TIPMIP-OCEAN experimental protocol phase 1: Tipping dynamics of the AMOC

Swingedouw, Didier; Jackson, Laura; Hu, Aixue; Romanou, Anastasia; Laureanti, Nicole C.; Weijer, Wilbert; Loriani, Sina; Otto-Bliesner, Bette; Abe-Ouchi, Ayako; Almeida, Lucas; Bellucci, Alessio; Börner, Reyk; Danabasoglu, Gokhan; Dennis, Donovan P.; Devilliers, Marion; Drijfhout, Sybren; Donges, Jonathan; Fröb, Friederike; Frölicher, Thomas L.; Gastineau, Guillaume; Goelzer, Heiko; Guo, Chuncheng; Hofmann, Urs; Höse, Anna; Jones, Colin; Koenigk, Torben; Klose, Ann Kristin; Lembo, Valerio; Licon-Salaiz, Jose; Mankoff, Ken; Meccia, Virna; Melnikova, Irina; Mehling, Oliver; Menviel, Laurie; Mignot, Juliette; Robson, Jon I.; Schmidt, Gavin A.; Smith, Robin; Sun, Yuchen; Trombini, Irene; Willeit, Matteo; Wood, Richard; Wu, Fanghua; Zhaohui, Lin; Winkelmann, Ricarda

doi:10.5194/egusphere-2026-1698

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https://doi.org/10.5194/egusphere-2026-1698

Preprints

08 Apr 2026

| 08 Apr 2026

Status: this preprint is open for discussion and under review for Geoscientific Model Development (GMD).

TIPMIP-OCEAN experimental protocol phase 1: Tipping dynamics of the AMOC

Didier Swingedouw, Laura Jackson, Aixue Hu, Anastasia Romanou, Nicole C. Laureanti, Wilbert Weijer, Sina Loriani, Bette Otto-Bliesner, Ayako Abe-Ouchi, Lucas Almeida, Alessio Bellucci, Reyk Börner, Gokhan Danabasoglu, Donovan P. Dennis, Marion Devilliers, Sybren Drijfhout, Jonathan Donges, Friederike Fröb, Thomas L. Frölicher, Guillaume Gastineau, Heiko Goelzer, Chuncheng Guo, Urs Hofmann, Anna Höse, Colin Jones, Torben Koenigk, Ann Kristin Klose, Valerio Lembo, Jose Licon-Salaiz, Ken Mankoff, Virna Meccia, Irina Melnikova, Oliver Mehling, Laurie Menviel, Juliette Mignot, Jon I. Robson, Gavin A. Schmidt, Robin Smith, Yuchen Sun, Irene Trombini, Matteo Willeit, Richard Wood, Fanghua Wu, Lin Zhaohui, and Ricarda Winkelmann

Abstract. This paper describes the experimental protocol for a set of coordinated simulations involving oceanic surface freshwater flux perturbations, conducted as part of the international Tipping Points Modelling Intercomparison Project (TIPMIP). These simulations constitute the first phase of the TIPMIP-OCEAN domain. We propose this protocol for inclusion in the Coupled Model Intercomparison Project Phase 7 (CMIP7), although it can also be implemented within CMIP6+ or other types of coupled or ocean standalone models. This initial phase focuses primarily on the dynamics of the North Atlantic Ocean, particularly the Atlantic Meridional Overturning Circulation (AMOC). The different experiments are designed to (i) evaluate the impacts of a potential major AMOC weakening under a 2 °C global warming scenario, (ii) assess the sensitivity of the AMOC to combined global warming and freshwater forcing, (iii) investigate the potential recovery of the AMOC following the reversal of forcings, and (iv) compare past AMOC variations with available climate observations and reconstructions. Four categories of experiments are included. Experiment group A examines the effect of freshwater release around Greenland under ramp-up, stabilization, and ramp-down scenarios in both CO₂ emissions and freshwater input. Experiment group B complements this idealized set by using historical climate simulations and projections for 1850–2100, incorporating realistic estimates of Greenland Ice Sheet melt based on observations for the historical period and ice-sheet model projections for the future. Experiment group C extends the existing North Atlantic Hosing Model Intercomparison Project (NAHosMIP) by applying large freshwater perturbations to both control and 2 °C-warming simulations to assess how global warming influences AMOC reversibility. Finally, experiment group D imposes freshwater inputs, consistent with those inferred for the 8.2 kyr before present event, under pre-industrial conditions, in order to constrain model sensitivity to freshwater forcing using paleoclimate reconstructions. Together, these coordinated experiments will allow systematic evaluation of how different climate models respond to identical freshwater perturbations—an essential step toward better understanding the wide inter-model spread in North Atlantic dynamics and projected future AMOC changes.

