Quantifying the eddy diffusivity of the Southern Ocean I: temporal variability

The robustness of Nakamura’s effective diffusivity diagnostic for quantifying eddy diffusivities of tracers in a streamwise-average framework is carefully examined in an oceanographic context and the limits of its applicability are detailed. The nearsurface Southern Ocean geostrophic flow, obtained from altimetric observations, is chosen for particular examination. The temporal variability of the effective diffusivity is examined over the period 1996–2001. Significant variability is observed on seasonal timescales with a magnitude of ±10–15%, however this variability does not appear to be everywhere well correlated with variations in the eddy kinetic energy (EKE), or relatedly, with changes in the wind stress associated with the Southern Annular Mode: in the region of the ACC variations in the mean flow u are more significant. It is shown that both in the ACC and equatorward of it there is a reasonable correlation between the eddy diffusivity and √ EKE/u. JULY 2008 SHUCKBURGH ET Al. 3