewing Wide Field-of-view Sensor (SeaWifs) data were made

available through a collaborative arrangement with David Griffin of the CSIRO

Division of Marine Research (see also Griffin et al., 2001).

56

Figure 3.2: Bathymetry off the Gascoyne continental shelf and offshore regions. Changes in the continental shelf and slopes of selected transects (B, E, G, I and J – see Figure 3.1) are shown to a maximum depth of 1000m with the location of the 200m isobath.

57

Figure 3.3: Wind data (arrows point with direction of wind flow) collected on board the vessel and together wind-roses from land based stations: Learmonth, Shark Bay and Abrolhos Island for November 2000.

58

3.3 Results and Discussion

The extensive data set provides detailed insight into the dynamics of the coastal

circulation regime on the Gascoyne continental shelf and upper slope during the early

summer of 2000. It includes the Leeuwin, Capes and Ningaloo Currents as well as the

outflow from Shark Bay. Figure 3.4 shows the T/S diagram constructed from CTD data

taken from the 100 m surface layer across all 11 transects, from the coast out to 1000 m

isobath. Notably, signatures of Shark Bay and Capes Current waters were both

conspicuously separate from that of the LC. In contrast, the NC comprised re-circulated

Leeuwin water, so its signature and that for the LC overlapped. The signature for the LC

itself appeared elongated due to its ‘ageing’ as it progressed along the coast. The LC

changed its T/S characteristics markedly through the study area, and also its velocity

structure through acceleration, deceleration and directional changes.

3.3.1 Topographic Controls

Within the study region, the continental shelf and slope change remarkably and exhibit a

range of continental shelf shapes (Figure 3.2, see also James et al., 1999). North of the

coastal promontory at Point Cloates (Figure 3.1), the continental shelf4 is extremely

narrow (< 10 km). It descends in a cliff-like manner into the 5 km deep Cuvier Abyssal

Plain without any visible slope-steepening at the 200 m isobath to indicate the shelf

break (Figure 3.2). South of Point Cloates, the coastline veers sharply eastward away

from the 200 m contour, resulting in a broader shelf (~ 40 km at Transect E). At Shark

Bay (Figure 3.1), the shelf width is at a maximum (~85 km offshore Naturaliste

Channel), and the shelf break becomes an increasingly pronounced feature. To the south

of Shark Bay, the shelf width decreases with a distinct shelf break and the continental

slope steepens (Figure 3.2). These changes in topography largely influence the

circulation patterns within the region.

4Continental shelf along the Gascoyne shelf is taken to be the seabed shoreward of the 200m-isobath.

59

Figure 3.4: T/S diagram for 100m surface layer of water, from the coast to the 1000m isobath, including all 11 transects. Larger filled circles indicate LC at its strongest poleward flow on each transect.

60

Figure 3.5: Surface sea temperature (SST) image together with surface current vectors measured from the ship-borne ADCP.

61

3.3.2 The Leeuwin Current

The continental shelf-break circulation off Western Australia is dominated by the LC

and was clearly identified in the field data collected during the study. The satellite

derived SST indicated a large raft of warm water (> 24oC) present in the north (Figure

3.5), reflecting the source waters of the LC. The surface ADCP data collected along the

cruise-path indicated that the strongest southward LC flow, i.e. the ‘core’, was located

along the 200 m isobath (Figure 3.5). As the location of the 200 m isobath changed, the

location and the flow direction of the ‘core’ also changed (see below).

The surface temperature distribution showed higher temperature water in the north of

the study area and a general cooling towards the south. For example, at the northern

Transect A, the SST was 22-23oC whilst to the south of Shark Bay (Transects I and J),

the SST was 21-22oC (Figure 3.5). The presence of several meanders and eddies

entraining the warmer LC water offshore can also be identified from the SST imagery

(Figure 3.5).

The surface salinity distribution also indicates changes in salinity as the LC flows

southwards. In the northern region, the surface salinity is 34.6 increasing to 35.2 at the

southern end of the study region (Figure 3.6).

The cross-shelf CTD and ADCP transects provide detailed information on the cross-

sectional structure of the LC. For example, Transect D (Figure 3.7) demonstrates that

the LC can exist as a distinct core that has lower salinity (< 35 for this transect), is

warmer (> 22oC for this transect) than the surrounding water and flows strongly

poleward (up to 0.45 ms-1), transporting 1.67 Sv. Temperature and salinity isopleths

beneath the LC appear depressed, suggesting downwelling. Transect J (Figure 3.8)

displays similar features of a relatively warm, low salinity core, except that the

temperature and salinity values of the LC are now > 21.50C and < 35.25 respectively,

with the maximum current up to 0.75 ms-1 transporting 2.23 Sv.

62

Figure 3.6: Surface salinity distribution obtained from thermosalinograph data.

63

Figure 3.7: Transect D: cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow.

64

Figure 3.8: Transect J cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow.

65

As the LC flows southward along the Gascoyne shelf, it traverses over a region where

the width of the continental shelf varies greatly; from an extremely narrow cliff-like

structure in the north, to a very wide and gradual slope extending from Shark Bay, to a

shelf which narrows with a steeper slope towards the south (Figure 3.2). Consequently,

the structure and strength of the LC also changed (Figure 3.9). Examining the

alongshore surface flows in ADCP records taken across the 11 transects (Figure 3.9b),

the LC was observed to be narrow and strong in the north, flowing in a south-westerly

direction (Transect D), broadening and decelerating as it rounded the wide section of the

shelf flowing southwards (between Transects F and DH) and then rapidly narrowing and

accelerating to the south of Shark Bay, along a steepening shelf aligned south-south-east

(Transects I through J). Thus, at its narrowest, its surface width measured only 36 km

(Transect J) with a maximum surface current of 0.6 ms-1, while at its broadest its width

was in excess of 100 km with the maximum surface current 0.3 ms-1. Examining the

water column beneath the maximum LC surface poleward flow, it was also seen (by

tracking the -0.3 ms-1 contour) that where the LC decelerated and broadened (between

Transects F and DH), it also decreased in depth (Figure 3.9a). Between Transects A to

E, the -0.3 ms-1 contour was located at ~150 m. Its depth decreased to < 50 m as the

current flowed past Shark Bay and then increased again south of Shark Bay to ~200 m

(Figure 3.9a).

Through the extraction of CTD data collected at the station closest to the strongest

poleward flow, a CTD pseudo-transect line was constructed, slicing through the length

of the LC ‘core’ (Figure 3.10). These results indicate that the LC core became cooler

and more saline as the current progressed southward. Examining the CTD data, taken

from successive transects, it was impossible to define the LC presence merely by

tracking water of a fixed T/S signature, because as the LC moves southwards, the T/S

signature is continuously modified through mixing. The Leeuwin Current T/S signature,

beginning at the top left corner of the T/S diagram (i.e. higher temperature and lower

salinity), gradually moved towards the bottom right corner with lower temperature and

increased salinity (Figure 3.4) with a corresponding increase in density. Hence, through

the course of its passage within the study area, the core temperature and salinity of the

LC reduced by ~3oC, and increased ~0.6, respectively (see Figure 3.4).

66

Figure 3.9: (a) Alongshore velocities along the line of maximum LC poleward flow. Shaded region indicates poleward flow >0.3 ms-1. (b) Surface alongshore surface velocity across individual transects.

67

Figure 3.10: (a) Salinity and (b) temperature properties along the length of the Leeuwin Current; and (c) geostrophic flow relative to 300db across the 1000m isobath. Unshaded areas indicate flow towards the Leeuwin Current (ie eastward flow).

68

Evaporation and atmospheric cooling most probably contributed to the changes in water

properties. However, the major reason for the LC ‘ageing’ is the entrainment and

mixing with the offshore waters. The LC is driven by an alongshore geopotential

gradient coupled with an eastward geostrophic flow from the central Indian Ocean

(Smith et al., 1991). Using the CTD data collected during the present study, the

geostrophic inflow, relative to the 300db level across the 1000 m and 500 m isobaths,

was estimated to be 4 Sv and 2 Sv, respectively, with the highest inflow of water

between Transects F and I (see Figure 3.10c). These values are similar to previous

estimates of the geostrophic inflow predicted using field data (between 2-4.5 Sv from

Smith et al., 1991) and numerical modelling (2.2 Sv by Meuleners et al., 2005). The

source waters of the inflow are characterised by cooler, more saline water originating

from Indian Ocean Central Water. Meuleners et al. (2005) estimated that, within the

study region, up to 40% of the total flow of the LC can be derived from the geostrophic

inflow and thus the entrainment of the relatively cooler, more saline water into the LC is

the dominant mechanism for the LC ‘ageing’.

Eddies and meanders are features of the LC (e.g. Pearce and Griffiths, 1991) and the

present study documented oceanic eddies on three transects. At Transect D, a large anti-

cyclonic eddy was observed (see Figure 3.11a) and resulted in the water at the offshore

end of the transect to flow equatorward at speeds of up to 0.65 ms-1 (Figure 3.5). The

eddy consisted of warmer, lower salinity water, similar to the ‘younger’ LC water found

further north. Eddies were also observed at the ends of Transects I and J, spinning

clockwise and anti-clockwise, respectively (Figure 3.11b). In addition to entraining LC

into the direction of their flow (seaward and coastward, respectively), these eddies also

introduced cooler (20.9oC), more saline surface waters (35.4) into the Leeuwin Current

region.

69

(a)

(b)

Figure 3.11: (a) Anticyclonic eddy at Transect D, transporting warm, low saline, tropical water. (b) Clockwise eddy at Transect I, anti-clockwise eddy at Transect J; both transporting cooler, saline water from offshore.

70

3.3.3 The Capes Current

The structure of the continental shelf circulation along the southwest coast of Australia

during the summer months has shown the existence of a cooler northward current, the

Capes Current (CC), with the southward LC, in general, located further offshore

(Gersbach et al., 1999; Pearce and Pattiaratchi, 1999). Pearce and Pattiaratchi (1999)

observed the CC moving equatorward along the southern WA coast from Cape Leeuwin

(34oS) to north of Perth (32oS), and suggested that the CC may extend as north as the

Houtman Abrolhos Islands (29oS).

A cooler, higher salinity signature found at the coast on Transect J provides a likely

indication that the CC had reached a little further north (28oS). Its properties can be

identified on the T/S diagram (Figure 3.4) as a band of water with salinity between

35.37 and 35.53 and temperature between 21.0oC and 21.4oC. The cooler water can also

be identified in the satellite SST image between latitudes 30.5oS and 27oS (Figure 3.5)

and is also associated with higher salinity (Figure 3.6). The cross-sectional properties of

the current can be seen in Figure 3.8. Here, the continental shelf water, inshore of the

LC, indicates a northward current carrying cooler, more saline water northward at a

maximum alongshore velocity of 0.15 ms-1 or 0.2 Sv. There is no evidence of upwelling

along this transect. However, as the local winds were southerly, it is likely that the

cooler, more saline water resulted from both upwelling and advection into the study

region from the south. Along this transect enhanced biological productivity was

observed (Hanson et al., 2005) and is further evidence of upwelling.

71

Figure 3.12: Transect A cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow.

72

3.3.4 The Ningaloo Current

The Ningaloo Current, defined as a northward flowing current on the continental shelf

(Taylor and Pearce, 1999), was observed at the northernmost region of the study area,

between 21oS and 24.50S. As is typical of the coastal currents found along the coastline

further south of our study area, the NC at Transect A was also confined to within 35 km

of the coast, inshore of the LC (Figure 3.9b) (with exceptions where coastal offshoots at

Transects B and C occurred during the study). CTD and ADCP data indicated that the

coastal flow at Transect A comprised of colder (< 23oC) saline water (34.92) when

compared to offshore waters. This surface water mass, with a depth of 50 m, was

moving northward with the prevailing wind at a maximum surface alongshore velocity

of 0.35 ms-1 (Figure 3.12). The upward-sloping isohalines and isotherms beneath the

NC are indicative of coastal upwelling (Figure 3.12). Hanson et al. (2005) found that

the inshore region of Transect A contained higher nutrient concentrations which were

reflected in higher phytoplankton biomass and maximum regional primary production

rate of 1310 mg C m-2 d-1. This confirms the observation of coastal upwelling along this

transect with the source waters for upwelling originating from depths of ~100m (Figure

3.12; see also Hanson et al., 2005).

