Nordic Overturning Increases as AMOC Weakens in Response to Global Warming

Roewer, Sasha; Fiedler, Lukas; Årthun, Marius; Huiskamp, Willem; Rahmstorf, Stefan

doi:10.5194/egusphere-2025-6172

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https://doi.org/10.5194/egusphere-2025-6172

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23 Dec 2025

| 23 Dec 2025

Status: this preprint is open for discussion and under review for Ocean Science (OS).

Nordic Overturning Increases as AMOC Weakens in Response to Global Warming

Sasha Roewer, Lukas Fiedler, Marius Årthun, Willem Huiskamp, and Stefan Rahmstorf

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is weakening in response to global warming, while Nordic Seas Overturning Circulation (NOC) is projected to increase. So far, no causal link has been proposed between these two opposing trends. Here we propose that a density reduction in the subpolar North Atlantic will weaken the AMOC by reducing the density difference with lighter waters further south, while at the same time strengthening the NOC by increasing the density difference with the heavier waters further north. Using high resolution climate model data and a box model, we find that in response to combined global warming and freshwater input the NOC initially increases moderately as the AMOC weakens, while a tipping point may be reached later if deep convection in the Nordic Seas shuts down and the NOC collapses together with the AMOC.

Received: 10 Dec 2025 – Discussion started: 23 Dec 2025

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Sasha Roewer, Lukas Fiedler, Marius Årthun, Willem Huiskamp, and Stefan Rahmstorf

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RC1:
'Comment on egusphere-2025-6172', Anonymous Referee #1, 13 Jan 2026 reply
Review of egusphere-2025-6172: “Nordic Overturning Increases as AMOC Weakens in Response to Global Warming”

General comments
This study investigates the joint evolution of the Atlantic Meridional Overturning Circulation (AMOC) and the Nordic Seas Overturning Circulation (NOC) under future climate change scenarios.
In a first part, a transient CO2 forcing simulation with a high-resolution climate model is analyzed to determine the changes in meridional heat transport (MHT) into the subpolar North Atlantic and Nordic Seas in response to increasing atmospheric CO2, including a decomposition into dynamic vs. thermodynamic contributions as well as overturning vs. gyre components. The authors find an increasing MHT into the Nordic Seas attributed to dynamic changes in the overturning circulation.
In a second part, a simplified 7-box model of the Atlantic ocean circulation is specified and used to study the impact of surface warming and freshwater hosing on the overturning circulation. A two-part response is found: first, the AMOC decreases while the NOC slightly increases; second, both the AMOC and NOC collapse as convection shuts down. This behavior is interpreted through an intuitive mechanism relating the decreasing subpolar Atlantic surface density to both the Atlantic density further south and the Nordic Seas density further north.
Overall, this study is a novel and valuable contribution to ongoing debates about a potential AMOC decline/shutdown and the fate of the North Atlantic circulation more generally. The methodology is mostly sound and the results are presented in an understandable fashion, except where noted below. While the Discussion mentions various caveats for the box model results, the climate model results should also be handled with care.
I can recommend this manuscript for publication in Ocean Science after the comments below have been addressed.

Specific comments
Abstract: given the limitations of the box model (appropriately discussed in the Discussion section, Ln. 321-351) and of the climate model simulation (not yet discussed), the Abstract should at least briefly acknowledge these caveats or perhaps tone down some of the confident language.
Ln. 30: for completeness, this paragraph should mention that the historical evolution and potential weakening of the AMOC is currently still a subject of scientific debate (whereas a future AMOC decline is fully expected). In particular:
Worthington, E. L. et al. A 30-year reconstruction of the Atlantic Meridional Overturning Circulation shows no decline. Ocean Sci. 17, 285–299 (2021). https://doi.org/10.5194/os-17-285-2021

Latif, M., Sun, J., Visbeck, M. et al. Natural variability has dominated Atlantic Meridional Overturning Circulation since 1900. Nat. Clim. Chang. 12, 455–460 (2022). https://doi.org/10.1038/s41558-022-01342-4

Terhaar, J., Vogt, L. & Foukal, N.P. Atlantic overturning inferred from air-sea heat fluxes indicates no decline since the 1960s. Nat Commun 16, 222 (2025). https://doi.org/10.1038/s41467-024-55297-5

