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La Niña responsible for megadroughts in North and South America, study finds

La Niña, the climate event that causes water to be colder than normal in the eastern Pacific, has now been shown by new research released Monday to be responsible for simultaneous megadroughts in the North and South American Southwest over the past 1,000 years. 

Megadroughts are extended periods of drought that last at least 20 years. In a study published in the journal Nature, researchers found that these megadroughts occurred simultaneously in the North American and South American Southwest “regularly” and often during a La Niña event.

The researchers used paleoclimate proxies, archival data about rainfall temperature in the ancient past, and created a climate model that compared the data to past megadrought events.

They discovered over the past millennium that 9 megadroughts occurred in the North American Southwest, with 12 occurring in the South American Southwest. On average, the North American megadroughts lasted for 31.8 years while the South American megadroughts lasted for 21.2 years.

The researchers found that seven of the megadroughts happened simultaneously in both North America and South America due to the colder eastern Pacific surface water temperatures brought about by La Niña. 

“The co-occurrence of these events cannot be explained by chance,” the researchers stated in the study.

La Niña events occur when trade winds push warm water in the Pacific Ocean toward Asia. As a result, colder waters off the coast of the Americas rise to the surface, push the jet stream northward and create drier conditions for the Southwest regions. They occur regularly and last at least five months.

The American Southwest is currently undergoing drought conditions, causing water shortages in the Colorado River and contributing to the several forest fires burning across the American West.

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Climatologists believe a La Niña event will set up for the fall and winter, with the National Weather Service predicting a 70% chance between November and January. If it occurs, it will be the second consecutive winter under a La Niña pattern — though models currently predict it will be a weak event.

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Julie Girard/CC BY 2.0

The Great Salt Lake recedes from Antelope Island in 2020, near Salt Lake City. Smoke from wildfires blocks the mountains in the background from view.