Rare ‘triple’ climate event in La Niña seems likely – what’s next?

Rare ‘triple’ climate event in La Niña seems likely – what’s next?

Rare ‘triple’ climate event in La Niña seems likely – what’s next?

Trash and damaged furniture piled up on a flooded road on March 3, 2022 in Brisbane, Australia.

La Niña contributed to flooding in eastern Australia earlier this year.Credit: Peter Wallis/Getty

An ongoing La Niña event that has contributed to flooding in eastern Australia and exacerbated droughts in the United States and eastern Africa could last until 2023, according to the latest predictions. The occurrence of two consecutive La Niña winters in the Northern Hemisphere is common, but three in a row is relatively rare. A ‘triple dip’ La Niña – lasting three years in a row – has only happened twice since 1950.

This particularly long La Niña is likely just a random blip in climate, scientists say. But some researchers warn that climate change could make La Niña-like conditions more likely in the future. “We’re increasing the likelihood of these triple events occurring,” said Matthew England, a physical oceanographer at the University of New South Wales in Sydney, Australia. England and others are now working towards reconciliation discrepancies between climate data and the output of key climate models – efforts that could clarify what lies ahead for the planet.

More La Niña events would increase the likelihood of flooding in Southeast Asia, increase the risk of drought and wildfires in the southwestern United States, and a different pattern of hurricanes, cyclones and monsoons across the Pacific and Atlantic on other regional changes.

La Niña and its counterpart, El Niño, are phases of the El Niño Southern Oscillation (ENSO) that occur every two to seven years, with neutral years in between. During El Niño events, the usual Pacific winds that blow from east to west along the equator weaken or reverse, causing warm water to flow into the eastern Pacific Ocean, increasing the amount of rainfall in the region. During La Niña, those winds get stronger, the warm water shifts west, and the eastern Pacific Ocean becomes cooler and drier.

The consequences are far-reaching. “The tropical Pacific Ocean is huge. If you shift the rainfall, it has a ripple effect on the rest of the world,” said Michelle L’Heureux, a physicist at the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center in College Park, Maryland. During La Niña years, the ocean absorbs heat to its depths, so global air temperatures tend to be cooler.

cold snap

The current La Niña started around September 2020 and has been mostly mild to moderate since then. From April 2022, it intensified, leading to a cold snap over the eastern equatorial Pacific Ocean not seen since 1950 at that time of year. “That’s pretty impressive,” says England.

The latest World Meteorological Organization forecast, released on June 10, puts a 50-60% chance that La Niña will last through July or September. This is likely to increase hurricane activity in the Atlantic, which will plague eastern North America through November, and decrease the hurricane season in the Pacific, which mainly affects Mexico. NOAA’s Climate Prediction Center predicts a 51% chance of La Niña in early 2023.

The weird thing, L’Heureux says, is that, unlike previous triple dips, this prolonged La Niña didn’t come after a strong El Niño, which tends to build up a lot of ocean heat that takes a year or two to dissipate.1† “I keep wondering, where’s the dynamic for this?” says L’Heureux.

climate correlation

The big questions that remain are whether climate change will change ENSO and whether La Niña conditions will become more common in the future.

Researchers have noticed a shift in ENSO in recent decades: The latest report from the Intergovernmental Panel on Climate Change (IPCC) shows that strong El Niño and La Niña events have been more frequent and stronger since the 1950s than in the centuries before, but the panel could not say whether this was caused by natural variability or by climate change. Overall, the IPCC models point to a shift toward more El Niño-like states as climate change warms the oceans, says climate modeler Richard Seager of Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York. Mysteriously, Seager says, observations over the past half-century have shown the opposite: While the climate has warmed, a tongue of upwelling water in the eastern equatorial Pacific has remained cold, creating more La Niña-like conditions.2

Some researchers argue that the data is simply too sparse to show clearly what’s going on, or that there’s too much natural variability in the system for researchers to spot long-term trends. But it could also be that the IPCC models are missing something big, L’Heureux says, “which is a more serious problem”. Seager thinks the models are indeed wrong and the planet will experience more La Niña-like patterns in the future3† “More and more people are taking this a little bit seriously that the models might be biased,” because they don’t capture this cold eastern Pacific water, Seager says.

Cold Water Injection

England has another possible explanation as to why the IPCC models could get future La Niña-esque conditions wrong. As the world warms and the Greenland ice sheet melts, the fresh cold water is expected to slow down a dominant conveyor belt of ocean currents: the Atlantic Meridional Overturning Circulation (AMOC). Scientists largely agree that AMOC flow has declined in recent decades4but can’t agree on why, or how much it will slow down in the future.

In a study published in Nature Climate Change on June 65England and his colleagues model how an AMOC collapse would leave excess heat in the tropical South Atlantic, triggering a series of barometric pressure changes that eventually amplify trade winds in the Pacific. These winds push warm water westward, creating more La Niña-like conditions. But England says current IPCC models don’t reflect this trend because they don’t account for the complex interactions between ice sheet melting, freshwater injections, ocean currents and atmospheric circulation. “We continue to add bells and whistles to these models. But we have to add the ice caps,” he says.

Michael Mann, a climatologist at Pennsylvania State University at State College, has also argued:2 that climate change will both slow down the AMOC and create more La Niña-like conditions. He says the study shows how these two factors can reinforce each other. Making the models better reflect what’s happening in the ocean, Seager says, “remains a very active research topic.”

“We need to better understand what’s going on,” L’Heureux agrees. For now, she adds, whether, how and why the ENSO might change “is a very interesting mystery.”