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