The distribution and evolution of supraglacial lakes on 79°&thinsp;N Glacier (north-eastern Greenland) and interannual climatic controls

Turton, Jenny V.; Hochreuther, Philipp; Reimann, Nathalie; Blau, Manuel T.

doi:https://doi.org/10.5194/tc-15-3877-2021

Articles | Volume 15, issue 8

The Cryosphere, 15, 3877–3896, 2021

https://doi.org/10.5194/tc-15-3877-2021

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

The Cryosphere, 15, 3877–3896, 2021

https://doi.org/10.5194/tc-15-3877-2021

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

Articles | Volume 15, issue 8

Research article 20 Aug 2021

Research article | 20 Aug 2021

The distribution and evolution of supraglacial lakes on 79° N Glacier (north-eastern Greenland) and interannual climatic controls

Jenny V. Turton et al.

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Final revised paper (published on 20 Aug 2021)
Supplement to the final revised paper
Preprint (discussion started on 02 Mar 2021)
Supplement to the preprint

Interactive discussion

Status: closed

RC1:
'Comment on tc-2021-45', Anonymous Referee #1, 04 Apr 2021
Summary

Turton et al. present a new analysis of the seasonal development of supraglacial lakes at 79ËN Glacier between the 2016 and 2019 melt seasons. Integrating optical satellite observations, meteorological data, numerical modelling, and reanalysis products, they find that extensive ponding observed in 2016 and 2019 were likely related to high air temperatures, boosted by a high liquid precipitation fraction, which increased the limit of lakes to higher elevations on the ice sheet. In contrast, 2018 displayed very low lake coverage, likely related to cold temperatures restricting melt and a large, accumulated snowpack acting as a reservoir for meltwater. The factors that control lake formation in the study area appear to be correlated, at least for surface temperature, with the North Atlantic Oscillation, with high lake extents appearing in years with a strongly negative JJA NAO index.

I believe this paper is a unique contribution to the literature as it is rare to see observational studies in the northeast of the ice sheet. It is also interesting to see a more nuanced consideration of separate climatic variables and wider teleconnections, which aren’t often considered in large-scale remote sensing studies of SGLs. Before publication, it would be useful to see more clarity in the communication of the climate data, in order that the reader might form a better narrative of inter-annual variability. I have also outlined further recommendations below.

My area of expertise trends towards ice sheet hydrology and remote sensing rather than climate modelling. However, this paper largely synthesises previously-peer-reviewed datasets, so I will assume that where datasets fall outside my ability to critique them, they are methodologically sound.

Specific Comments

I struggled to follow Section 3.3 - partly because it can be quite dense but mostly, I believe, because it strays from the narrative structure of the rest of the results. Sections 3.1, 3.2, and 3.4 each address individual variables (lakes, topography, and SMB respectively), describing how each of these properties vary between 2016-2019. In contrast, section 3.3.1-3.3.4 each address individual years (2016 to 2019 respectively), describing how a variety of properties (temperature, SWin, precipitation) behave in each year. This abrupt inversion to the logical structure of narrative makes it difficult to follow how the climatic variables change across years. The structure should be consistent across the results, and I believe the paper would be better served by continuing to treat variables separately, highlighting the narrative of interannual change. This would further benefit the paper by making sure each year and variable receives equal treatment: for instance, section 3.3.1 (2016) deals with observational Ta, TSK, and onset, as well as PWRF data, but section 3.3.4 (2019) discusses only observational Ta (for what it’s worth, I found section 3.3.4 more focussed and easier to follow).

As mentioned, a lot of the results are quite dense, which is understandable as a large quantity of data is being described in parallel. However, this can make it hard for a reader to follow all the moving parts, and I believe there could be some changes to the presentation of the data to aid the reader in qualitatively assessing the controls on lake development. A key point of interannual comparison is the total area of lakes, but the way this is visualised in Figure 2 is hard to interpret. It would be easier to follow a line graph, which would be simple to add as a second axis of the Figure 2 panels. Additionally, it might be useful to present lake area data alongside the later data, so that the reader does not have to continually refer back to earlier figures/text on lake development. One way (although I don’t mean to prescribe here, so certainly not the only or best way) of doing this might be to split Figures 5 and 8 into four panels - each representing individual years (mirroring Figure 2) - and including lake area as a second axis. Splitting Figure 5 into panels of individual years would also make it possible to include, perhaps as vertical lines or shaded boxes, the data presented in Table 2 (periods of Ta>0ËC and melt ponding). This work would allow for the easy visualisation and comparison of a lot of data that is currently only accessible via text or table, or by comparing disparate figures and tables on different pages. In turn, the authors may be able to simplify much of the denser text that is currently spent describing the temporal variations in these data.

