Feedback Processes Causing an AMOC Collapse in the Community Earth System Model

Abstract

The Atlantic meridional overturning circulation (AMOC) is recognized as a tipping element within the global climate system. Central to its tipping behavior is the salt-advection feedback mechanism, which has been extensively studied in box models and models of intermediate complexity. However, in contemporary, highly complex climate models, the importance and functioning of this feedback mechanism is less clear due to the intricate interplay of numerous ocean–atmosphere–sea ice feedbacks. In this study, we conduct a detailed mechanistic analysis of an AMOC collapse under quasi-equilibrium forcing conditions using the Community Earth System Model (CESM). By reconstructing the AMOC strength from the meridional density contrast across the Atlantic Ocean, we demonstrate that AMOC stability can be related to the Atlantic freshwater budget, revealing several important feedbacks. The dominant contribution is the destabilizing salt-advection feedback, which intensifies as the overturning freshwater transport at 348S, denoted by FovS, becomes more negative. Other feedbacks are related to changes in North Atlantic sea ice melt (destabilizing), ocean–atmosphere freshwater fluxes (destabilizing), and gyre transports (stabilizing). Our study clarifies the role of FovS as an indicator of the background state stability of the AMOC. As many modern climate models have a too positive FovS bias, this implies that their AMOC is too stable which leads to an underestimation of the risk of an AMOC collapse under climate change.