Mountain snowpack melting earlier in the Western US

Timing of melt of mountain snowpack has significant implications for streamflow, reservoir water-level management, and recreational activities. Stewart et al. (2005) report analysis of streamflow data for the 55 year period from 1948-2002 in which they find shifts toward ealier snowmelt. Timing of significant streamflow indicators (monthly fractional flows, spring pulse onset, date of the "center of mass" of annual flow) has moved earlier in spring by one to four weeks in the last few decades relative to decades of the 1950s through the 1970s. The authors have examined whether the Pacific Decadal Oscillation (PDO), a low frequency change in atmospheric pressure patterns over the northern Pacific Ocean, is responsible for this shift. While a shift in the PDO from a cool phase to a warm phase in 1976-77 and back to a cool phase in 1999 accounts for part of the streamflow variability, it cannot account for all of it, particularly the continued trend toward earlier snowmelt after the most recent shift back to the cool phase. These results suggest at least part of this earlier melting is due to global (regionally in the western US) temperature increases.

Reference

Stewart, I. T., D. R. Cayan, and M. D. Dettinger, 2005: Changes toward earlier streamflow timing across western North America. J. Climate, 18, 1136-1155.