Conditions brew for prolonged wet weather

La Nina and other systems point to wet weather ahead

Weather
Rain could be higher than average down the east coast until autumn 2021 on the back of La Nina and factors that facilitate availability of moisture and something to activate that moisture.

Rain could be higher than average down the east coast until autumn 2021 on the back of La Nina and factors that facilitate availability of moisture and something to activate that moisture.

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Rain could be higher than average down the east coast until autumn 2021.

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The long expected La Nina event is now in place and this typically means that above average rainfall is favoured for at least the coming three months over most of eastern Australia.

But a few other issues that can affect rainfall potential have to be considered.

These include the strength of the La Nina event, the time it lasts and the 'support' it gets from other indicators, especially the Indian Ocean and the Indian Ocean Dipole.

The last major La Nina event occurred in 2010-12. Although, there was a weak event around 2017, when the IOD was negative or - in other words - strongly supportive.

The 2010-12 event persisted for some time and was one of the four strongest La Nina systems ever. The rainfall in the eastern half of Australia in those years was the heaviest on record overall.

Initially, the 2020 La Nina appeared likely to be fairly weak.

But there have been a few indications lately of continued cooling of the tropical ocean temps in the eastern Pacific. This would indicate it could end up a little stronger than originally thought and, therefore, it could last a little longer than expected as well.

But it is still not expected to be as strong as that 2010-12 event.

Although the IOD is still not offering much support at the moment, the slight majority of international models believe it will be negative until the end of spring, after which it has minimal effect on our rainfall patterns before next April.

The Southern Annular Mode (SAM) to the south usually follows the La Nina and becomes positive, which in turns helps moisture to feed into eastern Australia from the Pacific.

Despite all this positivity, there remains the well known premise that there are two things necessary for rain events - the availability of moisture and something to activate that moisture.

A La Nina event looks after that first factor, increasing the availability of moisture over eastern Australia significantly in late spring, summer - and probably early autumn as well.

Activating mechanisms can be a little harder to come by regularly. But often, increased moisture leads to increased cloud and more variability in upper air temperatures - thus providing the necessary mechanisms.

Such mechanisms have occurred already this season over central and South Australia, Victoria, Tasmania and inland New South Wales - and it is probably only time before they move into north eastern NSW and Queensland.

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