SIO 210 Talley Topic 1: Introduction to Ocean Circulation

Lynne Talley, 2000
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The notes given here are in expanded outline form. The purpose of the notes is mainly to provide many of the figures used in class. The notes are not complete, and are not meant to replace the required text reading. All references are given in the study guide and bibliography .

1. What is physical oceanography?

What phenomena do physical oceanographers study? (e.g. surface and internal waves, air-sea exchanges, turbulence and mixing, acoustics, heating and cooling, wave and wind-induced currents, tides, tsunamis, storm surges, large-scale waves affected by earth's rotation, large-scale eddies, general circulation and its changes, coupled ocean-atmosphere dynamics for weather and climate).

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)

(zonal = east-west and meridional = north-south).

2. Geographical setting (see texts and study maps)

Earth is 70.8% water-covered. In the zonal direction, there is no land between 85-90 degrees N and between 55-60 degrees S. At latitudes 45-70N, there is more land than water. At latitudes 70-90S there is only land (Antarctica).

The areas of the oceans are: Pacific (179 x1e6 km2), Atlantic (106x1e6 km2), Indian (75 x1e6 km2). The order of magnitude of the horizontal length scales that we associate with these oceans are: Pacific (15,000 km), Atlantic (5,000 km), Indian (5,000 km).

The earth's radius is approximately 6371 km. (The earth is actually not a sphere, but this is close enough for us.) The average depth of the ocean is about 4000 m (actually 3795 m). Thus the ocean is a thin skin on the outside of the earth. The average height of land is 245 m. The maximum elevation is about 9,000 m (Mt. Everest) and the maximum ocean depth is about 11,500 m (Mindanao Trench).

We divide the ocean regions into:
Oceans - Atlantic, Pacific, Indian. We often call the region south of about 40S or 30S the "Southern Ocean".
Mediterranean seas - Mediterranean, Arctic, Gulf of Mexico, Red Sea, Persian Gulf.
Marginal seas - many. Examples: Pacific (Bering Sea), Atlantic (Caribbean), Indian (Andaman).
Inland seas - Black, Caspian Seas, Lake Baikal, Great Lakes.
Areas of open oceans are sometimes referred to as "seas", mainly for historical reasons and geographical convenience. There are no rules about whether some areas should have names and others should not. Examples are: Arctic (Greenland, Norwegian, iceland, Kara, Barents, Chukchi Seas), N. Atlantic (Labrador, Irminger, Sargasso Seas), Southern Ocean (Weddell, Ross Seas), Indian (Arabian Sea, Bay of Bengal), South Pacific (Tasman Sea), North Pacific (Gulf of Alaska).

3. What is the general circulation?

  1. Spatial scales: 100s to 1000s of kilometers
  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. The vertical velocity for the general circulation is on the order of 10-4 cm/sec and can only be inferred, not measured.
  4. Force balance: ma = F
    acceleration + Coriolis acceleration = pressure gradient force + forcing + mixing

    Geostrophic balance: in the ma=F equation, the dominant terms are the Coriolis acceleration and the pressure gradient. The time changes, advection and forcing/dissipation are much smaller. Geostrophic flow in the northern hemisphere is clockwise around high pressures (and counterclockwise around low pressures), and is the opposite in the southern hemisphere. I think of it this way: water is being pushed "down the pressure gradient" (that is, from high pressure to low pressure), and the Coriolis force turns it to the right in the northern hemisphere. A purely geostrophic flow has ONLY the Coriolis part and NONE of the part that goes down the pressure gradient.

  5. The other important equations:
    Mass conservation: what does into an enclosed box must come out
    Equation of state: how density depends on temperature, salinity and pressure
    Equation for density change: how density changes as a result of heating/cooling and evaporation/precipitation. (Or equivalently, have separate equations describing how salinity and temperature each change and then use the equation of state to get the density changes.)
Wind-driven circulation: The winds drive the general circulation associated with the subtropical and subpolar gyres, and the Antarctic Circumpolar Current. The mechanisms are described in topic 3 (Dynamical quantities and forcing). The wind-driven circulation consists of (1) the part of the geostrophic flow from the surface to the ocean bottom which is driven, indirectly, by the wind stress, and (2) the frictional wind-driven currents of the thin surface layer (50-100 meters) - the frictional Ekman layer.

Surface current maps from Tomczak and Godfrey or Pickard and Emery showing the large-scale geostrophic flow. Major similarities between the various ocean basins. Note asymmetry of the gyres: strong western boundary currents and weaker flow in the interior; weak and shallow eastern boundary 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)

The geostrophic flow in these gyres consists of narrow, swift western boundary currents and gentler flow in the ocean interior away from the western boundary. The western boundary currents and the ACC extend to the ocean bottom. The wind-driven gyre flow in the interior away from the western boundaries extends to about 2000 m depth.

The gyres have eastern boundary currents that apparently extend to no more than about 500 m depth.

There is also an intense wind-driven circulation towards the east around Antarctica, called the Antarctic Circumpolar Current (ACC). It extends to the ocean bottom. It consists of a series of fronts, described in the later lecture on the Southern Ocean.

There are major east-west currents at the equator, which reverse direction every few hundred meters from the top of the ocean to the bottom. (See later lecture on equatorial currents for much better description of these reversing jets, whose vertical scale changes with depth.)

Thermohaline circulation: Heating/cooling and to a lesser extent evaporation/precipitation drive global and basin-scale circulations characterized by overturn (sinking of dense water and upwelling). The current driven by this are slower than the wind-driven currents in most places. It is useful to think of this circulation as being imposed separately from the wind-driven circulation, although there are likely some nonlinear interactions. Deep western boundary currents and slow interior deep flow are thermohaline.

Conveyor belt diagram (Broecker, 1991) based on Gordon (1986) for the North Atlantic Deep Water cell gives the sense of the global scale of the overturning, but is completely missing the Antarctic Bottom Water cell, and is likely not to be correct in the locations and implied magnitudes and path of the return flows to the North Atlantic.

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). Note however that this cartoon does not yield the actual flowpaths.