Surges of Harald Moltke Bræ, north-western Greenland: seasonal modulation and initiation at the terminus

Müller, Lukas; Horwath, Martin; Scheinert, Mirko; Mayer, Christoph; Ebermann, Benjamin; Floricioiu, Dana; Krieger, Lukas; Rosenau, Ralf; Vijay, Saurabh

doi:https://doi.org/10.5194/tc-15-3355-2021

Articles | Volume 15, issue 7

https://doi.org/10.5194/tc-15-3355-2021

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

https://doi.org/10.5194/tc-15-3355-2021

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

Articles | Volume 15, issue 7

Research article

|

21 Jul 2021

Research article |

| 21 Jul 2021

Surges of Harald Moltke Bræ, north-western Greenland: seasonal modulation and initiation at the terminus

Lukas Müller, Martin Horwath, Mirko Scheinert, Christoph Mayer, Benjamin Ebermann, Dana Floricioiu, Lukas Krieger, Ralf Rosenau, and Saurabh Vijay

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Final revised paper (published on 21 Jul 2021)
Preprint (discussion started on 06 Nov 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Comments on Müller et al., "Surges of Harald Moltke Bræ, north-west Greenland: Seasonal modulation and initiation at the terminus"', Vanessa Round, 13 Dec 2020
- AC1: 'Reply on RC1', Lukas Müller, 19 Feb 2021
RC2: 'Review of tc-2020-256 by Müller et al.', Anonymous Referee #2, 18 Jan 2021
- AC2: 'Reply on RC2', Lukas Müller, 19 Feb 2021

Peer-review completion

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

ED: Publish subject to revisions (further review by editor and referees) (19 Feb 2021) by Daniel Farinotti

AR by Lukas Müller on behalf of the Authors (17 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to revisions (further review by editor and referees) (20 Apr 2021) by Daniel Farinotti

ED: Referee Nomination & Report Request started (21 Apr 2021) by Daniel Farinotti

RR by Anonymous Referee #2 (30 Apr 2021)

Suggestions for revision or reasons for rejection

This paper provides a useful review of the frequent surges of Harald Moltke Brae, and their strong seasonal variability. These velocity patterns haven’t been reported before, so the paper and the comprehensive data that it contains are a useful addition to the literature. I’m satisfied that the authors have addressed the substantive parts of my comments from the previous version.

My comments below are generally minor in nature, and are mainly aimed at improving explanations and removing potential ambiguity in wording in a few places. I’m happy for the authors and editor to decide between them as to which of these will be addressed; I do not need to review the paper again.

L18-L23 & Fig. 1: I still find this first paragraph and figure strange as they dive straight into the results without providing any kind of methods or context for them. So I would still prefer to see them placed in the Results section where they properly belong, but from the author’s comments I can see that they disagree with my viewpoint. So I’m happy to leave the decision up to the editor as to whether anything should be changed here.

L24-32: as stated in my original review, this describes the Study Area and would be better placed later in the text. I believe that a more general overview of the topic of surging and its history at Harald Moltke Brae needs to be provided first, before getting into these specific details.

L31: change to ‘northern side of Harald Moltke Bræ at a centerline distance of about 20km’

L45: I think that ‘typically flows slower than the upper part’ would be a more accurate description

L80: I think that ‘always’ is unnecessary here and can be deleted

L81: ‘always 2 years’ would be better expressed as ‘both 2 years’

L95: would be useful to add a short explanation as to why monthly averaging is deemed to be appropriate here (e.g., to enable clear identification of seasonal velocity variations?)

L99: provide a brief explanation as to why the black triangle is chosen as the best place (indeed, only place) to plot the velocity time series. E.g., is this because it’s on the glacier centreline? near the terminus? undergoes the most dramatic velocity variations?

Table 1: will be useful if you can provide a URL for each of the data sources listed in the last row if one is available

L103: change to ‘by applying a method…’

L124: describe what kind of line you fit to the ice thickness profiles

L127-128: provide some more information about the CReSIS ground penetrating radar measurements. E.g., When were they measured? What’s their resolution? Where did you download the data from, and what’s the name of the dataset that you downloaded?

L131: provide a few more details about the RACMO data that you used: e.g., spatial resolution? where was it downloaded from?

