Atlantic Water warming increases melt below Northeast Greenland’s last floating ice tongue

Domain: earth_science
Published: 2024-02-20
Editorial outcome: minor_revision

Abstract
The 79 North Glacier (79NG) features Greenland’s largest floating ice tongue. Even though its extent has not changed significantly in recent years, observations have indicated a major thinning of the ice tongue from below. Both ocean warming and an increase in subglacial discharge from the ice sheet induced by atmospheric warming could increase the basal melt; however, available observations alone cannot tell which of these is the main driver. Here, we employ a global simulation which explicitly resolves the ocean circulation in the cavity with 700 m resolution to disentangle the impact of the ocean and atmosphere. We find that the interannual variability of basal melt below 79NG over the past 50 years is mainly associated with changes in the temperature of the Atlantic Intermediate Water inflow, which can be traced back across the Northeast Greenland continental shelf to the eastern Fram Strait with a lag of 3 years.

Introduction
While ice shelves in Antarctica are ubiquitous extensions of the Antarctic Ice Sheet, in Greenland only three glaciers possess a floating ice tongue: the Nioghalvfjerdsfjorden Glacier (79 North Glacier, 79NG), Ryder Glacier, and Petermann Glacier. The largest of these glaciers is the 79NG in Northeast Greenland (Fig. 1a). Its ice tongue stretches over 80 km in length in a 20 km wide fjord (exceeding the internal Rossby deformation radius of 3–4 km, Supplementary Fig. 1c), which has a maximum water depth of 900 m1 as a result of glacial overdeepening. At the mouth of the fjord, the shallowest connection between the fjord and the continental shelf is a sill of 340 m depth. Moreover, islands along the fjord mouth act as pinning points. The floating ice tongue is anchored to the sides of the fjord and progressively thins from around 500 m at the grounding line to 100 m at the pinning points (Supplementary Fig. 1f). The 79NG and its neighboring glacier, the Zachariæ Isstrøm (ZI), drain the Northeast Greenland Ice Stream which covers 12% of the Greenland Ice Sheet area2. Its complete melt would lead to a 1.1-m global sea level rise3. In contrast to the ZI, which lost its floating ice tongue in the 2010s4, the extent of the 79NG has not changed significantly in the past decades. This is possibly due to the pinning points at the calving front of the glacier3. However, the ice tongue has been thinning in the past two decades4–6.

There are several hints that the thinning of the 79NG ice tongue can be attributed to the ocean, in particular to an increase in the temperatures of the Atlantic Intermediate Water (AIW) that occupies the deeper parts of the Northeast Greenland (NEG) continental shelf7–9. This year-round flow of warm (>1 °C) and salty waters through a narrow channel into the cavity is hydraulically controlled by an upstream sill, as revealed by one year of mooring measurements at the 79NG calving front10. The supply of AIW leads to melting at the base of the ice tongue and undercutting of grounded ice3. Studies based on satellite imagery5 estimated a negative mass balance for the ice tongue of the 79NG during the time period 2011–2015, and revealed that basal melt rates exceeded the ice discharge across the grounding line.

Apart from the ocean which carries the warm AIW into the cavity of the 79NG, the atmosphere impacts the stability of the glacier as well via subglacial discharge11. Runoff at the ice sheet surface results from ablation and liquid precipitation and drains to the bed of the ice sheet. It then flows towards the grounding line and, if this is below the sea surface, enters the ocean as subglacial discharge12. The outflowing freshwater vigorously entrains ambient waters, speeds up a buoyant plume that develops downstream of the grounding line at the base of the ice tongue, and thereby locally increases basal melt. This freshwater source is typically most pronounced around Greenland in summer and is almost absent otherwise. Runoff has been increasing during the past decades, a result of enhanced surface air temperatures over the Greenland ice sheet13.

Covered by an ice tongue of several 100 m thickness, the ocean circulation in the cavity of the 79NG is thus far largely unknown. Measurements with an Ice Tethered Mooring14 and CTDs15 allow a glimpse into the cavity but are constrained to single locations. Results from refs. 7,10,16,17 showed the important role that topographically steered subsurface currents play in transporting the warm AIW across the continental shelf and into the cavity. Yet these continuous observations were only started in 2016, and do not allow to investigate multi-annual to decadal changes in ocean heat supply. In addition, there is yet no realistic model simulation that resolves the subsurface flow on the shelf, and that simulates the 3D circulation of AIW within the cavity where it drives the basal melt. Modeling studies exploring the ocean circulation at the 79NG and in other cavities of Greenland glaciers (e.g. Petermann and Ryder) have been so far restricted to idealized setups in two dimensions18–20.

Here we present a dynamically consistent framework that connects the impact of both ocean warming and subglacial discharge with the melt rates below the ice tongue of the 79NG on interannual to decadal timescales. We apply a global 3D multi-resolution sea ice–ice shelf–ocean model, the Finite-volumE Sea ice-Ocean Model (FESOM2.1,21), which resolves the cavity of the 79NG with a dynamically appropriate horizontal resolution of 700 m (Methods, Supplementary Fig. 1). With this methodology we seamlessly connect the global and regional ocean circulation to the circulation in the cavity. Our simulation with realistic bathymetry and ice shelf geometry covers the time period 1970–2021. With this simulation we disentangle the drivers of the upward trend and interannual variability of the basal melt, and find that ocean warming conveyed via ocean currents has played a dominant role over the past 50 years.

Results
The Northeast Greenland continental shelf: ocean conditions
The 79NG is connected to the large-scale ocean circulation via a C-shaped trough system on the NEG continental shelf, consisting of the Norske Trough in the south and the Westwind Trough in the north (Fig. 1a). Originating from the North Atlantic, warm and salty Atlantic Water is transported northwards by the West Spitsbergen Current (WSC) toward the Arctic Ocean22. A fraction of it recirculates in the central Fram Strait23, subducts underneath cold and fresh Polar Water (PW), and finally enters the NEG shelf via Norske Trough7. There, it mixes with the colder Arctic Atlantic Water (AAW) that circulates through the Arctic Ocean24, forming AIW (defined here as waters with potential temperature > 1 °C). The water column on the NEG continental shelf is thus strongly stratified, with the relatively warm and salty AIW below a layer of cold and fresh PW. Our simulation reveals a current that transports the AIW from the mouth of Norske Trough northwestwards through the trough system (Fig. 1d), in agreement with mooring measurements in central Norske Trough which revealed a 10-km-wide bottom-intensified jet carrying AIW towards the glaciers16. AIW is present in Norske Trough, while fading out towards Westwind Trough in agreement with historic observations7. This indicates that AIW is being cooled by mixing with the overlying PW on its way to the inner shelf and that it does not enter the continental shelf through Westwind Trough. The flow of PW (defined by T < 1 °C, in agreement with ref. 25), forms an anti-cyclonic loop on the continental shelf. It flows northwards along the Greenlandic coast, continues eastwards through Westwind Trough, and then joins the boundary current flowing southwards along the continental slope as the East Greenland Current (EGC) (Fig. 1e).

As in-situ hydrographic observations on the NEG continental shelf are sparse due to harsh weather conditions and the year-round presence of sea ice, we compiled decadal means of hydrographic measurements in southern Norske Trough from the 1970s to the 2010s. Time-series of observed AIW temperature (vertical maximum of temperature in the water column) demonstrate a marked warming trend, reproduced by our model (Fig. 1c). Mean temperatures increased by 0.9 °C in the observations and by 0.8 °C in the model from the 1970s to the 2010s. Averaged over the entire time period 1970–2021, a slight positive AIW temperature bias of around 0.3 °C in the mean state of the model simulation on the NEG shelf compared to the observations is found. This bias can be traced back to the Nordic Seas with a warmer-than-observed Norwegian Atlantic Current. However, the simulated AIW temperatures are mostly within the observational uncertainty range and correctly represent the increasing trend. Moreover, mooring measurements conducted in Fram Strait revealed a positive trend in the WSC temperatures of 0.5 °C decade−1 for the time period 1997–201826, replicated by the FESOM2.1 simulation. We are thus confident that the hydrography on the NEG shelf and in the wider Fram Strait is well represented in the model, which can then be used to study the circulation in the cavity.

