Stability of the Atlantic overturning circulation: Competition between Bering Strait freshwater flux and Agulhas heat and salt sources

The role played by interocean fluxes of buoyancy in stabilizing the present -day overturning circulation of the Atlantic Ocean is examined. A 2D model of the Atlantic overturning circulation is used, in which the interocean fl uxes of heat and salt (via the Bering Strait, Drake Passage, and Agulhas Le akage) are represented by sources and sinks. The profiles and amplitudes of these sources are based mainly on the heat and salt fluxes in a high-resol ution ocean model (OCCAM). When applying realistic sources and sinks, a circulation is favored that is characterized by major downwelling in the Northern Hemisphere (northern si nking pole to pole circulation, NPP), and resembles the present-day Atlanti c overturning circulation. The Southern Ocean sources appear to stabilize t his circulation, whereas Bering Strait freshwater input tends to destabiliz e it. Already a small buoyancy input at southerly latitudes is enough to pr ohibit the existence of a southern sinking circulation (SPP), leaving the N PP circulation as a unique and stable solution. A large, factor 3 increase in Bering Strait freshwater import would be necessary to bring the SPP circ ulation back into existence. Especially the Indian-Atlantic transfer of heat and salt, brought about by Agulhas Leakage, contributes considerably to the strength and, in particula r, the stability of the northern sinking circulation. According to this mod el, shutting off the Agulhas Leakage, and consequently the so-called warm w ater route for North Atlantic Deep Water (NADW) compensation, leads to a re duction of the overturning strength by 10% at most. These results imply tha t the way in which the NADW renewal takes place has implications for both t he strength and stability of the Atlantic overturning circulation, giving t he discussion about the warm versus cold water route for NADW compensation dynamical significance. Moreover, when the stabilizing effect of the Agulhas Leakage on the overtur ning disappears, the destabilizing influence of the Bering Strait freshwate r input becomes more effective. The system is then close to a regime where the northern and southern overturning circulations coexist as stable soluti ons. Perturbations in Bering Strait inflow may then easily lead to switches between the two circulation states. These results suggest that the absence of the Agulhas Leakage during the last ice age may have contributed to wea kening of the glacial overturning circulation in the Atlantic. It may have made the thermohaline circulation vulnerable to variability caused either b y regime switches, or by the excitation of oscillatory modes. The sudden re start of the Atlantic overturning circulation at the beginning of the Holoc ene may well have been stimulated by the coincident reopening of the Agulha s Gap. Presence of the Agulhas Leakage may contribute to the relative stabi lity of Holocene climate. Present-day climate may thus be more stable than previously thought.