Persistent warm-eddy transport to Antarctic ice shelves driven by enhanced summer westerlies

Domain: earth_science
Published: 2024-01-22
Editorial outcome: major_revision

Abstract
The offshore ocean heat supplied to the Antarctic continental shelves by warm eddies has the potential to greatly impact the melting rates of ice shelves and subsequent global sea level rise. While featured in modeling and some observational studies, the processes around how these warm eddies form and overcome the dynamic sub-surface barrier of the Antarctic Slope Front over the upper continental slope has not yet been clarified. Here we report on the detailed observations of persistent eddies carrying warm modified Circumpolar Deep Water (CDW) onto the continental shelf of Prydz Bay, East Antarctica, using subsurface mooring and hydrographic section data from 2013-2015. We show the warm-eddy transport is most active when the summer westerlies strengthen, which promotes the upwelling of CDW and initiates eddy formation and intrusions. Our study highlights the important role of warm eddies in the melting of Antarctica’s ice shelves, both now and into the future.

Introduction
The Southern Ocean plays a critical role in heat uptake and redistribution in the context of global warming in recent decades 1–4. Warm intrusions of modified Circumpolar Deep Water (mCDW), the most important southward oceanic heat transport to the Antarctic margin, have the potential to greatly increase the supply of oceanic heat to the cavities of Antarctic ice shelves, and therefore enhance basal melting and its subsequent contribution to global sea-level rise 5–18. These intrusions are sourced from warm, saline and nutrient-rich CDW, at depth in the eastward-flowing Antarctic Circumpolar Current (ACC).

To reach the ice shelves, the CDW needs to migrate poleward, upwell over the upper continental slope to the depth range of the continental shelf break and overcome the dynamical barrier of the Antarctic Slope Front (ASF). The ASF is a dynamic sub-surface barrier between the cold shelf water and the warmer offshore water around Antarctica, which inhibits the transport of mCDW onto most of the continental shelves 19–21. It is important to note that the characteristics of the continental shelf regions surrounding East Antarctica differ significantly from the West Antarctic regions. In the western areas, there are primarily warm shelves (bathed in waters with temperatures exceeding 0°C), and the ASF is either weak or absent 21. Conversely, in the eastern regions, the waters on the shelves are cold and dense, and the ASF is well-defined 21. The ASF is often associated with the westward Antarctic Slope Current (ASC) and the easterly Antarctic coastal wind regime 20,21. Thereafter, mCDW intrusions need to negotiate the shelf break and move across the continental shelf to penetrate beneath Antarctic ice shelves. The lower the ‘modification’ of the intrusion, the greater the oceanic heat supply for potential basal melting.

Many hydrographic observations have captured the warm water intrusions onto the continental shelves around Antarctica, including the Prydz Bay 22,23, Adélie Land 24, and the Amundsen and Bellingshausen Seas 9,11,25–27. Concurrently, several modeling studies have successfully simulated CDW intrusions and elucidated their role in ocean heat transport to the continental shelf and ice shelves 6,13,28–32. Eddy-resolving models indicate that poleward mesoscale eddies, which have emerged due to the intensified mid-latitude westerly winds and their concurrent poleward shift over the past 50 years 33–35, play a crucial role in the Antarctic overturning circulation. These eddies are a significant factor in overcoming the barrier imposed by the ASF and enabling an enhanced transfer of relatively warm mCDW towards the Antarctic ice shelves 36.

And yet despite this poleward intensification of the westerly winds, both hydrographic observations and climate models show little change in Southern Ocean stratification or net ACC transport over the recent decades. This lack of change is attributed to an enhanced poleward eddy transport 37–40, after several modeling studies proposed that stronger eddy fluxes could compensate for the larger northward Ekman flow caused by the additional input of westerly momentum 41–46.

Several observational studies have described cross-slope eddies near the Antarctic continental shelves 47–49. However, precisely how eddies traverse the ASF barrier to facilitate the transfer of relatively warm waters to the shelf regions remains poorly understood. Here we report on the detailed observations of persistent warm-eddy transport to an Antarctic ice shelf from a subsurface mooring and ancillary data from hydrographic sections and instrumented seals. We link the processes of warm-eddy formation and transport to enhanced westerly winds in summer and speculate on the future of these processes should the westerlies strengthen further as predicted.

Results
Observations of warm eddies onto the continental shelf
Oceanographic Conductivity-Temperature-Depth (CTD) sections, subsurface mooring and elephant seal CTD data were collected in the Prydz Bay region of East Antarctica across 2013–2015 (Fig. 1a). There are six meridional hydrological sections across the continental slope and shelf with 2.5 degrees spacing in longitude, one zonal transect across the Prydz Channel along 67.25oS and one additional transect along the Amery Ice Shelf. A subsurface mooring located in the Prydz Channel (72.11oE, 67.18oS) was deployed in March 2013 and recovered in February 2015. In addition, there are numerous seal CTD profiles available in this region, with many concentrated in and around the coastal polynya areas 23,50.

The potential temperature and salinity data of the 73oE section and zonal section are presented in Fig. 1b–e. An inflow of relatively warm (>-1.7°C) and salty (34.4-34.6) water (hereafter defined as mCDW) intrudes onto the continental shelf during the observational period in February 2013 (Fig. 1b, d). The mCDW reaches up to 100m depth and extends southward to 67.8oS along the 73oE section, mixing with the relatively cold shelf waters (Fig. 1b). The main intrusion of mCDW across the zonal section occurred in the 200-400m layer between 72oE-76oE. The maxima in potential temperature (−0.2°C) and salinity (34.6) are found at 73.3oE at a depth of 350m (Fig. 1d). In February 2015, the observed mCDW intrusion seemed absent (Fig. 1c, e). The inflowing mCDW was constrained below 250m on the continental shelf break north of 67oS. The distribution of temperature and salinity across this section over the two years shows the substantial temporal and spatial variability of mCDW intrusions in this region.

The two year-round subsurface mooring in the Prydz Channel enables further understanding of the temporal variability of the mCDW intrusions (Fig. 2). Hourly potential temperature records at the five depth levels (248m, 270m, 325m, 449m, and 520m) of the mooring show a clear seasonal cycle (Fig. 2a, Supplementary Figs. 1 and 2). Warm water (with maxima over −1°C) appears during the austral summer (November-April), and the signal is intensified in the upper three levels. The warmest record (−0.26°C) occurred at 325m in April 2014. During the austral winter (May-October), Winter Water (WW) reached the bottom layer and the potential temperatures at all depths were near −1.89°C with little variation, suggesting strong mixing due to deep convection across the sea-ice growth season.

Following previous studies 7,25, we identify 28 eddy-like warm events from the two-year records of temperature and current velocity at 270m (Fig. 2b). All warm events occurred during the austral summer season, with the highest frequency of events in April 2014 and January 2015. The cross-stream velocity ve (perpendicular to the background velocity, see Materials and Methods) is a suitable parameter to identify the characteristic signature of eddies, as initially positive (negative) velocities are indicative of anticyclonic (cyclonic) circulation (Fig. 3a, b). The cross-stream velocity derived from the current meter at 270m (Fig. 2b) shows that the amplitude is 3-8 cm/s during the entire time series compared to the mean background velocity (2.4 ± 0.3 cm/s). Unlike the seasonal cycle of potential temperature, the cross-stream velocity varies throughout the year, likely reflecting an intra-seasonal oscillation. This distinction will be explained in more details later.

The potential temperature signature and associated heat flux are calculated to estimate the thermal effect of the warm events (Fig. 2c, d). Figure 2d shows that while most of the warm signals are above 450m, some can reach the seafloor. The warmest event occurred in April 2014. Most of the warm events (~75%) resulted in a positive heat flux (between 50-100 w/m2) southward (positive values) into the Prydz Bay basin, although the remaining warm events (~25%) removed heat (<50 w/m2) from the basin (negative values). Analysis of the background velocity at 270m suggests that the warm events propagating out of the basin are not the same well-mixed warm eddies retreating back to the continental shelf break (Supplementary Fig. 3). The strongest heat fluxes (>150 w/m2) occurred in January 2015. The warmest signal (in April 2014) shows a relatively small heat flux (nearly 50 w/m2), resulting from the weak background velocity. Assuming that all of the eddy heat is available for basal melting of the area beneath the Amery Ice Shelf that the mCDW can potentially access (about 20% of the total area), the poleward heat flux associated with the eddy intrusions (0.79 ± 0.15 Sv) would result in an average basal melt rate of 3.1 ± 0.7 m of ice per year over this section of the ice shelf (see Methods). This is roughly 50% more than the previous estimate of annual basal melt rate (2.0 ± 0.5 m/year) in the same area driven by mCDW flux into the Amery Ice Shelf cavity 16. Despite the heat lost over the continental shelf through interaction with polynyas 50, some mCDW can still reach the Amery ice shelf 16,51. This suggests that the eddy-induced heat transport into Prydz Bay has an important impact on the mass balance of the Amery ice shelf.

