EFFECTS OF WINTER ATMOSPHERIC CIRCULATION ON TEMPORAL AND SPATIAL VARIABILITY IN ANNUAL STREAMFLOW IN THE WESTERN UNITED-STATES

Winter mean 700-hectoPascal (hPa) height anomalies, representing the a verage atmospheric circulation during the snow season, are compared wi th annual streamflow measured at 140 streamgauges in the western Unite d States. Correlation and anomaly pattern analyses are used to identif y relationships between winter mean atmospheric circulation and tempor al and spatial variability in annual streamflow. Results indicate that variability in winter mean 700-Hpa height anomalies accounts for a st atistically significant portion of the temporal variability in annual streamflow in the western United States. In general, above-average ann ual streamflow is associated with negative winter mean 700-Hpa height anomalies over the eastern North Pacific Ocean and/or the western Unit ed States. The anomalies produce an anomalous flow of moist air from t he eastern North Pacific Ocean into the western United States that inc reases winter precipitation and snowpack accumulations, and subsequent ly streamflow. Winter mean 700-hPa height anomalies also account for s tatistically significant differences in spatial distributions of annua l streamflow. As part of this study, winter mean atmospheric circulati on patterns for the 40 years analysed were classified into five winter mean 700-hPa height anomaly patterns. These patterns are related to s tatistically significant and physically meaningful differences in spat ial distributions of annual streamflow.