Hysteretic transitions between Rayleigh–Bénard and horizontal convection in an experimental model of subglacial lakes

We investigate experimentally the flow structure in a fluid layer of thermal conductivity $k$ , heated from below with a uniform heat flux $F$ , and exposed to a horizontal temperature gradient, $\lambda$ , 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. We evidence a hysteretic transition from a RBC flow structure to a HC flow structure when the non-dimensional control parameter, $\varLambda = k\lambda /F$ , is $4\times 10^{-4}$ when $\varLambda$ is decreasing, and $7\times 10^{-4}$ when $\varLambda$ is increasing. These values are lower than the threshold value found in recent two-dimensional direct numerical simulations (Couston et al. 2022 J. Fluid Mech. , vol. 947, p. A13), of order $10^{-2}$ , highlighting the importance of exploring three-dimensional dynamics at higher realistic Prandtl numbers, and suggesting that HC may be more common in subglacial lakes than previously predicted. For large values of $\varLambda$ , we observe that the warmest part of the top plate has approximately the same temperature as the bottom plate, such that 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.