Extreme Precipitation Events in the Western United States Related to Tropical Forcing

Abstract

Three-day accumulations of precipitation for 2.5° long ? 2.0° lat areas along the west coast of the United States are used to rank precipitation events. Extreme precipitation events (those above the 90th percentile) occur at all phases of the El Niño?Southern Oscillation (ENSO) cycle, but the largest fraction of these events (for the West Coast as a whole) occur during neutral winters just prior to the onset of El Niño. In the tropical Pacific these winters are characterized by enhanced activity on intraseasonal (roughly 20?60 day) timescales and by relatively small sea surface temperature anomalies compared to ENSO winters. For these winters, lagged composites are used to document a coherent relationship between the location of extreme precipitation events along the West Coast and the location of enhanced tropical convection on intraseasonal timescales. The evolution of the atmospheric circulation patterns associated with the extreme precipitation events is described and a physical mechanism relating tropical intraseasonal oscillations, the ?pineapple express,? and the extreme precipitation events is proposed and illustrated.