THE INFLUENCES OF TYPE-1 EL-NINO AND LA-NINA EVENTS ON STREAMFLOWS INTHE PACIFIC-SOUTHWEST OF THE UNITED-STATES

Streamflows in the Pacific Southwest of the United States in relation to the tropical Type I El Nino-Southern Oscillation (Tl ENSO) and La N ina events are examined using composite and harmonic analyses for each event during a 24-month evolution period. The hydroclimatic signals a ssociated with either extreme phase of the Southern Oscillation (SO) a re explored based on data from 50 streamflow stations in California, A rizona, New Mexico, Colorado, and Utah. A significant level for the re sults is assessed by the use of a hypergeometric distribution. Highly significant, coherent signals are demonstrated to exist for both event s, with opposite sign and almost identical timing. Pacific Southwest s treamflow responses to the Tl ENSO thermal forcing are characterized b y a wet December-July season in the subsequent year of the event. Simi larly, a dry February-July season is detected as a period at which the La Nina-streamflow relationship is strong and spatially coherent. An index time series is plotted to determine the temporal consistency of the signal. It was found that the respective seasonal signal for each event was confirmed by all episodes. Amplification (suppression) of th e regional annual streamflow cycle is noticed during the subsequent ye ar of the typical Tl ENSO (La Nina) event. A lag cross-correlation ana lysis is conducted between the time series of the seasonal December-Ju ly streamflow index and the SO index. The March-May season in the prev ious year of the seasonal Tl ENSO signal was determined to be the logi cal period in which the SO index can be averaged to obtain the highest correlation and the maximum time lag. A Mann-Whitney U test reveals s tatistically significant differences in the means of seasonal streamfl ows associated with Tl ENSO and La Nina events. Plausible explanations for the observed teleconnections are presented.