Characteristics and evolution of the warm-eddy intrusions
Composite analysis of the warm events helps understand the structure and process of the eddy intrusions (Fig. 3). Importantly, all 28 warm events are strongly related to eddy rotation. The cross-stream velocity ve at 270m determines whether an eddy is cyclonic (17 events) or anticyclonic (11 events). The amplitude of the cross-stream velocity signal in the cyclonic events varies between 1-5 cm/s, with an average peak value of less than 2.8 cm/s. The current velocities of the anticyclonic events have comparable amplitudes (2.5 cm/s in average) to those of the cyclonic events, but with the opposite phase (Fig. 3a–c). Assembling composites of the temperature evolution during the eddy period (Fig. 3d–f) provides the associated eddy structure, which is inherently difficult to observe directly. On average, the cyclonic events occur above 450m with a warm core (>-1.4°C) located above 300m. The anticyclonic events are also concentrated in the same layer, but in contrast, the warm core (>-1.4°C) is deeper at a depth of 350m. The vertical structure in temperature (Fig. 3g–i) shows that the anticyclonic events are warmer than the cyclonic events across the full-depth. The maximum difference is around 0.13°C over the 270-320m layer. In general, anticyclonic rotation results in convergence and downwelling, and may be responsible for the relatively warm eddies observed. The opposite is the case for the cyclonic eddies.

The monthly distribution of warm-eddy intrusions reveals that the warm events occur across the October-April season (1.2 events per month on average), consistent with Fig. 2b. The maximum number of cyclonic (anticyclonic) events occurs in January (April–see Fig. 4a). The duration of these events, as they pass the mooring location, ranges from 1–7 days, most of which are within 2-3 days (78.6%, 22 of 28 events). A total of 63% of the anticyclonic (7 out of 11) events take 3 days to pass the mooring area (Fig. 4b).

As it is unlikely that many eddies pass directly over the mooring, the horizontal scale of the eddy chords can be estimated from the duration and background velocity. Figure 4d indicates that the eddy chords vary within 13.5 km as a decreasing function. Events with a horizontal scale of 0-5 km, 5-10 km and 10-15 km account for 71.4%, 21.4% and 7.2% of the data, respectively. Compared with the mesoscale eddies in the tropical ocean, the horizontal scales of eddies in polar region are relatively small, associated with the smaller Rossby deformation radius (3-4 km) at higher latitudes. Based on the potential temperature from the mooring records (Fig. 4c), about 11.2% (20.4%) of the temperature records are warmer than -1.7°C (-1.8°C). This indicates there is the equivalent of 1.4-2.4 months of warm-eddy intrusions every year.

It is worth noting that the eddy chords estimated from the mooring record are around 10%~50% of the full eddy size observed from the altimetry data. There are four possible explanations for this. Firstly, the mooring record used to estimate the eddy chords is observed by a current meter located at 270m (Figs. 2b and 3). Since the three-dimensional shape of eddies are vertical-taper structures (Fig. 3d–f), the eddy chords at 270m must be much smaller than the length scales at the sea surface. Secondly, the majority of the eddy chords do not cross through the center of the eddies (assumed to be circular in the horizontal plane). Thirdly, the mooring is located at 67.18oS, approximately 100 km from the continental shelf break. Intruding eddies could move eastward with the Prydz Bay Gyre, preventing the entire eddy from passing over the mooring location. Finally, since the merged Sea Surface Height (SSH) data is provided on a 0.25° grid, the smaller eddies just are not resolved by the altimetry product. All four reasons explain the reduction in eddy chord lengths estimated from the mooring record.

While the warm events only occur during austral summer, the cross-stream velocity ve varies throughout the year (Fig. 2b and Fig. 4a). To explain this difference, we consider changes in temperature, salinity and the mixed layer depth. Gradients in both potential temperature and salinity across the shelf break of Prydz Bay present a clear seasonal cycle, first increasing in January, peaking in April and lastly decreasing in June (Fig. 4e, f). The high (low) gradients in austral summer (winter) make the warm events conspicuous (inconspicuous). In other words, the mixed layer is shallow across the austral summer but deepens in the austral winter, consistent with a previous study 52. Temperature in the deepening mixed layer remains uniform near the freezing point when there is active sea ice formation driving oceanic convection. The maximum winter mixed layer depth occurs after it is cut-off from the atmosphere by total sea ice cover, with the temperature remaining uniform thereafter. Therefore, no warm events appear in the temperature records during austral winter, despite the continuing presence of eddy intrusions onto the continental shelf. An earlier study 24 suggested a similar process resulted in ongoing, but highly modified CDW intrusions occurring through winter in the Adélie Depression. This supports our argument that the eddies are likely to be occurring through winter, but without a strong temperature signal to identify them.

Physical processes of warm-eddy formation and transport
A typical warm intrusion event was observed during the hydrological survey in February 2013 (Fig. 1b, d). The evolution of SSH, geostrophic flow, and wind forcing anomalies describe the process of eddy formation and intrusion onto the continental shelf (Fig. 5a–f and Supplementary Fig. 4). A previous study described the eddy formation mechanism in response to large negative vorticity imparted by the presence of a storm in terms of classical Ekman layer theory 53. In the upper ocean, strong wind stress curl drives radial outward Ekman transport, resulting in an SSH decrease. Because of the continuity of mass, there must be a strong upwelling underneath the center of the storm to supply this mass flux. Assuming the frictional force is negligible, the flow is primarily cyclogeostrophic below the surface layer, and thus a cyclonic mesoscale eddy formed in the subsurface layer. There are also other examples in the literature that show that wind stress curl can be a mechanism for eddy generation 54,55. Similarly, two strong cyclones passed north of Prydz Bay from west to east during 26-30 January 2013 (Supplementary Fig. 4), and caused cyclonic eddy formation near the continental slope. The cyclonic winds drove the surface water divergence by Ekman transport, resulting in a continuous decrease in SSH (~3 cm) and an increase of geostrophic flow (15 cm/s) around the eddy in the following week. The intensified cyclonic eddy upwelled the warm mCDW and moved southward into the Prydz channel. The westward ASF near the continental shelf break bent southward influenced by the topographic troughs, and subsequently the eddy deformed and traversed the ASF barrier to transfer relatively warm waters to the shelf (Fig. 5a–f).

The regional mean changes of SSH and wind forcing help explain the process of the eddy formation and intrusions (Fig. 5g). The consistent cyclonic winds during 26-30 January caused surface divergence and a decrease in SSH that triggered a cyclonic eddy formation. The persistent positive westerly wind from 31 January (~5 m/s) increased the offshore Ekman transport and associated upward Ekman pumping near the continental slope, which drove the eddy onto the continental shelf and overcame the ASF barrier. Changes along the 73oE transect between a typical warm intrusion event (mid-February 2013) and the ‘normal’ case (mid-February 2015) help to further understand the warm-eddy intrusion process (Fig. 5h). Positive temperature anomalies (maxima ~1.5°C) appear in the 100-500m layer on the continental shelf between 66.6oS-67oS, while negative temperature anomalies (minima ~−1.3°C) are distributed in the 150–700m depth range on the continental slope between 65.5oS–66.6oS. This suggests that the negative wind stress curl and enhanced westerly winds offshore (Fig. 5g) raise the density surface and decrease the isopycnal slope near the continental shelf break (66.2oS–67oS), driving both the mCDW and ASF upward and southward onto the continental shelf (Fig. 5h).

Here we investigate the influence of zonal winds as a potential physical mechanism for the warm-eddy intrusions. Almost all of the warm events that carry heat onto the continental shelf are associated with wind stress curl changes and enhanced westerlies (Supplementary Figs. 5 and 6). Strong cyclones passed north of Prydz Bay in the first week of the observations (Supplementary Fig. 5b), driving surface water divergence by Ekman transport, resulting in a continuous decrease in SSH (2–5 cm) and an increase in cyclonic eddy formation near the continental slope (Supplementary Fig. 5a). The persistent positive westerly wind (Supplementary Fig. 5c) increased the offshore Ekman transport and associated upward Ekman pumping near the continental slope, which drove the eddies onto the continental shelf and overcame the ASF barrier. The warm anticyclonic eddy events that usually accompany the cyclonic eddies due to the multi-meander of ASF are also driven into the Prydz Bay by the enhanced westerly winds (Supplementary Fig. 6). Regression analyzes between the zonal wind speed, wind stress curl and warm events (warm signals at 270m) show that the zonal wind (wind stress curl) had a statistically significant (above the 95% confidence level) effect on eddy events with a lead-time of 4-14 days (3-7 days). The maxima of mean regression values over different lead times (4-8 days) for the warm events relative to the zonal wind appear over the 68oE-78oE, 60oS-68oS region, which covers the transition area between the climatological westerly and easterly wind regimes (Fig. 6a).

Most of the warm events occur 4-8 days after the westerly winds strengthened in this transition area (Supplementary Fig. 7a). Negative maximum regression values of warm events relative to wind stress curl appear in the 66oE–78oE, 65oS–68oS region, just over the continental shelf break of the Prydz channel (Fig. 6b). The cyclonic wind stress curl in this area tends to pump the warm signal upwards onto the Prydz Bay continental shelf (Supplementary Fig. 7b). This analysis suggests that the increasing and poleward shifting westerlies, alongside the resulting cyclonic wind stress curl, are responsible for the warm-eddy intrusions.

Additionally, during the 1979-2018 period, both the austral summer (DJF) westerly winds in the 50oS-67oS region and the coastal easterly winds south of 67oS, have strengthened and thus more cyclonic wind stress curl has occurred over the continental shelf break of the Prydz channel (Fig. 6c, d, Supplementary Figs. 8 and 9). Such changes would drive more warm eddies towards the front of the Amery Ice Shelf (north part), where thinning/mass loss is occurring at a significant rate 56.

