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BAMOS November 2025
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Figure 1: left: 2025 temperature over the South Pole( black line) compared to climatology( gray line). Credit: Japan Meteorological Agency Right: 2025 polar cap ozone( red line) compared to 1979-2024( black). Credit: Ozonewatch
When applying the same definition in the Southern Hemisphere, only the 2002 event satisfies the criterion in the entire satellite record( dating back to 1979). That’ s mainly because the southern polar vortex is surrounded by stronger winds, making it harder to disturb.
Another slightly weaker event occurred in 2019 and has been linked to the devastating Black Summer bushfires. It is now widely accepted to be as important as the 2002 event, although it didn’ t meet the criterion of polar vortex disruption.
Efforts are underway to adopt alternative definitions better adapted to the Southern Hemisphere, and early results from this work suggest that there were some important events in the 1980s and 1990s.
But even with a new classification, we would not expect the two consecutive events we just had( 2024 and 2025), nor three events in seven years( 2019, 2024, 2025).
So, climate change after all?
After the 2019 event, Thomas Reichler, Darryn Waugh and I conducted a study on the expected frequency of SSW under present-day conditions( 1990 conditions) and by the end of the century. We found a frequency of 1 in 22 years for the present, consistent with the two events observed in the 40 years of satellite records( from 1979 to 2019).
For the future, however, we found that these events would become incredibly rare, with only 1 event in over 300 years. The reason is that when greenhouse gases are added to the atmosphere, the stratosphere radiates heat more efficiently out to space and cools. As a result, the polar vortex gets stronger and more difficult to disturb.
What that study didn ' t do, however, was to include the transient response to greenhouse gas forcing. And that is the period we are currently in, where the entire climate system is still adjusting to the steadily rising CO 2 concentrations in the atmosphere.
Now, during that time, if the surface heats up faster than the stratosphere cools( for instance, if we not only increase CO 2 but also remove aerosols thanks to pollution regulations), it is possible that the warmer surface causes more perturbations, and SSWs become more frequent for a while. But currently, this is just me guessing, so don ' t take my word while the scientific community tries to figure this out.
How did this year ' s SSW impact our weather?
At first, I thought any surface impact would be minor this year— as mentioned to the media in early spring— because the ocean temperatures around Australia are so warm that they would override any effect the stratosphere might have.
But, now that we’ re halfway through spring and have more data available, let ' s have a look at the Bureau of Meteorology’ s seasonal forecasts and how they have changed since the event started around 5 September.
The September and early October forecasts for the period October – December all show above-average rainfall in eastern Australia( Figure 2, a – e). However, by mid-October( Figure 2, e – f; panels now show November – January forecasts), the wet signal seems to gradually disappear. The last panel shows actual rainfall anomalies for 1 to 20 October, with a clear pattern of drier-than-usual weather throughout the southeast. This might well be the effect of the SSW as it gets assimilated into the forecast system.