Wednesday, October 28, 2020
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Sudden stratospheric warming is the unusual climate variation affecting ozone, heat and wind

Computer generated image of the globe focused on Antarctica - only a small patch of yellow representing the ozone hole
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We just had the smallest ozone hole since 1982, the drought is dragging on and the fire danger is forecast to be up. What connects all these events? Sudden Stratospheric Warming (SSW).

South Australia sizzled on Thursday before relief arrived in the form of a cold front making its way across the country.

Victoria then had a warm night before the change, and New South Wales was forecast to be in the hot seat on Friday.

Pinning one event on a single climate driver is fraught, but this bout of warm weather fits with what would be expected as one of the strongest SSWs in 40 years starts impacting Earth’s surface.

The official outlook is for warm dry conditions in the months ahead.

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Smallest ozone hole since 1982

NASA announced the hole in the ozone layer over the South Pole, which usually expands during spring as a result of pollutants in the atmosphere, was the smallest in decades.

While it might be tricky to officially assign this cold front to the SSW, the link between SSW and the ozone hole is clear-cut according to Harry Hendon, senior principal research scientist at the Bureau of Meteorology.

“We can say conclusively that the lack of the ozone hole this year was a result of the stratospheric warming,” he said.

Typically in winter, the temperature difference between the warm tropics and the cold pole result in a strong vortex of westerly winds that circle the South Pole.

The warming of the vortex in late spring inhibited the destructive chemistry that acts the destroy the ozone.

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This is how sudden stratospheric warming occurs(Supplied: National Institute of Water and Atmospheric Research (NIWA))

“That destructive chemistry requires very cold temperatures in the vortex, and those cold temperatures result in ice crystals — it’s on those ice crystals that the chemistry happens,” Dr Hendon said.

“Once the vortex warmed up, those ice crystals melted and so that chemistry couldn’t happen.”

The weakened vortex also helped because it meant more ozone-rich subpolar air could mix in and block up the hole, he added.

Global action under the Montreal Protocol to ban pollutants that cause ozone depletion is often touted as an international environmental success story.

Dr Hendon, however, said the smaller ozone hole this year was probably not the result of anything humans had done.

“The lack of the ozone hole this year is purely the result of just a really strong natural event that caused the vortex to warm.”

What are the effects on the ground?

In a normal year as we move into summer and the pole starts to warm, the temperature difference with the tropics reduces and the vortex slowly weakens.

“But when we have stratospheric warming, that happens much more abruptly,” Dr Hendon said.

This sudden warming, he said, started in the stratosphere — the layer above the troposphere, where our normal weather takes place — but after about a month, its effects began making their way down towards the surface.

“The net effect is to act to weaken the westerly winds at the surface at high latitudes, which then results in the westerly winds essentially shifting towards the equator slightly.”

So for high-latitude locations underneath the westerly winds, like southern New Zealand, southern South America and western Tasmania, they will actually see colder and wetter conditions.

“But for [southern] Australia, which sits in the subtropics, that shift of the westerly winds towards the equator actually results in warmer and drier conditions,” Dr Hendon said.

What is in the picture
Discrepancies in (a) maximum temperatures, (b) minimum temperatures, (c) rainfall during SSW years.(Supplied: Lim et al. (2019) Nature Geoscience)

This north-south shift in the westerly wind belt over the Southern Ocean is referred to as the Southern Annual Mode (SAM).

“The stratospheric warming typically results in a shift to the low phase or the negative polarity of the SAM during the springtime.”

Dr Hendon said the main areas affected by SSWs were New South Wales, the northern parts of Victoria and the southern half of Queensland, but the effects could spread into surrounding regions.

“The peak time should be October and November, still some effect in December, and fade away as we get into January.”

Increased fire weather

Then, of course, hot dry conditions lead to other problems.

“Historically we’ve made the connection between a breakdown or a warming of the vortex in late winter and subsequent increases in fire weather in central NSW and central Queensland in the following spring,” Dr Hendon said.

“So there’s a historical relationship between the two, and of course we think the connection is through the promotion of negative SAM.

“Negative SAM results in drier conditions and warmer conditions which are both conducive to more fires.”

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Climate drivers do battle

To add yet more complexity, the SSW isn’t the only climate driver in the mix at the moment.

Dr Hendon said the positive Indian Ocean Dipole (IOD), whose warming and drying effects have been strongly felt over eastern Australia this winter, had been masking the downward progression of the stratospheric warming.

“It’s just a simple interference phenomenon that the IOD is sort of radiating out a response, which at high latitudes is interfering with what the stratospheric warming is trying to do at the surface.

“It looks like in the next couple of weeks, the stratospheric warming is going to win out and we’re going to get much more expression of this negative SAM.

“But the IOD is so strong that it’s going to linger and probably still have impacts into December.”

Whichever wins, it’s not great news.



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