The role of preconditioning in the evolution of open-ocean deep convection

Large-eddy simulation of open-ocean deep convection shows that the evolution of convection appears in a fundamentally different pattern, depending on the precondition of the ocean and the magnitude of the surface buoyancy flux. As the intensity of the cyclonic gyre in the ocean under the cooling increases, the pattern of convection is transformed from "distributed convection'' to "localized convection.'' In localized convection the typical pattern of open-ocean deep convection appears, such as the generation of baroclinic instability and large lateral buoyancy transfer, secondary circulation, restratification, and the breakup of the original cyclonic gyre. On the other hand, in distributed convection small-scale convective plumes appear uniformly over the whole surface similarly to the convective boundary layer without generating the typical features of open-ocean deep convection. Hence, an enormous difference in the generated eddy kinetic energies exists between the two cases. It is also found that a stronger cooling at the sea surface suppresses the transition from distributed convection to localized convection. Dimensional analysis provides the parameters to characterize the pattern of convection, and the critical condition for the transition is estimated by analyzing the numerical results.