Dynamics of up-gradient eddy fluxes of potential vorticity near the subtropical jet and synoptic-planetary scale interactions

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 atmosphere or along isopycnal surfaces in the ocean. The eddy fluxes are determined primarily by the eddy diffusivities and are necessarily down-gradient of the basic state PV field. As a result, the eddies tend to homogenize PV, except near wave guides (jets) where the eddy diffusivities are small.

Here, an analysis of eddy PV fluxes along isentropic surfaces in the upper troposphere and lower stratosphere is presented based on modern reanalysis data. The eddy PV fluxes are predominantly down-gradient throughout this domain with a strong tendency to homogenize PV. A notable exception occurs near the subtropical tropopause / jet core in both hemispheres where eddy PV fluxes are significantly up-gradient (i.e. notionally ‘anti-diffusive'). These up-gradient eddy PV fluxes exist in a region of strong and positive background PV gradient and thus enhance the angular momentum of the mean flow.

Analyses of the enstrophy budget suggest that the up-gradient PV fluxes are maintained by poleward enstrophy fluxes. Finite-amplitude effects thus represent leading order contributions to the enstrophy budget, whereas dissipation is only of secondary importance locally. The up-gradient PV fluxes are found to be predominantly due to planetary scale waves, which are generated through scale interactions during synoptic scale Rossby wave breaking along the tropopause wave guide. The nature of this upscale cascade and its fundamental role in wave-mean flow interaction phenomena in geophysical fluid dynamics will be discussed.