Record-breaking rain in California was caused by abnormally high temperatures on the other side of the world, study finds

Abnormally high temperatures on the Tibetan Plateau triggered two catastrophic rainfall events halfway around the world in California and nearby states in early 2017 and 2023, a new study reports.

This is the first time that researchers have identified this link, known as a climate teleconnection. The results could improve forecasts of extreme weather in the western U.S., which, in turn, could improve warnings for coastal communities and potentially save lives as well as billions of dollars in property damage, the study's authors say.

In December 2016 and February 2023, the Tibetan Plateau was warmer than usual for these times of the year. Roughly one month after each of these hotspots appeared, parts of California, Nevada, Oregon, Utah and Arizona received record-breaking precipitation, causing a total of around $6.6 billion in damage, according to the study, and killing at least nine people .

State-of-the-art forecast systems, which rely heavily on ocean conditions like El Niño, failed to predict this extreme rainfall, said study first author Yongkang Xue , a distinguished professor of atmospheric and oceanic sciences at UCLA. The record precipitation occurred during La Niña years, which are typically associated with dry conditions in California and neighboring regions, further complicating the picture.

"This study provides the first evidence from observational data analysis and model simulations that abnormally high early-winter temperatures on the Tibetan Plateau led to stronger atmospheric rivers along the U.S. Pacific Coast, driving heavy regional precipitation," Xue told Live Science in an email.

Atmospheric rivers are narrow, elongated corridors of intense water vapor transport. They are key drivers of precipitation and floods in the western U.S., and in this case, they interacted with giant waves in Earth's atmosphere, called Rossby waves, to produce devastating rainfall in the winters of 2017 and 2023.

As temperatures increased over the Tibetan Plateau in late 2016 and early 2023, a strong gradient formed with the regions north and south of the plateau. This generated an atmospheric disturbance that propagated eastward along the jet stream via Rossby waves, triggering what is known as a Rossby wave train between the Tibetan Plateau and the Rocky Mountains, Xue said.

Get the world’s most fascinating discoveries delivered straight to your inbox.