The diffusive approximation for eddy fluxes in baroclinically unstable jets

A scries of statistically steady states for baroclinically unstable jets in a two-layer quasigeostrophic model is examined, in order to evaluate diffusive approximations to the eddy potential vorticity or heat fluxes. The flow is forced by thermal relaxation to an unstable "radiative equilibrium" temperature gradient. The statistically steady states arc studied as a function of the width of the radiative equilibrium jet. A local diffusive "theory" for the eddy fluxes is obtained from integrations of a homogeneous, doubly periodic model with prescribed environmental potential vorticity gradients. The flux-gradient relationship generated by the homogeneous model predicts the magnitude and shape of the eddy fluxes in the unstable jet Hows remarkably well, as long as the jet is not too narrow. These results confirm the relevance of diffusive closures for eddy potential vorticity and heat fluxes in such flows. For narrow jets that produce eddy fluxes with uhalf width of one to two radii of deformation, this local theory underpredicts the fluxes.