Australia Weather News

The temperature high above Antarctica has climbed more than 30 degrees Celsius in the past week.

This is known as Sudden Stratospheric Warming (SSW), an event that has the potential to disrupt weather patterns across Australia for months.

SSWs are extremely rare in the Southern Hemisphere, with only two major events documented in the past 60 years — one in 2002 and the other in 2019 — and both resulted in some of the most devastating bushfires in Australia's history.

While a wetter background environment should prevent a repeat of Black Summer in 2025, the warming has thrown a major spanner in the works of the spring forecast — a shift already being felt throughout the country, including the current run of unseasonable heat along the east coast.

Sudden Stratospheric Warming explained

An SSW is a rapid warming over either pole, and while they occur on average every two years in the Northern Hemisphere, the uniformity of geography reduces their frequency in the Southern Hemisphere.

The warming sets off a chain reaction of events that ripple through the atmosphere, including a weakening of the polar vortex — a belt of semi-permanent fierce westerly winds spinning around the poles.

The consequence of a weaker polar vortex varies around the globe — for example, in February 2018, a major SSW over the North Pole resulted in the deadly "Beast from the East" winter storm across Europe.

The effect of an SSW on general Southern Hemisphere and Australian weather is reasonably consistent between events:

Westerly winds in spring bring warm and dry weather to Australia's east, while the only region guaranteed to see more rain is western Tasmania.

Cold fronts embedded in the westerlies also increase in number, leading to frequent bursts of strong winds and high fire danger days, and potentially an enhanced contrast in weather between the country's east and south coast.

Impact of SSW already being felt

The SSW has already manipulated Southern Hemisphere weather patterns this month.

Back in August, a wet spring was highly favoured across Australia.

And while the first 10 days of September brought relatively widespread rain, skies have mostly cleared up during the past fortnight, especially over the east, as dry westerlies become the predominant flow.

Counterintuitively, the prevalence of westerly winds and cold fronts over southern Australia has also resulted in unseasonably high temperatures over the north, interior and along the east coast.

For example, this week Sydney is on track to average 26C for maximums, 9C warmer than Melbourne's highs of 17C — and that's well above the usual 3C September disparity between the two cities.

Competing influences for the rest of spring

The critical question is, how will the stratospheric warming manifest itself on our weather maps for the remainder of 2025?

In 2019, the SSW event coincided with a record drought, a positive Indian Ocean Dipole (another dry climate driver), and the result was the most widespread forest fires in modern Australia's history.

In 2025, severe drought is restricted to parts of SA, Victoria, and Tasmania, and competing wet influences are present across the Indian and Pacific oceans — a negative Indian Ocean Dipole and potential emergence of a weak La Niña in the Pacific.

However, a 2019 study led by Eun-Pa Lim from the Bureau of Meteorology (BOM) found stratospheric warming episodes exerted a greater control over Australia's weather in spring and early summer than the traditional climate drivers from the Indian or Pacific.

Unsurprisingly, the paper therefore linked a weaker vortex to a substantial increase in the chances of hot and dry extremes and associated fire-conducive weather across subtropical eastern Australia.

This trend is consistent with short-term modelling, which predicts that the switch to drier and warmer weather will continue — as indicated by the BOM's two-week forecast above, which favours below-medium rainfall across most of the country.

"We are seeing a less positive Southern Annular Mode (SAM), so that signal you would expect from a negative IOD and La Niña, that is, a generally positive SAM, is easing back a bit," the BOM's Felicity Gamble said.

The Southern Annular Mode is the index that indicates the extent of westerly winds, and a continued trend away from a positive value is now likely through October.

Again, this downgrade from earlier forecasts of above average rain is reflected in the BOM's latest outlook, which now shows no clear swing favouring wetter conditions.

According to Ms Gamble, a weak, relatively unknown SSW event in October 1988 may also offer clues to how the 2025 event will unfold.

"1988 was a moderate to strong La Niña, with generally above average rainfall during the winter and spring months; however, October was particularly dry," Ms Gamble said.

November's rain prospects have also eased in recent model updates, and research suggests the dominance of warmer and drier weather from an SSW can linger until January.

But regardless of how this event may disrupt Australia's weather for the remainder of 2025, Ms Gamble did offer up a positive consequence.

"We typically see a smaller ozone hole over the Antarctic, due to the suppression of super-cold polar stratospheric ice clouds, which are key to the chemical process that destroys ozone," she said.