ENSO and Hydrologic Extremes in the Western United States
Daniel R. Cayan (1,2), Kelly T. Redmond (3), and Larry Riddle (1)
(1) Climate Research Division, Scripps Institution of Oceanography
La Jolla, California  92093-0224
(2) U.S. Geological Survey, La Jolla California  92093-0224
(3) Western Regional Climate Center, PO Box 60220
Reno, Nevada  89506-0220
Frequency distributions of daily precipitation in winter and daily streamflow from late winter to early summer, at several hundred sites in the western United States, exhibit strong and systematic responses to the two phases of ENSO. Most of the streamflows considered are driven by snowmelt. The Southern Oscillation Index (SOI) is used as the ENSO phase indicator. Both modest (median) and larger (90th percentile) events were considered. In years with negative SOI values ("El Nino"), days with high daily precipitation and streamflow are more frequent than average over the Southwest and less frequent over the Northwest. During years with positive SOI values ("La Nina"), a nearly opposite pattern is seen. A more pronounced increase is seen in the number of days exceeding climatological 90th percentile values than in the number exceeding climatological 50th percentile values, for both precipitation and streamflow. Streamflow responses to ENSO extremes are accentuated over precipitation responses. Evidence suggests that the mechanism for this amplification involved ENSO-phase differences in the persistence and duration of wet episodes, affecting the efficiency of the process by which precipitation is converted to runoff. The SOI leads the precipitation events by several months, and hydrologic lags (mostly through snowmelt) delay the streamflow response by several more months. The combined 6-12 month predictive aspect of this relationship should be of significant benefit in responding to flood (or drought) risk, and in improving overall water management in the western states.