The Impacts of Changing Winter Warm Spells on Snow Ablation over Western North America

An increase in winter air temperature can amplify snowmelt and sublimation in mountain regions with implications to water resources and ecological systems. Winter Warm Spells (WWS) are defined as a winter period (December to February, DJF) of at least 3 consecutive days with daily maximum temperature anomaly above the 90th percentile (using a moving-average of 15 days between 2001 and 2013). We calculate WWS for every 4-km grid cell within an atmospheric model over western North America to characterize WWS and analyze snow ablation and their changes in a warmer climate. We find that days with ablation during WWS represent a small fraction of winter days (0.6 days), however, 49% of total winter ablation (33.4 mm/DJF) occurs during WWS. Greater extreme ablation rates (99th percentile) occur 18% more frequently during WWS than during non-WWS days. Ablation rates during WWS in humid regions are larger (9 mm d-1) than in dry regions (7 mm d-1) in a warmer climate, which can be explained by differences in the energy balance and the snowpack's cold content. We find that warmer (0.8 degrees C), longer (1.8 days) and more frequent (3.7 more events) WWS increase total winter ablation (on average 109% or 18 mm/DJF) in a warmer climate. Winter melt during WWS in warm and humid places is expected to increase about 3 times more than in the cold and dry region. This study provides a comprehensive description of WWS and their impact on snowpack dynamics, which is relevant to reservoir operations and water security. Extreme maximum winter temperatures (Winter Warm Spells, WWS) can affect the snow in winter by increasing snowmelt, with consequences for mountain water supplies. In this study we quantify snowpack response to WWS in the mountains of western North America and its sensitivity to a warmer future climate using numerical models, observations, and statistical tools. We find that 49% of snow lost in winter happens during WWS, despite having only 0.6 days with WWS per winter. Across the mountains, the humid climatic regions are more affected by WWS than arid regions. We show that snowpack ablation during WWS will increase in a warmer climate, which will put more stress on reservoir and water security planning. Winter Warm Spells account for 49% of snowpack winter ablation (16.5 mm), despite only representing 0.6 days on average Historical Winter Warm Spells increase in frequency (3.7 more events), duration (1.8 days) and magnitude (0.8 degrees C) in a warmer climate Winter snow ablation driven by melt (93.6%) is expected to increase in a warmer climate (147%), especially in humid regions