La Nina weather event confirmed for Australia this summer, but Bureau of Meteorology says event will mostly impact eastern states

A La Nina weather event means more rain and a cooler summer for Australia – but what about SA?

Australians can expect a stormy, cool and wetter than usual summer with a La Nina weather event officially confirmed.

The Bureau of Meteorology on Tuesday afternoon declared a La Nina was under way in the tropical Pacific after weeks of anticipation.

Last summer was also affected by the climate driver, meaning back to back La Ninas for Australia.

The weather bureau issued a La Nina “watch” on September 14, which it ramped up to a La Nina “alert” on October 12.

Much of eastern Australia has been lashed over the past two months by heavy rain and thunderstorms, which last week flooded the Lachlan River catchment in the NSW central west.

The La Nina weather phenomenon, linked to the shifting pattern of sea surface temperatures through the Pacific and Indian Oceans, affects rainfall and temperature variations in Australia.

The La Nina phenomenon will affect the east coast more than South Australia. Wet weather is forecast for Sydney this week. Picture: David Swift

Typically it is associated with heavier rainfall for eastern, northern and central parts of the country, as well as a higher likelihood of tropical cyclones.

BoM South Australia duty forecaster Hannah Marsh agreed the La Nina pattern would affect the east coast more than South Australia.

However, she said a combination of the La Nina and other climate drivers, such as the Indian Ocean Dipole (IOD), would still contribute to changed summer conditions.

The IOD refers to changes in the difference in sea surface temperatures between the western and eastern Indian Ocean.

“The fact that the Indian Ocean Dipole is weakening means it might not be as wet as we have seen recently over summer,” Ms Marsh said.

The West Terrace weather station has recorded high rainfall over the past two months.

The city recorded 61.8mm in October, compared to the average monthly rainfall of 44.2mm.

A total 25.8mm has so far been recorded this month, compared to an average of 30.5mm, but Ms Marsh said up to 10mm is forecast to fall on Wednesday.

It's shaping up to be a wet and humid end to spring for eastern Australia ☔

Flooding is ongoing in multiple states, and with more rain on the way later this week, some communities could face further impacts.

Latest forecasts and warnings: https://t.co/Q5BdCfPAYCpic.twitter.com/3fW28fYmSF

— Bureau of Meteorology, Australia (@BOM_au) November 22, 2021

During La Nina, waters in the central or eastern tropical Pacific become cooler than normal, persistent southeast to north-westerly winds strengthen in the tropical and equatorial Pacific, and clouds shift to the west, closer to Australia.

Andrew Watkins, the weather bureau’s head of operational climate services, said the last significant La Nina event occurred in 2010-2012, bringing widespread flooding and Australia’s wettest-two year periods on record.

“La Nina also occurred during spring and summer of 2020-21. Back-to-back La Nina events are not unusual, with around half of all past events returning for a second year,” he said.

The first tropical cyclone of the 2021-22 Australian season has been named —Tropical Cyclone Paddy 🌀 #CyclonePaddy is south-east of Christmas Island and poses no immediate threat to the Australian mainland or offshore communities.

Latest forecast: https://t.co/FlU9xRmxUjpic.twitter.com/ysRdhxAi9D

— Bureau of Meteorology, Australia (@BOM_au) November 22, 2021

Dr Watkins said that this year’s La Nina was not predicted to be as strong as the 2010-12 event and may even be weaker than the one that occurred last summer.

The weather bureau says this La Nina event is likely to persist until at least the end of January 2022.

Meanwhile, three flood plains in the South Australian Riverland are operating concurrently for the first time in history, signalling a boost in the health of the state’s River Murray system.

The Chowilla, Pike and Katarapko flood plains all hold water for the first time.

A report into the nine-year Riverine Recovery Project released on Tuesday showed the $83.3m initiative had returned 7.245GLof water for the environment. Federal Resources and Water Minister, Keith Pitt, said the project had “delivered for communities as well in jobs and economic activity”.