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Date: March 10, 2014
Place: FL 2 Room 1001
Speakers: Charles Luce, John Abatzoglou, and Zachary Holden
Declining streamflows in Pacific Northwest streams have been noted by several authors in recent years, but the causality has been unclear. While trends in April 1 snow water equivalent and streamflow timing have also been noted, they have generally been attributed to increased temperature, because precipitation data at COOP climate stations in the region have shown little to no change over the last 60 years. Closer examination has revealed that precipitation recorded at COOP stations may not reflect precipitation in mountain environments, and suggests that observed streamflow declines are likely driven by mountain precipitation declines. Lower-troposphere winter (Nov-Mar) westerlies are strongly correlated to high elevation precipitation but weakly correlated with low elevation precipitation. Decreases in lower-tropospheric winter westerlies across the region from 1950-2012 are hypothesized to have reduced orographic precipitation enhancement, yielding the previously unexplored differential trends in precipitation across elevations and contributing to the decline in annual streamflow. Climate projections show weakened lower troposphere zonal flow across the region under enhanced greenhouse forcing, highlighting an additional stressor relevant for climate change impacts on hydrology.