On the physics of upgradient momentum transport in unstable eastward jets

Abstract

The weakly nonlinear finite amplitude evolution of mixed baro-clinic / barotropic instabilities of an eastward zonal jet is considered in a two-layer QG-model on a midlatitude beta-plane. Linear friction is included and is essential to the momentum transport. The focus is two parameter 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 transport and therefore sharpens the jet. The analysis of this example leads to a clear description of the physics of this process. It also indicates why upgradient momentum transport does not occur in the Gulf Stream regime.