For Antarctic coastal regions with broad continental shelves, increasing and poleward shifting westerlies increase the offshore Ekman transport and associated upward Ekman pumping near the continental slope, and thus facilitate to shift the mCDW layer upward. Meanwhile, the intensified westerly (easterly) winds offshore (onshore) generate more cyclonic wind stress curl near the continental slope, which also enhances the associated upward Ekman pumping, resulting in an elevated density surface and upward-tilting of the mCDW layer. Topographic troughs in the shelf break region make the ASF bend southward and form loop currents and meanders, with the Ekman pumping contributing further to the cyclonic wind stress curl over the continental shelf break, thus enhancing the onshore warm-eddy intrusions (Figs. 5 and 6). Thereafter, the warm-eddy intrusions join the basin circulation gyres, which supply the ocean heat to the local Antarctic ice shelves. It is worth noting that although most of the eddy intrusion events can be explained by the wind forcing, a number of the eddy events occurred without the trigger of a strong wind stress curl anomaly (Supplementary Fig. 5b). An alternative explanation is that the observed eddies are simply part of the background eddy field, maintained through baroclinic instability, and interact independently with the passing storms.

Methods
Data collections
Hydrological sections data and subsurface mooring observations (Lamont-Doherty Earth Observatory–First Institute Oceanography collaboration) were collected and conducted by the Chinese National Antarctic Research Expedition (CHINARE) from 2013 to 2015. Elephant seal CTD data was accessed from the Marine Mammals Exploring the Oceans Pole to Pole (MEOP) Consortium 60. All of the associated wind-forcing analyzes were based on daily wind data from the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis-interim (ERA-interim) products, which were able to capture both the high-frequency variability (e.g., changes in storminess) and long-term changes associated with the wind forcing. Two-satellite merged SSH and geostrophic flow data with spatial resolution 0.25°×0.25° was collected from the Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO).

Eddy flow estimation
Following previous studies 61,62, all mooring records were de-tided using T_TIDE and subsequently low-pass filtered with a 20-hour period to subtract the inertial frequency variability (Supplementary Fig. 10). Given that the observed horizontal flow u can be recognized as the sum of background flow U and eddy flow ue, the eddy flow then becomes

ue = ue + ive = (u − U)e−iϕ(t) (1)

where ϕ is the direction of background flow, and ue and ve are the along-stream component and cross-stream component of the eddy flow, respectively. The background flow U is taken as the 5-day low-pass velocity 7,63.

To compare the eddy signature over different durations, we normalized the time coordinate, where the temperature signal peaks, as

t0 = (t − tm) / Tvmax (2)

where t is the time, tm is the center time between the two absolute maximum cross-stream velocities in a single event, and Tvmax is the time period between those two velocities’ maxima.

Eddy heat flux estimation
The poleward heat flux is calculated based on the background flow and the potential temperature anomalies. Assuming the area of mCDW (~4.95×107 m2) intrusion into the Prydz Channel observed in 2013 (Fig. 1d) as the mean state. Given the meridional component of background velocity (1.6 ± 0.3 cm/s) during the warm events, the poleward mCDW flux associated with eddy intrusions is around 0.79 ± 0.15 Sv.

To estimate the possible heat gain on the Antarctic ice shelf, we assume that all the eddy heat is available for the Amery Ice Shelf melt in Prydz Bay. Following a previous study 16, the annual basal melt rate of the section of the Amery Ice Shelf directly impacted by mCDW inflow due to the eddy heat transport is estimated as

m = ρFcp(Tf − T)t / (ρiLiAi) (3)

