For the past three years, summers haven’t been the same in Australia, with much of the country experiencing extensive rainfall and floods.
With the weather event that caused these conditions - La Niña - officially over, climate forecasters around the world have been monitoring what will happen next.
After , there are growing chances the opposite climate pattern - El Niño - will form during winter (June to August) this year, according to Australia’s Bureau of Meteorology.
The United Nations' weather agency has already declared the arrival of El Niño conditions, warning of a likely surge in global temperatures and disruptive weather and climate patterns.
This would mean warm, dry conditions for Australia's east coast and an increased risk of bushfires.
But the bureau is yet to make the same call. Here’s what we know so far.
What is El Niño?
In recent months, below the surface of the eastern Pacific Ocean off Central and South America, a “blob” of warm water has been gathering - and growing larger, senior CSIRO research climate scientist Nandini Ramesh explained to SBS News in late April.
“That’s literally what we’re watching. That’s what is telling us that an El Niño is possible,” she said.
El Niño events occur when sea surface temperatures in the central and eastern tropical Pacific are warmer than usual, while La Niña events are characterised by cooler-than-usual waters. These changes cause a shift in atmospheric circulation.
“These warm and cool events last for several months at a time - a typical timeframe would be six to nine months for an El Niño event,” Ramesh said.
“It’s the warm or cool temperatures over that part of the ocean that affect the atmosphere. That allows that disturbance to spread, and it changes weather conditions all over the world.”
These changes in the Pacific Ocean and overlying atmosphere form part of a cycle called the El Niño-Southern Oscillation (ENSO).
What is the likelihood of El Niño forming?
El Niño and La Niña are the two ends of the spectrum, said Dr Tom Mortlock, an adjunct fellow at the UNSW’s Climate Change Research Centre and senior catastrophe research analyst at risk-mitigation consulting firm Aon, speaking to SBS News in late April.
“We’ve just come out of a pretty rare triple-dip La Niña event - that was three La Niña events in a row - the first time in 22 years, and only the fourth in historical records that we can see.”
“That’s something that we all know brought a lot of rain, wetter-than-normal conditions, floods and flood-associated damage across the east coast of Australia.
“El Niño is the opposite to that.”
The World Meteorological Organization has declared the arrival of El Niño conditions. Source: AAP / Jeremy Ng
Its latest update, which combines forecasts and expert guidance from around the world, estimates a 90 per cent probability of such an event continuing during the second half of this year. It's expected to be at least of moderate strength, the WMO says.
“The onset of El Niño will greatly increase the likelihood of breaking temperature records and triggering more extreme heat in many parts of the world and in the ocean,” WMO Secretary-General Professor Petteri Taalas said.
“The declaration of an El Niño by WMO is the signal to governments around the world to mobilise preparations to limit the impacts on our health, our ecosystems and our economies."
This follows a declaration by the United States' National Weather Service, which is part of the US government's National Oceanic and Atmospheric Administration (NOAA), last month.
But Australia is yet to do the same, and the bureau's latest ENSO outlook on 4 July remained at alert level.
The bureau upgraded its El Niño watch, issued in March with the official end of La Niña, to an alert last month.
Three out of its four criteria for an El Niño have been met, including warming sea surface temperatures, international modelling and the required difference of air pressure in the Pacific Ocean between Tahiti and Darwin.
The bureau is yet to observe its fourth criteria, which is a weakening of equatorial trade winds.
"This means the Pacific Ocean and atmosphere have yet to become fully coupled, as occurs during El Niño events," it said.
The bureau's alert level means there is up to a 70 per cent of an El Niño developing later in the winter months.
What are climate scientists looking out for?
Ramesh explained climate scientists were waiting to see how the ocean and atmosphere will interact.
“Right now, we're able to see that warm water has gathered under the surface of the Pacific Ocean. What we don’t know is whether the atmosphere will respond to that, because that is what we need for it to turn into an El Niño event,” she said.
That is called a feedback loop.
“If you get warm water at the surface, that then changes the pattern of the winds in such a way that it reinforces the warmth of the surface, which reinforces that same pattern of the winds."
“It can start from something very small. We’re at the stage of waiting to see if that happens so the cycle can take hold.”
In its July update, the WMO said, "some uncertainty remains because of only weak ocean-atmosphere coupling, which is crucial for the amplification and sustenance of El Niño".
It could take another month to see "fully established coupling in the tropical Pacific", it said.
What kind of weather would El Niño bring, and when?
Ramesh said the impacts of El Niño events typically start to be felt around spring in Australia (September to November), before reaching their peak in summer (December to February).
She said the eastern and northern parts of the country are usually affected the most.
For the east coast, Mortlock said El Niño events bring warmer- and drier-than-normal conditions, along with increased bushfire risk.
An overview of how a typical El Niño event usually develops in Australia, according to senior CSIRO research climate scientist Nandini Ramesh. Source: SBS News
When it comes to bushfire risk, Mortlock said there are some other variables or climate drivers that come into play.
For example, the Indian Ocean Dipole (IOD) refers to changes in sea surface temperatures of the tropical western and eastern Indian Ocean. A positive IOD can suppress winter and spring rainfall over much of Australia, and potentially exacerbate the drying effect of El Niño, the bureau says.
“Certainly, El Niño does increase the background risk associated with bushfires. But it's not the only thing going on,” Mortlock said.
He said fuel growth associated with vegetation and grass growth in recent years is also a risk factor.
“Fuel growth preconditions the landscape, to a certain extent, towards high bushfire risk, irrespective of whether or not we get an El Niño declared.”
For the east coast, typical El Niño events bring warmer and drier-than-normal conditions, along with increased bushfire risk. Credit: Getty
The bureau says the date of the monsoon onset in tropical Australia is generally between two to six weeks later during El Niño years than in La Niña and rainfall is typically well below average during the early part of the wet season.
“Typically, there’s less rainfall there than usual,” she said.
She said there isn’t a clear correlation between the strength of an El Niño - where temperatures in the eastern part of the Pacific are a lot higher than normal - and its impacts, for example bushfires.
“Historically, we’ve had big bushfires in small El Niño events, too. It’s not really a clean mapping of those things - there are a lot of other factors involved,” she said.
Mortlock said typical characteristics of El Niño also differ across the globe.
“In some parts of the world, El Niño does mean more storms, cyclones and rain.”
Mortlock said we shouldn’t be too focused on whether an El Niño event is called or not.
“There are certainly other regional climate drivers of what goes on with our weather in Australia, ” he said.
The bureau says the status of its ENSO outlook does not change its long-range forecast of warmer and drier conditions across much of Australia from August to October.
Earlier this year, a WMO report predicted a 98 per cent likelihood that at least one of the next five years will be the warmest on record, beating the record set in 2016 when an "exceptionally strong El Niño" took place.