Mean ocean circulation in the cavity and basal melt rates
The FESOM2.1 simulation reveals warm and salty AIW entering the cavity through its deepest channel, leaning against its northern flank (Fig. 2a, c). The inflow of AIW amounts to 69 mSv, higher than the 46 ± 11 mSv obtained from mooring-based estimates at the calving front10. The inflow occurs primarily at the main calving front and not through Dymphna Sound whose sill is too shallow for AIW to enter 17,27. The flow of AIW into the cavity is constrained by a sill at the calving front, as shown by a transect of simulated mean temperatures following the AIW pathway from the continental shelf into the cavity (Fig. 2e). The bottom-intensified gravity current accelerates down the slope of the sill, reaching flow speeds greater than 0.3 ms−1, accompanied by decreasing temperatures. This is presumably due to hydraulic control and the associated mixing and entrainment of AIW with ambient water10,28, which reduces the thermal forcing that reaches the grounding line. This is comparable to Ryder Glacier in North Greenland, which is shielded by a bathymetric sill as well29. The warm AIW then circulates cyclonically around the cavity. In particular, the jet turns north, loops into the entrance of Dymphna Sound and continues along the northern slope of the cavity. On its way towards the grounding line, the jet decelerates and becomes broader (Fig. 2d). The flow is thus clearly affected by rotation, deviating strongly from a two-dimensional estuarine circulation.

The inflow of AIW results in ocean temperatures above the freshwater freezing point in the cavity throughout the year, in agreement with CTD measurements15, which leads to strong and persistent melting at the base of the ice tongue. In the model, melting (or refreezing) at the base of the ice is computed based on the three-equation system30 and depends on velocity-dependent salt/heat exchanges (see Methods). The simulated long-term mean basal melt rate is highest with a maximum value of 85 myr−1 close to the grounding line (Fig. 2b), where the ice tongue is thickest (Supplementary Fig. 1f). This means that the densest (and thus warmest) waters in the cavity interact with the ice there. Near the grounding line, the ice base slopes steeply upward, allowing for (and shaped by) the rising meltwater plume. The simulated spatial pattern compares well with basal melt derived from satellite imagery (Ref. 5, Supplementary Fig. 2). The mean simulated basal melt water flux amounts to 17.0 ± 7.9 km3yr−1 (mean ± standard deviation of monthly means, equivalent to 0.54 mSv). This simulated value is higher than the remote sensing-based estimate (ref. 5, 11.9 ± 1.6 km3yr−1), but compares well to the estimate based on heat transport into the cavity from mooring measurements (ref. 10, 17.8 ± 5.2 km3yr−1). Note that the remote sensing based estimate may be on the lower side as it does not take into account basal melting close to the grounding line.

Both basal meltwater and subglacial discharge (the latter contributing around 20% of the overall freshwater input to the cavity) sustain a meltwater plume. Driven by its buoyancy and the effect of the Coriolis force, the plume accelerates along the southern flank of the cavity, reaching mean velocities > 0.3 ms−1 at section b-b' (Fig. 2d). On its upper boundary, freshwater from basal melt is injected along the plume’s path, whereas on its lower boundary, warm and saline ambient water of AIW origin is entrained. When reaching a level of neutral buoyancy, the plume detaches from the base of the ice tongue. The bulk of the submarine meltwater exits the cavity at around 150 m depth (Fig. 2c). This agrees with measured noble gas concentrations, indicative of submarine meltwater, which peak between 100 and 150 m at the 79NG calving front31. We find that most of the export occurs through the main calving front (mean transport of 62 mSv, 82%), whereas the export through the narrow Dymphna Sound is smaller (14 mSv, 18%). When reaching the continental shelf, the meltwater in the plume is strongly diluted (considering the freshwater input of 0.54 mSv and an outflow of 76 mSv, the plume contains around 1–2% meltwater).

Trend and interannual variability of basal melt rate
The simulated basal melt of the 79NG shows pronounced interannual variability between 1970 and 2021 (standard deviation of annual means: 3.9 km3yr−1), superimposed on a multi-decadal increase yielding a linear trend of 2.3 km3yr−1 decade−1 (Fig. 3a). The trend is even stronger during the last two decades (3.4 km3yr−1 decade−1). In the following we will disentangle the impacts of two main drivers of basal melt, namely ocean heat supply and subglacial discharge. Concurrent to the increase in basal melt rates, the simulated maximum temperature of AIW at the calving front (standard deviation of 0.36 °C) increased by 0.19 °C decade−1 during 1970–2021. This warming is consistent with the warming in the Atlantic Water layer upstream at Fram Strait (Fig. 4a, b). Besides the warming trends in the ocean, increasing air temperatures over the NEG ice sheet in the last decades13,32 have led to higher ablation and thus increased subglacial discharge entering the 79NG cavity. The subglacial discharge rate (standard deviation of 2 km3yr−1) has more than doubled with values of 3.3 km3yr−1 in the 1970s and 6.7 km3yr−1 in the 2010s, featuring a linear trend of 0.9 km3yr−1 decade−1. Given the simultaneous increase of AIW temperature and subglacial discharge, we will next quantify their relative contributions to the evolution of the basal melt of the 79NG, considering interannual and long-term variability.

The annual mean basal melt of the 79NG shows a significant positive correlation with the maximum AIW temperature at the calving front for the time period 1970–2021 (r = 0.85, 99% significant, Fig. 3a). The latter can be traced back into Norske Trough (with a lag of one year), the continental shelf break (two years lag) and into the wider Fram Strait area (three years lag), as revealed by point-wise lagged correlations (Fig. 4c, d). The correlation map thus visualises the pathway that Atlantic Water takes before reaching the NEG glaciers. Note that a mooring array across Fram Strait has been continuously monitoring ocean temperatures in the WSC since 199722; this data source could thus be exploited to predict basal melt rates at the 79NG three years in advance. Anomalously high basal melt rates are not only associated with increased temperatures but also with an acceleration of the entire cavity circulation. Composites of cross-sectional velocity at the calving front during years of low/enhanced basal melt show a pronounced decrease/increase in the speed of both the inflow and outflow respectively (Supplementary Fig. 3).

As a side note, we find a similar high positive correlation of basal melt and AIW temperatures at ZI, 79NG’s neighboring glacier (r = 0.64 for the time period 1970–2009, 99% significant). However, our ocean model does not resolve its transition into a tidewater glacier that started in the 2010s. An ocean model with changing ice tongue geometry would be needed here, a task left for future research.

To isolate the effect of ocean temperatures on basal melt rates, we conducted a 22-year long sensitivity experiment in which the input of interannually-varying subglacial discharge (light blue line in Fig. 3a) is replaced by a climatological seasonal cycle (experiment CLIM, Methods). The resulting basal melt rates and maximum AIW temperature at the calving front are shown in Fig. 3a (dashed lines). By eliminating the effect of interannual variability of subglacial discharge, we obtain a higher correlation of basal melt rate and ocean temperatures of r = 0.89 (99% significant, time period 2000–2021), compared to r = 0.77 (99% significant, time period 2000–2021) in experiment REF. Melt rates increase quadratically with the increase of ocean temperatures in the absence of subglacial discharge variation (black line in Fig. 3b). The reference run does not reveal a significant correlation between basal melt and subglacial discharge (r = 0.22, not significant at a 5% error probability, Fig. 3a). We conducted a suite of sensitivity experiments with reduced (0%, 25%, 50%, and 75%) and enhanced (125%, 150%) subglacial discharge (Fig. 5a), branched off from the reference experiment in 2010 and run for another 5 years (see Methods). The changes in freshwater input associated with altered buoyancy lead to a response in the speed of the meltwater plume in the cavity relative to the reference run. As the basal melt rate is computed based on turbulent heat and salt fluxes with coefficients depending on the friction velocity, the melt rate responds on the order of days. A reduction/increase in subglacial discharge decreases/enhances basal melt rates (Fig. 5b). In the absence of summer subglacial discharge, basal melt rates are nearly constant throughout the year. The relationship between basal melt rate and subglacial discharge, averaged over the five years of the experiment, reveals a square root dependency (Fig. 5c).

