We study the appearance of large scale mean motion sustained by stochastic forcing on a rotating fluid (in the quasigeostrophic approximation) flowing over topography. As in other noise rectification phenomena, the effect requires nonlinearity and absence of detailed balance to occur. By application of an analytical coarse graining procedure we identify the physical mechanism producing such effect: It is a forcing coming from the small scales that manifests in a change in the effective viscosity operator and in the effective noise statistical properties.

4 pages revtex, including 5 figures. Related material at http://www.imedea.uib.es/Nonlinear and http://www.imedea.uib.es/Oceanography Figure 4 replaced by a slightly better one