On-shelf circulation of warm water toward the Totten Ice Shelf in East Antarctica

Domain: earth_science
Published: 2023-08-17
Editorial outcome: accept

Abstract
The Totten Glacier in East Antarctica, with an ice volume equivalent to >3.5 m of global sea-level rise, is grounded below sea level and, therefore, vulnerable to ocean forcing. Here, we use bathymetric and oceanographic observations from previously unsampled parts of the Totten continental shelf to reveal on-shelf warm water pathways defined by deep topographic features. Access of warm water to the Totten Ice Shelf (TIS) cavity is facilitated by a deep shelf break, a broad and deep depression on the shelf, a cyclonic circulation that carries warm water to the inner shelf, and deep troughs that provide direct access to the TIS cavity. The temperature of the warmest water reaching the TIS cavity varies by ~0.8 °C on an interannual timescale. Numerical simulations constrained by the updated bathymetry demonstrate that the deep troughs play a critical role in regulating ocean heat transport to the TIS cavity and the subsequent basal melt of the ice shelf.

Introduction
While the contribution to global sea-level rise from West Antarctica has received the most attention (6.9 ± 0.6 mm for the period 1979–2017, dominated by the Amundsen and Bellingshausen Sea sectors), East Antarctica has also been a major contributor (4.4 ± 0.9 mm over the same period)1. The greatest discharge of ice from the East Antarctic Ice Sheet occurs via the Totten Glacier, which drains the Aurora Subglacial Basin in Wilkes Land. The Totten Glacier has an ice volume equivalent to >3.5 m of global sea-level rise2,3 and is grounded below sea level4,5, making the Aurora Subglacial Basin vulnerable to oceanic thermal forcing. The Totten Ice Shelf (TIS) experiences an area-averaged basal melt rate of 10.5 ± 0.7 m yr−1, the highest of all East Antarctic ice shelves, and a basal melt amount of 63.2 ± 4 Gt yr−1 for 2007‒2008 (ref. 6). Ocean heat transport to the ice shelf cavity drives basal melt of ice shelves. High melt rates and ice loss from the West Antarctic Ice Sheet are driven by intrusions of warm Circumpolar Deep Water (CDW)7,8. East Antarctic continental shelves are typically cold and fresh9 because access of warm CDW to the shelf is limited by the presence of the Antarctic Slope Current (ASC) associated with the Antarctic Slope Front (ASF) over the upper continental slope9,10. Knowledge of the factors that regulate the transport of ocean heat from offshore to the ice shelf cavity is essential to assess the vulnerability of the Totten Glacier and its large potential contribution to sea level rise. Substantial gaps remain because most of the continental shelf near the Totten Glacier has not been sampled due to heavy sea-ice cover. Previous observations on the Totten continental shelf are limited to the relatively shallow Dalton Polynya region and a single voyage to the front of the TIS11,12. Those observations showed that relatively warm modified CDW (mCDW) is present on the continental shelf and reaches the TIS11,12, through deep troughs that provide access to the ice shelf cavity3. However, from these limited observations, it was not possible to determine the pathways and processes regulating the transport of warm water from the offshore ocean to the TIS cavity, nor was it possible to assess variability in the temperature of mCDW.

Our observations span the Totten embayment, including broad helicopter-based measurements, detailed multibeam bathymetric surveys of the deep troughs that provide warm water access to the cavity, and hydrographic measurements from the unsampled western side of the ice front, where glacial meltwater is speculated to leave the cavity. We collected comprehensive observations of bathymetry and water properties off the Sabrina Coast during three cruises (March 2018, December 2019/March 2020, and March 2022) extending from the upper continental slope/shelf break across the continental shelf to the TIS front (see Methods) and including many regions not previously sampled. Here we combine our bathymetric data with existing measurements to create a more realistic and detailed bathymetric dataset for the region (Methods; Supplementary Fig. 1). We then analyze the distribution and circulation of warm mCDW revealed by the hydrographic data. Multi-year hydrographic observations spanning the period from 2015 to 2022 and moored time series from the deep troughs also allow us to assess temporal variability in the temperature of mCDW reaching the ice shelf cavity. Using the updated bathymetry data, we configure a high-resolution ocean-sea ice-ice shelf model and use the numerical simulation, validated against our ocean observations, to provide additional insight into the pathways of mCDW from the open ocean to the ice shelf cavity. Specifically, we utilize the numerical simulations to estimate ocean heat transport, basal melt, the ocean circulation in the TIS cavity, the connection to the ocean circulation over the continental shelf, and the sensitivity of ocean heat transport to the representation of bathymetry. The observations and model simulations delineate the pathways of warm mCDW from the shelf break to the ice shelf cavity, highlighting the importance of bathymetry and regional circulation in regulating heat transports toward the TIS cavity.

Results
Updated bathymetry off the Sabrina Coast
Due to the presence of heavy sea ice, most of the continental shelf offshore of the Sabrina Coast is difficult to access by ship and the bathymetry is therefore poorly known. Previous ship-based observations were mostly limited to the relatively ice-free Dalton Polynya13, east of the TIS (Supplementary Fig. 1). We conducted a multibeam bathymetric survey in the uncharted regions over the continental shelf and margin of the Sabrina Coast, from the shelf break to the TIS front, including the sea-ice covered region west of the Dalton Polynya (Fig. 1; Supplementary Fig. 1; see Methods for details).

The updated bathymetry reveals several bathymetric features that facilitate the transport of warm mCDW to the TIS (Fig. 1). The shelf break is relatively deep (bottom depth of 500‒600 m) along the full extent of the embayment, allowing warm offshore deep waters to access the continental shelf14,15. A broad and deep (600‒900 m) depression (the “Sabrina Depression”; Fig. 1) spans most of the continental shelf, allowing warm water that reaches the shelf to spread towards the continental margin. Closer to the coast, the depression shoals to depths of 400‒500 m. A previous study11 had identified the presence of several deep glacial troughs at the TIS front, but it was unclear if the troughs extended north to the continental shelf. The multibeam sonar data show that these troughs (the “Totten Troughs”) connect the Sabrina Depression to the TIS cavity (Fig. 1), providing a pathway for warm water at depth to reach the ice shelf cavity. The updated bathymetry also reveals the detailed structure of the troughs, including several sills that may act to partially block the flow of warm water along the seafloor. The troughs range in depth from 600 to 1200 m and in width from 10 to 20 km. Central Totten Trough (C-TT) at the central TIS front near 116.5°E, extending at least 40 km to the northeast from the TIS front, is the most significant, with a maximum depth of ~1100 m and width of ~20 km. C-TT connects the TIS cavity and the Sabrina Depression, with the minimum depths along the channel exceeding 600‒700 m (Fig. 1b). East Totten Trough (E-TT) at the eastern TIS front near 117.3°E is also deep (maximum depth >1000 m) but appears to be surrounded by shallower topography (<500‒600 m depth, Fig. 1b) and, therefore, may be less connected to the deep waters of the Sabrina Depression.

Pathways of warm water from the shelf break to the TIS front
The distribution of water temperature over the continental slope, shelf break, and inner shelf illustrates how and where mCDW spreads onto and across the shelf (Fig. 2). Over the upper continental slope, distinct subsurface warm cores (0.8‒1.3 °C) are observed at 400‒500 m depth (Fig. 2a), which are associated with semi-permanent cyclonic eddies16 (Supplementary Fig. 2) as discussed in previous work15,17. Although the upper slope section east of 119°E (Fig. 2a, d) is somewhat distant from the shelf break, the warm water cores over the slope are shallower than the shelf break (500–600 m deep) and can be traced across the shelf break and onto the continental shelf, where mCDW is widespread near the seafloor, with maximum temperatures of ~0.8 °C (Fig. 2).

The water column on the shelf exhibits a two-layer structure, with Winter Water (WW, cold, fresh, and high dissolved oxygen (hereafter DO)) overlying mCDW (warm, saline, and low DO) near the seafloor. These contrasting water masses are separated at ~500‒600 m depth by a sharp interface in temperature and salinity (hence density, noting that seawater density is determined mainly by salinity in the Antarctic coastal region) (Fig. 2c; Supplementary Fig. 3). A zonal temperature section crossing the deep basin on the shelf shows that relatively warm (0‒0.5 °C) mCDW occupies the bottom layer of the entire Sabrina Depression (Figs. 2c, 3a; Supplementary Fig. 3). Absolute ocean velocity (estimated by referencing geostrophic calculations to surface velocity derived from satellite altimetry, see Methods) shows a consistent pattern at the shelf break and on the inner shelf (Fig. 2e‒f), with southward flows (5‒20 cm s−1) on the eastern flank of the Sabrina Depression and weaker northward flow further west. Our comprehensive observations confirm the presence of a cyclonic gyre on the shelf, consistent with the circulation inferred from limited float data in an earlier study18, as well as satellite-derived sea surface velocity (Supplementary Fig. 2) and iceberg drift tracks (Supplementary Fig. 4).

Along the continental margin, including the TIS front region, mCDW is limited to the bottom layers in specific deep troughs (Figs. 3, 4; Supplementary Fig. 5a), as first observed by Rintoul et al.11. Each trough receives mCDW with different temperatures, with the warmest water (0.24 °C) observed in C-TT (station T5, March 2018, Figs. 4a, 5a) and cooler water (<−0.5 °C) in E-TT (station 43, February 2020, Supplementary Fig. 5a). The temperature of the mCDW arriving at the deep troughs near the ice front is lower (−0.5 to +0.24 °C) than that observed in the depression and at the shelf break (0‒0.8 °C) (Figs. 2, 3; Supplementary Fig. 3), suggesting as discussed below that interaction between the thermocline depth and the shallowest sill regulate the temperature of the water reaching the ice front. Note, however, that the water reaching the ice front is still warm enough to drive rapid basal melt at the deep (>2000 m)3 grounding line. For example, the warmest water observed in C-TT (Figs. 4a, 5a) would exceed the freezing point by more than 3.7 °C if it can access the grounding line (the in situ freezing point decreases as pressure increases).

Near the continental margin, warm (>−0.5 °C) mCDW is only observed where the water depth exceeds 600‒700 m (purple-shaded region in Fig. 3b). In particular, warm mCDW is largely constrained to C-TT and the Sabrina Depression. In contrast, the maximum temperature of mCDW at the bottom of E-TT is cooler (<−0.5 °C), despite depths there being >1000 m (Figs. 1b, 3b; Supplementary Fig. 5a). This suggests that exchange with the depression is more restricted for E-TT than C-TT, consistent with our observations of shallow topography adjacent to E-TT (noting that our survey did not cover all of the regions between E-TT and the depression, Figs. 1b, 3b). The temperature of water reaching the grounding lines is determined not only by the temperature of the water supplied to the trough from the depression but also likely by an interaction between the depth of the thermocline and the height of the shallowest topography (sills and ridges) in the deep troughs and beneath the ice shelf18–20. The WW/mCDW interface (i.e., thermocline) is typically at ~500‒600 m depth in the Totten region (Figs. 2c, 4; Supplementary Figs. 3, 5a), across which temperature increases with depth by up to ~1.4 °C over ~100 m. At C-TT, where the shallowest measured sills have depths of ~600‒700 m, all but the warmest water in the Sabrina Depression can pass through the trough. At E-TT, the shallowest measured sills are shallower (<500‒600 m deep), apparently restricting the passage of warm water through the trough. Therefore, the distribution of mCDW along the continental margin suggests that C-TT is the primary route by which warm water reaches the TIS cavity.

