Unprecedented weather event showing signs of 'slowing down'

After the Bureau of Meteorology (BoM) released its latest Climate Driver Update this week, a leading Australian weather expert said it's now less likely a La Niña weather system will eventuate later in 2024, meaning the probability of less rain than first feared for the second half of the year.

The phrases El Niño and La Niña have been tossed around a lot in Australia in recent years. El Niño brings with it dryer and warmer-than-average conditions and La Niña above average rainfall and cooler daytime temperatures for most.

In May, the Bureau said there was a 50 per cent chance that a La Niña system would sweep across the east in the latter half of the year — while that's certainly still possible, it's less likely.

Australia experienced three La Niñas from 2020 to 2023 followed by an El Niño so another La Niña would mean an extreme weather event has been declared five years in a row — something experts have previously said is unprecedented and unusual.

Speaking to Yahoo News Australia, Dr Tom Mortlock of UNSW's Climate Change Research Centre explained what's changed from the Bureau's May report through to its July update. He said while we're currently in an ENSO neutral phase — meaning neither El Niño nor La Niña — it doesn't mean that won't change come spring.

Chances of La Niña developing slightly less likely

"The only change that I can see is in May, of the seven global climate models that the BoM surveys, three, were suggesting we'd get a La Niña system by the start of spring," Mortlock told Yahoo. "Whereas now, if you have a look at those same set of models, only two of them are.

"If anything, it's slowing down a bit. The BoM are still maintaining that we're in a 'watch' phase, but now we've got two of those seven models — so that's five of them suggesting ENSO neutral. Although they say 'watch' is a 50 per cent chance of La Niña developing by spring, I think compared to May, the consensus is things are slowing down."

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While many anticipated the heavy rainfall experienced in NSW in recent weeks, particularly around Sydney, may indicate a looming La Niña, Mortlock said weather conditions in the lead-up to spring "aren't really relevant" when it comes to determining whether a La Niña will form.

Australia won't make the call for some time yet

"What these global models are looking at are temperature anomalies in the Central Pacific," he explained. "They're looking at a part of the Pacific Ocean and the sea surface temperatures there, and whether the temperatures of that sea surface is above or below the long-term average.

"If it's minus 0.8C below the long-term average, then they officially call a La Niña, and likewise, if it's point eight of a degree above the long-term average, they'll call El Niño.

"Now, that's just the international definition... but that doesn't necessarily mean we're going to get more floods, more cyclones, more rain in eastern Australia because of that."

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The Bureau, Mortlock said, are often one of the last agencies in the world to call El Niño or La Niña across Australia because they have a "slightly different, more complex" and more accurate definition.

"They use something called the Southern Oscillation Index, which is the pressure differential between an observation station in Darwin and one in Tahiti," he said. "Essentially what that does is it looks at the atmosphere as well as the ocean, and really, once both those things are aligning, then we know we really are in learning territory."

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