PARAMETERIZATION OF MESOSCALE EDDY FLUXES IN ZONAL OCEAN FLOWS

Quasi-geostrophic dynamics in an eddy-resolving zonal re-entrant chann el in the Southern Hemisphere have been studied for east-and westward wind forcing scenarios. The main difference is seen in the zonally ave raged velocity profiles. In the case of eastward forcing, transient ed dies strongly intensify the flow in the channel center into a jet, a f eature totally absent in the westward forcing cases. The free jet is a ssociated with a five times higher available potential energy compared to the westward flow. We have used these two distinctly different flo w regimes to investigate possible parameterizations of the eddy fluxes in both situations. Parameterizations using a diffusion concept for t he quasi-geostrophic potential vorticity (QPV) fluxes, based on earlie r work by Green and Welander, have raised a number of questions concer ning the transfer (or diffusion) coefficients. These coefficients must satisfy three basic integral constraints, the balances of momentum, e nergy and enstrophy. It is shown that the constraints related to momen tum and energy conservation are associated with Pedlosky's instability conditions. An analytical solution developed in this paper shows that the transfer coefficients have a theoretical upper limit in the eastw ard-forcing scenarios, resulting in a greater Reynolds number than a t heoretically derived critical Reynolds number. A general parameterizat ion scheme, based on the quasi-geostrophic eddy enstrophy balance, is presented, which accounts for both scenarios, a weakly baroclinic west ward how and a strongly baroclinic eastward flow. This new parameteriz ation reproduces the main difference in the east-and westward flows; i .e., a strong jet in the eastward-forcing case and a broad smooth flow in the westward-forcing case, in agreement with the numerical results of the eddy-resolving experiments.