Quantifying the eddy diffusivity of the Southern Ocean II: spatial variability and the role of the mean flow

A diagnostic framework is presented, based on the Nakamura effective diffusivity, to investigate the regional variation in eddy diffusivity. Comparison of three different diffusivity calculations enables the effects of locally enhanced tracer diffusion to be distinguished from the streamwise average and the distinction between locally generated complexity in the tracer structure and between that advected into a particular domain. The results highlight the important role the mean flow plays in determining the eddy diffusivity. The effective diffusivity is not simply related to the eddy kinetic energy: in the presence of a strong mean flow the eddy diffusivity can be suppressed even in the presence of moderately strong eddy activity, conversely in a region of weak mean flow the eddy diffusivity can be enhanced even in the presence of only weak eddy activity. This casts doubt on the ability of paramterizations based solely on the eddy kinetic energy to adequately characterize the eddy diffusivity in regions of strongly varying mean flow such as the Southern Ocean. The results are, however, consistent with the eddy transport and mixing variability predicted by potential vorticity based arguments. JULY 2008 SHUCKBURGH ET Al. 3