Southern Hemisphere Sudden Stratospheric Warmings Continue to Be Relevant Under Global Warming

Abstract. In 2019, a stratospheric warming event strongly disrupted the stratospheric circulation in the Southern Hemisphere. This disruption led to negative anomalies in the Southern Annular Mode that propagated down to the troposphere and contributed to hotter and drier conditions in Australia, culminating in a devastating fire season. Although such events have been rare in the past, it remains unclear how climate change will affect their occurrence and surface impacts in the coming decades. We conducted climate model time slice experiments to investigate how frequency and associated surface responses of Southern Hemispheric stratospheric warmings change under different global warming levels (present-day, 1.5 K, 2 K, 3 K, and 4 K). Additionally, we used an ensemble of historical experiments to confirm that our model simulations accurately capture the observed response to stratospheric warmings in the Southern Hemisphere. To investigate the change in stratosphere-troposphere coupling, we distinguish between downward- and non-propagating stratospheric events based on the Southern Annular Mode response on different atmospheric levels. Our findings show that the frequency of major stratospheric warmings remains close to the historical frequency up to a warming of 2 K before decreasing more significantly. The coupling between the stratosphere and the troposphere shows a large decrease only at the 4 K warming level. Hence, as long as global warming levels remain at intermediate levels, the possibility of sudden stratospheric warmings contributing to extreme events continues to be relevant and would add to heat-related stress in Australia and Southern Africa, making adaptation to global warming more challenging.