Counter-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. In this paper, eddy fluxes of PV along isentropic surfaces in the vicinity of the subtropical jet are studied using modern reanalysis data. A localized region of significant counter-gradient eddy PV flux is found on the poleward flank of the jet core during the respective winter and spring seasons in both hemispheres. This region of counter-gradient (i.e. notionally 'anti-diffusive') eddy PV flux is in conflict with the turbulence approach described above. Relationships to Rossby wave breaking along the subtropical jet, as well as to the width and position of the jet will be discussed.