Seasonal Persistence of Circulation Anomalies in the Southern Hemisphere Stratosphere and Its Implications for the Troposphere

Abstract

Previous studies have highlighted an important organizing influence of the seasonal Southern Hemisphere stratospheric vortex breakdown on the large-scale stratospheric and tropospheric circulation. The present study extends this work by considering the statistical predictability of the stratospheric vortex breakdown event, using reanalysis data. Perturbations to the winter stratospheric vortex are shown to persist into austral spring and to lead to a shift in the statistics of the breakdown event during austral summer. This is interpreted as evidence for the potential for seasonal predictability of the vortex breakdown event in the stratosphere. Coupled variability between the stratosphere and troposphere is then considered. The semiannual oscillation of the tropospheric midlatitude jet is discussed, and evidence for a connection between this behavior and variations in the stratosphere is presented. Based on this connection, an argument is made for the concomitant potential for seasonal predictability in the troposphere, assuming knowledge of the stratospheric initial state. Combining these various results, a nonstationary, regime-based perspective of large-scale extratropical Southern Hemisphere circulation variability between late winter and summer is proposed. The implications of this perspective for some previous studies involving annular modes of the circulation are discussed. In particular, the long annular mode time scales during austral spring and summer should not be interpreted as an increased persistence of perturbations to some slowly varying seasonal cycle, but instead as a reflection of a phase shift of the seasonal cycle induced by stratospheric variability.