Rare ‘triple’ La Niña climate event looks likely – what does the future hold?


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 East Africa could persist into 2023, according to the latest forecasts. The occurrence of two consecutive La Niña winters in the Northern Hemisphere is common, but having three in a row is relatively rare. A ‘triple dip’ La Niña – lasting three years in a row – has happened only twice since 1950.

This particularly long La Niña is probably just a random blip in the climate, scientists say. But some researchers are warning that climate change could make La Niña-like conditions more likely in the future. “We are stacking the odds higher for these triple events coming along,” says Matthew England, a physical oceanographer at the University of New South Wales in Sydney, Australia. England and others are now working to reconcile discrepancies between climate data and the output of major climate models – efforts that could clarify what’s in store for the planet.

More La Niña events would increase the chance of flooding in southeast Asia, boost the risk of droughts and wildfires in the southwestern United States, and create a different pattern of hurricanes, cyclones and monsoons across the Pacific and Atlantic oceans, as well as give rise. to 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 east to west along the Equator weaken or reverse, causing warm water to gush into the eastern Pacific Ocean, increasing the amount of rain in the region. During La Niña, those winds strengthen, warm water shifts west and the eastern Pacific becomes cooler and drier.

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

Cold snap

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

The latest forecast from the World Meteorological Organization, issued on 10 June, gives a 50–60% chance of La Niña persisting until July or September. This will probably increase Atlantic hurricane activity, which buffets eastern North America until November, and decrease the Pacific hurricane season, which mainly affects Mexico. NOAA’s Climate Prediction Center has forecast a 51% chance of La Niña in early 2023.

The weird thing about it, says L’Heureux, is that this prolonged La Niña, unlike previous triple dips, hasn’t come after a strong El Niño, which tends to build up a lot of ocean heat that takes a year or two to dissipate1. “I keep wondering, where’s the dynamics for this?” says L’Heureux.

Climate correlation

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

Researchers have noticed a shift in the ENSO in recent decades: the latest report by the Intergovernmental Panel on Climate Change (IPCC) shows that strong El Niño and La Niña events have been more frequent and stronger since 1950 than they were in the centuries before that. , but the panel couldn’t tell whether this was caused by natural variability or by climate change. Overall, the IPCC models indicate a shift to more El Niño-like states as climate change warms the oceans, says climate modeller Richard Seager at the Lamont – Doherty Earth Observatory of Columbia University in Palisades, New York. Puzzlingly, Seager says, observations have shown the opposite over the past half-century: as the climate has warmed, a tongue of upwelling waters in the eastern equatorial Pacific Ocean has stayed cold, creating more La Niña-like conditions.2.

Some researchers argue that the record is simply too sparse to show clearly what is going on, or that there is 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, says L’Heureux, “which is a more serious issue”. Seager thinks the models are indeed wrong, and that the planet will experience more La Niña-like patterns in the future3. “More and more people are taking this a bit seriously that maybe the models are biased,” because they don’t capture this cold eastern Pacific water, says Seager.

Cold-water injection

England has another possible explanation for why the IPCC models could be getting future La Niña-like conditions wrong. As the world warms and the Greenland ice sheet melts, its fresh cold water is expected to slow down a dominant conveyor belt of ocean currents: the Atlantic Meridional Overturning Circulation (AMOC). Scientists mostly agree that the AMOC current has slowed down in recent decades4but don’t agree on why, or how much it will slow down in future.

In a study published in Nature Climate Change on 6 June5, England and his colleagues model how an AMOC collapse would leave an excess of heat in the tropical South Atlantic, which would trigger a series of air-pressure changes that ultimately strengthen the Pacific trade winds. These winds push warm water to the west, thus creating more La Niña-like conditions. But England says that the current IPCC models do not reflect this trend because they do not include the complex interactions between ice-sheet melt, freshwater injections, ocean currents and atmospheric circulation. “We keep adding bells and whistles to these models. But we need to add in the ice sheets, ”he says.

Michael Mann, a climatologist at Pennsylvania State University in State College, has also argued2 that climate change will both slow the AMOC and create more La Niña-like conditions. He says the study shows how these two factors can reinforce each other. Getting the models to better reflect what’s going on in the ocean, says Seager, “remains a very active research topic”.

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

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