The Atlantic Meridional Overturning Circulation’s Response to CO₂ Increase: Assessing the Roles of Surface Flux and Oceanic Advection Feedbacks

Journal Article · Mon May 05 00:00:00 EDT 2025 · Journal of Climate

DOI:https://doi.org/10.1175/jcli-d-24-0143.1· OSTI ID:2563471

Garuba, Oluwayemi A. ^[1]; Liu, Wei ^[2]; Stuivenvolt-Allen, Jacob ^[3]; Balaguru, Karthik ^[1]; Hagos, Samson ^[1]; Wang, Hailong ^[1]

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1. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
2. Univ. of California, Riverside, CA (United States)
3. Yale Univ., New Haven, CT (United States)

The Atlantic meridional overturning circulation (AMOC) is projected to slow down in climate models due to greenhouse gas emissions. However, the physical mechanisms determining the rate of the projected AMOC slowdown remain unclear. Accordingly, this study isolates the roles of oceanic advection and surface flux feedbacks that might accelerate or decelerate the AMOC’s weakening using carbon dioxide (CO₂) quadrupling simulations in the CESM1.2 model. Surface flux feedbacks are isolated in partially coupled experiments in which either all surface flux components or the momentum flux responses to AMOC’s weakening that might provide feedback are suppressed, while a tracer decomposition of ocean density anomalies isolates the advection feedbacks. Comparing the ocean density components in the experiments shows that the AMOC’s response is initially determined by CO₂-induced anomalous surface heat fluxes, and afterward, feedbacks determine its response. In the fully coupled case, surface heat flux feedback strongly promotes AMOC slowdown and causes its near shutdown, while a weaker but active AMOC is maintained when the surface flux feedback is inhibited in the partially coupled case. The positive surface heat flux feedback works by canceling out the negative oceanic heat advection feedback on deep-water formation in the subpolar North Atlantic (SPNA). With the heat advection feedback thus reduced, the positive salinity advection feedback becomes the dominant contributor to SPNA density changes and deep-water formation. In the partially coupled case, negative ocean heat advection feedback and the CO₂-induced subtropical Atlantic saline anomalies imported into the SPNA play a stabilizing role. The results highlight the importance of SPNA salinity gradients and gyre circulation strength in determining the AMOC’s response rate or recovery.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); USDOE Office of Science (SC), Biological and Environmental Research (BER); USDOE Office of Science (SC), Biological and Environmental Research (BER). Earth & Environmental Systems Science (EESS)
• Grant/Contract Number: AC02-05CH11231; AC05-76RL01830
• OSTI ID: 2563471
• Alternate ID(s): OSTI ID: 2566874
• Report Number(s): PNNL-SA--195998
• Journal Information: Journal of Climate, Journal Name: Journal of Climate Journal Issue: 10 Vol. 38; ISSN 0894-8755
• Publisher: American Meteorological Society
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
Atmosphere-ocean interaction
Coupled models
Greenhouse gases
Gyres
Thermohaline circulation
Tracers