Causes of recent changes in western North American snowpack

Abstract

Monthly snow water equivalent (SWE) station observations and gridded temperature data are used to identify mechanisms by which
warming affects the temporal and geographical structure of changes in western North American mountain snowpack. We first exploit
interannual variability to demonstrate the sensitivity of snowpack to temperature during the various phases of the snow season.
We show that mechanisms whereby temperature affects snowpack emerge in the mid to late portion of the snow season (March through
May), but are nearly absent during the earliest phase (February), when temperatures are generally well below freezing. The
mid to late snow season is precisely when significant loss of snowpack is seen at nearly all locations over the past few decades,
both through decreases in snow accumulation and increases in snowmelt. At locations where April 1st SWE has been increasing
over the past few decades, the increase is entirely due to a significant enhancement of accumulation during the earliest phase
of the snow season, when the sensitivity analysis indicates that temperature is not expected to affect snowpack. Later in
the snow season, these stations exhibit significant snowpack loss comparable to the other stations. Based on this analysis,
it is difficult to escape the conclusion that recent snowpack changes in western North America are caused by regional-scale
warming. Given predictions of future warming, a further reduction in late season snowpack and advancement in the onset of
snowmelt should be expected in the coming decades throughout the region.

KeywordsSnow water equivalent–Climate change–Climate sensitivity–Trends–Surface observations