We study the appearance of large-scale mean motion sustained by stochastic forcing on a rotating fluid (in the quasigeostrophic approximation) flowing over topography, We show that the effect is a kind of noise-rectification phenomenon, occurring here in a spatially extended system, and requiring nonlinearity, absence of detailed balance, and symmetry breaking to occur. By application of an analytical coarse-graining procedure, we identify the physical mechanism producing such an effect: It is a forcing coming from the small scales that manifests itself in a change in the effective viscosity operator and in the effective noise statistical properties. Numerical simulations confirm our findings. [S1063-651X(98)14211-3].