How to cite. Swingedouw, D., Jackson, L., Hu, A., Romanou, A., Laureanti, N. C., Weijer, W., Loriani, S., Otto-Bliesner, B., Abe-Ouchi, A., Almeida, L., Bellucci, A., Börner, R., Danabasoglu, G., Dennis, D. P., Devilliers, M., Drijfhout, S., Donges, J., Fröb, F., Frölicher, T. L., Gastineau, G., Goelzer, H., Guo, C., Hofmann, U., Höse, A., Jones, C., Koenigk, T., Klose, A. K., Lembo, V., Licon-Salaiz, J., Mankoff, K., Meccia, V., Melnikova, I., Mehling, O., Menviel, L., Mignot, J., Robson, J. I., Schmidt, G. A., Smith, R., Sun, Y., Trombini, I., Willeit, M., Wood, R., Wu, F., Zhaohui, L., and Winkelmann, R.: TIPMIP-OCEAN experimental protocol phase 1: Tipping dynamics of the AMOC, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-1698, 2026.

Received: 25 Mar 2026 – Discussion started: 08 Apr 2026

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'Comment on egusphere-2026-1698', Anonymous Referee #1, 01 May 2026 reply
The manuscript by Swingedouw and co-authors describes an experimental protocol to systematically analyse AMOC sensitivity under external forcing from both freshwater input and climate change. The authors propose four interesting experiments, which will primarily be performed with Earth System Models (CMIP6+), and which are well suited to address the main scientific questions. The overarching aim is to obtain a better understanding of AMOC sensitivity and, in doing so, to assess AMOC-related impacts that are relevant for society.
I strongly recommend this manuscript for publication in Geoscientific Model Development (GMD). The experimental design is well motivated and clearly presented, with supporting figures and tables. This TIPMIP-OCEAN protocol will provide valuable information for the AMOC community and for assessing the risk of AMOC tipping under future climate change.
That being said, I have a few moderate and minor comments that should be addressed prior to publication.
Moderate comments and suggestions:
General aim of Experiments A, and specifically the reversibility component (Lines 191–192): I find the proposed experiments useful for studying AMOC weakening due to the joint contributions of global warming and freshwater input. The imposed forcing leads to a “major AMOC weakening” (Line 548; could this be quantified?) during the first 150 model years. Thereafter, forcing conditions are ramped down over 100 years and continued for an additional 100 years under no external forcing.

Given the typical multi-centennial adjustment timescale of the AMOC (Bonan et al., 2022, already cited) and results from existing pulse-forcing experiments (Jackson et al., 2023, already cited; van Westen et al., 2025, https://doi.org/10.1029/2025JC022651), it is unlikely that a coherent picture of AMOC reversibility can be obtained from the outlined experimental design (Fig. 2). Addressing reversibility would require substantially longer simulations for experiment “Stabilized (Ad)”, which are not mandatory under Tier 1 (Table 1). The 250-year experiment “Stabilized (Ab)” is likely sufficiently long in duration, but because it includes a modified background forcing, it cannot be used to assess reversibility.

My main point is that some aspects of AMOC stability cannot be addressed using simulations that are shorter than the AMOC adjustment timescale. This also applies to Experiments C and D, where the minimum required “stabilization” periods are 50 and 100 years, respectively. Only if modelling groups are willing to extend the stabilization (i.e. no external forcing) phases beyond 100 years can these questions be meaningfully addressed.

I would therefore suggest using more careful language and tempering expectations regarding AMOC reversibility, which is one of the central scientific questions in the TIPMIP-OCEAN protocol (Line 551).

The proposed experiments involve relatively rapid changes in forcing conditions (typically 100 years), which are short compared to the multi-centennial adjustment timescale of the AMOC. As a result, the simulated AMOC responses are likely to be dominated by the imposed forcing rather than by intrinsic AMOC dynamics. While the forcing may still trigger destabilising feedbacks (e.g., salt-advection), the experimental design appears less suitable for investigating true “tipping dynamics of the AMOC”, as suggested by the manuscript’s title. I therefore suggest reconsidering the title, for example:“Tipping sensitivity of the AMOC.”

I note that the manuscript appears to miss a substantial portion of relevant AMOC literature, despite the authors being highly active in this research area and publishing influential work. While the AMOC literature is rapidly evolving and extensive, I have included a number of relevant (non-exhaustive) recent studies in the minor comments below to help complement the discussion. I do not expect the authors to include all of the listed references, but I encourage them to consider the most relevant ones.

