Penetrative convection in a horizontal layer of seawater near its freezing point

Abstract

A series of numerical experiments are performed to investigate steady thermosolutal convection induced by the partial freezing of a horizontal pool of seawater from the top. This study is aimed at achieving a better understanding of the role of convection in transporting oceanic heat into the atmosphere during the formation of a polynya. The mathematical model considered accounts for the density anomaly of seawater and for the dependence of the freezing point on salinity. A new solutal regime is isolated wherein solutal convection is present even though the solutal expansion coefficient is set to zero. This convection process arises as a result of the coupling between the fluctuations of temperature and salinity at the ice–water interface. A parametric study is undertaken as function of the main dimensionless groups characterizing the problem, namely the thermal Rayleigh number, the density ratio and an extremum parameter which quantifies the effect of the nonlinear equation of state. Several indicators of the convection process, such as the Nusselt and Sherwood numbers, the kinetic energy in the fluid layer and the buoyancy are calculated. It is found that a two-layer convection regime generally predominates for relatively large Rayleigh numbers. These results imply that the penetrative convection which results from the coupling between the nonlinear equation of state and the freezing process acts in such a way as to delay the release of heat to the atmosphere during the formation of a polynya.