The influence of high‐latitude surface forcing on the global thermohaline circulation

The basin‐averaged, latitude‐depth ocean model of Wright and Stocker (1992) is used to simulate the deep circulation of the world ocean. Under present‐day surface forcing, sinking occurs in the North Atlantic and the southern ocean, and realistic temperature and salinity structures are obtained in the Atlantic, Pacific, and Indian oceans. “Color” tracers and radiocarbon are used to identify the composition of the deepwater masses and the associated renewal time scales. While broad agreement with observations is found in all basins, the water masses in the southern ocean are too young. The global thermohaline circulation and the composition of the deepwater masses are sensitive to the buoyancy contrast between the southern ocean and the North Atlantic. This contrast can be modified by changing relaxation values of temperature and salinity at the northern and southern high latitudes. If the model is forced with the zonal averages of the observed surface salinity, North Atlantic Deep Water is the dominant deep ocean water mass, and hardly any Antarctic Bottom Water flows into the Atlantic. Choosing instead the observed salinities of the newly formed deep water as the restoring values, the model realistically simulates the penetration of Antarctic Bottom Water into the different ocean basins. This has a global effect through reducing both strength and depth of North Atlantic Deep Water formation. If higher surface salinity values are applied in the southern ocean, a steady state is obtained whose tracer distributions and overturning are consistent with reconstructions of the deep circulation during the last glacial maximum. The two states are stable also under mixed boundary conditions and transitions are possible by smoothly varying the surface freshwater flux of one state to that of the other. These experiments suggest the importance of modified high‐latitude forcing in glacial‐to‐interglacial transitions.