As El Niño looms forecasters remain uncertain about impact of “off the charts” ocean temperatures on weather

While other national weather agencies have already declared El Niño, Australia’s Bureau of Meteorology is awaiting a sustained change in trade wind patterns before it makes an El Niño declaration.

The latest Bureau modelling has surprised climate watchers on the social media platform Twitter because the data shows a mean rise of 3°C above average sea surface temperatures in a specific region of the Pacific.

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Source: Bureau of Meteorology.

Dr Karl Braganza, the Bureau’s Manager of Climate Services, explains that increases above the average in the ‘Niño  3.4’ region of the Pacific Ocean – spanning a rectangle of more than six million square kilometres – is usually the best indicator that the phenomenon will eventuate later in the year.

But it’s not the only signal that the Bureau uses to declare the event in the western Pacific, which is why Australia is yet to follow the lead of the US in ‘certifying’ El Niño.

Explainer: El Niño, La Niña, Indian Ocean Dipole: what are the climate patterns that affect Australian weather?

That signal relates to the Southern Oscillation Index – the SOI – which is a measure of surface air pressure in Darwin and Tahiti.

Strong negative indices on the SOI indicate weakening trade winds, a key change that results in warm ocean water being dragged away from the western Pacific towards the Americas.

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The location of Nino 3.4 in the Pacific Ocean. Plotted using Google Maps.

The Bureau will continue monitoring this atmospheric response before rubber-stamping an El Niño event.

“We’re seeing it now, we’re seeing the atmosphere starting to respond,” Braganza tells Cosmos.

“We’re going to keep watching that now for the next month essentially, and we’ll make an update at the scheduled time.

“Once [the atmosphere measure] is locked in, it sustains those changes: so once the trade winds shift they actually contribute to the warmer water continuing to pool in the [Eastern] Pacific.”

The Niño 3.4 averages aren’t markedly higher than those witnessed by the Bureau in its modelling previously, according to Braganza. Once declared, he says, predicting the intensity of El Niño on the basis of average sea surface temperatures is less certain.

Instead, the Bureau looks to its long-range forecasting to predict rainfall and temperature impacts, which provide accurate predictions looking up to three months ahead.

At the same time, global sea temperatures have warmed markedly in 2023, with temperature records tumbling for some parts of the world. Braganza says hasn’t typically happened in the lead-up to past El Niño events.

“There is something interesting going on at the moment,” Braganza says. “Global ocean temperatures are really off the charts.”

“It’s a fact that we haven’t seen an El Niño develop with ocean temperatures this warm in the ocean, generally.

“If we had an El Niño this year, you’d find that the next year you’d get a spike in global mean temperatures, largely due to the influence that El Niño has on sea surface temperatures, but also to some degree on terrestrial temperatures.

“It’ll be very interesting to see how this develops with that backdrop of extremely warm SSTs.”

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