Rare stratospheric warming event develops over Antarctica, weakening polar vortex circulation
A rare stratospheric warming event is disrupting the Antarctic polar vortex in September 2025, with forecasts suggesting wetter southern Australia, drier eastern Australia, and a higher chance of abnormally hot days.
Extended-range forecast of stratospheric temperature anomalies at 10 hPa for South Pole - September 29, 2025. Credit: ECMWF, Copernicus Climate Change Service
A sudden stratospheric warming is developing above Antarctica in late September 2025, weakening the southern hemisphere polar vortex.
The polar vortex is essentially a large whirl of cold air high above Antarctica, held together by powerful winds that strengthen in winter and fade in spring. Right now, instruments are detecting unusual conditions — stratospheric temperatures at 50 and 100 hPa have surged into the upper 90th percentile compared with 1979–2024 records, while eddy heat flux has dropped into the lowest 10th percentile, a clear sign of strong poleward heat transport driving the warming.
At the same time, NASA’s MERRA-2 reanalysis shows the zonal-mean wind at 60°S and 10 hPa weakening significantly compared to the long-term average, confirming that the Antarctic polar vortex is undergoing disruption at altitudes between 10 and 50 km (6–31 miles).
If the warming propagates downward into the troposphere, the tropospheric vortex may weaken, shifting the Southern Annular Mode (SAM) into a negative phase.
In that case, rainfall may increase and westerly winds may strengthen across Tasmania, Victoria, South Australia, and parts of Western Australia. Rainfall may be reduced in eastern Australia, while the probability of abnormally hot days may rise in southern regions.
The Bureau of Meteorology reported the SAM index turned negative around September 13, with forecasts showing a return toward neutral in late September, consistent with a possible but not guaranteed downward influence from the stratosphere.
The effects could partly offset the influence of the negative Indian Ocean Dipole, which is strongly active with the weekly index at −1.17°C in mid-September and expected to persist through spring. A negative IOD typically promotes wetter conditions across large areas of Australia.
Monitoring by DCCEEW, CSIRO, and BoM confirms a sharp September warming, with 60–90°S mean stratospheric temperatures rising into the upper percentile bands and heat flux suppressed.
NASA’s MERRA-2 time series shows a clear weakening of the polar night jet at 10 hPa. As the warming intensified, the Antarctic ozone hole temporarily contracted to 15.8 million km² on September 5, accompanied by a rise in minimum ozone levels.
Major sudden stratospheric warmings, defined by a reversal of zonal winds at 60°S and 10 hPa, are frequent in the Northern Hemisphere but very rare in the Southern Hemisphere, occuring roughly once every two to three decades.
The most notable Antarctic event occurred in 2002, when the vortex split into two distinct systems, followed by another significant weakening in 2019. The Antarctic vortex is generally more stable due to weaker planetary wave forcing compared to the Northern Hemisphere, making downward coupling into the troposphere less consistent.
For a stratospheric disturbance to affect surface climate, anomalies must descend into the troposphere. When this occurs, the jet stream may shift equatorward, storm tracks may be displaced, and the SAM often trends negative.
This can promote polar air intrusions into midlatitudes, alter rainfall distribution, and increase surface temperature variability.
However, many Southern Hemisphere events do not produce a strong surface signal. Coupling strength depends on season, vortex state, and anomaly structure, making surface impacts less predictable than in the Northern Hemisphere.
References:
1 Exploring the link between austral stratospheric polar vortex anomalies and surface climate in chemistry-climate models – Baldwin et al. – 2022 – Atmospheric Chemistry and Physics – https://doi.org/10.5194/acp-22-13915-2022 – OPEN ACCESS
2 Negative Indian Ocean Dipole event underway – Bureau of Meteorology – September 16, 2025
3 Stratospheric warming is weakening the southern hemisphere’s polar vortex – Weatherzone – September 25, 2025
4 Long-range forecast: October–December 2025 – Bureau of Meteorology – September 25, 2025
5 Antarctic Ozone Hole 2025 – Report #7 – DCCEEW/CSIRO/BoM – September 22, 2025
6 MERRA-2 Reanalysis: 60°S, 10 hPa zonal wind – NASA/GSFC – Accessed September 25, 2025
7 The 2019 Southern Hemisphere Stratospheric Polar Vortex Weakening and Its Impacts – Lim et al. – 2020 – Bullletin of the American Meteorological Society – – https://doi.org/10.1175/BAMS-D-20-0112.1 – OPEN ACCESS
8 The sudden stratospheric warming events in the Antarctic in 2024 – Zi et al. – 2025 – Geophysical Research Letters – https://doi.org/10.1029/2025GL115257 – OPEN ACCESS
9 Trends and anomalies in the coherence of the Southern Polar Vortex: A 26 year meta-study – Blachut et al. – 2025 – Scientific Reports – https://doi.org/10.1038/s41598-025-12923-6 – OPEN ACCESS
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