Evolution of the firn pack of Kaskawulsh Glacier, Yukon: meltwater effects, densification, and the development of a perennial firn aquifer

Ochwat, Naomi E.; Marshall, Shawn J.; Moorman, Brian J.; Criscitiello, Alison S.; Copland, Luke

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

Articles | Volume 15, issue 4

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

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

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

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

Articles | Volume 15, issue 4

Research article

|

23 Apr 2021

Research article |

| 23 Apr 2021

Evolution of the firn pack of Kaskawulsh Glacier, Yukon: meltwater effects, densification, and the development of a perennial firn aquifer

Naomi E. Ochwat, Shawn J. Marshall, Brian J. Moorman, Alison S. Criscitiello, and Luke Copland

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Final revised paper (published on 23 Apr 2021)
Supplement to the final revised paper
Preprint (discussion started on 25 May 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review Ochwat et al.', Anonymous Referee #1, 02 Jul 2020
- AC1: 'reply to reviewer #1', Naomi Ochwat, 14 Aug 2020
RC2: 'Review of Ochwat et al.', Anonymous Referee #2, 14 Jul 2020
- AC2: 'reply to reviewer #2', Naomi Ochwat, 14 Aug 2020

Peer-review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (18 Aug 2020) by Etienne Berthier

Dear authors,

I would like to thank first the two referees for their thoughtful and timely comments. They recognized the potential of your findings but also identified several major shortcomings and weaknesses in your initial submission. Through the interactive discussion, you provided first responses and suggestions of how to address their comments. Just based on these responses, it is not entirely clear yet whether you will be in position to address all major comments.

To continue with the publishing process towards TC, I would like to ask you to upload a revised manuscript (ideally track-changed) for a second assessment, first by me. If I find that you managed to address all major comments, the review process will go on and the revised manuscript will be sent back to the original (and possibly fresh) referees.

At this stage, I expect to receive a high quality manuscript, free of inconsistencies between tables and texts (as listed by reviewer#2), with proper error propagation for the mean ice core density. (Can you back up the fact that all uncertainties are random as you suggest in your response? This is crucial because this assumption leads to very tiny errors for the average density). I also agree with reviewer#1 that an observation-based assessment of the surface lowering due to firn densification can only be made if the mean firn density is known sometime in the past (ideally in 2005). It seems you have new data to make this assessment and I look forward to read how you can integrate them in a deeply revised and improved article.

The present decision concludes the interactive discussion phase and allows me to request a revised manuscript but does not at all anticipate future publication in TC.

Given the amount of additional work that is required to respond to all reviewers’ concerns, do not hesitate to come back to me if you feel that you will not have time to perform these revisions in a reasonable (less than 1 month) time spam.

Best regards,

Etienne Berthier

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AR by Anna Mirena Feist-Polner on behalf of the Authors (28 Oct 2020) Author's response

ED: Referee Nomination & Report Request started (28 Oct 2020) by Etienne Berthier

RR by Anonymous Referee #3 (25 Nov 2020)

Suggestions for revision or reasons for rejection

The manuscript describes firn evolution at a high elevation site on Kaskawulsh Glacier, St. Elias Mountains, Yukon. This is a highly relevant topic and the presented results contribute to an improved understanding of ongoing trends in firn density, temperature and potential development of firn aquifers in mountainous environments. Generally, I find that the observational datasets are well described and interpreted. Also, the comparison with model output is valuable. Still, I have some moderate to major concerns that I would like to see addressed, primarily related to the lack of model calibration and the interpretation of surface lowering. If it would be an option to perform some additional model experiments, I would highly recommend to perform some additional runs to calibrate melt rates e.g. by minimizing the misfit between modelled and observed subsurface temperatures. Right now, discrepancies between the model and observations are hardly discussed and the lack of model calibration makes it difficult to draw strong conclusions on trends in firn conditions. My specific comments are listed below.

Specific comments

L39-41: This needs to be reformulated. The phrase "If the surface continues to melt" should be removed, since refreezing will happen directly when melt water enters cold snow/firn. 'Warming firn' does not necessarily lead to more refreezing, rather the opposite.

L42: A firn aquifer only forms if the water does not directly run off through moulins/crevasses.

L42-44: It would be good to consistently use “perennial firn aquifer” rather than “firn aquifer”, since here long-term (multi-annual) storage of water in firn is meant.

L51-52: Another useful reference for the Canadian Arctic is Noël et al. (2018; https://doi.org/10.1029/2017JF004304), and for Svalbard references to Van Pelt et al. (2019; https://doi.org/10.5194/tc-13-2259-2019) and Noël et al. (2020; https://doi.org/10.1038/s41467-020-18356-1) could be relevant to add.

L55: Here Machguth et al. (2016; https://doi.org/10.1038/nclimate2899) could be cited.