Variability of warm water reaching the TIS cavity
Multi-year hydrographic profiles and mooring time series demonstrate that changes in thermocline depth on a range of timescales are associated with large changes in the thermal forcing available to drive melt of the TIS. Repeat hydrographic stations in C-TT were occupied on four

Methods

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is a very good, well-written paper presenting interesting results. It concerns the Totten Ice Shelf in East Antarctica, a less famous (but nonetheless important) sister of the Amundsen Sea ice shelves. The processes revealed are of interdisciplinary interest, being relevant to oceanographers and glaciologists wanting to understand what leads to ice shelf mass loss, and/or the biogeochemical or climate impacts of the resulting meltwater. The primary results are the insights about the circulation revealed by a new swath bathymetry data set, and by hydrographic surveys, supported by a cutting-edge high resolution model.\nThe paper describes and synthesises some great observations, but would benefit from more clearly identifying what is new/surprising, and why it matters, as it goes along. For example the section on the pathways of warm water was clear and well described, but I was left wondering whether any of the features were different to previous papers. It’s fine if there’s nothing new there, but in that case make it clearer that the section is just setting the scene for subsequent sections?\nI think the key results are those stated in lines 220 to 225. These are clear, important and valuable. If the authors confirm my understanding, probably these key results should be stated more strongly at the end of the paper as the take-home message?\nRecommendations for strengthening:\n1. Many of the figures have multiple (fairly unrelated) panels and this means that the fonts on many of the panels are impossible to read. There are 16 figure panels, which in a longer-format journal would probably be presented in 10-12 separate figures; this makes it feel like a longer paper trying to squeeze into a high profile format. I recommend cutting down the number of figures to those that are essential to support the key message(s) of the paper, and enlarging them so that fonts are legible (e.g. contour labels, axis labels) and the figures themselves are useful to readers.\n2. The new bathymetry data are a major new result of your work, I think? This might be sold more strongly. Identify clearly what is new in the section on the updated bathymetry. So much literature is cited here that it is difficult to decipher what the new bathymetry has added to our knowledge.\nMinor suggestions:\nLine 30 and 32 – are the glacial troughs in the first sentence the same as the deep passages in the second sentence? If so, use the same terminology throughout.\nLine 42 – I wasn’t sure what it means by a basin being vulnerable to oceanic thermal forcing? Is this an ocean basin? Or part of the ice shelf or ice sheet?\nLine 43. In what sense is the “expansive ice behind” the glacier? Or is it behind the ice shelf?\nLine 47. Is there another source of oceanic thermal forcing (to justify calling this the strongest)?\nLine 80. Repeats what was said in lines 66-67\nLine 95. I wasn’t sure what was meant by “during at least the last ice retreat timing”?\nLine 96. Seem not seems\nLine 94-94. Why does this the bathymetric data support this? I couldn’t follow the reasoning.\nLine 101. How do you know that these are eddies and not just the section cutting across a meandering along-slope current?\nLine 169. In where?\nLine 172. Did you consider using the O18 measurements to calculate the meltwater distribution?\nLine 186. This would benefit from rephrasing: “characterized with an order of 5-10 cm s-1 of ocean currents”\nLine 206. I couldn’t follow “circulates over the Dalton Polynya from autumn to winter”. Why over? Can you circulate from one season to another?\nLine 208-211. I couldn’t follow this sentence. How does an inflow suggest increasing mCDW?\nLine 219. More CDW than what?\nIs the westward advection of meltwater a surprise? Don’t we expect it from previous (observational and model) studies?\nLines 240-246. These model studies are interesting but not really related to the discussion of your own results, so I cannot see why they are mentioned at all. The need for long-term mooring data is based on these published studies, not your own work. It means that the final paragraph of the paper is a bit speculative and not really based on the strong results of your own paper. I think it would be better to end the paper with a clear statement of the take-home messages based upon your own work (observations and model).\nFigure 1. It would make it easier for readers if both panels had the same colorbar. Is this colorbar colorblind-accessible?\nFigure 2. Panels a-c are too small to do justice to the observations. Since one of the strengths of your paper is your combined / updated bathymetry, it would be good to add this beneath the CTD sections?\nShouldn’t conservative temperature be used rather than potential temperature, following TEOS10?\nFigure 3. Panels and fonts are too small to read.\nFigure 4. Panel d is a super schematic but too small – I can’t read the text. Maybe it's just my old eyes!\nMaps 1a, 1b, 2d, 3a, 4a are all different regions/projections/aspect ratios. It would be helpful to aim for more consistency to help your readers.\nKaren J. Heywood", "reviewer_2": "Review of: On-shelf circulation of warm water toward the Totten Ice Shelf, East Antarctica\nAuthors: Daisuke Hirano, et al.\nThis manuscript documents new observations collected over the Antarctic continental shelf in the Sabrina Coast region in front of the Totten Ice Shelf. Specifically, a multi beam sonar survey provides details of deep glacial troughs that control the circulation around and under the Totten Ice Shelf. These bathymetric observations are coupled with a mooring time series and high-resolution numerical simulations to update the lateral circulation and heat transport pathways over the continental shelf.\nFirst, I thank the authors for a well written manuscript. This is important field work and the new observations will undeniably improve the ability to model heat and tracer transports in this region of the Antarctic continental shelf as well as improve predictions of future change. However, my opinion is that the results in this manuscript do not rise to the level of a Nature Communications publication. In particular, many of the key points have already been described in previous high-impact publications by some of the co-authors, including both observational (Rintoul et al. 2016, Hirano et al. 2021) and numerical (Nakayama et al. 202) studies. More importantly, the figures in this short manuscript do not reflect a deep analysis of the data acquired during the recent field work. In the absence of evidence of warm water under and around Totten, the results in this manuscript would be truly novel. However, based on recent work, I was hoping to see a more thorough discussion of the physical processes that impact heat transport over the continental shelf. My feeling is that a longer-format article in a disciplinary journal would provide a better opportunity for the authors to fully explore the exciting data set that they have acquired.\nAdditional details that support this opinion are provided below.\nMajor comments\n1) Figure 1 shows the major advance in this study, which is the identification of deep troughs in front of Totten. While the multi-beam data is impressive, to a certain extent, the presence of this deep trough was already apparent from the hydrographic section in Rintoul et al. (2016) — their station 36 was > 1000-m showing large heat content. Additionally, the multi-beam data is unable to capture the full meridional extent of these troughs so there remains some ambiguity about how the inflow is connected to the Sabrina Depression and ultimately the shelf break.\n2) Were there any current measurements made during the field program, either with an ADCP or via current meters on the mooring? The abstract of this manuscript discusses the importance of heat transport and circulation pathways, but there are no velocity fields provided from the observations, and only the barotropic streamfunction from the numerical model is provided in the main text of the manuscript. It would be nice to see the analysis go beyond a simple comparison of hydrographic sections between the model output and the numerical model. In particular, there is a nice time series of T/S and CDW thickness variability — are the same temporal scales captured by the numerical model?\n3) As a related point, the abstract states, “These results provide insight into the pathways and processes regulating heat transport to the TIS, …” First, I felt that the authors could have been more explicit about how the new bathymetry modifies either the transport pathways or the heat flux towards Totten, as compared to previous modeling efforts. There are no quantitative estimates of heat fluxes or transports in this manuscript. The mooring time series is another example where a deeper analysis would have been appreciated. Rather than simply plotting the time series, the paper would be strengthened by a consideration of what processes set this variability. Perhaps looking at a frequency spectra would help attribute changes to short-term surface wind and buoyancy forcing (appreciating that the forcing might be poorly constrained) or eddies. The study by Webber et al. (Nat. Comm. 2017) is a nice example of this approach applied to mooring data in front of Pine Island Glacier.\n4) Supplementary Figure 8: This is probably the most important figure in the study. It would have been nice to see this in the main text. However, there are still some confusing aspects in this figure. First, the caption only says “cumulative transports” — is this a heat or a volume transport or both? The figure has units of (mSV) labeled, but it is hard to judge a magnitude because there are no numbers on the y-axis. It would be nice to see a similar panel with heat transports. The caption says the transports are binned by density, but it is hard to judge the vertical structure with this presentation. The traditional way to produce an overturning circulation like this is to simply plot the volume transport in density space — it would be nice to see this figure. It would also make it easier to see if/how the overturning transport closes across the different sides of this box.\nMinor comments\nLine 69: “We also examine the variability in the temperature of mCDW reaching the TIS, which is directly related to the magnitude of TIS melting …” A reference linking the observed melt rates at Totten to the on-shelf temperature content would be helpful. I expect this to be true based on detailed studies in West Antarctica, but are there sufficient observations to confirm this on the Sabrina Coast?\nLine 94: “While the Sabrina Depression has very likely not been affected by glacial erosion during at least the last ice retreat . . .” Please add a citation here or at least justify these comments.\nPathways of warm water section: Even if ADCP data is not available from the cruise, it would be helpful to see section of the geostrophic velocities. While the barotropic component may be uncertain, either a clearly stated assumption about a level or no motion or use of the numerical model output would provide a clearer picture of the circulation. Related to this is that there is no discussion of the overturning circulation on the shelf in this manuscript and how that might be influenced by the new bathymetry observations.\nLine 127: “This suggests that TTR1 is the main conduit by which ocean heat is delivered for TIS melting.” Without showing the velocity fields, I am not sure how the authors reach this conclusion — this can not be deduced by only looking at the temperatures. Both the circulation and heat content contribute to the total heat flux.\nSpatiotemporal variability section: There is no discussion or speculation about possible causes for the sub-seasonal variability. Tides are mentioned at one point, but the authors do not provide information about the expected magnitude of the tides in this region.\nLine 179: “showing a reasonable model representation of ice shelf-ocean interaction in this region.” Is the parameterization for ice-shelf melt in this model tuned to agree with this value?\nSimulation section: Overall, the modeling work is very nice and provides a useful complement to the observations. However, it feels like the analysis here is somewhat superficial because of the length of the manuscript. The only real comparison in the main text are two mean sections of T and S shown in Figure 4b. There are some significant differences between the thickness of the CDW layer in these panels, but this is not discussed in detail. Can these be attributed to seasonal fluctuations, uncertainty in surface forcing in the model, other?\nFigure 4c: Units are not provided for the streamfunction."}, "author_response": "Response to the specific comments and suggestions from Reviewer #1\nDear Prof. Karen Heywood,\nWe very much appreciate your positive assessment and constructive/helpful comments. Your comments were invaluable for improving our manuscript. We have addressed all your comments/suggestions and have substantially modified the paper with the enhanced in-situ observations and simulations (Koji Saito, Japan Coast Guard, was newly added as a co-author due to the addition of the latest observation data.) Our response to each comment is written in red italic and we have highlighted the changes made to the revised manuscript. We now believe that the revised manuscript meets the criteria for publication in Nature Communications.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThis is a very good, well-written paper presenting interesting results. It concerns the\nTotten Ice Shelf in East Antarctica, a less famous (but nonetheless important) sister of the\nAmundsen Sea ice shelves. The processes revealed are of interdisciplinary interest, being\nrelevant to oceanographers and glaciologists wanting to understand what leads to ice\nshelf mass loss, and/or the biogeochemical or climate impacts of the resulting meltwater.\nThe primary results are the insights about the circulation revealed by a new swath\nbathymetry data set, and by hydrographic surveys, supported by a cutting-edge high\nresolution model.\nThe paper describes and synthesises some great observations, but would benefit from\nmore clearly identifying what is new/surprising, and why it matters, as it goes along. For\nexample the section on the pathways of warm water was clear and well described, but I\nwas left wondering whether any of the features were different to previous papers. It’s\nfine if there’s nothing new there, but in that case make it clearer that the section is just\nsetting the scene for subsequent sections?\nØ Previous papers relevant to this study have already provided some of the following\nobservational findings: poleward (eddy-induced) warm water inflow across the shelf\nbreak (Nitsche et al., 2017; Hirano et al., 2021) and subsequent warm water\naccess/distribution to the Sabrina Depression (Silvano et al., 2019) and Totten Ice Shelf\n(TIS) front (Rintoul et al., 2016). However, the linkages between the key process\nmentioned above have not been defined due to the insufficient (spotty) in-situ\nhydrographic and bathymetric data in the region. Thus, to date, there has been no\nintegrated description/knowledge of warm-water distribution and circulation from\noffshore to the TIS front regions in association with realistic bathymetry, one of the\nessential pieces of information for understanding ocean circulation. Therefore, we\nconducted comprehensive shipborne and airborne observations from the upper slope,\ncontinental shelf, to the TIS front, aiming for an integrated understanding of the warm\nwater distribution/circulation that would control the magnitude of oceanic thermal\nforcing for the TIS basal melt.