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Transient and Equilibrium Responses of the Atlantic Overturning Circulation to Warming in Coupled Climate Models: The Role of Temperature and Salinity

Bonan, David B. and Thompson, Andrew F. and Newsom, Emily R. and Sun, Shantong and Rugenstein, Maria (2022) Transient and Equilibrium Responses of the Atlantic Overturning Circulation to Warming in Coupled Climate Models: The Role of Temperature and Salinity. Journal of Climate, 35 (15). pp. 5173-5193. ISSN 0894-8755. doi:10.1175/jcli-d-21-0912.1.

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The long-term response of the Atlantic meridional overturning circulation (AMOC) to climate change remains poorly understood, in part due to the computational expense associated with running atmosphere–ocean general circulation models (GCMs) to equilibrium. Here, we use a collection of millennial-length GCM simulations to examine the transient and equilibrium responses of the AMOC to an abrupt quadrupling of atmospheric carbon dioxide. We find that GCMs consistently simulate an AMOC weakening during the first century but exhibit diverse behaviors over longer time scales, showing different recovery levels. To explain the AMOC behavior, we use a thermal-wind expression, which links the overturning circulation to the meridional density difference between deep-water formation regions and the Atlantic basin. Using this expression, we attribute the evolution of the AMOC on different time scales to changes in temperature and salinity in distinct regions. The initial AMOC shoaling and weakening occurs on centennial time scales and is attributed to a warming of the deep-water formation region. A partial recovery of the AMOC occurs over the next few centuries, and is linked to a simultaneous warming of the Atlantic basin and a positive high-latitude salinity anomaly. The latter reduces the subsurface stratification and reinvigorates deep-water formation. GCMs that exhibit a prolonged AMOC weakening tend to have smaller high-latitude salinity anomalies and increased Arctic sea ice loss. After multiple millennia, the AMOC in some GCMs is stronger than the initial state due to warming of the low-latitude Atlantic. These results highlight the importance of considering high-latitude freshwater changes when examining the past and future evolution of the AMOC evolution on long time scales.

Item Type:Article
Related URLs:
URLURL TypeDescription ItemLongRunMIP project, including data ItemCode
Bonan, David B.0000-0003-3867-6009
Thompson, Andrew F.0000-0003-0322-4811
Newsom, Emily R.0000-0002-3436-3833
Sun, Shantong0000-0002-6932-5589
Rugenstein, Maria0000-0002-4541-3277
Additional Information:© 2022 American Meteorological Society. (Manuscript received 29 November 2021, in final form 6 April 2022) The research would not have been possible without the efforts of the contributors to the LongRunMIP project, which is freely available at The authors thank Sarah Ragen for helpful comments on an earlier version of this paper. D.B.B. was supported by an American Meteorological Society (AMS) Graduate Fellowship and the National Science Foundation Graduate Research Fellowship Program (NSF Grant DGE-1745301). A.F.T. and S.S. were supported by NSF Grant OCE-1756956 and OCE-2023259. E.R.N was supported by NERC Project NE/P019218/1 on transient tracer-based Investigation of Circulation and Thermal Ocean Change (TICTOC). Data availability statement. The code for this study is available at The data for this study are freely available at
Funding AgencyGrant Number
American Meteorological SocietyUNSPECIFIED
NSF Graduate Research FellowshipDGE-1745301
Natural Environment Research Council (NERC)NE/P019218/1
Subject Keywords:Ocean; Atmosphere-ocean interaction; Meridional overturning circulation; Ocean circulation; Thermocline circulation; Climate change; Climate models; General circulation models
Issue or Number:15
Record Number:CaltechAUTHORS:20220804-250049000
Persistent URL:
Official Citation:Bonan, D. B., Thompson, A. F., Newsom, E. R., Sun, S., & Rugenstein, M. (2022). Transient and Equilibrium Responses of the Atlantic Overturning Circulation to Warming in Coupled Climate Models: The Role of Temperature and Salinity, Journal of Climate, 35(15), 5173-5193. Retrieved Aug 4, 2022, from
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:116124
Deposited By: George Porter
Deposited On:09 Aug 2022 14:43
Last Modified:09 Aug 2022 14:43

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