The outlook reflects a range of climate drivers, including a negative Indian Ocean Dipole event and warmer than average waters around Australia.
With wet soils, high rivers and full dams, and the outlook for above average rainfall, elevated flood risk remains for eastern Australia, the bureau said.
BOM has moved from 'La Niña WATCH' to 'La Niña ALERT' with the likelihood of La Niña returning this spring increasing to around three times the normal risk.
“A La Niña ALERT means the chance of a La Niña developing in the coming season has increased,” BOM said.
When La Niña criteria have been met in the past, a La Niña event has developed about 70 per cent of the time.
“La Niña refers to changes in sea surface temperatures in the tropical Pacific Ocean, with waters in the eastern Pacific being cooler than normal, and waters in the western tropical Pacific being warmer than normal,” BOM said.
“Trade winds strengthen, increasing the water moisture in the air, which usually brings rainfall to eastern and central Australia and a wetter start to the northern wet season.”
University of NSW senior research associate Agus Santoso said the probability that a La Niña may re-emerge in summer had increased from 50-60 per cent in the past months to about 70 per cent, thus an increased chance for a third La Niña since 2020.
“A triple La Niña is quite rare in the historical record,” Dr Santoso said.
“This elevated likelihood is concurrent with the development of negative Indian Ocean Dipole.
“A negative IOD, marked by warming north-west of Australia and over Indonesia, can enhance La Niña development through strengthening the Pacific trade winds that support a La Niña.
“The negative IOD, expected to peak in spring, will likely bring higher-than-normal rainfall across the southern parts of Australia, including NSW and Victoria, in the coming months.
“This is in addition to the prospect for the triple La Niña and the backdrop of already saturated catchments.”