To summarize, the basal melt rates exhibit different sensitivities to ocean temperatures and subglacial discharge. Our experiments revealed that basal melt has a quadratic dependency on AIW temperature and a square-root dependency on subglacial discharge (Figs. 3b and 5c). Similarly, model studies19,20 and observations33 pointed to an above linear dependency of basal melt on thermal forcing, and a square-root dependency on subglacial discharge. These relationships, derived for ice tongues and ice shelves, vary slightly from the ones derived for tidewater glaciers where subglacial discharge can exceed the subsurface ice melt. Here, a linear relationship with thermal forcing and a cubic relationship with subglacial discharge has been obtained12,34–36. Implications of the sensitivities found in our study for the future state of the ice tongue will be discussed now.

Methods
Ocean-ice shelf-sea ice model FESOM2.1
In this study, we employ the Finite-volumE Sea ice-Ocean Model (FESOM2.1), which solves the standard set of hydrostatic primitive equations under the Boussinesq approximation with the finite volume method21. FESOM2.1 is formulated on unstructured meshes, allowing for mesh refinement in areas of interest. The sea ice model operates on the same mesh as the ocean model, and its details can be found in ref. 41. FESOM2.1 performs well on a global scale42, and the global model has been used for several regional applications such as eddy dynamics in the Arctic Ocean43, heat transport into the Barents Sea44 and variability of AIW on the NEG continental shelf45.

FESOM2.1 includes an ice shelf component which resolves the 79NG and ZI cavities. The treatment of cavities closely follows their implementation in the previous version FESOM1.446. The interaction between ocean and ice shelf is simulated using the three-equation system30. It computes melting and refreezing at the ice shelf-ocean boundary layer caused by velocity-dependent salt/heat exchanges.

In our setup we set the dimensionless drag coefficient Cd to 1.25 × 10−3, which is close to the optimal value determined from sensitivity tests for simulations of the Petermann Glacier19. Note that the simulated basal melt rates are sensitive to the choice of the drag coefficient. Sensitivity experiment with drag coefficients of 1.875 × 10−3 and 2.5 × 10−3 carried out for one year revealed that the basal melt was increased by 27% and by 48% respectively relative to the reference run, whereas the variability did not change (Supplementary Fig. 4).

In the regions with coarse resolution, isoneutral tracer diffusion47 and the Gent-McWilliams (GM)48 eddy parameterization are applied. The GM parameterization is switched off when the horizontal mesh is finer than 30 km. The vertical mixing parametrization used here is the turbulent kinetic energy mixing computed by the CVMix package49. We apply a surface salinity restoration to the PHC climatology50 with a restoring velocity of 1.929 × 10−6 ms−1. The restoration helps to avoid local salinity trends that can occur in response to inaccuracies in, for example, precipitation.

Model setup
The unstructured triangular mesh designed for this study was generated with JIGSAW-GEO51. The background resolution in the global oceans is set to 1° and refined to 25 km north of 40° N. In the Arctic Ocean and Nordic Seas, the mesh is refined to 4 km, and further refined to 2.5 km on the Northeast Greenland continental shelf. In the vicinity of the 79NG, mesh resolution is set to 700 m. This resolution is appropriate to resolve the Rossby radius of deformation and thus the geostrophic circulation with at least 3 grid points (Supplementary Fig. 1c,d). For vertical discretization, 86 z-levels are used, with 5 m layer thickness in the top 100 m, and coarser layer thickness towards the ocean floor. Until 900 m water depth, which is the maximum depth below the ice tongue of the 79NG, the coarsest layer thickness is set to 25 m. Bathymetry and ice shelf topography are taken from the RTopo-2.0.4 data set1. In our model setup, the ice shelf topography of the 79NG represents present day conditions, and does not change with time. While this assumption is appropriate for the 79NG, where the grounding line and ice shelf topography did not change significantly in the last decades6, it does not hold for the ZI which strongly retreated starting from the 2010s4.

Atmospheric data from JRA55-do-v1.4.052 was used to force the model. The dataset has a spatial resolution of 55 km and temporal resolution of 3 hours. The JRA55-do-v1.4.0 dataset also includes global river runoff. For Greenland, we replace the river runoff by data sets of solid ice and liquid water discharge from refs. 53, 54, respectively. Solid ice discharge from the Greenland ice sheet is defined as the product of ice sheet surface velocity and thickness along an outlet glacier flux gate. As the dataset only covers the years from 1986 onwards, we use an average over the 1986–1990 period prior to 1986 in the simulation. The dataset of liquid water discharge contains runoff (the sum of ablation and rainfall minus retention and refreezing) from the regional climate model RACMO, in particular ice sheet runoff and land runoff, that has been routed to ice margin outlets and coast outlets, respectively, with the topography datasets ArcticDEM and BedMachine. Both runoff datasets have daily temporal resolution and high spatial resolution (~100 m; resolving individual streams). For every data point of the Mankoff et al. data sets, the nearest model grid point around Greenland is identified and solid and liquid discharge is then spread at the ocean surface over a prescribed radius of 50 km. In the cavities, the treatment of discharge is different. Here we inject the liquid discharge (now termed subglacial discharge) evenly along the grounding line, and then spread it over a radius of 10 km. Thus, at the 79NG, we inject the liquid discharge at a depth of around 500 m, whereas it is spread at the surface model layer for all other Greenland glaciers.

A control simulation was carried out for the time period 1960–2021 (experiment REF), with a timestep of 3 min. The first 10 years are considered as spin-up, and are not included in the analysis. With 2880 cpus used, a throughput of 3 model years per day was achieved on the NHR@ZIB supercomputer. The calculations for this research were conducted with computing resources under the project hbk00099.

Limitations of the model
There are a number of limitations of the model that will be listed here.
• In our model setup we assume a fixed geometry of the ice tongue, neglecting thereby the ice-ocean interaction. The floating ice tongue of the 79NG has been thinning in the last two decades5,6, which is thus not represented in the model geometry.
• Subglacial discharge is implemented in the model as a line plume, i.e. it is spread evenly along the grounding line. Another approach would be to inject the discharge as axisymmetric plume which mimics the inflow through subglacial channels, assuming a fixed outlet width. Ref. 55 pointed out that the geometry of the plume (line plume vs axisymmetric plume) has implications for entrainment and basal melting and thus the structure of the meltwater plume (line plumes leading to higher basal melt rates and entrainment than axisymmetric plumes). For the 79NG, detailed observations of subglacial discharge outlets are missing. Still, the method of injecting the discharge in the model has an impact on the basal melt rates.
• Measurement campaigns at e.g., Petermann Glacier showed that the base of the ice shelf is characterized by subglacial channels of 1–2 km width transporting glacially modified water towards the calving front56. Observations at Thwaites Glacier, Antarctica, show that these channels play an important role for basal melting57. These features are however not represented in our model, leading to possibly underestimated melt rates.
• In our model setup, we do not account for tidal motions in the cavity. Observations with an ice tethered mooring revealed that tidal currents in the cavity are weak14, being one order of magnitudes smaller than the mean inflow velocity of warm AIW.
• The model presented here has a positive temperature bias of around 0.3 °C, which can be traced back to the Nordic Seas with a warmer than observed Norwegian Atlantic Current. The warm bias can presumably be related to model deficiencies in resolving lateral ocean heat fluxes due to the fact that mesoscale eddies are not fully resolved with the applied resolution in the Nordic Seas. These lateral heat fluxes play a larger role in cooling the Atlantic Water carried by the Norwegian Atlantic Slope Current northwards58. We acknowledge that this has an effect on the simulated mean basal melt rates which are rather on the higher side; it is probably not critical for the interannual variability and sensitivities that we find.