Elsewhere in the NC system, complexities to the flow are observed, most notably as a

result of interactions with a coastal promontory and its associated bathymetry, as well as

with the LC, which flows close to the coastline of the Exmouth peninsula.

The seaward extension of the coastal promontory at Point Cloates effectively blocks off

the broad, gradual southern shelf, leaving only a narrow, extremely steep shelf to the

north (see Figures 3.1 and 3.2). The reduction in the cross-sectional area, to the 50 m

contour, between to the south and to the north of the promontory is ~80%. Satellite SST

imagery and ADCP measurements showed that on the southern side of Point Cloates,

the NC flowed northward along the coast (Figures 3.5 and 3.7). On approaching the

promontory at Point Cloates, it turned anti-clockwise with the curvature of the

promontory, moving westward across the shelf at 0.2 ms-1 onto the shelf-break (see

Figures 3.5 and 3.13). This resulted in relatively cool, higher salinity (mean values:

22.7oC, 35.0) NC water being present over the inner shelf at Transect D (Figure 3.7). On

reaching the seaward margin of the promontory, part of the NC continued northward

73

towards North West Cape, inshore of the 200m contour and the Leeuwin Current, whilst

the rest of the current flowed poleward along with the LC (Figure 3.13). The dynamics

of this recirculation pattern using satellite imagery and numerical modelling is described

in Chapter Five.

In the satellite SST imagery, the arm of the NC proceeding northward along the edge of

the peninsula was seen as a narrow strip of cooler water along the coast (Figures 3.5 and

3.13). This flow, which is also reflected in the CTD data, transported water with

different TS characteristics to the local LC flowing offshore. However, its T/S signature

bore close resemblance to that of the ‘older’ LC found further south at Point Cloates.

This indicates re-circulation of the ‘older’ LC northward within the NC.

Whilst moving northward along the constricted shelf off the peninsula, the NC

interacted with the LC (which flowed in the opposite direction at the shelf-break very

close to the coast) and created two coastal offshoots along Transects B and C, extending

beyond the 1000 m contour (Figures 3.5 and 3.13). These offshoots, with surface

currents (cross-shore) up to 0.35 ms-1 flowed perpendicular to the coast (and thus the

Leeuwin Current) transporting colder, high productive water offshore (Hanson, et al.

2005). These two offshoots also had an influence on the geostrophic flow at the 1000 m

contour which reflects the offshore movement of water (Figure 3.10).

The arm of the NC that did not progress northward of the promontory was swept

southward along the LC against the wind. This produced the poleward NC observed in

Transect E (Figure 3.14). Here, although all the coastal water is moving southward, T/S

analysis clearly revealed that apart from LC water, there was the cooler, more saline NC

water present near the coast. Notably, the NC water detected both here and at the

previous transect (i.e. at Transects D and E) bore close resemblance to the ‘aged’ LC

water found in the next transect south (Transect F). It is likely that the water from the

LC at Transect F had been re-circulated northward up the coast as the NC. This receives

some support from the finding that the surface waters at the coastal boundary of the LC

were seen turning towards the coast and flowing northward. A northward flowing

coastal current was not identified at Transect E. It is possible that it had traveled

undetected to Transect D as a narrow coastal current beyond the coastal extent of the

transect. Alternatively, it may have resulted from previous (wind induced) flow-

reversals in the coastal waters.

74

(a)

(b)

Figure 3.13 (a) Schematic diagram depicting detail of Ningaloo Current flow near Point Cloates. (b) Shipboard ADCP data showing surface current flow.

75

Figure 3.14: Transect E cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow.

76

3.3.5 Shark Bay Outflow

Shark Bay is a semi-enclosed coastal embayment with open, deeper waters to the north

and two shallower Gulfs to the south, bounded by three large islands to the west (Figure

3.1). The Bay is open to the ocean through two main channels; Geographe and

Naturaliste Channels and a smaller channel, South Passage (Figure 3.1). Nahas et al.

(2005) demonstrated the existence of a two layer flow at the Geographe and Naturaliste

Channels with the less dense oceanic water (lower salinity) flowing into the Bay at the

surface and the higher density Bay water (higher salinity) exiting the Bay near the sea

bed. James et al. (1999) postulated the presence of a southward flowing plume of higher

salinity water on the continental shelf near the seabed. Data collected during this study

confirm these findings. The higher salinity of the outflow is clearly identified from the

T/S diagram as a distinct surface water mass on the continental shelf, with a temperature

range between 21.2oC and 22.9oC and salinity up to 36.1 (Figure 3.4).

At Transect F, which extended offshore from Geographe Channel (Figure 3.1), higher

salinity water (> 35.3) can be seen to flow from Geographe Channel out onto the shelf,

with the highest salinity water (36.0) present near the seabed (Figure 3.15). Similarly,

Transects G, H, DH and I, located to the south of Shark Bay, all contained higher

salinity water (up to 35.4) near to the seabed at coastal stations up to 30 km offshore. A

similar southward distribution of higher salinity waters on the continental shelf offshore

Shark Bay was reported by James et al. (1999). This indicates that the higher salinity

water after exiting Shark Bay flows southward due to the influence of the Coriolis force.

77

Figure 3.15: Transect F cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow.

78

Figure 3.16: Transect G cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow.

79

From close examination of the surface velocity map (Figure 3.5) and transect cross-

sections (Figure 3.16), it can be seen that at Transect G, whilst the LC continued to flow

poleward at the shelf-break, the current along the shelf was also flowing poleward. CTD

measurements show this coastal current to transport water of a distinctive 35.2 salinity

signature (Figure 3.17), well mixed in the vertical water column at the coastal CTD

stations landward of the 100m-isobath. This well mixed 35.2 water was subsequently

detected on the coastal side of the LC, through all the southern transects of the study

area (i.e. Transects G to J, Figure 3.1), hence existing as ‘common shelf water’. The

water did not result from mixing between the CC and LC, since its position at 25oS

could not be explained; the CC did not extend far enough north, and the ‘common shelf

water’ was flowing poleward. This coastal water would not have resulted from local

upwelling as the poleward current does not reflect upwelling.

35 35.05 35.1 35.15 35.2 35.25 35.3 35.35 35.4 35.45 35.5 20

20.5

21

21.5

22

22.5

salinity

Tem

pera

ture

(o C)

24.1 24.2

24.324.4

24.5

24.6

24.7

24.8

24.9

25

25.1

Figure 3.17: TS-diagram for surface waters at Transect G, showing the presence of a pronounced 35.2 water as well as Leeuwin Current water.

80

The most plausible explanation emerged from ship-based meteorological data, which

showed the wind streamed eastward on the shelf at Transect G, and then poleward on

approaching the land (Figure 3.3). Moreover, meteorological data from the land based

stations at both Shark Bay and the Abrolhos (North) Island showed that the northerly

wind persisted for 2 consecutive days. This wind pattern would have been conducive to

the development of a wind-driven southward coastal current, which in turn may have

promoted mixing between the Shark Bay outflow and the coastal edge of the LC,

forming the 35.2 water on the shelf. Transect H was performed a week after Transect G

was completed. In the time that elapsed in between, the winds had swung back around

to a southerly direction, and it could be seen from salinity profiles (Figure 3.18) that the

vertically mixed structure of the coastal water column had begun to collapse. Here,

although the 35.2 water was still present on the shelf at the 6 coastal stations (furthest

station being at 135m-isobath), apart from the well-mixed water column at the nearest

station, the upper 40 m of the other 5 coastal stations clearly exhibited more influence

by LC water.

Transect I extended seaward from South Passage, the southernmost and smallest

opening out of Shark Bay (see Figure 3.1). Here, inshore of the LC, evidence of the

‘common shelf water’ was also seen at the coast (Figure 3.19). However, salinity of the

water here was 0.02 higher than in the previous transects, due to the proximity to the

hypersaline bay water through South Passage. At Transect J, at the southern extent of

the research area, the ‘common shelf water’ was still present, albeit not immediately

adjacent to the coast, since this was now where the CC flowed (Figure 3.8). The 35.2

water was detected at the stations in the coastal edge of the LC. It is likely that the

coastal water had been swept along in the LC and taken offshore with it. It should be

noted, however, that because sustained northerly winds are generally not a regular

occurrence at the Gascoyne in summer, specifically 35.2 ‘common shelf water’ might

not be a frequently encountered phenomenon.

A schematic of the generalised surface flow regime, during the summer, based on the

field measurements presented here is shown on Figure 3.20. The main features of the

currents and associated water masses defined in this study are presented. The Leeuwin

Current flowed from north to south and was located along the 200 m isobath. Inshore of

the Leeuwin Current, three different currents and water types were identified: (1) the

Capes Current flowing northwards advecting cooler, higher salinity water from the

81

south; (2) Shark Bay outflow, warmer higher salinity water exiting from Shark Bay and

flowing southward; and, (3) the Ningaloo flowing northward, re-circulating Leeuwin

Current water as well as a contribution from upwelling of colder water. In addition to

the demarcation of the water types from the temperature/salinity characteristics, each of

these current systems has a unique biological identity with different primary production

regimes and phytoplankton species composition (Hanson et al., 2005).

Figure 3.18 (a) T/S diagram at Transect H shows the presence of 35.2 water at 6 coastal stations. (b) Salinity profiles show the position of the 35.2 water to be throughout the water column at the shallowest coastal station, and at the deeper parts of 5 subsequent stations offshore.

82

Figure 3.19: Transect I cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow.

83

Figure 3.20: Schematic of the general surface circulation pattern of the major currents along the Gascoyne continental shelf. Isobaths drawn at 100m increments.

84

3.4 Conclusions

The coastal circulation in the Gascoyne region, during the summer, comprises of a

complex system of currents and their interaction with oceanic eddies, wind patterns,

varying shelf-widths, a coastal promontory and a hypersaline bay.

The Leeuwin Current (LC) is the major current flowing along the Gascoyne continental

shelf. It transported lower salinity, warmer water along the 200 m isobath, poleward.

However, because the current is continually ‘ageing’ as it moves southward, cooling

and gaining salt, it could not be traced as a ‘river’ of water with fixed properties. The

signature at its core (i.e. maximum poleward flow) is gradually transformed from a

warm (24.70C), lower salinity (34.6) water in the north to a cooler (21.90C), more saline

(35.2) water in the south. Although evaporation and atmospheric cooling are

contributing factors to these changes, inflow of offshore waters due to geostrophic

balance is the main process that controls this transformation.

The LC also featured several meanders, eddies and offshoots. On its seaward side,

offshore eddies were observed interacting with the current; and on the landward side,

bathymetry and coastal topography also had an influence. Where unimpeded by seaward

jets or outflows from the bay, the LC flowed strongly at 0.6–0.75 ms-1. In contrast, it

slowed to about 0.2–0.4 ms-1 when flowing past the wider continental shelf offshore of

Shark Bay. Downwelling events were persistently associated with the LC.

The Ningaloo Current (NC) flowed along the northernmost coast of the Gascoyne

within 35 km from the coast. Although upwelling was detected, its water properties

were also clearly suggestive of re-circulation of LC water from more southern locations.

A recurring anticlockwise flow pattern, detected south of the promontory of Point

Cloates, would have caused such a re-circulation. Some indication of possible re-

circulation was also observed just north of Shark Bay at Transect F. The impact of such

a system on the coastal ecosystem may be significant, as it would imply that coastal

substances swept away by the LC could be re-circulated and returned up the coast in the

NC.

