Modeling circulation and seasonal fluctuations in perennially ice-covered and ice-walled lake untersee, antarctica

Abstract

Lake Untersee, Antarctica, is a freshwater perennially ice covered lake bounded along its north by the Anuchin glacier. The Massachusetts Institute of Technology general circulation model, used on a representative wedge-shaped lake and actual bathymetry for Lake Untersee, produces estimates for circulation and longterm temperature and mixing trends. Modeled circulation is dominated by an anticyclonic gyre in front of the glacier, with slower ̴ 3 mm s21 flow exhibited around the lake’s perimeter, allowing effective mixing throughout most of the lake with time scales of one month. Estimated velocities bound maximal glacial flour particle size at ̴ 10 µm for effective transport throughout the lake, consistent with the sediment’s mostly fine composition observed in field studies, and mixing time scales mean nonuniformities in measured O2 concentration likely require recent or ongoing sources. Areas in which large temperature gradients prevent exchange of fluid demonstrate minimal mixing, such as the lake’s upper water layers and the anoxic basin in the south, and circulation is consistently slowed in the northern sheltered bay area. Mean flow velocities fluctuate by about one fifth of their magnitude between summer and winter, and the lake’s almost homothermal body temperature varies by about one tenth of a degree over the same period. While calculated temperature profiles qualitatively agree with field data, the model’s long-term equilibrium temperature differs substantially, likely due to poor description of heat transfer with the glacier. Model robustness tests show results differ by ̴ 10% when either grid scale or water temperature are halved.