Ocean circulation plays an important role in the global climate and involves length scales ranging from small-scale convection near the poles, meso-scale eddies and boundary currents, and basin-scale gyres. Surface fluxes (buoyancy, wind, etc.) and tides act as primary energy inputs to the circulation while, in an equilibrated ocean, the energy sinks are predominantly turbulent mixing and viscous dissipation. We hypothesize that buoyancy plays a substantial role in the global circulation and in ocean heat transport, turbulent mixing and CO2 uptake. However, much of the fundamental dynamics of convection relevant to the oceans (from small to global scales) is not well understood and is not captured by Global Ocean Models. Here, we examine an idealized model of ocean circulation with flow driven by surface buoyancy in a closed basin under planetary rotation, using both laboratory experiments and numerical simulations. We show that, even in the absence of wind stress, the flow becomes three-dimensional with a turbulent boundary layer, small-scale deep convection, and broad basin-scale gyres. For the first time, numerical simulations that fully resolve convection and turbulence are used to model this circulation and quantify the ocean heat transfer and flow energetics. In recent work, we study the circulation forced by both wind and surface buoyancy forcing in the presence of turbulent convection.