Estimate of lateral eddy diffusivity upstream of Drake Passage: an update

An estimate of the rate at which geostrophic eddies mix potential vorticity (PV) across the Antarctic Circumpolar Current upstream of the Drake Passage is presented. The estimate is based on a combination of data collected as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) and a 1/20 degree resolution numerical model of the DIMES region. The estimates for meridional PV diffusivity at various depths are in agreement with earlier estimates based on mixing of a passive tracer as well as those calculated from dispersion of floats. The PV diffusivity is then compared and contrasted with the GM diffusivity, which is not enhanced at critical levels. Finally it is shown that PV mixing is weak upstream of Drake Passage and strong in the Scotia Sea. The implications of these results for our understanding and representation of geostrophic eddy mixing will be discussed.