Seasonal variability of oceanographic conditions on the Pacific

continental shelf of the southern Baja California peninsula

O. Zaitsev*, C.J. Robinson** and O. Sánchez-Montante***

* Interdisciplinary Center of Marine Sciences, NacionalPolitechnical Institute, La Paz, Baja California Sur, 23096 Mexico.

** Institute of Marine Sciences and Limnology, Nacional Autonomous University of Mexico, D.F., Mexico

*** Center of Investigations in Applied Sciences and Advanced Technology, Politechnical Institute, Altamira, Tamaulipas, Mexico

Map and bathymetry of the study area

-116 -114 -112 -11022

24

26

28

118 116 114 112 110Longitude, W

22

24

26

28

30

32

Latit

ude,

N

B a j a C a l i f o r n i a

M e x i c o

P a c i f i cO c e a n

Gulf of California

Punta Eugenia

Ensenada

UlloaGulf

Magdalena Bay

Study area

• Area of interest is about 350 km long and up to 80 km wide.

• The Bahia Magdalena – Almejas lagoon system is located in the central part of the study area (24.5º N).

• A width of the continental shelf varies from 20-30 km (Magdalena lagoon system) to 70-80 km (Gulf of Ulloa).

• The average continental shelf gradient varies from 0.001 to 0.004. That of the continental slope from 0.03 to 0.06.

• The IMECOCAL grid of stations only covers the northern part of the Gulf of Ulloa.

Principal goals of the study

Seasonal variability of the thermal structure and circulation pattern on the continental shelf of the southern part of the Baja California peninsula

Can tropical and subtropical waters penetrate northward along the coast as far as Punta Eugenia?

Evaluation of upwelling intensity on the Pacific shelf of the southern part of the Baja California Peninsula.

Introduction

The most important forcing mechanism that affect both the California Current system and shelf circulation is wind stress.

Seasonally averaged sea surface wind in 2000.(PFEL/FNMOC data, www.pfeg.noaa.gov)

Jun-Mar 2001-05 Apr-Jun 2001-05

Oct-Dic 2001-05Jul-Sep 2001-05

8.1 m/s

7.6 m/s5.7 m/s

5.6 m/s20

24

28

32

20

24

28

32

124 W 120 116 112 108 124 W 120 116 112 -108

N

N

Left: Roden (1971) described for November 1969 the poleward flows of theTropical Surface Waters up to the Punta Eugenia and Equatorial Subsurface Waters along all the peninsula.

Right: Durazo and Baumgartner (2002) (CalCOFI and IMECOCAL data for 1997-99) assumed that during the El Niño years this northward advectionintensified, especially for Equatorial Subsurface Water at depth.

Data used

Averaged weekly and monthly SST distributions calculated fromAVHRR data for the period from 1996 to 2003 (provided by NOAA)

Sea height anomalies, calculated from altimetry data, and geostrophiccurrents calculated for a 10-day period using Ocean Circulation and Climate Advanced Model (OCCAM) model with Levitus climatology. These data are provided on the page of the NOAA/AOML/CoastWatchproject (www.aoml.noaa.gov/phod/ dataphod/work/trnanes/INTERFACE/index.html).

Averaged daily and monthly series of Coastal Upwelling Index (CUI) are provided on the PFEL Website (www.pfeg.noaa.gov).

Four hydrographic surveys (CTD) were made on board of R/V “El Puma”(March and December of 2003 and 2004) on the continental shelf in front of the Magdalena-Almejas lagoon system.

Three CTD transects were made in Apr 2000, Feb 2001 and Apr 2002in front of the main inlet of Magdalena Bay.

Average monthly SST anomalies for the period from 1996-2003(AVHRR data)

Sea height anomalies (SHA) calculated from satellite altimetry data

(NOAA/AOML/Coast Watch data)

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Oct 2006

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-16

-8

0

8

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40

H, cm

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32Apr 2006

Average seasonal geostrophic currents calculated from SHA using the OCCAM model with the Levitus climatology

for the period from 2003 to 2006(NOAA/AOML/Coast Watch data)

Jan-Mar Apr-Jun

22

23

24

25

26

27

28 Jan-Mar

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Scale (cm/s)

125

Apr-Jun

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Scale (cm/s)

125

Average seasonal geostrophic currents calculated from SHA using the OCCAM model with the Levitus climatology

for the period from 2003 to 2006(NOAA/AOML/Coast Watch data)

Jul-Sep Oct-Dec

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24

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28 Jul-Sep

Scale (cm/s)

125

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Oct-Dec

Scale

125

Seasonal cycle of off-shore Ekman transport, with 0.5 latitude resolution, for entire Pacific coast (left) and for the latitude of

the Magdalena bay (right), calculated using PFEL methodology.

