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The Missing Mountain Water: Slower Westerlies Decrease Orographic Enhancement in the Pacific Northwest USA

Science  13 Dec 2013:
Vol. 342, Issue 6164, pp. 1360-1364
DOI: 10.1126/science.1242335

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Climate change is expected to have significant impacts on Earth's hydrological cycle, owing to changes, for example, in air temperature, weather patterns, and land surface evaporation. In the Pacific Northwest of the United States, decreases in stream flow have been attributed to increased temperatures. Luce et al. (p. 1360, published online 29 November) show that these changes in stream flow are not likely to have been caused by temperature change, however, but rather by decreasing precipitation in the mountains where the streams originate.

Abstract

Trends in streamflow timing and volume in the Pacific Northwest United States have been attributed to increased temperatures, because trends in precipitation at lower-elevation stations were negligible. We demonstrate that observed streamflow declines are probably associated with declines in mountain precipitation, revealing previously unexplored differential trends. Lower-troposphere winter (November to March) westerlies are strongly correlated with high-elevation precipitation but weakly correlated with low-elevation precipitation. Decreases in lower-tropospheric winter westerlies across the region from 1950 to 2012 are hypothesized to have reduced orographic precipitation enhancement, yielding 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 that is relevant for climate change impacts on hydrology.

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