the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jun 2024
Disentangling the drivers behind the post-2000 retreat of Sermeq Kujalleq, Greenland (Jakobshavn Isbrae)
Abstract. Ocean temperatures have warmed in fjords surrounding the Greenland Ice Sheet, which is causing increased melt along their ice fronts, rapid glacier retreat, and contributes to rising global sea levels. However, there are many physical mechanisms which may mediate the glacier response to ocean warming and variability. Warm ocean waters can directly cause melt at horizontal and vertical ice interfaces or promote iceberg calving by weakening proglacial mélange or undercutting the glacier front. Sermeq Kujalleq (also known as Jakobshavn Isbræ) is the largest and fastest glacier in Greenland and has undergone substantial retreat starting in the late 1990s. In this study, we use a large ensemble modeling approach to disentangle the dominant mechanisms driving the retreat of Sermeq Kujalleq. Within this ensemble, we vary the sensitivity of three different glaciological parameters to ocean warming: frontal melt, subshelf melt and a calving stress threshold. Comparing results to the observed retreat behavior from 1985–2018, we select a best-fitting simulation which reproduces the observed retreat well. In this simulation, the arrival of warm water at the front of Sermeq Kujalleq in the late 1990s leads to enhanced rates of subshelf melt, leading to the disintegration of the floating ice tongue over a decade. Retreat into a substantially deeper bed trough around 2010 accelerates retreat, which continues nearly unabated despite local ocean cooling in 2016. An extended ensemble of simulations with varying calving threshold shows evidence of hysteresis in calving rate, which can only be inhibited by a substantial increase in calving stress threshold beyond values suggested for the historical period. Our findings indicate that accurate simulation of rapid calving-driven glacier retreats requires more sophisticated models of iceberg mélange and calving evolution coupled to ice flow models.
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RC1: 'Comment on egusphere-2024-1435', Richard Parsons, 25 Jul 2024
Overview
This study seeks to determine which mechanism of frontal and submarine melt and ice-ftont calving primarily drove the retreat of Sermeq Kujalleq glacier between 1985 and 2018. Ensemble modelling was performed whereby the magnitude of previously published frontal and submarine melt rates were adjusted alongside the calving stress threshold of a tensile stress based calving law. The calving law was further adapted in an attempt to account for backstresses imparted by ice melange. Optimised parameters were then used to assess potential future behaviour of Sermeq Kujalleq up to 2100.
Although the question of whether oceanic melting or calving processes were the ultimate drivers of Sermeq Kujalleq’s retreat is of great interest, there could be more detail given in methods used in the study in order to justify how physically meaningful the employed parameters are. Further, improvement to existing figures as well as the addition of further figures may help with the clarity of results interpretation and the conclusions drawn from the study. With further clarification added to address the comments below I believe this manuscript would be valuable to the community and recommend it for publication.
Comments
Line 8: Are all the drivers coming from the ocean? There is no mention of atmospheric drivers. Can it be discussed in the methodology why atmospheric drivers are not considered here and the title of the paper should be more specific on this.
Line 49-50: ‘the process of acquiring necessary observations…’ Which observations? And why do icebergs make it more challenging?
Line 77: I’d find it informative to include a figure of the study area. Where are the fast flowing regions? Where is ‘deep in the catchment area’ and what are the flow speeds here?
Line 78: How are different areas of the domain chosen for coarsening / refinement?
Line 82 – 85: ‘A linear-viscous sliding relation…’ How does the inversion arrive at appropriate coefficients for the sliding law? I don’t understand what is meant by ‘which have been scaled down to give a smooth transition between both data sets’, please expand on this.
Line 86-87: What are the boundary conditions elsewhere in the domain?
Line 95: Can you explain the correction that has been applied in more detail? I don’t understand what has been done from this sentence.
Line 112: Is the linear decrease in calving stress threshold supported by previous studies, observations or physics?
Line 114: Equation 3 – explain what sigma_max and sigma_min used in this equation are.
Line 129: Equation 4 – A and B are not explained. Are these tuning parameters? What are their values?
Line 139: Equation 5 – Add a table with values of parameters such as gamma_T etc.
Lines 144-150: How the parameter space is varied may be better suited in the respective sections 2.2 and 2.3 rather than in the section detailed the mismatch score.
