Response of stratospheric circulation and stratosphere-troposphere exchange to changing sea surface temperatures

We examine the influence of sea surface temperatures (SSTs) on stratospheric circulation and extratropical stratosphere-troposphere exchange (STE) in the GEOS-4 general circulation model. The simulation runs from 1949 to 1998 using Hadley Centre observed SSTs. The SST influence is artificially separated from the atmospheric greenhouse gas (GHG) burden by holding the GHG sources interannually constant. We find that the sub-tropical tropospheric temperature gradient increases with time and the upper-tropospheric zonal winds strengthen consistent with the thermal wind relationship. Vertically propagating waves from the troposphere into the stratosphere are refracted more poleward resulting in a stronger stratospheric residual circulation. This is manifest as a net annual mass STE increase of 2.7% per decade in the Northern Hemisphere and 2.0% per decade in the Southern Hemisphere. The STE trend in a second simulation with an interannually increasing GHG burden is not significantly different. The upper-tropospheric response is also similar. The second simulation also shows that the trend in the radiative cooling is greater in magnitude than the dynamical warming of the polar stratosphere. The results of this study imply that realistic SSTs, a response to the atmospheric GHGs, are required to accurately model the circulation of the stratosphere. It follows that the fidelity of simulations of the 21st century atmosphere strongly depends on the realism of the SSTs, whether prescribed or interactive.