Impact of North Atlantic – GIN Sea exchange on deglaciation evolution of Atlantic Meridional Overturning Circulation

The Bølling-Allerød (BA) warming is the most pronounced abrupt climate change event during the last deglaciation. Two notable features of the BA onset are found in our transient simulation of the last deglaciation with CCSM3: the first is the occurrence of an overshoot in the Atlantic Meridional Overturning Circulation (AMOC, about 20 Sv as to 13 Sv at Last Glacial Maximum) and the second is the subsequent transition of AMOC from a glacial (about 13 Sv) to an interglacial mean state (about 18 Sv). Here, we present two new sensitivity experiments to explicitly illustrate the impact of North Atlantic – GIN Sea exchange on the deglaciation evolution of the AMOC. In these sensitivity experiments, the oceanic exchange during the BA onset is inhibited by introducing a Partial Blocking scheme. In response to this, the deep-water formation in the GIN Sea is reduced by 80% compared to the transient simulation. This in turn results in a reduced AMOC overshoot followed by a lower mean state of the AMOC. Our results therefore suggest that, oceanic processes were more important than the external forcings and atmospheric processes for the AMOC evolution during the BA onset.