What chance of a sudden stratospheric warming in the southern hemisphere?

Abstract

Sudden stratospheric warmings (SSWs) are amongst the most dramatic events in the Earth’s atmosphere and they drive extreme surface weather conditions. They have been recently linked to the hot and dry weather conditions that favour wildfires over Australia. However, the chance of a southern hemisphere event is unknown because it has only been observed once. Legitimate estimation of the frequency of SSW events requires a large sample of realistic model simulations. Here we show that the chance of an event is close to 4% per year, implying that an event will occur, on average, every 25 yr, using a state-of-the-art model that simulates SSWs accurately. It is thus not surprising that there was a near miss in the September prior to the Australian wildfire of 2019, given the 40 yr of comprehensive satellite records and just one observed Antarctic event. According to this new estimate, it would also not be surprising to see a second SSW event in the coming years in the southern hemisphere. Such a stratospheric warming event might bring further extreme surface weather conditions and natural hazards, as it may raise the risk of increased rainfall in the latitudinal band of 35–50°S. Meanwhile, the associated hot and dry weather conditions over austral subtropical continents might increase the risk of wildfires over these regions.

Keywords

• sudden stratospheric warming
• southern hemisphere
• polar vortex
• wildfire