On the Role of Sea Ice and Convection in a Global Ocean Model

Abstract

An earlier estimate regarding the possible impact of sea ice on deep-ocean water mass properties and the global thermohaline circulation in a coupled sea ice?ocean general circulation model (OGCM) is updated. Compared to the earlier application, the main upgrade is a subgrid-scale plume-convection parameterization that replaces conventional grid-cell-wide convective adjustment. The different treatment of convection leads to some noticeable differences in some of the repeated sensitivity experiments. For example, in an experiment where sea ice salinity is assumed to be that of the upper ocean, thus neglecting the primary effect of sea ice formation and melting on the ocean's buoyancy forcing, Antarctic Bottom Water formation comes essentially to a halt, the global deep-ocean properties and thermohaline circulation thus being almost solely determined by North Atlantic Deep Water. The much weaker impact in the earlier estimate turns out to be mainly due to excessive open-ocean convection in the Southern Ocean, rendering that region susceptible to the open-ocean polynya mode. The associated melting in such polynyas leads to an enhancement of convection in a salty sea ice experiment, thus obscuring the effect of neglected brine release in coastal polynyas. Besides underscoring the necessity of a careful treatment of sea ice and convection in the Southern Ocean of a global OGCM, this study indicates that new-ice formation around Antarctica has a much larger effect on global deep-ocean properties and circulation than previously estimated.