New research from UC San Diego's Scripps Institution of Oceanography, supported by the Southwest CASC, challenges traditional reliance on seasonal El Niño-Southern Oscillation (ENSO) patterns for predicting precipitation in the Southwestern United States. ENSO typically brings wet (El Niño) or dry (La Niña) conditions to the region, but 2023 - a La Niña year - was California's 10 wettest year on record.
The new study points to atmospheric rivers - powerful air currents carrying large amounts of water vapor - as the driving force behind these precipitation anomalies. Analyzing over 70 years of weather data, researchers found that atmospheric rivers explained 70% of anomalous years (when precipitation did not match ENSO expectations) and, in some years, accounted for up to 65% of annual precipitation in Northern California and 40% in Southern California. In 2023, nine atmospheric rivers brought significant rainfall to the region, altering the usual dry influence of La Niña.
While ENSO patterns are predictable months in advance, atmospheric rivers can currently only be forecast about 3 weeks ahead of time, making it more difficult to anticipate how they may affect precipitation patterns each year. Climate change may increase the role of atmospheric rivers in determining annual precipitation in the Southwestern United States, potentially reducing the reliability of El Niño and La Niña predictions. Researchers highlight the need to improve atmospheric river forecasting, and to integrate those forecasts with seasonal ENSO predictions to help water managers, farmers, and policymakers make informed decisions on reservoir planning, water allocation, and agricultural planning.