The Influences of Type 1 El Niño and La Niña Events on Streamflows in the Pacific Southwest of the United States

Abstract

Streamflows in the Pacific Southwest of the United States in relation to the tropical Type 1 El Niño-Southern Oscillation (T1ENSO) and La Niña events are examined using composite and harmonic analyses for each event during a 24-month evolution period. The hydroclimatic signals associated with either extreme phase of the Southern Oscillation (SO) are explored based on data from 50 streamflow stations in California, Arizona, New Mexico, Colorado, and Utah. A significant level for the results is assessed by the use of a hypergeometric distribution. Highly significant, coherent signals are demonstrated to exist for both events, with opposite sign and almost identical timing. Pacific Southwest streamflow responses to the T1ENSO thermal forcing are characterized by a wet December-July season in the subsequent year of the event. Similarly, a dry February-July season is detected as a period at which the La Niña-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 the regional annual streamflow cycle is noticed during the subsequent year of the typical T1ENSO (La Niña) event. A lag cross-correlation analysis is conducted between the time series of the seasonal December-July streamflow index and the SO index. The March-May season in the previous year of the seasonal T1ENSO signal was determined to be the logical 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 statistically significant differences in the means of seasonal streamflows associated with T1ENSO and La Niña events. Plausible explanations for the observed teleconnections are presented.