Thermal convection in subglacial lakes: insights from simulations of the competition between Rayleigh-Bénard and horizontal convection

Subglacial lakes are liquid water environments trapped between the bedrock and ice sheets of Greenland and Antarctica. They are subject to geothermal heating, but they also experience horizontal temperature gradients along their top ice–water interface, when tilted, because of the pressure-dependence of the freezing temperature. A key question is: what dynamics result from these two sources of buoyancy? In this presentation I will show results from direct numerical simulations of idealized lake geometries (rectangular) covering a wide range of buoyancy-forcing control parameters (top temperature gradient, lake length, water depth). I will show that a single dimensionless parameter controls the type of dynamics dominating, i.e., which is either Rayleigh-Bénard or horizontal convection. I will then demonstrate that the dynamics is bi-stable (resp. bi-modal) close to the threshold for low (resp. large) buoyancy forcing. Finally, I will explain why Rayleigh-Bénard convection forced by geothermal heating should dominate in real subglacial lakes, even though our study discards the nonlinearity of the equation of state for freshwater and the Coriolis force. Our results on water movements will help understand sediment and nutrient transports and distributions in subglacial lakes, which are key to paleoclimate and ecosystem studies.