where ρ is ocean density (1027 kg m-3), F is the mCDW flux into the ice shelf cavity (~0.79 ± 0.15 Sv), cp is the ocean heat capacity (4000 J kg-1 C-1), Tf is the pressure freezing temperature (−2.14°C) at 345 dbar (the mean ice shelf draft for the area of the ice shelf base where mCDW can potentially drive basal melt), T is the mean temperature of the mCDW (waters with temperatures over −1.8°C), t is the number of seconds when the temperature above −1.8°C in a year, ρi is the density of ice (920 kg m-3), Li is the latent heat of ice (334,000 J kg-1), and Ai is the area of the ice shelf base where mCDW can potentially drive basal melt (12,773 km2). Negative melt rate means the volume of ice is decreasing. Assuming that all the eddy heat associated with this inflow is available for melt, it would result in an average basal melt rate of 3.1 m of ice per year.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of \"Persistent warm-eddy transport to Antarctic ice shelves driven by enhanced summer westerlies\", by Libao Gao, Xiaojun Yuan, Wenju Cai, Guijun Guo, Weidong Yu, Jiuxin Shi, and Guy D. Williams.\nGeneral Comments\nThis study primarily uses data from a two year mooring in the Prydz Channel on the Antarctic continental shelf, but also data from hydrological sections, instrumented seals, satellite altimetry, and atmospheric reanalysis, to study the characteristics of episodic intrusions of relatively warm modified Circumpolar Deep Water (mCDW) onto the continental shelf. The authors use the mooring data to identify individual \"eddy-like warm events\" and estimate the rotation, frequency, and size of these eddy driven impulses of mCDW. The warm events only occur during the summer months and the authors show that that is due to the temperature gradient along the trough (driven by surface mixed layer processes) rather than eddies only occurring during the summer. The authors then present correlations between the off shore westerlies, and the wind stress curl at the shelf break, with the strength of the individual warm events to show that strengthened winds promote these intrusions.\nBecause of the importance of mCDW intrusions on, not only the melting of the Amery Ice Shelf that is local to this study, but on the melting of Antarctic ice shelves in general, I feel the subject is certainly worth the attention of Nature Communications. The mooring is a fantastic data set to study this issue. While this is not the first high temporal resolution data on eddies advecting across an Antarctic shelf (see comment on lines 286-287 below), there are very few moorings that have captured this data, and much (most?) of the previous results are along the west Antarctic Peninsula, which does not have the dynamic barrier of the Antarctic Slope Front that exists here.\nHowever, I think more could be done to put these results in a broader context in terms of whether the eddies are a significant contributor to the heat budget on the continental shelf. For example, the authors do compute a mean heat flux (lines 126-128) for the warm events. Can the authors estimate a total heat flux due to these warm events across all of Prydz Channel from this? Is this total heat flux comparable to the heat known to be lost through the nearby polynyas? The heat lost to melting of the near ice shelf front portions of the Amery Ice Shelf?\nAlso, I'm a little confused by the authors discussion of the formation of the off-shelf cyclonic eddy described on lines 189-201. Are the authors proposing that most of the mesoscale eddies that carry heat onto the continental shelf originate off the shelf due to wind divergence or was this a one time event? I could easily be wrong, but while winds can impact the ability (as also shown in this paper) of eddies to get onto the shelf, I was under the impression that most of the mesoscale eddy formation relevant to heat advection onto the Antarctic continental shelf was thought to be due to baroclinic instability (e.g. Stewart and Thompson, GRL, 2015; McKee et al., JPO, 2019). If this is not the case, I think this is an interesting finding that could be emphasized more. Do the authors know of any literature supporting the idea of these particular eddies originating more from wind divergence?\nI have several more specific comments and suggestions below, although many of them are very minor editorial comments. I think this paper can eventually be published, but it does require some significant revision first.\nSpecific Comments\nLine 22: I think \"contribution\" in this context implies that the ice shelf melt water directly increases sea level. If the authors agree, then I suggest changing \"contribution to\" to \"impact on\" or something that does not directly imply that ice shelf meltwater raises sea level.\nLine 39: Same comment about \"contribution\" as above.\nLine 42: Since the ASF does not go all the way around Antarctica (see the schematic in ref. 18, Thompson et al., 2018), I suggest changing \"onto the continental shelf\" to \"onto most of the continental shelf\".\nLine 84: Suggest changing \"southward at\" to \"southward to\".\nLine 93: I think \"moorings\" should be \"mooring\".\nLine 97: A negative sign is missing before \"1 deg C\".\nLine 106: I don't think \"between April 2014 and January 2015\" is correct for the highest frequency of events. Several months in that time frame (May-Sep 2014) have no events.\nLine 111: Is the 3-8 cm/s amplitude during the entire time series or just the warm events?\nLines 115-116: I think the authors should add a sentence (perhaps not here, but in the methods section or Figure 2 caption) about exactly how the heat fluxes are calculated.\nLines 126-128: I don't understand how the comparison of the lateral heat flux at one specific point onto the continental shelf to the estimated heat gain of SAMW, computed on a much much coarser (2 degrees by 4 degrees I think) scale is relevant. As mentioned above, it would be nice to see a comparison of this to other estimates of heat gain/loss on the Antarctic continental shelf though.\nLine 130-131: As mentioned above, I think more needs to be done to show that the \"eddy-induced heat transport into Prydz Bay has an important impact on the mass balance of the Amery ice shelf\".\nLine 137: Suggest adding \"cross-stream\" before \"velocity signal\" (if this is referring to the cross-stream velocity).\nLines 163-165: What is the Rossby deformation radius from the observations in this area?\nLines 171-174 and Figure 4e and f: What direction is the gradient in? It makes more sense to me if it is in the cross shelf break direction and the denominator of the y-axis label in the figure is \"dy\". However, the figure caption says \"across Prydz Channel\". Also (for Figure 4e-f), what are the units for the gradients?\nLines 177-180: How deep do the winter mixed layers get here? Do they drop down into the 200-400 m depth range used for computing the gradients?\nLines 222-227 and Figure 6a (similar question for Figure 6b): Apologies if this should be obvious, but I'm not sure what is meant by the wind leads warm events by 4-8 days with respect to the figure. Is the plotted regression for a specific lead time (e.g. 6 days)? Is it a regression computed over several different lead times from 4 to 8 days. Is the plot showing the regression for the lead time with the maximum correlation at each point with (potentially) a different lead time at each location?\nLines 231-233: I agree that this analysis suggests a relation between increased westerlies and changes in the wind stress curl with warm intrusions. However, have the authors shown a relationship with the poleward shift of the westerlies? I realize the poleward shift will change the curl, but I don't know if that has been explicitly demonstrated at this point in the manuscript.\nLine 233: Suggest adding \"the\" before \"1979-2018 period\".\nLines 237-238: Is there a paper or any data showing if the melt rate of the Amery Ice Shelf has been increasing over some portion of the last 40 years?\nLine 256: Most of these references are looking at the eddies off the continental shelf. Do any of these actually discuss \"eddy-mediated transport of CDW across the Antarctic Shelf Break\"? If not, suggest adding '33' to the references here, since that one does explicitly look at eddy transport across the shelf break.\nLine 260-261: Is \"seasonally enhanced\" the best wording? The authors do show that the summer winds and wind stress curl are increasing in time. However, \"seasonally enhanced\" implies (to me anyway) that the winds are stronger in the summer than during the rest of the year and that is why the intrusions come in during the summer. There is no discussion of the summer wind strength vs. other seasons (apologies if there is and I missed it) and the authors show (lines 171-182) that the seasonality of the intrusions is driven by the seasonality of the temperature gradient.\nLine 283: Suggest changing \"role of eddies play\" to either \"role of eddies\" or \"role eddies play\".\nLines 286-287: These are not the first observations of persistent eddies across an Antarctic shelf break: Moffat et al., 2009 (referenced in this manuscript), Martinson and McKee, 2012 (also referenced here), and Couto et al., 2017 all have nice detailed observations of eddy transport of warm mCDW across an Antarctic shelf. What might be different here is that all those observations were on the WAP, where the ASF does not exist. There are other locations where the ASF exists where authors have observationally studied the impact of eddies transporting mCDW onto the shelf (Nost et al., 2011; Foppert et al., 2019), but I don't think they had the fine temporal resolution of measurements this study does. I think the authors need to be a little more specific than just stating that this is the \"first observations of persistent eddies across an Antarctic shelf break\".\nLine 292: Suggest changing \"likely increase\" to \"likely to increase\".\nFigure 1 caption, Line 522: Are the authors sure reference 17 (Jacobs, 1991) is the correct one for the polynya definitions?\nExtended Data Fig. 3: What are the units for the wind anomaly vectors and the SLP anomaly contours?\nExtended Data Fig. 4: Why are there are so many more data points in Figure 4a vs. 4b? Both are using the same set of warm events, correct?", "reviewer_2": "The manuscript by Dr. Gao and collaborators presents evidence of seasonal warm water intrusions in the Prydz Bay channel based on temperature observations from a 2-year moored record and hydrographic sections collected in the vicinity of the mooring 2 years apart. Con-current velocity observations from the mooring further show warm intrusions to be associated with either cyclonic and anticyclonic eddies. The authors explained the presence/absence of this warm intrusions is linked to two factors: (1) local westerly winds and the associated negative wind stress curl upwelling lifting CDW onto the continental shelf, (2) the seasonal cycle of surface buoyancy loss eroding the warm temperature signal despite the eddies being present year-round.\nThe authors have done a great job extracting information from the in-situ record and combining their results with the remote sensing analysis. I believe results presented in this paper will be of interest to the Nature Communications readership. But the manuscript still needs a fair bit of work before it can be published. The introduction and discussion lack focus, there are a number of arguments that are unclear or incomplete, and there is missing information throughout the manuscript. Detailed comments are provided below.