\nThe above is the novelty associated with the first submitted version. Furthermore, in the\nrevised manuscript we have included newly acquired in-situ observation data\n(bathymetric and hydrographic data) from the 2021/22 season (JARE63) in the Totten\nregion (see figures below), which we believe further strengthens the novelty of this paper\non warm water distribution and circulation towards TIS cavity controlled by deep\nbathymetric features. Our multi-beam sonar survey, for the first time, revealed the\nrelationship with detailed bathymetry. Thus, we could clarify the comprehensive warm\nwater circulation from the upper slope to the TIS front regions. Of particular note is that\nthe TIS cavity is connected to the continental shelf (depression) by the TTR1’s deeper\ntopography of at least >600‒700 m, and warm water flows along this deep channel\ntoward the TIS. This indicates, together with the simulation results, that TTR1 is the main\nconduit by which ocean heat is delivered for TIS melting.\n)Co(\nerutarepmet\nlaitnetoP\n)m(\nhtpeD\nBathymetric & hydrographic data WITHOUT2021/22 season data (previous version)\nTotten Ice Shelf\nTIS\nMUIS\nSabrina Coast\n)Co(\nerutarepmet\nlaitnetoP\n)m(\nhtpeD\nBathymetric & hydrographic data WITH2021/22 season data\nTotten Ice Shelf\nTIS\nMUIS\nSabrina Coast\nIn addition, by building a high-resolution model incorporating the realistic updated\nbathymetry data, we could simulate water mass distribution and ocean circulation,\nconsistent with in-situ observations, even near the TIS front. The model simulation further\nallowed us to illustrate the picture of vertical circulation beneath the TIS cavity (i.e., ice-\npump circulation) for the first time. (Line: 63-71; 200-243)\nWe believe the above points are the major novelty of this study and advance the field\nfrom previous studies.\nI think the key results are those stated in lines 220 to 225. These are clear, important and\nvaluable. If the authors confirm my understanding, probably these key results should be\nstated more strongly at the end of the paper as the take-home message?\nØ Thank you for your valuable suggestion. We have modified the take-home message to one\nestablished on our new findings (circulation and temporal variability of the mCDW\ninflowing into TIS) and have highlighted the necessity for long-term monitoring of the\nocean state in the Totten region. (Line: 276-289)\nRecommendations for strengthening:\n1. Many of the figures have multiple (fairly unrelated) panels and this means that the fonts\non many of the panels are impossible to read. There are 16 figure panels, which in a\nlonger-format journal would probably be presented in 10-12 separate figures; this makes it\nfeel like a longer paper trying to squeeze into a high profile format. I recommend cutting\ndown the number of figures to those that are essential to support the key message(s) of\nthe paper, and enlarging them so that fonts are legible (e.g. contour labels, axis labels)\nand the figures themselves are useful to readers.\nØ Based on the comment, we have reorganized the overall figure structure by decomposing\nthe multiple unrelated panels, moving figures from the Supplementary Information, and\ncreating new figures. The revised manuscript now consists of ten figures. We have also\nenlarged the figures for ease of reading (Figures 1-10)\n2. The new bathymetry data are a major new result of your work, I think? This might be\nsold more strongly. Identify clearly what is new in the section on the updated bathymetry.\nSo much literature is cited here that it is difficult to decipher what the new bathymetry\nhas added to our knowledge.\nØ We agree with your comment. We believe that new bathymetry data obtained by\nmultibeam sonar is one of the major highlights of this paper (major new result). To\nfurther strengthen this, we have added the in-situ bathymetric data newly acquired in the\n2021/22 season (JARE63) in this revised manuscript. With the first bathymetry dataset\nfrom the multibeam sonar around the TIS, we could reveal almost the full extent of TTR1,\nas the main conduit by which ocean heat is delivered for TIS melting and have significantly\nimproved our understanding of the warm water circulation toward the TIS, which is\ncontrolled by the bathymetry. (Line: 74-95; 131-142; 226-234)\nMinor suggestions:\nLine 30 and 32 – are the glacial troughs in the first sentence the same as the deep passages\nin the second sentence? If so, use the same terminology throughout.\nØ No, they are not the same. We have deleted the word “passage” to avoid\nmisinterpretation throughout the revised paper.\nLine 42 – I wasn’t sure what it means by a basin being vulnerable to oceanic thermal\nforcing? Is this an ocean basin? Or part of the ice shelf or ice sheet?\nØ It is not an “ocean” basin but a “subglacial” basin below the ice base of the ice\nsheet/shelf. For clarity, we have modified this sentence. (Line: 40-42)\nLine 43. In what sense is the “expansive ice behind” the glacier? Or is it behind the ice\nshelf?\nØ Based on your comment, we have modified this part by deleting “expansive ice behind.”\n(Line: 40-42)\nLine 47. Is there another source of oceanic thermal forcing (to justify calling this the\nstrongest)?\nØ It is seasonal, but Antarctic Surface Water is also a strong oceanic thermal forcing for the\nice-shelf melting (e.g., Aoki et al., 2022). Based on your comment, we have modified this\nsentence to avoid misunderstanding. (Line: 45-46)\nLine 80. Repeats what was said in lines 66-67\nØ Thank you for pointing this out. We have deleted the redundant section (Line 58-60).\nLine 95. I wasn’t sure what was meant by “during at least the last ice retreat timing”?\nLine 96. Seem not seems\nLine 94-94. Why does this the bathymetric data support this? I couldn’t follow the\nreasoning.\nØ We cannot show the timing relationship between the glacial activity and topographic\nformation using data or proper references. We have retained minimal wording in this part\nbecause it does not inherently affect the conclusions of the study. (Line: 94-95)\nLine 101. How do you know that these are eddies and not just the section cutting across a\nmeandering along-slope current?\nØ Thank you for pointing this out. For this revision, we created the absolute ocean velocity\n(Fig. 2d-f) from the geostrophic velocity calculation, referenced to the satellite-derived\nsurface ocean velocity, following the same method as Hirano et al. (2021). Since the\nobservations along the upper slope section (Figs. 2a, d) do not follow the isobath, the\nresults might partly include features such as a meandering slope current; however, at\nleast the warm water core at ~119.5E corresponds to the downstream of the southward\nflowing region of the eastern half of the West Sabrina Eddy. The subsequent southward\ninflow is still observed in the shelf break and inner shelf sections east of 119E (Fig. 2b, c, e,\nf), where warm water widely spreads near the seafloor. In other words, it is an important\nobservational fact that at least the offshore eddies contribute to the poleward warm-\nwater transport across the shelf break, followed by the continuous southward warm\nwater inflow to the Sabrina Depression in the inner shelf. (Line: 97-114)\nLine 169. In where?\nØ We have deleted this sentence in the revised manuscript because we described the ice-\npump circulation along with the simulation results.\nLine 172. Did you consider using the O18 measurements to calculate the meltwater\ndistribution?\nØ Thank you for the comment. We have added a brief description of the glacial meltwater\ndistribution estimated from a three-end-member mass balance formulation (Meredith et\nal., 2008; Silvano et al., 2018). (Line: 190-193)\nLine 186. This would benefit from rephrasing: “characterized with an order of 5-10 cm s-1\nof ocean currents”\nØ Thank you for pointing this out. We have modified this sentence. (Line: 217-220)\nLine 206. I couldn’t follow “circulates over the Dalton Polynya from autumn to winter”.\nWhy over? Can you circulate from one season to another?\nLine 208-211. I couldn’t follow this sentence. How does an inflow suggest increasing\nmCDW?\nØ For the simulation results, we decided to remove the discussion on seasonal variability\nand focus on the mean fields (climatology) in this revision because the discussion on\nseasonal variations could not be compared and verified with observations and included\nsome speculation. Therefore, we have deleted these parts.\nFurthermore, as already mentioned above, we have enhanced simulation results along the\nice-front section (Fig.8) and in the TIS cavity (Fig.9), which is the additional novelty of the\npaper. (Line: 226-243)\nLine 219. More CDW than what?\nØ Thank you for pointing this out. We have modified this sentence. (Line: 247-249)\nIs the westward advection of meltwater a surprise? Don’t we expect it from previous\n(observational and model) studies?\nØ The westward advection itself is not unexpected, as Antarctic Coastal Currents are\ngenerally westward, but the meltwater signal concentrated to the west originating from\nthe TIS cavity was only speculated based on hydrographic data (Rintoul et al., 2016;\nSilvano et al., 2017).\nAdditionally, this study offers a multifaceted evaluation from oxygen isotope ratio data\nand simulation results, which indicate that the meltwater exits the cavity along the TIS\nbase northward, making the western TIS front region fresh/less dense. This is a better\npresentation of meltwater outflow from TIS and an advance from speculations by\nprevious studies. (Line: 188-193; 226-243)\nLines 240-246. These model studies are interesting but not really related to the discussion\nof your own results, so I cannot see why they are mentioned at all. The need for long-term\nmooring data is based on these published studies, not your own work. It means that the\nfinal paragraph of the paper is a bit speculative and not really based on the strong results\nof your own paper. I think it would be better to end the paper with a clear statement of\nthe take-home messages based upon your own work (observations and model).\nØ We agree with your comment. As described above, we have modified the take-home\nmessage section to the one based on our new findings (circulation and temporal\nvariability of the mCDW inflowing into TIS) and have highlighted the necessity for long-\nterm monitoring of the ocean state in the Totten region. (Line: 276-289)\nFigure 1. It would make it easier for readers if both panels had the same colorbar. Is this\ncolorbar colorblind-accessible?\nØ Based on your comment, we have made Figure 1 easier to read. Additionally, the colorbar\nused for Figure 1 is commonly used in Marine Geology (“Haxby” colormap of GMT\nsoftware). Furthermore, we checked whether the main topographic features, such as\ndeep Totten troughs, are discernable with a color-blind simulator\n(https://pilestone.com/pages/color-blindness-simulator-1), and we confirmed they are\ndiscernable with this color space.\nFigure 2. Panels a-c are too small to do justice to the observations. Since one of the\nstrengths of your paper is your combined / updated bathymetry, it would be good to add\nthis beneath the CTD sections?\nØ Revised Figure 2 differs from the previous one because we have reorganized the overall\nfigure structure. Current Figure 2 consists of temperature and absolute ocean velocity\nsections. We also added ocean bottom topography extracted from the multibeam sonar\ndataset for each panel.\nShouldn’t conservative temperature be used rather than potential temperature, following\nTEOS10?\nØ We used the potential temperature in this study, considering the consistency with the\nocean-sea ice-ice shelf model (Kusahara and Hasumi, 2013).\nFigure 3. Panels and fonts are too small to read.\nØ Revised Figure 3 differs from the previous one, but we have made all figures easier to\nread.\nFigure 4. Panel d is a super schematic but too small – I can’t read the text. Maybe it's just\nmy old eyes!\nØ We are happy with your compliment (and sorry for the inconvenience). The schematic\npicture is now Figure 10 (with no multiple panels). We have also made the text larger\nso that readers would be able to read it easily.\nMaps 1a, 1b, 2d, 3a, 4a are all different regions/projections/aspect ratios. It would be\nhelpful to aim for more consistency to help your readers.\nØ Thank you for your comment. We have tried to unify the regions/projections/aspect\nratios of maps shown in Figures 1, 3, and 7 as much as possible.\nKaren J. Heywood\nResponse to the specific comments and suggestions from Reviewer #2\nWe appreciate your constructive/helpful comments very much. Your comments were invaluable for\nimproving our manuscript. We have considered almost all your comments/suggestions and have\nsubstantially modified the paper with the enhanced in-situ observations and simulations (Koji\nSaito, Japan Coast Guard, was newly added as a co-author due to the addition of the latest\nobservation data.) Our response to each comment is written in red italic and we have highlighted\nthe changes made to the revised manuscript. We now believe that the revised manuscript meets\nthe criteria for publication in Nature Communications.\nReviewer #2 (Remarks to the Author):\nReview of: On-shelf circulation of warm water toward the Totten Ice Shelf, East Antarctica\nAuthors: Daisuke Hirano, et al.\nThis manuscript documents new observations collected over the Antarctic continental\nshelf in the Sabrina Coast region in front of the Totten Ice Shelf. Specifically, a multi beam\nsonar survey provides details of deep glacial troughs that control the circulation around\nand under the Totten Ice Shelf. These bathymetric observations are coupled with a\nmooring time series and high-resolution numerical simulations to update the lateral\ncirculation and heat transport pathways over the continental shelf.\nFirst, I thank the authors for a well written manuscript. This is important field work and\nthe new observations will undeniably improve the ability to model heat and tracer\ntransports in this region of the Antarctic continental shelf as well as improve predictions\nof future change. However, my opinion is that the results in this manuscript do not rise to\nthe level of a Nature Communications publication. In particular, many of the key points\nhave already been described in previous high-impact publications by some of the co-\nauthors, including both observational (Rintoul et al. 2016, Hirano et al. 2021) and numerical\n(Nakayama et al. 2021) studies.\nØ Previous papers relevant to this study have already provided some of the following\nobservational findings: poleward (eddy-induced) warm water inflow across the shelf\nbreak (Nitsche et al., 2017; Hirano et al., 2021) and subsequent warm water\naccess/distribution to the Sabrina Depression (Silvano et al., 2019) and Totten Ice Shelf\n(TIS) front (Rintoul et al., 2016). However, the linkages between the key process\nmentioned above have not been defined due to the insufficient (spotty) in-situ\nhydrographic and bathymetric data in the region. Thus, to date, there has been no\nintegrated description/knowledge of warm-water distribution and circulation from\noffshore to the TIS front regions in association with realistic bathymetry, one of the\nessential pieces of information for understanding ocean circulation. Therefore, we\nconducted comprehensive shipborne and airborne observations from the upper slope,\ncontinental shelf, to the TIS front, aiming for an integrated understanding of the warm\nwater distribution/circulation that would control the magnitude of oceanic thermal\nforcing for the TIS basal melt.\nThe above is the novelty associated with the first submitted version. Furthermore, in the\nrevised manuscript we have included newly acquired in-situ observation data\n(bathymetric and hydrographic data) from the 2021/22 season (JARE63) in the Totten\nregion (see figures below), which we believe further strengthens the novelty of this paper\non warm water distribution and circulation towards TIS cavity controlled by deep\nbathymetric features. Our multi-beam sonar survey, for the first time, revealed the\nrelationship with detailed bathymetry. Thus, we could clarify the comprehensive warm\nwater circulation from the upper slope to the TIS front regions. Of particular note is that\nthe TIS cavity is connected to the continental shelf (depression) by the TTR1’s deeper\ntopography of at least >600‒700 m, and warm water flows along this deep channel\ntoward the TIS. This indicates, together with the simulation results, that TTR1 is the main\nconduit by which ocean heat is delivered for TIS melting.