The results tend to treat lake behaviour in bulk, rather than considering the behaviour and/or heterogeneity of individual lakes. I think this is largely fine for the purposes of the study, but it would be nice to see some consideration at specific points. First, Section 3.1 discusses late-season increase in lake area. Is this a general trend common to all lakes (due to, e.g., a melt day / rainfall event)? Or is it a result of a few individual lakes rapidly increasing in area (due to, e.g., reorganisation of the surface drainage system). If the latter, does this also occur at other points, but is masked in the bulk data? Second, it is not discussed how the lakes are draining (rapid vs slow drainage). Can the authors quantify (or at the very least, comment qualitatively on) the relative dominance of drainage modes? Recent work has begun to take an interest in how lake drainage may differ between land- and marine-terminating sectors of the Greenland Ice Sheet (e.g. Williamson et al. 2018a, Chudley et al. 2019), but this is still largely focussed in the SW/W of Greenland. Further observations from this unique sector would be of considerable interest.

On a different note, considering how much time is spent considering the influence of teleconnections in the discussion, I am surprised by their relative lack of attention in the abstract, introduction, and conclusion (and perhaps, also, their lack of inclusion graphically). On a basic level of critique, this means that their inclusion in the discussion comes a bit out of nowhere. However, more interestingly, I do not think that many (any?) observational studies of lake variation consider these modes of climate variability, so some time spent introducing them and their context may be useful for those coming from other areas of the discipline (e.g. optical remote sensing / computer vision), as well as highlighting them in the abstract/conclusion so that the interesting conclusions of this paper can reach the widest audience!

Minor Comments

Abstract

L10: “Together with two neighbouring glaciers…”. Perhaps a broad statistic for a study focussing on just 79N? Why not just say 8%, as per L25?

L16: “2014 to 2019”. The primary ablation seasons considered are 2016-2019, so this might set expectations high. Somehow make it clear that whilst you examine lakes over 2016-2019 melt seasons, you examine influences up to however many months prior?

L18: “Over 1400 m” > “Up to [x] m”

Intro

L34 “Low-elevation” is subjective, considering lakes can extend away from the outlet glaciers and up onto the ice sheet proper. Perhaps just ‘on the ablation zone of the ice sheet’?

L38-41 This paragraph focuses on short-term accelerations: perhaps make it clearer that meltwater has also been shown to have negative feedbacks on ice velocity in the medium-long (seasonal-decadal) timescales, at least in land-terminating sectors (e.g. Tedstone et al. 2015; Sundal et al. 2011, etc.).

L55-56 “...likely underestimated the lake area by 12%”. Was this shown in the Sundal 2009, or did a later high-resolution study quantify this? A bit unclear as written.

L58-59 Willamson et al. 2018b also provide a prior example of Sentinel-2 automated lake detection.

L80 I always understood VHR to be ~1m resolution - does Sentinel-2 really make the cut?

Data and Methods

L91-95 This introduction to the Sentinel-2 mission is generously detailed, and can probably be safely removed if the intention is to provide a brief overview (as per L91).

L100-101 It would be nice to at least reference by name the pre-processing steps applied, so that those familiar with the techniques do not have to consult another paper. Those who are still interested in the nuts and bolts then have the option to delve further.

Paragraph beginning L108 - From the paragraph it seems that the data was cropped to the grounded ice, but how was the spatial limit determined? A fixed outline (e.g. GIMP), user-determined annual grounding lines, etc.?

Results

L170: These lakes are all rather small, quite close to the cutoff size of 0.015 km^2. Could the dataset be sensitive to this chosen limit?

L174-179: It would be nice to also see absolute as well as relative changes in this text (see also my comments about Figure 2).