\nMajor comments\n1. The introduction and parts of the discussion need to be brought into focus. The authors spend a significant amount of text discussing the response of the ACC to changes in westerly winds. While the ACC and the ASC may share some similarities in terms of their eddy overturning circulations, they are distinct features of the Southern Ocean circulation and to my knowledge there is no direct link between increased eddy activity in the ACC and increase eddy shedding of the ASC. If the authors feel there is such link, they need to articulate it much more clearly. Furthermore, the analysis the authors present is based on a relatively short time series, and while it adds to our understanding of the processes that govern cross-shelf exchange, it can’t in itself address the question of long-term changes in the system (see also comment 3).\nThe introduction could also spend more time framing the question about delivering heat to the Antarctic margin in the context of East Antarctica specifically. A lot of the existing literature on the topic of cross-shelf exchange, including modelling and observational evidence of the role eddies play, is based on the West Antarctic Peninsula and the Amundsen Sea. The margins around East Antarctica are significantly different with primarily warm shelves in the west, and weak or missing ASF, and cold/dense in the East, with a well-defined ASF.\nLast with regards to the introduction I would encourage the authors to be more selective on the references they used. Throughout the introduction the authors cite long lists of papers on relation to one topic, without explicitly describing what the findings are, and at times including references that are not really appropriate for the topic. For instance, in line 43 the authors say “have captured this process” following a discussion about the role of the ASF as a barrier to cross-shelf transport, while the references that follow described either papers that focus on the circulation and processes within the continental shelf, or in the case of the Amundsen Sea, the role of the undercurrent delivering heat to the shelf. Another clear example is found in lines 60-61. Other references that would be quite relevant to the subject of eddy transport across the shelf-break, such as Stewart et al 2015, are in the reference list but are actually not cited in the text.\n2. In my view, there are two processes that are tangled throughout the manuscript. The first is the wind driven uplift of the warm CDW that delivers the water up to the shelf break, and the speculated eddies that can then carry the warm water across the shelf break. Collectively, these two processes deliver heat onto the continental shelf. The correlation analysis provides some evidence about the forcing mechanism behind the first (the uplifting of warm water). However, the manuscript lacks a clear explanation for the second (the formation and evolution of the eddies).\n3. Throughout the manuscript the authors appear to link observed trends in the Southern Ocean winds (figures 6 d and e) to warm water intrusions in Prydz bay. But the manuscript provides no evidence of what the link is. The in situ observations capture a seasonal process, and with just 2 years’ worth of data it would be quite challenging to detect a trend. And then in line 238 the authors mistakenly claim that the Amery Ice Shelf is thinning at a significant rate and cite the Smith et al 2019!! When in fact Smith and collaborators conclude that the Amery has a relatively small height change, and it’s of mass gain not of mass loss. The manuscript would be much more convincing if the authors simply concentrated in the processes they observe, and how they operate at a regional scale, rather than try to connect them with long term changes in the large scale winds.\nMinor comments\n1. Perhaps more appropriate than “Persistent” in the title would be “Seasonal”, as the intrusions are only observed in the summer time.\n2. Line 21. The word “prescribed” implies that this is the only factor controlling the delivery of heat, when in reality the circulation within the shelf, buoyancy fluxes, and the geometry of the ice-shelf cavity are just as important. Word-count permitting, I would suggest the authors rephrase this first sentence in the abstract.\n3. Line 78 – Mentioning the seal data gives the reader the impression it is another source of data used in this study. Please remove.\n4. Line 88-89- Here the intrusion is described as “weaker”, when later on with regards to figure 5, it’s considered as the “normal” conditions when the intrusion is absent. Please correct for consistency.\n5. Line 103, the reader would benefit from a brief description (in relation to the relevant literature cited) of how these events are detected.\n6. Lines 108-109. Could the authors clarify how would the method would work in the presence of a northward background flow? Wouldn’t a method based on the sequential detection of a pair of opposite sign velocity anomalies depend on the direction of the background flow? Please clarify.\n7. Line 119, with regards to heat fluxes the reader would benefit from a brief description of how these fluxes are calculated. Estimating heat transported by and eddy, derived from a single point eulerian measurement is not a trivial. This is due to the fact that in the time mean, the velocity of an eddy advected past a mooring site is zero (that is for a perfectly symmetric eddy). Some other non-trivial aspects of the calculation would be over which length scale to integrate the single point measurement.\n8. Lines 127-128. It’s unclear what the reference to Subantarctic Mode Water is here for. Consider removing.\n9. Line 191. Typo: replace describes by describe.\n10. Lines 192-194. Can the authors elaborate on how exactly they observe the “passing” cyclones in Figure 5? I suspect the main signal in the SSH anomaly field has a larger footprint than the small domain being plotted. My guess is that if the authors replotted the same six panels, removing the local mean value, all panels would look much more similar. This large-scale signal is superimposed on the eddy pattern could be the response to the large storm the authors show in the extended data figure 3.\n11. Lines 198-200. The authors should consider rescaling panels a-f in figure 5. Due to the small size of the vectors, and the dark shades of blue, it is quite hard to see what the authors describe here as the ASF bending southward and the eddies traversing the ASF.\n12. Line 204. Could the authors clarify the mechanism for how surface divergence triggers cyclonic eddies?\n13. Line 222. What does the 3-7 day lag in parenthesis refer to? And why is it different than the 4-14 day?\n14. Line 284. Typo. Please remove “the” (in “the Antarctica”).\n15. At the time of review the link to the mooring/CTD didn’t work.\n16. Can the authors please supply DOI’s for the complete, revised reference list?\nFigures\nFigure 1\n- Could the authors use a different marker type to show in the map which exact stations are plotted in the sections in panels b-e. Having some markers along the top of the sections illustrating where the data actually is would also help pick up features that may have result from interpolation.\n- Figure 5, is quite important to the authors arguments. Instead of plotting the small area around the stations in the dashed blue line, they could plot the domain used in Figure 5. It could help providing context to Figure 5.\nFigure 5\n- Vectors in the southern part of panels a-f are not distinguishable from the background. Could the authors clarify why are different averaging boxes used for the different variables.\n- And in line 558, please clarify what the “southern boundaries of the ASF” and how are they specified.\n- Please, add units to panel g.\nFigure 6\n- Correlations values in figure 6b seem odd. In Panel b these range from [-1,1] why [-4,4] in panel b? And could the authors clarify what exactly is plotted in these two panels? Is it the maximum correlation from a lagged-correlation calculation? The authors briefly mention the lag in the manuscript. If this is the case, it would be useful to provide the corresponding figure with the lags, at least as supplementary material.\n- Also, is the correlation analysis carried out only for the summer months? It is not described in the text nor in the figure caption. If it was only calculated using summer months, how did the authors handle the lag? Please clarify. If in the other hand, the correlation included all months, then can the authors explained how can they rule out the role of the seasonal buoyancy loss in the correlation (I would expect seasonal winds would correlate well with seasonal winter heat loss).", "reviewer_3": "This paper presents the observations of warm eddies with subsurface mooring near Prydz Bay. The eddy-induced heat transport is important to Antarctic ice shelf melt. This is an interesting topic. This manuscript is well organized. However, further process studies are required to define the formation and recurrence of eddies on the mooring site and the characteristics of ASF flow and exchanges.\nComments:\n1. The eddies’ characteristics are derived from the spiking patterns of temperature time series and spiral velocity records. The salinity is also important to trace the water mass of eddies. The T-S diagram might be useful to show the intrusion of warm water and warm eddies.\n2. Line 118-121: The poleward heat flux is calculated from the warm events. From Fig.3c and Extended Data Fig.1c,d, the net cross-stream and meridional component of background velocity are very small. It might be overestimated or underestimated without consideration of the path and movement speed of eddies.\n3. Line 133-134： the warm-eddy intrusion is assumed from the mCDW cross ASF. More detailed numerical modeling will be needed to understand the exchanges between the eddy field and upwelling and cross-shelf transport. (Ref. 53 Liu et al’s model results are useful. )\n4. Line 188-195: Fig. 5a-f. The mooring is almost located at the center of the saddle shape of SSH. The rotation of the velocity at the mooring site was related to large-scale SSH oscillation. The mesoscale variability near mooring also could be filaments.\n5. Line 264-280: “We show that increasing and poleward shifting westerlies, in conjunction with the cyclonic wind stress curl, cause upward-tilting of the mCDW layer that contribute to facilitating warm-eddy intrusions.” Both the cyclone and anticyclone eddies were observed. However, the mechanism of formation cyclone and anticyclone eddy is not clarified."}, "author_response": "Point-by-point response to Reviewer #1:\nReviewer #1 (Remarks to the Author):\nReview of \"Persistent warm-eddy transport to Antarctic ice shelves driven by\nenhanced summer westerlies\", by Libao Gao, Xiaojun Yuan, Wenju Cai, Guijun Guo,\nWeidong Yu, Jiuxin Shi, and Guy D. Williams.\nGeneral Comments\n1. This study primarily uses data from a two-year mooring in the Prydz Channel on\nthe Antarctic continental shelf, but also data from hydrological sections,\ninstrumented seals, satellite altimetry, and atmospheric reanalysis, to study the\ncharacteristics of episodic intrusions of relatively warm modified Circumpolar Deep\nWater (mCDW) onto the continental shelf. The authors use the mooring data\nto identify individual \"eddy-like warm events\" and estimate the rotation,\nfrequency, and size of these eddy driven impulses of mCDW. The warm events\nonly occur during the summer months and the authors show that that is due to the\ntemperature gradient along the trough (driven by surface mixed layer processes)\nrather than eddies only occurring during the summer. The authors then present\ncorrelations between the off shore westerlies, and the wind stress curl at the shelf\nbreak, with the strength of the individual warm events to show that strengthened\nwinds promote these intrusions.