\n)Co(\nerutarepmet\nlaitnetoP\n)m(\nhtpeD\nBathymetric & hydrographic data WITHOUT2021/22 season data (previous version)\nTotten Ice Shelf\nTIS\nMUIS\nSabrina Coast\n)Co(\nerutarepmet\nlaitnetoP\n)m(\nhtpeD\nBathymetric & hydrographic data WITH2021/22 season data\nTotten Ice Shelf\nTIS\nMUIS\nSabrina Coast\nIn addition, by building a high-resolution model incorporating the realistic updated\nbathymetry data, we could simulate water mass distribution and ocean circulation,\nconsistent with in-situ observations, even near the TIS front. The model simulation further\nallowed us to illustrate the picture of vertical circulation beneath the TIS cavity (i.e., ice-\npump circulation) for the first time. (Line: 63-71; 200-243)\nWe believe the above points are the major novelty of this study and advance the field\nfrom previous studies.\nMore importantly, the figures in this short manuscript do not reflect a deep analysis of the\ndata acquired during the recent field work. In the absence of evidence of warm water\nunder and around Totten, the results in this manuscript would be truly novel. However,\nbased on recent work, I was hoping to see a more thorough discussion of the physical\nprocesses that impact heat transport over the continental shelf. My feeling is that a\nlonger-format article in a disciplinary journal would provide a better opportunity for the\nauthors to fully explore the exciting data set that they have acquired. Additional details\nthat support this opinion are provided below.\nØ The novelty and advance of this paper are noted above. Furthermore, we have performed\nthe following additional analyses for this revision: the estimation of the absolute ocean\nvelocity from the geostrophic and satellite-derived surface velocity (Fig. 2d-f) and\nspectrum (Fig. S4) and trend (Fig. 6c) investigations of the mooring time series. Please\ntake further note of the response below for details.\nMajor comments\n1) Figure 1 shows the major advance in this study, which is the identification of deep\ntroughs in front of Totten. While the multi-beam data is impressive, to a certain extent,\nthe presence of this deep trough was already apparent from the hydrographic section in\nRintoul et al. (2016) — their station 36 was > 1000-m showing large heat content. Additionally,\nthe multi-beam data is unable to capture the full meridional extent of these troughs so there\nremains some ambiguity about how the inflow is connected to the Sabrina Depression and\nultimately the shelf break.\nØ Indeed, the presence of a deep trough with a depth of >1000 m at the TIS front and warm\nwater access to the trough had already been observed (Rintoul et al., 2016). However, the\nstudy was based on just-below bathymetry data from a single beam sonar; thus, the\ndetails of the spatial structure of the Totten trough were unknown. In contrast, our first\nmultibeam-surveyed bathymetric data near the TIS reveals the detailed spatial structure\nof the Totten trough, especially the most significant TTR1. However, as you pointed out,\nthere may still be ambiguity regarding the topographic connection between the Totten\ntrough and the Sabrina Depression/shelf break.\nThis revised manuscript includes newly acquired in-situ observation data\n(bathymetric and hydrographic data) from the 2021/22 season (JARE63) in the Totten\nregion, which we\nbelieve have overcome the concern about ambiguity. We have revealed almost the full\nextent of TTR1 as the main conduit by which ocean heat is delivered for TIS melting and\nhave significantly improved our understanding of the warm water circulation toward the\nTIS, which is controlled by the bathymetry. Specifically, we successfully showed that the\nTIS cavity directly connects with the Sabrina Depression (and to the shelf break) by the\nTTR1’s deeper topography of at least >600‒700 m. (Line: 74-95; 131-142; 226-234)\n2) Were there any current measurements made during the field program, either with an\nADCP or via current meters on the mooring? The abstract of this manuscript discusses the\nimportance of heat transport and circulation pathways, but there are no velocity fields\nprovided from the observations, and only the barotropic streamfunction from the\nnumerical model is provided in the main text of the manuscript. It would be nice to see\nthe analysis go beyond a simple comparison of hydrographic sections between the model\noutput and the numerical model. In particular, there is a nice time series of T/S and CDW\nthickness variability — are the same temporal scales captured by the numerical model?\nØ There are no available ocean current data either from the mooring or ADCP. Based on the\ncomments, we have carried out the current best practice to estimate the absolute ocean\nvelocity on the upper slope, shelf break, and the inner shelf of the Sabrina Depression\nsections (Fig. 2d-f) from the geostrophic velocity calculation, referenced to the satellite-\nderived surface ocean velocity, following the same method as Hirano et al. (2021). The\nresults show a cyclonic circulation, with warm water flowing southward in the eastern\nhalf of the Sabrina Depression and northward in the western half (Fig. 3), reinforcing the\nobservational evidence for the on-shelf cyclonic circulation of the warm water. (Line: 97-\n114)\nFurthermore, the observation of the warm-water cyclonic circulation is consistent with\nthe simulation results (Fig. 7).\nWith the high-resolution model incorporating the realistic updated bathymetry data,\nwe\ncould simulate the water mass distribution and ocean circulation, consistent with in-situ\nobservations, even near the TIS front. We believe the enhanced observations of the\nbathymetry and hydrography near the TIS allow us to compare the results from in-situ\nobservations and numerical simulations for the first time, which in itself is a simple but\nsignificant advance. (In other words, until now, it has not been possible to compare even\n“simple\" observation-simulation results.) (Line: 63-71; 200-243)\nIn addition, we decided to remove the discussion on seasonal variability and focus on the\nmean fields (climatology) in the revised manuscript. Instead, a separate numerical\nsimulation paper (in preparation) will present an in-depth discussion of variability,\nincluding a timescale with more than a seasonal scale (Kusahara et al., in prep.).\n3) As a related point, the abstract states, “These results provide insight into the pathways\nand processes regulating heat transport to the TIS, …” First, I felt that the authors could\nhave been more explicit about how the new bathymetry modifies either the transport\npathways or the heat flux towards Totten, as compared to previous modeling efforts.\nThere are no quantitative estimates of heat fluxes or transports in this manuscript. The\nmooring time series is another example where a deeper analysis would have been\nappreciated. Rather than simply plotting the time series, the paper would be\nstrengthened by a consideration of what processes set this variability. Perhaps looking at\na frequency spectra would help attribute changes to short-term surface wind and\nbuoyancy forcing (appreciating that the forcing might be poorly constrained) or eddies.\nThe study by Webber et al. (Nat. Comm. 2017) is a nice example of this approach applied to\nmooring data in front of Pine Island Glacier.\nØ We have added in-situ observations from the 2021/22 season in the Totten region to\nstrengthen our original novelty. Our high-resolution model, incorporating realistic\nupdated bathymetry data, could simulate water mass distribution and ocean circulation,\nconsistent with in-situ observations, even near the TIS front. The model simulation\nallowed us to illustrate a picture of vertical circulation beneath the TIS cavity for the first\ntime. Integrating results from the realistic simulation and in-situ observations, we found\nthe TTR1 as the main conduit by which ocean heat is delivered for TIS melting and\nsignificantly improved our understanding of the warm water circulation toward the TIS,\nwhich is controlled by the successive deep channels from the upper slope, depression, to\nthe TIS cavity. This is the main progress from previous studies.\nBased on your comments, we additionally performed the spectral analysis of the\ntemperature time series and ERA5 wind data (Fig. S4) to examine the relationship\nbetween inflowing warm water property into the TIS and tides or wind forcing. If we\ndefine trends over the mooring period (~3 months) as ranges of seasonal variability, it is\nreasonable to conclude that the range of mCDW temperature reflects interannual\nvariability rather than seasonal one. In addition, we conclude, from the frequency\nanalysis, a contribution from the wind-driven Ekman upwelling/downwelling on the\nWW/mCDW interface (i.e., mCDW thickness) variation (e.g., Ohshima et al., 1996; Hirano\net al., 2020) is not evident in the Totten region, at least below the seasonal timescale.\nCertainly, Webber et al. (2017) is an excellent example, but their conclusions are\nestablished on investigations with long-term five-year mooring records. In contrast, our\ntime series data are shorter than the seasonal window (summertime only). Furthermore,\nwind/buoyancy forcing typically has seasonality; therefore, it is difficult to examine the\nrelationship between warm-water properties and wind/buoyancy forcing in a sub-\nseasonal timescale. (Line: 150-185)\nWhen we obtain more than year-round time series data, discussing variability with more\nthan a seasonal scale will be optimal through comparison and integration with model\nresults.\n4) Supplementary Figure 8: This is probably the most important figure in the study. It\nwould have been nice to see this in the main text. However, there are still some confusing\naspects in this figure. First, the caption only says “cumulative transports” — is this a heat\nor a volume transport or both? The figure has units of (mSV) labeled, but it is hard to\njudge a magnitude because there are no numbers on the y-axis. It would be nice to see a\nsimilar panel with heat transports. The caption says the transports are binned by density,\nbut it is hard to judge the vertical structure with this presentation. The traditional way to\nproduce an overturning circulation like this is to simply plot the volume transport in\ndensity space — it would be nice to see this figure. It would also make it easier to see\nif/how the overturning transport closes across the different sides of this box.\nØ Thank you very much for your constructive comments. In this revision, we removed\nresults and discussion of seasonal variability to focus more on the mean fields\n(climatology). To take account of your suggestion on the overturning circulation, we have\nadded new figures for the overturning circulations under/near the ice-shelf cavity (Fig. 9)\nand along TIS front temperature/salinity/ocean velocity sections (Fig. 8). These new\nfigures allow us to clearly show ice-pump circulation and the connection to ocean\ncirculation/condition over the continental shelf region. (Line: 226-243)\nIn addition, we considered the seasonal-to-interannual variability in ice-ocean interaction\nand ocean circulation in the model to be addressed. Thus, we are working on a study that\nwill provide more details about this (Kusahara et al. in prep).\nMinor comments\nLine 69: “We also examine the variability in the temperature of mCDW reaching the TIS,\nwhich is directly related to the magnitude of TIS melting …” A reference linking the\nobserved melt rates at Totten to the on-shelf temperature content would be helpful. I\nexpect this to be true based on detailed studies in West Antarctica, but are there sufficient\nobservations to confirm this on the Sabrina Coast?\nØ Thank you for the comment. Rignot et al.\n(2002) showed a linear relationship\nbetween the basal melt rates vs oceanic\nthermal forcing (T minus in-situ freezing\npoint). This might address your comment [redacted]\non the circumpolar view, but it might not\nRignot et al. (2002, Science)\nbe valid for the Totten region because of\ninsufficient in-situ observations, at least\nwhen Rignot et al. (2002) was published.\nThus, we have modified this sentence to\navoid misunderstanding. (Line: 61-63)\nLine 94: “While the Sabrina Depression has very likely not been affected by glacial erosion\nduring at least the last ice retreat . . .” Please add a citation here or at least justify these\ncomments.\nØ We cannot show/suggest the timing relationship between glacial activity and topographic\nformation using data or proper references. We have retained minimal wording in this part\nbecause it does not inherently affect the conclusions of the study. (Line: 94-95)\nPathways of warm water section: Even if ADCP data is not available from the cruise, it\nwould be helpful to see section of the geostrophic velocities. While the barotropic\ncomponent may be uncertain, either a clearly stated assumption about a level or no\nmotion or use of the numerical model output would provide a clearer picture of the\ncirculation. Related to this is that there is no discussion of the overturning circulation on\nthe shelf in this manuscript and how that might be influenced by the new bathymetry\nobservations.\nØ Thank you for the suggestion. As I mentioned above, for this revision, we created the\nabsolute ocean velocity (Fig. 2d-f) from the geostrophic velocity calculation, referenced to\nthe satellite-derived surface ocean velocity, following the same method as Hirano et al.\n(2021). Since the observations along the upper slope section (Figs. 2a, d) do not follow the\nisobath, the results may partly include features such as a meandering slope current, but at\nleast the warm water core at ~119.5E corresponds to the downstream of the southward\nflowing region of the eastern half of the West Sabrina Eddy. The subsequent southward\ninflow is still observed in the shelf break and inner shelf sections east of 119E (Fig. 2b, c, e,\nf), where warm water widely spreads near the seafloor. In other words, it is an important\nobservational fact that at least the offshore eddies contribute to the poleward warm-\nwater transport across the shelf break, followed by the continuous southward warm\nwater inflow to the inner shelf. (Line: 97-114)\nFurthermore, we have enhanced simulation results along the ice-front section (Fig.8) and\nin the TIS cavity (ice-pump overturning circulation, Fig.9). The enhanced observations and\nsimulations significantly improve the picture of the warm water circulation all the way\nfrom the offshore into the TIS cavity through the deep channels. (Line: 220-243)\nLine 127: “This suggests that TTR1 is the main conduit by which ocean heat is delivered for\nTIS melting.” Without showing the velocity fields, I am not sure how the authors reach\nthis conclusion — this can not be deduced by only looking at the temperatures. Both the\ncirculation and heat content contribute to the total heat flux.\nØ Based on your comment, we have added simulated data along the ice-front sections of\ntemperature, salinity, and ocean velocity (inflow/outflow) (Fig. 8) to complement the lack\nof ocean velocity observations at the TIS front. We established that the most significant\nmCDW inflow, marked by the thicker mCDW layer along with the bottom-intensified\nvelocity of 4‒8 cm s-1, occurs in the bottom layer (deeper than 500‒600 m) centered at the\neastern side of the TTR1. Then, we could conclude from the observations (Figs. 3, 4, 5) and\nsimulations (Fig. 8) that the TTR1 directly connects the TIS cavity and the Sabrina\nDepression (Figs. 1, 3) and is the main conduit by which ocean heat is delivered for TIS\nmelting. (Line: 131-135; 226-234)\nBesides, the TTR2 is also a deep trough with a maximum depth of >1000 m but it seems to\nbe surrounded by relatively shallower topography with <500‒600 m depth; therefore, its\ntopographic connection to the Sabrina Depression is undefined from the existing\nbathymetric data. The shallow topographic context would prevent the inflows of the\nwarmer mCDW present at the deeper layer of the depression into the TTR2, as inferred by\nSilvano et al. (2019). (Line: 135-142)\nSpatiotemporal variability section: There is no discussion or speculation about possible\ncauses for the sub-seasonal variability. Tides are mentioned at one point, but the authors\ndo not provide information about the expected magnitude of the tides in this region.