Shark Bay influenced the coastal system mainly through Geographe and Naturaliste

Channels. While the most hypersaline water was found on the seabed within 30 km

85

seaward of the Channels, the surface water mixed with the LC, forming a distinctive

vertical salinity profile of 35.2. This mixing was likely encouraged by persistent

northerly winds at the start of the voyage. The 35.2 coastal water was then swept

poleward and out seaward along with the LC. Because sustained northerly winds are

generally not a regular occurrence at the Gascoyne in summer, specifically the 35.2

coastal water may not be a recurring phenomenon.

A cool, relatively saline coastal current flowed equatorward along the southernmost end

of the research area. This was the northern continuation of the Capes Current (CC), a

wind-driven current originating from further south.

86

Chapter Four: Hydrography and water masses off the West

Australian Coast

Abstract

The water mass characteristics of the eastern Indian Ocean margin, between latitudes

21° and 35°S adjacent to the coastline of Western Australia are described using field

measurements. Results indicated the presence of five different water masses, in the

upper 1 km of the ocean, as interleaving layers of salinity and dissolved oxygen

concentrations. These included: (i) lower salinity tropical surface water (TSW); (ii)

higher salinity South Indian Central Water (SICW); (iii) higher dissolved oxygen

Subantarctic Mode Water (SAMW); (iv) lower salinity Antarctic Intermediate water

(AAIW); and, (v) lower oxygen North West Indian Intermediate (NWII) water. Through

comparison of the present data (collected in 2000 and 2003) with historical data (1987),

the interannual variability of the tropical surface water and subantarctic Mode Water

were identified and were linked to ENSO events.

Within the study region, the circulation pattern can be described as the Leeuwin Current

system consisting of three major currents: the Leeuwin Current (LC); Leeuwin

Undercurrent (LU) and shelf current systems consisting of the Capes and Ningaloo

Currents. Within the northern region of the study area, geopotential gradients were

found to drive both the Leeuwin Current (LC) and Leeuwin Undercurrent (LU) with a

(negative) sea surface slope of 4x10-7 driving the LC poleward, whilst a (positive) slope

of 1x10-7 beneath the LC was found to be the driving force of the LU equatorward. It

was also found, from cross-shelf geopotential anomalies, that the surface layer (in the

upper 300 m) sloped seaward at a gradient of 1.7x10-6, whilst the subsurface layer

(between depths of 300 and 730 m) sloped coastward at a gradient of 6.3x10-7. This

arrangement of geopotential slopes, together with the positions of the LU relative to the

LC, indicated a dynamical relationship between the LU and the LC. The data indicated

that the core of the LU to be located at a depth of 400 m, transporting SAMW water

equatorward.

The water mass distribution at the continental margin was influenced by the presence of

the Leeuwin Current and Leeuwin Undercurrent at the continental shelf break and slope,

87

respectively. Downwelling caused by the LC resulted in the surface and subsurface

water mass (SICW) and the upper edge of the SAMW to slope downward toward the

shelf break, whilst subsurface upwelling beneath the LU moved AAIW and the bottom

edge of the SAMW upward.

Figure 4.1: Location map of the research area including the positions of CTD transect lines performed during voyages SS09/2003 and FR10/00, and CTD stations (over 1000m-isobath) taken from voyages FR87/03 and FR87/04.

88

4.1 Introduction

The eastern Indian Ocean region, between latitudes 21° and 28°S, flanking the north-

central Gascoyne coastline of Western Australia (Figure 4.1) has received little focus.

Due an absence of data collected from the local waters, the hydrography of the region

has been inferred from transoceanic sections performed across other parts of the South

Indian Ocean. These include: (a) meridional sections along 110°E (Rochford, 1969a);

(b) zonal sections along 18°S (Warren, 1981; Field, 1997); and, (c) zonal sections along

32°S (Wyrtki, 1971; Toole and Warren, 1993). Within the study region, while the

accepted classical model of water masses for the upper 1 km of ocean consists of South

Indian Central Water (SICW) and Antarctic Intermediate Water (AAIW) (Pinet, 1992),

data from other studies (see a-c above) have revealed the presence of additional water

masses including lower salinity tropical surface water, Subantarctic Mode Water

(SAMW), and North West Indian Intermediate (NWII) water.

Along the eastern boundary margin of the Indian Ocean the circulation pattern is

described as the Leeuwin Current system, a system of three currents: the Leeuwin

Current; Leeuwin Undercurrent and shelf current systems consisting of the Capes and

Ningaloo Currents (Chapter Three). The Leeuwin Current (LC is an anomalous eastern

boundary current that carries warm relatively fresh tropical waters poleward along the

continental shelf break) (see Section 3.3.2). Extensive observational and modelling

studies have shown that the LC results from an alongshore geopotential gradient that

overwhelms the opposing equatorward wind stress (Thompson, 1984, 1987; Godfrey

and Ridgway, 1985; Weaver and Middleton, 1989; Batteen and Rutherford, 1990;

Pattiaratchi and Buchan, 1991). The LC flows weakly (1.4 Sv) against maximum

southerly (opposing) winds in October–March (summer) (Pearce, 1991) and strongly (7

Sv) against weaker southerly winds in April–September (winter) (Smith et al., 1991). It

is persistently associated with downwelling (see Chapter Three).

The Leeuwin Undercurrent (LU) flows northwards beneath the LC at depths of 200–400

m transporting 5 Sv of higher salinity (> 35.8) oxygen-rich nutrient-depleted water at a

rate of 0.32–0.40 ms-1 northward (Thompson, 1984). An equatorward undercurrent was

also apparent in steric height data at 500db/3000db (Wyrtki, 1971) and 450db/1300db

(Godfrey and Ridgway, 1985), as well as in current meter data (at 250–450 m) from the

Leeuwin Current Interdisciplinary Experiment (LUCIE) (Smith et al., 1991). The LU

89

has been postulated to be driven by an equatorward geopotential gradient located at the

depth of the Undercurrent (Thompson, 1984), and its water to be advected northward

underneath the LC transporting cooler, higher salinity, oxygen rich water from the

surface of the South Indian Ocean (Thompson and Cresswell, 1983).

Inshore of the LC, higher salinity wind-driven currents have been recorded along

various locations on the Western Australian coastline (Rochford, 1969b; Cresswell and

Golding, 1980; Cresswell et al., 1985; Cresswell et al., 1989; Smith et al., 1991; Pearce,

1997; Gersbach et al., 1999; Pearce and Pattiaratchi, 1999; also see Chapter Three).

Generally, the northward coastal currents are strongest in summer when the wind

pattern is predominantly southerly and has the greatest contribution from sea breezes

(Pattiaratchi et al., 1997).

In order to obtain a direct understanding of the ocean off the Gascoyne region of

Western Australia, a research cruise was conducted at the beginning of the austral

summer of 2000. In this chapter, we use CTD and ADCP data to describe the

hydrography of the upper ocean and to investigate the coastal circulation processes.

4.2 Data Collection

The data reported here were collected during the austral summer (between 13 and 27

November 2000) on eleven standard transects across the continental shelf between 21°S

and 28°S (see Figure 4.1). Instrumentation deployed onboard RV Franklin (a 55 m

Australian National Research Facility vessel) included a Neil-Brown Conductivity-

Temperature-Depth (CTD) recorder with a 24 x 5L-bottle Niskin rosette for calibration

and water sampling, a 150-kHz RDI Acoustic Doppler Current Profiler (ADCP) linked

to the Global Positioning System (GPS), a Turner Designs Fluorometer, meteorological

sensors, and a near surface thermosalinograph.

CTD data was also similarly obtained from FRV Southern Surveyor voyage SS09/2003

southward to 35.26°S. This voyage was completed over the October/November months

of the austral summer of 2003.

90

Additional historical CTD data5 obtained during the January–March period of the 1987

austral summer were retrieved from RV Franklin voyages FR87/03 and FR87/04 for

comparison purposes.


Five different water mass types were detected in the upper Indian Ocean along the West

Australian coast (see table 4) and they correspond with accepted classical water masses

of the Indian Ocean (Wyrtki, 1971; Warren, 1981). These were observed in the vertical

distribution of salinity and dissolved oxygen as interleaving layers of salinity and

dissolved oxygen. In terms of increasing depth these water masses were:

(i) lower salinity tropical surface water (TSW)

(ii) higher salinity South Indian Central Water (SICW)

(iii) higher oxygen Subantarctic Mode Water (SAMW)

(iv) lower salinity Antarctic Intermediate Water (AAIW)

(v) lower oxygen North West Indian Intermediate water (NWII)

Table 4: The different characteristics of each of the water masses found in the 1km-deep water column defined. This table combines data from voyage FR10/00 (21.30-27.90S) and voyage SS09/2003 (28.10-350S).

Water Mass Temperature

Range

Salinity Range Dissolved Oxygen

Range

Tropical surface water (TSW) 22 – 24.5 0C 34.7 – 35.1 200 – 220 μM/L

South Indian Central Water

(SICW)

12 – 22 0C 35.1 – 35.9 220 – 245 μM/L

Subantarctic Mode Water

(SAMW)

8.5 – 12 0C 34.6 – 35.1 245 – 255 μM/L

Antarctic Intermediate Water

(AAIW)

4.5 – 8.5 0C

34.4 – 34.6

115 – 245 μM/L

North West Indian

Intermediate water (NWII)

5.5 – 6.5 0C ~ 34.6 100 – 110 μM/L

5 CSIRO online database: http://www.marine.csiro.au/

91

http://www.marine.csiro.au/

Comparing the temperature-salinity diagram and a temperature-oxygen diagram

obtained from measurements from the northern study region, it is evident that the

salinity and dissolved oxygen distributions have an inverse relationship (Figure 4.2).

This is highlighted when examined through a three-dimensional view. A north-south

cross section along the 1000 m-isobath line, parallel to the shore is shown on Figure 4.3.

The salinity distribution along the cross-section indicates a salinity-maximum and two

minima (Figure 4.3a) whilst the dissolved oxygen distribution also indicates an oxygen-

maximum and two minima, but located at different depths to the depths of the salinity

maximum and minima (Figure 4.3b).

Figure 4.2: Temperature-salinity (with σT contours) and temperature-oxygen diagrams exhibit interleaving positions of property extrema.

92

(a)

(b)

Figure 4.3: Three-dimensional ‘blocks’ of ocean depicting the cross-shelf (across Transect J, the southernmost transect made by FR10/00) and along-shelf (along 1000 m-isobath) distribution of (a) salinity extrema, and (b) dissolved oxygen extrema.

93

Figure 4.4: Major water masses observed at the 1000 m-isobath along the Western Australian shelf. Asterisks on the surface indicate CTD stations positions. This chart combines data from voyage FR10/00 (21.3°–27.9°S) and voyage SS09/2003 (28.1°–35.2°S).

The location of each the five water masses and their relative position relative to each

other can be identified for the whole length of the coastline from North West Cape

(21oS) to Point D’Entrecasteaux (35oS) (see Figure 4.1 inset) using both salinity and

oxygen (Figure 4.4). In the following sections, the characteristics of each of the water

masses are discussed in detail.

94

4.3.1 Water Masses

4.3.1.1 Tropical Surface Water (TSW) – Salinity Minimum

A layer of lower salinity (< 35.1) warmer (> 22°C) tropical water was found in the

surface water in the northern region and corresponded with the temperature/salinity

characteristics of the Leeuwin Current water. This water mass is derived from the

Australasian Mediterranean Water (AAMW), a tropical water mass with origins in the

Pacific Ocean Central Water and formed during transit through the Indonesian

archipelago (Tomczak and Godfrey, 1994). Field data revealed that this surface water

mass was associated with lower nutrient (near zero) and higher dissolved oxygen

concentrations. (The reader is referred to Chapter Three for detailed discussions on the

different coastal surface water types and their dynamics).

