SIO 210 Talley Lecture 1: Introduction to Ocean Circulation

Lynne Talley, September 26, 1996

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Figures

1. Climatological surface temperature and salinity
2. Vertical sections of T, S and O2 in the Atlantic
3. Annual mean net heat flux maps
4. Annual mean net evaporation-precipitation map
5. Example of global wind field

Outline

What is physical oceanography?

1. What forces act on the ocean? (e.g. wind [waves, turbulence, large scale waves, circulation], heating due to the sun and geothermal energy, cooling, evaporation due to sun and wind, precipitation, tidal potential [the moon and sun], earthquakes, gravity)
2. How do these forces work on the ocean? Most of them create pressure gradients which cause water to try to flow from high pressure to low pressure. The earth's rotation introduces a Coriolis acceleration which induces flow in the frame of reference of the earth which is to the right of the motion in the northern hemisphere and to the left in the southern hemisphere, if the flow persists for about a day or longer such that it is affected by the rotation. Details for each follow in Hendershott's lectures.
3. What equations do physical oceanographers use to describe the ocean?
   ma=F (vector form; F = pressure gradient, body forces and dissipation and a includes time change of velocity, advection and Coriolis acceleration)
   mass conservation (whatever amount goes into a box must come out)
   equation of state (dependence of density on temperature, salinity and pressure)
   density changes as a function of heating/cooling and evaporation/precipitation, and pressure

   Density of seawater: mass/volume. The density of pure water is about 1000 kg/m3. The density of seawater is about 1020 to 1050 kg/m3, with the full range mostly due to pressure effects. Ignoring pressure effects, the range (at sea level pressure) is about 1020 to 1028 kg/m3.

What is the general circulation?

1. Spatial scales: 100s to 1000s of kilmoeters
2. Time scales: small seasonal, interannual to decadal/century changes in flows which basically retain the same patterns as long as the land configuration doesn't change and the atmosphere is dominated by trades and mid-latitude westerlies.
3. Average current speeds: 1-5 cm/sec (horizontal) in the interior of the ocean, and 100-150 cm/sec (horizontal) in the fastest currents such as the Gulf Stream and Antarctic Circumpolar Current. Vertical velocity for the general circulation is on the order of 10-4 cm/sec and can only be inferred, not measured.
4. Geostrophic balance: in the ma=F equation, the largest terms are the Coriolis acceleration and the pressure gradient. The time changes, advection and forcing/dissipation are much smaller.

Description of upper ocean currents.

Subtropical gyres in every ocean basin (high pressure in the middle so flow is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere)

Subpolar gyres in the two northern hemisphere basins and in the Weddell and Ross Seas (low pressure in the middle)

Antarctic Circumpolar Current which is nearly unimpeded flow around Antarctica

Complicated zonal currents in the tropics

Wind forcing: westerlies and trades (see example or Hellerman and Rosenstein [1983] figures in lecture handout or other wind products) (Go to wind example.)

Description of large-scale overturning (thermohaline) circulation

Schmitz (1995) diagram: better sense of the complexity of the overturning pathways. Division of the ocean into 4 layers is sensible (upper ocean to pycnocline, intermediate layer, deep water layer, bottom water layer).

Description of temperature and salinity distributions

Surface salinity, showing highest salinity in the subtropical evaporation cells and lowest salinity in the precipitation-dominated subpolar regions.

(Go to vertical section figures.) Vertical section of temperature (example at 20W in the Atlantic): warm at the surface to cold down below. Warm waters reach a bit deeper in the bowls of the subtropical gyres. Coldest deep water extending northward from Antarctica. Large rise in isotherms south of 40S associated with the Antarctic Circumpolar Current. Temperature inversion layer in the South Atlantic, must be balanced by salinity since on these large scales true density inversions are never seen.

Vertical section of salinity (example at 20W in the Atlantic): "Four" layers are fairly clear. (1) Saline near-surface layer, with bowls in the subtropical gyres, (2) intermediate layer with fresh tongues extending equatorward from the south (500-1500 meters deep) and north (1200-2200 meters deep) and a saline layer in the North Atlantic injected from the Mediterranean (500-2500 meters deep), (3) deep water layer marked by high salinity extending southward into the South Atlantic, (4) bottom water layer of lower salinity extending northward from Antarctica into the North Atlantic.

Vertical section of oxygen (example at 20W in the Atlantic): showing the same four layers as are evident in salinity.

Thermohaline forcing

Figures