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Abstract

The Atlantic Meridional Overturning Circulation (AMOC) plays a crucial role in shaping climate conditions over the North Atlantic region and beyond and its future stability is a matter of concern. While the stability of the AMOC to surface freshwater forcing (FWF) has been investigated in numerous model simulations, its equilibrium response to changing CO2 remains largely unexplored and precludes a comprehensive understanding of AMOC stability under ongoing global warming. Here we use a fast Earth system model to explore the stability of the AMOC to combined changes in FWF between -0.25 and +0.25 Sv in the North Atlantic and atmospheric CO2 concentrations between 180 and 560 ppm. We find four different AMOC states associated with qualitatively different convection patterns in the North Atlantic. Apart from an Off AMOC state and a Modern-like AMOC with deep water forming in the Labrador and Nordic Seas, we find a Weak AMOC state with convection occurring south of 55° N and a Strong AMOC state characterized by deep water formation extending into the Arctic. Several of these AMOC states can be stable under the same boundary conditions for specific combinations of CO2 and FWF. Generally the model shows an increase in equilibrium AMOC strength for higher CO2 levels. It is also noteworthy that, while under preindustrial conditions the AMOC off state is not stable in the model, it becomes stable for CO2 concentrations above ~400 ppm, suggesting that an AMOC shutdown in a warmer climate might be irreversible.