Rapid transitions in the Atlantic thermohaline circulation triggered by global warming and meltwater during the last deglaciation



In a series of sensitivity experiments, using a three-dimensional ocean general circulation model, rapid climate shifts during the last deglaciation are interpreted in terms of gradual temperature changes and freshwater perturbations, which impact on the Atlantic thermohaline circulation (THC). We show that increasing global temperature leads to a rapid intensification of the THC. The transition to an interglacial THC mode is preconditioned by a decrease of the subsurface temperatures due to an increase in ventilation of the subsurface water in the northern North Atlantic, which enhances the merdional transport of salt to the northern high latitudes and gradually erodes the halocline. This process enables the remaining temperature inversion to overcome the salinity stratification in the northern North Atlantic, which causes a kick start of vigorous convection and a rapid intensification of the Atlantic THC. As a result of the abrupt THC amplification and the deglacial warming and sea ice retreat in the Southern Ocean, enhanced transport of relatively salty surface and intermediate-depth waters from the Indian Ocean provides an additional source of salt to the North Atlantic, which changes the stability behavior of the THC with respect to freshwater perturbations. A warming-induced transition from a weak glacial THC to a stronger THC state, with different North Atlantic freshwater hysteresis characteristics, provides a concept that might explain the sequence of events, characterizing the last glacial termination as recorded in proxy data.