Weather researchers have a new theory on what caused rare triple-dip La Nina

Scientists say the global event upended conventional wisdom on what caused the phenomenon from late 2020 to mid-2022 which brought record rain to most parts of south and eastern Australia and widespread flooding.

They already know it was only the third triple La Nina for more than a century following others from 1998 to 2000 and from 1973 to 1975.

Global forecasters are now expecting the drying El Nino with influence Australia's weather later this year.

Michael McPhaden of the US National Oceanic and Atmospheric Administration is trying to learn whether such events will become more likely as Earth warms.

Dr McPhaden will present his team's findings this week during the European Geosciences Union general assembly.

Scientists previously considered multiyear La Nina events were triggered as a rebound from preceding extreme El Nino events.

The conventional thinking was that extreme El Ninos "drain" the tropical Pacific of heat, causing a deficit which takes years to recover - hence, multiyear La Ninas can occur.

But the recent three-year La Nina was not preceded by an extreme El Nino.

Dr McPhaden said to think of it like a pendulum where a big swing in one direction meant a big swing in the other direction.

"Nature has a way of tripping you up when you think you know the answers," he said.

"The easy formula 'Big El Nino followed by multiyear La Nina' was a nice rule of thumb. But it doesn't work all the time."

READ MORE: Our children are eating more meat, not less

One possible cause of the last three years of La Nina was extreme conditions far from the tropical Pacific, Dr McPhaden and his team hypothesize.

Their analyses indicate that anomalous warmth in the tropical Indian and Atlantic oceans energized the La Nina, leading to the three-year event.

"Something needed to kick the system in the direction of La Nina so as to trigger the self-reinforcing feedbacks that characterize El Nino and La Nina variations," Dr McPhaden said.

"We think that kick may have first come from the Indian Ocean with a later boost from the Atlantic."

Whether the world will experience more of these events in the future, Dr McPhaden admits the question will need more investigation.

But, he says, the tropical Pacific has been trending toward a more La Nina-like background state over the past 40 years, which may have set the stage for the triple-dip La Nina.

Scientists are working to discover whether that trend is a natural variation or caused by climate change.