Abstract
Models project weakened Atlantic meridional overturning circulation (AMOC) under increasing CO2, often attributed to increased ocean stratification caused by surface warming and freshening in the subpolar North Atlantic and by sea-ice melting. Here I analyze novel coupled model simulations with and without large Arctic amplification (AA)—the enhanced Arctic warming relative to the rest of the world under increasing CO2—to show that AA largely explains AMOC’s weakening as CO2 increases to above 570 ppm, due to increased runoff from Greenland and large evaporation decreases caused by reduced ocean-air temperature gradients from the Labrador Sea to the Nordic Seas. Together with a small role from ocean warming, the freshening reduces surface salinity, ocean density and mixed-layer depth, weakening deep water formation and AMOC. AMOC weakens only slightly under increasing CO2 when AA is small due to less runoff from Greenland and smaller evaporation decreases. The results suggest a dominant role of Arctic and subpolar surface forcing over ocean dynamics in controlling AMOC’s strength and that enhanced Arctic warming, rather than changes at lower latitudes, will largely determine future AMOC’s fate under large CO2 increases.
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Availability of data and material
The model data used in this study is available from the author upon request.
Code availability
The CESM1 model code is available from https://www.cesm.ucar.edu/models/cesm1.2/.
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Acknowledgements
I thank Dr. Mirong Song for her assistance in making the FixeIce run. Dai was supported by NSF (Grants AGS-2015780 and OISE-1743738).
Funding
This work was funded by University at Albany of SUNY, NSF (Grants AGS-2015780 and OISE-1743738).
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A Dai did all the work for this study.
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Dai, A. Arctic amplification is the main cause of the Atlantic meridional overturning circulation weakening under large CO2 increases. Clim Dyn 58, 3243–3259 (2022). https://doi.org/10.1007/s00382-021-06096-x
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DOI: https://doi.org/10.1007/s00382-021-06096-x