the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 May 2026
Multidecadal North Atlantic Circulation Shifts under Historical Anthropogenic Forcing in CESM2-LE
Abstract. The North Atlantic subpolar gyre is a critical region for global climate, yet the mechanisms driving its multidecadal circulation variability under anthropogenic forcing remain poorly understood. This study investigates the physical processes underlying large multidecadal shifts in density overturning strength at 55°N using a 100-member ensemble of the Community Earth System Model version 2 (CESM2). Using change point and composite analyses, we identify three distinct categories of circulation shifts: strengthening events, and two types of weakening events separated by a 1985 regime shift. Strengthening shifts are driven by an internal positive feedback where enhanced surface heat loss triggers deep convection and strengthened horizontal gyre circulation, subsequently increasing northward heat and salt transport that sustains the anomaly. Weakening shifts before 1985 are primarily driven by internal density-gradient adjustments between the subpolar and subtropical gyres. In contrast, post-1985 weakening events are characterized by basin-wide thermodynamic changes, where greenhouse gas-induced warming and reduced surface buoyancy loss suppress convection across the Irminger Sea and eastern subpolar North Atlantic. Our results reveal that these shifts are non-linear and asymmetric, reflecting a transition from a salinity-dominated internal variability regime to a forced, temperature-driven regime. These findings suggest that the North Atlantic circulation is undergoing a fundamental change in its governing dynamics, highlighting the increasing influence of anthropogenic forcing on the stability of the Atlantic Meridional Overturning Circulation.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2588', Anonymous Referee #1, 24 Jun 2026
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RC2: 'Comment on egusphere-2026-2588', Anonymous Referee #2, 13 Jul 2026
Review of “Multidecadal North Atlantic Circulation Shifts under Historical Anthropogenic Forcing in CESM2-LE” by Ina Nagler, Helene Asbjørnsen, and Andreas Born.
This study investigates the processes associated with multidecadal shifts toward strengthened and weakened AMOC phases using a 100-member ensemble from CESM2 simulations. Based on composites of hydrographic properties, circulation, and air–sea heat flux changes, the authors identify one class of strengthening events and two classes of weakening events. The interpretation of strengthening events—positive feedback between surface density and overturning, triggered by initial surface heat loss—is plausible. However, I am not so convinced by some arguments. Overall, the analysis is logically organized and offers useful insights into mechanisms driving multidecadal AMOC variability. Several conclusions would benefit from stronger supporting evidence and clearer explanations. My main comments are below.
General comments
Methodology
1) Fig. 2b: Circulation-shift occurrences are labeled every 5 years. How are “years” defined? Where does the 20-year mean window start, and why are labels shown every 5 years?
2) Equations (1)–(3): Several points need clarification:
- Eq. (1): The upper density limit appears to correspond to the densest class. Please either adjust the sign convention or clarify that this is the density at which the streamfunction reaches its maximum.
- Eq. (2): The overturning streamfunction depth range appears to be integrated from bottom to surface. Is this intentional or a typo?
- Eq. (3): The barotropic streamfunction is defined via zonal accumulation of meridional transport (west to east). How should this be interpreted for the SPNA, where flow is not simply bounded west-to-east? How is the spatial map (e.g., Fig. 3c) constructed? If this is effectively a vertically integrated transport at each grid point, please state so explicitly. Also define what is meant by the “minimum barotropic streamfunction within the SPNA box” (l. 119).
3) Do you use March alone to represent winter conditions? Please justify this choice and indicate whether results differ when using a seasonal mean (e.g., December–March).
4) Please clarify whether potential density is averaged over the upper 500 m or over the full water column (l. 114–116, l. 124, Fig. 4).
5) If the AMOC metric is defined in density space at 55°N, why is northward heat transport at 45°N used for process analysis? Using consistent latitudes may improve interpretability.
6) The pre-1985 weakening group includes only 12 cases (l. 128), fewer than the other groups, and the weakening signal appears weak (Fig. 11). Please discuss robustness and whether including this group is physically justified.
7) AMOC at 26°N is introduced without sufficient explanation. Does it vary in phase with 55°N, and what is its role in the argument?
In general, clearer methodological justification would improve readability and confidence in the results.
Shifts toward increased overturning
- Phase relation: The analysis assumes AMOC strength is controlled by density anomalies. However, density and AMOC do not appear fully in phase (Fig. 4; l. 150–152; l. 161–162). How do you explain continued AMOC strengthening after density anomalies stop increasing?
- Stratification evidence: In Fig. 5, positive density anomalies are widespread in the upper ocean, which does not necessarily imply reduced stratification. It would be more convincing to show buoyancy frequency (N²) or vertical density anomaly differences (surface minus subsurface) as a direct stratification metric. Likewise, reduced full-column density anomaly alone is not sufficient evidence for weakened convection.
- Dynamical linkage: Please elaborate on the mechanism linking deep convection, baroclinic flow anomalies, and strengthened horizontal circulation (l. 144–145).
Shifts toward decreased overturning (pre-1985)
- Meridional density gradients: For l. 174–175, please provide explicit evidence for reduced meridional density gradients, since this is presented as the key mechanism. Showing the relevant density anomalies earlier (currently in Fig. 10) may help.
- Initiation of heat-transport decline: At l. 177, if gradual decline in northward heat transport drives density and AMOC decreases, what initiates the heat-transport decline?
Comparison of mechanisms
Please make the comparison more explicit and process-based. It would help to clearly state feedback loops and their signs (surface heat loss/gain, horizontal transport strengthening/weakening, overturning response, and thermohaline contributions).
Potential inconsistency (l. 236–241)
Fig. 11 suggests density anomalies are mostly salinity-dominated, which appears inconsistent with some composite interpretations (e.g., Figs. 4, 7). Please clarify this apparent discrepancy.
