Colloquially known as “rivers in the sky”,atmospheric rivers are fast-moving bands of moisture that supply the westernU.S. with a large portion of its annual precipitation, but also sometimes cause devastating flooding, as occurred in California this past winter. Improving our capability to predict atmospheric river threats far in advance would greatly support water and emergency managers in the West.
A new study by Hye-Mi Kim (Stony Brook University) and other researchers has found that the El Niño-Southern Oscillation, which changes slowly and is usually predictable many months in advance, affects how frequently atmospheric rivers make landfall along the western U.S. coast. During El Niño, when the the eastern Pacific Ocean is warmer than normal, atmospheric rivers are more likely to impact the western U.S. Conversely, during La Niña, when the eastern Pacific is cooler than normal, atmospheric rivers are less likely to impact the western U.S. These findings indicate that research to improve the modeling and prediction of the El Niño-Southern Oscillation is a viable pathway to improving monthly and seasonal precipitation forecasts for the western U.S.
This study was supported by the CPO Modeling, Analysis, Predictions, and Projections (MAPP) Program.
The year-to-year changes in atmospheric rivers (ARs) and moisture transport over the northeast Pacific and western North America are investigated during December to February (DJF) from 1979/80 to 2015/16. Changes in AR frequency, intensity, and landfall characteristics are compared between three ENSO phases: central Pacific El Niño (CPEN), eastern Pacific El Niño (EPEN), and La Niña (NINA). During EPEN events, the subtropical jet extends to the south and east with an anomalous cyclonic flow around a deeper Aleutian Low. More moisture is transported towards North America and AR frequency is increased over western North America. In CPEN events, the Aleutian low shifts further southward relative to its position in EPEN, resulting in an increase in the frequency and intensity of landfalling ARs over the southwestern US. In NINA events, the landfalling AR frequency is reduced associated with anomalous anticyclonic circulation over the eastern North Pacific. We diagnose the contribution of multiple factors to the seasonal mean moisture transport using moisture budgets. During the three ENSO phases, the change in low-frequency circulation (dynamical process) is the leading contributor to the seasonal mean moisture flux divergence, while the contributions of the synoptic anomalies and the change in moisture anomaly (thermodynamic process) are not significant along the west coast of North America.
This paper is a contribution to the special collection on ENSO Diversity. The special collection aims at improving understanding of the origin, evolution, and impacts of ENSO events that differ in amplitude and spatial patterns, in both observational and modeling contexts, and in the current as well as future climate scenarios. This special collection is coordinated by Antonietta Capotondi, Eric Guilyardi, Ben Kirtman and Sang-Wook Yeh.