L132-133: I find it a little difficult to follow the wording for the area over which your RACMO data is averaged. You specify that it’s a ‘fixed area’ and the ‘drainage area of a cross section which is located close to the glacier front at about 15km’, so do you mean that it’s the entire area of the glacier basin upglacier of the cross section at 15km? If so, then update the text to say this.

L134: provide some more detail as to how you computed the ice-mass flux through the cross section. E.g., did you assume that surface velocity was the same as depth-averaged velocity? Did you account for any change in ice thickness over time?

Figure 3 caption: change to ‘The documented surges are marked…’

Figure 4a & 6a: would be useful to add a dotted 0 line to part (a), so that it’s clearer as to which years are positive and negative SMB

Figure 4 & 6: if there is sufficient space, I wonder if these two figures could be merged into a single continuous timeline? This would make it easier to see how the surges compare to each other. But if this will end up with data points being too small to clearly see, then no problem to leave them as separate figures.

Figure 5c and 5d and others (e.g., Fig. 7, 9): specify in the x-axis label or the caption that distance is from the 1916 terminus, and indicate where the current terminus is (i.e., similar to how the terminus is labelled in Fig. 13b)

L176: change to ‘one order of magnitude’

Figure 12: add shading to this figure to indicate the timing of the surge events, similar to shading used on Figs. 3, 4, 6.

L267: change to ‘pronounced seasonality in velocity simultaneously with a surge…’

L285: to avoid any ambiguity, I suggest changing the wording to ‘A significant variation in quiescent velocities cannot be detected…’ (at least I assume that’s what you’re referring to in this sentence)

Fig. 16: this new figure is very helpful!

L385/6: change ‘hydrological surge’ to ‘hydrological mechanism’

L403 & L408: these arguments would be more convincing if you can demonstrate that crevasses on the glacier actually changed over the periods that you describe. I know that you mentioned earlier that the resolution of Landsat imagery is insufficient to detect these changes, but have you tried looking at higher resolution free imagery from sensors such as Sentinel 2 or Planet? From my own quick search of Planet imagery, for example, there are many good scenes available since 2016, at resolutions as high as 3 m, which show clear crevasse patterns.

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ED: Publish subject to minor revisions (review by editor) (02 May 2021) by Daniel Farinotti

Dear Lukas,

as agreed, your submission has now been re-seen by the reviewer that had requested to do so.
I am happy to inform that your manuscript can be accepted for publication after a last set of minor revisions. These revisions will not be reviewed again, and only be checked by me.

Most of the reviewer’s comments seem straightforward to address. For two of them, my view is as follows:

L18-23 (contents of the first paragraph of the introduction): Here I’m tempted to agree with the reviewer. I understand the reasoning that you provided in the last rebuttal letter, but isn’t this “attracting start” something that happens in the abstract already? If it was my paper, I would probably start with what is now lines 52ff, i.e. by providing some general information about surges, and about surges in Greenland in particular, and only then focus on Harald Moltke Bræ. I’m happy to adopt your own suggestion if you feel strongly about it, but would imagine that a significant part of the readership will have a similar reaction to the paper’s start as the one shown by the reviewer.

L24-32 (place where the study site is introduced): Also in this case, I would agree with the reviewer. As per suggestion above, it would seem more “natural” to start with the general introduction presently provided at lines 52ff, and to tighten the focus after that. Again, I just feel that the readership might be better guided through your work that way, thus increasing the appreciation towards it.

Finally, I would also like to back up the reviewer’s request for some additional words of information in a few instances (e.g. comments for lines 95, 99, or 127). Although there is certainly no need to go into length with these, I am sure that the readers will appreciate them as they provide additional clarity to your work.

I’m looking forward to the manuscript being finalized and for your work to be successfully completed.

Best regards
Daniel

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AR by Lukas Müller on behalf of the Authors (22 May 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (27 May 2021) by Daniel Farinotti

AR by Lukas Müller on behalf of the Authors (28 May 2021)

Short summary

Harald Moltke Bræ, a marine-terminating glacier in north-western Greenland, undergoes remarkable surges of episodic character. Our data show that a recent surge from 2013 to 2019 was initiated at the glacier front and exhibits a pronounced seasonality with flow velocities varying by 1 order of magnitude, which has not been observed at Harald Moltke Bræ in this way before. These findings are crucial for understanding surge mechanisms at Harald Moltke Bræ and other marine-terminating glaciers.