Overturning Pathways Control AMOC Weakening in CMIP6 Models

• Main Authors: Baker, JA, Bell, MJ, Jackson, LC, Renshaw, R, Vallis, GK, Watson, AJ, Wood, RA
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley / American Geophysical Union (AGU) 2023
• Subjects: Pacific; Southern Ocean
• Online Access: http://hdl.handle.net/10871/137371; https://doi.org/10.1029/2023gl103381

This is the final version. Available on open access from Wiley via the DOI in this record Data Availability Statement: The CMIP6 data used in this study is available from ESGF (https://esgf-index1.ceda.ac.uk/search/cmip6-ceda/) with references listed in Supporting Information S1 (Table S1). MOC estimates from ECCOv4, a robust diagnostic simulation and an inverse model were taken from Cessi (2019), Lee et al. (2019), and Lumpkin and Speer (2007) respectively. MOC estimates from GloRanV14 (funded by the E.U. Copernicus Marine Service) are available through Zenodo: https://doi.org/10.5281/zenodo.7649266 (Baker & Renshaw, 2023). Future projections indicate the Atlantic Meridional Overturning Circulation (AMOC) will weaken and shoal in response to global warming, but models disagree widely over the amount of weakening. We analyze projected AMOC weakening in 27 CMIP6 climate models, in terms of changes in three return pathways of the AMOC. The branch of the AMOC that returns through diffusive upwelling in the Indo-Pacific, but does not later upwell in the Southern Ocean (SO), is particularly sensitive to warming, in part, because shallowing of the deep flow prevents it from entering the Indo-Pacific via the SO. The present-day strength of this Indo-Pacific pathway provides a strong constraint on the projected AMOC weakening. However, estimates of this pathway using four observationally based methods imply a wide range of AMOC weakening under the SSP5-8.5 scenario of 29%–61% by 2100. Our results suggest that improved observational constraints on this pathway would substantially reduce uncertainty in 21st century AMOC decline. European Union Horizon 2020 Met Office Hadley Centre Climate Programme