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
Abstract
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.