Rayleigh-Bénard convection is what happens when you heat a fluid from below: hot fluid rises, cool fluid sinks, and the system organizes into convection cells. Horizontal convection is what happens when you heat the fluid unevenly along a horizontal boundary: fluid flows laterally from warm to cool regions. What happens when both operate simultaneously?
Rein, Llewellyn Smith, and Young (arXiv: 2603.23770) show that horizontal convection can suppress and even reverse Rayleigh-Bénard convection. When horizontal forcing is strong enough, it creates a stable stratification that prevents the vertical overturning that Rayleigh-Bénard requires. The bottom-heated fluid, which should rise, is held in place by the horizontal circulation's density gradient.
Two regimes emerge. In the neutral state, horizontal convection merely offsets Rayleigh-Bénard — the two are in balance. In the strong stratification regime, horizontal convection dominates entirely, producing a stably stratified layer where none should exist based on the vertical heating alone. The top boundary layer controls everything — its structure determines the mean stratification of the interior.
The through-claim: two convective drives can cancel rather than add. Naively, more heating should mean more convection. But heating from the side and heating from below impose competing density gradients. The horizontal gradient creates stratification; the vertical gradient tries to destroy it. When the horizontal drive wins, the result is less convection than either drive would produce alone. This applies to subglacial lakes, extraterrestrial oceans — any system with competing thermal boundary conditions.
Rein, Llewellyn Smith & Young, 2603.23770. Fluid dynamics / convection / geophysics / thermal stratification.