Minor comments and suggestions:
Lines 68 – 69: I also recommend including the more recent studies by Armstrong McKay et al. (2022) and Wunderling et al. (2024), which are already cited in the manuscript.

Line 74: The term “exact thresholds” is somewhat vague and would benefit from clarification; please specify what is meant here. In addition, it may be helpful to explicitly identify the relevant ocean tipping elements in this context, particularly the two major ones: the AMOC and the North Atlantic subpolar gyre.

Line 77: Please write out AMOC here; this is the first time the acronym is mentioned in the main text.

Lines 77 – 78: Either quantify “warm and salty” and “cold and fresh” or add ‘relatively’ here.

Line 80: after “Labrador, Irminger, and Nordic seas”. This should have a reference, e.g., Katsman et al. (2018, https://doi.org/10.1029/2017JC013329).

Lines 81 – 82: “The Atlantic nortward upper limb of the AMOC”, the term “Atlantic” is redudant here and could be removed.

Line 84: I would also add the study by Orihuela-Pinto et al. (2022, https://doi.org/10.1038/s41558-022-01380-y) here, which nicely provides an overview of the relevant global climatic responses (their Figure 6).

Line 85: The term “climatological equator” is unclear in this context. Do you mean that the ITCZ and/or the maximum zonally-averaged temperature is located north of the geographic equator? Please clarify.

Lines 85 – 89: These climate impacts require substantially more comprehensive and appropriate referencing, as a single citation is clearly insufficient. Moreover, the study by Ben-Yami et al. (2024) focuses primarily on ITCZ shifts under different AMOC strengths and does not cover the broader range of climate impacts discussed here. A number of recent modelling studies have addressed AMOC-related climate impacts more comprehensively, including the mentioned effects here.

For clarity, I provide below a (non-exhaustive) overview of relevant and recent studies:

General impacts:

- Orihuela-Pinto et al. (2022, https://doi.org/10.1038/s41558-022-01380-y)

- Bellomo et al. (2023, https://doi.org/10.1007/s00382-023-06754-2)

- van Westen et al. (2024, already cited)

- Bellomo & Mehling (2024, https://doi.org/10.1029/2023GL107624)

Temperature impacts and winter storms:

- Meccia et al. (2024, https://doi.org/10.1088/1748-9326/ad14b0)

- Meccia et al. (2025, https://doi.org/10.1088/1748-9326/ada3e7)

- van Westen & Baatsen (2025, already cited)

Hydroclimate:

- Saini et al. (2025, https://doi.org/10.1029/2024PA004967)

- van Westen et al. (2025, https://doi.org/10.5194/hess-29-6607-2025)

Sea level:

- Volkov et al. (2023, https://doi.org/10.1038/s41467-023-40848-z)

- Howard et al. (2024, https://doi.org/10.1088/2515-7620/ad3368)

- van Westen et al. (2026, https://doi.org/10.5194/os-22-1353-2026)

Line 91 – 92: A recent study by Nian et al. (2026, https://www.nature.com/articles/s43247-026-03427-w) might be worth including/discussing here.

Line 98: Perhaps you could roughly quantify the “large freshwater outburst” for the general reader, for example by expressing it in Sverdrups and/or as a multiple of the present-day Greenland Ice Sheet (GrIS) melt rate. I do acknowledge that this is being discussed in greater detail in lines 505 – 518.

Line 108: I can’t find the (Dijkstra, 2024) entry in the reference list, please include.

Lines 117–119: There is indeed a significant weakening trend across all SSP scenarios (Weijer et al., 2020, already cited; Dijkstra & van Westen, 2026;https://doi.org/10.1146/annurev-marine-040324-024822). However, more recent studies provide additional nuance. For instance, Bonan et al. (2025, https://doi.org/10.1038/s41561-025-01709-0) suggest that the decline is more coherent across CMIP6 models, while Portmann et al. (2026, https://www.science.org/doi/10.1126/sciadv.adx4298) report even larger declines when accounting for model biases. It may be worthwhile to also discuss these more recent findings here.

Lines 119 – 120: The 21st century climate change-induced AMOC weakening is primarily driven by changing surface heat fluxes, with a limited contribution by surface freshwater fluxes. See the surface buoyancy flux analyses in van Westen et al. (2025, https://doi.org/10.1029/2025JC022651) and Drijfhout et al. (2025, already cited).

