Competition between Rayleigh-Bénard and Horizontal Convection: an experimental model for subglacial lakes

We investigate experimentally the flow structure in a fluid layer heated from below and cooled from above, where, additionnally, a horizontal temperature gradient is imposed on the top plate. This is a model system for the dynamics in subglacial lakes where such a competition between Rayleigh-Bénard Convection (RBC) and Horizontal Convection (HC) is thought to happen, and an experimental realisation of the numerical work of Couston et al. (2022). We evidence a hysteretic transition from a RBC flow structure to a HC flow structure when the ratio of the horizontal heat flux to the vertical heat flux, Λ, is 4e-4 when Λ is decreasing, and 7e-4 when Λ is increasing. These values are lower than the threshold value found in the two-dimensional Direct Numerical Simulation (DNS), of order 1e-2, which has an impact on the flow structure prediction for several subglacial lakes. Additionnally, for larger values of Λ, we observe that the warmest part of the top plate becomes warmer than the bottom plate, and a stable temperature gradient settles below the warm side of the top plate. Thermal plumes are no longer visible in this region, and seem to be replaced by internal gravity waves.