Lynne Talley, 1997
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Reading, references and study questions for topic 1 - click here
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 .
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).
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)
Antarctic Circumpolar Current which is nearly unimpeded flow around Antarctica, extending to ocean bottom.
Complicated zonal currents in the tropics.
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).
As has already been described in the first half of the course (Hendershott) and as will be discussed in topic 3, most of our knowledge of the circulation is somewhat indirect, using the geostrophic method to determine velocity referenced to a known velocity pattern at some depth. If the reference velocity pattern is not known well, then we must deduce it.
One assumption that we make is that flow is least impeded along isentropic surfaces. Our approximation to isentropic surfaces is isopycnal or neutral surfaces (topic 2). We thus often study patterns of tracers and relative circulation along isopycnals. Cross-isopycnal velocities are very much smaller, although over the largest scales and also in restricted coastal or equatorial regions, they are of course important (thermohaline circulation and upwelling, respectively).
Deduction of the absolute velocity field is based on all of the information that we can bring to bear. This includes identifying sources of waters, by their contrasting properties, and determining which direction they appear to spread on average. We use the concept of water masses as a convenient way to tag the basic source waters. The definition of a "water mass" is somewhat vague, but is in the sense of "cores" of high or low properties, such as salinity or oxygen, in the vertical and along isopycnal surfaces. A range of densities (depths) is usually considered for a given water mass. Water mass definitions may change as a layer is followed from one basin or ocean to another. Many examples of water masses will be given in topics 4 and following.