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.

• Received 12 February 1998

DOI:https://doi.org/10.1103/PhysRevE.58.7279