Impact of Interannual Ozone Variations on the Downward Coupling of the 2002 Southern Hemisphere Stratospheric Warming

The Southern Hemisphere experienced its first recorded major sudden stratospheric warming during September 2002, which subsequently resulted in strong low polarity of the Southern Annular Mode (low SAM) and extreme daily mean maximum temperatures and low rainfall over eastern Australia during October 2002. The warming and weakening of the polar vortex were accompanied by anomalously high values of polar stratospheric ozone, which possibly could have constructively sustained the weakened vortex and subsequent development of low SAM. We explore the impact of this ozone variation by conducting an idealized forecast experiment using the Australian Bureau of Meteorology's operational subseasonal to seasonal prediction system (Australian Community Climate and Earth System Simulator-Seasonal forecast system version 1, ACCESS-S1), whose atmospheric model well resolves the stratosphere. The ACCESS-S1 control forecasts are generated with prescribed climatological monthly mean ozone, whereas the observed monthly mean ozone during 2002 is prescribed during the forecast for the experiment. While the control forecasts initialized on 1 August 2002 demonstrate good skill in predicting the weakening of the polar vortex and the resultant occurrence of low SAM during October, the extremity of the SAM anomaly and associated extreme high temperatures and low rainfall over eastern Australia were significantly underpredicted. Prescribing the observed ozone results in more realistic weakening of the stratospheric vortex and stronger development of low SAM and extreme warm conditions in eastern Australia during October 2002. These results suggest that polar stratospheric ozone variations are a potential source of long lead climate variability, which can be tapped with future ACCESS-S development.