Diffusive Versus Nondiffusive Properties of Coherent Ocean Eddies

Ocean mesoscale eddies play an important role in transport and mixing in the ocean. With the advent of satellite observations and high-resolution ocean models, the observed eddies are increasingly considered as individual coherent structures. Coherent eddies can trap fluid inside their cores and physically move water and tracers over potentially long distances and may be important in determining the mixing and dispersion properties of turbulent flows. The modern ocean climate models are of insufficient resolution to resolve mesoscale eddies. Eddy tracer transport is typically parameterized as a diffusive process whereby the tracer flux is related to the large-scale tracer gradient by an eddy diffusivity. However, a diffusive parameterization does not fully account for transport by coherent eddies, which can be nonlocal or nondiffusive. It is therefore necessary to understand the dynamics and transport properties of coherent eddies in order to appropriately parameterize their effects.

This study investigates the contribution of coherent eddies to tracer transport in a two-layer quasigeostrophic model of geophysical turbulence. The coherent eddies are identified by closed contours of the Lagrangian-averaged vorticity deviation (LAVD) obtained from Lagrangian particles advected by the flow. The PV transport by coherent eddy cores is systematically upgradient due to their meridional beta drift. This indicates that the PV transport by coherent eddies is significantly different from a diffusive process. However, estimates of the Taylor (1921) diffusivity— which assumes that particle motion is independent of the tracer distribution—shows that particle dispersion due to coherent eddies leads to positive diffusivities, which is inconsistent with the observed upgradient coherent PV flux. The coherent eddies are—almost by definition—PV extrema and their meridional motion depends on the sign of their PV, which invalidates the assumptions behind the Taylor diffusivity. Transport by the stirring effect of coherent eddies is also estimated using the piecewise PV inversion method. The PV transport by the flow induced by the PV of the coherent eddies is downgradient and several times larger than the trapping transport inside coherent eddy cores. This study shows that the trapping and stirring transport by coherent eddies are distinct, and that the latter is more significant and more consistent with the diffusion process. The results of this study have implications for the transport of heat or biological tracers, which are correlated with eddy PV anomalies.