Paleoclimate records indicate that winter-time polar temperatures at the Earth's surface during the Eocene (55-38 Ma) were dramatically warmer than they are today. The inability of global climate models to simulate above-freezing polar winter temperatures for Eocene-like boundary conditions has led to the hypothesis that polar stratospheric clouds (PSCs) may have played a major role in the radiative balance of the polar regions. For this to be true, stratospheric water vapor concentrations would have to have been considerably higher than at present. A recent study has suggested that the high greenhouse gas concentrations of the Eocene may have resulted in a slower stratospheric overturning circulation, thereby warming the tropical tropopause, allowing more water vapor to enter the stratosphere, while simultaneously cooling the polar stratosphere, leading to more frequent PSC condensation. One key assumption in this study was that the propagation of planetary-scale waves into the stratosphere would decrease (thereby slowing the overturning circulation) in response to a decreased equator-to-pole temperature gradient at the surface resulting from increased greenhouse gas emissions.

We present a mechanistic study of changes in planetary-scale wave propagation and the stratospheric overturning circulation in response to changes in the tropospheric meridional temperature gradient, using the dynamical core of a general circulation model. The model is forced by Newtonian cooling to a specified radiative equilibrium temperature distribution and Rayleigh friction in the boundary layer and near the model top. We use a realistic radiative equilibrium temperature distribution that is typical of Northern Hemisphere winter. Topography is used to generate planetary-scale waves. We vary the basic state near-surface temperature structure through the radiative equilibrium temperature field and examine the resulting steady-state stratospheric response. Our results are applicable not only to the Eocene climate, but also to future scenarios with increased greenhouse gas concentrations.