The relationship between the El Nino/Southern Oscillation (ENSO) and unimpaired streamflow over the contiguous United States was studied. The ENSO is a warm event in the tropical Pacific Ocean and is considered a significant perturbation of the general atmospheric circulation. EI Nino events have been observed and recorded since 1726. They occur approximately once every 4 years; however, the time interval between successive events varies from 2 to 10 years.

ENSO-related events have important consequences for U.S. atmospheric and weather patterns, but the nature of these consequences depends on the type ofEI Nino. In 1941 and 1983, the major ENSOs of this century, heavy rains were experienced in the West and Southwest and the Colorado River basin. However, most ENSO episodes produce dry conditions in these regions (e.g., that of 1986-1987).

In these studies there was an identification of regions of land that appear to have strong and consistent ENSO-related streamflow signals. Coherent and significant streamflow responses to ENSO forcing are found in four regions of the United States: the gulf of Mexico, the Northeast, the North Central, and the Pacific Northwest.

Streamflows in the Pacific Southwest of the U.S. in relation to the tropical Type 1 EI Nino-Southern Oscillation and La Nina events were specifically studied. The Pacific Southwest streamflow responses to the Type 1 ENSO 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 Nina-streamflow relationship is strong and spatially coherent.

Once an ENSO event sets in, a long-range forecasting utility may be available for these regions. The results of this analysis, which are consistent with previous studies on precipitation and temperature, demonstrate the mid-latitude hydrologic response to the tropical ENSO phenomena.