Eddy diffusion and the Upper Meridional Cell of the Southern Ocean

Eddy diffusivity (K) in the Southern Ocean has been estimated from several different angles in order to quantify the role of eddies on mixed layer heat and mass budgets, and to revisit the role of eddies on the upper cell of the meridional overturning circulation. Estimates were made with surface drifters, both implicitly (K=u'^2*T; where T= an integral time scale) and directly from dispersion in 5 deg X 1 deg boxes. Additional estimates were obtained with altimetry-derived velocity fields. Comparisons in 3 areas, including highly energetic (Aghulas Retro, Campbell Plateau, Brazil current) and lower energy regions (SE indian, SE Pacific, SE Atlantic) suggests that in areas of high diffusivity the implicit methods overestimate K compared to the direct method, whereas in low diffusivity areas the results from the 2 methods are in good agreement. In energetic regions, non-gaussian pdfs of velocity, and associated large kurtosis is probably associated with coherent structures. The pattern and the strength of K is compared to other studies.

These eddy diffusivity estimates are used to revise the main balances of the 2-d overturning cell, and it is found that horizontal buoyancy mixing plays an important role locally near the main fronts of the ACC, though it tends to disappear in zonal averages. To close the mass budget the eddy-driven mass flux is estimated from mixed layer properties alone using a new CPT parameterization. Errors are discussed; within the 2-d framework a consistent balance for the circulation cell is obtained which is centered on the SAF and the denser SAMW.