Generalized stability landscape of the Atlantic Meridional Overturning Circulation

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.