Sensitivity experiments
In addition to the control simulation, we conducted several sensitivity experiments. Firstly, to test the impact of ocean temperatures on basal melt rates, we carried out a 22-year simulation (2000–2021) with a climatological seasonal cycle of subglacial discharge (constructed by averaging the subglacial discharge by ref. 54 that is injected at the grounding line of the 79NG over the same time period). This simulation is termed CLIM. Secondly, to test the impact of subglacial discharge on basal melt rates, we conducted a suite of 5-year long simulations branched off in 2010 where the subglacial discharge at 79NG was set to zero, reduced (to 25%, 50% and 75% of the original data), and enhanced (to 125%, 150% of the original data).

Correlation coefficients
Prior to computing the correlation coefficient, all time series were linearly detrended. Correlations are regarded as significant when they are different from 0 at a 95% confidence level (two-sided hypothesis test).

Hydrographic measurements
To assess the simulated temperature variability in Norske Trough, we use CTD measurements compiled by ref. 45. It includes processed CTD data from refs. 7, 59, 60, and from the World Data Center PANGAEA61, 62, covering the time period from 1970 to 2020. As the overall number of CTD profiles is relatively low, we computed decadal averages and the corresponding standard deviation.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review for “Atlantic Water warming increases melt below Northeast Greenland’s last floating ice tongue”\nThis manuscript uses a global simulation to resolve the ocean circulation in the cavity of the 79NG with 700m resolution to explain the impact of ocean and atmosphere. The authors find that, the interannual variability and long-term trend over the past 50 years of the basal melt below the 79NG are mainly associated with changes in the temperature of the AIW. This manuscript is clearly expressed the whole story, and it is recommended to accept based on my opinion.\nSuggestion:\nThe methods parts seem not to be the exact format with this journal.\nAnother thing I am very curious about is, since the authors did the FESOM2.1 among the entire northeast Greenland, and for ZI, it did not collapse until 2014, so did the author analyze the same situation for ZI before it collapsed?", "reviewer_2": "Comments to authors\nManuscript NCOMMS-23-49679-T “Atlantic Water warming increases melt below Northeast Greenland’s last floating ice tongue” by Claudia Wekerle, Rebecca McPherson, Wilken-Jon von Appen, Qiang Wang, Ralph Timmermann, Patrick Scholz, Sergey Danilov and Torsten Kanzow\nThis manuscript investigates using a regional and global model the melting of the 79North glacier ice tongue and addresses the role of the ocean (i.e. warming of the Atlantic intermediate Water (AIW)) and the atmosphere (i.e increased discharge due to the warming of the atmosphere).\nThe authors find a dominant role of the ocean in the melting of the the 79North ice tongue.\nThe work used a combination of numerical model and observations both to validate the results obtained by the numerical model but also to corroborate them.\nThe question addressed by the authors is a long-standing question and the contribution brought by this work is relevant both for the oceanographic and glaciological community.\nIn general, I find the work well written, the methodology used is mostly clear, the results are clearly presented and the discussion and perspective well highlighted.\nI think the manuscript is worth publishing but I have a few questions for the authors.\n1) Line 47-48 I find this sentence hard to read. The concept is clear, but I would suggest rephrasing\n2) Line 51-54 plumes do not form only in cavities but also at the margin of outlet glaciers. The impression here is that they form only in cavities. Please rephrase accordingly.\n3) Line 61 I think the “as” should be deleted\n4) The authors clearly state that one of the limitations of the numerical model is the warm bias of the AIW that can be traced back to the Norwegian current. Can the author speculate on the possible reasons of this warm bias?\n5) Line 131 the Three equation system used in the model is strongly dependent on the temperature/salinity and velocity estimated by the model. Previous work cited by the authors (Xu, 2012) and others (Sciascia, 2013) have shown that high resolutions, both vertical and horizontal, are needed to correctly represent the melting at the base of the ice. The resolution here is high for a regional model but not high to correctly capture the velocity required by three equation model. Do the author have an idea on how sensitive their model is to the horizontal resolution? Earlier regional modelling studies (Cowton 2015, https://doi.org/10.1002/2014JC010324) tried to overcome this issue by incorporating theoretical models of plumes into the regional numerical model. Have the authors considered this option to be confident of the melting obtained in their simulations?\n6) Line 252 the drag coefficient used here is slightly higher than the usual valued used in previous work. How sensitive is the model to this choice.\n7) Line 270 the geometry of the ice shelf does not change in time, but which geometry did the authors use? The one of the present days? Please clarify.\n8) Line 271 Maybe it would be useful for the reader to know the resolution of the atmospheric forcing to know how it compares with the resolution of the ocean model. And especially to force the surface dynamics in the most resolved areas of the model.\n9) Line 282-284 the injection of liquid discharge used in the cavities is also used for the other tidewater glaciers in Greenland? Please clarify.\n10) Figure S3 panel c and d is the cross-section used here the a-a’ section used in the other figures of the text? Please clarify.", "reviewer_3": "This paper examines how ocean conditions affect the subsurface melt on the ice tongue of the 79 North Glacier, which is the largest floating ice tongue in Greenland. For this purpose, the authors uses a global ocean-circulation model, forced by an atmospheric reanalysis dataset, and with a magnified resolution in the fjord of ice tongue and adjacent open ocean regions. The model resolves the ice cavity below the ice tongue, and subsurface ice melt is represented by boundary conditions that accounts also for effects of subglacial discharge, which stems from surface glacial melt and is forced by the atmospheric conditions. The simulation focuses on the time period 1970–2021. Additional sensitivity experiments are conducted to examine the roles of oceanic temperatures and atmospherically-driven subglacial discharge for driving basal melt on the ice tongue. The results suggest that between 1970–2021 the inter-annual variations and trend in the subsurface melt are chiefly driven by temperature changes in the Intermediate Atlantic Water off Northeast Greenland.\nBased on the results and climate model scenarios, a discussion is given on possible basal melt rates for 79 NG in the late part of the present century.\nThe paper is interesting and addresses important questions related to the dynamics of the Greenland Ice Sheet and its contribution for future sea-level rise. It is generally well written and the figures are illustrative. However, the paper reports a rather complex study in a rather short format. Therefore, I deem that a few additional clarifications are needed to convey the limitations as well as the original new contributions of this work.\nMajor points\nThe model yields a mean basal melt of ~17 km^3 yr^{-1}, which very close to the ~18 km^3 yr^{-1} that Schaffer et al. (2020) estimated from moored observation. It would be relevant to explain and discuss any possible tuning that was required to get this remarkably close correspondence. For example are tides simulated in FESOM2.1? Or have possible tidal motions in the ice cavity been accounted for in the melt parameterization? Even if no tuning was made, it is still relevant to mention and discuss how well the model reproduces the observations and possibly why.