The Diffusive Approximation for Eddy Fluxes in Baroclinically Unstable Jets

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  • 1 Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey
  • | 2 Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, New Jersey
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Abstract

A series of statistically steady states for baroclinically stable 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 are 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 flows 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 a half-width of one to two radii of deformation, this local theory underpredicts the fluxes.

Abstract

A series of statistically steady states for baroclinically stable 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 are 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 flows 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 a half-width of one to two radii of deformation, this local theory underpredicts the fluxes.

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