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Bureau of Meteorology says La Niña's an even chance to return for a second consecutive spring

Wednesday September 15, 2021 - 08:12 AEST
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A return of even a mild La Niña could pose a flood risk to already wet catchments this summer. - ABC

The Bureau of Meteorology (BOM) has moved to a La Niña watch, indicating there is now a 50 per cent chance of the rain-bearing climate driver returning this spring.

While that is double the normal chance, there is no guarantee it will eventuate.

Conditions remain neutral but the eastern Pacific Ocean has been slowly cooling in the past few months.

Now, five of the seven climate models the bureau monitors are indicating La Niña thresholds will be met in November and December. 


What could a La Niña bring? 

La Niña is the phase of the El Niño Southern Oscillation (ENSO) over the Pacific Ocean that often leads to wetter-than-average conditions for eastern Australia. 

The last big La Niña event in 2011 brought devastating floods on the east coast, notably in Brisbane, Toowoomba, and the Lockyer Valley.

But not all La Niña events are as extreme. Last summer was also a La Niña season and, while it brought a welcome return to wetter-than-average conditions for many, it was well below infamous La Niña years like 2011 or 1974, when Brisbane was inundated.

Andrew Watkins, head of BOM'S operational climate services, said 2010?2012 was the wettest two-year period on record, thanks to "one of the stronger La Niña events that we've seen".

By comparison, he said, the 2020?2021 La Niña was "moderate at most" and if it returned this year it was not expected to be strong.

But he warned already wet catchments could still pose a flood risk this summer. 

"Unfortunately, if we get rain in the coming months on top of that already sodden landscape then it increases the risk of flood because the soils and the rivers and the dams can't absorb as much water," Dr Watkins said. 

Is it going to rain? 

What's also interesting about our climatic indicators now is that four drivers are pushing towards wetter conditions, according to Dr Watkins: 

Even without full La Niña conditions, Pacific Ocean patterns are encouraging moisture

A negative Indian Ocean Dipole (IOD) means warm waters around Indonesia encouraging evaporation and increased odds of rain

In fact, waters across the north of Australia are warm ? again encouraging rain

A positive Southern Annular Mode (SAM) in the Southern Ocean is drawing westerly winds and high-pressure ridges south, encouraging on-shore flows for the east coast

"All four of those things tend to bring above-average rainfall and hence why our outlook is looking fairly wet at the moment," Dr Watkins said. 

But in Australia's south west, it was a different story.

Recently, warm waters in the Indian Ocean had been feeding moisture into cloud bands reaching the south west and beyond, Dr Watkins said.

But the south west's rainfall is largely dependent upon rain-bearing fronts in the westerly wind band that circle the Southern Ocean. 

The positive SAM will draw those winds south, meaning many will likely fail to reach the continent. 


© ABC 2021

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