\n(around L184) Schaffer (2020) reported that on weakly-to-monthly time scales the basal melt was closely correlated with the height of the ~1 °C isotherm close to the glacier (their rationale was that the inflow was hydraulically controlled). Is this correlation also found in the present model and are there any trends or decadal variability in the ~1 °C isotherm height near 79 NG or at the shelf break? Could a figure similar to S3 be used to illuminate this? The fundamental question is perhaps how closely the AIW temperature is correlated to the ~1 °C isotherm height.\nMinor points and comments\nThe title is fine, but maybe \"Atlantic Water warming increases melt below Greenland’s largest floating ice tongue\" is a good alternative.\nL12: discharge -> subglacial discharge\nL63: Recently, Reinert et al. (2023) published an idealised two-dimensional model study of flow and basal melt in the 79 NG cavity. This paper is relevant to cite.\nL102: Writing 0.88 C gives a sense that the accuracy here is 1/100 of a degree C. I would write 0.9 C instead (i.e. one significant decimal); and the same for trends on L106\nL123: It would be relevant to also cite Jakobsson et al. (2020), who reported observations on a hydraulically-controlled inflow toward Ryder Ice Tongue.\nL193: In Fig 5c I recommend that you add a curve showing how the annual-mean basal melt rate responds to the changes in subglacial discharge. (Showing only JJA inflates the role of subglacial discharge, and is misleading for appreciating the annual mean melt.)\nL205: It may be relevant to cite Jenkins et al. (2018), who infer a quadratic dependence of basal melt on temperature from Antarctic ice shelf observations. (My understanding is that more liner melt–temperature relationships tend to be found at tidewater glaciers – where the SDG can be much larger than the subsurface ice melt – rather than at ice tongues and shelves.)\nL224: (I assume that this is inferred from the results in Fig 3b? If so it could be good to state that.)\nHere I suggest that you write something like: \"Assuming that the extent of the 79 NG Ice Tounge does not change, the quadratic dependence of melt on AIW temperature (Fig 3b) suggests that the POTENTIAL basal melt would be ....\nHowever, it seems rather unlikely that the ice tongue would reamin unchanged or even exist if the basal melt would reach ~140 km^3 yr^{-1}, which is 20 times larger than today. Therefore, it make sense to talk about potential melt rates.\nAn additional caveat, I believe, is that in projections of the potential melt rates there are implicit assumptions of the AIW height in the future: if the AIW height would decrease, the potential melt could be lower.\nL234: What if the 79 NG tongue disintegrates and it become a tidewater glacier? Does this statement still hold?\nReferences:\nJakobsson et al., 2020: Ryder Glacier in northwest Greenland is shielded from warm Atlantic water by a bathymetric sill. https://doi.org/10.1038/s43247-020-00043-0\nJenkins et al., 2018: West Antarctic Ice Sheet retreat in the Amundsen Sea driven by decadal oceanic variability, Nature Geoscience, VOL 11, 733–738 .\nReinert, M., Lorenz, M., Klingbeil, K., Büchmann, B., and Burchard, H. (2023). High-resolution simulations of the plume dynamics in an idealized 79°N glacier cavity using adaptive vertical coordinates. Journal of Advances in Modeling Earth Systems, 15, e2023MS003721. https://doi. org/10.1029/2023MS003721"}, "author_response": "Response to reviewers\nDearReviewers,\nOn behalf of all co-authorsIwouldliketothankyouallforyoureffortandfortakingthetimetoevaluate\nour manuscript.Yoursupportiveandconstructivecommentsarehighlyappreciated.Yoursuggestionswill\nimprovethequalityandclarityofourwork.\nPlease find below our answers (in black) to your questions/comments (in blue). The red color indicates\ntextthathasbeenaddedinthemanuscript.\nReviewer #1\nThis manuscript uses a global simulation to resolve the ocean circulation in thecavityofthe79NG\nwith 700m resolution to explain the impact of ocean and atmosphere. The authors find that, the\ninterannual variabilityandlong-termtrendoverthepast50yearsofthebasalmeltbelowthe79NG\nare mainly associated with changes in the temperature of the AIW. This manuscript is clearly\nexpressedthewholestory,anditisrecommendedtoacceptbasedonmyopinion.\nThemethodspartsseemnottobetheexactformatwiththisjournal.\nWe checked the author’s guide of Nature Communications and our methods section carefully, and could\nnot find problems with the format. However, in the first submission we did not add a proper data\navailability statement. We now added a link to a data repository where the necessary model output is\nstored to reproduce the figures. The final format of the Method section will be adjusted following the\neditorialcommentofthejournalifthepaperisaccepted.\nAnother thing I am very curious about is, since the authors did the FESOM2.1 among the entire\nnortheast Greenland, and for ZI, it did not collapse until 2014, so did the author analyzethesame\nsituationforZIbeforeitcollapsed?\nWe analyzed the processes at ZI as well. Fig. A1 shows the bathymetry and icebasetopographyatZIin\nour FESOM2.1 simulation.Timeseriesofmaximumtemperatureinthewatercolumnatthedeepestpoint\nclose to the southern calving front (orange dot in Fig. A1) and basal melt revealasignificantcorrelation\nfor the time period 1970-2009 (r=0.64,Fig.A2). Bothtimeseriesshowstrongpositivelineartrendsover\nthe 40 years. However, since the floatingicetonguedisappearedinthe2010s,wedecidednottofocuson\nthe ZI. A realistic simulation would require representing thethinninganddisappearingofthefloatingice\ntongue, which was not possible with our current model setup. We agree nonetheless that it is very\ninterestingtostudythetransitionperiodoftheZI,whichisthusapossibleresearchtopicforthefuture.\nWeaddedthefollowingparagraphinthemanuscript(lines182-185):\n1\n“As a side note, we find a similar positive correlation of basal melt and AIW temperature at ZI,79NG’s\nneighboring glacier (r=0.64 for the time period 1970-2009, 99% significant) before its transition into a\ntidewater glacier that started in the 2010s. An oceanmodelwithchangingicetonguegeometrywouldbe\nneededforthetransitionperiod,ataskleftforfutureresearch.“\nFigure A1: Water depth (left) and ice base topography (right) in the model based on data from\nRTopo-2.0.4 (Schaffer et al. 2019). The red line shows the calving front. The orange dot indicates the\nlocation where the maximum temperature in the water column, shown in Figure A2, has been extracted.\nThegreyandlightgreybackgroundshowsbarelandandgroundedice,respectively.\nFigure A2: Annual mean basal melt rate of the ZI (blue) and maximum AIW temperature in the water\ncolumn at the calving front of the glacier (orange) simulated by FESOM2.1.Thincoloredlinesshowthe\nrespectivelineartrendsofthetimeseriesforthetimeperiod1970-2009.\nThis manuscript investigates using a regional and global model the melting of the 79North glacier\nice tongue and addresses the role of the ocean (i.e. warming of the Atlantic intermediate Water\n(AIW)) and the atmosphere (i.e increased discharge due to the warming of the atmosphere). The\nauthorsfindadominantroleoftheoceaninthemeltingofthethe79Northicetongue.\nThe work used a combination of numerical model and observations both to validate the results\nobtained by the numerical model but also to corroborate them. The question addressed by the\nauthors is a long-standing question and the contribution brought by this work is relevant bothfor\nthe oceanographic and glaciological community. In general, I find the work well written, the\nmethodology used is mostly clear, the results are clearly presented and the discussion and\nperspective well highlighted. I think the manuscript is worth publishing butIhaveafewquestions\nfortheauthors.