How wind-forcing and air-sea heat exchange determine the meridional temperature gradient and stratification for the Antarctic Circumpolar Current

Bryden, Harry L. and Cunningham, Stuart A. (2003) How wind-forcing and air-sea heat exchange determine the meridional temperature gradient and stratification for the Antarctic Circumpolar Current. Journal of Geophysical Research, 108 (C8), 3275. (doi:10.1029/2001JC001296).

Abstract

Mesoscale oceanic eddies have a profound effect on the meridional circulation in the Antarctic Circumpolar region. Previous studies have shown that eddies transport heat poleward to balance the heat lost by the ocean to the atmosphere in the waters around Antarctica and also transport eastward momentum downward at a rate comparable to the amount of momentum put into the water column by the wind stress. Using the poleward eddy heat fluxes to relate air-sea heat loss and wind stress, we find that the meridional temperature gradient in the circumpolar region is determined by the ratio of the air-sea heat loss to the zonal wind stress. Including a widely used parameterisation where poleward eddy heat fluxes are proportional to the mean meridional temperature gradient, we find that the slope of isotherms (or isopycnals) across the circumpolar zone is determined by the wind stress. Further assuming no cross-isopycnal mixing in the interior ocean, we find that the stratification in the deep ocean is determined by the ratio of air-sea heat loss to zonal wind stress squared. The poleward eddy heat fluxes also represent a southward eddy mass transport in the upper water column in density coordinates. The vertical gradient of this eddy mass transport is the meridional component of the eddy Stokes drift which is equal to the difference between the Lagrangian and Eulerian mean velocities in the circumpolar region. Because there can be no zonally averaged geostrophic Eulerian meridional velocity across the circumpolar region above the topography, this southward eddy Stokes drift provides a mechanism by which circumpolar deep water can flow southward across the circumpolar zone. Eddy heat fluxes, which are central to the mass, momentum, heat and energy balances in the circumpolar region, provide a catalyst for relating the roles of wind and buoyancy forcing in setting the overall circulation for the Southern Ocean.

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Contributors

Author: Harry L. Bryden

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