Title: Up-gradient eddy fluxes of potential vorticity near the subtropical jet

Abstract: The role of eddy fluxes in the general circulation is often approached by treating eddies as (macro)turbulence. In this approach, the eddies act to diffuse certain quasi-conservative quantities, such as potential vorticity (PV), along isentropic surfaces in the free atmosphere. The eddy fluxes are determined primarily by the eddy diffusivities and are necessarily down-gradient of the basic state PV field. Support for the (macro)turbulence approach stems from the fact that the eddy fluxes of PV in the free atmosphere are generally down-gradient in the long-term mean. Here we call attention to a pronounced and significant region of up-gradient eddy PV fluxes on the poleward flank of the jet core in both hemispheres. The up-gradient eddy PV flux corresponds to divergent Eliassen-Palm flux and equatorward residual flow. The region of up-gradient
(i.e. notionally 'anti-diffusive') eddy PV fluxes is most pronounced during the winter and spring seasons, and partially contradicts the turbulence approach described above. Analyses of the enstrophy budget suggest that the up-gradient PV fluxes represent wave decay and are maintained by poleward enstrophy fluxes. The appearance of up-gradient PV fluxes in the long-term mean is associated with the poleward shift of the jet - and thus the region of wave decay relative to wave growth - following wave breaking events.
Wave-driven jet shifts play a fundamental role in climate variability across a range of timescales - the region of up-gradient PV fluxes appears to be an integral part of the associated underlying dynamics.

Speaker's website: http://birner.atmos.colostate.edu/index.html