Right now, a broad plume of cool water has pooled in the Pacific Ocean, west of South America.
It's the signature of La Niña.
While Australia waits to see whether La Niña will be declared, the United States has already called it, using slightly less stringent criteria.
"La Niña conditions are present," the US National Oceanographic and Atmospheric Administration (NOAA) wrote last week.
It will officially be a third La Niña if it continues until summer.
La Niña or not, cold water in the Pacific is right now driving floods in Australia and drought in Africa and the United States and may continue to do so for some time.
That's according to NOAA Senior Scientist Mike McPhaden, perhaps the world's top expert on La Niña and El Niño.
Dr McPhaden is something of a legend in climate science.
In the 1980s, he drove the building of one of the planet's largest scientific instruments, the Global Tropical Moored Buoy Array.
It's a network of floating sensors that encircle the globe, like a Hubble Telescope for the oceans.
The instruments relay the oceans' temperature, particularly in the "Niño" regions of the eastern tropical Pacific, providing the backbone for seasonal climate forecasts.
"ENSO [El Niño Southern Oscillation] is the 800-pound gorilla in the climate system," Mike McPhaden said.
"When you prod it, it's going to react and it's going to have climate impacts that are felt all over the world like we are seeing now."
Incredibly, those climate impacts are much bigger than just the floods that devastated Australia's east coast this year.
La Niña and IOD triggering drought in East Africa
As the rains began in Australia shortly after the Black Summer in early 2020, the rains failed in East Africa, across some of the poorest countries on earth.
This year, the Horn of Africa faces a record fifth failed wet season.
The region has two wet seasons a year.
The cause — cool seas off Africa and warm seas off Australia, leading to rain falling over Australia instead of Africa.
Scientists call this pattern the Indian Ocean Dipole, which is connected to and influenced by La Niña in the Pacific Ocean.
"It's a perfect ocean for drought," according to Chris Funk, director of the Climate Hazards Centre at the University of California.
In recent years, East Africa has experienced the flip side of Australia's abrupt U-turn from drought to floods.
"We're seeing the same kind of whipsaw climate volatility," Dr Funk said.
"In 2019, you were having your Black Summer and all the horrible fires. That October, November, December, East Africa was receiving the other side of the Indian Ocean Dipole with incredible flooding and rainfall. That led to an outbreak of locusts."
As Australians face another wet summer ahead, farmers in the Horn of Africa stare down the barrel of another failed crop and almost inevitable famine.
"I have never seen a situation like the one that I saw during my recent trip to Ethiopia," Caritas Australia chief executive Kirsty Robertson said.
"Many communities had secure access to water before this drought. But now, these communities are walking further and further to water. In one community we visited, the women there were walking overnight.
"It is staggering, the impact that has on your daily life, because if you used to spend an hour or two getting water, and you're now spending a day or two getting water, that completely changes your life and your livelihood," she said.
For Dr Funk, the perfect ocean for drought offers little good news for those in the Horn of Africa already on the brink.
"We know there won't be rain at least until November in that region, so that poor woman walking for water — no matter what — is going to be in a lot of hurt.
"And then if there's yet another bad season, it's going to be bad and I think, unfortunately, tragically, you're going to start seeing pictures of people dying."
Dr Funk said famines in the region had become much less common since the Ethiopian food crisis of the 1980s that prompted the Live Aid movement.
"That's a pretty good achievement. But the magnitude of these repeated shocks is really likely to overwhelm humanitarian relief capabilities unless countries really step up," he said.
La Niña deepening drought in the United States
If East Africa's climate in recent years has mirrored Australia's, so too has the western United States.
For a brief few wet months in 2019, it looked like the worst drought in American history might break.
Then La Niña arrived.
Dry conditions returned in 2020 and intensified through 2021.
Since then, the megadrought has been recognised as the worst in at least 1,200 years. And the long-term forecast there is for dry months ahead.
"There's two things going on here," Dr McPhaden said.
"One is the slow rise in temperatures associated with climate change.
"The other hammer that's dropped is two years of prolonged La Niña. That's why we're in such an extreme state in this past couple of years in the Western US," he said.
In Reno, Nevada, Tim Brown from the Desert Research Institute has been watching the climate literally change around him.
"We're heading into something new," he said.
"Using tree ring data, we've seen multi-decade-scale droughts before, and there's been a number of these historically going back to about the year 800.
"The current drought, which really now has been about 20 years when you aggregate it over time, has exceeded the greatest magnitude of any of the previous megadroughts," he said.
When will La Niña stop?
With Australia, East Africa and the Western United States all in a La Niña-driven climate crisis, the million-dollar question is, When is it going to stop?
"With the potential for a third year of La Niña, people are really concerned that there'll be no relief in sight, at least in the near term," Dr McPhaden said.
With so much cool water in the eastern tropical Pacific Ocean pooling for so long, scientists such as Dr McPhaden are wondering whether the rains in Australia and the droughts in the US and Africa could persist much longer than just 2022.
"The ocean is very slow to change, unlike the atmosphere. It's a much denser fluid. So we know the current conditions that we observe in the ocean will persist for at least a couple seasons, just because the ocean is slow to change," he said.
Climate scientists have observed periods where cool tropical Pacific Ocean temperatures have persisted year after year, driving extended rainfall in Australia and some of the nation's greatest floods.
The mid-1950s was one such phase, culminating in the historic 1955 Maitland floods.
The mid-1970s was another, culminating in the 1974 Brisbane floods.
"The impacts are very significant and it's concerning," Dr McPhaden said.
"Are we in now a phase where we will see even more severe and prolonged drought in the western US? Is it going to be unusually wet for a much longer period of time in Australia? These are big questions."
Big questions indeed.
There is, of course, another directly related big question — what role is climate change playing?
La Niña and climate change
"We expect the swing from one extreme to the next is going to be enhanced by climate change," Agus Santoso from the University of New South Wales said.
Dr Santoso is one of the world's top experts on the impact of climate change on ENSO and the IOD.
He said it was best to think of the impact of climate change as something that occurred over decades, rather than about single events.
"It's very difficult to isolate the role of climate change on one particular La Niña for example, or one east coast low. It takes time for us to be sure whether climate change has played a role in an event," he said.
Despite this, hard evidence is now coming in connecting recent rainfall extremes to climate change, according to Dr Santoso.
"It's not just a hunch. There are good reasons to believe what's been happening for the past decade is attributed to climate change. But of course, we cannot be 100 per cent sure. Because there are uncertainties in the computer models themselves. They're not perfect tools," he said.
A hallmark of the megadrought in the western United States has been record heat, fuelled by climate change.
"We're calling this particular drought a 'hot drought'. Because these dry conditions are associated with increased warming," Dr Brown said.
In East Africa, Dr Funk said human-caused climate change was amplifying climate drivers like La Niña and the IOD.
"In 2016 and 2017, we had a La Niña that came with exceptionally warm ocean waters in the western Pacific that are related to climate change. And we had two droughts in East Africa, then we had this massive Indian Ocean Dipole event in 2019.
"That helped to produce the Black Summer and flooding in Kenya. And then we've just had the sequence of La Nino related droughts four times in a row.
"And so in each of those scenarios, you can find where climate change is making the waters much more warmer than they used to be," he said.