Similarly, the past evolution of the Nordic MOC should be briefly mentioned for context, e.g.
Rossby, T., Chafik, L., & Houpert, L. (2020). What can hydrography tell us about the strength of the Nordic Seas MOC over the last 70 to 100 years?. Geophysical Research Letters, 47, e2020GL087456. https://doi.org/10.1029/2020GL087456

Ln. 36: “the AMOC has demonstrated the capacity to...”: since this is referring to model results, care should be taken to use wording that does not imply any directly observed “capacity” of the real-world AMOC
Ln. 48-49: “This increased overturning...”: this sentence is too vague, it is unclear where exactly those warm and salty waters appear, and what kind of changes in water mass transformation and circulation the authors have in mind
Ln. 88: “initial simulation”: does this refer to a spin-up simulation to quasi-equilibrium before starting the preindustrial and 2xCO2 experiments? If so, that should be stated, since on Ln. 86 it is only said that “the model is initialized” with certain conditions, which does not sound like a proper spin-up simulation.
Equation (1): introduce all symbols and the values of the constants
Sections 2.2 - 2.3: it could be good to cite Msadek et al. (2013) which seems share a lot of the methodology used here:
Msadek, R., W. E. Johns, S. G. Yeager, G. Danabasoglu, T. L. Delworth, and A. Rosati, 2013: The Atlantic Meridional Heat Transport at 26.5°N and Its Relationship with the MOC in the RAPID Array and the GFDL and NCAR Coupled Models. J. Climate, 26, 4335–4356, https://doi.org/10.1175/JCLI-D-12-00081.1.

Fig. 1 caption: “winter” in what hemisphere (what months specifically)? And why winter?
Equation (5): what are the values of the constants in this equation?
Ln. 141: “the light blue arrow flips”: specify which arrow exactly, since several of them are light blue
Box model equations (Eqs. 6-21): this collection of equations is a bit hard to parse. I would suggest putting it in an Appendix to save some space in the main text. Also, since each term in each equation belongs to one of several categories (i.e., restoring terms with lambda, overturning terms with m, gyre terms with K, convection terms with c, diffusion terms with eta), it could be beneficial to align these terms vertically so that e.g. all terms involving lambda are vertically aligned, then all overturning terms with m, etc. Lines with e.g. no restoring term (for boxes below the surface) would then just have empty space in the restoring column, if that makes sense.

Maybe this would take up too much space but it would certainly increase the readability of the model equations.
Ln. 195: “The weights for the salinity and temperature differences are the values of α and β in the linear EOS.” This sounds intuitive, but is there really a physical motivation to choose the loss function weights in this way? I would assume that the weights are arbitrary hyperparameters that could in principle be tuned, even if the chosen values (1.7 and 7.9) may happen to give good results.
Uncertainties (section 3.1): all values reported in tables 1-3 come without uncertainty estimates/confidence intervals. This makes statements about particular decompositions and “dominant” processes etc. less convincing. If more than one ensemble member has been run for this configuration, the ensemble spread should be stated. If similar runs from other climate models are available (e.g., from the CMIP archive), these could be used to get some sense of the variance in these results. Otherwise, the Discussion section should fully discuss these caveats, not only the limitations of the box model.
Ln. 228-229: “Hence, the findings imply an anticorrelation between AMOC and NOC-driven MHT changes under increased CO2 forcing in CM2.6.” Since there are really only single data points presented in these tables (e.g. a decreasing AMOC and an increasing NOC), “anticorrelation” might not be the most accurate term. It would be interesting to test whether the evolution of the AMOC and the NOC are correlated in time on decadal (or even inter-annual) time scales in CM2.6, as well as across hosing/warming experiments in the box model in later sections.
Table 2: the heat budget terms for the subpolar North Atlantic contain a large compensating effect of the dynamic vs. nonlinear term (-0.36PW vs. +0.35PW). Is it thus really possible to interpret that circulation changes “dominate” (Ln. 221 and 228)? It could also be said that nonlinear effects dominate, or that dynamic and nonlinear effects cancel and the thermodynamic effect dominates.
Ln. 238: “...the weakening of the overturning circulation is the dominant factor...”: I would argue that a 57.5% contribution is not very “dominant”, especially since no uncertainty bounds are stated (see comment about uncertainties further above).
Ln. 243: It would be helpful to have a paragraph briefly summarizing the climate model results before starting the box model section.
Ln. 252: “...the imposed warming accelerates convection shutdown due to decreasing surface buoyancy”: this is not apparent from Fig. 3a/c/e, it seems that the high- and low-end warming scenarios (red and light blue lines) bracket the response of the intermediate warming scenarios. Additionally, the red high-end warming line seems to behave qualitatively differently from the other scenarios. In any case, the differences due to different warming rates seem to be relatively small.
Ln. 314: “warming in the Nordic Seas”: it appears that Li and Liu (2025) are concerned more with the subpolar gyre warming hole than with Nordic Seas warming? See their Fig. 3a
Ln. 371: “warming and freshening of the northern Atlantic caused by an AMOC slow-down” Since both warming and freshening are actually imposed in the box model, to what extent can one really conclude that the mechanism involves the AMOC itself driving further warming and freshening? Can this feedback contribution be quantified?