\n1)Line47-48Ifindthissentencehardtoread.Theconceptisclear,butIwouldsuggestrephrasing\nWerephrasedthesentenceasfollows(lines44-45):\n“Apart from the ocean which carries the warm AIW into the cavityofthe79NG,theatmosphereimpacts\nthestabilityoftheglacieraswellviasubglacialdischarge(Slateretal.2022).”\n2) Line 51-54 plumes do not form only in cavities but also at the margin of outlet glaciers. The\nimpressionhereisthattheyformonlyincavities.Pleaserephraseaccordingly.\nWereformulatedthesentence,anditishopefullymoreclearnow (lines48-50):\n“The outflowing freshwater vigorously entrains ambient waters, speeds up as a buoyant plume that\ndevelops downstream of the grounding line at the base of the ice tongue, and thereby locally increases\nbasalmelt.”\n3)Line61Ithinkthe“as”shouldbedeleted\nCorrected.\n4) The authors clearly state that one of the limitations of the numerical model is the warm bias of\nthe AIW that can be traced back to the Norwegian current. Can the author speculate on the\npossiblereasonsofthiswarmbias?\nThe Norwegian Atlantic Current, and in particular the Norwegian Atlantic Slope Current (NwASC),\ntransports warm Atlantic Water along the Norwegian coast towards the Arctic Ocean, which is the main\nsource of the AIW that eventually reaches the 79NG. A recent study revealed that the cooling of the\nNwASC along its pathway from the Greenland Scotland Ridge up north is rather due to oceanic lateral\nheat transfer than air-sea heat fluxes (Huang et al. 2023). Eddies play an important role for lateral heat\nfluxes. Our model resolution in the Nordic Seas (4 km) can be considered as eddy-permitting, but not\neddy-resolving. We speculate that higher mesh resolutionwouldincreaselateralheatfluxesintheNordic\nSeas, leading to stronger heat loss as the AWtravelsnorthwards,whichmightreducethewarmbias.The\n3\nissue of too large AW transport into the Nordic Seas can be discarded because it is in the range of\nobservations.Weaddedthefollowingsentencesinthesectionaboutmodellimitations(lines331-335):\n“The warm bias can presumably be related to model deficiencies in resolving lateral ocean heat fluxes\ndue to the fact that mesoscale eddiesarenotfullyresolvedwiththeappliedresolutionintheNordicSeas.\nThese lateral heatfluxesplayalargeroleincoolingtheAtlanticWatercarriedbytheNorwegianAtlantic\nSlopeCurrentnorthwards(Huangetal.2023).”\nReference:\nHuang, J., Pickart, R.S., Chen, Z. et al. Role of air-sea heat flux on the transformation ofAtlanticWater\nencirclingtheNordicSeas.NatCommun14,141(2023).https://doi.org/10.1038/s41467-023-35889-3\n5) Line 131 the Three equation system used in the model is strongly dependent on the\ntemperature/salinity and velocity estimated by the model. Previous work cited by the authors(Xu,\n2012) and others(Sciascia,2013)haveshownthathighresolutions,bothverticalandhorizontal,are\nneeded to correctly represent the melting at the base of the ice. The resolution here is high for a\nregional model but not high to correctly capture the velocity requiredbythreeequationmodel.Do\nthe author have an idea on how sensitive their model is to the horizontal resolution? Earlier\nregional modelling studies (Cowton 2015, https://doi.org/10.1002/2014JC010324) tried to overcome\nthis issue by incorporating theoretical models of plumes into the regional numerical model. Have\ntheauthorsconsideredthisoptiontobeconfidentofthemeltingobtainedintheirsimulations?\nDue to limitations in computing time we were not able to perform simulations with higher mesh\nresolution than used in our study. However, the fact that we are gettingarathergoodagreementwiththe\navailable observational data gives us certain credence in our results. Besides, ourhorizontalresolutionis\nhigh enough to resolve the Rossby radius of deformation (Fig. S1), so that we are able to resolve\nrotational effects on the currents in the cavity, which clearly play a role as revealed in Fig. 2 a, c and d.\nIdealised 3d plume modeling of melt processes underneath the 79NG (personal communication, Markus\nReinert, IOW, Germany) shows the same signature of a thick meltwater plume at thesouthernmarginof\nthe ice tongue when rotational effects areincorporated,andaverythinplumeinthecasewithoutrotation\n(Reinert et al., 2023). Thus, once the plume has been established, we expect the vertical resolutiontobe\nsufficient to model it realistically. Overall, the good model performance indicates that our model\nconfiguration (with the bestresolutionwecanafford)issufficientforourcurrentpurposeandwethusdid\nnot consider other modeling options. However, limited realism is expected very close to the grounding\nline. This is firstly due to the largely unknownspatialdistributionoftheinjectionofsubglacialdischarge\nat the grounding line (something, a plume model would not be able to fix). Secondly, we use the\nhydrostatic approximation which might be violated in regions where the ice tongue is particularly steep.\nHere a plume model might help. We did notconsiderincorporatingaplumemodelintheoceanmodelso\nfar,butitisdefinitelyagreatideaforfuturework.\n6) Line 252 the drag coefficient used here is slightly higher than the usual values used in previous\nwork.Howsensitiveisthemodeltothischoice?\nThank you for this question. As far as we know, there is no good observational evidence of the valueof\nthe drag coefficient. This leaves some freedom to choose anappropriatevalue.Thus,thedragcoefficient\n4\nis often used to tune the model to reproduce a reasonable basal melt rate. We chose a drag coefficient\nfollowing Cai et al. 2017, who determined that a coefficient of 1.2·10-3 isoptimalforPetermannGlacier\n(NW Greenland)andbestreproducestheobservedbasalmelt.Typicalvaluesofthedragcoefficientrange\nfrom 1.2·10-3 to 3.0·10-3 (see Gwyther et al. 2015, https://doi.org/10.1016/j.ocemod.2015.09.004).\nLargervaluesofC aremostlyappliedfor“cold”iceshelvessuchastheonesinAntarctica,whichfeature\nD\nstronger friction. In contrast, smaller values are appliedfor“warm”iceshelvessuchastheGreenlandice\ntongueswhichhaveasmoothericesurfaceduetohigherbasalmelting.\nWe carried out two additional sensitivity experiments with larger drag coefficients (C =1.875·10-3 and\nD\nC =2.5·10-3, see Fig. A3). Indeed, the simulated basal melt rate is sensitive to the choice of the drag\nD\ncoefficient. The overall melt rate for the year 2000 increased by 27% with C =1.875·10-3 and by 48%\nD\nwith C =2.5·10-3 relative to the reference run. Changing the drag coefficient adds an offset to the time\nD\nseries,whereasthevariabilitydoesnotchange.\nWe think it is good to show the sensitivity of the model tochangesinthedragcoefficient,andaddedthe\nfigure in the Supplementary material, and also added a short description in the methods section (lines\n263-267):\n„Note that the simulated basal melt rates are sensitive to the choice of the drag coefficient. Sensitivity\nexperiments with drag coefficients of 1.875·10-3 and 2.5·10-3 carried out for one year revealed that the\nbasal melt was increased by 27% and by 48% respectively relative to the reference run, whereas the\nvariabilitydidnotchange(Fig.S4).“\nFigure A3: Impact of basal friction. Time series of daily mean basal melt rate in the year 2000 in\nexperiment REF (green line, using a drag coefficient of C =1.25·10-3), andexperimentswithhigherdrag\nD\ncoefficient(C =1.875·10-3,turquoiselineandC =2.5·10-3,blueline).\nD D\n5\n7) Line 270 the geometry of the ice shelf does not change in time, but which geometry did the\nauthorsuse?Theoneofthepresentdays?Pleaseclarify.