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

The Goddard Earth Observing System, version 4 general circulation model (GCM) is used to examine the influence of sea surface temperatures (SSTs) on stratospheric circulation and extratropical stratosphere-troposphere exchange (STE). The simulation uses observed SSTs from 1949 to 1998 at the lower boundary. The atmospheric greenhouse gas (GHG) burden is interannually constant to artificially decouple the radiative influence from the SST forcing. A stronger SST gradient from the tropics through the middle latitudes results in a greater meridional temperature gradient in the subtropical troposphere. The upper tropospheric zonal winds increase consistent with the thermal wind relationship, and vertically propagating waves are refracted more poleward into the stratosphere. As a consequence, the stratospheric residual circulation is strengthened. 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. Likewise, the characteristic residence time of air in the lowermost stratosphere decreases by about a month. The STE trends and upper tropospheric temperature gradients are not significantly different in a second simulation that specifies an increasing GHG burden. The second simulation also shows that the radiative cooling from an increased GHG burden is greater in magnitude than the SST-induced dynamical warming in the polar stratosphere. The results of this study imply that the modeled circulation of the stratosphere can be significantly affected by the SSTs. It follows that the fidelity of simulations of the 21st century atmosphere strongly depends on the SSTs, whether prescribed or interactive.