At the North West Cape (21oS), the northern extent of the study region, this water mass

extends to 180m (Figure 4.4) with the surface salinity < 34.9. The depth of the water

mass decreases southwards with the passage of the Leeuwin Current and at ~26oS its

salinity signature (< 35.1) disappears. This is due to the dynamics of the Leeuwin

Current. The Leeuwin Current is driven by an alongshore geopotential gradient and

entrainment of cooler more saline South Indian Central Water (see below) from offshore

due to geostrophic inflow is a feature of the Leeuwin Current (Chapter Three).

4.3.1.2 South Indian Central Water (SICW)—Salinity Maximum

South Indian Central Water (SICW) is identified here as a salinity maximum layer (35.1

- 35.9). Along the 1000m bathymetric contour, ADCP data revealed its core moving

northward along 26.8 σT, with a maximum speed of 0.3 ms-1. However, near the shelf

break this same water mass is part of the Leeuwin Current flowing southwards (Chapter

Three). Here, ADCP data indicated that the Leeuwin Current extends up to 300m water

depth which is the total depth of this water mass (Figure 4.4). SICW had a temperature

range of 12 - 22°C and was associated with weak minima of dissolved nitrate, silica, and

phosphate. It was found at the surface south of 29.0°S and the depth of the salinity

maximum increased northward: from the surface at 29.0°S to 245 m at 21.5°S. In the

95

northern latitudes of the study region, the water mass subducted underneath the tropical

surface water derived from Australasian Mediterranean Water (AAMW). The

observation of surface salinity maximum is in agreement with Wyrtki (1971) who found

higher salinity water was found across the breath of the surface Indian Ocean at latitude

range 25°–35°S. At these latitudes, an excess of evaporation over precipitation forms the

higher salinity water at the sea surface (Baumgartner and Reichel, 1975). This is then

subducted below the surface water (Karstensen and Tomczak, 1997), extending

northward until 12°–16°S (Church et al., 1989) where it meets the lower salinity

Australasian Mediterranean Water (AAMW), flowing westward from the Indonesian

Archipelago in the South Equatorial Current (Sharma, 1972; Tomczak and Godfrey,

1994).

In addition to being termed ‘South Indian Central Water’ (Webster et al., 1979;

Rochford, 1969a) and ‘Indian Central Water’ (Karstensen and Tomczak, 1997), this

high salinity band has also been referred to as ‘southern subtropical surface water’

(Muromtsev, 1959), ‘tropical surface waters’ (Ivanenkov and Gubin, 1960) and

‘subtropical surface water’ (Wyrtki, 1973).

4.3.1.3 Subantarctic Mode Water (SAMW)—Oxygen Maximum

Beneath the South Indian Central Water (SICW), a water mass with high dissolved

oxygen concentrations of 245–255 µM/L can be identified as Subantarctic Mode Water

(SAMW), whose core occurred at 400–510 m. The data revealed that SAMW consisted

of water with temperature range of 8.5°–12°C and salinity range of 34.6–35.1. Its σT

value ranged between 28.9 and 29.5.

SAMW is formed by deep winter convection at 40°–50°S in the zone between the

Subtropical Convergence and the Subantarctic Front to the south of Australia (Wyrtki,

1973; Colborn, 1975; McCartney, 1977; Toole and Warren, 1993; Karstensen and

Tomczak, 1997). It is postulated that the SAMW formed to the south of Australia is

transported westward by the Flinders Current (Middleton and Cirano, 2002) and is the

source waters for the Leeuwin Undercurrent transporting water northward along the

WA coast (see below).

96

As SAMW is formed by deep convection rather than subduction, newly formed SAMW

penetrates to a greater depth than the newly subducted SICW (thus, is comparatively

better ventilated) and then moves northward from its formation region. Due to its high

oxygen content, the SAMW plays an important role in ventilating the lower thermocline

of the southern hemisphere subtropical gyres (McCartney, 1982).

SAWM also corresponds to the Indian Ocean Central Water (ICW) defined by Sverdrup

et al. (1942). SAMW and ICW often have similar temperatures and salinities;

consequently SAMW has been thought to contribute to the depth range of ICW

(Karstensen and Tomczak, 1997). According to Karstensen and Tomczak (1997), the

source characteristics of SAMW differ from region to region depending on prevailing

atmospheric conditions during its formation. Evidence of the SAMW make-up changing

with prevailing atmospheric conditions (as mentioned above) is found by comparing our

data, which was collected during a non-El Niño-Southern Oscillation (ENSO) years

(2000 and 2003), with data from 1987, an ENSO, year. The CTD data from RV Franklin

voyages FR03/87 and FR04/87 (Figure 4.1), a composite image of water masses

following the 1000 m-isobath along the West Australian coastline was constructed

(Figure 4.5). Although relatively low resolution compared to Figure 4.4 due to scarcity

of data, the overall picture from 1987 clearly shows the change in SAMW; its dissolved

oxygen signature was 5–10 µM/L higher in 2000 than it had been in 1987 (Figure 4.6a).

Additionally, the lower boundary of the SICW overlapped with the top of the SAMW

(Figure 4.5), the interaction resulted in lower salinity at that interface (Figure 4.6b).

Overall, the positions of all the major water masses remained relatively unchanged

between the years, and there were no changes to the core-defining signatures of any

other subsurface water mass. Understandably, because atmospheric conditions have

direct influence on the water, changes were seen in the surface water, i.e. the surface

salinity minimum was less intense, and surface water with as low salinity did not appear

to extend as far south in 1987 as in 2000. The Leeuwin Current would have been a

contributing factor to this observation since it transports more lower-salinity water

southward during ENSO years than non-ENSO years (Cresswell et al., 1989;

Pattiaratchi and Buchan, 1991).

97

Figure 4.5: Sparse CTD data indicate the major water masses observed at the 1000 m-isobath in 1987. SAMW was less ventilated in 1987 than in 2000/3 (Figure 4.4). Asterisks on the surface indicate CTD stations positions.

98

22 24 26 28 30 32

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dept

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-10

-10

-10

-10 -10

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(b)

Figure 4.6: Differences between observations from 1987 (Figure 4.5) and 2000/3 (Figure 4.4) of (a) dissolved oxygen, and (b) salinity. Shaded areas indicate values were greater in 1987 than in 2000/3.

99

4.3.1.4 Antarctic Intermediate Water (AAIW)—Salinity Minimum

Below the SAMW, a salinity minimum (34.4 - 34.6) was observed, indicating the

presence of Antarctic Intermediate Water (AAIW) along the coast. The water was cold

(4.5°–8°C) and the position of its core became shallower northward (core depth of 875

m at 27.5°S and 520 m at 21.5°S). Its σT values spanned 30.3–31.0.

It has been reported the AAIW extends northward from the Antarctic Polar Front to

latitudes 10°–15°S, and is thought to flow more slowly than the oxygen maximum layer

above it (Warren, 1981).

4.3.1.5 Northwest Indian Intermediate (NWII) Water—Oxygen Minimum

An oxygen minimum signature of < 110 µM/L in the northern region (21.3°–24.5°S)

indicated the presence of Northwest Indian Intermediate (NWII) water immediately

beneath the AAIW. Occupying depths of 800–1175 m, with σT values of 31.8–32.4, its

orientation implied southward deepening. As such, it is possible it extends further south

into the deeper ocean. The temperature of the NWII water was recorded at less than 5°C

and its salinity ranged 34.55–34.65. NWII water was associated with maxima of

dissolved nitrate, silica, and phosphate.

A similar water mass of Red Sea origin (Rochford, 1964) was observed by Rochford

(1961), Newell (1974), Webster et al. (1979), Warren (1981), and Toole and Warren

(1993) in other regions of the Indian Ocean. The low oxygen values are the result of in-

situ consumption of dissolved oxygen in water that has not been in contact with the

atmosphere for a long time, presumably due to much slower overall horizontal flow at

such depths (Warren, 1981).

100

4.3.1.6 Shallow Oxygen Minimum

Although not associated with a particular water mass, an oxygen minimum layer was

observed occurring beneath the surface layer throughout the study region (Figure 4.4). It

was associated with maxima in nitrate, silica, and phosphate concentrations. Its core

followed the 26.1σT level, reaching depths of 100–200 m. The intensity of the oxygen

minimum increased northward from 204 µM/L at 27.5°S to 175 µM/L at 21.5°S. A

similar oxygen minimum layer was reported by Rochford (1967, 1969a), Webster et al.

(1979), Warren (1981), and Church et al. (1989). Rochford (1969a) also found a salinity

minimum associated with the oxygen minimum layer and Rochford (1969a) postulated

that as the oxygen minimum strengthened southward, he concluded that the layer had

been formed by a southward advection of lower salinity, lower oxygen tropical water. It

is likely that the southward motion recorded by Rochford (1969a) in closer to the shelf

edge was actually the result of the Leeuwin Current’s southward motion, which

dominated the flow at the shelf break and slope (Section 3.3.2). In contrast to Rochford

(1969a), the data collected during this study does not indicate that the shallow oxygen

minimum layer was associated with a lower salinity layer. In fact, the oxygen minimum

layer was associated with higher salinity south of 23°S and lower salinity to the north.

This is because the depth of the shallow oxygen minimum’s core remained fairly

constant at approximately 160 m throughout the study region (Figure 4.4). Depending

on the latitude at which observations were made, the oxygen minimum layer was

present in two different water masses. To the north it existed within the surface tropical

water whilst to the south it was associated with the higher salinity SICW sometimes

within its core (i.e. at 26°–31°S).

The data indicate that the shallow oxygen minimum layer was closely related to a deep

chlorophyll maximum (DCM), centred at a depth approximately 80 m, found

throughout the study area. The shallow oxygen minimum layer was located directly

beneath the DCM at each station (Figure 4.7). This finding is important when coupled

with the detection of increased level of nitrate, silica, and phosphate concentration, as

well as an increasing minimum northward, as this provides evidence to reject

Rochford’s (1969a) hypothesis. Warren (1981) proposed that the water could be

northward flowing, with an intensifying oxygen minimum occurring when sinking

detritus gradually decayed and released nutrients. Photosynthetic organisms of the DCM

101

live at depths with an optimal balance of radiance from the surface and nutrients from

below (Hanson et al., 2005). Hence, the depth of the oxygen minimum layer forms

beneath the DCM regardless of the type of water mass present.

22 24 26 28 30 32 34

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250

300

latitude 0 E

dept

h m

fluorometer units

1015 20

15

253530

4035

30 25 3535

25 2015

1510

30

30

oxygen min line

oxygen min line

(a)

(b)

Figure 4.7: 1000 m-isobath cross-sections of (a) dissolved oxygen, and (b) chlorophyll. The dashed line in both charts traces the core of the band of shallow oxygen minimum water.

102

4.3.2 Surface and Sub-surface Current Systems

Along the West Australian continental shelf margin, there are three main current

systems which are collectively defined here as the Leeuwin Current System (Figure

4.8):

(1) a wind-driven equatorward coastal surface currents, the Capes and Ningaloo

Currents, on the continental shelf present mainly during the summer months under

strong southerly wind stress;

(2) Leeuwin Current (LC), a poleward surface current generally located along the

shelf break;

(3) Leeuwin Undercurrent (LU), an equatorward subsurface undercurrent located on

the continental slope.

The dynamics of the Capes and Ningaloo Currents as well as the Leeuwin Current have

been discussed in detail in Chapter Three. Here, we examine the dynamics of the

Leeuwin Undercurrent, which has received very little attention in the literature.

These three current systems can be identified clearly from a cross-shore ADCP transect

obtained along Transect I (Figures 4.1 and 4.9). Due to the depth limitation of the

ADCP (to approximately 300 m), only the upper portion of the LU can be identified;

nonetheless, the pattern was indicative of an undercurrent at greater depth. The pattern

observed in the ADCP data was closely correlated with the numerical modelling results

of the LU performed by Meuleners et al. (2005) which indicated the core of the LU to

be centred at a depth of 400 m (Figure 4.9).

