Air temperature partitioning of snow accumulation, erosion and melt: a regime shift occurring on Mt. Ortles (Eastern Italian Alps)

Glacier mass balance measurements and models are key tools for understanding the glacier response to climate change and specific processes occurring at the glacier surface. Snow accumulation and wind-driven erosion are among the most difficult processes to measure and model in high-altitude alpine terrain and on glaciers, due to their high variability in space and time, and to the scarcity of in situ observations. In addition, snow accumulation and erosion are key processes in the formation and preservation of ice core archives located on high-altitude accumulation areas of mountain glaciers, yet their impact on these paleoclimatic archives is still unquantified. In this study we used a rare dataset of nivo-meteorological and mass balance observations collected between 2011 and 2015 at 3830 m a.s.l. (meter above sea level) on Mt. Ortles (Eastern Alps) to investigate snow accumulation and erosion processes in close proximity to an ice core drilling site located in the upper accumulation area of Alto dell'Ortles Glacier. We applied the physics-based snow cover model SNOWPACK, constrained by field data, to reproduce the local mass balance and to explicitly simulate snow erosion by wind. The model reproduced the observed seasonal and annual mass balance variability with good accuracy over the four-year study period. Results indicate that wind erosion was the dominant ablation process at the study site, removing 21 % of the snowfall, whereas melt played a minor role removing only 3 %. Erosion was most effective in winter, during or shortly after snowfall events, and its efficiency was controlled by air temperature, with dry snow being much more susceptible to erosion (91 % of total erosion) than wetted snow (9 % of total erosion). Sensitivity experiments to air temperature perturbations demonstrated that wind erosion provides a negative feedback to the mass balance, because increasing temperature accelerates snow metamorphism and makes the snow surface less erodible. However, a further 1 °C warming would promote a transition from an erosion-dominated to a melt-dominated mass balance regime. Our findings emphasize the importance of accounting for wind erosion in projections of glacier mass balance on high-elevation wind exposed glacierized areas. They also highlight the relevance of snow erosion for the interpretation of ice core records, because long-term variations in snow erosion may have affected the formation of the seasonal paleoclimatic signal and its preservation.