Declining westerlies and Pacific Northwest hydrology: a hypothesis

Charlie Luce from the U.S. Forest Service’s Boise Aquatic Sciences Laboratory and colleagues have a paper in the most recent Science examining the question of whether declining westerlies are behind the changing snowpack in the mountains of the Pacific Northwest:

Decreases in lower-tropospheric winter westerlies across the region from 1950-2012 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 relevant for climate change impacts on hydrology.

Previous work has found no significant decline in precipitation, but a decline in stream flows. Luce argues that’s because most of the rain and snow measurement stations on which that conclusions is based are at lower elevations, rather than the higher elevations that contribute the bulk of the snowpack.

It’s important to recognize that this is a hypothesis, but luckily for science we’re going ahead with the greenhouse emissions experiment, so we’ll know the answer soon enough. Well, not exactly soon enough if you’re a resident of the Pacific Northwest who has built a society based on the current runoff patterns.