103

Figure 4.8: Schematic diagram illustrating the general flow patterns at the continental margin.

104

The depth of the LU core can be confirmed by estimate of the geostrophic flow obtained

from CTD data (Figure 4.10). The LU is closely associated with the Subantarctic Mode

Water (SAMW) identified in section 4.3.1.3. Along Transect I, the longest cross-shore

CTD transect (Figure 4.1), a complete cross-section of the LU core can be identified

from the dissolved oxygen distribution: the core of the current consisted of a dissolved

oxygen maxima (252 μM/L) centred at a depth of approximately 400 m (Figure 4.11).

Although the LU may contain some SICW in the upper layers the data presented here

clearly shows that the LU consists of SAMW water and not a tongue of higher salinity

SICW water as reported by Thompson (1984) (Section 4.1).

Figure 4.9: Transect I cross-section of ADCP alongshore velocities (ms-1) shows equatorward LU and coastal current, and a poleward LC. The trace lines show the positions of the LC and LU as numerically modelled by Meuleners et al. (2005).

105

111.5 112 112.5 113

0

100

200

300

400

500

600

700

800

900

1000

longitude 0E

dept

h m

0

0.1

0.20.3 0.4

0.5 0.6

0.7

0.8

0.9

1

-0.1-0.1

-0.2

0

0

Figure 4.10: Transect I cross-section of geostrophic flow (ms-1) relative to the surface shows the LU flowing equatorward at a depth of 400 m.

Figure 4.11: Cross-section of dissolved oxygen levels for Transect I shows the presence of a > 252 microM/L core at 400 m depth.

106

The Leeuwin Current is driven by an alongshore geopotential gradient (Thompson,

1984; Godfrey and Ridgway, 1985; Smith et al., 1991) which through geostrophy

results in onshore flow and downwelling. This is clearly seen as downward-sloping

isotherms and isohalines at the shelf break (Figure 4.12) and results in the surface water

together with the SICW and the upper edge of the SAMW to be depressed downward

approaching the shelf break (Figure 4.3). Since the LU flows in the opposite direction,

the opposite occurs, resulting in concomitant upwelling. This is seen as upward-sloping

isotherms and isohalines beneath 400 m (Figure 4.12). Hence, the AAIW and NWII

water are drawn upward approaching the shelf slope, as is the bottom edge of the

SAMW (Figure 4.3).

The alongshore geopotential gradients estimated from the data from the present study

along the 1000 m isobath (Figure 4.13) indicate a negative sea surface slope of 4 x10-7

(from north to south) at the surface 0 to 300db level which is the driving force of the

Leeuwin Current (see above). In the 300 to 1000 db layer the slope was reversed with a

value 1 x10-7 which is the driving force of the LU equatorward. These values are

comparable with previous studies: Thompson (1984) found that the surface slope to be

3.6 x 10-7 whilst Smith et al. (1991) reported it be 2.6 x 10-7. Similarly, for the Leeuwin

Undercurrent, sub-surface slopes of 0.4 x 10-7 and 0.2 x 10-7 were reported by

Thompson (1984) and Smith et al., respectively. These results confirm that the

alongshore geopotential gradient remains almost constant through both seasonal and

inter-annual time-scales (Godfrey and Ridgeway, 1985).

The cross-shelf geopotential anomalies indicate (Figure 4.14) thicker surface layer

coastward and a thicker subsurface layer seaward. We have limited the calculations for

the subsurface layer to a maximum depth of 730 m in order to have sufficient data

points to plot a line with. To counter the scarcity of data points, calculations were

repeated for Transects E, F, G, and H, and average values for geopotential slope were

subsequently obtained (The other transects were excluded as their data proved

unsuitable for calculations due to interference from jets and eddies, or from insufficient

measurement depths). It was found that the surface layer (6–300 m depth) sloped

seaward at a gradient of 1.7x10-6, and the subsurface layer (300–730 m depth) sloped

coastward at a gradient of 6.3x10-7. In geostrophic balance, this arrangement of

geopotential slopes contributed to the poleward flow in the surface layer and the

equatorward flow in the subsurface layer.

107

Figure 4.12: Transect I cross-sections of (a) salinity, and (b) temperature. Circles along the surface indicate CTD stations positions.

108

21 22 23 24 25 26 27 286

6.5

7

7.5

8

8.5

9

9.5

10D

6/3

00geopotential anomaly m2s-2

21 22 23 24 25 26 27 287

7.2

7.4

7.6

7.8

8

8.2

latitude 0S

D 3

00/9

96

Figure 4.13: Geopotential anomaly in m2s-2 plotted versus latitude from CTD stations recorded along the 1000 m-isobath with straight-line fits. (a) Calculated between depths 6 and 300 m, and (b) between depths 300 and 996 m.

109

112.7 112.8 112.9 113 113.1 113.2 113.3 113.4 113.5 113.6 113.7 8.5

9

9.5

10

10.5

D 6

/300

geopotential anomaly m 2s-2

112.7 112.8 112.9 113 113.1 113.2 113.3 113.4 113.5 113.6 113.7 5

5.05

5.1

5.15

5.2

5.25

5.3

5.35

D 3

00/7

30

longitude 0E

Figure 4.14: Geopotential anomaly in m2s-2 plotted versus longitude from CTD stations recorded along Transect E with straight-line fits. (a) Calculated between depths 6 and 300 m, and (b) between depths 300 and 730 m.

110

Due to the Coriolis force, the LC remains adjacent to the coast but the dynamics which

control the LU, causing it to be adjacent to the continental slope is unclear. Thomson

(1984) suggested that the presence of the continental slope allows the constraint of the

earth’s rotation to be broken, although no theoretical explanation was presented. Based

on the geopotential gradients discussed above, however, we suggest a mechanism based

on Margule’s equation (Figure 4.15). At the shelf edge, the LC has an onshore

component of flow which results in the depression of the lower layer, particularly

beneath the LC. The cross-shore gradient of the subsurface layer is thus increased,

causing an associated LU flow close to the continental slope. This implies that

anywhere along the continental slope wherever the LC flow results in downwelling, an

undercurrent is induced to flow closely beneath it.

Along the southern shelf of the Australian continent, the Flinders Current, an

undercurrent along the continental slope has also been identified beneath the eastward

flowing LC. Here, the Flinders Current is flowing westward (Middleton and Cirano,

2001). Similar to the LU, the Flinders Current is also found along the shelf slope and

centred at a depth of 400 m (Middleton and Cirano, 2001). It is most likely that the

Leeuwin Undercurrent is a continuation of the Flinders Current transporting

subantarctic mode water (SAMW) from its generation region northward.

= flow out of paper

⎟⎟⎠

⎞⎜⎜⎝

⎛−−

=12

11222 ρρ

ρρ vvgfiρ1

ρ2

= flow into paper

Figure 4.15: Relationship among density, geostrophic velocity, and the slope of the interface between layers, as given by Margule’s equation. (Adapted from Knauss, 1997.)

111

4.4 Conclusions

Along the coast of Western Australia, five different water masses were identified as

interleaving layers of salinity and dissolved oxygen extrema. From the surface these

water masses were: (i) low salinity tropical surface waters; (iii) high salinity SICW;

(iv) low oxygen SAMW; (v) low salinity AAIW; and, (vi) low oxygen NWII water.

The shallow oxygen minimum layer was formed as a result of sinking detritus from a

gradually decaying DCM layer. This was confirmed in the detection of depleted oxygen

levels that intensified northward with the direction of flow, raised nutrient levels, lack

of a fixed salinity signature, and the persistent presence of a DCM immediately above

the shallow oxygen minimum band.

With regard to changes in the water masses due to prevailing atmospheric conditions, it

was found that the extrema for both the oxygen minimum of the SAMW and the salinity

minimum of the surface waters were weaker during the 1987 ENSO year than during

the anti-ENSO years of 2000/3, while attributes of the other subsurface water masses

remained relatively unchanged.

With regard to changes in the water masses upon approaching the West Australian

continental margin, it was found that coastal currents drew the coastward edges of the

water masses upward or downward depending on their location in the water column.

The LC flowing poleward at the surface along the shelf break caused water to

downwell. Consequently, beneath it (to a depth of 350 m), the surface water, SICW, and

the top part of the SAMW were depressed. The core of the LU carrying SAMW

equatorward flowed at 400 m, resulting in upwelling, and causing the uplifting of NWII

water, AAIW, and the bottom part of SAMW.

Geopotential gradients were found to drive both the LC and LU. In the study area, a

-4x10-7 slope drove the LC poleward, while a 1x10-7 slope drove the LU equatorward.

Cross-shelf geopotential anomalies show a 1.7x10-6 seaward gradient for the surface

layer (depth 6–300 m) and a 6.3x10-7 coastward gradient for the subsurface layer (depth

300–730 m). This orientation of geopotential slopes allows the direction of both

currents, and the position of the LC adjacent to the coast, to be explained by the Coriolis

effect; this cannot, however, explain the position of the LU at the shelf slope.

112

Invoking Margule’s equation, an explanation for the latter is proposed: when the added

surface mass at the coast (due to the LC flow) reduces the thickness of the lower layer,

the resulting increase in the subsurface layer’s cross-shore gradient produces a stronger

LU positioned close to the shelf slope.

113

114

Chapter Five: Dynamics of the Ningaloo Current off Point

Cloates, Western Australia

Abstract

The Ningaloo Current (NC) is a wind driven, northward flowing current present during

the summer months along the continental shelf between the latitudes of 22º and 24ºS off

the coastline of Western Australia. The southward flowing Leeuwin Current is located

further offshore and flows along the continental shelf break and slope, transporting

warm relatively fresh tropical water poleward. A recurrent feature, frequently observed

in satellite images (both thermal and ocean colour), is an anti-clockwise circulation

located offshore Point Cloates. Here, the seaward extension of the coastal promontory

blocks off the broad, gradual southern shelf, leaving only a narrow, extremely steep

shelf to the north. The reduction in the cross-sectional area, coast to the 50 m contour,

between southward and northward of the promontory is ~80%. Here, a numerical model

study is undertaken to simulate processes leading to the development of the

recirculation feature offshore Point Cloates. The numerical model output reproduced the

recirculation feature and indicated that a combination of southerly winds and coastal

and bottom topography off Point Cloates is responsible for the recirculation. The results

also demonstrated that stronger southerly winds generated a higher volume transport in

the NC and that the recirculation feature was dependent on the wind speed, with

stronger winds decreasing the relative strength of the recirculation.

115

Figure 5.1: Locality map showing the position of the research area that was used as the numerical-modelling domain in this study.

116

5.1 Introduction

The Leeuwin and Ningaloo Currents dominate the summer continental shelf dynamics

between 22º and 24ºS (Figure 5.1) off the coastline of north-central Western Australia.

The Leeuwin Current (LC) is stronger and flows along the continental shelf break and

slope, transporting warm relatively fresh tropical water poleward (Smith et al., 1991).

Over the past two decades, extensive observational and modelling studies have revealed

that LC is generated by a meridional pressure gradient that overwhelms the opposing

equatorward wind stress (Thompson, 1984, 1987; Godfrey & Ridgway, 1985; Weaver

& Middleton, 1989; Batteen & Rutherford, 1990; Pattiaratchi & Buchan, 1991). The

seasonal change in the LC is generally attributed to regional wind stress variability: in

summer the LC is weaker (~1.4 Sv) as it flows against maximum southerly (opposing)

winds and flows strongly (~7 Sv) in winter in the absence of strong southerly winds

(Godfrey & Ridgway, 1985; Pearce, 1991).