L69: I suppose this refers to air temperature? Please clarify.

L107-117: This part fits better at the start of the next section (3.2).

L137: I suppose L_unc should be dL (or dL in the equation should be L_unc).

L186-190: “Surface lowering associated with refreezing” is confusing. Surface melting leads to thinning and gravitational settling of the snow as well, but refreezing just adds mass to the existing vertical column and does not (directly) cause any thinning. See also my later comment.

Section 3.4: It appears that no calibration of the model has been done, presumably because there were no melt observations to compare to (?). This currently makes it very hard to trust the model output, especially since there appear to be major biases in modelled subsurface temperatures, which may indicate an underestimation of melt rates. See also my later comment.

Section 3.4: Most likely the subsurface model also simulates density evolution, but this is not mentioned here and no graphs of it are shown in the results. It would be an important validation of the model results if it could be shown that simulated density evolution matches the observed densities well.

Section 3.4: I am missing a description of how the model and in particular the subsurface conditions were initialized, i.e. if some spin up has been done.

L249: “heat advection from melt water percolation” seems odd, since melt water typically is at 0 degrees C and if it encounters cold snow it will just refreeze thereby releasing heat. This is the only way in which heat is “advected”, but that is probably not meant.

L251-252: The symbols k_h and k_w are mixed.

L284-293: It remains unclear how melt-affected and not melt-affected firn are distinguished. I think this is an important aspect, because if there is indeed non melt-affected firn with a firn aquifer below, that probably implies that fast deep melt water percolation through piping is an important process here. I would like to see additional discussion on this in the manuscript.

L306-313: What is the density difference between 1964 and 2018 when considering the mean density between the LSS and 15m depth?

L318-319: This is comparing snapshots of subsurface temperature to extract trends. Since subsurface temperatures may vary strongly from year to year, care should be taken to determine long-term trends based on only two snapshots in time.

L344-345: These kind of statements about melt trends are hard to defend without any calibration or validation of melt estimates against observations.

L375-376: “due to increased presence of ice layers”: Is there any information on ice content in the 1964 core?

L381-383: The modelled subsurface temperature trends may be reasonable, but the absolute values are quite a bit off when comparing temperatures in Fig. 5 and Fig. 7. This discrepancy is important to discuss in much more detail in the manuscript, especially since many of the conclusions for example on when firn became temperate and when the PFA may have formed are based on the modelled temperature evolution.

L382-383: “The ERA5 climate analysis”: This is rather an analysis of snow model output. Please reformulate.

L388: “increases” and “effect”

L391-394: I would suggest to reformulate this. It is unclear what this "first stage of densification" is. In my view there are two processes that affect densification 1) gravitational settling (which will go faster at higher temperatures) and 2) refreezing. Refreezing will increase subsurface temperatures, which in turn may increase the densification rate by gravitational settling/packing. That is a completely different sequence of processes than described in L391-394.

L397: “effects”

L411-412: “The firn model predicted the development of wet,temperate conditions in the deep firn following the 2013 melt season, although it took two years to fully develop (Figure 7).” But the observations reveal that the firn was already temperate in 2006. This should be acknowledged.

L440: “Kuipers Munneke et al. (2014)”

L445-446: Kuipers Munneke et al. (2014) indicate what accumulation and melt conditions favour the development of firn aquifers. So in addition to the accumulation comparison it would be good to also compare melt rates with rates observed in southeast Greenland.

L453: Temperature of the firn will not have a major impact on the perennial firn aquifer. Typically once a perennial firn aquifer has formed the firn above it is temperate and the winter cold wave does not penetrate deep enough to case any refreezing. A factor that is important though is how easily the water can runoff via moulins and crevasses.

L454-455: Internal accumulation commonly refers to the amount of refreezing below the last summer surface, which is probably not what is meant here. See for example Cogley et al. (2011; https://wgms.ch/downloads/Cogley_etal_2011.pdf).

L469-470: Ice layers in snow and firn happen in any accumulation zone that experiences some melt, which is the case for the vast majority of glaciers on Earth. Hence, the presence of ice layers in firn is not something special. Please reformulate.

L474-475: This is an important notion. I understand that there may not be any melt observations to make use of, but I would instead strongly suggest to perform new modelling experiments where one or more parameters affecting the modelled melt rates are calibrated such that a best match between modelled and observed subsurface temperature is achieved. Right now, it seems that modelled melt rates are underestimated, which would result in too little water percolation and refreezing in snow and firn, thereby explaining the currently underestimated subsurface temperatures. With a calibrated model, confidence in modelled melt rates and firn conditions would considerably increase!

L477: “with most of the meltwater refreezing”: It is unclear if this is still the case. The subsurface temperatures reveal that firn was already temperate in 2006 implying that already then some melt water did not refreeze.

