Abstract

The seasonal variability of the polar stratospheric vortex is studied in a simplified AGCM driven by specified equilibrium temperature distributions. Seasonal variations in equilibrium temperature are imposed in the stratosphere only, enabling the study of stratosphere–troposphere coupling on seasonal time scales, without the complication of an internal tropospheric seasonal cycle. The model is forced with different shapes and amplitudes of simple bottom topography, resulting in a range of stratospheric climates. Hemispheric differences in stratospheric seasonal variability are recovered in the model with appropriate choices of wave-2 topography. In the model experiment with a realistic Northern Hemisphere–like frequency of stratospheric sudden warming events, the distribution of the intervals between these events suggests that the model has no year-to-year memory. In addition to tropospheric impacts of stratospheric ‘events’, it is shown that there are tropospheric impacts of the stratospheric seasonal cycle itself.

Additionally, the model is used to investigate the sensitivity of model responses to the timing of imposed polar stratospheric cooling, intended to mimic the radiative effects of ozone depletion. The model exhibits circulation responses to springtime cooling that qualitatively match both observations and the responses of comprehensive chemistry climate models. The model’s surface response is sensitive to the timing of the cooling, suggesting that the meteorology plays an important role. The model’s responses are not always annular mode-like. Larger and more persistent surface responses at certain times are consistent with the model’s seasonal cycle of tropospheric annular mode timescales. It is shown that lower stratospheric variability impacts the persistence of the tropospheric jet and storm tracks.