Monsoon and Glacial Meltwater Input Drive Seasonal Cooling of a Himalayan Ice‐Contact Lake

Thousands of glacial lakes exist across the Himalaya. However, the physical characteristics of these lakes that drive changes in glacier mass balance and meltwater delivery downstream are poorly understood. We measured water temperature with depth in Thulagi Lake, Nepal, between May and October 2023 to give the first observations of the thermal dynamics of a Himalayan ice-contact glacial lake spanning the entirety of the glacier melt season. During the pre-monsoon and early monsoon periods, we observed lake temperatures greater than 9°C as high incoming shortwave radiation and wind-driven vertical mixing drove warming at the lake surface. Lake temperature consistently cooled with depth, indicating that the lake was stratified (74% of days within this period). However, these conditions were short lived, with a curtailed summer stratification period after which the lake cooled and vertical mixing was more common. During the pre-monsoon and early monsoon periods (May–July), consistently higher temperatures were measured near the glacier front than at distal locations (mean differences of 0.30°C–0.96°C at the lake surface) indicating intense convection and the delivery of heat to the ice front resulting in subaqueous melt. Our results show that monsoon conditions (increased precipitation, reduced incoming solar shortwave radiation and lower wind speed) and the input of glacial meltwater inhibit prolonged lake warming, suggesting that subaqueous melt-driven frontal ablation may play a less important role in driving glacier mass loss here than it does in other glacierised regions.