\nØ Based on the comment, we have added possible causes for the sub-seasonal variability in\naddition to tide-related variations (semidiurnal/diurnal to fortnightly): offshore semi-\npermanent West Sabrina Eddy with characters with a relatively short variability in 2-6\nmonths (Mizobata et al., 2020) and wind/buoyancy forcing. However, we cannot\nquantitatively evaluate each contribution to the inflowing mCDW variation with the\ncurrent dataset because of the reason noted above. (Line: 158-170)\nIn addition, we have added pressure time series data as tidal variation in the region (see\nFig. 6b).\nLine 179: “showing a reasonable model representation of ice shelf-ocean interaction in this\nregion.” Is the parameterization for ice-shelf melt in this model tuned to agree with this\nvalue?\nØ No, we did not perform tuning to match the simulated basal melting to the satellite-based\nestimate. In this study, we used an ice-ocean parameterization of Hellmer and Olbers\n(1989) based on an observation of ice-ocean interaction. In the revised manuscript, we\nhave updated the model simulation using ERA5 forcing. The modeled basal melting\namount at the TIS becomes smaller than that included in the previous manuscript (forced\nwith ERA-Interim). However, the model estimate of 47 Gt/yr is not far from the\nobservational estimate, 63 Gt/yr. The applicable sentences have been rephrased. (Line:\n209-210).\nSimulation section: Overall, the modeling work is very nice and provides a useful\ncomplement to the observations. However, it feels like the analysis here is somewhat\nsuperficial because of the length of the manuscript. The only real comparison in the main\ntext are two mean sections of T and S shown in Figure 4b. There are some significant\ndifferences between the thickness of the CDW layer in these panels, but this is not\ndiscussed in detail. Can these be attributed to seasonal fluctuations, uncertainty in surface\nforcing in the model, other?\nØ Thank you very much for your encouragement on our modeling component. We consider\nthat there is a novelty in a direct comparison between the in-situ observation and\nmodeling results. The lack and ambiguity of the bathymetry map off the Sabrina Coast,\nparticularly near the TIS, has hampered the previous modeling studies from simulating\nocean circulation and the subsequent TIS-ocean interaction. Our simulation with the\nupdated bathymetry for the first time demonstrated the overall ocean circulation from\nshelf break, continental shelf, to the region in front of/underneath the TIS. (Line: 63-71; 200-243)\nWe acknowledge the differences between the observation and model results; the model\nbiases probably come from integrated uncertainties in snapshot-/averaged-fields,\nuncharted bathymetry, atmospheric forcing, and model configurations (as you pointed\nout). As mentioned in earlier responses, we are working on a study that describes the\nmodel results, in which we focus on seasonal-to-interannual variability in the model. Thus,\nwe consider that the simple comparison between observation and model climatological\nresults is sufficient to support the observational findings in this study.\nFigure 4c: Units are not provided for the streamfunction.\nØ Thank you for your correction. The figure was removed in this revision. We have carefully\nchecked the details (units, legends, and captions) of all the figures in the revised\nmanuscript.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors have considered the suggestions made by both reviewers, and made some good\nand useful revisions. They have added a substantial amount of new data from the most recent\nseason, which is great. I would like to be more enthusiastic about this paper, but I feel it could\nstill be strengthened to make the most of this great data set. I hope these suggestions are helpful,\nas I would like to see this work published.\nMy primary concern of the original paper was that it needed to make clear what was new or\nsurprising, and why it mattered. This was also raised by Reviewer 2. The authors’ response states\n“to date, there has been no integrated description/knowledge of warm-water distribution and\ncirculation from offshore to the TIS front regions in association with realistic bathymetry” which I\ndidn’t find very convincing. I am still unclear exactly what insight this paper has added to our\nknowledge. Figure 10 is lovely, but couldn’t this schematic have been produced before any of the\nwork described in this paper, based on pre-existing papers? The paper argues that this paper\npresents “more detail” but why are these details important (and for whom/what)? Maybe new\ninsights are presented, but I was not sure what they were. If something is a new finding, say so\nexplicitly.\nI still find it hard to identify what is new. The abstract, for example, is unchanged. It makes a\nseries of statements with which everyone can agree, I think, but doesn’t make clear what\nspecific bit of knowledge was added through the analysis presented in the paper (that merits\npublication in Nature Comms). Sentences 1-3 are scene setting, that’s fine. The 4th sentence\nmight be new results, or might not – can you clarify for your future readers exactly what is the\nnew finding in this sentence? The 5th sentence is surely obvious and could be written about any\nice shelf even with no observations? And the final sentence seems a bit of a stretch – what\nexactly in the results/analysis has “highlighted the glacier’s vulnerability” that we did not know\nbefore?\nIt is good that the authors talked more about the circulation in the revised version. It surprises\nme that there are no velocity data – does the ship not have a shipboard ADCP? Did the moorings\nthat you mention not have velocity? Heat transports would greatly strengthen the paper. In the\nabstract and again in line 45 you introduce the concept of ocean heat transport as being key, but\nyou never calculate this so it is unclear why you introduce it. You could quantify it in your super\nmodel simulation?\nIn lines 117-120, you talk about there being a cyclonic circulation of mCDW. If this is the first\npaper to identify this, then say so. You have the data to quantify it more – how many Sv? If it’s\nnot new, then shorten the description so that the paper focuses on the things that are new – this\nwill help your readers to grasp your take-home message.\nReviewer 2 made a very valid suggestion that you should do more than just present data (e.g.\ntime series or sections) (their point 3). I wasn’t really convinced by your response. The\nparagraph added about the mooring time series (p8-9) simply describes the variability in the\ntime series. But what is the message, why do we care about this, is any of this new or surprising?\nPointing out the surprises would be very helpful.\nWhat would really demonstrate the value of the revised bathymetry, to me, would be a\ncomparison of a numerical simulation with the “old” bathymetry, and a simulation with the\nnew/revised bathymetry. You state (line 65) that the model “reasonably reproduces the ocean\ncirculations and mCDW pathways from the shelf break to the TIS front regions, consistent with\nthe observational findings”. This raises two questions in my mind – first, how do you know this,\nand do you mean that it agrees with the observations of this paper, or of previous work? Second,\nif you regard the observations as “truth”, what are you gaining from the use of the model? What\nkey result comes from the model? The model is great, but you could use it to quantify or shed\nlight on something? Is it the melt rates? If so, sell this new finding better (e.g. before our new\nbathymetry, models couldn’t get the melt rate correct, but now…..”?). You state (line 205) that the\nmodel representation (of water masses? Fluxes? Or what?) is “substantially improved” with the\nnew bathymetry, but how do we know this? What was it like before the new bathymetry?\nIn my original review, I asked \"Did you consider using the O18 measurements to calculate the\nmeltwater distribution?\" The authors’ response is to state that there is a 0.2% increase in\nmeltwater in the outflow compared with the inflow. The distribution is not shown. Since we’re\nnot given the numbers, this isn’t very useful. And 0.2% increase is surely not significant given the\nuncertainties in the measurements and in the calculations of meltwater? In addition, I was\npuzzled by the references given here? The first reference is about the WAP – it is by no means\nthe earliest paper to discuss calculating the freshwater components from O18 using this method,\nnor is it from this region, so why is this particular paper an appropriate reference?\nLine 263, you state that the “observations provide compelling evidence that warm water reaches\nthe TIS to drive rapid basal melt”. But did we not know this already from other papers? If this is\nnew, say so.\nLines 267-275 – I cannot see the relevance of these sentences. They talk about other people’s\nresults, and are nothing to do with the implications of your own results. You need to say how\nyour new results affects these speculations. Or remove them.\nThe figures are much better, thank you, though I find some of the fonts extremely small (e.g.\ncontour labels and station numbers in figure 2).\nFigure 2, it would be good if you used colorbarfill or similar (as you did in Figure 3) so that the\ncolorbar is consistent will the filled contours.\nFor all figures, it would be helpful for readers if there was more consistency, e.g. in colorbars,\ncolormaps, colorbar ranges.\nFor example:\n- Figure 2 e does not have a comparable colorbar range as d and f for velocities\n- Figure 7 uses different colormaps and colorbar ranges from the other figures of T and S, and\nFigure 8 is different again. Then Figure 9 uses yet another temperature colorbar.\n- Figure 8 c does not use the same velocity colormap or colorbar range as the earlier velocity\nsections from observations. It is also given in cm/s rather than m/s – units should be consistent\nthroughout.\n- Is the colormap used in Figure 8 a,b colour-blind friendly?\nMinor points:\nLine 94-95, the sentence about glacial erosion seems irrelevant to this paper and should be\nremoved.\nLine 285, I would say suggests rather than offers\nKaren J. Heywood\nReviewer #2 (Remarks to the Author):\nReview of: On-shelf circulation of warm water toward the Totten Ice Shelf, East Antarctica\nAuthors: Daisuke Hirano, et al.\nThis manuscript documents new observations collected over the Antarctic continental shelf in the\nSabrina Coast region in front of the Totten Ice Shelf. Specifically, multi beam sonar surveys from\nmultiple research cruises (including a cruise from March 2022 that was not included in the original\nmanuscript) provides details of deep glacial troughs that control the circulation around and under\nthe Totten Ice Shelf. These bathymetric observations are coupled with a mooring time series and\nhigh-resolution numerical simulations to update the lateral circulation and heat transport pathways\nover the continental shelf.\nI thank the authors for the thorough response to my original review. I appreciate that multiple\nchanges and additions have been implemented in the revised manuscript, which I feel has\nstrengthened the paper. The new observations collected during the March 2022 cruise also\nstrengthen the main messages of the paper. While I feel that there is still scope for a deeper\nexploration of the physical processes dictating the seasonal to interannual variability in the heat\ntransport, the partitioning of heat between different troughs and the inclusion of a more\nthorough analysis of the numerical simulations are important stories. If the authors are able to\naddress my remaining comments and suggestions below, then I would recommend acceptance\nof this manuscript.\nMajor comments\n• The manuscript now includes estimates of the velocity field in Figure 2, which have a strong\nbarotropic component, especially over the shelf. Therefore the velocity field is strongly dependent\non the ADCP processing and analysis. This information is currently missing in the main text as well\nas in the Methods section.\nRe-reading the replies, I now realize that the absolute velocity field is derived from estimating\nsurface velocities from remote sensing data. I could not find this stated in the manuscript. If this\nis indeed the case, it would be very helpful to know what the spatial resolution of this SSH\nproduct. In fact, a more complete description of the altimetry product should be included in the\nMethods section. It appears to originate from Mizobata et al. (2018), but this is not cited in the\nmanuscript.\n• The horizontal resolution of the numerical simulation is 3-4 km but the trough widths are only\n10-20 km wide (line 88). Therefore, boundary currents will be poorly resolved in the model, if at\nall. At most there will be only a couple of points representing both inflow and outflows in the\ntroughs. While I appreciate that it will not be possible to run the model at higher resolution for\nthis study, caveats related to this grid size should be discussed. Does the model capture the\ntemporal variability seen in the mooring data?\n• The velocity sections in Figure 2 are a nice addition to the paper — is it possible to include data\ncloser to the coast? It is hard to see from Figure 3 exactly how many CTDs profiles there are\nfrom the 2022 work, but it appears that a crude section could be made that spans TTR1 and TTR2.\n• The heat transport estimates from the model are still not presented in the manuscript. It seems\nthat a simple comparison between the observations and the model is whether the frequency\nspectra of temperature variations are similar between the two. Then, the model could test whether\nvariations in mCDW temperature or mCDW layer thickness are correlated with changes in cross-\nshelf heat transport. It seems odd that a paper detailing heat transport towards Totten Ice Shelf\ndoes not diagnose heat transport in the model.\nMinor comments\nLine 31: “The temperature of mCDW reaching TIS varies on tidal-to-interannual timescales,\nindicating that ocean-driven melt is sensitive to changes in forcing.” Again, I thank the authors\nfor digging a bit deeper into the time series analysis in the manuscript. This particular statement\nin the abstract is not terribly surprising — a more substantial comment about what timescales are\ndominant (the authors seem to suggest interannual) would make it clearer to the reader the\nkey result(s) of this study.\nLine 38: “has also contributed significantly to sea-level rise in recent decades.” Quantify\nsignificant?\nParagraph starting on line 45: The second and third sentences say the same thing.\nLine 88: “Totten Trough 1 (TTR1)” It feel it would be more intuitive to readers if these were\nlabeled as TT-E and TT-W for east and west. I found I had to keep referring to the map to\nremember which was trough 1 and trough 2.\nLine 109: “a sharp interface in temperature and salinity (hence density).” You should mention\nhere that the water is salinity stratified, so it is mostly the halocline (and not the thermocline)\nthat generates a pycnocline.\nLine 137: This is nice — in addition to the weaker exchange between the depression and TTR2,\nthere may also be more recirculation within TTR2 that gives rise to further homogenization of\nproperties.\nThe paragraph ending on line 163 is a nice addition to the paper!\nLine 165: It would be good to define “West Sabrina Eddy” briefly.\nLine 166: “Suppose the season is from autumn to winter. In that case . . . deprive the available\noceanic heat of the water column.” These sentences needs to re-phrased. I think you simply\nmean that “Buoyancy forcing may also contribute to sub-seasonal variations in autumn to\nwinter, since deepening . . . removes heat from the water column.” Although, why does this only\nhave to be during fall/winter months?\nLine 169: What does “their” refer to in this sentence?\nLine 170”. “If we define trends”. This also should be re-worded. You do calculate trends — they\nare just small.\nLine 188 and following: Oxygen isotopies are introduced here without really explaining how\nthey are interpreted. You need at least one sentence explaining this proxy so the study is clear\nto non-experts.\nLine 195: “These signals reflect the mixing of glacial meltwater …” What type of mixing are you\nenvisioning here: small-scale vertical mixing, lateral stirring along isopycnals, instabilities at the\nice-shelf face?