A split in the southern vortex – not seen since 2002 – could lead to sudden warming of the Antarctic stratosphere and hotter weather in Australia and South America.
Antarctica’s polar vortex could be about to split in two for the first time in more than 20 years after a series of sudden spikes in stratospheric temperatures. If that happens, it could lead to significant warming in Antarctica and an unusually hot summer in Australia and South America.
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| A satellite view of Antarctica Science History Images / Alamy |
The southern polar vortex is a clockwise swirl of winds that trap a cold air current above Antarctica during the southern hemisphere winter. The vortex is usually very stable at this time of year, with temperatures inside its core in the stratosphere normally being about -80°C (-112°F).
But there are growing signs that this year’s vortex is increasingly unstable. In mid-July, wind speeds in the southern polar vortex slowed from 300 to 230 kilometres per hour, allowing cold air to descend and triggering a record spike in stratospheric temperatures. Temperatures inside the vortex were about 20°C (36°F) above the long-term average of about -80°C.
Another slowdown in wind speed was recorded in early August, says Steven Keates at the UK Met Office, accompanied by another rapid spike in stratospheric temperatures. “We started to see a marked warming in the very highest layers of the stratosphere,” he says.
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| The graph shows mean temperatures inside the vortex high in the stratosphere. The red line shows observed temperatures in 2024, the pink section is provisional temperature estimates, and orange is forecast temperatures. The pink spikes show the sudden surges in vortex temperatures in recent weeks, well outside the normal range for the time of year. |
As a result, the vortex has been pushed out of its place above the South Pole, causing cold polar air at the surface to seep out towards Australia, New Zealand and South America, and warm air to creep over Antarctica, causing a heatwave on the continent.
If the wind slows too dramatically, the vortex could suddenly change direction and swirl anticlockwise, in what is known as sudden stratospheric warming. Repeated slowdowns in wind speeds could increase the chances of a vortex split and sudden stratospheric warming taking place this year, says Keates.
In 2002, the last time the polar vortex split, there were repeated sharp spikes in temperature and slowdowns in wind speed throughout the winter, before the weakened vortex split in September of that year and collapsed much earlier than usual. It is the only time sudden stratospheric warming has been observed in the southern polar vortex, although other years, including 2019, have come close.
Simon Lee at the University of St Andrews in the UK says even relatively small disruptions to the vortex can have a cumulative effect. “Sometimes minor warmings can set the vortex up for something major later on,” he says. “Because Antarctic vortex variability is small, if something even slightly unusual happens it can very quickly grow to become an extreme event.”
Sudden stratospheric warming could lead to higher ground-level temperatures in Antarctica, and may trigger hot, dry summers in Australia and South America.
The reason for this is that the weaker winds put an atmospheric system called the southern annular mode into a negative phase, says Martin Jucker at the University of New South Wales in Sydney, Australia. That brings cold polar air over Australia in the winter, and hotter, drier air over the country during the summer.
Jucker describes the state of the southern annular mode at the beginning of August as “really extreme”. What is happening now with the polar vortex is “definitely a newsworthy event”, he says, although wind speeds have returned to near normal in the past few days.
Chantelle Blachut at the University of Adelaide, Australia, studies the behaviour of the southern polar vortex and says this year’s structure is very unusual, with competing wind patterns stretching and warping its shape. “What you can see is this kind of stretching of the polar vortex by two structures either side,” she says. “Something unusual is happening this year.”
Keates stresses that models are predicting a “huge spread” of potential outcomes for the vortex in the coming weeks. Most model outcomes suggest wind speeds will soon pick up again to above-normal levels, although a minority indicate a further slowdown and the most extreme model outcomes show a near collapse of wind speeds later this month. “It’s very uncertain,” says Keates.
Emerging research suggests climate impacts could be at least partially to blame for a weakened southern polar vortex. Winds in the stratosphere are slower than normal in years with lower than average levels of sea ice, according to a study published last week. Antarctic sea ice cover hit a record low in 2023, and has remained well below the long-term average throughout 2024.
Jucker suggests the massive increase in water blasted into the stratosphere by the eruption of the Hunga Tonga-Hunga Haʻapai volcano in 2022 could also be a contributing factor, as well as record sea temperatures.
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