ON THE PHYSICS OF UPGRADIENT MOMENTUM TRANSPORT IN UNSTABLE EASTWARD JETS

The weakly nonlinear finite amplitude evolution of mixed baroclinic/ba rotropic instabilities of an eastward zonal jet is considered in a two -layer QG-model on a midlatitude beta-plane. Linear friction is includ ed and is essential to the momentum transport. The focus is two parame ter regimes, one corresponding to the Gulf Stream and the other to the Antarctic Circumpolar Current. In the latter case, the nonlinear self -interaction of the most unstable mode causes upgradient momentum tran sport and therefore sharpens the jet. The analysis of this example lea ds to a clear description of the physics of this process. It also indi cates why upgradient momentum transport does not occur in the Gulf Str eam regime.