Impact of Subgrid-Scale Convection on Global Thermohaline Properties and Circulation

Abstract

In most ocean general circulation models the simulated global-scale deep-ocean thermohaline properties appear to be chronically colder and fresher than observed. To some extent, this discrepancy has been known to be due to excessive open-ocean deep convection in the Southern Ocean (SO) caused by crude ?convective adjustment? parameterizations on scales typically two orders of magnitude larger than the actual convection scale. To suppress the strength of open-ocean convection and to thereby eventually improve the global deep-ocean water properties, the authors first reduced convection in the SO in an ad hoc manner by activating it every 10 days rather than every model time step (20 hours). Second, a more physically based subgrid-scale convection in the SO was introduced by applying the penetrative plume convection scheme of Paluszkiewicz and Romea. With both treatments, SO convection decreased by about 30%, and the globally averaged deep-ocean potential temperature and salinity increased substantially to within 0.2°C and 0.02 psu of observed estimates. Furthermore, the plume convection scheme led to more realistic vertical temperature and salinity sections with more distinct Circumpolar Deep Water extension toward the south and a significant improvement of SO sea ice in terms of its thickness and its seasonality. The results of this study confirm that in order to obtain more realistic deep-ocean properties, open-ocean convection in the SO must be substantially weakened and shallower. This can be achieved by adopting a more physical plume convection scheme.