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Abstract

For a barotropic ocean it is shown that the effect of the eddy field on the general cir- culation can be parametrized by a spatial diffusion term under four premises: (1) The eddy field can be regarded as a superposition of weakly nonlinear Rossby waves. (2) Mean flow and eddy field are separated by a spectralgap. (3) The effect of the mean flow on the wave field can be regarded as a small perturbation. (4) There exist relaxation pro- cesses within the Rossby wave field which maintain a basic state of the wave field and counteract the forcing by the mean flow. The validity of these assumptions restricts the applicability to mid-ocean regions of low eddy intensity. The diffusion coefficients depend on the characteristic time scale of the relaxation processes and on the energy density spectrum of the wave field and can be evaluated if both are known. For the spe- cial case of a purely zonal mean flow we show.that scattering at bottom topography of a specific orientation provides a suitable relaxation mechanism, The diffusion coefficient is explicitly determined for this case and found to be positive and of the order of 104 m 2 s -1. For arbitrary mean flows the diffusion coefficient depends sensitively on the wavenumber structure of the relaxation time and energy density spectrum, although plausible argu- ments suggest a positive value in this case as well