\nLine 200: Remove or re-word “lack” — observations are lacking, not the bathymetric map.\nLine 204: I realize that the model is described in Methods, but it is nice to least state what model\nis being used in the main text.\nLine 209: “The simulated amount of TIS basal melting …” Is this value tuned in the simulation?\nAlso, the Methods section should provide information on the parameters that were selected to\ndetermine melt rates.\nLine 271: Remove “Besides,”\nLine 340: The model name/type should be included in the Methods section, e.g. MITgcm, ROMS,\netc.\nFigure 3: Consider using different color scales for bathymetry and temperature.\nFigure 4: It would be very nice to see temperature/salinity diagrams for these profiles — not\njust temperature. Also, I did not understand from the caption what the triangles in panel (b)\nwere supposed to illustrate.\nFigure 6: Consider flipping the y-axis in Figure 6 to make the figure more intuitive (colder water\non top)? Maybe this would not be intuitive for everyone!\nFigure 8: I do not understand what panel (d) is showing (why are there different colors? How\nare the “areas” defined? Why is the distance not just continuous?). Also, I am not sure where\nthe ice-shelf front is — does it coincide with the bold red line?\nFigure 9: This is really nice! It is interesting that the Eulerian, depth-based overturning is much\nlarger than the overturning in density space — is this a Deacon-cell like effect? One small request\nis to include the maximum streamfunction value in the caption.\nREVIEWER COMMENTS\nWe thank both reviewers for their insightful and helpful comments, which have helped us to\nsubstantially improve the manuscript. We have responded to each of the comments raised by the\nreviewers, as discussed below. The reviewer comments are shown in bold black font and our\nresponse in blue italics. Changes to the manuscript are highlighted in the attached revision of the\nmanuscript.\nReviewer #1 (Remarks to the Author):\nThe authors have considered the suggestions made by both reviewers, and made some good\nand useful revisions. They have added a substantial amount of new data from the most recent\nseason, which is great. I would like to be more enthusiastic about this paper, but I feel it could\nstill be strengthened to make the most of this great data set. I hope these suggestions are helpful,\nas I would like to see this work published.\nDear Prof. Karen Heywood, We thank you for the time reading our revised manuscript and for the\nhelpful comments provided. Your comments were invaluable for further improving our manuscript.\nWe believe we have addressed all your comments/suggestions and substantially improved the paper.\nMy primary concern of the original paper was that it needed to make clear what was new or\nsurprising, and why it mattered. This was also raised by Reviewer 2. The authors’ response\nstates “to date, there has been no integrated description/knowledge of warm-water\ndistribution and circulation from offshore to the TIS front regions in association with realistic\nbathymetry” which I didn’t find very convincing. I am still unclear exactly what insight this paper\nhas added to our knowledge. Figure 10 is lovely, but couldn’t this schematic have been produced\nbefore any of the work described in this paper, based on pre-existing papers? The paper argues\nthat this paper presents “more detail” but why are these details important (and for\nwhom/what)? Maybe new insights are presented, but I was not sure what they were. If\nsomething is a new finding, say so explicitly.\nØ We have carefully revised the manuscript to address your concern. We have added a section to\nthe Introduction to highlight the novel aspects of our work. (Line: 48-62, 68-79)\nWe have also made sure to explicitly highlight new results and their significance throughout the\ntext. Novel aspects of our work include:\n• The first comprehensive hydrographic and bathymetric observations spanning the Totten\ncontinental shelf. These observations revealed for the first time the pathway of warm\nmCDW from the open ocean, across the shelf, and to the front of the ice shelf. The\ncombined hydrographic and bathymetric observations also showed how specific\nbathymetric features control the transport of warm water from the open ocean to the ice\nshelf cavity.\n• The first hydrographic observations (including oxygen isotope measurements) at the\nwestern TIS front, confirming that the outflow of glacial meltwater from the TIS cavity is\nconcentrated in the west.\n• The first measurements of variability in the properties of warm mCDW reaching the ice\nfront. These observations showed that the temperature of mCDW varies strongly from\nyear-to-year. The moored time series (the first collected near the Totten ice front) helped\nexplain why: variations in mCDW temperature near the ice front are associated with\nchanges in thermocline depth.\n• The first numerical simulations of the region with realistic bathymetry and sufficient\nresolution to represent the circulation, water mass distributions, ocean heat transport,\nand ocean – ice shelf interaction. The model simulations, carefully validated against the\nnew observations, allowed us to quantify the circulation on the shelf and in the ice shelf\ncavity for the first time, including ocean heat transport, and confirmed the importance of\nthe Central Totten Trough as the primary conduit for warm mCDW to reach the ice shelf.\n• An additional experiment has been added to this revision, in which the deep troughs are\nfilled in. Comparison of the two simulations provides a quantitative demonstration of the\ncritical role of the deep troughs in allowing warm mCDW to reach the ice shelf cavity.\nI still find it hard to identify what is new. The abstract, for example, is unchanged. It makes a\nseries of statements with which everyone can agree, I think, but doesn’t make clear what\nspecific bit of knowledge was added through the analysis presented in the paper (that merits\npublication in Nature Comms). Sentences 1-3 are scene setting, that’s fine. The 4th sentence\nmight be new results, or might not – can you clarify for your future readers exactly what is the\nnew finding in this sentence? The 5th sentence is surely obvious and could be written about any\nice shelf even with no observations? And the final sentence seems a bit of a stretch – what\nexactly in the results/analysis has “highlighted the glacier’s vulnerability” that we did not know\nbefore?\nØ Thank you for the suggestion on the abstract. We have substantially modified the abstract to\nconvey to readers the paper's novelty based on your comment. (Line: 24-34)\nIt is good that the authors talked more about the circulation in the revised version. It surprises\nme that there are no velocity data – does the ship not have a shipboard ADCP? Did the moorings\nthat you mention not have velocity? Heat transports would greatly strengthen the paper. In the\nabstract and again in line 45 you introduce the concept of ocean heat transport as being key, but\nyou never calculate this so it is unclear why you introduce it. You could quantify it in your super\nmodel simulation?\nØ Our icebreaker, “Shirase,” has an SADCP. But unfortunately, we found the SADCP data to be of\npoor quality on this voyage, probably because the vessel was passing through heavy sea ice for\nmost of the time on the continental shelf. No current meters were installed on the mooring\nsystem at East Totten Trough.\nTherefore, as you suggested, we evaluated ocean heat transports from the model simulations\nin this revision. Notably, a new \"No Trough Experiment (see below) \" allows us to quantitatively\ndiscuss the importance of Totten Troughs in ocean heat transport/partitioning to the TIS cavity.\nIn lines 117-120, you talk about there being a cyclonic circulation of mCDW. If this is the first paper\nto identify this, then say so. You have the data to quantify it more – how many Sv? If it’s not new,\nthen shorten the description so that the paper focuses on the things that are new – this will help\nyour readers to grasp your take-home message.\nØ A previous paper by Silvano et al. (2019) used trajectories of a small number of under-ice Argo\nfloats to infer a cyclonic circulation in the Sabrina Depression, but the actual shape of the\ndepression was unknown at that time. We have revised the text to make it clear that a previous\nstudy had suggested the presence of a cyclonic circulation, which our more complete\nhydrographic and bathymetric observations confirm. (Line: 126- 129)\nWe now use the model to quantify the strength of the cyclonic circulation around the rim of the\ndepression. (Line: 228-230)\nReviewer 2 made a very valid suggestion that you should do more than just present data (e.g.\ntime series or sections) (their point 3). I wasn’t really convinced by your response. The\nparagraph added about the mooring time series (p8-9) simply describes the variability in the\ntime series. But what is the message, why do we care about this, is any of this new or surprising?\nPointing out the surprises would be very helpful.\nØ We have carefully revised the manuscript throughout to ensure the message is clear. We have\narticulated the new aspects of the work above and in the revised manuscript text. The revised\ntext provides quantitative analysis of the circulation on the shelf, ocean heat transport,\nbasal\nmelt, and the sub-ice shelf circulation. Comparison of the numerical simulations with and\nwithout deep troughs provides a quantitative demonstration of the importance of the Central\nTotten Trough as a pathway for warm mCDW to reach the ice front. We thank both reviewers\nfor helping us strengthen the analysis and clarify the message.\nWith regard to the mooring data, we have now moved this to the Supplementary Information\n(Supplementary Fig. 5). The 3-month duration of the moored time series is useful (e.g., for\nshowing that tides influence the temperature of mCDW at the ice front), but does not address\nthe issue of interannual variability, which makes the dominant contribution to variability of\nmCDW at the ice front and hence is our focus here. Furthermore, our numerical model does not\nincorporate a tide scheme, so it does not make sense to compare the short mooring record, in\nwhich the variability is dominated by tides, with the numerical model results.\nWhat would really demonstrate the value of the revised bathymetry, to me, would be a\ncomparison of a numerical simulation with the “old” bathymetry, and a simulation with the\nnew/revised bathymetry. You state (line 65) that the model “reasonably reproduces the ocean\ncirculations and mCDW pathways from the shelf break to the TIS front regions, consistent with\nthe observational findings”. This raises two questions in my mind – first, how do you know this,\nand do you mean that it agrees with the observations of this paper, or of previous work? Second,\nif you regard the observations as “truth”, what are you gaining from the use of the model? What\nkey result comes from the model? The model is great, but you could use it to quantify or shed\nlight on something? Is it the melt rates? If so, sell this new finding better (e.g. before our new\nbathymetry, models couldn’t get the melt rate correct, but now…”?). You state (line 205) that the\nmodel representation (of water masses? Fluxes? Or what?) is “substantially improved” with the\nnew bathymetry, but how do we know this? What was it like before the new bathymetry?\nØ Thank you for your suggestion. To demonstrate the real value of the new bathymetry from our\nmultibeam sonar survey, we conducted a new experiment, in which the deep troughs near the\nice front are artificially filled in (the \"No Trough\" experiment, Line: 471-476 in Methods).\nBy comparing the results between experiments with and without troughs, we quantitatively\nevaluated the significance of the deep troughs for ocean heat transports into the TIS cavity (Fig.\n8). Note that the \"No Trough experiment\" imitates a numerical simulation with the old\nbathymetry dataset that you suggested (as mentioned, essential topographic features such as\nthe Totten Troughs and Sabrina Depression are not correctly expressed in any previous\nbathymetric datasets). The comparison shows clearly that the Central Totten Trough, first\nmapped by our intensive bathymetric survey, is the most important conduit for ocean heat to\nreach the TIS cavity and drive basal melt of the ice shelf. (Line: 250-264 & 274-276)\nIn my original review, I asked \"Did you consider using the O18 measurements to calculate the\nmeltwater distribution?\" The authors’ response is to state that there is a 0.2% increase in\nmeltwater in the outflow compared with the inflow. The distribution is not shown. Since we’re\nnot given the numbers, this isn’t very useful. And 0.2% increase is surely not significant given the\nuncertainties in the measurements and in the calculations of meltwater? In addition, I was\npuzzled by the references given here? The first reference is about the WAP – it is by no means\nthe earliest paper to discuss calculating the freshwater components from O18 using this method,\nnor is it from this region, so why is this particular paper an appropriate reference?\nØ We apologize for the previous insufficient response. We now compare the actual values, rather\nthan referring to their difference, to avoid any ambiguity. In response to your comment, we\nhave also added a new figure (Fig. 5d) and a description regarding the spatial distribution of the\nglacial meltwater fraction. We also added Price et al. (2008) as a more appropriate reference\nfor East Antarctic ice shelf as well. (Line: 197-200)\nWe have also added an explanation of the error evaluation of meltwater fraction estimates\ncaused by the O18 uncertainty in glacial meltwater endmember to the Method section.\nAlthough the absolute values in meltwater fraction have substantial uncertainty in WW layer,\nwe believe their spatial difference has far less uncertainty and the western-central TIS contrast\nis robust. (See Methods for “Estimation of glacial meltwater fraction”, the new section from\nthis revision)\nLine 263, you state that the “observations provide compelling evidence that warm water reaches\nthe TIS to drive rapid basal melt”. But did we not know this already from other papers? If this is\nnew, say so.\nØ Based on your comment, we have deleted that sentence and modified this part: \"Satellite\nobservations show that the Totten Glacier has thinned and the grounding line has retreated in\nrecent decades34, 35. These changes may reflect an increase in ocean heat transport to the TIS\ncavity, but this hypothesis cannot be tested with observations because oceanographic\nmeasurements at the TIS front (ref. 11 and this study) span a short time interval.\" (Line: 310-313)\nLines 267-275 – I cannot see the relevance of these sentences. They talk about other people’s\nresults, and are nothing to do with the implications of your own results. You need to say how\nyour new results affects these speculations. Or remove them.\nØ Thank you for the comment. We have added an explanation clarifying the relevance of our own\nresults with those sentences. (Line: 317-320)\nThe figures are much better, thank you, though I find some of the fonts extremely small (e.g.\ncontour labels and station numbers in figure 2).\nØ Thank you, too. We have made the font size as large as possible for figures with small fonts.\nFigure 2, it would be good if you used colorbarfill or similar (as you did in Figure 3) so that the\ncolorbar is consistent will the filled contours.\nØ We have modified Fig. 2 as suggested.\nFor all figures, it would be helpful for readers if there was more consistency, e.g. in colorbars,\ncolormaps, colorbar ranges. For example:\n- Figure 2 e does not have a comparable colorbar range as d and f for velocities\n- Figure 7 uses different colormaps and colorbar ranges from the other figures of T and S, and\nFigure 8 is different again. Then Figure 9 uses yet another temperature colorbar.\n- Figure 8 c does not use the same velocity colormap or colorbar range as the earlier velocity\nsections from observations. It is also given in cm/s rather than m/s – units should be consistent\nthroughout.\n- Is the colormap used in Figure 8 a,b colour-blind friendly?