Units are metric tons per second per 100 m coastline.

Jan Feb Mar AprMay Jun Jul Aug Sep Oct NovDec0

20

40

60

80

100

120

CU

I, m

3 s-

1 / 1

00 m

1 2 3 4 5 6 7 8 9 10 11 12Month

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32

Latu

tude

a

Typical SST anomaly at the latitude of Magdalena Bay (April 2002)

-112.4 -112.2 -112.0 -111.8 -111.6 -111.424.2

24.4

24.6

24.8

25

25.2

SST AnomalyApril, 2002. AVHRR data. 24.5° N.

Longitude

Latit

ud

Comparison of Coastal Upwelling Index (CUI) with difference in SST between the upwelling zone and the adjacent sea.

-113 -112

24

25Maximum UpvellingZone

Adjacent Zone

Pacific ocean

BM

PeninsulaBaja California

60 km

200 misobath

-2

0

2

4

SST

Dife

renc

e (º

C)

-80

-40

0

40

80

120

160

CU

I (m

3 s-1

/ 1

00 m

)

-4

0

4

SST

Ano

maly

(ºC)

5

10

15

20

25

30

SST

(ºC)

1996 1997 1998 1999 2000 2001

SST CUI

1996 1997 1998 1999 2000 2001

Sampling grid used during the 2000-2004 surveys(R/V “El Puma”, hydrographic boats).

BM

112.6W 112.4W 112.2W 112W 111.8W 111.6W 111.4W 111.2W24N

24.2N

24.4N

24.6N

24.8N

25N

Apr00Apr02Feb01

Dec04Mar04Dec03Mar03

Typical stratification patterns in front of Magdalena Bay in Spring (Mar 2003)

24.4 24.8 25.2 25.6 26 26.4 26.8Sigma-t

-200

-160

-120

-80

-40

0

Dep

th, m

24.4 24.8 25.2 25.6 26 26.4Sigma-t

-200

-160

-120

-80

-40

0

a b

BM

112.6W 112.4W 112.2W 112W 111.8W 111.6W 111.4W 111.2W24N

24.2N

24.4N

24.6N

24.8N

25N

Mar03

b

a

Field observations of the coastal upwelling pattern in front of Magdalena Bay in March 2003

0 5 10 15 20 25 30 35150

120

90

60

30

0

Dep

th, m

0 5 10 15 20 25 30 35150

120

90

60

30

0

Dep

th, m

Salinity

Temperature,

33.93434.134.234.334.434.534.6

1112131415161718

oC

Comparision between TS- diagrams in the Spring and Fall (2003 data).

33.5 34 34.5 355

10

15

20

25

Tem

pera

ture

SAW

StSW

ESsW

TrW

200 m

100 m

0 m

33.5 34 34.5 35Salinity

5

10

15

20

25

SAW

StSW

ESsW

TrW

200 m

100 m

0 m

March 2003 December2003

Concluding remarks

Analysis of the 2001-06 data shows that, from January to June, relatively cold and low salinity waters (TrW and sometimes SAW) are present on the continental shelf in front of the Magdalena lagoon system.

In Summer and Fall, subtropical surface water (StSW), eastern tropical Pacific waters(TSW) and, ocasionally, Equatorial subsurface Water (EsSW) penetrate along the coast northward as far as Punta Eugenia, including the continental shelf in front of Magdalena bay.

Inspite of relatively low upwelling intensity (CUI<130 m**3/s/100m)in the southern part of the peninsula, the favorable bottom topography provokes a well-developed upwelling pattern in SST distributions over the continental shelf, including the area in front of the Magdalena Bay.

Thank you.