Total and Scale-specific Eddy Diffusivities in Energetic Regions with Jets: Spatio-temporal Patterns and Theories.

Eddies, including both mesoscale and submesoscale components, have scales ranging from O(1)km to O(100)km. They play a key role in the climate system by stirring and mixing water masses, and thus much effort has been devoted to estimating total eddy diffusivities. Climate-scale oceanic models are entering the eddy-permitting regime, and only subgrid scale eddies need to be parameterized in these models. The goal of our research is to estimate and interpret scale-specific eddy diffusivities, which are the diffusivities induced by eddies with scales smaller than the smallest resolvable scale in a climate model.

Using one million numerical floats that were deployed in a high-resolution (nominal 0.1^o) global eddying ocean model (the Parallel Ocean Program), we calculated and compared the spatio-temporal structures of scale-specific eddy diffusivities with those of total eddy diffusivities. We focus on the Kuroshio Extension and Southern Ocean regions, both of which are eddy rich and have intense jets. Results indicate that scale-specific eddy diffusivities have very different spatial structures and magnitudes from those of the total eddy diffusivities. To understand the mixing patterns, we developed a multiwavenumber theory, whose formulation is motivated by the broad range of spatial scales of oceanic motions. This theory can be used to represent both total and scale-specific eddy diffusivities in the jet regions. Calculations indicate that the multiwavenumber theory better represents the spatial variation of total eddy diffusivities in the cross-jet direction than the single-wavenumber theory. The validity of other eddy-mixing theories and parameterizations in representing scale-specific eddy diffusivities will also be assessed.