Specific comments and language edits
- l. 101–102: “around 1895 and 1955” → “starting around 1895 and around 1955”; “from the 1970s onwards” → “from the 1980s onwards”
- l. 147: Please use consistent terminology throughout (e.g., “strengthening shifts” vs. “increased shifts”; see also l. 127 and l. 140).
- Fig. 3: Please add latitude/longitude labels for readability, and clarify the year indexing (including what “year 25–35” means).
- l. 197–198: The deepest anomaly appears first in the central SPNA and Labrador Sea, rather than in the Irminger Sea.
- l. 199–200: The salinity-chain argument seems somewhat ad hoc; please clarify the physical linkage between convection and SPG strength.
Citation: https://doi.org/10.5194/egusphere-2026-2588-RC2 - RC3: 'Comment on egusphere-2026-2588', Anonymous Referee #3, 16 Jul 2026
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EC1: 'Comment on egusphere-2026-2588', Karen J. Heywood, 18 Jul 2026
Thank you for submitting this thought-provoking paper to Ocean Science. I am grateful to all three reviewers for their constructive comments and helpful suggestions to strengthen the analysis. I encourage the authors to consider these carefully in preparing your responses and revisions.
The next stage is your online responses in this discussion forum. It is acceptable to post what you will do during revisions, even if you have not yet done that. You have about a month for posting those. After posting these responses, you have about another month for submission of the revised text and the final responses. These final responses can be the same as you posted online, or may need to be updated. If you require additional time at any stage, please do not hesitate to ask.
Karen J. Heywood, co-editor-in-chief
Citation: https://doi.org/10.5194/egusphere-2026-2588-EC1
Model code and software
Analysis code for: Multidecadal North Atlantic Circulation Shifts in CESM2-LE Ina Nagler et al. https://doi.org/10.5281/zenodo.19979604
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- 1
Dear editor and authors,
The well-written manuscript under consideration presents important results about multidecadal periods of strengthening and weakening of the North Atlantic circulation. The authors furthermore make a qualitative distinction between the pre- and post-1985 weakening events with compelling evidence. The emphasis in this paper is on subpolar gyre, inter-gyre, and basin-wide thermodynamic drivers of circulation anomalies.
However, I think the authors should also discuss the synchronous, coherent coevolution of the maximum overturning between the subpolar and subtropical latitudes seen very clearly in their results. The authors should furthermore discuss the role of the salinity anomalies along the western boundary of the Labrador Sea in driving these coherent overturning anomalies. These salinity anomalies stand out in some of the figures showing the authors’ results. In addition to their present analysis of inter-gyre thermodynamic gradients, the authors should also examine in greater detail the horizontal density gradients between the western boundary and the interior of the subpolar gyre. I therefore recommend minor but strongly recommended revisions.
Major comments:
Figures 4 and 9: The authors should discuss and interpret the synchronous, coherent coevolution of the maximum overturning between 55N and 26N seen in these figures.
Figures 3, 6, and 8: The authors should discuss the role of salinity anomalies along the western boundary of the Labrador Sea seen in these figures, and the role of these anomalies in driving meridionally coherent overturning variability in the context of Kostov et al. (2023).
Figure 10: Once again, in this figure, the western boundary of the Labrador Sea is a region of particularly strong density anomalies that also extend along the intergyre boundary. Please discuss in the context of Kostov et al. (2023) and the possible fast mechanism that give rise to the coherent response you see between 55N and 26N.
Lines 225-229, 257-260, and 275-276: You talk about “density within the SPG”, “in the STG”, the “inter-gyre density gradient,” and “the meridional gradient.” However, you do not mention the dynamical importance of density anomalies along the western boundary. What about the horizontal density gradients between the western boundary and the interior?
Lines 293-294: Finally, there is a brief mention of the western SPG margin in this single sentence. Could you also cite Kostov et al. (2023) here in the context of fast north-to-south connectivity along the western boundary?
Minor comments:
Lines 41-42: “in the subtropical gyre (STG), where it loses heat and salt.” – How is salt lost in the subtropical gyre? I think freshwater is lost there, evaporatively.
Lines 51-52: “it is ultimately density that governs the movement of water masses” – The statement should be qualified in the context of the study. Otherwise, winds play a first order role.
Line 85: “a pressure of 2000 dbar” – Is this reference level appropriate for subpolar North Atlantic overturning
Lines 92-93: “by taking the maximum of the annual mean overturning strength at 55N in the annual mean” – Is the maximum overturning strength close to a dept of 2 km or in a density layer that includes that depth.
Line 110: “sea surface height” – Cite Yeager et al.(2021) and Kostov et al. (2023) who discuss the importance of sea surface height anomalies in the North Atlantic and their role in setting the variability in overturning.
Figures 4 and 9: Define the positive direction.
Lines 300-301: Could you please cite Buckley and Marshall (2016) in the context of the Mann Eddy and inter-gyre exchange. Could you please cite Buckley et al. (2023) in the context of eddy exchange between the boundaries and the gyre interior?
Additional references:
Buckley, M. W. and J. Marshall (2016), Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review, Rev. Geophys., 54, doi:10.1002/2015RG000493.
Buckley MW, Lozier MS, Desbruyères D, Evans DG. 2023 Buoyancy forcing and the subpolar Atlantic meridional overturning circulation.Phil. Trans. R. Soc. A 381: 20220181. https://doi.org/10.1098/rsta.2022.0181
Kostov, Y., Messias, MJ., Mercier, H. et al. Fast mechanisms linking the Labrador Sea with subtropical Atlantic overturning. Clim Dyn 60, 2687–2712 (2023). https://doi.org/10.1007/s00382-022-06459-y