The impact of CO2 and ice sheet changes on the deglacial AMOC sensitivity to freshwater perturbations in three different Earth System Models

Abstract

During deglaciation disintegration of large-scale continental ice sheets represents a continuous threat to reduce the strength of the Atlantic meridional overturning circulation (AMOC) via meltwater perturbations to the northern high latitudes. Nevertheless, an abrupt AMOC recovery is detected half-way through the last deglaciation and  a growing number of studies using Earth System Models (ESMs) of varying complexity have shown that atmospheric CO2 concentrations and ice sheet volume can influence the operational mode of the AMOC, eventually including the coexistence of multiple states and associated threshold behavior for intermediate climate states between full glacial (e.g. Last Glacial Maximum, LGM) and full interglacial (e.g. pre-industrial, PD)  conditions. In this study we present results from coordinated sensitivity experiments conducted as part of the German climate modeling initiative (PalMod), using three complex ESMs (AWI-ESM, CESM and MPI-ESM). Besides differences in the impact of CO2 and ice volume changes, we also investigate how variations in these boundary conditions control the AMOC sensitivity to deglacial meltwater injections in the North Atlantic. We find that the AMOC strength responds to ice sheet and/or CO2 changes in all models, with partly opposing effects.  A similar AMOC strength for PD and LGM conditions is detected in AWI-ESM and MPI-ESM, while CESM shows a weaker LGM AMOC. This weaker LGM state is also characterized by a relatively pronounced AMOC sensitivity to freshwater perturbations. Our inter-comparison experiments suggest that this specific behavior in CESM can be detected for atmospheric concentrations between LGM and intermediate levels of ~220 ppm. This further corroborates in particular the impact of CO2 changes to modulate the trajectory of deglacial climate changes by an alteration of the AMOC susceptibility to meltwater injections as recently suggested (Sun et al., Glob. Planet. Change, 2021; Barker & Knorr, Nat. Commun., 2021).