The northward flowing Ningaloo Current (NC) is located along the inner-shelf between

the LC and the coast. It is driven by the strong southerly wind stress (Taylor & Pearce,

1999) similar to the Capes Current along the Southwestern Australian coastline (Pearce

& Pattiaratchi, 1999; Gersbach et al., 1999). The early evidence for the presence of the

Ningaloo Current was obtained from aerial whale shark surveys undertaken in 1990–

1992 and from surface current plume observations from boats during 1987–1992

(Taylor & Pearce, 1999). These preliminary observations showed the NC moving

northward along the Ningaloo reef front, forming a distinct line in the water, separating

the coastal waters from the Leeuwin Current flowing southward (against prevailing

southerly winds) some 2 km offshore. Subsequently, satellite imagery (1991–1996)

revealed that the NC was predominant at Ningaloo from September through April,

although along the coastal segment north of Point Cloates where the shelf is very

narrow, the constricted (2 km wide) current was often indiscernible in satellite imagery

(Taylor & Pearce, 1999).

The structure of the continental shelf circulation during the summer months, along the

southwest coast of Australia, has been addressed in several recent studies using limited

field data and satellite imagery (Cresswell and Peterson, 1993; Pearce and Pattiaratchi,

1997; Pearce and Pattiaratchi, 1999; Gersbach et al., 1999). All of these studies have

117

shown the existence of a cooler northward current on the continental shelf with the

southward Leeuwin Current, in general, located further offshore. The Capes Current,

generally located along the southern part of the west coast appears to be well established

around November when winds in the region become predominantly southerly due to the

strong sea breezes (Pattiaratchi et al., 1997) and continues until about March when the

sea breezes weaken. The dynamics of the Capes Current are such that the southerly

wind stress overcomes the alongshore pressure gradient resulting in the surface layers

moving offshore, colder water upwelling onto the continental shelf, and the Leeuwin

Current to migrate offshore (Gersbach et al., 1999). Numerical model results have

shown that a wind speed of 8 ms-1 is sufficient to overcome the alongshore pressure

gradient on the inner continental shelf with the northward flowing Capes Current

generally limited to depth less than 50 m (Gersbach, 1999). The dynamics of the

Ningaloo Current is similar – i.e. a wind driven current, generally limited to depth

<50m. Based on records of cold water anomalies at Ningaloo coast recorded by

Simpson & Masini (1986), Taylor & Pearce (1999) suggested the possibility of coastal

upwelling at Ningaloo. This has now been confirmed by recent field data which

indicated that the NC, which has similar water characteristics to the Leeuwin Current, is

also associated with upwelling and high primary productivity with distinct

phytoplankton species (Hanson et al., 2005; also see Chapter Three).

A recurring feature, revealed by satellite imagery (Taylor & Pearce, 1999) and field

measurements (Chapter Three), was an anti-cyclonic circulation pattern located

immediately to the south of Point Cloates. Here, the Ningaloo Current appears to move

across the shelf, and then flow southward parallel to the coast and the LC (Figure 5.2).

Taylor and Pearce (1999) intimated that the cross-shelf exchange/re-circulation is

important to the dispersal of coral larvae and the retention of planktonic biomass within

the Ningaloo ecosystem. This recirculation pattern is the focus of this study, which aims

to examine the processes that may be responsible for the formation of this recirculation

feature through the application of a numerical model.

The study area (Figure 5.1) is situated on the Tropic of Capricorn in the northwest of

Western Australia. The winds are predominantly southeasterly throughout much of the

year. In July (austral winter), the wind blows moderately, approximately 3 ms-1 near the

shelf edge. It subsequently strengthens from July through November, and remains

strong through summer (January–March), frequently maintaining a velocity over 7 ms-1

118

for several consecutive days. In May, a weaker more variable winter wind pattern is

again re-established (Godfrey & Ridgway, 1985; Hearn et al., 1986; Taylor & Pearce,

1999).

Figure 5.2: Arrows indicate the general surface circulation pattern observed on a Sea-Surface Temperature (SST) satellite image from Ningaloo, 18th November 2004. Isobaths are 200m and 1000m.

Topography of the study region is non-uniform. In the northern section (Figure 5.1),

between Point Cloates and North West Cape, the shelf (200 m contour) runs parallel to a

straight coast along the edge of the peninsula. Here, the shelf is narrow (about 10 km

wide) and extremely steep. However, south of the peninsula, immediately beyond the

foreland at Point Cloates, the coast veers sharply eastward, allowing space for a gentler

much wider shelf. In this study, we focus on the area at Point Cloates where the

transition in bathymetry occurs most abruptly. From the detail provided in 1 km x 1 km

bathymetry data (Figure 5.3), it is apparent that north of the coastal promontory the

shelf descends into the abyss (Cuvier Abyssal Plain—see Figure 5.1) without any

distinct change in slope gradient to indicate a shelf break. Whereas southward

immediately beyond the foreland of the peninsula, the bathymetry is quite transformed,

with the inner shelf (shore to 20 m-isobath) exhibiting a more gradual slope, and a

terrace appearing at a depth of 65 m. The seaward extension of the coastal promontory

at Point Cloates effectively blocks off the broad, gradual southern shelf, leaving only a

narrow, extremely steep shelf to the north (see Figure 5.3). The reduction in the cross-

119

sectional area, to the 50 m contour, to the south and to the north of the promontory is

~80% and therefore would have a major influence on the dynamics of the Ningaloo

Current in this region.

Satellite imagery (Figure 5.2) revealed that at Point Cloates, the wind-driven coastal

current (Ningaloo Current) moved offshore in response to the westward extension of the

coastline, subsequently interacting with the southward-flowing Leeuwin Current and

recirculating in an anticlockwise direction. With this observation in mind, we applied a

numerical model to determine if wind, topography and the Leeuwin Current could

combine to simulate a wind-driven NC and the associated recirculation patterns

observed in shipboard data and satellite imagery (of sea-surface temperature and surface

chlorophyll distribution) recorded south of Point Cloates. The numerical modelling was

undertaken using HAMburg Shelf Ocean Model (HAMSOM), which was initially

developed by Backhaus (1985) for the North Sea shelf area. It is shown that the re-

circulating pattern is controlled by the topographic feature at Point Cloates and the

prevailing winds.

Figure 5.3: A 3-dimensional bathymetry chart detailing the shelf structure in the vicinity of the Point Cloates

120

5.2 Methodology

5.2.1 Numerical Model

HAMSOM is a three-dimensional, semi-implicit, finite-difference primitive equation

model developed by Backhaus (1985). It is governed by equations of i) mass

conservation, ii) momentum conservation in x- and y-directions, iii) hydrostatic

equilibrium, iv) conservation of temperature and v) conservation of salinity.

(i) Mass conservation:

0=∂∂

+∂∂

+∂∂

zw

yv

xu

(ii) Momentum conservation in x-direction:

⎟⎠⎞

⎜⎝⎛

∂∂

∂∂

+⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

∂∂

+⎟⎠⎞

⎜⎝⎛

∂∂

∂∂

+∂∂

−=−∂∂

+∂∂

+∂∂

+∂∂

zuAv

zyuA

yxuA

xxPfv

zuw

yuv

xuu

tu

HHρ1

Momentum conservation in y-direction:

⎟⎠⎞

⎜⎝⎛

∂∂

∂∂

+⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

∂∂

+⎟⎠⎞

⎜⎝⎛

∂∂

∂∂

+∂∂

−=+∂∂

+∂∂

+∂∂

+∂∂

zvAv

zyvA

yxvA

xyPfu

zvw

yvv

xvu

tv

HHρ1

(iii) Hydrostatic equilibrium:

0=+∂∂ g

zP ρ

(iv) Conservation of temperature:

⎟⎠⎞

⎜⎝⎛

∂∂

∂∂

+⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

∂∂

+⎟⎠⎞

⎜⎝⎛

∂∂

∂∂

=∂∂

+∂∂

+∂∂

+∂∂

zTK

zyTK

yxTK

xzTw

yTv

xTu

tT

vHH

121

(v) Conservation of salinity:

⎟⎠⎞

⎜⎝⎛

∂∂

∂∂

+⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

∂∂

+⎟⎠⎞

⎜⎝⎛

∂∂

∂∂

=∂∂

+∂∂

+∂∂

+∂∂

zSK

zySK

yxSK

xzSw

ySv

xSu

tS

vHH

here

u(x,y,z,t) = velocity component in the x-direction

x,y,z

A y viscosity coefficient for momentum

y viscosity coefficient for mass

AMSOM has been successfully applied to coastal and estuarine systems worldwide

w

v(x,y,z,t) = velocity component in the y-direction

w(x,y,z,t) = velocity component in the z-direction

f = Coriolis parameter

P( ,t) = pressure

ρ(x,y,z,t) = density

H = horizontal edd

Av = vertical eddy viscosity coefficient for momentum

g = gravitational acceleration

T(x,y,z,t) = temperature

S(x,y,z,t) = salinity

KH = horizontal edd

Kv = vertical eddy viscosity coefficient for mass

H

(Backhaus, 1985; Stronach et al., 1993; Pattiaratchi et al., 1996; Ranasinghe and

Pattiaratchi, 1998; Burling et al., 2003; Nahas et al., 2003). The model uses horizontal

layers of variable thickness with fixed permeable interfaces so that governing equations

are vertically integrated over the thickness of each layer to allow the horizontal velocity

components to become depth mean transports over the specified depth range. Implicit

algorithms enter an approximation for external gravity waves and vertical shear stress to

each space co-ordinate. In this study a kinematic boundary condition is applied at the

surface while a non-linear bottom stress condition is used at the bottom. At the lateral

boundaries, a no-slip condition was specified whilst at the open boundaries a clamped

boundary condition (fixed sea surface elevation in time) was used with the open

boundaries located some distance away from the region of interest (Figure 5.1).

122

The model domain included an internal 1 km x 1 km grid flanking the Ningaloo coast.

Bathymetry for this region (21.8°–24.4°S, 112.4°–114.1°E) was obtained from

Geoscience Australia (GA). The domain boundaries were then extended in 2 km grid

intervals northward to 20.5°S, southward to 25.5°S and westward to 106.4°E.

Boundaries were thus moved far from the domain of interest for the purpose of

minimising the boundary-effects upon the flow in the study area. Horizontal layer

thicknesses of 20, 50, 100 and 200 m, were prescribed.

A spatially and temporally invariant alongshore geopotential gradient was applied in the

north south direction to simulate the presence of the Leeuwin Current. The magnitude of

the gradient, equivalent to a sea surface gradient of 2 x10-7 was based on the field data

reported by Smith et al. (1991) and was successfully utilised by Pattiaratchi & Backhaus

(1992) for studies along the Perth Metropolitan coastline. A summary of parameters

used in the model is presented in Table 5.1.

Table 5.1: Summary of general constants used in HAMSOM modelling work.

Constant Value Description

CD 2.5 x 10-3 bottom drag coefficient

Hmax 200 m maximum depth of numerical domain

Δt 60 s time step

F 8.2 x 10-5 s-1 Coriolis parameter

G 9.81 ms-1 acceleration of gravity

123

Five numerical runs, each using the same bathymetry data and elevation gradient, were

performed, only with a different wind intensity blowing constantly from the south for

two simulated days (Table 5.2).

Table 5.2: Forcing combinations for each of the five runs.

Run Elevation Gradient Wind Forcing ms-1

R1 2x10-7gΔt 0

R2 2x10-7gΔt 2

R3 2x10-7gΔt 3

R4 2x10-7gΔt 4

R5 2x10-7gΔt 5

5.2.2 Field Data

The results of the numerical modelling were compared to field data collected between

13 and 27 November 2000 (during the austral summer) onboard the RV Franklin

(Chapter Three). Instrumentation used onboard included a 150-kHz RDI Acoustic

Doppler Current Profiler (ADCP) linked to the Global Positioning System (GPS) and a

Neil-Brown Conductivity-Temperature-Depth (CTD) recorder with a 24 x 5 L-bottle

Niskin rosette for calibration and water sampling. Winds during the voyage were

continuously logged by an underway meteorological station onboard, and monitored at

half-hourly intervals from three land-based meteorological stations at Learmonth, Shark

Bay and the Abrolhos (North) Island.

