UC Irvine

Extreme precipitation events, commonly associated with “Atmospheric Rivers,” are projected to increase in frequency and severity in western North America; however, the intensity and landfall position are difficult to forecast accurately. As the isotopic signature of precipitation has been widely utilized as a tracer of the hydrologic cycle and could potentially provide information about key physical processes, we utilize both climate and precipitation isotope data to investigate these events in California from 2001 to 2011. Although individual events have extreme isotopic signatures linked to associated circulation anomalies, the composite across all events unexpectedly resembles the weighted mean for the entire study period, reflecting diverse moisture trajectories and associated teleconnection phases. We document that 90% of events reaching this location occurred during the negative Arctic Oscillation, suggesting a possible link with higher-latitude warming. We also utilize precipitation data of extreme precipitation events across the entire western U.S. to investigate the relationships between key tropical and Arctic climate modes known to influence precipitation in this region. Results indicate that the wettest conditions occur when the negative Arctic Oscillation, negative Pacific/North American pattern, and positive Southern Oscillation are in sync and that precipitation has increased in the southwestern U.S. and decreased in the northwestern U.S. relative to this phase combination’s 1979-2011 climatology. Furthermore, the type of El Niño-Southern Oscillation event, Central Pacific or Eastern Pacific, influences the occurrence, landfall location, and isotopic composition of precipitation.