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

Abstract

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 34°S, denoted by F ov S , 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 F ov S as an indicator of the background state stability of the AMOC. As many modern climate models have a too positive F ov S bias, this implies that their AMOC is too stable which leads to an underestimation of the risk of an AMOC collapse under climate change. Significance Statement The Atlantic meridional overturning circulation (AMOC) is a key component and a tipping element in the climate system. Recent climate model simulations demonstrated that the AMOC can tip under the input of freshwater in the Atlantic Ocean. A potential AMOC tipping event has severe climate and societal impacts, and it is, therefore, important to understand the feedback mechanisms that cause the AMOC tipping event. This study identifies and quantifies these feedback mechanisms and their effects on AMOC stability. Our findings show that the dominant salt-advection feedback is responsible for AMOC tipping and is only activated when the AMOC carries net salinity into the Atlantic Ocean. This finding establishes a physical connection between the Atlantic salt transport and the salt-advection feedback. Understanding this connection is important for assessing the risk of AMOC tipping within the twenty-first century.

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