We are still technically in an El Niño period, and the world's first meteorological agency has put the world under an El Niño watch.
The US National Oceanic and Atmospheric Administration (NOAA) announced that it has officially come under a La Niña watch, stating that the current El Niño phenomenon is weakening and will likely end this fall.
Australia's latest La Niña “triple dip” – between 2020 and 2022 – contributed billions of dollars in damage from flooding and heavy rain across much of eastern Australia.
For Australians already reeling from a summer that defied expectations, news that we could be facing La Niña again soon could raise concerns about next year.
But before you panic, it's important to understand what these forecasts entail, and why Australian climate scientists are still monitoring the situation before issuing any local warnings.
What is the La Niña phenomenon?
La Niña, El Niño and their lesser-known cousin “neutral” are phases of the natural climate cycle that affect weather around the world, especially in Australia. This phenomenon is known as the El Niño-Southern Oscillation, or ENSO.
In Australia, La Niña is traditionally associated with cooler, wetter weather, while El Niño usually brings drier and hotter conditions.
La Niña occurs when tropical trade winds across the Pacific Ocean become stronger, changing the ocean's surface currents and pulling deep, cold water from below, causing ocean surface temperatures near Australia to rise.
Currently, we are still in the El Niño phenomenon, although the factors driving this phenomenon are weak.
Girl it's not guaranteed to happen
On Thursday, the US Meteorological Agency classified the chances of the end of the current El Niño phenomenon and the return of the neutral phase in the Australian autumn at 79 percent. Those are good odds.
But when it comes to the chances of a return to La Niña in the winter, the odds are as high as 55 percent.
The odds of a La Niña occurring are a “flip of a coin,” says Kim Read, a climate scientist at Monash University's Center of Excellence for Climate Extremes.
“All these weather and climate forecasts are about probabilities. They tell you what is most likely,” Reed said.
“As people know, the favorite horse doesn't always win the race. Even if there's a little more than a 50 percent chance of something happening, it doesn't necessarily mean it will happen.”
Why is there no BOM on the La Niña screen?
NOAA is a respected agency when it comes to weather and climate forecasting. However, it emphasizes different indicators when considering the chances of the next La Niña event compared to the Australian Bureau of Meteorology (BOM).
La Niña and El Niño are caused by what happens in the oceans and also in the atmosphere.
In Australia, BOM standards reflect this.
“To reach this stage, Met Office climatologists assess whether a set of atmospheric and ocean criteria have been met and the current state of the climate system has reached this level,” the Met Office says.
In contrast, NOAA only requires that certain conditions be met in the oceans before they are on “watch” for a looming La Niña.
This is because the severe weather impacts these phases have on the United States – especially hurricanes – are driven more by changes in the ocean.
A Bureau of Meteorology spokesperson told the ABC: “In general, weather agencies have designed their services around the impact of La Niña/El Niño in their country, and the Bureau considers its service to be best suited to Australia.”
The BOM also has a higher threshold for sea surface temperature variations that must be observed to initiate the “watching” phase of La Niña.
This was the same situation last year when the BOM made a markedly different prediction in May regarding the chances of an El Niño event — 50 percent compared to the 90 percent predicted by the National Oceanic and Atmospheric Administration (NOAA) — and waited until later to announce that El Niño was underway.
However, when it comes to declaring a La Niña event (as opposed to just monitoring a single event), BOM says the differences between the two agencies are narrower.
“Differences between agencies’ assessments of a La Niña or El Niño event are most pronounced during the development phase of the event, or when the event is relatively weak,” a BOM spokesperson said.
The timing of the forecast is important
Dr. Reed says long-term weather forecasting is less accurate this time of year.
“We tend to have strong relationships between these climate factors such as El Niño/La Niña and the weather in Australia in winter and spring,” she explained.
“Whereas in the summer and fall, there are a lot of extremes in the weather. It's a known predictability barrier, that the models are not accurate in the fall.”
The BOM says that a La Niña phase develops 40 to 50 percent of the time after an El Niño phase.
“Don't panic”
Even if La Niña returns later this year, there is no guarantee that we will see extreme weather events and natural disasters like those that occurred during the recent triple La Niña.
La Niña was not the only climate driver causing widespread flooding and heavy rain at that time.
There was also what is known as a negative Indian Ocean dipole, which is similar to La Niña but for the Indian Ocean, and a positive southern annular mode, all of which combined to cause extreme weather and flooding.
“I hope people don't get spooked by the possibility of La Niña yet, because it's not the only thing affecting the weather in Australia,” Reid said.
How climate change is messing with all this
Adding heat to this complex system, as we do on a global scale, makes the task of climate prediction even more complex.
For example, La Niña phases have traditionally been associated with cooler global temperatures, but the last three phases were still hotter than historical El Niño years.
Burning fossil fuels has increased heat in the atmosphere as well as in the oceans, with the oceans absorbing up to 90 percent of the warming so far.
Currently, global sea surface temperatures remain at a record high, although the National Oceanic and Atmospheric Administration's (NOAA) La Niña watch has observed cooling water deeper in the ocean.
Dr. Reid says warming ocean temperatures are affecting how these natural weather events occur, such as a cooler-than-normal El Niño this summer.
“Normally with El Niño, ocean temperatures are cold near Australia, so that's why we stop getting rain,” she said.
“But because of climate change and warming oceans everywhere, the ocean pattern has not been what we normally expect.
“We're basing this on our historical observations, what we've seen in the past, but what we're seeing now we've never seen before. So we're venturing into uncharted territory.”
With so much emphasis on ocean temperatures when observing these weather patterns, there is debate in climate science circles about whether baselines need to be adjusted to account for rising temperatures.
“Historically, we used to associate La Niña with cooler, below-average global temperatures,” Reid said.
But we have reached the point of global warming where a La Niña year is hotter than an El Niño year, say, 30 years ago.
