An analysis of the physical processes controlling observed spatial trends in glacier mass balances across High Mountain Asia

Alpine glaciers are valuable climate change indicators, but the complexity of glacier systems and lack of direct measurements often make it difficult to establish a clear causal relationship between glaciers and regional climate. This is particularly true in High Mountain Asia (HMA) where the topography, climate, and glacier systems are especially heterogeneous, and where few glaciers have meaningful in-situ mass balance (MB) data available. Recently, multi-decadal geodetic MB estimates have become available for numerous glaciers across HMA, providing for the first time an opportunity to study the physical processes controlling the spatial patterns in glacier MB across the region. New gravity estimates of regional glacier MB compliment the geodetic estimates, providing higher temporal resolution estimates of regional glacier MB. These MB estimates cover sufficient timespans to capture climatic trends despite inherent methodological noise and heterogeneous glacier response times.

In this study we model the MB of glaciers across HMA using an energy- and mass-balance model (accounting separately for clean and debris-covered ice). The model is calibrated using geodetic MB estimates for glaciers across HMA for the period 1974-2006. It is run with a daily timestep at 90m resolution over the entire glacierized area for this same time period, forced by 50km resolution meteorological outputs from the Forecast-oriented Low Ocean Resolution model - a global climate model developed by NOAA's Geophysical Fluid Dynamics Laboratory. The model results capture the large-scale spatial patterns observed in the geodetic MB estimates reasonably well. Notably, the preliminary modeling results also show accelerating glacier mass loss in the latter half of the study period, as hypothesized in other studies. Spatiotemporal trends in glacier energy budgets during the time period of the study suggest that the accelerating mass loss throughout the region can largely be explained by the observed increase in air temperature, while the spatial variability in MB is controlled more by glacier hypsometry and local climate. These results establish a clearer relationship between changes in climate and glacier responses in HMA and will help provide insights on how glaciers in the region are likely to respond to future changes in climate.