Climate forecasters said Thursday that the world had entered La Niña, the opposite phase of the climate pattern that also brings El Niño and affects weather across the globe. Among other impacts, La Niña has the potential this winter to worsen what are already severe drought conditions in the American Southwest.
The Climate Prediction Center, a part of the National Oceanic and Atmospheric Administration, said in its monthly forecast that sea-surface temperatures in the central and eastern tropical Pacific Ocean had cooled, signifying La Niña conditions, and that there was a 75 percent likelihood that La Niña would continue through the winter.
Like El Niño, which results from warmer-than-normal ocean temperatures in the tropical Pacific, La Niña occurs every two to seven years on average. And like El Niño, it leads to changes in atmospheric circulation that can affect weather in unconnected parts of the world.
La Niña’s strongest influence is usually felt in winter. And while the precise effects are unpredictable, La Niña can result in warmer and drier conditions across the Southern United States and cooler conditions in southeastern Alaska, the Northern Plains and western and central Canada. It can also lead to a wetter winter in the Pacific Northwest.
Mike Halpert, deputy director of the Climate Prediction Center, said that as a result of La Niña, Southern California, as well as most of Arizona and New Mexico, could “tilt toward dry” this winter.
Southern California, which gets most of its rainfall from late fall to early spring, is already abnormally dry, according to the United States Drought Monitor. Those conditions have contributed to numerous wildfires this summer. All of Arizona and New Mexico are in varying stages of drought, from moderate to severe.
But La Niña can have effects around the globe. The most consistent impact is in Indonesia, which usually sees increased rainfall. La Niña can also lead to dry conditions in Eastern China and East Africa and cool and wet conditions in Southern Africa and Southeastern Brazil.
NOAA scientists said this summer that the decreasing sea-surface temperatures in the tropical Pacific were a factor in their prediction that the North Atlantic hurricane season would be an active one. La Niña influences atmospheric conditions in the North Atlantic that would otherwise tend to disrupt hurricanes as they form.
Emily Becker, associate director of the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami, said that since the last El Niño ended in 2019, ocean temperatures in the tropical Pacific had been “neutral,” neither abnormally warm or cool. But that began to change this summer. “We saw some pretty substantial easterly winds,” she said. “It might have cooled a little faster than we would have expected, but not radically so.”
These west-to-east trade winds cooled the ocean surface and also led to upwelling of deep, colder water to the surface, Dr. Becker said.
The resulting shift of warmer water to the western tropical Pacific affects the jet stream, the high-altitude river of air that moves west to east and serves to separate colder and warmer air. It is this change in the jet stream that can modify the North American winter, Dr. Becker said.
El Niño affects the jet stream, too, although in different ways, and leads to changes that are often the opposite of La Niña’s, including wetter conditions across the Southern United States. El Niño tends to last longer than La Niña as well. Dr. Becker said current models suggested that this La Niña would not persist through the spring.