L80-81: To what extent are these late-season increases spread across all lakes (e.g. due to a high melt day) or more heterogenous (due to, say, reorganisation of the surface drainage system allowing a few lakes to fill)? Is it possible to separate the data by individual lakes?

L396 - Contextual sentence, probably better belongs in intro or discussion (alongside a citation)

Discussion

Paragraph beginning L488: Strange absence of reference to any Greenlandic literature here. One of the unique aspects of this study, as identified in the introduction, is the lack of prior SGL studies in the NE of the ice sheet. It is a shame not to see more comparison and contrast to the established SGL literature in the SW and W of the ice sheet.

Conclusion

Paragraph beginning L577: This paragraph probably belongs at the end of the discussion?

Figures and Tables

Figure 1: Currently only the land extent is demarcated in Figure 1a - could the ice extent and grounding line(s) be done also?

Figure 6/7: (i) What do the vectors represent? (ii) Could these two figures be combined into one figure of six panels, so that it is easy for the reader to compare June/July of the same years as well as individual months of different years?

Figure 10 - Notable compression artefacts in this image - possibly just my download, but it doesn’t seem to appear elsewhere so thought I’d mention in case it’s common.

References not in the main text

Chudley, T. R., Christoffersen, P., Doyle, S. H., Bougamont, M., Schoonman, C. M., Hubbard, B., & James, M. R. (2019). Supraglacial lake drainage at a fast-flowing Greenlandic outlet glacier. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.1913685116

Sundal, A. V., Shepherd, A., Nienow, P., Hanna, E., Palmer, S., & Huybrechts, P. (2011). Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage. Nature, 469(7331), 521–524.

Tedstone, A. J., Nienow, P. W., Gourmelen, N., Dehecq, A., Goldberg, D., & Hanna, E. (2015). Decadal slowdown of a land-terminating sector of the Greenland Ice Sheet despite warming. Nature, 526(7575), 692–695. https://doi.org/10.1038/nature15722

Williamson, A. G., Willis, I. C., Arnold, N. S., & Banwell, A. F. (2018a). Controls on rapid supraglacial lake drainage in West Greenland: an Exploratory Data Analysis approach. Journal of Glaciology, 1–19. https://doi.org/10.1017/jog.2018.8

Williamson, A. G., Banwell, A. F., Willis, I. C., & Arnold, N. S. (2018b). Dual-satellite (Sentinel-2 and Landsat 8) remote sensing of supraglacial lakes in Greenland. The Cryosphere, 12(9), 3045–3065. https://doi.org/10.5194/tc-12-3045-2018
Citation: https://doi.org/10.5194/tc-2021-45-RC1
- AC1: 'Reply on RC1', Jenny Turton, 26 Apr 2021
  
