El Niño, La Niña Are Poor Predictors Of CA Rain Forecast, Study Finds

CALIFORNIA — California weather experts have long looked to El Niño and La Niña patterns to predict rainy seasons, but a new study suggests these climate phenomena may not be as reliable as once thought.

El Niño is a weather phase that is associated with heavy rain seasons in the Golden State that are often driven by warmer ocean temperatures in the central and eastern tropical Pacific Ocean. La Niña is a cooler weather phase that often predicts drier conditions.

Together they are called ENSO, which stands for El Niño Southern Oscillation, and they have been thought to have some of the largest natural effects on climate.

However, California researchers in the Feb. 7 study argue that atmospheric rivers are the true predictor of a season's precipitation outlook — not ENSO.

This is because these storms, characterized by narrow corridors of concentrated moisture in the atmosphere, can significantly disrupt the expected precipitation associated with El Niño and La Niña events, researchers said.

The study — published in Climate Dynamics by University of California and Scripps researchers — analyzed historical rainfall data and identified "heretical" years when precipitation levels deviated from typical ENSO patterns.

These years were termed heretical because they deviated from the patterns of rainfall expected from La Niño and La Niña.

For instance, in 2023, despite being a La Niña year, California experienced one of its wettest seasons due to an influx of atmospheric rivers.

Currently, many parts of Southern California have fallen back into drought conditions as the region recorded its driest start ever to a rainy season. Last month, the National Oceanic and Atmospheric Administration officially declared La Niña.

But this new study argues that atmospheric rivers — storms described as rivers in the sky — can override ENSO's influence on climate.

"Atmospheric rivers don’t dance to the tune of ENSO," said Alexander Gershunov, a climate scientist at Scripps and co-author of the study.

What's more, the study says that atmospheric rivers are actually key to the state's water supply. They deliver on average up to 65 percent of annual precipitation in Northern California and 40 percent to the Southland.

Although they can be a great disrupter of ENSO predictions, how much rain they bring varies greatly from year to year, researchers said.

For instance, atmospheric rivers in California accounted for just 5 percent of precipitation in 1977 and 71 percent in 1956.

"Atmospheric rivers are the precipitation wildcards in the Western U.S.,” said Rosa Luna-Niño, lead author of the study who is a postdoctoral scholar at Scripps. "One or two atmospheric rivers can turn it into a wet year, but a weak atmospheric river season can turn it into a dry year."

"This means we can’t trust El Niño and La Niña completely to make accurate water year predictions."

Under climate change, scientists believe atmospheric rivers could become increasingly more important for bringing much needed annual precipitation in the Western U.S.

Typically, water managers in California rely on ENSO's predictions, but researchers say they shouldn't rely on these weather patterns for seasonal planning.

As research indicates, climate change may further amplify the influence of atmospheric rivers on annual rainfall patterns, potentially diminishing the usefulness of ENSO for seasonal forecasting.

Looking to the future, Gershunov and the other co-authors said they will work on merging seasonal forecasts based on ENSO with shorter term sub-seasonal forecasts that predict atmospheric rivers.

"We need to keep improving our ability to predict atmospheric river landfalls, and the better we get at that the more we can use that information to help us interpret the seasonal forecast and vice versa," Gershunov said.