WESTERN US STREAMFLOW AND ATMOSPHERIC CIRCULATION PATTERNS DURING EL-NINO SOUTHERN OSCILLATION

Using principal component analysis (PCA), cluster analysis, and jackkn ife analysis, we investigated the spatial and temporal modes that domi nate streamflow variability in the western US in response to El Nino-S outhern Oscillation (ENSO) events. Spatial variability was investigate d with data only from ENSO years and with rotated PCA on 79 streamflow stations in the western United States. Eight regions, or clusters, we re thus pinpointed as areas where streamflow tends to co-vary similarl y following ENSO events; traditional cluster analysis confirmed the id entification of these regions. The ENSO response in streamflow was the n further evaluated by forming an aggregate ENSO composite for each re gion. Temporal variability of western US streamflow in the PCA-identif ied regions was evaluated with a 'T-mode' PCA that isolated the differ ent responses in streamflow following ENSO events. The T-mode PCA brea ks the 13 ENSO events that occurred from 1932 to 1993 into five subset s. It is interesting to note that the events in the dominant mode, PC1 (+), occurred before 1976, and next mode, PC2(+), included events prio r to 1976. Finally, we investigated the atmospheric circulation patter ns over the North Pacific Ocean and much of North America that are ass ociated with the various US streamflow responses. The circulation patt erns vary according to the prescribed ENSO forcing. The results of thi s study contribute to a better understanding of the varied ENSO-stream flow relationship in the western US and the use of ENSO for long-range streamflow forecasting. (C) 1997 Elsevier Science B.V.