Physical and Dynamical Oceanography

CLIM712

Wednesday: 10:30am – 1:30pm

Bohua Huang

Department of Climate DynamicsCollege of Science

George Mason University

Center for Ocean-Land-Atmosphere Studies

Phone: 301-902-1246Email: huangb@cola.iges.org

bhuang@gmu.edu

References

Text books: • Pond, S., and G.L. Pickard, 1983: Introductory Dynamical Oceanography.

2nd edition, 329pp, Butterworth-Heinemann.• Pickard, G.L., and W.J. Emery, 1993: Descriptive Physical Oceanography,

5th enlarged edition, 320pp, Pergamon Press.

Other titles of interest:• Mellor, G.L., 1996: Introduction to Physical Oceanography, 260pp, AIP

Press.• Knauss, J.A., 1997: Introduction to Physical Oceanography, 309pp, 2nd

edition, Prentice-Hall.

More readings:• Tomczak, M., and J.S. Godfrey, 1994: Regional Oceanography: An

Introduction, 422pp, Pergamon Press.• Apel, J.R., 1987: Principles of Ocean Physics, 634pp, Academic Press.• Warren, B.A., and C. Wunsch, (ed.,) 1981: Evolution of Physical

Oceanography, 623pp, The MIT Press.• Pedlosky, J., 1996: Ocean Circulation Theory,453pp, Springer.

Useful Websites Physical Oceanography Courses

• M. Tomczak: Introduction to Physical Oceanography (http://gaea.es.flinders.edu.au/~mattom/IntroOc/newstart.html)

• F. Webster: Introduction to Physical Oceanography (http://www.cms.udel.edu/mast602)

• R. H. Stewart: Introduction to Physical Oceanography(http://oceanworld.tamu.edu/resources/ocng_textbook/contents.html)

The slides of this course (CLIM712) will be put on:ftp://grads.iges.org/pub/huangb/Fall07

Requirement

• Attending class (advance notice of absence)

• Homework: 4-5 assignments

• Mid-term exam (close book)

• Final exam (open book)

• (Project or term paper)

Major Topics• Properties of seawater • T-S forcing and conservation laws • Global T-S distribution • Fluid dynamics on rotating sphere • Description of large-scale gyres • Barotropic dynamics of large-scale gyres• Mixing, turbulence, surface layer • Large-scale overturning and thermohaline circulation • Surface gravity waves (nonrotating and rotating) • Tides • Internal gravity waves • Rossby waves, instability and mesoscale eddies • Coastal processes: currents, fronts, estuaries • El Nino

Course Outline

• Properties of seawater [Des 2, 3, 6] – composition – equation of state – measurement: T, S, pressure

• Global T-S distribution [Des 4 ] – surface profiles – vertical profiles – static stability– annual cycle and interannual variability

• T-S Forcing and conservation laws [Des 5] – heat flux components – heat flux distribution – evaporation, precipitation, runoff – box models

• Fluid dynamics on rotating sphere [Dyn 6, 8, 9.1-9.4] – Coriolis force – equations of motion – geostrophy – Ekman layers

• Description of large-scale gyres [Des 7] – wind patterns and gyres – western and eastern boundary currents – polar currents – equatorial currents

• Barotropic dynamics of large-scale gyres [Dyn 9.5-9.14] – vorticity dynamics – gyres and western boundary currents – Sverdrup, Stommel, and Munk

[Numbers in brackets give chapters to read in Descriptive Physical Oceanography, 5th Ed.(Des), and Introductory Dynamical Oceanography, 2nd Ed.(Dyn). Lectures do not cover the entirety of all chapters assigned; students will only be responsible for material covered in lectures. For some topics, additional reading materials will be supplied with class notes]

• Mixing, turbulence, surface layer [supplied reading] - descriptive Kelvin-Helmholtz instability - surface mixed layer dynamics - sources of subsurface mixing

• Large-scale overturning [supplied reading] - thermohaline structure and meridional overturning - advective-diffusive balance and overturning - Stommel-Arons patterns - subduction and shallow cells

• Surface gravity waves (nonrotating and rotating) [Dyn 12.1-12.8, 12.10.1-12.10.3] - short and long nonrotating SGWs - Poincare and Kelvin waves - nonlinear effects

• Tides [Dyn 13.1-13.7] - tidal forcing - equilibrium theory - forced response

• Internal gravity waves [Dyn 12.9] - two-layer fluid - rotational effects - continuous fluid

• Rossby waves, instability and mesoscale eddies [supplied reading] - Rossby wave dynamics - observations of eddies

• Coastal processes: currents, fronts, estuaries [Des 8] • El Nino [supplied reading]

- air-sea feedbacks - equatorial waveguide - ENSO description

Introduction

Why is ocean important for climate?

