Physical characterization of the boundary separating safe and unsafe AMOC overshoot behaviour

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is an important tipping element within the climate system as it may collapse due to a changing surface buoyancy forcing. Under scenarios of future greenhouse gas emission reductions, it has been suggested that the AMOC may undergo a safe overshoot. However, this was based on a rather conceptual model limiting the physical characterization of the boundary between safe and unsafe AMOC overshoot behaviour. Here, using a fully-implicit global ocean model, we investigate the AMOC overshoot behaviour under different piecewise linear transient freshwater forcing scenarios. We clarify the physics of the collapse and recovery behaviour of the AMOC and show that the potential for a safe overshoot is tightly linked to a delicate balance of salt fluxes in the North Atlantic. More specifically, the sign of the time derivative of the integrated salt content in the northern North Atlantic is identified as an adequate indicator of the type of AMOC overshoot behaviour. The insights gained are relevant to inform climate policy strategies regarding emission reductions, highlighting the necessity for thoughtful scenarios to prevent an AMOC collapse.