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Adios La Nina: BOM declares weather event over, for now

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The La Nina weather event that brought Australia’s costliest ever floods has reached an end, with most climate indicators currently neutral, although the Bureau of Meteorology (BOM) says some of its modelling suggests the ocean-atmosphere phenomenon may form again later this year.

The Bureau, which first declared the flood-inducing weather pattern was underway in November, now has La Nina at “watch” status, meaning there is around a 50% chance of La Nina forming again in 2022 - around double the normal likelihood.

In February and March, many parts of NSW and Queensland received more than half their average annual rainfall in just a week and river heights set new records to be Australia’s worst ever flood event, with insured losses well above $4 billion.

BOM head of long-range forecasting Andrew Watkins says the new watch stance “does not change the outlook of above average rainfall for most of Australia over coming months”.

Sea surface temperatures are currently warmer than average for much of the Australian coastline, he says, particularly to the north and west, and that pattern is likely to increase the chance of above average winter-spring rainfall for Australia.

“The Bureau's long-range outlook remains wetter-than-average,” Mr Watkins said.

La Nina, Spanish for “little girl,” is the colder counterpart to “little boy” El Nino. These two forces have the strongest influence on year-to-year climate variability for most of Australia.

La Nina occurs when equatorial trade winds become stronger, changing ocean surface currents and drawing cooler water up from below. The last significant La Nina was in 2010-11.

Neither La Nina nor El Nino are likely to persist during the southern hemisphere's winter, though rainfall across eastern and southern Australia is typically above average during winter and spring during a negative Indian Ocean Dipole - which the Bureau says is likely to form in the coming months.

That would also increase the chances of warmer days and nights for northern Australia.

For now, water temperatures are close to average and trade winds close to average strength, though cloudiness along the equator and the Southern Oscillation Index is a “La Nina-like” signal.

The Bureau says southern Australia has seen a reduction of 10–20% in April–October rainfall in recent decades, and there has been a trend towards a greater proportion of rainfall from high intensity, short duration rainfall events, especially across northern Australia.