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

TL;DR

This paper investigates the feedback mechanisms behind AMOC collapse in a complex climate model, emphasizing the salt-advection feedback and its implications for climate change risk assessments.

Contribution

It provides a detailed mechanistic analysis of AMOC stability and collapse in the CESM, highlighting the role of freshwater transport and feedbacks, especially the salt-advection feedback.

Findings

Salt-advection feedback is the dominant destabilizing mechanism.

Modern climate models tend to overestimate AMOC stability due to positive $F_{ovS}$ bias.

AMOC stability can be effectively indicated by the sign of $F_{ovS}$ at 34°S.

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 destabilising salt-advection feedback, which is quantified through a negative sign of the overturning freshwater transport at 34$^{\circ}$S, indicated by $F_{\mathrm{ovS}}$. Other feedbacks are related to changes in North Atlantic sea-ice melt (destabilising), ocean-atmosphere freshwater fluxes (destabilising) and gyre transports (stabilising). Our study clarifies the role of $F_{\mathrm{ovS}}$ as an indicator of the background state stability of the AMOC. As many modern climate models have a positive $F_{\mathrm{ovS}}$ bias this implies that their AMOC is too stable which leads to an underestimation of the risk of an AMOC collapse under climate change.