THE DIFFUSIVE APPROXIMATION FOR EDDY FLUXES IN BAROCLINICALLY UNSTABLE JETS

A series of statistically steady states for baroclinically unstable je ts in a two-layer quasigeostrophic model is examined, in order to eval uate diffusive approximations to the eddy potential vorticity or heat fluxes. The flow is forced by thermal relaxation to an unstable ''radi ative equilibrium'' temperature gradient. The statistically steady sta tes are studied as a function of the width of the radiative equilibriu m jet. A local diffusive ''theory'' for the eddy fluxes is obtained fr om integrations of a homogeneous, doubly periodic model with prescribe d environmental potential vorticity gradients. The flux-gradient relat ionship generated by the homogeneous model predicts the magnitude and shape of the eddy fluxes in the unstable jet flows remarkably well, as long as the jet is not too narrow. These results confirm the relevanc e of diffusive closures for eddy potential vorticity and heat fluxes i n such flows. For narrow jets that produce eddy fluxes with a half-wid th of one to two radii of deformation, this local theory underpredicts the fluxes.