Wet and wild weather conditions are expected to continue in the eastern parts of Australia, with the Bureau of Meteorology officially declaring a La Niña.
Over the next three days, 60 per cent of the country is set to see rainfall, with most of the east coast and the Northern Territory expecting at least 40mm of rain.
According to the Bureau, in a La Niña event rainfall becomes focused in the western tropical Pacific which leads to a wetter than normal period for eastern, northern and central parts of Australia.
BoM said the La Niña is likely to persist until at least the end of January 2022.
“La Niña also increases the chance of cooler than average daytime temperatures for large parts of Australia and can increase the number of tropical cyclones that form," Andrew Watkins, the Bureau of Meteorology's head of operational climate services said in a press conference on Tuesday.
Last significant La Niña was over a decade ago
A La Niña event is also associated with the earlier first rains of the northern wet season, which Dr Watkins says the bureau has observed across much of tropical Australia this year.
The last significant La Niña hit Australia in 2010 to 2012, leading to the nation's wettest two years on record, with widespread flooding.
"La Niña also occurred during spring and summer of 2020-21. Back-to-back La Niña events are not unusual, with around half of all past events returning for a second year," he said.
Bureau senior meteorologist Dean Narramore said parts of Victoria, Queensland and NSW should expect heavy rain and thunderstorms from Wednesday.
Mr Narramore said eastern Queensland, eastern and central NSW, and Victoria's north and east could receive between 50 and 100 millimetres of rain, with rain and flooding predicted to be at its worst between Thursday and Friday.
Dr Watkins said this year's event is not predicted to be as strong as the 2010-12 event.
The Bureau has declared that a #LaNiña has developed in the tropical Pacific. Typically during La Niña, there is above average rainfall for eastern, northern and central parts of Australia. pic.twitter.com/4KJeKsVI6A
— Bureau of Meteorology, Australia (@BOM_au) November 23, 2021
“Every La Niña has different impacts, as it is not the only climate driver to affect Australia at any one time," he explained.
"That's why it is important not to look at it in isolation and use the bureau’s climate outlooks tools online to get a sense about likely conditions for the months ahead."
What is a La Niña?
La Nina is part of a cycle known as the El Nino-southern oscillation (ESNO) involving a natural shift in ocean temperatures and weather patterns in the Pacific Ocean, bringing high levels of rain, floods and cyclones.
The BoM website says El Niño and La Niña have perhaps the strongest influence on year-to-year climate variability in Australia and are a natural part of the global climate system which occurs when the Pacific Ocean and the atmosphere above it change from their 'normal' state for several seasons.
El Niño events are associated with a warming of the central and eastern tropical Pacific, while La Niña events are the reverse, with a sustained cooling of these same areas.
In a La Niña event, the winds are stronger than usual, it makes the water near the equator of the Pacific colder than usual and pushes the warmer water west towards Australia, resulting in a higher rainfall.
The bureau says a La Niña can also mean cooler days, more tropical cyclones, and an earlier onset of the first rains of the wet season across the northern part of the country.
According to BoM, La Niña typically means:
Increased rainfall across much of Australia
Cooler daytime temperatures (south of the tropics)
Warmer overnight temperatures (in the north)
Shift in temperature extremes
Decreased frost risk
Greater tropical cyclone numbers
Earlier monsoon onset
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