Where Does the World's Highest Glacier Get Its Moisture?

Abstract:

Himalayan glaciers act as a critical water tower to hundreds of millions of people living downstream and their widespread and ongoing retreat is a pressing concern. The potential response of these glaciers to future warming is, however, highly uncertain due to limitations in climate and glacier modelling at the required spatial resolution, compounded by a lack of observations to constrain empirical methods. Improved understanding of the physical processes influencing glacier mass balance can help reduce this uncertainty by identifying the key controls that must be accurately represented in models. We address this here by focusing on mass inputs through snowfall on to the Khumbu Glacier – the highest glacier in the world that descends from Mount Everest in the Nepal Himalaya. Using new observations from a high-quality weighing precipitation gauge surrounded by a double-alter shield near Everest Base Camp, paired with hourly ERA5 reanalysis, we identify annual snowfall events. Source regions for the snow are traced using a Lagrangian model (FLEXPART) and an algorithm to explicitly quantify evaporation. Almost 80% of the snowfall occurs in the monsoon and the main source region is the Bay of Bengal. Precipitation amounts and lapse rates vary considerably among snowfall events, and we identify that both of these parameters are related to sea surface temperatures within the moisture source regions. These findings place an important process-based constraint on the use of climate model simulations for projecting future glacier extent, and they may also help facilitate seasonal forecasting of regional glacier mass balance and streamflow.