Additionally, Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Advanced Very High

Resolution Radiometer (AVHRR) and Coastal Zone Color Scanner (CZCS) satellite

images were used to identify instances of the re-circulation occurring at Point Cloates,

and to examine the surface structure of these events when they occurred.

124


Sea-surface temperature (SST) and surface-chlorophyll satellite imagery showed that

throughout the year there were changes to the structure of the coastal processes at Point

Cloates. There were many instances when there appeared to be no evidence of the

Ningaloo Current flowing along the Ningaloo coast, let alone the NC re-circulation

pattern (Figure 5.4a).

Occasionally, the NC was clearly seen flowing along the coast toward Point Cloates,

sweeping into its familiar anticlockwise pattern (Figure 5.4b); and yet on other

occasions the arm of the NC continued northward away from Point Cloates, flowing

broadly enough to be seen via satellite (Figure 5.4c).

Through numerical modelling undertaken using HAMSOM, it was found that re-

circulation at Point Cloates may result from the combined effects of a southerly wind, a

poleward LC (through the introduction of a surface elevation gradient) and local

bathymetry. Furthermore, altering the intensity of the southerly wind simulated the

differences in flow patterns during the re-circulation (as seen in satellite images—

Figure 5.4).

Figure 5.4a: CZCS satellite image from March 1980 showing no evidence of a NC recirculation event in surface chlorophyll patterns south of the promontory at Point Cloates. Surface wind speeds were low.

125

(i) (ii)

Figure 5.4b: (i) CZCS image from September 1980, and (ii) SeaWiFS image from November 1997. Both show an anticlockwise re-circulation feature in the surface chlorophyll south of Point Cloates, but no coastal current proceeding north along the peninsula’s edge.

(i) (ii)

Figure 5.4c: (i) CZCS image from May 1980 showing surface chlorophyll levels, and (ii) AVHRR image from January 1991 showing sea-surface temperature. Both display anticlockwise re-circulation features south of Point Cloates, as well as the NC along the coast on both sides of the promontory.

126

As stated in Table 5.2, the initial simulation R1 was performed with surface elevation

forcing alone. As expected, the results indicated a poleward surface flow (not shown),

which represented the LC. Subsequently, in simulation R2, the introduction of a

constant 2 ms-1 southerly wind clearly produced the NC in the surface 20 m adjacent to

the shoreline (mostly within the 20 m isobath) south of Point Cloates (Figure 5.5a). The

water then followed the curvature of the coastline westward at the promontory and

extended out across the shelf terrace (Figure 5.3). The movement of the NC as it

proceeded northward along the steep edge of the peninsula was not evident.

When the wind-forcing was increased to 3 ms-1 in simulation R3, the NC broadened and

was apparent along the narrow shelf north of Point Cloates (Figure 5.5b). While the NC-

flow strengthened within the surface 20 m layer all along the coast, it was now also

detected in the next 50 m-layer underneath. Moreover, there appeared to be a small

amount of upwelling, with the surface waters moving offshore and the deeper waters

moving onshore. Meanwhile, the cross-shelf motion of the NC at Point Cloates

continued to occur (Figure 5.5b).

Increasing the wind-speed to 4 ms-1 in R4 caused the events observed in R3 to become

more pronounced (Figure 5.5c). The surface NC broadened to the 65 m isobath.

Additionally, a circular shape began to form over the width of the shelf-terrace,

enabling a cross-shelf exchange south of the promontory at Point Cloates. This was

evident in the 20–70 m and 70–170 m layers.

Finally, strengthening the wind-forcing to 5 ms-1 in run R5 generated the strongest NC

flow up the coast to Point Cloates and beyond; as well as the clearest anticlockwise

sweep of the NC in the 20–70 m and 70–170 m layers at the foreland.

127

Figure 5-5a: Model run R2. Simulated (depth mean) flow velocities resulting from 2 days’ forcing by a constant 2m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3). Fi

gure

5.5

a: M

odel

run

R2.

Sim

ulat

ed (

dept

h m

ean)

flo

w v

eloc

ities

res

ultin

g fr

om 2

day

s’ f

orci

ng b

y a

cons

tant

2m

s-1 s

outh

erly

win

d an

d a

surf

ace

elev

atio

n gr

adie

nt. A

rrow

s poi

nt in

dir

ectio

n of

flow

. Iso

bath

s at 2

0m, 6

5m,1

10m

and

200

m (s

ee F

igur

e 5.

3).

latitude N

0 - 2

0m d

epth

-200-110

-65

-20

Sca

le:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-23.

2

-23

-22.

8

-22.

6

-22.

4

-22.

2

long

itude

E

20 -

70m

dep

th

-200

-110

-65-20

Scal

e:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-23.

2

-23

-22.

8

-22.

6

-22.

4

-22.

2

70 -

170m

dep

th

-200

-110

-65

-20

Sca

le:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-22.

2

-22.

4

-22.

6

-22.

8

-23.

2

-23

128

Figure 5-5b: Model run R3. Simulated (depth mean) flow velocities resulting from 2 days’ forcing by a constant 3m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3).

Figu

re 5

.5b:

Mod

el r

un R

3. S

imul

ated

(dep

th m

ean)

flow

vel

ociti

es r

esul

ting

from

2 d

ays’

forc

ing

by a

con

stan

t 3m

s-1 s

outh

erly

win

d an

d a

surf

ace

elev

atio

n gr

adie

nt. A

rrow

s poi

nt in

dir

ectio

n of

flow

. Iso

bath

s at 2

0m, 6

5m,1

10m

and

200

m (s

ee F

igur

e 5.

3).

latitude N

0 - 2

0m d

epth

-200

-110

-65-20

Sca

le:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-23.

2

-23

-22.

8

-22.

6

-22.

4

-22.

2

long

itude

E

20 -

70m

dep

th

-200

-110

-65

-20

Scal

e:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-23.

2

-23

-22.

8

-22.

6

-22.

4

-22.

2

70 -

170m

dep

th

-200

-110

-65-20

Sca

le:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-22.

2

-22.

4

-22.

6

-22.

8

-23.

2

-23

129

Figure 5-5c: Model run R4. Simulated (depth mean) flow velocities resulting from 2 days’ forcing by a constant 4m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3). Fi

gure

5.5

c: M

odel

run

R4.

Sim

ulat

ed (

dept

h m

ean)

flo

w v

eloc

ities

res

ultin

g fr

om 2

day

s’ f

orci

ng b

y a

cons

tant

4m

s-1 s

outh

erly

win

d an

d a

surf

ace

elev

atio

n gr

adie

nt. A

rrow

s poi

nt in

dir

ectio

n of

flow

. Iso

bath

s at 2

0m, 6

5m,1

10m

and

200

m (s

ee F

igur

e 5.

3).

latitude N

0 - 2

0m d

epth

-200

-110

-65-20

Sca

le:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-23.

2

-23

-22.

8

-22.

6

-22.

4

-22.

2

long

itude

E

20 -

70m

dep

th

-200

-110

-65-20

Scal

e:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-23.

2

-23

-22.

8

-22.

6

-22.

4

-22.

2

70 -

170m

dep

th

-200

-110

-65

-20

Sca

le:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-22.

2

-22.

4

-22.

6

-22.

8

-23

-23.

2

130

Figure 5-5d: Model run R5. Simulated (depth mean) flow velocities resulting from 2 days’ forcing by a constant 5m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3).

Figu

re 5

.5d:

Mod

el r

un R

5. S

imul

ated

(de

pth

mea

n) f

low

vel

ociti

es r

esul

ting

from

2 d

ays’

for

cing

by

a co

nsta

nt 5

ms-1

sou

ther

ly w

ind

and

a su

rfac

e el

evat

ion

grad

ient

. Arr

ows p

oint

in d

irec

tion

of fl

ow. I

soba

ths a

t 20m

, 65m

,110

m a

nd 2

00m

(see

Fig

ure

5.3)

.

latitude N0

- 20m

dep

th

-200

-110

-65-20

Sca

le:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-23.

2

-23

-22.

8

-22.

6

-22.

4

-22.

2

long

itude

E

20 -

70m

dep

th

-200

-110

-65

-20

Scal

e:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-23.

2

-23

-22.

8

-22.

6

-22.

4

-22.

2

70 -

170m

dep

th

-200

-110

-65

-20

Sca

le:

0.05

m/s

113.

311

3.4

113.

511

3.6

113.

711

3.8

-22.

2

-22.

4

-22.

6

-22.

8

-23.

2

-23

131

In order to examine the spatial distribution of the NC circulation and its response to

wind speed, data was extracted from each simulation to constitute vertical transects at

four latitudes across the model domain (Figure 5.6).

Initially, volume transport of NC, generated by different wind speeds, was calculated

for each transect. The results showed that, at each location along the coast, the NC

strengthened with increased wind-forcing (Figure 5.7).

Figure 5.6: Location of four transect lines across the numerical modelling domain. Volume transports were calculated for the region coastward of the 70m isobath.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 1 2 3 4 5 6

wind velocity (m/s)

volu

me

tran

spor

t (Sv

)

a-a'

b-b'

c-c'

d-d'

transect C (field data)

Poly. (c-c')

0.0836

5.52

Figure 5.7: Relationship between northward NC volume transport and wind-forcing velocity, at four locations, i.e. 22.350S (a-a’), 22.60S (b-b’), 22.90S (c-c’) and 23.1750S (d-d’). The NC volume transport recorded in field data taken close to the location of c-c’ is also shown.

132

The greatest perturbation to the northward flow of the NC occurred between transects b-

b’ and c-c’ where the re-circulation pattern was situated (Figure 5.8). Tracking the

volume of NC transported northward from transect d-d’, it was apparent that for wind

speeds higher than 2 most of the NC contributed to the re-circulation pattern at Point

Cloates, while a much smaller portion reached the coast further north. The reason for

this pattern was the topography/bathymetry feature at Point Cloates. The presence of the

promontory at Point Cloates acted as a barrier to any northward flow, thus the water was

forced westward with the curve of the land. As the section of shelf continuing on from

the promontory was very constricted, the volume of NC able to filter through was also

very restricted. Hence, most of the water only continued out across the shelf until it

encountered the LC and was swept poleward along with it. This was especially true at

lower wind speeds. For instance, when a 2 ms-1 southerly wind was only able to

generate a shallow coastal flow within the 20 m isobath, the NC north of Point Cloates

(at transect a-a’) was confined to a 3.5 km-wide strip along the shore, which was the

distance from the 20 m-isobath to the shore. The last row of columns in Figure 5.8

reveals that another effect of intensifying wind-forcing was a redistribution of the NC-

flow so that the percentage of it that had pushed northward past Point Cloates increased.

Conversely, the percentage of it that turned off into recirculation dropped.

0

10

20

30

40

50

60

70

80

90

% of total volume

lost b

etwee

n d-d'

& c-

c'

lost b

etwee

n c-c'

& b-b'

lost b

etwee

n b-b'

& a-

a'

volum

e gett

ing th

rough

2m/s

3m/s

4m/s

5m/s

wind speed

Figure 5.8: The fate of NC water travelling northward through the model domain.

133

This redistribution of the NC-flow was apparent on closer examination of the area at the

promontory, i.e. focusing on the transects immediately south (b-b’) and north (c-c’) of it

(see Figure 5.9). It was found that with the initial constant wind speed of 2 ms-1, only

12.7% of the northward flow remained along the coast, while 87.3% went out in to re-

circulation. However, increasing the wind-forcing raised the portion of NC directed to

b-b’, so that at winds of 5 ms-1 some 34.3% of the NC had emerged from c-c’.

Conversely, the portion of NC that turned westward off the coast lessened. Only 65.7%

of it contributed to the re-circulation in 5 ms-1 winds.

