Warming could nearly double rate of severe La Niña events

Thanks to climate change, El Niño’s meteorological sister will strike more intensely over the next century, a new study predicts.

Based on 21 climate simulations, researchers project that as rising temperatures around Southeast Asia outpace central Pacific Ocean warming, the frequency of weather-disrupting severe La Niña events will nearly double. Roughly 75 percent of these additional extreme La Niñas will immediately follow an intense El Niño season, prompting dramatic weather shifts worldwide, the researchers report January 26 in Nature Climate Change.

“Countries such as Australia are going to get big swings in extreme weather,” says lead author Wenju Cai, a climate scientist at the Commonwealth Scientific and Industrial Research Organisation in Aspendale, Australia. “After an extreme drought caused by El Niño, farmers could plant a lot of crops only to have them wiped out just before harvest by floods caused by La Niña.”

El Niño and La Niña are opposite stages of a naturally occurring climate cycle in the Pacific Ocean and occur every two to seven years. During El Niño, winds push warm water into the eastern Pacific, boosting rainfall in North and South America and parching the western Pacific. La Niña, on the other hand, occurs when unusually strong eastward winds cause relatively cold water to gather in the tropical central Pacific. This cool seawater intensifies the temperature difference between the hot landmasses of Southeast Asia, Indonesia and the Philippines and the cooler Pacific Ocean to the east, causing tropical precipitation to shift westward.

In addition to torrential western Pacific downpours, La Niña causes far-reaching atmospheric changes that can trigger South and North American droughts and supercharge the Atlantic hurricane season. The extreme 1998–1999 La Niña prompted severe weather events worldwide, such as floods in China and powerful hurricanes in the Caribbean, killing tens of thousands and displacing hundreds of millions more.

To project how climate change will affect the likelihood of another La Niña on par with the 1998–1999 event, Cai and colleagues initially looked at 32 different global climate computer simulations. Of these, the researchers selected 21 that could adequately simulate the complex changes in the ocean and atmosphere that spark La Niña. They then compared each simulation’s reconstruction of 1900 through 1999 La Niña events with its predictions for 2000 through 2099 under the current greenhouse gas emission rate.

While the simulations forecast that the total number of La Niña events would stay relatively constant, the frequency of especially severe events increased on average from about one every 23 years to one every 13 years. Only four models forecast a decrease.

“Global warming is altering the background state of the Pacific Ocean, creating a situation where it’s easier to develop very strong La Niña events,” says coauthor Michael McPhaden, an oceanographer at the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle.

That result seemed paradoxical at first, Cai says, because the eastward winds that help La Niña develop are projected to weaken with global warming. However, investigating further, the team discovered several factors that caused the increase.

First, just like an empty skillet heats up faster than a water-filled pot, land temperatures around the western Pacific are increasing faster than water temperatures in the central Pacific Ocean. This imbalance increases the temperature difference between the two regions, which can boost the strength of temporary eastward gusts, Cai says.

In addition, the same research team reported last year that the frequency of extreme El Niño events will also increase over the next century. While El Niño is often described as La Niña’s opposite, the end of a strong El Niño season can bring cold subsurface water closer to the surface in the central Pacific, helping jump-start a successive La Niña event.

“At first, our result was very surprising,” Cai says, “but once we identified the mechanisms involved, it began to make sense.”

Predicting changes in severe weather is important, says climate scientist Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colo. But climate simulations often struggle to accurately portray the small-scale ocean and atmosphere changes that drive La Niña, he says. While lacking confidence in the simulations, Trenberth says predictions of more severe impacts of El Niño and La Niña make sense. “Warming temperatures will make extreme weather, from droughts to floods, worse,” he says. “The whiplash of going from one weather extreme to another will get quite bad.”