Nonlinear Wave-Mean Flow Interaction Near the Subtropical Jet

The conventional quasi-linear treatment of wave-mean flow interaction under quasi-geostrophic scaling predicts that eddy fluxes of potential vorticity (PV) are down-gradient of its basic state field, corresponding to wave dissipation and reduction of the angular momentum of the mean flow in the interior where background PV gradients are positive. However, eddy PV fluxes have been shown to be significantly up-gradient near the subtropical tropopause / jet core in both hemispheres. These up-gradient eddy PV fluxes exist in a region of strongly positive background PV gradient and thus enhance the angular momentum of the mean flow. It is confirmed that this implies that finite-amplitude / nonlinear effects play an important role for wave-mean flow interaction in this region of the atmosphere. The up-gradient PV fluxes are found to correspond to up-scale transfer via wave-wave interactions from synoptic to planetary scale Rossby waves. A potential enstrophy budget-based analysis of this scale interaction will be presented and the fate of the planetary scale waves generated through this interaction will be discussed.