Variability of the polar stratospheric vortex and its impact on surface climate patterns
Abstract
This thesis investigates various aspects of the variability of the stratospheric polar vortex and the effect of this variability on tropospheric weather and climate patterns on various timescales. In the first part of this work, an improved idealized model was developed to study the coupled stratosphere-troposphere system. The model is forced by relaxation to a specified equilibrium temperature profile, which varies seasonally only in the stratosphere. This model setup permits the investigation of stratosphere-troposphere interactions on seasonal timescales, 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. The effect of these different kinds of topography on the seasonal variability of the strength of the polar vortex, the average timing and variability in timing of the final breakup of the vortex (final warming events), the conditions of occurrence and frequency of midwinter warming events, and the impact of the stratospheric seasonal cycle on the troposphere are explored. The inclusion of wavenumber 1 and wavenumber 2 topographies results in very different stratospheric seasonal variability. 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 midwinter warming events, the distribution of the intervals between these events suggest that the model has no year to year memory. When forced with wave-1 topography, the gross features of seasonal variability are similar to those forced with wave-2 topography, but the dependence on forcing magnitude is weaker. Further, the frequency of major warming events has a non-monotonic dependence on forcing magnitude, and never reaches the frequency observed in the northern hemisphere. In the second part of the thesis, the impact of stratospheric ozone depletion on the Antarctic polar vortex and its subsequent influences on southern hemisphere surface climate patterns is investigated. It is verified that stratospheric final warming events have an impact on tropospheric circulation in a simplified GCM with seasonal variations in the stratosphere only. The model produces qualitatively realistic final warming events whose influence extends down to the surface, much like what has been reported in observational analyses. The hypothesis that recent observed trends in surface westerlies in the Southern Hemisphere are directly consequent on observed trends in the timing of stratospheric final warming events is tested. It is confirmed that there is a statistically significant shift towards later final warming events in the years with large ozone depletion. However it is found that the observed trends in surface westerlies cannot be attributed simply to this shift towards later final warming events. Finally, responses of the idealized AGCM to polar stratospheric cooling that mimics the radiative effects of stratospheric ozone depletion are studied. It is found that there are two factors that play a role in setting the magnitude and persistence of the model's surface response to cooling: the seasonal cycle of tropospheric annular mode timescales, and whether or not the imposed cooling leads to the presence of stratospheric westerlies at a time when easterlies were prevalent in the control run. That is, the surface response is sensitive to the timing of the imposed polar stratospheric cooling.