Simulated stability of the Atlantic Meridional Overturning Circulation during the Last Glacial Maximum

Pöppelmeier, Frerk; Scheen, Jeemijn; Jeltsch-Thömmes, Aurich; Stocker, Thomas F.

doi:https://doi.org/10.5194/cp-17-615-2021

Articles | Volume 17, issue 2

https://doi.org/10.5194/cp-17-615-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/cp-17-615-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 2

Research article

|

11 Mar 2021

Research article |

| 11 Mar 2021

Simulated stability of the Atlantic Meridional Overturning Circulation during the Last Glacial Maximum

Frerk Pöppelmeier, Jeemijn Scheen, Aurich Jeltsch-Thömmes, and Thomas F. Stocker

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Final revised paper (published on 11 Mar 2021)
Preprint (discussion started on 21 Oct 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review of Poppelmeier et al.,', Anonymous Referee #1, 08 Nov 2020
- AC1: 'Response to reviewers', Frerk Pöppelmeier, 18 Dec 2020
RC2: 'Review of Poeppelmeier et al. 2020', Anonymous Referee #2, 23 Nov 2020
- AC1: 'Response to reviewers', Frerk Pöppelmeier, 18 Dec 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (28 Dec 2020) by Qiuzhen Yin

AR by Frerk Pöppelmeier on behalf of the Authors (29 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (07 Jan 2021) by Qiuzhen Yin

RR by Anonymous Referee #1 (19 Jan 2021)

Suggestions for revision or reasons for rejection

Poppelmeier et al. have adequately answered the previous round of comments and have appropriately revised the manuscript. There remain a few ambiguities regarding the novelty of the work that needs to be taken into account in a revised version.
From the title, abstract and discussion, the novelty of the work is not clear and a few statements can be misleading as they would imply that “realistic boundary conditions” have not been used in previous LGM studies: the Bering Strait is closed in most (if not all) LGM simulations. LGM simulations performed with coupled models also include glacial winds (apart from a few which do not include a complex enough atmospheric model). A few LGM studies, particularly model-data comparisons, have also added meltwater into the North Atlantic to weaken the glacial AMOC (similar to the flux adjustment, apart from the fact that no salt was added to the North Pacific, but a debate onto whether there should also be enhanced runoff into the North Pacific during glacial times is out of topic here). The new part in this study is the change in tidal dissipation, which is currently not mentioned in the Abstract, and should be made more obvious throughout. Below are a few suggestions to make sure there is no confusion, particularly for readers who are not numerical modellers. Line numbers refer to the track-change version.

Title: I would suggest to remove the “under realistic boundary conditions” as this implies that other studies have not been done “under realistic boundary conditions”, which is not the case.

Abstract: It would be appropriate to mention the tidal dissipation in the abstract as this effect has received little attention.

L. 346-348 needs to be amended to make it clear that most LGM simulations already include a closed Bering St and appropriate glacial wind fields. This statement could cause confusion with readers that are not numerical modellers.

Similarly L. 348-352 should be amended as numerical studies investigating abrupt climate transitions of the last glacial period are performed with a closed Bering Strait, and the impact of Bering St closing is not new.

L. 360-362: This statement needs to be amended as you cannot reach that conclusion in your study. You can say what the Bern3D simulates under the forcing applied, and particularly I would suggest to explicitly state the NADW depth you find, but you cannot say if it is the “true” LGM NADW depth. This is also particularly true as you are not showing/simulating oceanic d13C and comparing it with proxy records. You can also discuss the limitations of your study, since the ocean model of the Bern3D is not an OGCM, and is of coarse resolution both horizontally and vertically.

L. 362-363: This statement needs to be more precise: What was the previous depth, and what is the new one?

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ED: Publish subject to minor revisions (review by editor) (26 Jan 2021) by Qiuzhen Yin

AR by Frerk Pöppelmeier on behalf of the Authors (02 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (03 Feb 2021) by Qiuzhen Yin

AR by Frerk Pöppelmeier on behalf of the Authors (04 Feb 2021)

Short summary

The stability of the Atlantic Meridional Overturning Circulation (AMOC) critically depends on its mean state. We simulate the response of the AMOC to North Atlantic freshwater perturbations under different glacial boundary conditions. We find that a closed Bering Strait greatly increases the AMOC's sensitivity to freshwater hosing. Further, the shift from mono- to bistability strongly depends on the chosen boundary conditions, with weaker circulation states exhibiting more abrupt transitions.