Measuring Eddy Driven Transport in a Zonally Inhomogeneous Flow

The theory and modeling work related to ocean mesoscale tracer transport has been developed primarily in the context of axisymmetric or periodic models; however, many of the most important regions for mesoscale transport exhibit curvature in the mean flow and inhomogeneity in eddy statistics. Here we report on transport characteristics of passive and active tracers in a channel forced by steady winds and surface buoyancy, where the zonal symmetry is broken and inhomogeneity is introduced by a meridional ridge.

The turbulent transport properties are quantified by a diffusivity tensor, which is diagnosed by obtaining a best fit estimate of the diffusivity tensor by using flux and gradient information from multiple tracers that are advected by the turbulent flow. This 3D field of a 3X3 tensor captures the advective and diffusive behavior of eddy tracer transport. This diagnosed diffusivity tensor reproduces buoyancy and QG PV fluxes with a surprisingly high degree of accuracy, even though these tracers are not used in the fitting procedure. Thus, the diffusivity tensor effectively able to separate the kinematics of the turbulent flow from dependence on any tracer orientation.

We analyze the diffusivity tensor further to assign physical meaning to its subparts. The symmetric part of the tensor is composed of influence of the enhanced dissipation due to turbulent stirring, and a part that is associated with divergence of transport variance, which can only be studied in simulations having inhomogeneities. The antisymmetric part of the tensor captures the advective eddy-driven transport, which is composed of a component that acts to flatten isopycnals, releasing APE, and another that advectively transports tracers along isopycnals. Finally, we diagnose the scalar diffusivity coefficients corresponding to the Redi diffusivity and GM diffusivity, which turns out to be laterally anisotropic. The structure of these coefficients is probed under the lens of extant theories, and we test if a relationship exists between the Redi and GM coefficients.