Noise-sustained currents in quasigeostrophic turbulence over topography

We study the development of directed mean currents in a nonlinear model of fluid flow appropriate for large-scale ocean dynamics in the presence of bottom topography and dissipation, and forced with a noise term. We show that the presence of noise leads to persistent average currents directed along isobaths. The effect requires the presence of both the nonlinear and the random terms, and can be though of as a rectification of the stochastic energy input by the combined effect of nonlinearity and topography. At variance with previous works we use a scale-unselective dissipation, so that the phenomenon can not be explained in terms of minimum enstrophy states. Despite the fact that the system is out of equilibrium the large-scales are well described by a generalized canonical equilibrium with mean energy and enstrophy determined by a balance between random forcing and dissipation. This result allows predicting the strenght of the noise-sustained currents. Finally we discuss the relevance that these noise-induced currents could have on real ocean circulation.