Barotropic instability and the latitude of the surface westerlies

The barotropic eddy-driven jet stream is a prominent feature of the general circulation of Earth's atmosphere, and is an important influence on the global climate and ocean circulation. One of the robust circulation changes in simulations of future climate is a poleward shift of the eddy-driven jet stream and surface westerlies. Here a mechanism is proposed that may provide a dynamical interpretation of this shift.

Output two idealized models is used. The first is a stirred-barotropic model. It is shown that in this model, for realistic wind-speeds, barotropic shear instability occurs, and when it does so, it accelerates the poleward flank of the jet. An increase in the speed of the jet in this model causes the jet to shift poleward. This is shown to result from an increase in barotropic instability. The increase in barotropic instability accelerates the poleward flank of the jet, thereby broadening it. Subsequently, waves that are excited through other means (stirring) no longer dissipate on the poleward flank of the jet, and this results in a poleward shift of the jet. A comparison is made with a second model, a multi-level dry dynamical core. It is shown that this model exhibits similar dynamics during the transient phase of a poleward shift of the westerlies, and this appears to be independent of the change in the mean-state that instigates the shift.