Topographically generated steady currents in barotropic turbulence

Abstract

Steady currents develop in oceanic turbulence above topography even in the absence of steady forcing. Mesoscale steady currents are correlated with mesoscale topography with anticyclonic eddies above topographic bumps, and large scale westward flows develop when β is non-zero. The relationship between those two kinds of steady currents, as well as their dependence on various parameters, is studied using a barotropic quasi-geostrophic channel model. The percentage of steady energy is found to depend on the forcing, friction and topography in a non-monotonic fashion. For example, the percentage of steady currents grows with the energy level in the linear regime (low energies) and decreases when the energy level increases in the nonlinear regime (high energies). Mesoscale steady currents are the energy source for the steady westward flow U, and therefore U is the maximum when large scale and mesoscale currents are of the same order of magnitude. This happens when the ratio S of the large scale slope βH/f ₀ and the mesoscale rms topographic slope α is of order one. U decreases for both small and large values of S.