Full‐Depth Scalings for Isopycnal Eddy Mixing Across Continental Slopes Under Upwelling‐Favorable Winds

Abstract

AbstractMesoscale eddy mixing profoundly modulates the ocean tracer budgets, and is typically parameterized via the isopycnal eddy diffusivity in ocean climate models. However, relatively little is known about the magnitude/structure of isopycnal eddy diffusivity across continental slopes, which hinders the understanding and prediction of shelf‐open ocean exchanges. In this study, we quantify the isopycnal eddy diffusivity in a suite of eddy‐resolving, process‐oriented simulations of mesoscale turbulence over continental slopes under upwelling‐favorable winds, a configuration that commonly arises around the margins of subtropical gyres. Cross‐shore eddy diffusivity is found to be suppressed in the upper open ocean occupied by strong alongshore flows, but enhanced at depths where alongshore flows are weakened, a finding that is consistent with the enhancement of eddy mixing near the steering level. Over continental slopes, eddy diffusivity also strengthens at mid‐depths, but almost vanishes near the seafloor. To theoretically constrain the simulated eddy fluxes, we examine the scaling of eddy diffusivity proposed by Ferrari and Nikurashin (2010, https://doi.org/10.1175/2010JPO4278.1), which accounts for the suppression of eddy mixing induced by the relative propagation of eddies to the mean flow. We show that, apart from the mean‐flow suppression effect, the eddy anisotropy effect induced by steep topography shapes both the horizontal and vertical structures of cross‐shore eddy diffusivity. Finally, we propose prospective closures of the eddy propagation speed and eddy anisotropy effect over continental slopes using the large‐scale flow and bathymetric quantities. This work offers a basis upon which a “slope‐aware” parameterization of mesoscale eddy mixing can be developed.