  Dear Reviewer,
  In this response, we outline the larger changes that we will make to the manuscript in response to your specific comments. Then, following a decision by the editor, we will then upload a revised manuscript and point-by-point response to all critiques.
  Firstly, thank you for the positive feedback on the study and its contribution to the field. We appreciate your comments and questions. In general, we aim to improve the manuscript’s structure for better understanding, especially in section 3.3, and improve the communication of the results by amending the figures. We summarise your main concerns and address your specific comments below:
  Comment: Section 3.3: The structure should be consistent across the results, and I believe the paper would be better served by continuing to treat variables separately, highlighting the narrative of interannual change. This would further benefit the paper by making sure each year and variable receives equal treatment...
  Response: We will edit the structure of this section as you suggest, to focus on each variable individually and give a similar weight to each. We will also remove any repetition and density of this section to aid reading and understanding. The second reviewer also suggested some structural changes which we will incorporate.
  Comment: A key point of interannual comparison is the total area of lakes, but the way this is visualised in Figure 2 is hard to interpret. It would be easier to follow a line graph, which would be simple to add as a second axis of the Figure 2 panels. Additionally, it might be useful to present lake area data alongside the later data, so that the reader does not have to continually refer back to earlier figures/text on lake development… In turn, the authors may be able to simplify much of the denser text that is currently spent describing the temporal variations in these data.
  Response: A number of figure changes were also suggested by Reviewer 2, so we will modify all figures in the manuscript to better represent the results, synthesise the data and aid in understanding. Specifically: Figure 1 will now include a larger map of Greenland with labels for NEGIS and the grounding line. Figure 2 will include the absolute lake area values (as a line plot) as well as percentage change, with some alteration to the colour bar. Figures 3 and 5 will be split into 4 panels to represent each year, and table data will be included on figure 5. Figures 6 and 7 will be combined into one panelled figure. An additional figure is likely to be included to accompany a short discussion on the lake drainage characteristics- as mentioned by both reviewers.
  Comment: Some discussion of individual lake behaviour needed alongside discussion of lake drainage.
  Response: Both reviewers requested more information about lake drainage results. As the aim of this study was to link climate conditions with lake characteristics, and there is likely little relationship between climate indicators and drainage, we did not include this information initially. Furthermore, given the very cloudy conditions in northeast Greenland, it can often be difficult to quantify whether lakes have drained or are covered with clouds, especially when clouds persist for a number of days and lake drainage may have been missed. However, we will include some discussion of one or two lake drainage events in the form of case studies, so that we may infer the likely drainage mode in this region. Neckel et al. (2020) observed a lake drainage event in our study region and we will analyse this particular event, as we have observations to prove that the lake drained as opposed to being covered by clouds. Following this, we will assess a number of other lakes, to see whether individual lakes or a number of lakes drain in a similar fashion.
  Comment: I am surprised by the relative lack of attention to the influence of teleconnections in the abstract, introduction, and conclusion (and perhaps, also, their lack of inclusion graphically)… some time spent introducing them and their context may be useful for those coming from other areas of the discipline.
  Response: With just four years of data for comparison, it is difficult to draw strong conclusions to teleconnections, but we wanted to include them in the discussion due to the likely importance of such indices for record melting events over Greenland. We will consider how to best represent them graphically, and whether a broader discussion of teleconnections could be included. At the very least we will include them in the abstract and introduction, so that their discussion in section 4 does not come out of the blue, and to introduce them for others coming from a different field of research.
  Comment: A number of additional references are provided, with guidance that a more thorough discussion of Greenland should accompany that of the Antarctic in section 4.
  Response: We will ensure that more discussion of similar studies in other regions of Greenland are included and cite the suggested literature where necessary.
  The minor comments and a more thorough description of the changes made will be provided in a point-by-point response to reviewers, once the editor has provided guidance on whether we are to upload a revised manuscript.
  Once again, thank you for taking your time and providing your expert opinion on our research. We are hopeful that the editor invites us to provide a revised manuscript.
  Best wishes,
  Dr Jenny Turton, on behalf of all authors.
  
  Citation: https://doi.org/10.5194/tc-2021-45-AC1
RC2:
'Comment on tc-2021-45', Anonymous Referee #2, 07 Apr 2021

The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2021-45/tc-2021-45-RC2-supplement.pdf

Citation: https://doi.org/10.5194/tc-2021-45-RC2
- AC2: 'Reply on RC2', Jenny Turton, 26 Apr 2021
  
