Asymmetries of the oceanic thermohaline circulation and meridional heat transport

Recent studies have indicated that oceanic meridional heat transport is a major component of the global heat budget. The meridional heat transport patterns of the ocean are asymmetric; involving northward transport throughout the Atlantic and stronger poleward transport in the South Pacific than in the North Pacific. This study has examined relationships between these asymmetries and asymmetric thermohaline circulations using simple-geometry ocean general circulation models (OGCM's). It has been found that oceanic meridional heat transports are associated largely with thermohaline overturning involving high-latitude sinking and upwelling at mid and low latitudes. The results indicate that the observed heat transport asymmetry is associated with the spread of North Atalantic Deep Water (NADW) throughout the World Ocean and, therefore, with the asymmetric distribution of deep water formation regions. A relatively minor association between the asymmetry of Atlantic heat transports and circumpolar continuity in the Southern Hemisphere has also been identified. Experiments were also performed to assess the influence of high-latitude surface freshwater fluxes on deep water formation and meridional heat transports. However, this influence was not adequately tested as a feedback between surface salinities and thermohaline overturning, which occurs in OGCM's having flux surface salinity boundary conditions (Bryan (1986)), sustained the initial circulations of the experiments. Differences between flux surface salinity boundary conditions and linear restoring conditions, which couple surface salinities to imposed salinity fields, have been examined. Linear restoring conditions suppress the feedback between surface salinities and thermohaline overturning, but can result in unrealistic surface freshwater flux distributions. These factors should be considered when selecting surface salinity boundary conditions for OGCM studies. (DX86645)