Mixing by eddies in the Southern Ocean

The mixing by geostrophic eddies in the Southern Ocean is investigated by quantifying their diffusivities. At the surface, a technique is used whereby the “effective diffusivities” associated with the eddies are estimated by numerically monitoring the lengthening of idealized tracer contours as they are strained by surface geostrophic flow observed by satellite altimetry. Modifications to this technique provide an indication of the variation in the strength of the eddy mixing with depth. The resulting surface diffusivities show considerable spatial variability and are large (2000 m2/s) on the equatorward flank of the Antarctic Circumpolar Current and are small (500 m2/s) at the jet axis. Regions of high and low effective diffusivity are shown to be collocated with regions of, respectively, weak and strong isentropic potential vorticity gradients, and the depth structure is suggestive of mixing in the region of critical layers. The maps of diffusivity are used, along with climatological estimates of surface wind stress and air–sea buoyancy flux, to estimate surface meridional residual flows and the relative importance of Eulerian and eddy-induced circulation in the streamwise-averaged dynamics of the Antarctic Circumpolar Current.