Two episodes of record rainfall in California may have been fueled by an anomaly that emerged thousands of kilometers away, on the Tibetan Plateau.
Researchers focused on the winters of 2016–2017 and 2022–2023. California and nearby regions received exceptional precipitation during those periods. The result was all the more surprising because these episodes occurred during La Niña, a situation often associated with drier weather in this part of North America.
Rossby waves represented in an atmospheric model
By examining weather data from 1984 to 2023, the team identified a link. When the Tibetan Plateau was unusually warm at the beginning of winter, more rain fell over the western United States the following month. However, simple correlation is not enough to establish that one phenomenon causes another.
The scientists therefore used climate simulations. Without accurately reproducing the observed heat in Tibet, their model did not reproduce California’s extreme precipitation. After incorporating this temperature anomaly, it reproduced about 56% of the excess precipitation in January 2017 and 38% of that in March 2023.
How can heat in Asia alter rainfall in America? The atmosphere transports disturbances over very long distances. In this case, warming of the Tibetan Plateau changes vast undulations in the air that propagate eastward. Meteorologists call them Rossby waves.
This chain reaction then reaches the Pacific and influences atmospheric rivers. Despite their name, these are not waterways. They are long bands of very humid air capable of transporting enormous amounts of water vapor over the ocean. When they reach the California coast, they can cause heavy precipitation.
The simulations also indicate that the large atmospheric undulations deform over the northeastern Pacific. This mechanism and atmospheric rivers can then act together. Tibet’s heat therefore does not explain every flood on its own, but it appears to be one of the factors that contributed to these two record episodes.
This discovery could above all help improve forecasts several weeks in advance. California’s winter precipitation remains difficult to predict that far ahead. Monitoring temperatures on the high plateaus of Asia could provide an additional clue before certain events, an avenue that researchers can now test over more winters.