Technical corrections
Typesetting: the LaTeX could be improved in a number of places:
Uses of MHT’ and similar “math terms” in the text should be placed inside an inline equation ($...$)

2xCO2 should use \times and non-italic letters

Some citations should not be in parentheses: Ln. 133, 189, 198, 205

Ln. 1: “while Nordic...” -> while the Nordic...
Ln. 2: “increase” -> strengthen
Ln. 23: “winter temperatures”: specify that this refers to (surface) air temperature
Ln. 33: “above 90% significant”: the wording should be improved
Ln. 35: “hosing” -> freshwater hosing
Ln. 41: “southern shift” -> southward shift
Ln. 45: “to name a few”: this could be removed
Ln. 79-80: “much greater fidelity, ... more realistic”: compared to what? Other models of the same generation/resolution?
Ln. 106-107: “In the following, ...”: this sentence is redundant with the previous sentence and could be removed
Ln. 127: refer to Fig. 2 after “... eight boxes”
Ln. 185: “The parameter c”: there are actually several parameters c_i
Ln. 185: “The parameter c...”: although I can understand the meaning of this sentence, it would be easier to understand when put in equation form (with a large brace containing the value of c_i in the two cases)
Ln. 191: “eta is the heat diffusion constant”: it is also responsible for salt diffusion (Eq. 21)
Ln. 214: “surface heat flux”: specify the sign convention
Fig. 3 caption: “Grey lines”: since they are all grey, it is not possible to tell which line belongs to which warming rate.
Ln. 261: “because of the nonlinearity in the equation of state”: expand on this a bit, I would assume this refers to the temperature dependence of alpha and the different temperatures in the two regions.
Ln. 264: “salt advection feedback”: the accumulation of freshwater in the northern Atlantic after an AMOC decline is not yet a feedback, it is only one of the two “arrows” in the feedback loop. I would thus remove this parenthesis.
Ln. 290: “warming prevents the decline of the NOC”: it only “prevents” it in the warming-only experiments. The NOC does end up declining in the hosing+warming experiments, so a more accurate term could be “delays” or similar.
Ln. 304: “Labrador Sea”: it looks like the box covers the Irminger Sea more than it does the Labrador Sea?
Ln. 340: “dsicussed”: typo
Ln. 355: “in northern Atlantic” -> in the northern Atlantic
Ln. 358: “5 mSv/year”: should this be 4 mSv/year instead?

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Citation: https://doi.org/10.5194/egusphere-2025-6172-RC1

Sasha Roewer, Lukas Fiedler, Marius Årthun, Willem Huiskamp, and Stefan Rahmstorf

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Short summary

A density reduction in the subpolar North Atlantic might weaken the Atlantic Meridional Overturning Circulation (AMOC) by reducing the density difference with waters further south, while strengthening the Nordic Overturning Circulation (NOC) by increasing the density difference with waters further north. During combined global warming and freshwater input, the NOC increases moderately as the AMOC weakens, while the NOC and AMOC may collapse later if convection shuts down.


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