\nWe used the RTopo-2.0.4 dataset by Schaffer et al. 2019 to obtain the geometry of the ice tongue.\nCompared to the previous version RTopo-2.0.1 (Schaffer at. 2016), it contains new bathymetry data for\nthe Northeast Greenland continental shelf. The ice thickness data of the79NGinRTopo-2.0.4represents\npresent day conditions. However, the thickness of the Zachariæ Isstrøm (ZI) ice tongue in RTopo-2.0.4\ncorresponds to the state before it disintegrated. In contrasttothe79NG,itthusdoesnotrepresentpresent\nday conditions. We excluded the analysis of the ZI from this study. A realistic simulation of the ocean\ncirculation in the cavity of the ZI would require a more sophisticated approach with a changing (and\nvanishing) ice geometry over the last decades (see answer to Reviewer 1). To clarify this, we modified\nandaddedthefollowingsentencesinthemethodssection(lines284-287):\n“In our model setup, the ice shelf topography of the 79NG represents present day conditions, and does\nnot change with time. While this assumption is appropriate for the 79NG, where the grounding line and\nice shelf topography did not change significantly in the lastdecades(Mayeretal.2018),itdoesnothold\nfortheZIwhichstronglyretreatedstartingfromthe2010s(Mouginotetal.2015).“\n8) Line 271 Maybe it would be useful for the reader to know the resolution of the atmospheric\nforcing to know how it compares withtheresolutionoftheoceanmodel.Andespeciallytoforcethe\nsurfacedynamicsinthemostresolvedareasofthemodel.\nThe spatial resolution of the atmospheric forcing is 55 km, muchlargerthanthemeshresolutioncloseto\ntheglaciers(700m).Weaddedthisinformationinthemethodssection(lines288-289):\n“Thedatasethasaspatialresolutionof55kmandtemporalresolutionof3hours.”\nDespite being relatively coarse, the JRA-55 datasetiscommonlyusedforoceanmodels,andFESOM2.1\nproduces good results with this data set. We note that atmospheric and oceanic processes have different\ntypical spatial scales. Nonetheless, we aim toexploretheimpactofhigherresolutionatmosphericforcing\ndatainthefuture.\n9) Line 282-284 the injection of liquid discharge used in the cavities is also used for the other\ntidewaterglaciersinGreenland?Pleaseclarify.\nWe use the liquid discharge data from Mankoff et al. which contains data for entire Greenland, so for\nother tidewater glaciers as well. For 79NG,weinjecttheliquiddischargeatthegroundingline(ataround\n500 m depth), while for all other glaciers, the liquid discharge is injected in the surface model layerand\ndistributedwithinaradiusof50km.\nWereformulatedthesentencesabit(lines299-304):\n“For every data point of the Mankoff et al. data sets, the nearest model grid point around Greenland is\nidentified and solid and liquid discharge is then spread at the ocean surface over a prescribed radiusof\n50 km. In the cavities, the treatment of discharge is different. Here we inject the liquid discharge (now\ntermed subglacial discharge) evenly along the grounding line, and then spreaditoveraradiusof10km.\nThus, inthe79NGandZIcavities,weinjecttheliquiddischargeatadepthofaround500m,whereasitis\nspreadatthesurfacemodellayerforallotherGreenlandglaciers.”\n6\n10) FigureS3panelcanddisthecross-sectionusedherethea-a’sectionusedintheotherfiguresof\nthetext?Pleaseclarify.\nYes, the section a-a’ is also shown in panels c and d. We addedthisinformationinthecaptionandinthe\nfigures.\nReviewer #3\nThis paper examines how ocean conditions affect the subsurface melt on the ice tongue of the 79\nNorth Glacier, which is the largest floating ice tongue in Greenland. For this purpose, the authors\nuses a global ocean-circulation model, forced by an atmospheric reanalysis dataset, and with a\nmagnified resolution in the fjordoficetongueandadjacentopenoceanregions.Themodelresolves\nthe ice cavity below the ice tongue, and subsurface ice melt is represented by boundary conditions\nthat accounts also for effects of subglacial discharge, which stems from surface glacial melt and is\nforced by the atmospheric conditions. The simulation focuses on the time period 1970–2021.\nAdditional sensitivity experiments are conducted to examine the roles of oceanictemperaturesand\natmospherically-driven subglacial discharge for driving basal melt on the ice tongue. The results\nsuggest that between 1970–2021 the inter-annual variations and trend in the subsurface melt are\nchiefly driven bytemperaturechangesintheIntermediateAtlanticWateroffNortheastGreenland.\nBased on the results and climate model scenarios, a discussion is given onpossiblebasalmeltrates\nfor79NGinthelatepartofthepresentcentury.\nThe paper is interesting and addresses important questions related to the dynamics of the\nGreenlandIceSheetanditscontributionforfuturesea-levelrise.Itisgenerallywellwrittenandthe\nfigures are illustrative.However,thepaperreportsarathercomplexstudyinarathershortformat.\nTherefore, I deem that afewadditionalclarificationsareneededtoconveythelimitationsaswellas\ntheoriginalnewcontributionsofthiswork.\nThe model yields a mean basalmeltof~17km^3yr^{-1},whichveryclosetothe~18km^3yr^{-1}\nthat Schaffer et al. (2020) estimated from moored observation. It would be relevant toexplainand\ndiscuss any possible tuning that was required to get this remarkably close correspondence. For\nexample are tides simulated in FESOM2.1? Or have possible tidal motions in the ice cavity been\naccounted for in the melt parameterization? Even if no tuning was made, it is still relevant to\nmentionanddiscusshowwellthemodelreproducestheobservationsandpossiblywhy.\nFor this study, we used the standard FESOM2.1 version, and did not tune itspecifically.Ithastobesaid\nthat themodelhasareasonablerepresentationofobservations,buttheresultsarestillnotperfect.Thereis\na warm bias in the AIW layer on the Northeast Greenland continental shelf, which we describe in the\nmethods section. Reviewer #2 asked to speculate about possible reasons; amongotherissuestheremight\nbeunderestimatedlateraloceanheatfluxesintheNordicSeas.\nused by our colleagues to simulate the ocean-ice shelf interaction in the Southern Ocean, particularly\nFilchner-Ronne Ice shelf. Here we benefited from their experience.Reviewer#2pointedustotheroleof\nthe drag coefficient, andsensitivityexperiments(seeFigureA3,andnewFigureS4inthepaper)revealed\nthat the magnitude of basal melt strongly depends on the choice of this parameter. We chose a small\nparameter, following Cai et al. 2017,thatisbettersuitableforiceshelves/tonguescharacterizedbystrong\nbasal melting (“warm” ice shelves), such as the ones in Greenland. We added a description of the\nsensitivity experiments with varying drag coefficients in the Methods section. We are very glad that the\nsimulated basal meltfitswelltotheestimatefrommooringobservationsbySchafferatal.2020,butithas\ntobenotedagainthatthemagnitudeissensitivetothechoiceofthedragparameter.\nIn our model setup, we do not simulate tides, and we also do not account for tidal motions in the melt\nparameterization. We added this model deficiency to the “Limitations of the model” section (lines\n327-329):\n“(4) In our model setup, we do not account for tidal motions in the cavity., Observations with an ice\ntethered mooring revealed that tidal currents in the cavity are weak (Lindeman et al. 2020), being one\norderofmagnitudesmallerthanthemeaninflowvelocityofwarmAIW.”\n(around L184) Schaffer (2020) reported that on weakly-to-monthly time scales the basal melt was\nclosely correlated with the height of the ~1°Cisothermclosetotheglacier(theirrationalewasthat\nthe inflow was hydraulically controlled). Is this correlationalsofoundinthepresentmodelandare\nthere any trends or decadal variability in the ~1 °C isotherm height near 79 NG or at the shelf\nbreak?CouldafiguresimilartoS3beusedtoilluminatethis?Thefundamentalquestionisperhaps\nhowcloselytheAIWtemperatureiscorrelatedtothe~1°Cisothermheight.