Southern Ocean origin for the resumption of Atlantic thermohaline circulation during deglaciation

During the two most recent deglaciations, the Southern Hemisphere warmed before Greenland1,2. At the same time, the northern Atlantic Ocean was exposed to meltwater discharge3, which is generally assumed to reduce the formation of North Atlantic Deep Water4,5. Yet during deglaciation, the Atlantic thermohaline circulation became more vigorous, in the transition from a weak glacial to a strong interglacial mode6. Here we use a three-dimensional ocean circulation model7 to investigate the impact of Southern Ocean warming and the associated sea-ice retreat8 on the Atlantic thermohaline circulation. We find that a gradual warming in the Southern Ocean during deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation, triggered by increased mass transport into the Atlantic Ocean via the warm (Indian Ocean) and cold (Pacific Ocean) water route9,10. This effect prevails over the influence of meltwater discharge, which would oppose a strengthening of the thermohaline circulation. A Southern Ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of Southern and Northern hemispheric climate change at times of deglaciation, in agreement with the available proxy records.