The missing mountain water: Slower Westerlies decrease orographic enhancement in the Pacific Northwest USA
The topic will be of particular interest to researchers in hydrology and climate, but also has implications for land and water management, and forest ecology. This talk covers a paper that was published in Science earlier this year, and some more recent updates.
Dr Charles Luce, US Forest Service, Boise Idaho
Streamflow has been declining in U.S. Pacific Northwest streams over the last 60 years, but the causality has been unclear. Has there been less precipitation or more evapotranspiration? 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 long-term climate stations in the region have shown little to no change over the last 60 years. But those precipitation stations are located at lower elevations that are less affected by orographic precipitation enhancement as winds lift moist air over mountain ranges. In this research, we note that 1) increasing evapotranspiration is inadequate to explain more than a small fraction of regional changes, and 2) strong correlations between high elevation precipitation and westerly winds support a hypothesis wherein observed declines in westerly winds drive decreasing precipitation in the mountains. At the same time, the long-term precipitation gages have a poor correlation to the westerly winds, and do not carry information about westerly trends in the way that the mountain watersheds do. Climate projections show further weakened westerlies across the region under enhanced greenhouse forcing, highlighting an additional stressor relevant for climate change impacts on hydrology.
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