\nWe computed the depth of the 1°C isotherm at the calving front of the 79NG, and found, similar to\nSchaffer et al. (2020), a high correlation with the maximum temperature in the water column (r=0.96,\nFigure A4). The depth of the 1°C isotherm also reveals a strong positive linear trend during the time\nperiod 1970-2021 (21 m/decade). The vertical displacement of the 1°C isotherm is thus connected to\nchangesinthemaximumtemperatureinthewatercolumn.\nwatercolumn(orange)atthecalvingfrontofthe79NGsimulatedbyFESOM2.1.Thincoloredlinesshow\ntherespectivelineartrendsofthetimeseriesforthetimeperiodof1970-2021.\nThe title is fine, but maybe \"Atlantic Water warming increases melt below Greenland’s largest\nfloatingicetongue\"isagoodalternative.\nThanksalotforthissuggestion!However,wethoughttostaywiththeoriginaltitlebecauseitconveysthe\ngeographicalregion(NortheastGreenland).\nL12:discharge->subglacialdischarge\nCorrected.Notethatwehadtoshortentheabstractduetothewordlimit(max150wordsallowed).\nL63: Recently, Reinert et al.(2023)publishedanidealisedtwo-dimensionalmodelstudyofflowand\nbasalmeltinthe79NGcavity.Thispaperisrelevanttocite.\nThepaperbyReinertetal.isindeedveryrelevanttocite.Weaddedthecitationhere(lines61-63):\n“Note that modelling studies exploring the ocean circulation at the 79NG and in other cavities of\nGreenland glaciers (e.g. PetermannandRyderGlaciers)havebeensofarrestrictedtoidealizedsetupsin\ntwodimensions(Reinertetal.2023,Caietal.2017,Wiskandtetal.2023).“\nL102: Writing0.88Cgivesasensethattheaccuracyhereis1/100ofadegreeC.Iwouldwrite0.9C\ninstead(i.e.onesignificantdecimal);andthesamefortrendsonL106\nCorrected.\nL123: It would be relevant to also cite Jakobsson et al. (2020), who reported observations on a\nhydraulically-controlledinflowtowardRyderIceTongue.\nThanksforpointingustothestudybyJakobssonetal.(2020).Weaddedthisreference(lines120-121):\n9\n“This is comparable to Ryder Glacier inNorthGreenland,whichisshieldedbyabathymetricsillaswell\n(Jakobssonetal.(2020).”\nL193: In Fig 5c I recommend that you add a curve showing how the annual-mean basal melt rate\nresponds to the changes in subglacial discharge. (Showing only JJA inflates the role of subglacial\ndischarge,andismisleadingforappreciatingtheannualmeanmelt.)\nWe agree with the reviewer that it makes much more sense to show the annual meanbasalmeltrateand\nsubglacial discharge in Figure 5c, since the focus of this study is on interannual variability. We thus\ndecidedtoshowonlytheannualmeans,insteadofaddinganothercurve.\nThis changes slightly a statementintheDiscussionsection,whereweappliedthesquare-rootrelationship\nbetween basal melt and subglacial discharge to infer the sensitivity to a potential future subglacial\ndischarge.WethusmodifiedthecorrespondingsentenceintheDiscussionsection(lines239-241):\n“Given the square-rootrelationshipobtainedbyourmodel(Fig.5c),a3.3-timesincreaseinannualmean\nsubglacialdischargewouldleadtoa18%increaseinannualmeanbasalmeltratesofthe79NG.”\nL205: It may be relevant to cite Jenkins et al. (2018), who infer a quadratic dependence of basal\nmelt on temperature from Antarctic ice shelf observations. (My understanding is that more liner\nmelt–temperature relationships tend to be found at tidewater glaciers – where the SDG can be\nmuchlargerthanthesubsurfaceicemelt–ratherthanaticetonguesandshelves.)\nWe cited the study by Jenkins et al. (2018), and also addedarelevantpaperbySlateretal(2018).Thank\nyou for clarifying the difference between ice tongues/shelves and tidewater glaciers. We added this\ndistinctioninthemanuscript(lines208-213):\n“Similarly, model studies (Wiskandt et al. 2023, Cai et al. 2017) and observations (Jenkins et al. 2018)\npointed to an above-linear dependency of basal melt on thermal forcing, and a square-root dependency\non subglacial discharge. These relationships, derived for ice tongues and ice shelves,differslightlyfrom\nthe ones derived for tidewater glaciers wheresubglacialdischargecanexceedthesubsurfaceicemelt.In\nthe latter case, a linear relationship with thermal forcing and a cubic relationship with subglacial\ndischarge have been obtained (Xu et al. 2012, Slater et al. 2016, Cenedese et al. 2016, Jackson et al.\n2022).”\nL224:(IassumethatthisisinferredfromtheresultsinFig3b?Ifsoitcouldbegoodtostatethat.)\nHere I suggest that you write something like: \"Assuming that the extent of the 79 NG Ice Tongue\ndoes not change, the quadratic dependence of melt on AIW temperature (Fig 3b) suggeststhatthe\nPOTENTIALbasalmeltwouldbe....\nHowever, it seems rather unlikely that the ice tongue would remain unchanged or even exist if the\nbasal melt would reach ~140 km^3 yr^{-1}, whichis20timeslargerthantoday.Therefore,itmake\nsense to talk about potential melt rates. An additional caveat, I believe,isthatinprojectionsofthe\npotential melt rates there are implicit assumptions of the AIW height in the future: if the AIW\nheightwoulddecrease,thepotentialmeltcouldbelower.\nThanksalotforsuggestingamuchbettersentence.Wemodifieditaccordingly(lines229-232):\n10\n“Assumingthattheextentofthe79NGicetonguedoesnotchange,thequadraticdependenceofbasal\nmeltonAIWtemperature(Fig.3b)suggeststhatthepotentialbasalmeltwouldbe40km3yr-1,63km3yr-1,\n92km3yr-1and140km3yr-1,respectivelyinthesewarmingscenarios.”\nOf course it makes much more sense to talk about potential melt rates. This whole paragraph is\nspeculative, and certainly our discussion is not taking into account any dynamic response of the ice\ntongue.\nRegarding the height of the AIWlayer,weshowedabove(followingyoursuggestion),thatthemaximum\ntemperature of the AIW layer is highly correlated with the AIW layer thickness (represented by the1°C\nisotherm).Thiscorrelationmightofcoursebreakdowninthefuture.\nL234: What if the 79 NG tongue disintegrates and it become a tidewater glacier? Does this\nstatementstillhold?\nWe suppose that this statement still holds if the 79NG transitions into a tidewater glacier. The studies\ncited above (Xu et al. 2012, Slater et al. 2016, Cenedese et al. 2016, Jackson et al. 2022) dealing with\ntidewater glaciers still find a linear relationship of basal melt with ocean thermal forcing, and a\nbelow-linear relationship with subglacial discharge. To study the possible future shape ofthe79NGand\nitssensitivitytooceantemperaturesandsubglacialdischarge,acoupledocean-icesheetisneededthough.\nWemodifiedthelastparagraphslightly(lines243-247):\n“In the coming decades, we suppose that the ocean might still be the dominant factor forthestabilityof\nthe 79NG,evenifittransitionsintoatidewaterglacier.Wededucethisfromtheabovecitedstudies(Xuet\nal. 2012, Slater et al. 2016, Cenedese et al. 2016, Jackson et al. 2022) dealing with tidewater glaciers,\nthat obtained a linear relationship of basal melt with ocean thermal forcing, and a below-linear\nrelationshipwithsubglacialdischarge.”\nReferences:\nJakobsson et al.,2020:RyderGlacierinnorthwestGreenlandisshieldedfromwarmAtlanticwater\nbyabathymetricsill.https://doi.org/10.1038/s43247-020-00043-0\nJenkins et al., 2018: West Antarctic Ice Sheet retreat in the Amundsen Sea driven by decadal\noceanicvariability,NatureGeoscience,VOL11,733–738.\nReinert, M., Lorenz, M., Klingbeil, K., Büchmann, B., and Burchard, H. (2023). High-resolution\nsimulations of the plume dynamics in an idealized 79°N glacier cavity using adaptive vertical\ncoordinates. Journal of Advances in Modeling Earth Systems, 15, e2023MS003721. https://doi.\norg/10.1029/2023MS003721\n11"}, {"round": 2, "reviewer_comments": {"reviewer_2": "I thank the authors for addressing carefully all the comments of the reviewers and modifying the\nmanuscript accordingly.\nI think the manuscript is now suitable for publication.", "reviewer_3": "The authors have addressed all my comments constructively. I recommend that the paper is published\nin its present form."}, "author_response": ""}]

Decision letter