What is Physical Oceanography?

How do we do it?

A brief history

Ocean is a major component of the

earth climate system

Ocean plays important roles in maintaining the earth climate

• Ocean has large heat storage

-- 3 meters of sea water has about the same heat

capacity as the whole atmospheric column above it

-- Ocean heat storage modulates diurnal and

seasonal cycles and climate variations-- Maritime climate is

generally milder than continental climate

• Ocean transfers heat and freshwater over a wide range of time and space scales

-- The earth system is not in radiative balance

-- The tropics gaining and the polar regions losing heat

-- Meridional oceanic heat transport is comparable to that of the atmosphere

Sea surface temperature (SST) changes from year-to-year significantly. The SST anomalies can persist for a long time.

Nino3 and southern oscillation indices

Fluctuations within the ocean affect the climate significantly.

The SST anomalies have serious consequences to the weather and climate

Air-sea interaction is an important source for global climate variability (e.g., ENSO)

Ocean provides the “memory” of the low frequency fluctuations

The figure depicts atmospheric CO2 concentrations from 1958 to the present as measured at Mauna Loa, Hawaii. These data, obtained by Keeling and Whorf (1998), represent the longest continuous record of directly measured CO2 concentrations. As the graph of these data indicates, there has been a substantial and sustained rise in the air's CO2 content over the past four decades, from about 315 ppm to over 360 ppm.

The greenhouse effect tends to increase atmospheric temperature.

Ocean is a major part of global carbon cycle and our knowledge of oceanography may be important for estimating the trend of global warming.

ocean plays a significant role in the global change.

It is likely that much of the rise in sea level has been related to the concurrent rise in global temperature over the last 100 years. On this time scale, the warming and the consequent thermal expansion of the oceans may account for about 2-7 cm of the observed sea level rise, while the observed retreat of glaciers and ice caps may account for about 2-5 cm.

Global ocean circulation may be changed fundamentally by global

warming

And the oceanic circulation change will feedback seriously to the earth climate.

-- T and S distribution affects phytoplankton-- Current affects the concentration and dispersion-- Mixing and upwelling are important to provide nutrients-- Phytoplankton changes the ocean color-- Phytoplankton represents the first link in the marine food web-- Phytoplankton has a major role in the global carbon cycle-- An indicator of circulation change-- Biological feedback to circulation?

ocean circulation and marine biology

As indicated by the red (warm) region off the west coast of Peru (top image), El Niño was still going strong in February 1998. Phytoplankton were growing just to the north of the equator (bright blue green region in the image second from top). By February 1999 La Niña had replaced El Niño, and the equatorial Pacific had strong phytoplankton production (bottom pair of images). Images by Robert Simmon based on data from the Distributed Active Archive Centers at the JPL and GSFC

Ocean Color and El Niño

Knowledge of ocean circulation, especially coastal processes, is helpful for environmental sciences

-- pollution

-- oil drilling

-- oil spills

-- sewage outfalls

-- industrial waste

What is Physical Oceanography?

1). A description of the temperature, salinity, and density patterns in the ocean, including their variability.

2). The three dimensional water movement (the circulation: currents and vertical movements; also, waves and tides).

3). The transfer of mass, energy, and momentum between the ocean and the atmosphere.

4). The special properties of sea water (e.g., the propagation of sound and light energy).

5). The mechanisms of these properties and processes.

Simply:• What temperature is the water?• What salinity is the water?• Where is the water going?• Why is that?

The approach of physical oceanography research

• observations to get the basic phenomenon• applying laws of physics to explain the

features we find (hypothesis/theory)• theory leads us to find new information to

verify its predictions (more observation)• new observations test (verify, modify, or

disprove) the theory (improved theory)• general circulation models blurs the boundary

between traditional physical and dynamical branches

Gulf Stream: An Example

Questions:

Why does the Gulf Stream concentrate near the western boundary?

