Transitions between turbulent states in rotating Rayleigh-Bénard convection

Weakly rotating turbulent Rayleigh-Bénard convection was studied experimentally and numerically. With increasing rotation and large enough Rayleigh number a supercritical bifurcation from a turbulent state with nearly rotation-independent heat transport to another with enhanced heat transfer is observed at a critical inverse Rossby number 1/Ro_c≃0.4. The strength of the large-scale convection roll is either enhanced or essentially unmodified depending on parameters for 1/Ro < 1/Ro_c, but the strength increasingly diminishes beyond 1/Ro_c where it competes with Ekman vortices that cause vertical fluid transport and thus heat-transfer enhancement.