The Pacific Decadal Oscillation and Other High-Elevation Drivers of Snowpack in the American West

Mountain snowpack is one of the most vital sources of freshwater for millions of people worldwide. Recent changes in snowpack depth and duration pose serious problems for people that depend on them for irrigation, drinking water, and other water needs. Significant snowpack decreases (>80%) have been observed in many locations throughout western North America over the last century, threatening an important water source: snowmelt. In this study, data from the Snow Telemetry Network (SNOTEL) in the western United States from 1980-2022 were collected to answer two main questions: firstly, what environmental factors and watershed characteristics influence trends in peak snowpack date at high elevations, and secondly, do these high-elevation snowpacks correlate with the Pacific Decadal Oscillation (PDO) on the 10-20 year timescale? We tested the common assumption that April 1 can be used as the date of peak snowpack. We found that at elevations >2500 meters, this assumption can mask true snowpack trends. At lower elevations, rain and watershed morphology affected snowpack peak timing. The correlation between high-elevation snowpack characteristics and the PDO was tested via a Linear Inverse Modeling (LIM) approach. We found that the PDO explained variability in snowpack timing and magnitude at the highest latitudes and elevations in the American West. A potential relationship between the variability of the PDO and its link to snowpack characteristics was observed. We found that the LIM predicted observed snowpack data better in the intervals from 1990-1999 and from 2010-2022, when the PDO underwent large excursions from positive (warm) to negative (cold), than from 1980-1989 and 2000-2009. This study suggests that the assumption of April 1 for peak snowpack is not currently valid for high elevations and may become less so with global warming. Moreover, correlations between PDO phases and snowpack characteristics are variable on decadal timescales. These results improve understanding of elevation-driven trends in snowpack characteristics, and begin to link a powerful climate modulator to vital water resources in what is already a water-needy region.