Field data provided some support to the accuracy of the modelling results. During the

fieldwork undertaken in November 2000 (see Chapter Three), a re-circulation event was

detected in satellite SST imagery. Similar to the simulated NC flow directions, the

actual recirculation also rotated anticlockwise and was located south of Point Cloates.

Northward past the promontory, relatively cooler NC water was seen continuing along

the coast. Concurrent shipboard CTD and ADCP data corroborated with this

description. Additionally, upwelling was also indicated in the field data and simulated in

the numerical modelling. Finally, the magnitude of volume transport in the simulated

NC was verified with that observed in the field. Using data from a location

corresponding to that of transect c-c’ (i.e. Transect D in Chapter Three), it was found

that the transport was 0.1 Sv. Consistent with the trend of simulated volume-transport

along numerical transect c-c’, such a volume would be produced by a wind forcing at

5.5 ms-1 (see Figure 5.7). Cross-referencing with shipboard underway wind data, it was

found that a very similar average northerly wind component of 5.7 ms-1 had been

recorded over field Transect D.

134

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6

wind velocity (m/s)

port

ion

of c

-c' w

ater

pre

sent

(%)

volume left at b-b'volume 'lost'

Figure 5.9: NC distribution chart showing the percentage volume retained or lost through the re-circulation event alone, under different southerly wind velocities.

135

5.4 Conclusions

Numerical modelling in this study demonstrates that the combined effects of ocean

bathymetry, Leeuwin Current flow, and a southerly wind can produce a northward

Ningaloo Current along the Ningaloo coast, its associated coastal upwelling, and a re-

circulation event south of Point Cloates. Generally, three circulation scenarios can occur

at Ningaloo:

1) Absence of a Ningaloo Current: This occurs when there is no southerly wind present

to drive a coastal current.

2) Ningaloo Current flows northward along the southern Ningaloo coastline, and

deflects offshore across the shelf at Point Cloates. However, along the coastline north

of Point Cloates the NC is weak.

This scenario occurs under weak southerly winds (~2 ms-1), which only have sufficient

energy to drive a shallow (< 20 m deep) coastal current northward along the inner shelf

(inshore of the 20 m-isobath). The promontory at Point Cloates obstructs any northward

flow so the NC water is forced to follow the curve of the coastline westward. Upon

clearing the promontory, the water can either round the corner to continue with the wind

northward along the coast, or swing around southward with the LC at the shelf break.

However, as a result of the constricted nature of the coastal passage northward (due to

an extremely steep shelf along the edge of the peninsula), the portion of water that

filters onto the northern coast is only a small fraction of that which floods across the

shelf and re-circulates.

In response to changes in wind intensity, the overall NC volume changes, as well as the

distribution of water at the Point Cloates junction. The weaker the southerly wind, the

smaller the NC volume and the lesser the percentage of water filtering through to the

northern section of coast. Hence, in weak southerly wind conditions (e.g. 2 ms-1) when

the NC along the northern Ningaloo coast is only a small percentage of an already small

volume, the Current is too insignificant to be detected in satellite imagery.

136

3) Ningaloo Current flows as a broad coastal current northward along the southern

Ningaloo coastline. On reaching the promontory, it splits into two arms. One arm

results in the anticlockwise re-circulation returning south with the LC; the other arm

continues northward along the edge of the peninsula.

This scenario occurs in a similar manner to the previous scenario except that here, the

wind-forcing is higher. With stronger southerly winds, the driving force is strong

enough to generate a broader deeper coastal current. Where there is a split in the current

at Point Cloates, stronger winds increase the proportion of NC water directed

northward. This percentage is still much smaller than what leaves the coast at the Point

Cloates re-circulation feature. Nonetheless, because the total volume in the NC is large,

flow along the northern coast is sufficiently large to be clearly detected in satellite

imagery.

137

138

Chapter Six: Conclusions and Recommendations

The principal objective of this study was to investigate the oceanographic processes in

the Indian Ocean along the Gascoyne continental shelf. Prior to this study, very little

was known of this ocean environment, which comprised a complex system of currents

and their interaction with oceanic eddies, wind patterns, varying shelf-widths, a coastal

promontory and a hypersaline bay. Our study has coupled detailed analyses of field data

with numerical modelling to develop a clear understanding of the physical ocean and its

coastal processes along the Gascoyne coast.

6.1 Field Study

Of the surface circulation, it had been established that the main current was the Leeuwin

Current (LC); a warm, relatively fresh surface current flowing poleward along the

continental shelf break. However, little was known of the way that the current

continually ‘aged’ as it moved southward, cooling and gaining salt along the way. This

study has shown that the signature at the core (i.e. maximum poleward flow) of the LC

gradually transformed from a warm (24.70C), lower salinity (34.6) water in the north to

a cooler (21.90C), more saline (35.2) water in the south. And it was quantitatively

determined that a 2-4Sv geostrophic inflow of offshore waters contributed to this

transformation.

There were also changes to the structure and strength of the LC along various locations

along the Gascoyne coast. On its seaward side, offshore eddies interacted with the

current, while on the landward side, bathymetry and coastal topography exerted an

influence. Where unimpeded by seaward jets or outflows from the bay, the LC flowed

strongly at 0.6–0.75 ms-1. In contrast, when flowing past the wider continental shelf

offshore of Shark Bay, the current broadened and shallowed according to the shape of

the shelf, and slowed to a speed of only 0.2–0.4 ms-1. Downwelling events were

persistently associated with the LC.

This research has presented evidence of the Ningaloo Current (NC) along the

northernmost coast of the Gascoyne, less than 35 km from the shore. Previously, such a

current had only been inferred from visual sightings made during aerial whale shark

139

surveys, observations of differences in sea surface roughness from boats, and patterns of

Sea-Surface Temperature seen from satellites. In addition, our study has established the

presence of upwelling associated with the NC, and detected re-circulation of LC water

from more southern locations into the NC. Such re-circulation was caused by a

recurring anticlockwise flow pattern detected south of the promontory of Point Cloates.

Some indication of possible re-circulation was also observed just north of Shark Bay.

The impact of such a system on the coastal ecosystem may be significant, as it would

imply that coastal substances swept away by the LC could be re-circulated and returned

up the coast in the NC. While providing an explanation for the retention of nutrients in

ecosystems at Ningaloo, this re-circulation is also indicative of a possible threat

upcoming oil fields in the north could pose to the inner coastal ecosystems.

It was found that Shark Bay affected the coastal system through Geographe and

Naturaliste Channels. The most hypersaline water was detected on the seabed within 30

km seaward of the Channels. This observation provides physical evidential support for

the numerical modelling results of Nahas et al. (2004), which indicated the outflow of

hypersaline water along the seabed. Furthermore, our study has shown that the surface

water from Shark Bay mixed with the LC, forming a distinctive vertical salinity profile

of 35.2. This mixing was likely encouraged by persistent northerly winds at the start of

the voyage. The 35.2 coastal water was then swept poleward and out seaward along

with the LC. Because sustained northerly winds are generally not a regular occurrence

at the Gascoyne in summer, specifically the 35.2 coastal water may not be a recurring

phenomenon.

A cool, relatively saline coastal current flowed equatorward along the southernmost end

of the research area. Thus the northernmost extent (28oS) of the Capes Current (CC), a

wind-driven current originating from further south, was found.

Down to a depth of 1km, masses of five different water types were detected as

interleaving layers of salinity and dissolved oxygen extrema within the upper ocean

flanking the Gascoyne. From the surface downwards, these water masses were: I) low

salinity tropical surface waters, II) high salinity SICW, III) low oxygen SAMW, IV)

low salinity AAIW, and V) low oxygen NWII.

140

Additionally, a shallow oxygen minimum band was identified. The origin of this layer

had previously been a puzzle to scientists (Warren, 1981). However, our study has

provided evidence to confirm Warren’s (1981) hypothesis that this layer is formed as a

result of the decay of sinking detritus. A deep chlorophyll maximum (DCM) layer

(discovered during our survey) was found persistently positioned above the oxygen

minimum band, which intensified equatorward, and was associated with raised nutrient

levels, and a lack of a fixed salinity signature.

With regard to the effect of prevailing atmospheric conditions on the water masses, it

was found that the intensity of both the oxygen minimum of the SAMW and the salinity

minimum of the surface waters were weaker during the 1987 ENSO year than during

the anti-ENSO years of 2000/3. Attributes of the other subsurface water masses

remained unchanged.

With regard to changes to the water masses upon approach to the West Australian

continental margin, it was found that the coastward edges of the water masses were

drawn upward or downward, depending on their location in the water column. Above

the depth of 350m, water masses were depressed due to downwelling beneath the LC.

While beneath the depth of 400m, water masses were uplifted due to subsurface

upwelling associated with the LU. The LU was identified at the shelf-slope, carrying

SAMW equatorward at a depth of 400m.

LC and LU were both found to be driven by geopotential gradients. In the study area, a

-4x10-7 slope drove the LC poleward, while a 1x10-7 slope drove the LU equatorward.

Cross-shelf geopotential anomalies showed a 1.7x10-6 seaward gradient for the surface

layer (depth 6-300m), and a 6.3x10-7 coastward gradient for the subsurface layer (depth

300-730m). This orientation of geopotential slopes allows Coriolis effect to explain the

direction of both currents, as well as the position of the LC close to the coast; but not

the position of LU at the shelf slope.

An explanation for the latter was put forward by invoking Margule’s equation, i.e. when

the added surface mass at the coast (due to the LC flow) squeezes the lower layer into a

smaller thickness, the resulting increase in cross-shore gradient of the subsurface layer

causes a stronger LU positioned close to the shelf slope.

141

6.2 Numerical Modelling Study

Numerical modelling was performed to provide greater understanding of the

recirculation pattern found south of Point Cloates. Through this study, the combined

effects of ocean bathymetry, LC flow, and a southerly wind have been demonstrated to

produce a northward NC along the Ningaloo coast, an associated coastal upwelling and

a recirculation event south of Point Cloates. Moreover, it was found that an increase in

southerly wind velocities not only increases the width, depth and volume transport of

the coastal NC, but it also raises the percentage of NC that pushes through the re-

circulation system to continue up the northern coast. The modelled volume transport,

circulation directions and upwelling pattern were verified with field data. Using the

findings from the numerical modelling, different circulation scenarios observed in

satellite images of Point Cloates could subsequently be explained.

6.3 Recommendations for Future Work

This work has provided a detailed understanding of the coastal circulation specifically

during summer. Summer had been the period chosen for study because the strong

prevailing southerly winds produced wind-driven coastal currents, which in turn

interacted with other topographic and oceanographic features, producing very complex

overall circulations. In winter however, southerly winds are much weaker, so it was

presumed that a strong Leeuwin Current would simply dominate the coastal circulation.

However, there have been no actual field studies at the Gascoyne to corroborate with

this view. In light of our numerical modelling results, which indicate the possibility of a

weak coastal current in winds as slight as 2ms-1, it is recommended that further studies

be made along the Gascoyne during the winter months, so that a more complete picture

of the seasonal variations to coastal circulation may be established.

Our study of subsurface coastal circulation off Gascoyne constitutes one of the earliest

field studies made directly of the Leeuwin Undercurrent. As such, it is hoped that future

work may build upon our initial framework of understanding, so that the undercurrent’s

path, and its changes in flow strength, structure and composition along the shelf may be

revealed. Also, we have hypothesised the position of the undercurrent to be controlled

by the presence of the Leeuwin Current at the shelf break. It is thus recommended that

142

further work be undertaken to determine if this is indeed the case, and if so, to further

establish if the Flinders Current and the Leeuwin Undercurrent are the same. Finally,

our study has shown upwelling to be a feature closely associated with the undercurrent.

It is proposed that biological research be undertaken to study the implications of this

process on the regional ecosystem.

143

144

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Documents

Summer Circulation and Water Masses along the West ...Figure 4.1: Location map of the research area including the positions of CTD transect lines performed during voyages SS09/2003