Winter mean 700 millibar (700 mb) height anomalies, representing the average atmospheric circulation during the snow season, were compared with annual streamflow measured at 140 stream gauges in the western United States. Correlation analysis was used to identify relations between winter mean atmospheric circulation and annual streamflow, and to quantify the degree to which the temporal variability in annual streamflow can be attributed to variations in winter mean atmospheric circulation. Results indicate that winter mean 700 mb height anomalies account for a statistically significant portion of the temporal variability in annual streamflow in the western United States. In general, above-average annual streamflow is associated with negative winter mean 700 mb height anomalies over the eastern North Pacific Ocean and/or the western United States. These anomalies are indicative of anomalous cyclonic circulation which is associated with an anomalous flow of moist air from the eastern North Pacific Ocean into the western United States that increases winter precipitation and snowpack accumulations, and subsequently streamflow. Below-average annual streamflow is associated with positive 700 mb height anomalies over the eastern North Pacific Ocean and/or the western United States. These positive anomalies indicate anomalous anticyclonic circulation which prevents the intrusion of moist air from the eastern North Pacific Ocean into the western United States, increases subsidence, decreases winter precipitation, and results in decreased streamflow. Results also indicate that long-term trends in annual streamflow are related to long-term trends in winter mean 700 mb height anomalies.
