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Strong Antarctic polar vortex adds to south-east Australian rainfall and flood risk, BOM says

ABC News (AU) logo ABC News (AU) 22/10/2022

A band of high-altitude winds that's surrounding Antarctica, known as the stratospheric polar vortex, is adding to the risk of increased rainfall in Australia's south-east this spring, according to the Bureau of Meteorology (BOM).

"The polar vortex is currently forecast to be stronger and colder than normal," said Andrew Marshall, a senior climate researcher at the BOM.

This year, the stronger wind pattern will influence winds to the south of Australia and in turn drive a higher risk of flooding rains across the south-east of the continent, climate scientist Ariaan Purich says.

"Around Antarctica, the strong winds in the upper atmosphere can couple to stronger winds lower in the atmosphere," she said. 

"When this happens, westerly winds south of Australia can move closer to Antarctica."

In springtime, when those westerly winds move south of Australia it makes room for more easterly winds in the south-east. 

"And when they're more easterly than usual it means that more weather-bearing systems from the ocean can move over the land and cause wetter weather than usual over south-eastern Australia," Dr Purich said. 

Southern Annular Mode climate driver

Climate scientists call this movement of the belt of westerly winds over the Southern Ocean towards or away from Australia the Southern Annular Mode, or SAM.

When the westerly winds move away from Australia in spring, driving an increased chance of rain across south-eastern Australia, it's called a positive SAM.

When the westerly winds move towards Australia in spring, driving a decreased chance of rain, it's called a negative SAM.

SAM is one of Australia's big three climate drivers, alongside La Niña/El Niño (ENSO) and the Indian Ocean Dipole.

Right now, all three major climate drivers are in their rain-promoting phases across much of south-eastern Australia. 

More than any other reason, this trio of rain-promoting climate drivers are behind the current flooding rains. 

SAM a Black Summer driver

The connection between the Antarctic polar vortex, SAM and Australia's climate was demonstrated in devastating fashion in 2019 when polar winds in springtime played a significant role in driving the Black Summer.

In September 2019, something very strange occurred over Antarctica.

The atmosphere above the South Pole abruptly began rapidly heating in an event called sudden stratospheric warming.

These events aren't caused by global warming. They're effectively random and are quite rare in the Southern Hemisphere.

This warm air, up to 35 degrees Celsius above normal, broke down the polar vortex.

The polar vortex, which spins at up to 200 kilometres an hour, is powered by the difference in temperature between the pole, which sees little sun, and the Southern Ocean, which is warmed by the sun.

If the temperatures above the pole warm, then the polar vortex can weaken.

"In 2019, the polar vortex winds, which are usually very strong, became extremely weak," said climate scientist Martin Jucker, an expert in sudden stratospheric warming from the University of New South Wales.

At the time, scientists from the BOM warned the rare event could intensify the drought.

Unfortunately, they were right. 

Over the following weeks and months, the effects reached the Earth's surface, leading to westerly winds moving north over Australia — the reverse of what's happening right now.

And El Niño played a role as well.

"The strong El Niño event in the central Pacific Ocean during spring 2019 is also likely to have influenced the sustained negative SAM at that time," Dr Marshall said.

Through late spring and summer of 2019, and into early 2020, hot, dry westerly winds amplified drought conditions in south-eastern Australia and fuelled what became known as the Black Summer bushfires.

But then that summer itself impacted the Antarctic polar vortex and helped swing Australia back from drought to rain.

Bushfire smoke affected the global climate

The bushfires were so intense that millions of tonnes of smoke were injected up to 35km into the sky, high into the stratosphere.

That stratospheric smoke drifted south all the way to Antarctica.

"Just a year after this extremely weak polar vortex in 2019, it was extremely strong in 2020. Scientists have attributed that to bushfire smoke," Dr Jucker said. 

So how could smoke strengthen the polar vortex?

Ozone hole, polar vortex and SAM are all linked

It's all about the ozone hole.

"In springtime, over Antarctica, because there are human-emitted ozone-depleting chemicals … you start to get chemical reactions that destroy the ozone high up in the atmosphere," Dr Purich said.

"And one of the reasons that we have these chemical reactions over Antarctica is the strong polar vortex is so fast that it isolates the Antarctic air from the rest of the atmosphere.

"And so you have this unique environment for the ozone destruction to occur."

Since ozone-depleting chemicals like CFCs began being phased out in 1987, the hole in the ozone layer has been healing.

But ozone-depleting substances will remain for decades to come.

With ozone-depleting chemicals inside the polar vortex, under certain conditions, the ozone hole can get bigger.

Ozone intercepts the sun's heat and keeps it in the stratosphere, so if there's less ozone, then temperatures in the stratosphere can be colder than average, intensifying the polar vortex.

It's a classic feedback loop.

The ozone hole causes a stronger polar vortex and a stronger polar vortex causes a bigger hole in the ozone layer.

That in turn strengthens the polar vortex, which drives a positive SAM, promoting increased rainfall in south-eastern Australia.

So what's that got to do with bushfire smoke?

Bushfire smoke affects the chemical reactions that deplete ozone.

"In 2020, the bushfire smoke destroyed ozone which then cooled the polar vortex," Dr Jucker said.

2020 saw one of the largest ozone holes on record and a strong polar vortex.

Since then, for three springs in a row, the ozone holes have been large, the polar vortexes have been cold and strong, and positive SAMs have been promoting wetter weather in south-eastern Australia.

The Black Summer fires weren't the only recent event to inject huge amounts of aerosols and gases into the atmosphere.

Does a volcanic eruption affect our weather?

Earlier this year, the Tongan Volcano Hunga-Tonga-Hunga-Ha'apai erupted, producing the largest atmospheric explosion in recorded history.

The volcano, which was partially underwater, blasted 146 million tonnes of water vapour to a height of 56km, well into the stratosphere. 

Scientists have been researching what all that water vapour will do to the Earth's climate.

"There is now extra water in the stratosphere that's normally very dry," Dr Purich said.

"It can change the chemistry of the stratosphere and can change the dynamics of the stratosphere and the radiation balance.

"There has been some suggestion that the Hunga-Tonga volcanic eruption may influence ozone depletion, but really, at this stage, the science regarding this is unclear."

The SAM and La Niña connection 

There's another subtle connection between SAM and rainfall in Australia, according to Dr Marshall. 

"La Niña and El Niño can also influence the SAM in spring," he said.

"During La Niña, we tend to see more positive SAM events and during El Niño we tend to see more negative SAM events.

"These relationships between the SAM, the polar vortex and La Niña/El Niño help us to forecast the springtime SAM with higher skill up to three months ahead."

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