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Abstract

The future trajectory of the Atlantic Meridional Overturning Circulation (AMOC) and its past and future variability on timescales of centuries to millennia remain a subject of debate. For instance, the mechanism setting up the oscillatory behaviour of the climate system during Marine Isotope Stage 3 (MIS3; about 59.4 - 27.8 thousand of years before present), a period characterized by pronounced Dansgaard-Oeschger variability, has not been disentangled so far.In order to shed more light on millennial-scale variability from a modelling perspective, we have conducted a set of experiments with the Comprehensive Earth System Model v1.2 (CESM1.2). These simulations have been specifically designed with the aim to explore the parameter space of the AMOC with respect to greenhouse gas concentrations, continental ice-sheet topography and orbital parameters representative for the Last Glacial Maximum (LGM) and Marine Isotope Stage 3. Our experiments reveal an increase in mean AMOC strength in response to higher greenhouse-gas concentrations. Furthermore, the relative role of the various boundary conditions in shaping a possible oscillatory behaviour of the AMOC is investigated. Under full LGM conditions, unforced multi-centennial AMOC variations on the order of about 5 Sv are simulated. With the intermediate MIS3 ice-sheet topography, however, the model exhibits well-pronounced millennial-scale AMOC fluctuations of more than 12 Sv at a recurrence time of ~1500 years and reproduces the range of Dansgaard-Oeschger-type surface-temperature fluctuations of more than 10°C recorded in Greenland ice cores. The high temporal covariation between Greenland temperature and AMOC strength in our model setup further mimics MIS3 stadial-interstadial transitions.