Spanning more than 12,000km along the equator and between 100 and 200 metres below the surface of the Pacific Ocean, there is a massive blob of warm water.
If enough of that water makes its way to the surface, it could set off an El Niño event that can push up temperatures around the globe and raise the risk of Australia suffering heatwaves, droughts and bushfires.
“You can compare it to a loaded gun,” says Prof Axel Timmermann, the director of the IBS Center for Climate Physics at Pusan National University in South Korea. “The magazine is full, but the atmosphere hasn’t pulled the trigger yet.”
What is El Niño and is it definitely coming?
Because the Pacific Ocean is so large, the effects of El Niños can reverberate across the globe, causing floods and landslides in Central America and delayed and failed monsoons in the Indian subcontinent.
Australia has just been through three years of El Niño’s cooler cousin La Niña, where water temperatures in the central Pacific are lower than usual. For Australia, this had a cooling influence on temperatures but helped produce extreme rainfall and floods.
This cycle between El Niño and La Niña is known as Enso (the El Niño-Southern Oscillation) and it also has a neutral phase, where we now sit.
Climate models from weather agencies around the globe surveyed by the Bureau of Meteorology are all showing that, by August, temperatures in the central equatorial Pacific will have risen to at least 0.8C above average – putting them into El Niño territory.
On Friday the US National Oceanic and Atmospheric Administration released its latest outlook, saying there was a 62% chance of an El Niño developing by July.
An El Niño Watch (!) has been issued by @NWSCPC which means that conditions are favorable for the development of El Niño within the next 6 months. In fact, there’s a 62% chance of El Niño conditions for the May–July period.
— NOAA Climate.gov (@NOAAClimate) April 13, 2023
For more, head to the ENSO Bloghttps://t.co/lcgh1yhoCv pic.twitter.com/WH2IPY6WxI
But climate forecasters stress they have less confidence in their models during the southern hemisphere autumn. A predicted strong El Niño in 2014, for example, didn’t materialise.
Timmermann says to trigger El Niños, you first need several strong wind bursts from west to east and, so far, that hasn’t happened.
Even though there is wide agreement in the models that an El Niño is likely to form later this year, he says those models can’t predict winds. This is one reason why there is less confidence in the forecasts at this time of year.
Dr Andrea Taschetto, an Enso expert at the University of New South Wales, explains that for an El Niño to form, the atmosphere needs to react with those warm ocean waters.
“We need to remember this is a coupled system,” she says. “The ocean and the atmosphere both need to respond. The winds are really important.”
Once an El Niño does start to materialise, another strong sign is a weakening of trade winds that usually blow from east to west that allows that warmer deeper water to rise to the surface.
To be really confident, forecasters will be waiting until May or June.
Does El Niño always mean hot and dry for Australia?
If an El Niño does happen, its location could be important for Australia.
Taschetto says that El Niños that have warm surface water located in the central Pacific are more associated with lack of rainfall in Australia than those where the warmer water is further east.
Strong El Niños in 1997-98 and 2015-16 were centred further east, having less affect on Australia’s rainfall, Taschetto says.
“Australia is not as strongly affected by eastern pacific El Niño events but, elsewhere, those do have very strong impacts for places like South and North America. Australia tends to see more impacts with central Pacific El Niños.”
It is thought that central Pacific El Niños weaken the updrafts that help clouds and rain form over the east of Australia.
Taschetto says it’s too early to know where any El Niño might be located.
A study last month led by scientists at CSIRO looked at all El Niño events since 1950 – regardless of their location in the Pacific – and found that generally they doubled the chance of a dry spring for eastern Australia, but there were some exceptions.
While the drying affected the vast eastern two-thirds of the continent, areas along the populated eastern seaboard, including Brisbane and Sydney, were less affected.
How strong might this El Niño be?
Some models from national weather agencies – including Australia’s – include projections that raise the chance of an extreme El Niño where temperatures in the Pacific are about 2C above average.
As Dr Mike McPhaden, of the US National Oceanic and Atmospheric Administration, said this week: “The magnitude of the predicted El Niños shows a very large spread, everything from blockbuster to wimp.”
Noaa’s outlook this week said there was a 40% chance that, by the end of the year, a strong El Niño with ocean temperatures above 1.5C could be active.
But Timmermann and others have warned it’s too early to have much confidence in those predictions.
Dr Jaci Brown, the research director of CSIRO’s climate intelligence program, says the size of an El Niño “is not necessarily related to the size of the impacts”.
“Every El Niño is different. But for now, as they say in agriculture, it is telling us which way to lean but not which way to jump.”
What are the risks?
As well as cutting rainfall and increasing temperatures, marine biologists have said that El Niños increase the risk of severe coral bleaching along the Great Barrier Reef.
After three wet La Niña years in Australia, climate scientists and bushfire experts have said they are very concerned that all the grass and vegetation growth could create nightmarish bushfire conditions next summer.
Brown says the memories of the catastrophic and unprecedented black summer bushfires of 2019 and 2020 shouldn’t be forgotten.
“We are now primed for bushfires with this record [vegetation] growth. I feel we’re primed for one of the worst bushfire seasons ever.”
Taschetto and Brown say understanding the dynamics of Enso is an active area of research and there is still more to understand.
But Brown says studies have suggested that as the climate warms, “El Niños and La Niñas will become more extreme – that’s a real possibility”.
