Potential vorticity in models of the ocean circulation

Existing observations suffice to give a qualitative description of the wind‐driven and thermohaline components of the ocean circulation, but a hierarchy of analytical and numerical models is now needed for in coupled ocean‐atmosphere models of the earth's climate. Potentialvorticity is a more appropriate diagnostic field variable than angular momentum for the ocean circulation because of the complicated geometry of basins. Patterns of potential vorticity onsurfaces of constant density help validate ocean circulation models, and give physical insight into how the oceancirculation works. High resolution models suggest that the lateral mixing of potential vorticity by mesoscale eddies along isopycnal surfaces is of the same order as large‐scale advection, and that the assumption of inviscid, potential‐vorticity‐conserving flow in the thermocline is not appropriate for the real ocean. A satisfactory test of this conjecture will require the extensive measurements planned for the World Ocean Circulation Experiment. Models also indicate that the transport of water mass properties by mesoscale eddies is largely a mixing along isopycnal surfaces. The wcak temperaturc gradients along isopycnal surfaces in most areas of the ocean limit the effectiveness of the mesoscale eddies in transporting significant amounts of heat across latitude circles.