Australia has been experiencing freezing temperatures and extreme weather in the south and east this winter, while the rest of the continent and the globe continue to warm. One possible cause for these weather events is an event occurring high above Antarctica, which may have been triggered by a heatwave on the surface of the frozen continent.
The story begins in the stratosphere over Antarctica, where temperatures in July are typically around -80°C. The strong winds in this region, known as the stratospheric polar vortex, can be influenced by high air pressure in the lower atmosphere. This can cause the winds to slow down and the air above the pole to become warmer than usual. In extreme cases, the stratospheric winds can completely break down, resulting in a “sudden stratospheric warming” event.
When the polar vortex is disturbed, it can impact surface weather by steering weather systems from the Southern Ocean towards the Equator. However, this process is slow and the impact may not be felt until weeks or even months later. The weakening of the stratospheric polar vortex puts the Southern Annular Mode, an atmospheric system, into a negative phase. This brings westerly winds further north in winter, leading to more rain in southern Australia and snowfall in alpine regions. In spring and summer, it brings warm and dry air to southeastern Australia.
It is difficult to determine the exact cause of any weather event at the moment due to the unusual global weather patterns in the past year. However, there are indications that the stratosphere is influencing Australia’s winter weather. The stratospheric polar vortex has been warming and the winds have slowed down, although it does not technically qualify as a sudden stratospheric warming event. The evolution of southern hemisphere winds in recent weeks resembles what would be expected from a sudden stratospheric warming.
The disturbances in the polar vortex are likely caused by surface disturbances, such as near-record surface temperatures around Antarctica. These disturbances may be due to globally high ocean temperatures or the effects of the Hunga Tonga volcano eruption in 2022.
There are two possible scenarios for the rest of the year. One is that the stratospheric winds and temperatures return to normal and no longer influence surface weather. The other is that the stratosphere continues to warm and the winds remain slower, resulting in a persistent negative Southern Annular Mode. This would lead to warmer and drier weather over southeastern Australia in spring and potentially summer. Seasonal forecasting models suggest the second scenario is more likely.
