Wintertime Southern Hemisphere jet streams shaped by interaction of transient eddies with Antarctic orography

Abstract

The wintertime Southern Hemisphere extratropical circulation exhibits considerable zonal asymmetries. We investigate the roles of various surface boundary conditions in shaping the mean state using a semi-realistic, atmosphere-only climate model. We find, in agreement with previous literature, that tropical sea surface temperature (SST) patterns are an important contributor to the mean state, while mid-latitude SSTs and sea ice extent play a smaller role. Our main finding is that Antarctic orography has a first order effect on the structure of the mid-latitude circulation. In the absence of Antarctic orography, equatorward eddy momentum fluxes associated with the orography are removed and hence convergence of eddy momentum in midlatitudes is reduced. This weakens the Indian Ocean jet, making Rossby wave propagation downstream to the South Pacific less favourable. Consequently the flow stagnates over the mid to high-latitude South Pacific and the characteristic split jet pattern is destroyed. Removing Antarctic orography also results in a substantial warming over East Antarctica partly because transient eddies are able to penetrate further polewards, enhancing poleward heat transport. However, experiments in which a high latitude cooling is applied indicate that these temperature changes are not the primary driver of circulation changes in mid-latitudes. Instead, we invoke a simple barotropic mechanism in which the orographic slope creates an effective potential vorticity gradient which alters the eddy momentum flux.