Abstract

<div> <p>The Atlantic Meridional Overturning Circulation (AMOC) has been, and will continue to be, a key factor in the modulation of climate change both locally and globally. However, there remains considerable uncertainty in recent AMOC evolution. Here, we show that the multi-model mean AMOC strengthened by approximately 10% from 1850-1985 in new simulations from the 6th Coupled Model Inter-comparison Project (CMIP6), a larger change than was seen in CMIP5. Across the models, the strength of the AMOC trend up to 1985 is related to a proxy for the strength of the aerosol forcing. Therefore, the multi-model difference is a result of stronger anthropogenic aerosol forcing on average in CMIP6 than CMIP5, which is primarily due to more models including aerosol-cloud interactions. However, observational constraints - including a historical sea surface temperature fingerprint and shortwave radiative forcing in recent decades - suggest that anthropogenic forcing and/or the AMOC response may be overestimated.</p> </div>

Highlights

  • The Atlantic Meridional Overturning Circulation (AMOC) represents the Atlantic branch of the global overturning circulation

  • The multidecadal structure in Coupled Model Intercomparison Project Phase 5 (CMIP5) resembles that of Coupled Model Intercomparison Project 6 (CMIP6), but the strengthening toward the end of the 20th century is far less pronounced

  • In this paper we have shown that the multimodel mean AMOC in CMIP6 increases by ~10% from 1850 to the 1980s, in contrast to the CMIP5 multimodel mean, which shows little change over this time period

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Summary

Introduction

The Atlantic Meridional Overturning Circulation (AMOC) represents the Atlantic branch of the global overturning circulation. Various lines of modeling and paleo‐oceanographic evidence suggest the AMOC may weaken under the effects of greenhouse gases (GHGs) (Gregory et al, 2005; Stommel, 1961; Stouffer et al, 2006; Thornalley et al, 2018; Wood et al, 1999), leading to consistent projections that the AMOC will weaken in the century (Collins et al, 2013). Such a weakening could have severe climate impacts (Jackson et al, 2015). In this study, we revisit the simulation of historical AMOC changes in the new CMIP6 simulations

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