The nature of the steady buoyancy-driven circulation is investigated using multi-level numerical models. An ocean which extends over the northern and southern hemispheres is forced by cooling in a confined region and heating in the rest of the ocean through the sea surface. As is already known, the circulation and associated thermal structure strongly depend upon the effect of the vertical diffusivity. This nature of the buoyancy-driven circulation is found in the thermodynamic balance. The vertical diffusion plays an essential role in the whole ocean domain. In counterbalancing the vertical diffusion, the horizontal advection at the deepest levels and the vertical advection in the rest of the interior region plays a dominant role. Thus, horizontal transport of cold water from the convective (cooling) to the diffusive (heating) region occurs mainly in the lowest part of the deep water. It is a natural consequence of predominance of the vertical diffusion that the buoyancy-driven circulation has a significant vertical shear well below the thermocline; the Stommel and Arons pattern for the deep circulation tends to be confined in the lower part of the deep water. Details of a set of alternating zonal jets along the equator and associated meridional circulation are obtained and discussed, and dependence on diffusivity and viscosity is also discussed.