L484: It would be nice to have an additional figure showing the modelled density evolution.

L487: "0.73+/-0.23 m". If this is calculated from Eq 6 then, if I am correct, this is not the actual surface lowering, but rather the surface lowering relative to a snow/firn pack that would not experience any melting. I do not really see why this is relevant here. For me, the interesting thing to know would be how much additional refrozen mass sits in the firn column in 2018 compared to 1964, because that is a mass term that is missed by geodetic mass balance observations.

L487-488: “to have experienced a minimum of 0.73 ±0.23 m of surface lowering due to internal refreezing”: Refreezing does not lower the surface, melting does and gravitational settling of snow/firn. Please clarify and rephrase.

L487-497: I am missing a bit the point here. Surface elevation changes are the effect of long-term trends in melt and accumulation. How much of the melt water refreezes does not (directly) affect surface elevation or thinning. I would rather expect a discussion here on the impact of increased densification on geodetic mass balance estimates. Geodetic mass balance observations will just consider surface height changes and not any mass changes that result from an increasing density of firn.

L496: “liquid water retention processes cause the surface to lower”. This is not correct. Refreezing just adds mass to the existing firn column, which leads to densification, but not to thinning! Higher firn temperatures after refreezing do speed up the compaction (gravitational settling) process though.

Figure 1: It could be good to include coordinate axes.

Figure 6c: In addition to Fig. 7 also Fig 6c confirms that the modelled subsurface temperatures are much colder than observed (Fig. 5).

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RR by Anonymous Referee #1 (02 Dec 2020)

ED: Publish subject to revisions (further review by editor and referees) (03 Dec 2020) by Etienne Berthier

AR by Naomi Ochwat on behalf of the Authors (29 Jan 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to revisions (further review by editor and referees) (03 Feb 2021) by Etienne Berthier

ED: Referee Nomination & Report Request started (03 Feb 2021) by Etienne Berthier

RR by Anonymous Referee #3 (19 Feb 2021)

RR by Anonymous Referee #1 (28 Feb 2021)

Suggestions for revision or reasons for rejection

General Remarks

The study by Ochwat et al. has again been modified substantially. Once again, I would like to thank the authors for making a thorough effort in revising their study. I think my comments were addressed. The study has gained a much stronger focus on the modelling and the description of the modelling approach as well as the results has further matured.

I have few comments on details and one suggestion which is more general and concerns surface lowering. Assuming that thickening from accumulation and thinning due to ice flow are in balance, the surface at the plateau would remain at stable elevations. However, melt has increased over the recent years likely leading to an imbalance and surface lowering. The terms of the imbalance seem to have three components that all cause surface lowering but their impact on mass balance differs: (i) melt which runs off (actual mass loss; amount poorly known because runoff is difficult to observe), (ii) melt that refreezes (apparent mass loss; subject to uncertainties but better known than the other terms) and (iii) melt that makes it into the PFA (likely contributing to mass loss but this is not known with certainty; this term is not well constrained and overlaps with (i)). Finally, there is a potential fourth component, namely accelerated compaction of warming firn (apparent mass loss, difficult to quantify as there is substantial uncertainty to what degree the firn warmed). While I think the authors have now outlined these different components well, I still suggest distinguishing their influence on geodetic mass balance more clearly in the discussion, in particular in Section 5.3.

Detailed Remarks

Line 420: I suggest removing “perpetual”.
Line 558: “... has played ...” Why past tense? It still plays an important role.
Lines 586 – 587: This sentence is unclear. At the drill site, summer melting is the precondition to both refreezing and mass loss (through runoff). I think what is meant it that both refreezing and runoff will result in surface lowering (but only one of them is associated with mass loss).
Line 588: I do not understand “ ... remaining 27%”. Both 100 - 96 and 100 - 86 (percentages from previous sentence) do not yield 27 %.
Lines 612 – 619: In its current form, the study thoroughly discusses uncertainties in the measurements and in modelling. The statements here in the conclusion, however, do not reflect the considerable uncertainties anymore. No need for another detailed discussion, somewhat more cautious wording or a brief remark on uncertainties might suffice.

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ED: Publish subject to minor revisions (review by editor) (02 Mar 2021) by Etienne Berthier

AR by Naomi Ochwat on behalf of the Authors (12 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (15 Mar 2021) by Etienne Berthier

AR by Naomi Ochwat on behalf of the Authors (18 Mar 2021) Author's response Manuscript

Short summary

In May 2018 we drilled into Kaskawulsh Glacier to study how it is being affected by climate warming and used models to investigate the evolution of the firn since the 1960s. We found that the accumulation zone has experienced increased melting that has refrozen as ice layers and has formed a perennial firn aquifer. These results better inform climate-induced changes on northern glaciers and variables to take into account when estimating glacier mass change using remote-sensing methods.