  Dear Reviewer,
  In this response, we outline the larger changes that we will make to the manuscript in response to your specific comments. Then, following a decision by the editor, we will then upload a revised manuscript and point-by-point response to all critiques.
  Firstly, thank you for providing your time and expertise to review our manuscript. We greatly appreciate your comments and hope to improve the manuscript by incorporating the changes. In general, we aim to improve the manuscript’s structure for better understanding, especially in section 3.3, and improve the communication of the results by amending the figures. Furthermore, we will extend the description for the lake detection algorithm and include more discussion of lake drainage. We summarise your main concerns and address your key comments below:
  Comment: I would like to see more explanation and justification of the lake identification method including motivation over NDWI method, how to distinguish slush, key limitations and lower lake-area limit.
  Response: The mentioned aspects are critical aspects of the lake detection methodology and are discussed in length in the method paper by Hochreuther et al. (2021), including the paper you suggested (Williamson et al. 2017). The goal of this study is centred around the linkages between atmosphere, cryosphere and hydrosphere, thus we initially thought an in-depth discussion of the lake detection method would not only extend the page volume of the manuscript significantly, but also shift the balance of its aspects quite heavily towards methods. Nonetheless, as we agree these are key aspects of the lake detection method and thus the curiosity is well justified, we will extend the paragraph to include information on the bandratio method, including its limitations (the minimum lake size is one of these). Lake depth could not be calculated, as, due to missing ground truth data, no relationship could be built between the MSI-detected spectra and actual lake depth. Though numerous studies from West Greenland exist (including measurements from there), these values cannot simply be applied due to a number of reasons (different solar angle and azimuth, different lake bottom properties etc.). We will also include this.
  Comment: More Greenlandic studies are required in the discussion.
  Response: We will include more Greenlandic supraglacial lake studies in the introduction and discussion, this is a clear oversight. Both reviewers suggested literature that we will read and incorporate where possible.
  Comment: More information required on lake drainage.
  Response: Both reviewers requested more information about lake drainage results. As the aim of this study was to link climate conditions with lake characteristics, and there is likely little relationship between climate indicators and drainage, we did not include this information initially. Furthermore, given the very cloudy conditions in northeast Greenland, it can often be difficult to quantify whether lakes have drained or are covered with clouds, especially when clouds persist for a number of days and lake drainage may have been missed. However, we will include some discussion of one or two lake drainage events in the form of case studies, so that we may infer the likely drainage mode in this region. Neckel et al. (2020) observed a lake drainage event in our study region and we will analyse this particular event, as we have observations to prove that the lake drained as opposed to being covered by clouds or froze over. Following this, we will assess a number of other lakes, to see whether individual lakes or a number of lakes drain in a similar fashion.
  Comment: A few figures need substantial work.
  Response: A number of figure changes were also suggested by Reviewer 1, so we will modify all figures in the manuscript to better represent the results, synthesise the data and aid in understanding. Specifically: Figure 1 will now include a larger map of Greenland with labels for NEGIS and the grounding line. Figure 2 will include the absolute lake area values (as a line plot) as well as percentage change, with some alteration to the colour bar. Figures 3 and 5 will be split into 4 panels to represent each year, and table data will be included on figure 5. Figures 6 and 7 will be combined into one panelled figure. An additional figure is likely to be included to accompany a short discussion on the lake drainage characteristics- as mentioned by both reviewers.
  Comment: Structure of some paragraphs needs revision, including the discussion and conclusion.
  Response: We will ensure that some sections of the discussion are moved into the introduction, and that the conclusion does not bring any new results to light but includes the wider implications of the findings. Furthermore, we will adapt section 3.3 of the results following suggestions from reviewer 1.
  Suggestion: To include looking at the role of atmospheric rivers.
  Response: The first author of the paper is currently working with a number of collaborators (including the author of the suggested paper you referenced) to look at the role of atmospheric rivers on the whole surface mass balance and melt production in this region. As this is quite a substantial amount of work and results, we will not include it in this manuscript. However, it is likely that the high snowfall amounts in 2018 were related to a high frequency of strong atmospheric rivers, and we could include this in the discussion, and as a pointer to future areas of research.
  The line-by-line comments and a more thorough description of the changes made will be provided in a point-by-point response to reviewers, once the editor has provided guidance on whether we are to upload a revised manuscript.
  Once again, thank you for taking your time and providing your expert opinion on our research. We are hopeful that the editor invites us to provide a revised manuscript.
  Best wishes,
  Dr Jenny Turton, on behalf of all authors.
  
  Citation: https://doi.org/10.5194/tc-2021-45-AC2

Peer review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (12 May 2021) by Stef Lhermitte

AR by Jenny Turton on behalf of the Authors (23 Jun 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (24 Jun 2021) by Stef Lhermitte

RR by Anonymous Referee #1 (01 Jul 2021)

ED: Publish subject to minor revisions (review by editor) (05 Jul 2021) by Stef Lhermitte

AR by Jenny Turton on behalf of the Authors (07 Jul 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (15 Jul 2021) by Stef Lhermitte

Short summary

We assess the climatic controls of melt lake development, melt duration, melt extent, and the spatial distribution of lakes of 79°N Glacier. There is a large interannual variability in the areal extent of the lakes and the maximum elevation of lake development, which is largely controlled by the summertime air temperatures and the snowpack thickness. Late-summer lake development can be prompted by spikes in surface mass balance. There is some evidence of inland expansion of lakes over time.