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.
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