AMOC tipping under Climate Change in the Community Earth System Model

Recent quasi-equilibrium simulations with the Community Earth System Model (CESM) have shown that the Atlantic Meridional Overturning Circulation (AMOC) in a pre-industrial climate is a multi-stable system (van Westen & Dijkstra, 2023). By slowly increasing the surface freshwater forcing strength over the North Atlantic Ocean, the AMOC tips from a northward overturning state (strength of 17 Sv) to a fully  collapsed state (strength of 0 Sv). When reversing the freshwater forcing, the AMOC recovers at  smaller values of this forcing compared to the collapse, giving rise to hysteresis behaviour. Here we analyse AMOC tipping under climate change using the same CESM version. From the hysteresis experiment, we branch off simulations under fixed freshwater forcing values to find the statistical steady states. We follow these states under climate change up to 2100 (historical forcing followed by SSP5-8.5) and then run the simulation into equilibrium under constant year 2100 conditions. We find an AMOC tipping event during the 21st century and we compare this event to the one from the pre-industrial quasi-equilibrium simulation. The rate of AMOC changes and the AMOC-related impacts are comparable to the quasi-equilibrium simulation. However, the initial AMOC weakening and the collapsed AMOC state are very different under climate change. Temperature changes primarily drive the initial AMOC weakening and the collapsed state has a very weak (strength of 1 Sv) and shallow (< 1000 m) northward overturning circulation in the Atlantic Ocean. The results indicate that the strong northward overturning statistical steady states disappear under climate change and that only the collapsed AMOC state exists under a high-end emission scenario.