Studying "effective diffusivities" in the ocean using tracer stirring and adjoint techniques

Evidence is presented suggesting that ‘effective diffusivites', which characterize the stirring properties of geostrophic eddies at the sea surface, are displaced relative to, rather than collocated with, large-scale baroclinic zones. In studies of tracer stirring driven by altimetric-derived velocity fields, tracer contours are more efficiently stretched on the equatorward flank of the Antarctic Circumpolar Current and Gulf Stream than at theirs cores.

A complementary but completely different approach yields the same result. Diffusivities are deduced by fitting a global ocean model to observations using eddy stresses as control parameters. Again, the implied diffusivities are found to have elevated values in regions of weak potential vorticity gradients on the periphery of the jet, with much lower values in the core of the jet stream, where PV gradients are large.

Finally, implications for the dynamics of the Antarctic Circumpolar Current and Gulf Stream and the representation of eddies in models are discussed.