Constructing the Residual Circulation of the ACC from Observations

Abstract

The dynamics of the meridional overturning of the Antarctic Circumpolar Current (ACC) are best described in terms of a residual circulation that sums the transport of the wind-driven Ekman layer to the transport associated with eddies. Here an attempt is made to infer the residual circulation from observations by combining altimetric data and gridded hydrographic data to estimate eddy fluxes and winds to estimate Ekman transport. At the surface, a flow directed equatorward on the poleward flank of the ACC, and directed poleward on the equatorward flank of the ACC, is deduced. This convergence of flow into the axis of the ACC drives the subduction of the Antarctic Intermediate Water. Weak southward residual flow on the equatorward boundary of the ACC indicates that here Ekman transport is offset by eddy fluxes. The sense of the deduced residual circulation suggests that buoyancy is gained by the ocean through air–sea flux poleward of the ACC, in broad agreement with observations. The surface residual circulation is mapped down to depth to yield two counterrotating meridional cells associated with the transformation of North Atlantic Deep Water and Subantarctic Mode Water into Antarctic Intermediate Water. The circulation suggested by these cells agrees remarkably well with the subsurface distribution of salinity and dissolved oxygen. The dependence of the residual circulation estimate on the magnitude of assumed eddy transfer and mixing coefficients is discussed.