Climate change and the middle atmosphere 5. Paleostratosphere in cold and warm climates

The GISS Global Climate Middle Atmosphere Model is used to investigate how the stratosphere would have changed during two paleotime periods: the cold Last Glacial Maximum (similar to 21,000 years ago) and the warm Paleocene ( 58 million years ago). Uncertainties in sea surface temperatures and mounta in wave drag over the ice sheets are investigated in sensitivity experiment s. In many respects the climate and dynamical forcing of the stratosphere w as opposite in these time periods, with reduced CO2, increased topography, and increased latitudinal temperature gradients during the ice age, and inc reased CO2, reduced topography and latitudinal temperature gradients during the Paleocene, representative of much of the Tertiary. The results show th at the stratospheric response was often of an opposite nature as well, with the ice ages featuring a warmer stratosphere, increased residual circulati on in the lower stratosphere (and decreased above), and weakened polar vort ices, while the Paleocene simulation had a colder stratosphere, decreased r esidual circulation in the lower stratosphere (and increased above), with s trengthened polar vortices. Analysis shows that the stratospheric response is very individualistic to the particular climate regime, and the opposite effects are not necessarily produced by inversely related mechanisms. Of pa rticular importance in both climates is the reduced latitudinal gradient at high latitudes, which weakens high-latitude zonal winds and limits wave en ergy vertical propagation. Increased planetary wave. forcing in the lower s tratosphere accelerates the circulation during the ice ages. A strong incre ase in zonal winds during the Paleocene is the result of both decreased pla netary wave forcing, associated with the reduced topography, and decreased mountain wave drag. The sensitivity experiments show that if tropical sea s urface temperatures were warmer, the stratospheric residual circulation was enhanced, while stratospheric warmings are sensitive to the precise sea su rface temperature specifications and mountain wave drag.