Line 145: ‘we choose a range of 0-4x the empirical parametrisations’ What are the choices of range based on? Are there any observations / literature to support this?
Line 146: 220-360kPa – same comment as above. Please justify the choice of ranges. Figure 2 shows results up to 350kPa, should this be 360kPa?
Figure 1: It would be helpful to know where the grounding line is and how this moves through the simulation. A lot of the results interpretation talks about the ice tongue and it would be very helpful to visualise how the extent of the ice tongue changes and a note to emphasise when it disintegrates.
Figure 1: Is ‘Elevation’ shown colour map ‘Bed elevation’?
Figure 1: I find the figure somewhat difficult to interpret. As the modelled and observed calving fronts deviate quite widely it is difficult to tell which dashed lines should be compared to which solid lines from the colour scheme. Is it necessary to present data from every year or can the colour scheme be adjusted to make it more clear which modelled and observed calving fronts should be compared?
Line 154: No need to write ‘within’ as well as ‘less than’
Line 155-162: I don’t understand the scoring system from the description given here. Please can this be addressed and made more explicit?
Line 161: How is the error vector (starting and end point) defined?
Line 168: ‘thin lines’ should be dashed lines
Line 169: ‘thick lines’ should be solid lines
Line 177: For discussion on the disintegration of the ice tongue, no grounding line locations are shown in figure 1 so the location / extent of the ice tongue is not clear
Line 186-187: It would be interesting to see the evolution of tensile stress seen at the terminus of both branches over the years. Can a figure be used to illustrate this?
Figure 2. It is not clear why the specific parameters are presented in panels b – d.
Figure 2: Is M_f the same as M_fr (given in equation 4)?, Is M_s the same as M_sm (given in equation 5)?
Figure 2: What is the red dot displayed on panel b? Best overall fit? Please add explanation to caption.
Figure2: alpha_ms and alpha_mf are not defined anywhere, are these the same as M_s and M_f?
Line 198-199: ‘…than suggested by the two-equation parameterisation.’ Add citation for this.
Line 204: With the best fitting simulation requiring high sensitivity to frontal melting, how do the ablation rates m_fr and c compare in the parameter space? It would be interesting to see whether the calving rates and frontal melt rates vary on the same magnitude.
Figure 3: Panel B. Observed area change time series vs modelled area time series would be more informative.
Line 218: ‘SK maintained a floating ice tongue ahead of its terminus’. Is the terminus not the front of the ice tongue at this point?
Line 239: Need figures to support this statement
Line 230: Calving rate in this study is defined by speed, stress and stress threshold. Temperature can only play a role through these variables, why would we expect calving fluxes to be controlled by temperature? The calculation of ‘calving flux’ should also be defined as different definitions appear in the community (not as unique as ‘grounding line flux’)
Line 269: With the runs up to 2100, significant terminus retreat is observed. Is the same mesh considered as had been described previously? Is the mesh resolution at these retreated grounding line locations still 400m?
Line 272: How is the range of threshold values 260-437kPa chosen? Same comment as on Line 146.
Line 278-279: ‘even if ocean temperatures returned to the coldest values achieved during the historical period over the next 80 years, the rapid acceleration of calving and retreat would likely continue unabated’ – this may well be due to a limitation with the calving law which should be acknowledged, rather than necessarily being a direct indication of what may happen in the future.
Figure 4: caption – What is the 2100 run? The ensemble members running from 1985 – 2100? The caption could be more concise.
Line 305: sigma_max or sigma_thr?
Line 315-320: See ‘Parsons et al, 2024, Quantifying the Buttressing Contribution of Sea Ice to Crane Glacier’ for proposed methodology for assessing the terminus stress regime with/without adjoining melange elements. Could a similar method be used in this study to account for changes in the terminus stress regime with the presence of melange?
Figure 5: Panel A. Improve the quality of this figure. It appears to have been stretched.
Citation: https://doi.org/10.5194/egusphere-2024-1435-RC1 - AC1: 'Reply on RC1', Ziad Rashed, 30 Aug 2024
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RC2: 'Comment on egusphere-2024-1435', Anonymous Referee #2, 01 Aug 2024
Rashed et al. simulate SK evolution. They incorporate several parameterizations and through a number of sensitivity runs determine the combination of parameters that yields the most accurate evolution of the glacier extent. They identify different phases of retreat, depending on the predominant ice loss mechanism. They then continue some runs into the future, observe a runaway retreat along the southern SK trunk and argue that only sufficiently strong melange would be able to stop this retreat.
