the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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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Status: open (until 04 Aug 2024)
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RC1: 'Comment on egusphere-2024-1435', Richard Parsons, 25 Jul 2024
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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
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