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Why WA's south-west is drying out at one of the worst rates in the world

Tuesday December 28, 2021 - 02:34 AEDT
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Rainfall has declined significantly in the last 50 years over WA's south-west. - ABC

As parts of Western Australia are tipped for another scorching hot summer, climate change is leaving a worrying footprint on the south-west of the state.

Experts agree the region is drying out at a globally significant rate.

It was one of the first places on the planet to see a trend of rainfall reduction, and the latest Intergovernmental Panel on Climate Change (IPCC) report pinpoints it as one of the few regions in the world where the bulk of models agree drying will continue

So what's going on?

Since the late 60s, rainfall in the region has declined by up to 20 per cent overall.

Figures show in recent decades, the downward trend has gained momentum.

According to Bureau of Meteorology?s (BOM) Pandora Hope, who has been researching south-west WA for the last two decades, it ultimately comes down to two factors.

The cold fronts and lows that cross the region during the cool months of the year, bringing rainfall and storms, are getting fewer and weaker. 

High pressure systems, which help suppress the development of rain systems and create warm, sunny days, are getting stronger and more frequent.

Dr Hope said it was a pattern happening right across the subtropics of the southern hemisphere.

But she said it was particularly problematic for south-west WA because the winter cold fronts and lows were the primary source of rainfall.

?The fact is it?s a region where you really only get rainfall from one direction, one source,? she said.

?If we have a regional shift in where the rain falls, that will have a big impact on this small region of a big continent.?

Why are the weather systems changing?

To better understand what?s behind the changes, Dr Hope said you had to look above the surface and into the way the Earth circulated its air around the globe.

These circulation patterns bring us our daily weather patterns.

For southern Australia, a big part of it is a pattern known as the Hadley Cell.

Warm air rises at the equator, spreads southwards across the upper levels of the atmosphere and then descends as cool, dry air at about 30-40 degrees latitude.

The descending air creates a general belt of high pressure systems for the area beneath it.

In summer, this belt tends to sit over southern Australia, making it a hard task for rain systems like cold fronts to break through and reach south-west WA.

In the winter, it migrates north, extending over central parts of Australia and allowing rain-bearing systems like lows and cold fronts, to sweep over the South West.


But Dr Hope said heating caused by greenhouse gases had been tinkering with the delicate way air flows around the atmosphere.

"So with greenhouse gases we know you introduce them into the atmosphere and you increase the global temperatures," she said.

"But it also changes patterns of circulation simply by warming some parts of the atmosphere more than others."

Dr Hope said one of those changes was the downward branch of the Hadley Cell had been intensifying in recent years, and had also been expanding toward the poles. 

This meant the high pressure systems were stronger, and also had an influence for longer, allowing them to dominate the weather pattern. 

At the same time, circulation patterns to the south of Australia, which are linked to the growth of winter cold fronts and lows, were becoming weaker.

That meant fewer rain-bearing systems, with each delivering less rain.

Dr Hope said greenhouse gases had also created a generally "more stable" atmosphere over south-west WA.

A stable atmosphere relates to the vertical temperature profile of the atmosphere.

Like a hot air balloon, if the air parcel being lifted is warmer than its surrounding air ? known as an unstable atmosphere ? it will rise easily and is able to form rain clouds.

But if the air aloft is also warm ? known as a stable atmosphere ? it has the tendency to sink back down and will struggle to form clouds.

?So the whole structure of the atmosphere is different,? Dr Hope said.

"It's more difficult for the storms to really build to be quiet large so they don't bring as much rainfall as they they might have once."

Significant impact on south-west

Murdoch University atmospheric scientist Jatin Kala said the particularly unique thing about south-west WA, was the clear sign the drying trend would continue.


?With the latest climate projections from the IPCC, the one region in the world where most of the models agree on the changing precipitation in terms of magnitude and sign of the change, is south-west WA,? he said.

He said the projection was concerning for the region's eco-systems.

Already, the WA Water Corporation estimates the 20 per cent rainfall reduction had compounded to an 80 per cent reduction in streamflow.

?A hotter climate means you have more evaporation, drier soils soak up more waters and also trees need to use more water when it?s dry,? Dr Kala said.

?So there?s a whole range of factors that make the reduction in streamflow many magnitudes higher than the reduction in rainfall.?

How do researchers know it?s caused by greenhouse gases?

Research shows the changing patterns cannot be entirely pinned on greenhouse gases.

But Dr Hope said greenhouse gases were a big and crucial part of the story, which was clear when you looked at the climate models.

?If you force climate models with just natural variability, like the solar cycle and the volcanic eruptions, you simply don?t see the rainfall decline that we?ve experienced over the last 50 years,? she said.

?And then when you do add greenhouse gases, all the climate models agree that you do get a loss of that rainfall.?

To put it simply, the puzzle just doesn?t piece together without greenhouses gases in the mix.

Add them, and all of a sudden the picture falls into place.


© ABC 2021

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