\nBecause of the importance of mCDW intrusions on, not only the melting of the\nAmery Ice Shelf that is local to this study, but on the melting of Antarctic ice\nshelves in general, I feel the subject is certainly worth the attention of Nature\nCommunications. The mooring is a fantastic data set to study this issue. While this\nis not the first high temporal resolution data on eddies advecting across an\nAntarctic shelf (see comment on lines 286-287 below), there are very few\nmoorings that have captured this data, and much (most?) of the previous results\nare along the west Antarctic Peninsula, which does not have the dynamic barrier\nof the Antarctic Slope Front that exists here.\nResponse: Thanks for your comments. A few observational studies have described\ncross-slope eddies near the Antarctic continental shelves (Nost et al., 2011;\nThompsonet al., 2014; Foppert et al., 2019). But precisely how eddies traverse the\nASF barrier to facilitate the transfer of relatively warm waters to the shelf regions\nremains poorly understood, which is why we made it the focus of our study.\n(1) Nøst, O. A., Biuw, M., Tverberg, V., Lydersen, C., Hattermann, T., & Zhou, Q.,\nSmedsrud, L. H., and Kovacs, K. M. Eddy overturning of the Antarctic Slope\nFront controls glacial melting in the Eastern Weddell Sea. Journal of\nGeophysical Research: Oceans (2011). doi:10.1029/2011JC006965.\n(2) Thompson, A., Heywood, K., Schmidtko, S. et al. Eddy transport as a key\ncomponent of the Antarctic overturning circulation. Nature Geosci., 7,\n879–884 (2014). https://doi.org/10.1038/ngeo2289.\n(3) Foppert, A., Rintoul, S. R., & England, M. H. Along-slope variability of\ncross-slope eddy transport in East Antarctica. Geophys. Res. Lett., 46,\n8224-8233. (2019). https://doi.org/10.1029/2019GL082999.\n2. However, I think more could be done to put these results in a broader context in\nterms of whether the eddies are a significant contributor to the heat budget on the\ncontinental shelf. For example, the authors do compute a mean heat flux (lines\n126-128) for the warm events. Can the authors estimate a total heat flux due to\nthese warm events across all of Prydz Channel from this? Is this total heat flux\ncomparable to the heat known to be lost through the nearby polynyas? The heat\nlost to melting of the near ice shelf front portions of the Amery Ice Shelf?\nResponse: Agreed. We have compared our estimates with the heat transport of\nmCDW to the Amery ice shelf (Herraiz-Borreguero et al., 2015). Assuming that\nall of the eddy heat is available for melting the Amery Ice Shelf in Prydz Bay,\nthe poleward heat flux associated with eddy intrusions (0.79±0.15 Sv) would result in\nan average basal melt rate of 3.1±0.7 m of ice per year (see Methods). It is\nroughly 50% more than the previously estimated annual basal melt rate (2.0±0.5\nm/year) driven by mCDW flux into the Amery Ice Shelf cavity\n(Herraiz-Borreguero et al., 2015).\nThe poleward heat flux is calculated based on the background flow and the\npotential temperature anomalies. We assume that the area of mCDW (~4.95105\nm2) intrusion into the Prydz Channel observed in 2013 (Fig.1d) is the mean state.\nGiven the meridional component of background velocity (1.6±0.3 cm/s) during\nthe warm events, the poleward mCDW flux associated with eddy intrusions is\naround 0.79±0.15 Sv.\nTo estimate the possible heat gain on the Antarctic ice shelf, we assume that\nall the eddy heat is available for melting the Amery Ice Shelf in Prydz Bay.\nFollowing Herraiz-Borreguero et al. (2015), the annual basal melt rate of the\nAmery Ice Shelf due to the eddy heat transport is estimated as\nFc (T T)t\nm p f ,\nLA\ni i\nwhere  is ocean density (1027 kg m-3), F is the mCDW flux into the ice\nshelf cavity (~0.79±0.15 Sv), c is the ocean heat capacity (4000 J kg-1 C-1), T\np f\nis the pressure freezing temperature at 345 dbar where mCDW can potentially\ndrive basal melt (-2.14 ºC), T is the mean temperature above -1.8 ºC of the\nmCDW, t is the number of seconds when the temperature above -1.8 ºC in a\nyear,  is the density of ice (920 kg m-3), L is the latent heat of ice (334,000 J\ni\nkg-1), and A is the area of the ice shelf base where mCDW can potentially drive\ni\nbasal melt (12,773 km2). Assuming that all of the eddy heat associated with this\ninflow is available for melt, it would result in an average basal melt rate of 3.1 m\nof ice per year. We have added these in the Methods section in our manuscript.\nWe have revised our manuscript accordingly with changes shown in green color.\nHerraiz-Borreguero, L., R. Coleman, I. Allison, S. R. Rintoul, M. Craven, and\nG. D. Williams (2015), Circulation of modified Circumpolar Deep Water and\nbasal melt beneath the Amery Ice Shelf, East Antarctica, J. Geophys. Res. Oceans,\n120, 3098–3112, doi:10.1002/2015JC010697.\n3. Also, I'm a little confused by the authors discussion of the formation of the\noff-shelf cyclonic eddy described on lines 189-201. Are the authors proposing that\nmost of the mesoscale eddies that carry heat onto the continental shelf originate\noff the shelf due to wind divergence or was this a one time event? I could easily\nbe wrong, but while winds can impact the ability (as also shown in this paper) of\neddies to get onto the shelf, I was under the impression that most of the mesoscale\neddy formation relevant to heat advection onto the Antarctic continental shelf was\nthought to be due to baroclinic instability (e.g. Stewart and Thompson, GRL, 2015;\nMcKee et al., JPO, 2019). If this is not the case, I think this is an interesting\nfinding that could be emphasized more. Do the authors know of any literature\nsupporting the idea of these particular eddies originating more from wind\ndivergence?\nResponse: Thanks for your suggestion. We have conducted the composition\nanalysis of all warm events (Extended Data Fig. 5), which indicates that most of\nthe mesoscale eddies that carry heat onto the continental shelf originate off the\nshelf due to wind forcing. Strong cyclones (negative wind stress curl anomaly)\npassed north of Prydz Bay during the first week (Extended Data Fig.5b), driving\nthe surface water divergence by Ekman transport, resulting in a continuous\ndecrease in SSH (2-5 cm) and increase cyclonic eddy formation near the\ncontinental slope (Extended Data Fig.5a). The intensified cyclonic eddy\nupwelled the warm mCDW and moved southward into the Prydz channel. The\npersistent positive westerly wind (Extended Data Fig.5c) increased the offshore\nEkman transport and associated upward Ekman pumping near the continental\nslope, which drove the eddy onto the continental shelf and overcame the ASF barrier.\nThe westward ASF near the continental shelf break bent southward, influenced by\nthe topographic troughs, and subsequently the eddy deformed and traversed the\nASF barrier to transfer relatively warm waters to the shelf. This phase of cyclonic\neddy formation and transport onto the continental shelf lasted for almost three\nweeks (Extended Data Fig.5a, Fig. 5a-f). This analysis suggests that the\nincreasing and poleward shifting westerlies, alongside the resulting cyclonic wind\nstress curl, are responsible for the warm-eddy intrusions (Extended Data Fig.5a-c).\nMoreover, in a manner similar to Fig. 5a-f and Extended Data Fig.4, Figure S1\nand Figure S2 show another case of eddy formation and intrusion on to the\ncontinental shelf driven by wind forcing.\nThe increased upwelling and southward transport driven by wind forcing\ncauses a baroclinic instability, and spawns eddies that counteract the steepening of\nthe isotherms due to wind-driven Ekman pumping in the surface mixed layer\n(Marshall and Radko, 2003). The process we described is consistent with previous\nstudies (e.g. Stewart and Thompson, GRL, 2015; McKee et al., JPO, 2019). Wang\net al. (2016) observed a three-dimensional structure of ocean cooling and eddy\nformation induced by Pacific tropical cyclones. Furthermore Marshall and Radko\n(2003) and Stewart and Thompson (2012) also support the concept that the\ncross-shelf eddies originate more from wind forcing.\n(1) Wang, G., Wu, L., Johnson, N., & Ling, Z. (2016). Observed\nthree-dimensional structure of ocean cooling induced by Pacific tropical\ncyclones. Geophysical Research Letters, 43(14), 7632-7638.\ndoi:10.1002/2016gl069605.\n(2) Marshall, J., and T. Radko (2003), Residual-mean solutions for the Antarctic\nCircumpolar Current and its associated overturning circulation, J. Phys.\nOceanogr., 33(11), 2341–2354.\n(3) Stewart, A. L., & Thompson, A. F. (2012). Sensitivity of the ocean's deep\noverturning circulation to easterly antarctic winds. Geophysical Research\nLetters, 39(18).\nExtended Data Fig. 5 | Composition analysis of all warm events. a, Evolution of\nSSH anomaly (averaged in the 70-74oE, 65.7-67.1oS region, black line, unit: cm). b,\nEvolution of wind stress curl anomaly (averaged in the 65-85oE, 56-66oS region, red\nline, unit: 210-10 m·s2). c, Evolution of zonal wind anomaly (averaged in the\n65-85oE, 56-66oS region, cyan line, unit: 3 m·s-1). Red lines are the ensemble mean.\nX-axis is days.\nFigure S1. Evolution of SSH anomaly (shade and contours, cm) and geostrophic flow\nanomaly (arrows) during 20-January~28-January in 2014. The mooring location is\nshown with red star. The 500m, 1000m, 2000m and 3000m bathymetric contours are\nshown in red.\nFigure S2. Evolution of wind forcing anomaly during 16-January~27-January in\n2014. (wind stress curl anomaly, shade, unit: 10-10 m·s2; wind anomaly, arrows; Sea\nLevel Pressure anomaly, contours).\nI have several more specific comments and suggestions below, although many of\nthem are very minor editorial comments. I think this paper can eventually be\npublished, but it does require some significant revision first.\nSpecific Comments\n4. Line 22: I think \"contribution\" in this context implies that the ice shelf melt water\ndirectly increases sea level. If the authors agree, then I suggest changing\n\"contribution to\" to \"impact on\" or something that does not directly imply that ice\nshelf meltwater raises sea level.\nResponse: As suggested by Reviewer #1 and Reviewer #2, we have rephrased the\nfirst sentence in the abstract as “The offshore ocean heat supplied to the Antarctic\ncontinental shelf by warm eddies has the potential to greatly impact the melting\nrates of ice shelves and subsequent global sea level rise”.\n5. Line 39: Same comment about \"contribution\" as above.\nResponse: we have rephrased this sentence.\n6. Line 42: Since the ASF does not go all the way around Antarctica (see the\nschematic in ref. 18, Thompson et al., 2018), I suggest changing \"onto the\ncontinental shelf\" to \"onto most of the continental shelf\".\nResponse: agreed.\n7. Line 84: Suggest changing \"southward at\" to \"southward to\".\nResponse: we have changed the text accordingly.\n8. Line 93: I think \"moorings\" should be \"mooring\".\nResponse: Yes.\n9. Line 97: A negative sign is missing before \"1 deg C\".\nResponse: Yes.\n10. Line 106: I don't think \"between April 2014 and January 2015\" is correct for the\nhighest frequency of events. Several months in that time frame (May-Sep 2014)\nhave no events.