What determines its width and speed?

Why are there meanders and rings?

Any climate significance?…….

A Brief History of Oceanographic Exploration

Surface Oceanography- major approach prior to 1873

Systematic collection of phenomena observable from the deck of sailing ships (marine winds, currents, waves, temperature etc.)

Examples:Halley’s charts of the trade-winds (1685); Hadley(1735)Franklin’s map of the Gulf Stream (1769)Maury's Physical Geography for the Sea (1847)Pillsbury's measurements of the Florida Current (1885)

The 1786 version of Franklin-Folger map of the Gulf Stream

Oceanographic ExpeditionsWide range survey of surface and subsurface oceanic conditions

Examples: Challenger Expedition (British, 1872-1876)

Main interest in marine life below 600 m but also collected large amount of physical measurements in the Atlantic and Pacific

Fram Expedition (Norway, 1895-1896)Leaded by Nansen, polar sea exploration

Meteor Expedition (German, 1925-1927)Leaded first by Merz and later by Wüst, concentrated on overturning

circulation. The ship travels 67,000 miles, made 14 sections across the Atlantic, 310 hydrographic stations, 33,000 depth sounding

Other AcchivementsThe Scandinavian Scientists developed the “dynamical method” to

derive geostropic currents from T-S observationsReversing thermometer gives more accurate subsurface temperature

measurements

Upper panel: Track of the H.M.S. Challanger during the British Challanger Expedition 1872-1876. From Wüst (1964).

Right panel: Track of the R/V Meteor during the German Meteor Expedition. From Wüst (1964).

International Programs: 1957-1978

• Multi-national surveys of oceans and studies of oceanic processes.

Example: International Geophysical Year cruises

• Multiship studies of oceanic processes: e.g., MODE, POLYMODE experiments

•New Technologies improve observations significantlyBruce Hamon and Neil Brown develop the CTD for measuring

conductivity and temperature as a function of depth in the ocean (1955).

Sippican Corporation (Tim Francis, William Van Allen Clark, Graham Campbell, and Sam Francis) invents the Expendable Bathy Thermograph

(XBT), now perhaps the most widely used oceanographic instrument (1963).

The sections in the International Geophysical Year Atlantic Program 1957-1959. From Wüst (1964).

Satellite Remote Sensing (since 1978)

Examples Seasat (1978)

NOAA 6-17(1979-2002)

NIMBUS-7 (1978-1994) Geosat (1985-1990)Topex/Poseidon (1992-)ERS 1 & 2(1991-00, 1995)

Global surveys of oceanic processes from space

Topex/Poseidon tracks in the Pacific Ocean during a 10-day repeat of the orbit. From Topex/Poseidon Project.

Earth System Study Global studies of the interaction of biological,

chemical, and physical processes in the ocean, the atmosphere and the land using in situ and space data as well as coupled models.

Oceanic examples Tropical Ocean-Global Atmosphere (TOGA)

Program (1985-1995)World Ocean Circulation Experiment (WOCE,

1991-1996)Joint Global Ocean Flux Study (JGOFS)

World Ocean Circulation Experiment: Tracks of research ships making a one-time global survey of the oceans of the world

Some Theoretical Milestones• 1775 Laplace's published his theory of tides. • 1800 Rumford proposed a meridional circulation of the ocean with water

sinking near the poles and rising near the Equator.• 1905 Ekman published his paper on wind-driven oceanic boundary layer.• 1910-1913 Vilhelm Bjerknes published Dynamic Meteorology and

Hydrography which laid the foundation of geophysical fluid dynamics. • 1942 Publication of The Oceans by Sverdrup, Johnson, and Fleming, the

first comprehensive survey of oceanographic knowledge. • 1947-1950 Sverdrup, Stommel, and Munk publish their theories of the wind-

driven circulation of the ocean. Together the three papers lay the foundation for our understanding of the ocean's circulation.

• 1958 Stommel publishes his theory for the deep circulation of the ocean.• 1969 Kirk Bryan and Michael Cox develop the first numerical model of the

oceanic circulation.• ……