The seasonal cycle of planetary waves in the winter stratosphere

A simple mechanistic model, with time-independent planetary wave forcing at the lower boundary, is used to investigate the observed differences between the Northern Hemisphere (NH) and Southern Hemisphere (SH) stratospheric evolution. Considered here are differences in both the zonal mean and the zonal wavenumber 1 structure of the winter vortex throughout its seasonal evolution. In particular, for small and intermediate forcing values the model robustly produces the midwinter minimum in geopotential wavenumber 1 amplitudes observed in the SH lower and middle stratosphere. This feature is considered from the different viewpoints of transitions between stable equilibria, wave transmission properties of the mean flow, and resonant type interactions with the lower boundary wave forcing, to assess the extent to which these viewpoints, previously studied in the context of models representing variation with height only, are relevant to a more complicated model representing variation in both height and latitude. Results show that resonance with the lower boundary forcing in early and late winter, leading to early and late winter wave maxima and hence to an associated midwinter minimum, is the dominant mechanism in the model considered. For large forcing amplitudes more typical of the NH, two possible winter and late winter evolutions are found to exist for the same forcing amplitudes. Which of these is selected depends on the details of the forcing in early winter. Note that there is no constant value of wave forcing amplitude that produces both a SH-like, undisturbed winter evolution as well a strong final warming, suggesting that time-varying lower boundary wave forcing is necessary to simulate these aspects of the SH evolution.