\nResponse: sorry to cause confusion. We mean that the highest frequency of events\noccurred in April 2014 and January 2015. The text should be \"in April 2014 and\nJanuary 2015\".\n11. Line 111: Is the 3-8 cm/s amplitude during the entire time series or just the warm\nevents?\nResponse: Yes, we have added \"during the entire time series\" in the revision.\n12. Lines 115-116: I think the authors should add a sentence (perhaps not here, but in\nthe methods section or Figure 2 caption) about exactly how the heat fluxes are\ncalculated.\nResponse: Agreed. We have added sentences about the heat flux calculation in the\nmethods section.\n13. Lines 126-128: I don't understand how the comparison of the lateral heat flux at\none specific point onto the continental shelf to the estimated heat gain of SAMW,\ncomputed on a much much coarser (2 degrees by 4 degrees I think) scale is\nrelevant. As mentioned above, it would be nice to see a comparison of this to\nother estimates of heat gain/loss on the Antarctic continental shelf though.\nResponse: Agreed. As the response to point 2 above, we have compared our\nestimates with the heat transport of mCDW to the Amery ice shelf\n(Herraiz-Borreguero et al., 2015). Assuming that all of the eddy heat is available\nfor the melting of Amery Ice Shelf in Prydz Bay, the poleward heat flux\nassociated with eddy intrusions (0.79±0.15 Sv) would result in an average basal\nmelt rate of 3.1±0.7 m of ice per year (see Methods). This is roughly 50% greater\nthan the previous estimate of annual basal melt rate (2.0±0.5 m/year), driven by\nthe mCDW flux into the Amery Ice Shelf cavity (Herraiz-Borreguero et al., 2015).\n14. Line 130-131: As mentioned above, I think more needs to be done to show that\nthe \"eddy-induced heat transport into Prydz Bay has an important impact on the\nmass balance of the Amery ice shelf\".\nResponse: Agreed. As mentioned above, we have compared our estimates with the\nheat transport of mCDW to the Amery ice shelf (Herraiz-Borreguero et al., 2015).\nAssuming that all of the eddy heat is available for the melting of Amery Ice Shelf\nin Prydz Bay, the poleward heat flux associated with eddy intrusions (0.79±0.15\nSv) would result in an average basal melt rate of 3.1±0.7 m of ice per year (see\nMethods). It is roughly 50% greater than the previous estimate of annual basal\nmelt rate (2.0±0.5 m/year), driven by mCDW flux into the Amery Ice Shelf\ncavity (Herraiz-Borreguero et al., 2015). This suggests that the eddy-induced heat\ntransport into Prydz Bay has an important impact on the mass balance of the\nAmery ice shelf.\n15. Line 137: Suggest adding \"cross-stream\" before \"velocity signal\" (if this is\nreferring to the cross-stream velocity).\nResponse: Agreed. We have changed the text accordingly.\n16. Line 163-165: What is the Rossby deformation radius from the observations in\nthis area?\nResponse: The Rossby deformation radius from the observations in this area is\naround 3~4km.\n17. Lines 171-174 and Figure 4e and f: What direction is the gradient in? It makes\nmore sense to me if it is in the cross shelf break direction and the denominator of\nthe y-axis label in the figure is \"dy\". However, the figure caption says \"across\nPrydz Channel\". Also (for Figure 4e-f), what are the units for the gradients?\nResponse: Yes, the direction of the gradient is in the across shelf break direction.\nThe units for the gradients (for Figure 4e-f) are ºC/latitude (temperature) and\npsu/latitude (salinity). We have changed the text accordingly.\n18. Lines 177-180: How deep do the winter mixed layers get here? Do they drop\ndown into the 200-400 m depth range used for computing the gradients?\nResponse: The winter mixed layer here is thicker than 400m, so the 200-400m\ndepth range normally used for computing averaged gradients in potential\ntemperature and salinity is too small in austral winter (see Figure 4e-f).\n19. Lines 222-227 and Figure 6a (similar question for Figure 6b): Apologies if this\nshould be obvious, but I'm not sure what is meant by the wind leads warm events\nby 4-8 days with respect to the figure. Is the plotted regression for a specific lead\ntime (e.g. 6 days)? Is it a regression computed over several different lead times\nfrom 4 to 8 days. Is the plot showing the regression for the lead time with the\nmaximum correlation at each point with (potentially) a different lead time at each\nlocation?\nResponse: We calculated the regression between the zonal wind speed and the\nwarm events for several different lead times, respectively, and found that the\nregression shows a statistically significant (above the 95% confidence level) effect\non eddy events with a lead-time of 4-8 days in the transition area. Then we use the\nmean regressions over different lead times (4, 5, 6, 7, 8 days) to show the\nrelationship between the zonal wind speed and warm events (Figure 6a).\n20. Lines 231-233: I agree that this analysis suggests a relation between increased\nwesterlies and changes in the wind stress curl with warm intrusions. However,\nhave the authors shown a relationship with the poleward shift of the westerlies?\nI realize the poleward shift will change the curl, but I don't know if that has been\nexplicitly demonstrated at this point in the manuscript.\nResponse: Here we mainly concentrated in the regional processes that increased\nthe westerlies and the changes in the wind stress curl driving warm eddy\nintrusions, rather than connecting them with long-term changes in the large-scale\nwinds. The zonal wind trend in austral summer (Figure 6c) is only used for\ndiscussion: the poleward shift winds during the 1979-2018 periods might have\ndriven more warm-eddy intrusion into the Prydz Bay.\n21. Line 233: Suggest adding \"the\" before \"1979-2018 period\".\nResponse: Agreed.\n22. Lines 237-238: Is there a paper or any data showing if the melt rate of the Amery\nIce Shelf has been increasing over some portion of the last 40 years?\nResponse: Smith et al. (2020) provided unified estimates of Antarctic ice mass\nchange from 2003 to 2019 using satellite laser altimetry. Their work showed that\nthe northern part of Amery Ice Shelf was losing mass at an increasing rate (~1\nm/year) during the 2003-2019 periods. We have now cited the paper in our\nmanuscript.\n23. Line 256: Most of these references are looking at the eddies off the continental\nshelf. Do any of these actually discuss \"eddy-mediated transport of CDW across\nthe Antarctic Shelf Break\"? If not, suggest adding '33' to the references here, since\nthat one does explicitly look at eddy transport across the shelf break.\nResponse: Agreed. We have added the reference here.\n24. Lines 260-261: Is \"seasonally enhanced\" the best wording? The authors do show\nthat the summer winds and wind stress curl are increasing in time. However,\n\"seasonally enhanced\" implies (to me anyway) that the winds are stronger in the\nsummer than during the rest of the year and that is why the intrusions come in\nduring the summer. There is no discussion of the summer wind strength vs. other\nseasons (apologies if there is and I missed it) and the authors show (lines 171-182)\nthat the seasonality of the intrusions is driven by the seasonality of the\ntemperature gradient.\nResponse: Thanks. We have replaced \"seasonally enhanced westerly winds\" with\n\"enhanced summer westerly winds\" to specify the season we focused on.\n25. Line 283: Suggest changing \"role of eddies play\" to either \"role of eddies\" or \"role\neddies play\".\nResponse: Agreed. We have changed \"role of eddies play\" to \"role of eddies\".\n26. Lines 286-287: These are not the first observations of persistent eddies across an\nAntarctic shelf break: Moffat et al., 2009 (referenced in this manuscript),\nMartinson and McKee, 2012 (also referenced here), and Couto et al., 2017 all\nhave nice detailed observations of eddy transport of warm mCDW across an\nAntarctic shelf. What might be different here is that all those observations were on\nthe WAP, where the ASF does not exist. There are other locations where the ASF\nexists where authors have observationally studied the impact of eddies\ntransporting mCDW onto the shelf (Nost et al., 2011; Foppert et al., 2019), but I\ndon't think they had the fine temporal resolution of measurements this study does.\nI think the authors need to be a little more specific than just stating that this is the\n\"first observations of persistent eddies across an Antarctic shelf break\".\nResponse: Agreed. We specified the text as \"Our two-year unique subsurface\nmooring and hydrographic section observations across 2013-2015 reveal the\nessential role that eddies play in mCDW intrusions onto the continental shelf of\nPrydz Bay\".\n27. Line 292: Suggest changing \"likely increase\" to \"likely to increase\".\nResponse: Agreed.\n28. Figure 1 caption, Line 522: Are the authors sure reference 17 (Jacobs, 1991) is the\ncorrect one for the polynya definitions?\nResponse: Apologies for the mistake. The reference should be 23 (Williams et al.,\n2016) here.\n29. Extended Data Fig. 3: What are the units for the wind anomaly vectors and the\nSLP anomaly contours?\nResponse: The units for the wind anomaly vectors and the SLP anomaly contours\nare (m/s) and (hpa) respectively.\n30. Extended Data Fig. 4: Why are there are so many more data points in Figure 4a vs.\n4b? Both are using the same set of warm events, correct?\nResponse: We show that the poleward shift in the westerlies and associated\ncyclonic wind stress curl promotes these eddy-induced intrusions. Figure 4b\nshows the data points with cyclonic wind stress curl, without the data points with\nanticyclonic wind stress curl.\nPoint-by-point response to Reviewer #2:\nReviewer #2 (Remarks to the Author):\nThe manuscript by Dr. Gao and collaborators presents evidence of seasonal warm\nwater intrusions in the Prydz Bay channel based on temperature observations from a\n2-year moored record and hydrographic sections collected in the vicinity of the\nmooring 2 years apart. Con-current velocity observations from the mooring further\nshow warm intrusions to be associated with either cyclonic and anticyclonic eddies.\nThe authors explained the presence/absence of this warm intrusions is linked to two\nfactors: (1) local westerly winds and the associated negative wind stress curl\nupwelling lifting CDW onto the continental shelf, (2) the seasonal cycle of surface\nbuoyancy loss eroding the warm temperature signal despite the eddies being present\nyear-round.\nThe authors have done a great job extracting information from the in-situ record and\ncombining their results with the remote sensing analysis. I believe results presented in\nthis paper will be of interest to the Nature Communications readership. But the\nmanuscript still needs a fair bit of work before it can be published. The introduction\nand discussion lack focus, there are a number of arguments that are unclear or\nincomplete, and there is missing information throughout the manuscript. Detailed\ncomments are provided below.\nMajor comments\n31. The introduction and parts of the discussion need to be brought into focus. The\nauthors spend a significant amount of text discussing the response of the ACC to\nchanges in westerly winds. While the ACC and the ASC may share some\nsimilarities in terms of their eddy overturning circulations, they are distinct\nfeatures of the Southern Ocean circulation and to my knowledge there is no direct\nlink between increased eddy activity in the ACC and increase eddy shedding of\nthe ASC. If the authors feel there is such link, they need to articulate it much more\nclearly. Furthermore, the analysis the authors present is based on a relatively short\ntime series, and while it adds to our understanding of the processes that govern\ncross-shelf exchange, it can’t in itself address the question of long-term changes\nin the system (see also comment 3).