A mechanism for up-gradient eddy fluxes of potential vorticity near the subtropical jet

The role of eddy fluxes in the general circulation has often been approached by treating eddies as (macro)turbulence. In this approach the eddies act fundamentally to diffuse certain quasi-conservative quantities, such as potential vorticity (PV) along isentropic surfaces in the free atmosphere. Eddy fluxes are determined primarily by the eddy diffusivities and are necessarily down-gradient of the basic state PV field in this picture. A localized region of significant up-gradient eddy PV flux has been found on the poleward flank of the jet core during the winter and spring seasons of both hemispheres. We present a detailed analysis of this feature based on modern reanalysis data. Analysis of the enstrophy budget suggests that the up-gradient PV flux represents wave decay and is maintained by poleward enstrophy fluxes. The poleward migration of the jet following wave breaking events appears to be crucial in understanding the dynamics of these up-gradient PV fluxes, as it leads to a latitudinal displacement of the region of wave decay relative to the region of wave growth. Decompositions in space and time reveal that the up-gradient PV fluxes primarily correspond to low-frequency planetary waves.