On the role of sea ice and convection in global ocean model

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 adjust&not

ment' 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, we approached in two ways. First, we 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, we introduced more physically based subgrid-scale convection in the SO by applying the penetrative plume convection scheme of Paluszkiewicz and Romea (1997). 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. Fur&not

thermore, 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 different treatment of ocnvection leads to some noticeable differences in some of the sensitivity experiments. For example, in an experiment where sea ice salinity is assumed to be that of upper ocean, thus neglecting the primary effect of sea ice formation and melting on the ocean's buoyancy forcing, Antarctic Bottom Water comes essentially to a halt, the global deep-ocean properties and thermohaline circulation thus being almost soley determined by the North Atlantic Deep Water.