\nØ Thank you for the comments. We have made colormaps and units consistent as much as\npossible. We use colormaps of “RdBu” for temperature and ocean velocity, “Spectral” for\nsalinity, and “RdYlBu” for dissolved oxygen (https://matplotlib.org/stable/tutorials/colors/\ncolormaps.html).\nWe have made colorbars consistent for Figs. 2d-f. For Fig. 2e, the ocean velocity out of range is\nrepresented by contours only.\nThe ranges of colorbar for the observations and simulation are still different because we think\nwe should highlight the values as obviously as possible for each result.\nFurthermore, we checked whether the figures (https://pilestone.com/pages/color-\nblindness-simulator-1), and we confirmed they are discernable with this color space.\nMinor points:\nLine 94-95, the sentence about glacial erosion seems irrelevant to this paper and should be\nremoved.\nØ As you suggested, we have deleted the sentence from this revision.\nLine 285, I would say suggests rather than offers\nØ Thank you for the suggestion. We have modified this part: \"The large interannual variability\nobserved in the temperature of mCDW reaching the TIS (Fig. 4a) highlights the need for long-\nterm monitoring of the oceanic thermal forcing driving basal melt of the TIS.\" (Line: 332-335)\nKaren J. Heywood\nReviewer #2 (Remarks to the Author):\nReview of: On-shelf circulation of warm water toward the Totten Ice Shelf, East Antarctica\nAuthors: Daisuke Hirano, et al.\nThis manuscript documents new observations collected over the Antarctic continental shelf in\nthe Sabrina Coast region in front of the Totten Ice Shelf. Specifically, multi beam sonar surveys\nfrom multiple research cruises, including a cruise from March 2022, which provides details of\ndeep glacial troughs that control the circulation around and under the Totten Ice Shelf. These\nbathymetric observations are coupled with a mooring time series and high-resolution\nnumerical simulations to update the lateral circulation and heat transport pathways over the\ncontinental shelf.\nI thank the authors for the thorough response to my original review. I appreciate that multiple\nchanges and additions have been implemented in the revised manuscript, which I feel has\nstrengthened the paper. The new observations collected during the March 2022 cruise also\nstrengthen the main messages of the paper. While I feel that there is still scope for a deeper\nexploration of the physical processes dictating the seasonal to interannual variability in the heat\ntransport, the partitioning of heat between different troughs and the inclusion of a more\nthorough analysis of the numerical simulations are important stories. If the authors are able to\naddress my remaining comments and suggestions below, then I would recommend acceptance\nof this manuscript.\nØ We thank you for the time reading our revised paper and for the helpful comments provided.\nWe also very much appreciate your positive assessment of the previous revision. Your\ncomments were invaluable for further improving our manuscript. We believe we have\naddressed all your comments/suggestions and substantially improved the paper. In particular,\nwe have estimated the ocean heat transports into the TIS cavity by the simulation and could\nfurther highlight through a new additional model experiment the role of regional circulation\nand bathymetry in regulating ocean heat transports into the TIS cavity for rapid melting in the\nTotten Glacier.\nMajor comments\n• The manuscript now includes estimates of the velocity field in Figure 2, which have a strong\nbarotropic component, especially over the shelf. Therefore the velocity field is strongly\ndependent on the ADCP processing and analysis. This information is currently missing in the\nmain text as well as in the Methods section.\nRe-reading the replies, I now realize that the absolute velocity field is derived from estimating\nsurface velocities from remote sensing data. I could not find this stated in the manuscript. If this\nis indeed the case, it would be very helpful to know what the spatial resolution of this SSH\nproduct. In fact, a more complete description of the altimetry product should be included in the\nMethods section. It appears to originate from Mizobata et al. (2018), but this is not cited in the\nmanuscript.\nØ Sorry for the confusion. Based on your comment, we have added a new section to the Methods\nwith further information on the SSH products (Mizobata et al., 2020) and now they are used to\nderive absolute ocean velocity from SSH and hydrographic data (according to Hirano et al., 2021).\n(See Methods for “Calculation of absolute ocean velocity from hydrographic and satellite-\nderived Dynamic Ocean Topography (DOT) data”, the new section from this revision)\n• The horizontal resolution of the numerical simulation is 3-4 km but the trough widths are only\n10-20 km wide (line 88). Therefore, boundary currents will be poorly resolved in the model, if at\nall. At most there will be only a couple of points representing both inflow and outflows in the\ntroughs. While I appreciate that it will not be possible to run the model at higher resolution for\nthis study, caveats related to this grid size should be discussed. Does the model capture the\ntemporal variability seen in the mooring data?\nØ Based on the suggestion, we have added a brief comment regarding the model grid size and\nhighlighted what the model can or cannot represent. (Line: 218-221)\nOur numerical model does not incorporate a tide scheme, so it does not make sense to compare\nthe short mooring record, in which the variability is dominated by tides, with the numerical\nmodel results. As our focus here is on the interannual variability that dominates the changes\nobserved in mCDW from repeat hydrography, we have moved the mooring data to the\nSupplementary Information (Supplementary Fig. 5).\nWe have added one figure presenting results from the new numerical model experiment\nwithout troughs to highlight the Central Trough's significance in ocean heat transports into the\nTIS cavity (Fig. 8).\n• The velocity sections in Figure 2 are a nice addition to the paper — is it possible to include\ndata closer to the coast? It is hard to see from Figure 3 exactly how many CTDs profiles there are\nfrom the 2022 work, but it appears that a crude section could be made that spans TTR1 and TTR2.\nØ Thank you for the comment. Unfortunately, it is impossible to accurately estimate SSH near the\ncoast due to the influence of sea ice and landfast ice. To obtain SSH, there must be open water\n(leads and cracks in sea ice areas) within the 15 km footprint of the CryoSat-2/SIRAL satellite\nradar altimeter (the area the sensor is looking at, i.e., the field of view). In the coastal area of\nthis research area, there is no open water surface due to the constant presence of sea ice and\nlandfast ice, so even if an icebreaker could make hydrographic observations, we cannot\ncalculate the absolute velocity because satellite data is not available.\nAdditionally, there are no hydrographic profiles across the Totten Troughs, so it is impossible to\ncreate an observation-based ocean velocity section along the ice front. Accordingly, in the last\nrevision, we added a simulated ocean velocity section along the ice front (Fig. 7). Further, in this\nrevision, we have estimated ocean heat transport through the Totten Troughs and added a\nquantitative reference to the ocean heat transport to the Sabrina Coast ice shelves (Fig. 8, see\nalso later)\n• The heat transport estimates from the model are still not presented in the manuscript. It\nseems that a simple comparison between the observations and the model is whether the\nfrequency spectra of temperature variations are similar between the two. Then, the model\ncould test whether variations in mCDW temperature or mCDW layer thickness are correlated\nwith changes in cross-shelf heat transport. It seems odd that a paper detailing heat transport\ntowards Totten Ice Shelf does not diagnose heat transport in the model.\nØ As mentioned above, our numerical model does not incorporate a tidal scheme, so a direct\ncomparison with the short mooring time series, which is dominated by tidal signals, does not\nmake sense. So, we moved the mooring time series to the Supplement Information as support\ndata (Supplementary Fig. 5).\nMotivated by this comment, we conducted an additional experiment (\"No Trough\" experiment\nLine: 471-476 in Methods) in this revision. In the new experiment, the simulation was conducted\nby filling the deep layers below 500 m along the Sabrina Coast (i.e., filling the troughs along the\nice fronts).\nBy comparing the results between experiments with and without troughs, we quantitatively\nevaluated the significance of the deep troughs for ocean heat transports into the TIS cavity (Fig.\n8). The comparison clearly shows that the Central Totten Trough, first mapped by our intensive\nbathymetric survey, is the most important conduit of ocean heat transport along the Sabrina\nCoast for driving the rapid TIS basal melt.\nMinor comments\nLine 31: “The temperature of mCDW reaching TIS varies on tidal-to-interannual timescales,\nindicating that ocean-driven melt is sensitive to changes in forcing.” Again, I thank the authors\nfor digging a bit deeper into the time series analysis in the manuscript. This particular statement\nin the abstract is not terribly surprising — a more substantial comment about what timescales are\ndominant (the authors seem to suggest interannual) would make it clearer to the reader the\nkey result(s) of this study.\nØ We agree with your comment. We have modified this part to focus more on interannual\nvariability. (Line: 30-31)\nLine 38: “has also contributed significantly to sea-level rise in recent decades.” Quantify\nsignificant?\nØ We have modified this part slightly: “has also been a major contributor to sea-level rise in recent\ndecades.” (Line: 37-38)\nParagraph starting on line 45: The second and third sentences say the same thing.\nØ Thank you for pointing this out. We have deleted the repetition here. (Line: 44)\nLine 88: “Totten Trough 1 (TTR1)” It feel it would be more intuitive to readers if these were\nlabeled as TT-E and TT-W for east and west. I found I had to keep referring to the map to\nremember which was trough 1 and trough 2.\nØ Thank you for the suggestion. We have named Totten Trough 1 (TTR1) as Central Totten Trough\n(C-TT) and Totten Trough 2 (TTR2) as East Totten Trough (E-TT). (Lines: 99-100 & 103-104)\nNote: The east trough is located on the eastern ice front (so it would be OK to refer to this\ntrough as suggested), but the western of the two troughs is located in the central ice front (so\nit could be confusing to refer to this trough as “TT-W”). To avoid any confusion, we refer to the\ntwo troughs as Central Totten Trough and East Totten Trough.\nLine 109: “a sharp interface in temperature and salinity (hence density).” You should mention\nhere that the water is salinity stratified, so it is mostly the halocline (and not the thermocline)\nthat generates a pycnocline.\nØ We have added a note that the seawater density in the Antarctic coastal region is determined\nmainly by salinity. (Line: 119-120)\nLine 137: This is nice — in addition to the weaker exchange between the depression and TTR2,\nthere may also be more recirculation within TTR2 that gives rise to further homogenization of\nproperties.\nØ Thank you. We agree with your suggestion of recirculation and a resulting homogenization of\nthe warm water properties within the E-TT. However, we do not have enough evidence, so we\nkept it here as the original (but added a brief explanation based on your comment). (Line: 144-145)\nThe paragraph ending on line 163 is a nice addition to the paper!\nØ Thanks! Due to other changes, we have substantially revised the content and structure of this\nsection, but the same statement still ends this section. (Line: 157-184)\nLine 165: It would be good to define “West Sabrina Eddy” briefly.\nØ To focus more on the discussion of interannual variation, we have removed this content\nregarding offshore eddies in this section.\nLine 166: “Suppose the season is from autumn to winter. In that case . . . deprive the available\noceanic heat of the water column.” These sentences needs to re-phrased. I think you simply\nmean that “Buoyancy forcing may also contribute to sub-seasonal variations in autumn to\nwinter, since deepening . . . removes heat from the water column.” Although, why does this only\nhave to be during fall/winter months?\nØ We agree with your comment, but to focus more on the discussion of interannual variation, we\nhave removed this content as well in this revision.\nLine 169: What does “their” refer to in this sentence?\nØ Same as above, we have deleted the relevant sentence in this revision.\nLine 170”. “If we define trends”. This also should be re-worded. You do calculate trends — they\nare just small.\nØ We agree with you. We have modified the relevant part: “much smaller variability (±0.1 °C).”\n(Line: 170-172)\nLine 188 and following: Oxygen isotopies are introduced here without really explaining how\nthey are interpreted. You need at least one sentence explaining this proxy so the study is clear\nto non-experts.\nØ We have briefly explained the oxygen isotope ratio and why we use this proxy. (Line: 192-195)\nLine 195: “These signals reflect the mixing of glacial meltwater …” What type of mixing are you\nenvisioning here: small-scale vertical mixing, lateral stirring along isopycnals, instabilities at the\nice-shelf face?\nØ Although it could not be determined, we have also mentioned some possibilities, such as\nboundary mixing and entrainment by buoyant meltwater plumes. (Line: 205-208)\nLine 200: Remove or re-word “lack” — observations are lacking, not the bathymetric map.\nØ Thank you for the correction. We have modified it here. (Line: 211-215)\nLine 204: I realize that the model is described in Methods, but it is nice to least state what model\nis being used in the main text.\nLine 340: The model name/type should be included in the Methods section, e.g. MITgcm, ROMS,\netc.\nØ Thank you very much for your advice. In the revised manuscript, we have added the model’s\nname (COCO) in the main text and Methods section. (Line: 215-217 & 429)\nIt should be noted that COCO originally stood for CCSR Ocean COmponent model, but the\nresearch center, CCSR, was merged into AORI, the University of Tokyo. Now we have used COCO\nas a proper noun for pointing out the ice-ocean model.\nLine 209: “The simulated amount of TIS basal melting …” Is this value tuned in the simulation?\nAlso, the Methods section should provide information on the parameters that were selected to\ndetermine melt rates.\nØ We used an observation-based parameter of the thermal and salinity exchange velocities\n(γ = 1.0×10\"# m s-1 and γ = 5.05×10\"% m s-1, Hellmer and Olbers (1989) in this\n! $\nstudy. (Line: 456-458)\nWe didn’t perform turning on the parameter, and the rate and amount of the model’s ice-\nshelf basal melting are estimated from ice-ocean interaction with the parameterization in\nthe model configuration.\nLine 271: Remove “Besides,”\nØ We have modified it here. (Line: 320)\nFigure 3: Consider using different color scales for bathymetry and temperature.\nØ Thank you for the suggestion. We have modified this figure with each colormap for bathymetry\nand water temperature. In addition, we originally displayed bathymetric data from the multi-\nbeam survey in the broad map (Fig. 3b, now it is Fig. 3a), but we have removed the bathymetry\nfrom the broad map for more facilitating visualization.\nFigure 4: It would be very nice to see temperature/salinity diagrams for these profiles — not\njust temperature. Also, I did not understand from the caption what the triangles in panel (b)\nwere supposed to illustrate.\nØ Since the T-S diagram does not allow us to highlight the interannual variations in mCDW that\nwe focus on here (all plots for the mCDW layer are almost along the same line), we have added\na vertical salinity profile as panel b of Figure 4.\nNote that we have moved the original Fig.4b to Supplementary Fig. 5a. The modified caption of\nSupplementary Fig. 5 is, “Right-pointing triangles indicate the depths of the moored\ntemperature time series plotted in panel (d), using the same colors (showing which depths the\nsensors measured).” (see Supplementary Fig. 5)\nFigure 6: Consider flipping the y-axis in Figure 6 to make the figure more intuitive (colder water\non top)? Maybe this would not be intuitive for everyone!