What Sets the AMOC's Sensitivity to CO₂ Forcing? The Role of Background Mean‐State and Density Balance
Abstract
The future evolution of the Atlantic Meridional Overturning Circulation (AMOC) under anthropogenic forcing remains highly uncertain, partly because the role of the background ocean state in conditioning AMOC sensitivity is not fully understood. Using CMIP6 simulations under abrupt‐4 × CO₂ and SSP‐585 forcing, we investigate how mean‐state thermal and haline density compensation in the western subpolar North Atlantic influences AMOC decline. Models are classified into thermallydominated, salinity‐dominated, and balanced‐compensation regimes based on their subsurface density structure in the western subpolar North Atlantic. Models farther from balanced thermal–haline compensation experience substantially larger AMOC weakening, associated with enhanced upper‐ocean stratification and reduced deep convection. Salinity‐dominated models tend to have the strongest initial AMOC and weakest stratification in the western subpolar North Atlantic, but have the largest absolute and percentage forced AMOC decline. Balanced‐compensation models retain persistent convection in the Nordic Seas and exhibit the smallest AMOC decline, whereas salinity‐dominated models undergo rapid stratification‐driven weakening linked to surface freshening and convection collapse. Thermally‐dominated models have the weakest initial AMOC state and weaken more gradually due to residual haline compensation at depth, but still represent a large percentage AMOC decline. While background AMOC strength explains much of the inter‐model spread in absolute AMOC decline, including thermal and haline density compensation consistently improves the explanation of inter‐model differences across both forcing scenarios. Together, these results demonstrate that AMOC sensitivity depends not only on its initial strength, but also on the underlying thermohaline structure of the subpolar North Atlantic.
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BibTeX
@article{ferster2026what,
title = {{What Sets the AMOC's Sensitivity to CO₂ Forcing? The Role of Background Mean‐State and Density Balance}},
author = {Brady Ferster and Rym Msadek and Jon Robson and Laurent Terray},
journal = {Journal of Geophysical Research. Oceans},
year = {2026},
month = nov,
doi = {10.1029/2025jc023947},
url = {https://hal.science/hal-05856816v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - What Sets the AMOC's Sensitivity to CO₂ Forcing? The Role of Background Mean‐State and Density Balance
AU - Brady Ferster
AU - Rym Msadek
AU - Jon Robson
AU - Laurent Terray
PY - 2026
DA - 2026/11/08
JO - Journal of Geophysical Research. Oceans
DO - 10.1029/2025jc023947
UR - https://hal.science/hal-05856816v1
AB - The future evolution of the Atlantic Meridional Overturning Circulation (AMOC) under anthropogenic forcing remains highly uncertain, partly because the role of the background ocean state in conditioning AMOC sensitivity is not fully understood. Using CMIP6 simulations under abrupt‐4 × CO₂ and SSP‐585 forcing, we investigate how mean‐state thermal and haline density compensation in the western subpolar North Atlantic influences AMOC decline. Models are classified into thermallydominated, salinity‐dominated, and balanced‐compensation regimes based on their subsurface density structure in the western subpolar North Atlantic. Models farther from balanced thermal–haline compensation experience substantially larger AMOC weakening, associated with enhanced upper‐ocean stratification and reduced deep convection. Salinity‐dominated models tend to have the strongest initial AMOC and weakest stratification in the western subpolar North Atlantic, but have the largest absolute and percentage forced AMOC decline. Balanced‐compensation models retain persistent convection in the Nordic Seas and exhibit the smallest AMOC decline, whereas salinity‐dominated models undergo rapid stratification‐driven weakening linked to surface freshening and convection collapse. Thermally‐dominated models have the weakest initial AMOC state and weaken more gradually due to residual haline compensation at depth, but still represent a large percentage AMOC decline. While background AMOC strength explains much of the inter‐model spread in absolute AMOC decline, including thermal and haline density compensation consistently improves the explanation of inter‐model differences across both forcing scenarios. Together, these results demonstrate that AMOC sensitivity depends not only on its initial strength, but also on the underlying thermohaline structure of the subpolar North Atlantic.
SN - 2169-9275
SN - 2169-9291
ER -