On the Mixing Coefficient in the Parameterization of Bolus Velocity

Abstract

Mesoscale eddies in the ocean play an important role in the ocean circulation. In order to simulate the ocean circulation, mesoscale eddies must be included explicitly or parameterized. The eddy permitting calculations of the Los Alamos ocean circulation model offer a special opportunity to test aspects of parameterizations that have recently been proposed. Although the calculations are for a model in level coordinates, averages over a five-year period have been carried out by interpolating to instantaneous isopycnal surfaces. The magnitude of ?thickness mixing? or bolus velocity is found to coincide with areas of intense mesoscale activity in the western boundary currents of the Northern Hemisphere and the Antarctic Circumpolar Current. The model also predicts relatively large bolus fluxes in the equatorial region. The analysis does show that the rotational component of the bolus velocity is significant. Predictions of the magnitude of the bolus velocity, assuming downgradient mixing of thickness with various mixing coefficients, have been compared directly with the model. The coefficient proposed by Held and Larichev provides a rather poor fit to the model results because it predicts large bolus velocity magnitudes at high latitudes and in other areas in which there is only a small amount of mesoscale activity. A much better fit is obtained using a constant mixing coefficient or a mixing coefficient originally proposed by Stone in a somewhat different context. The best fit to the model is obtained with a coefficient proportional to ?2/T, where ? is the radius of deformation, and T is the Eady timescale for the growth of unstable baroclinic waves.