Ice sheet - ocean interactions in simulations of abrupt glacial climate change

Analysis of deep cores from the Greenland ice sheet revealed that the North Atlantic climate was much more variable during the last glacial period than during the present interglacial. Rapid, large-amplitude transitions between cold (stadial) and warm (interstadial) states occured repeatedly on millennial timescales. It is widely believed that mode changes of the Atlantic meridional overturning (AMO) circulation were involved in these oscillations. The reason for the different variability between glacial and interglacial times, however, has remained enigmatic and the forcing mechanisms for the mode changes are still unknown. Here, using a coupled model of the ocean-atmosphere-cryosphere system, we show that the stability of the AMO is substantially reduced for a glacial background climate. With the inclusion of an interactively coupled dynamical land ice model we demonstrate that reduced calving of ice bergs into the North Atlantic, following a massive surge of northern hemisphere ice sheets, constitutes a trigger for an abrupt switch to a warmer climate state. Moreover, we identify a feedback mechanism between the AMO and the mass balance of the adjacent continental ice sheets that destabilises the warm circulation mode, consistent with the proxy record.