A multi-century, tree-ring-derived perspective of the North Cascades (USA) 2014-2016 snow drought

Cool-season precipitation is a critical component of western North American water supplies. In recent decades, snow water equivalent (snowpack) has declined dramatically, culminating in record lows along the northern Cascade Range, USA, during the winters of 2014 and 2015. Given the brevity of observational records, the extent to which these recent trends exceed historical ranges of variability remains poorly understood. Here, we use a network of 30 tree-ring chronologies to reconstruct April 1 snowpack in the northern Cascade Mountains (Washington, USA) over the past ca. 400 years that accounts for 62% of the instrumental period variability. The reconstruction is characterized by considerable interannual- to interdecadal-scale variability. Yet, the most conspicuous feature is a decline that began in the mid-1970s concurrent with a shift to above-average air temperature which is accelerated through to the most recent years of the record. The 2014-2016 snow drought, especially snow conditions during the 2015 winter, across the northern Cascades is unprecedented within the context of the last 400 years. The reconstruction is characterized by considerable interannual- to interdecadal-scale variability, but a conspicuous decline emerged in the mid-1970s and accelerated through to the most recent years of the record to levels below the pre-industrial envelope of variability. This decline culminated in the 2014-2016 northern Cascade Mountains snow drought, which is unprecedented within the context of the last 400 years. Not only is the twentieth-twenty-first century period characterized by the rapid decline in snowpack conditions, it is also highly volatile. Extreme flips in snowpack conditions (year-to-year changes between < 5th and > 95th percentiles) during the twentieth and twenty-first centuries were anomalous within the context of the record. These patterns described for the North Cascades are concurrent with anomalously low snowpack in the Sierra Nevada Mountains of California, and thus part of a broader trend toward reduced snowpack in western North America. This decline will fundamentally impact regional drought and hydrography in North Cascadia and is likely the combined result of rising temperatures, weakening