Line 124: The ‘substantial biases in CMIP-class model’ needs some references:

- van Westen & Dijkstra (2024, already cited)

- Dijkstra & van Westen (2024, https://doi.org/10.16993/tellusa.3246)

- Portmann et al. (2026, https://www.science.org/doi/10.1126/sciadv.adx4298)

Line 126 – 127: ‘with indications that a tipping point may have been crossed’, this has been suggested in the analysis by van Westen et al. (2025, https://doi.org/10.1029/2025JC022651).

Line 128 – 130: A reference to Vanderborght et al. (2025, https://doi.org/10.1175/JCLI-D-24-0570.1) is also appropriate here, as they quantify the ‘key feedbacks governing its stability and tipping behaviour’.

Line 131: entrance -> southern entrance

Line 132: Atlantic -> North Atlantic; as the salt-advection feedback convergences freshwater anomalies over the northern North Atlantic

Line 134: ‘presence of an AMOC tipping point’, the review study by Dijkstra et al. (2026, https://doi.org/10.1002/wcc.70049) points to the strong evidence that the present-day AMOC is indeed in a multi-stable regime (i.e., the presence of an AMOC tipping point).

Line 137: The statement “may not behave” is somewhat vague. This likely reflects that (quasi-)equilibrium conditions are not satisfied. Only under such conditions is an appropriate stability indicator, as it can then be meaningfully linked to the strength of the salt-advection feedback (Rahmstorf, 1996, already cited; Vanderborght et al., 2025, https://doi.org/10.1175/JCLI-D-24-0570.1). Please clarify this point.

Lines 141 – 142: A reference to Hankel (2025, https://doi.org/10.1073/pnas.2411357121) is also appropriate here.

Lines 145 – 147: What about the results from Weijer et al. (2012, https://doi.org/10.1029/2012GL051611), where a comparison is made between a high-resolution and low-resolution stand-alone ocean model simulation.

Lines 150–153: I do not fully agree with this statement. The pulse-forcing experiments in Jackson et al. (2023) (+0.3 Sv for 100 years) are relatively short in duration (mostly 200 years in total), which may be insufficient to assess AMOC stability. For example, van Westen et al. (2025,https://doi.org/10.1029/2025JC022651) show in their pulse-forcing experiments (their Fig. 6a, blue curve) that a 200-year integration is too short to robustly assess AMOC stability based on AMOC strength alone. This limitation should be acknowledged or discussed more carefully.

Lines 157–161: The statement regarding an overshoot does not appear to be supported by all relevant studies. For instance, the CO₂ hysteresis experiment in Willeit & Ganopolski (2024,https://doi.org/10.5194/esd-15-1417-2024) does not exhibit an overshoot, nor do the results presented by Gérard & Crucifix (2024, https://doi.org/10.5194/esd-15-293-2024).

Lines 233 & 469: The compensation indeed prevents model drift; however, more importantly, it is required to obtain equilibrium solutions (i.e., to ensure an autonomous system).

Line 235: Maybe you can already state here that you are proposing a volume compensation.

Line 361: ‘Impact the GMSAT’, maybe good to add a reference here? For example, Drijfhout (2015, https://www.nature.com/articles/srep14877) or van Westen et al. (2024, already cited).

Line 371: The more recent study by Pontes & Menviel (2024, https://doi.org/10.1038/s41561-024-01568-1) might be worth adding here.

Line 447: This claim needs some references, for example Dijkstra (2024, https://doi.org/10.1016/j.physd.2023.133984).

Line 451 (and elsewhere): It may be helpful to refer to this forcing protocol as a “pulse forcing” experiment, following the terminology used in Dijkstra & van Westen (2026,https://doi.org/10.1146/annurev-marine-040324-024822). Suggestion: “… constant freshwater flux for a fixed time, a so-called pulse forcing (Dijkstra & van Westen, 2026).”

Line 454: “there is an indication that a threshold has been crossed”, this is quite vague. Please clarify which threshold.

Line 482: There is a consensus on the sign (i.e., weakening) in AMOC projections (Weijer et al., 2020, already cited; Dijkstra & van Westen, 2026, https://doi.org/10.1146/annurev-marine-040324-024822), but there is much more uncertainty on the magnitude of AMOC weakening (Bonan et al., 2025, https://doi.org/10.1038/s41561-025-01709-0; Portmann et al., 2026, https://www.science.org/doi/10.1126/sciadv.adx4298). Maybe rewrite this sentence.

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Citation: https://doi.org/10.5194/egusphere-2026-1698-RC1

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Short summary

This study presents a plan for climate model experiments to better understand how changes in freshwater in the North Atlantic affect major ocean currents. We designed coordinated simulations to test their response to warming, added freshwater, and possible recovery after weakening. Comparing results across models and past climate evidence helps improve confidence in projections and assess risks of large ocean circulation changes.


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