Atmospheric rivers are jets of sky-born water vapor that transport tropical moisture to higher latitudes, and they can be a blessing.
In some years they provide 60% of California’s annual precipitation. But they are very hard to predict and can cause flooding, creating headaches for the people who manage the state’s water resources.
Alexander Gershunov is a research meteorologist at Scripps Institution of Oceanography at UC San Diego. He leads a research team that has described atmospheric rivers as great disruptors or wild cards in the effort to predict snow and rainfall in California.
When those jets of water vapor arrive in California, our mountains push them upwards and squeeze rain and snow from them. The huge amounts of water in atmospheric rivers can make our winter forecasts, based on La Niña and El Niño, practically obsolete.
Conditions connected to El Niño Southern Oscillation, or ENSO, have been the main source of seasonal precipitation predictability for California. Their forecasts are based on Pacific Ocean surface temperature. The warm waters of El Niño usually mean it’ll be a wet winter in the Southwestern U.S. and the colder waters of La Niña mean it’ll be dry.
But Gershunov said in the 21st century, strong atmospheric rivers in California have turned many La Niña predictions upside down.
“The really wet years have been La Niña years. And that was 2011, 2017, which were two of the wettest years on record in Southern California,” he said.
And then of course there was 2023, another La Niña winter that nonetheless brought record snowfall to the Sierra Nevada.
“There are certain storms that don’t care about El Niño and La Niña. And the atmospheric rivers are those storms, Gershunov added. ”They don’t dance to the tune of ENSO.”
Predicting wet and dry winters is vital to California’s water management, allowing reservoir operators to know when to release water or when to save it for the dry summer ahead.
National Weather Service meteorologist Alex Tardy said we can see atmospheric rivers coming maybe 10 days out. But predicting seasonal precipitation from them is, for now, just not possible. We just don’t know what path those atmospheric rivers will take and where they’ll make landfall.
“Your prediction might have been really good. You might have said, ‘Hey, we’re going to have a really active atmospheric river winter, and there are going to be 25 atmospheric rivers.’” Tardy said.
“But if they all miss Southern California or you only get one or two of them, that’s going to make a major difference. That’s going to determine the outcome of your season, whether you’re wet or dry or normal.”
Gershunov said predicting when and where atmospheric rivers arrive in our state is something we need to get better at.