\nØ Thanks for the suggestion. We have tried to flip the y-axis of the time series (see below).\nHowever, the flipped version would make warm and cold cycles of water temperatures\nmisleading (upward means cooler in this case). So, we think it should be kept original.\nThe flipped version of Supplementary Fig. 5d.\nFigure 8: I do not understand what panel (d) is showing (why are there different colors? How\nare the “areas” defined? Why is the distance not just continuous?). Also, I am not sure where\nthe ice-shelf front is — does it coincide with the bold red line?\nØ We have modified the caption (the current Figure 7d). Line sections are required to integrate\nvolume transport in lateral and vertical directions to calculate the overturning ocean circulation.\nSince the model’s horizontal resolution is about 3-4 km, we categorized the model grids with a\n5 km interval based on distance from the ice-front line, and the boundaries between the\ndifferent categorized grids were used for the line sections. Colors in the panel show the category\nand the boundaries. We have added orange lines showing the ice front on the figure.\nFigure 9: This is really nice! It is interesting that the Eulerian, depth-based overturning is much\nlarger than the overturning in density space — is this a Deacon-cell like effect? One small request\nis to include the maximum streamfunction value in the caption.\nØ Thank you so much. The targeted ice shelf is connected to the Sabrina Depression with several\ndeep troughs of different depths. Panel (a) in the depth-distance domain is intuitive, but a\ncombination of the realistic topography and the ocean circulation makes interpretation\ndifficult. We consider that the overturning circulation in the density-distance domain is more\nsuitable for understanding ocean thermohaline circulation within the cavity in the realistic\ntopography configuration. Following your suggestions, we have added numbers of the\nstreamfunction at the ice front and the position of the maximum absolute value in the density-\ndistance domain.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThank you for responding thoroughly and carefully to my suggestions and those of the other\nreviewer. I particularly appreciate the additional model run and the revised figures, thank you. The\nnovelty of the paper is now much clearer for readers. I am delighted to recommend publication.\nOur sincere gratitude to Prof. Karen Heywood:\nWe heartily thank you for your constructive and helpful comments up to this point, which were\ninvaluable for improving our manuscript.\nReviewer #2 (Remarks to the Author):\nReview of: On-shelf circulation of warm water toward the Totten Ice Shelf, East Antarctica\nAuthors: Daisuke Hirano, et al.\nThis manuscript documents new observations collected over the Antarctic continental shelf in\nthe Sabrina Coast region in front of the Totten Ice Shelf. Specifically, multi beam sonar surveys\nfrom multiple research cruises, including a cruise from March 2022, which provides details of\ndeep glacial troughs that control the circulation around and under the Totten Ice Shelf. These\nbathymetric observations are coupled with a mooring time series and high-resolution\nnumerical simulations to update the lateral circulation and heat transport pathways over the\ncontinental shelf. In the latest revised version the authors have focused more on inter-annual\nvariability in mCDW properties, as opposed to the tidal variations revealed in the mooring\ntime series, and they have added a new numerical simulation that highlights the impact of the\ntroughs on heat transport towards the Totten Ice Shelf.\nI again appreciate the thorough replies to my second round of reviews. The manuscript\ncontinues to improve. A few questions about the analysis still remain, which are documented\nbelow. However, it feels like a straightforward and important addition to the manuscript\nwould be to inform the reader whether the model reproduces the observed interannual\nvariability in mCDW properties — this would strengthen the paper significantly. It would be\ngreat to address the points below as the authors are preparing a final version of the\nmanuscript and the editor decides on suitability for publication.\n***************\nMajor comments\n• Throughout the manuscript (and in the reviewer replies) there are inconsistencies in\nwhether the changes in temperature of the mCDW or the thickness of the mCDW layer are\nmore likely to impact ice shelf melt rates. From Figure 4, it looks like the thermocline depth\nonly varies by ~100 m, whereas temperature varies by over a degree. Therefore temperature\ndifferences are likely as important, if not more important, than variations in\nthermocline/pycnocline depth. It is worth noting in the manuscript that this is a very different\nsystem from the Amundsen Sea, where mCDW temperatures have been largely unchanged\nover the past decade and a half, but the thermocline depth varies by 300-400 m.\n• Line 56-58: “to determine the pathways and processes regulating the transport of warm water …\nto assess the variability in the temperature of mCDW.” This is a nice addition to the\nmanuscript that clearly lays out the goals of the study. The revised manuscript makes a much\nclearer statement that the inter-annual variability in mCDW characteristics are more important\nthan the tidal fluctuations from the mooring data. However, there is no discussion in the\ncurrent version of the manuscript about what processes give rise to this interannual\nvariability, other than it is not due to Ekman transport at the shelf break. Even some\nspeculation about how these changes might be occurring would strengthen the paper. Also, I\nrealize that the model does not have tides, so comparison with the mooring data is not\npossible, but does the model reproduce the different mCDW regimes seen in the observations?\nAgain, these observations are very exciting and worthy of publication — however, for a Nat.\nComm. publication, I would expect a discussion of mechanism as well as reporting of the\nobservations.\nØ Thank you for your comments, which have helped us to further deepen and improve the\ndiscussion. Specifically, we have added a new discussion on the possible processes driving the\nvariability of temperature and thermocline depth on the Antarctic continental shelf (Line:\n292-328), including a comparison of the depth range of the thermocline on the Amundsen Sea\ncontinental shelf in West Antarctica (Line: 300-304). In addition, we have provided additional\nrelevant comments in some places (Line: 137-139; 150-151).\nØ Thank you also for the comment on the numerical model. We confirmed that our numerical\nmodel reproduces the different mCDW regimes observed at the TIS front to some extent (see\nthe figure below). Due to space and figure constraints, we decided to focus on the mean fields\n(climatology) for the simulation results in this paper, and it sufficiently supports our\nobservational findings. However, as you pointed out, we also consider that the interannual\nvariation of the cryosphere-ocean system is important and should be addressed using the\nmodel results. We have prepared a separate numerical simulation paper entitled “Modeling\nseasonal-to-decadal ocean-cryosphere interactions along the Sabrina Coast, East Antarctica”\nby Kusahara, Hirano, et al., and it is now ready for submission to a journal (The Cryosphere)\nafter the acceptance of this paper.\nFigure: A simulated different regime of inflowing mCDW property at the TIS front (shown by blue\ncircle), corresponding to observations shown in Fig. 4. Instead of Mar. 2022 in Fig. 4, the model\nresults for Dec. 2021 (the end of the model simulation) were used.\n• I appreciate the additional information provided about the SSH product that provides the\nreference values for the estimates of total geostrophic velocity and transport. There are some\nissues with this approach that need to be discussed in the manuscript. First, as mentioned\npreviously, the flow is almost purely barotropic, so the velocities are strongly influenced by\nthe choice of reference velocity. There is a mismatch between the SSH reference, which is a\ntime-averaged velocity over a period of almost a decade and the synoptic sections that were\ncollected during the cruises. During the time of the cruise, the barotropic velocity might be\nconsiderably different from what is inferred from the SSH product — this is why it is much\nbetter to reference to ADCP data, if available. I also appreciate that ADCP data is not available.\nThe authors should consider whether they want to continue to use the SSH velocity product —\nin which case it needs to be made clear in the manuscript that this is a climatological velocity\nfield and not directly related to the observation period. Alternatively, the authors could just\nshow the baroclinic component of the flow, which will be more closely linked to the ice-shelf\noverturning circulation (e.g. Wahlin et al. Nature 2020)\nØ Apologies for not being entirely clear in the Methods. We use ocean surface velocity derived\nfrom monthly, not climatological, SSH data, using the closest month to the observation\nperiod, as a reference velocity for calculating the absolute ocean velocity for Fig. 2.\nSpecifically, we used the monthly mean SSH in February 2020 because the observation data\nused for Fig. 2 were obtained from February to early March. Note that the choice of monthly\nSSH in March 2020 shows almost no change in the results. We have added one sentence to\nMethods for clarity. (Line: 430-432)\nMinor comments\n• As the authors are preparing a final version of this manuscript I would urge them to be more\nquantitative into the abstract. Rather than just saying that temperature varies, provide a\nrange; rather than just saying that bathymetry changes the heat transport in the simulations,\ngive a percentage of how much based on the new numerical simulation.\nØ Thank you for your suggestion. We have now stated the range of the observed mCDW\ntemperature reaching the TIS cavity in the Abstract (Line: 30-31). On the other hand, since the\nexperimental setup of the simulation with no troughs is somewhat arbitrary, the quantitative\nimpact of the bathymetry on ocean heat transport may change depending on the\nexperimental setup (i.e., how the troughs are filled). Therefore, we have not provided a\nquantitative value for the change in heat transport between the two simulations in the\nAbstract. However, readers can easily find the difference in ocean heat transport between the\nexperiments in Fig. 8.\n• Line 37: “a major contributor to sea level rise” I recognize the change from “significant” to\n“major” but I was hoping that you would be able to provide a quantitative value for the\nreader.\nØ Based on your comment, we have modified this sentence as “While the contribution to global\nsea-level rise from West Antarctica has received the most attention (6.9 ± 0.6 mm for the\nperiod 1979-2017, dominated by the Amundsen and Bellingshausen Sea sectors), East\nAntarctica has also been a major contributor (4.4 ± 0.9 mm over the same period)1. (Line: 36-\n38)\n• Line 43: “and a net mass loss of …” over which period?\nØ We have provided the period for which the net mass loss was estimated. (Line: 44)\n• Somewhere in the manuscript there should be a comment that the velocities in the upper\nslope section (Figure 2d) are all offshore. How does the heat access the continental shelf in\nthis case? Standing eddies are mentioned multiple times, but then I would expect to see a\npattern of onshore and offshore flow.\nØ Hirano et al. (2021) showed cross-slope mCDW intrusion onto the continental shelf with\nmeridional sections in this region, and we can see also from Supplementary Fig.2 that the\nTotten continental shelf is connected to the offshore via the standing eddy off the shelf;\nhowever, the hydrographic sections (Fig. 2) are not ideally suited for illustrating this\nconnection, as you pointed out. Therefore, we have revised the first paragraph of the section\n“Pathways of Warm Water from the Shelf Break to the TIS Front.” (Line: 110-116)\n• Line 182-183: “we do not find clear evidence of a relationship between zonal wind along the\nSabrina Coast and WW/mCDW interface variations. This suggests that wind-driven Ekman\nupwelling/downwelling is not the dominant process driving thermocline depth variations.” Be\ncareful here that you are not mixing Ekman transport (dependent on wind stress) and Ekman\npumping (dependent on wind stress curl). A recent paper by Kim et al. (Kim et al JGR, 2021)\nshows that wind stress curl, integrated over the entire continental shelf, does a good job of\nreproducing changes in the WW/mCDW interface in front of Dotson ice shelf.\nØ Thank you for your information. Kim et al. (2021), with long-term oceanographic surveys from\n2007 to 2018, found that the interannual mCDW variability is coherent with local Ekman\npumping integrated along the Dotson-Getz Trough.\nIn contrast, our observations themselves are too sparse to allow a statistically robust analysis\nof the causal link between oceanic interannual variability and forcing. Therefore, it would be\nbest to consider Kim’s approach when we obtain long-term observation data in the Totten\nregion. Instead, we have cited Kim et al. (2021) in the new discussion (Line: 317) and included\nrelevant comments (Line: 326-328; 356-358; 359-360).\n• Thank you for including the salinity profiles in Figure 4. It is fascinating the variations in\nsalinity (stratification) look quite different from what is observed in the Amundsen Sea.\nØ As in the earlier response to the first two major comments, we have included the comparison\nof the depth range of the thermocline on the Amundsen Sea continental shelf in West\nAntarctica (Line: 300-304).\n• Line 221: “special” should be “spatial”\nØ Thank you. We have corrected it. (Line: 224)\n• Line 258: I could not find where the acronym mMUIS was defined.\nØ Thank you for pointing this out. We have modified this sentence as “Warm water also\nreaches the cavities beneath the nearby eastern TIS (eTIS) and western Moscow\nUniversity Ice Shelf (wMUIS), but the heat transport is relatively small (118 GW at eTIS,\n• Figure 8: The x-axis should be labeled.\nØ We have added “Month” on the x-axis. (Revised Figure 8)\n• Line 291: “Because the depth of the thermocline is similar to the depth of the shallowest\nsills, heaving of the sharp thermocline between cold WW and warm mCDW drives large\nvariations in the temperature of mCDW reaching the cavity.” What causes this heaving?\nØ As in the earlier response to the first two major comments, we have added a new discussion\non the possible processes driving the variability of temperature and thermocline depth on the\nAntarctic continental shelf (Line: 292-328), including the change of this sentence (Line: 304-\n306).\nNote that variations in temperature and variations in thermocline depth are related, as\ndiscussed in the paper. The observations suggest that variations in temperature at the ice\nfront (i.e., of most relevance to basal melt) are strongly linked to changes in depth of the\nthermocline relative to the depth of the shallowest sills.\n• Line 326: “ In particular, our observations from the TIS front show that changes in depth of\nthe thermocline drive variability in thermal forcing on broad range of timescales (tidal to\ninterannual).” This statement seems to be different from the abstract which talks about\nchanges in the temperature of mCDW, rather than the thermocline depth.\nØ Following the abovementioned changes, we have slightly weakened this sentence's claim\n(Line: 351-354).\n• Line 430: “commponnt\" should be “component”\nØ Thank you for pointing this out. We have corrected it (Line: 458).\n• Line 470: Since you have run the model through the entire period where there are\nobservations, it would be really interesting to know if the model is able to reproduce the\nobserved variability in mCDW thickness and temperature!\nØ Thank you very much for your encouragement. As in the earlier response, we will submit\nanother modeling paper about the seasonal-to-decadal variability (Kusahara, Hirano et al.,\nto be submitted to The Cryosphere). We have examined the temporal variability of the\nmCDW in the modeling paper in detail."}]

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