Major comments are bellow. More comments (not necessarily minor) are in the attached pdf.
*Novelty
I think the main new addition in this paper is the parameterization of ice melange effects. The authors do that by making calving stress threshold a linear function of ocean temperature. It is not clear how significant this addition is, as there is no comparison to simulations without this feature. Previous studies using the same model (Bondzio et al) were able to reproduce SK evolution just as well, so it seems that the proposed form of melange parameterization is not necessary for reproducing the past. This is a major issue for the claims and findings in this paper with regard to the importance of melange to SK dynamics - essentially there is no control run presented here to evaluate this importance against.
*Goals and Scope*The paper needs to better clarify the questions it is addressing and how the answers it comes up with support, or contradict what is already known about the SK retreat.
There are multiple papers by Bondzio et al that concern SK, and I believe the authors here use the same model and try to build on this work. One of the papers of Bondzio focuses on disentangling drivers and mechanism of the retreat. How does your study compliment this work. Or other existing hypothesis of the retreat?
The authors write "there is still debate about which physical processes are responsible for mediating the glacier response to ocean warming" but the reader never learns about what the debate is and how this study contributes to this debate.
How about the papers of Nick et al. and similar, they seem to be relevant to this paper.
For an example how to clearly state hypothesis and place your study in existing context, see for example Bondzio et al. 2017: "The mechanisms behind Jakobshavn Isbræ’s acceleration and mass loss: A 3-D thermomechanical model study"
*Methods
There is no information about the temperatures used to prescribe melting. A mention is made about ECCO, but no detail what so ever about how are these temperatures propagated/extrapolated into the fjord, whether topography, namely the sill, is taken into account during this extrapolation. Some comparison of existing vertical temperature profiles with whatever vertical temperature profiles the authors come up with would be good to show too.
Clarify when is frontal melt prescribed. Only when there is no floating tongue? Or also on the vertical calving front of the floating tongue? If the latter is the case, then how do you account for the effect of convective forcing due to meltwater from basal melting emerging at the front - do you have an estimate for that?
There is no information about which parameters are varied throughout the sensitivity studies. These parameters then appear, unexplained, in figures, but they are nowhere introduced.
*ValidationThere are some strange unphysically looking calving front positions that may indicate problems with the calving algorithm. Specifically, I am referring to the appearance of two structural arches, well before the glacier splits from one through into two (I would give you a location but there are no axes on the plot). Can you explain this?
None of the simulations appears to be able to stabilize and readvance once it has begun to retreat, unlike what happened in observations around the 2017 cooling. This indicates that the model is more eager to retreat than in reality. This needs to be addressed.
*Results
The results are not in general supported by figures. There needs to be a clear separation between results and discussion/interpretation and 'general knowledge' statements which assume that the reader is inclined to agree with the authors. All results should have figure references, which is currently almost never the case. This is a major issue in assessing the manuscript contribution.
There is no stress quantification that would support several claims and arguments by the authors.
The results seem to suggest that the main ice tongue disintegration occurred via basal melting and not by calving (Fig 3a). Is there observational evidence for that? I was under the impression that there were some major calving events that diminished the floating tongue, rather than in just melting off.
*Hysteresis
The authors run experiments with different calving stress thresholds into the future(? unclear what temperature forcing they are using). And then claim the system experiences hysteresis, but they don't actually do experiments with reversal of the forcing conditions. While it seems perfectly plausible the system experiences hysteresis, it was not actually shown in this paper, and as such it remains a hypothesis.
*Discussion/conclusion
There are some relatively strong statements at the end of the discussion and conclusion sections advocating for a fully dynamic and coupled model of melange. I don't see how any of this need is supported by this paper. What was done here is only a very simplistic parameterization of calving rates on ocean temperature, and as admitted, no feedbacks between melange strength and calving rate were incorporated. There is probably still room for more sophisticated parameterizations (e.g. following Schlemm et al.)
- AC2: 'Reply on RC2', Ziad Rashed, 30 Aug 2024
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