Australia Weather News

An atmospheric "tug of war" for control of Australia's weather is taking place this spring, and it is a forecasting nightmare.

On the one hand, one of the strongest negative Indian Ocean Dipoles (IOD) on record, a developing La Niña, and ongoing record warm waters around Australia suggest we should be seeing widespread rain.

However, these wet influences are up against a sudden stratospheric warming (SSW) event high above Antarctica, which heavily favours dry and warm conditions.

This battle has thrown doubt on earlier long-range outlooks, including a potential increase in the bushfire risk across eastern states as we approach summer.

Promised rain overwhelmed by dry westerlies

The concept of a butterfly flapping its wings to trigger a hurricane is used to demonstrate how a small initial change in a physical system can lead to major difference in the future.

The butterfly effect theory certainly applies to the current environment across the southern hemisphere, although in this case the initial change — temperatures 30 degrees Celsius above average over Antarctica — is far from small.

It is therefore no surprise the impacts of polar warming on broadscale weather patterns can be substantial.

As expected, the polar vortex rapidly decelerated in September, with wind speeds in the stratosphere above the Antarctic circle as much as 100 kilometres per hour below average.

"SSW events are associated with a weaker polar vortex … for this SSW event, the stratospheric anomalies propagated down to reach the surface from late September," a Bureau of Meteorology (BOM) spokesperson said.

The filtering down into the troposphere — where weather occurs — has allowed westerly winds to escape high latitudes and spread across southern Australia. It is a consequence which is threatening the pre-spring forecasts for a wet September and October.

Instead of rain, enhanced westerlies are currently transporting dry and unseasonably warm air from the interior to the eastern seaboard.

Sydney is a prime example.

Only 10 millimetres has fallen since the middle of September, during which time the city has averaged daily maximums at nearly 26C — about 5C above normal — although the BOM is not convinced this can be attributed to the SSW.

Conversely, the westerlies are hammering Tasmania with rain and gales with gusts on Friday up to 163kph.

Polar warming out competes triple team

While the stratospheric warming is the dominant driver of the current weather, what makes the forecast for the remainder of 2025 so complex is one of the wet influences, the negative IOD, is at near-record strength.

A negative IOD can be thought of as the La Niña of the Indian Ocean, and the latest weekly IOD index has fallen to -1.49. That's well below the negative IOD threshold of -0.4.

In a normal year, a well-established negative IOD would produce frequent bands of widespread rain across Australia in spring, especially considering a borderline La Niña, also a wet driver, is developing in the Pacific Ocean.

Along with the broadscale influences from the Pacific and Indian Oceans, near-record-warm water temperatures around Australia would also typically support additional rain, however it appears this triple team of wet drivers is being overwhelmed by the SSW.

"Having multiple factors at play at the same time means that the current situation is complex," a BOM spokesperson said.

"For the best guidance on upcoming rainfall and temperature patterns in Australia, see the bureau's long-range forecasts."

Warming impacts could linger for months

The key question going forward is: How much longer will the SSW offset the wetter and cooler influences?

Starting with the source region, temperatures above Antarctica at an altitude of 30km have returned to near average.

However, the polar vortex remains substantially weak. The mean wind strength at a latitude of 60 south is still 70kph below normal.

With a significantly weaker vortex, the trickle-down effect into the troposphere continues.

Even well after the polar warming has passed, the disruption to Australia's weather could linger — a scenario well documented following SSW events in 2019 and 2002 when December rainfall remained well below average.

The 2002 event, the strongest southern hemisphere SSW on record, also produced a record dry January across parts of Australia's east coast.

How much longer this year's effects last is uncertain, especially since the SSW strength was moderate compared to the 2002 and 2019 events.

"Based on prior research, the impact on the southern hemisphere circulation is expected to last up to two months," a BOM spokesperson said.

"However, the impact may be difficult to observe given other current factors in the atmosphere."

Fire season forecast revisited

The turbulent outlook has also raised the possibility the bushfire season will have a much greater severity than originally expected.

The Australian and New Zealand Council for Fire and Emergency Services (AFAC) analysed that only small pockets of South Australia, Western Australia, along with the south-west half of Victoria, had a heightened risk of fires in spring.

Since that initial forecast, soils have dried out considerably across Victoria, New South Wales and central and southern Queensland, revealed by the changes in root zone soil moisture (RZSM).

RZSM, a measure of the amount of water in the top 1 metre of soil, is a crucial factor in the fire danger, as wet ground supplies plants with moisture which prevents combustion.

The maps below show that for much of eastern Australia, soil moisture had been average or above average in early September, but has now fallen below average in many regions.

Under the scenario where hot and dry weather persists into November, soil moisture could quickly drop to levels where fast-moving and uncontrollable fires become likely on hot and windy days.

However, while many previous severe fire seasons were also preceded by a rare SSW event, the wet climate drivers this year will hopefully dampen the threat and prevent a repeat of the scale and intensity of bushfires from Black Summer.