SURFACE CLIMATE AND STREAMFLOW VARIABILITY IN THE WESTERN UNITED-STATES AND THEIR RELATIONSHIP TO LARGE-SCALE CIRCULATION INDEXES

A statistical analysis was undertaken to determine the nature and magnitude of the relationship of precipitation, temperature and streamflow in the western United States to large-scale atmospheric circulation patterns. The Southern Oscillation Index (SOI) was used as an indicator of the E1 Nino/Southern Oscillation (ENSO) and the PNA index as an indicator of the Pacific/North America pattern. These indices were correlated with surface climate data and split sample analyses were conducted to determine climate response during the extreme phases of each index. October-March precipitation was shown to be most strongly correlated with SOI averaged over the July-November period. The analysis showed that there are two centers of opposite association with the SOI. During low values of the SOI (ENSO events) precipitation is low in the Pacific northwest and high in the desert southwest. Correlations between SOI and temperature were greatest in the Pacific northwest. The split sample analysis also revealed statistically significant differences in precipitation occurring during extremes of the SOI. The PNA pattern was related to precipitation and temperature over a concurrent time period. Especially strong associations were noted in the Pacific northwest for both precipitation and temperature. Streamflow showed associations with SOI similar to precipitation.