Earlier this July, the temperature more than 20 kilometres above the east Antarctic coastline suddenly warmed by about 50 degrees Celsius in a week — an event called a sudden stratospheric warming (SSW).
While SSWs occur every other year in the northern hemisphere, they are extremely rare in the southern hemisphere, and have never previously been observed in winter.
The rapid polar warming is now beginning to filter down towards the surface in the form of higher pressure over Antarctica, a set-up which has the potential to impact Australia's weather through August and possibly well into spring.
What is a sudden stratospheric warming event?
The term SSW is used to describe a rapid warming over either the Arctic or Antarctic, which reduces the temperature contrast between the cold poles and milder mid latitudes.
Since upper-level winds are driven by a thermal gradient, this warming leads to a breakdown of the polar vortex, or in other words, a significant weakening of the fierce stratospheric westerly winds that circumnavigate the poles.
Typically this sudden change over the stratosphere will then filter down and disrupt the troposphere (the layer of atmosphere where weather occurs), which can result in prolonged spells of extreme weather for weeks.
The strongest SSW events even cause a reversal in winds around the polar vortex from a westerly to an easterly, including the infamous "Beast From the East" winter storm in Europe through February 2018.
SSWs are almost solely a northern hemisphere phenomenon due to the alignment of land and sea in northern polar regions, occurring on average every two years, however in the southern hemisphere only two SSW episodes have been recorded through the past 65 years according to the Bureau Of Meteorology (BOM); in September 2002 and September 2019.
Technically the warming of the stratosphere is required to average at least 25C to be classified as a true SSW, and while this event's warming has doubled that figure near the east Antarctic coast, the average warming over the whole of Antarctica may have so far peaked just below the threshold.
While remaining below the intensity of the most severe SSWs, the current warming is the strongest event on record for a southern hemisphere winter and, according to Eun-Pa Lim from the BOM, has likely been triggered by a near stationary high-pressure system near the Amundsen Sea in May and June.
"A persistent weather pattern over the Amundsen Sea … can bring changes to the polar vortex strength and temperature … which is likely the cause of the intense stratospheric warming in the recent two weeks," she said.
Tumultuous spell of windy westerlies likely this August
Now that a SSW event is underway, the critical questions are: How long will it last and what impact will it have on our weather?
The link between Australia's weather and a weakening polar vortex is best explained with an analogy. Think of the polar vortex like a magnet; the stronger the magnet, the harder for an object to escape.
A weaker vortex therefore allows westerly winds to escape the poles and expand towards the equator, a pattern called a negative southern annular mode (SAM).
A northward migration of westerly winds and cold fronts from the Southern Ocean has historically brought windy, cold and showery weather to Australia's southern coastline from south-west WA to southern SA, Victoria and Tasmania.
Conversely, westerlies along the east coast of Australia bring warm and dry weather, explaining why the last Antarctic SSW in September 2019 played a major role in fuelling the Black Summer bushfires.
Thankfully, though, the current event is not coinciding with a record multi-year drought. And, occurring in winter when temperatures are cool, it's very unlikely, at least in the short term, to bring widespread bushfires.
The change in weather has already been observed during the past week, with last weekend's front driving gales and snow across south-east Australia, while a pair of fronts this week are currently bringing a second burst of showers and gusty winds.
In the meantime, the westerly winds have produced warmer and drier weather along the east coast — Sydney has already recorded three consecutive days above 20C.
So how much longer could then enhanced westerlies last for Australia?
A pair of leading international weather models are predicting the SSW footprint will continue into August, which could mean frequent stormy conditions for the southern states, and possibly renewed bursts of gale-force winds and heavy alpine snow in the weeks ahead.
However, as with all climate drivers, a negative SAM does not guarantee a deviation from average weather, it only swings the odds.
Ozone hole and weather under threat this spring
While the weather is already being impacted by the warm stratosphere over Antarctica, its ongoing presence could instigate a chain of events with the potential for long-lasting impacts on southern hemisphere atmospheric conditions.
A critical step in this prolonged influence would be a reduction in this year's ozone hole.
A key step in the annual growth of the naturally occurring hole is the formation of polar stratospheric clouds, however the high-altitude clouds only develop in extremely low temperatures
The suppression of clouds could then maintain or even enhance the warm Antarctic stratosphere as radiation from the sun is absorbed by the surplus ozone.
"Less formation of the polar stratospheric clouds means a limited development of the Antarctic ozone hole, which could positively feed back into the warming and weakening of the polar vortex in spring," Dr Lim said.
Essentially the more ozone the more heating occurs and the weaker the polar vortex, while a depleted ozone layer, like we saw in 2023, leads to a colder and stronger polar vortex.
The above process would lead to a similar expansion of westerly winds through spring, bringing further above-average rain to the southern coast and ranges, while again leading to warmer and drier conditions for the east coast, including Sydney and Brisbane.
However, accurate forecasts for spring, and even August, are problematic, as there is no guarantee a second anomalous weather system near Antarctica won't reverse the stratospheric warming through the coming weeks.
Another wildcard is the residual moisture from the Hunga-Tonga volcano, which could counterbalance the excess ozone.
Therefore it's probably safe to conclude forecasts issued now for the rest of the year still have a degree of uncertainty.
Posted, updated