\nResponse: Agreed. We have re-crafted the ‘Introduction’ to focus on warm water\nintrusions and increased eddy shedding of the ASC associated with the poleward\nintensification of the westerlies. It is all about reinforcing the main messages to\nmake it easier for the reader. Now it is much leaner, more focused and streamlined.\nWe have revised our manuscript accordingly with changes shown in green color.\nWe agree that it is better to concentrate in the processes observed at a regional\nscale. Indeed, we mainly focus on the eddy formation and transport on to the\nPrydz Bay continental shelf driven by regional wind forcing based on subsurface\nmooring and hydrological section data from 2013-2015. Figure 5, Extended Data\nFig. 5 and Figures 6a-b show the influence of zonal winds as a potential physical\nmechanism for the warm-eddy intrusions in Prydz Bay region. Figures 6c-d and a\nfew additional sentences in our manuscript are provided for speculation that the\nregional changes in westerly winds in past decades would drive more warm eddies\ntowards the front of the Amery Ice Shelf.\n32. The introduction could also spend more time framing the question about\ndelivering heat to the Antarctic margin in the context of East Antarctica\nspecifically. A lot of the existing literature on the topic of cross-shelf exchange,\nincluding modelling and observational evidence of the role eddies play, is based\non the West Antarctic Peninsula and the Amundsen Sea. The margins around East\nAntarctica are significantly different with primarily warm shelves in the west, and\nweak or missing ASF, and cold/dense in the East, with a well-defined ASF.\nLast with regards to the introduction I would encourage the authors to be more\nselective on the references they used. Throughout the introduction the authors cite\nlong lists of papers on relation to one topic, without explicitly describing what the\nfindings are, and at times including references that are not really appropriate for\nthe topic. For instance, in line 43 the authors say “have captured this process”\nfollowing a discussion about the role of the ASF as a barrier to cross-shelf\ntransport, while the references that follow described either papers that focus on the\ncirculation and processes within the continental shelf, or in the case of the\nAmundsen Sea, the role of the undercurrent delivering heat to the shelf. Another\nclear example is found in lines 60-61. Other references that would be quite\nrelevant to the subject of eddy transport across the shelf-break, such as Stewart et\nal 2015, are in the reference list but are actually not cited in the text.\nResponse: Thanks for your suggestions. We have re-crafted the ‘Introduction’ to\nfocus on warm water intrusions and increased eddy shedding of the ASC\nassociated with the poleward intensification of the westerlies. As suggested, we\nhave added some text to introduce that the ASF is especially strong and\nwell-defined around the cold and dense East Antarctic margins rather than around\nthe primarily warm shelves in the west. We have described the topics in\n‘Introduction’ more explicitly and selected appropriate references for the topics.\nStewart et al (2015) has been cited in the manuscript.\n33. In my view, there are two processes that are tangled throughout the manuscript.\nThe first is the wind driven uplift of the warm CDW that delivers the water up to\nthe shelf break, and the speculated eddies that can then carry the warm water\nacross the shelf break. Collectively, these two processes deliver heat onto the\ncontinental shelf. The correlation analysis provides some evidence about the\nforcing mechanism behind the first (the uplifting of warm water). However, the\nmanuscript lacks a clear explanation for the second (the formation and evolution\nof the eddies).\nResponse: Thanks for your suggestion. As the response to point 3 above, we\nhave done the composition analysis of all warm events (Extended Data Fig. 5),\nwhich indicates that most of the mesoscale eddies that carry heat onto the continental\nshelf originate off the shelf due to wind forcing. Strong cyclones (negative wind\nstress curl anomaly) passed north of Prydz Bay in the beginning week (Extended\nData Fig.5b), driving surface water divergence by Ekman transport, resulting in a\ncontinuous decrease in SSH (2-5 cm) and increase cyclonic eddy formation near the\ncontinental slope (Extended Data Fig.5a). The intensified cyclonic eddy\nupwelled the warm mCDW and moved southward into the Prydz channel. The\npersistent positive westerly wind (Extended Data Fig.5c) increased the offshore\nEkman transport and associated upward Ekman pumping near the continental\nslope, which overcame the ASF barrier and drove the eddy onto the continental\nshelf. The westward ASF near the continental shelf break bent southward, influenced by\nthe topographic troughs, and subsequently the eddy deformed and traversed the\nASF barrier to transfer relatively warm waters to the shelf. Thus the\ncyclonic eddy formation and transport on to the continental shelf lasted almost\nthree weeks (Extended Data Fig.5a, Fig. 5a-f). This analysis suggests that the\nincreasing and poleward shifting westerlies, alongside the resulting cyclonic wind\nstress curl, are responsible for the warm-eddy intrusions (Extended Data Fig.5a-c).\n34. Throughout the manuscript the authors appear to link observed trends in the\nSouthern Ocean winds (figures 6 d and e) to warm water intrusions in Prydz bay.\nBut the manuscript provides no evidence of what the link is. The in situ\nobservations capture a seasonal process, and with just 2 years’worth of data it\nwould be quite challenging to detect a trend. And then in line 238 the authors\nmistakenly claim that the Amery Ice Shelf is thinning at a significant rate and cite\nthe Smith et al 2019!! When in fact Smith and collaborators conclude that the\nAmery has a relatively small height change, and it’s of mass gain not of mass loss.\nThe manuscript would be much more convincing if the authors simply\nconcentrated in the processes they observe, and how they operate at a regional\nscale, rather than try to connect them with long term changes in the large scale\nwinds.\nResponse: Thanks for your suggestion. As the response to point 31 above, we\nagree that it is better to concentrate in the processes observed at a regional scale.\nIndeed, we mainly focus on the eddy formation and transport on to the Prydz Bay\ncontinental shelf driven by regional wind forcing based on subsurface mooring\nand hydrological section data from 2013-2015. Figure 5, Extended Data Fig. 5 and\nFigures 6a-b show the influence of zonal winds as a potential physical mechanism\nfor the warm-eddy intrusions in Prydz Bay region. Figures 6c-d and a few\nadditional sentences in our manuscript are provided for speculation that the\nregional changes in westerly winds in past decades would drive more warm eddies\ntowards the front of the Amery Ice Shelf.\nFigure S3 from Smith et al. (2020) shows the Mass loss from Antarctica\n(2003-2019). The whole Amery has a relatively small height change. However,\nthe north part of Amery Ice Shelf is thinning at a significant rate, which likely\nconsumes most of the heat inflow taken by the Prydz Bay Gyre. We have\nrephrased the sentence as ‘Such changes would drive more warm eddies towards\nthe front of the Amery Ice Shelf (north part), which is thinning/losing mass at a\nsignificant rate (Smith et al., 2020)’.\nREDACTED\nFigure S3. Mass loss from Antarctica (2003-2019), From Smith et al. (2020)\nMinor comments\n35. Perhaps more appropriate than “Persistent” in the title would be “Seasonal”,\nas the intrusions are only observed in the summer time.\nResponse: We are inclined to emphasize the“Persistent”warm-eddy transport in\nsummer compared with “rare” warm-eddy transport in other seasons. The title\nshows the “Seasonal intrusion”by the words “driven by enhanced summer\nwesterlies”.\n36. Line 21. The word “prescribed” implies that this is the only factor controlling the\ndelivery of heat, when in reality the circulation within the shelf, buoyancy fluxes,\nand the geometry of the ice-shelf cavity are just as important. Word-count\npermitting, I would suggest the authors rephrase this first sentence in the abstract.\nResponse: We have rephrased the first sentence in the abstract as “The offshore\nocean heat supplied to the Antarctic continental shelf by warm eddies has the\npotential to greatly impact the melting rates of ice shelves and subsequent global\nsea level rise”.\n37. Line 78 – Mentioning the seal data gives the reader the impression it is another\nsource of data used in this study. Please remove.\nResponse: The seal data was indeed used in this study. Please see Fig. 4e and Fig.\n4f.\n38. Line 88-89- Here the intrusion is described as “weaker”, when later on with\nregards to figure 5, it’s considered as the “normal” conditions when the\nintrusion is absent. Please correct for consistency.\nResponse: We have changed the text accordingly.\n39. Line 103, the reader would benefit from a brief description (in relation to the\nrelevant literature cited) of how these events are detected.\nResponse: The details about the eddy detection are presented in the “Eddy flow\nestimation” part of the “Methods” section.\n40. Lines 108-109. Could the authors clarify how would the method would work in\nthe presence of a northward background flow? Wouldn’t a method based on the\nsequential detection of a pair of opposite sign velocity anomalies depend on the\ndirection of the background flow? Please clarify.\nResponse:\nEddy composites: We search for eddies with equation (1) in the method part of\nu\nour manuscript. Given that the observed horizontal flow can be recognized as\nu\nU\nthe sum of background flow and eddy flow e, the eddy flow then becomes\nu  u iv  (u U)ei(t)\ne e e (1)\n u v\nWhere is the direction of background flow, e and eare the along-stream\ncomponent and cross-stream component of the eddy flow, respectively. The\nU\nbackground flow is taken as the 5-day low-pass velocity (Moffat et al., 2009;\nMartinson and Mckee, 2012).\nu v\nPlease note that e and e are the along-stream component (along the\nbackground velocity) and cross-stream component (cross the background velocity)\nof the eddy flow, respectively. The background velocity has been accounted for\nwhen we calculated the eddy flow. The coordinate axis changes with the\nu v\nbackground flow. The sketch below shows the difference. All the e and eare\nin their own coordinate axis (X-Y). When framed this way, we can get the eddy\nvelocity no matter which direction it is propagating from. See figure (c) and (d),\nthe eddy velocity is relative to the rotation but without any relationship to the\nbackground flow. This quantitative analysis can be found in previous studies\n(Moffat et al., 2009; Martinson and Mckee, 2012).\n(1) Martinson, D., McKee, D., 2012. Transport of warm Upper Circumpolar Deep\nWater onto the western Antarctic Peninsula continental shelf. Ocean Sci. 8,\n433–442.\n(2) Moffat C, Owens B, Beardsley R C. On the characteristics of Circumpolar\nDeep Water intrusions to the west Antarctic Peninsula Continental Shelf [J].\nJournal of Geophysical Research Oceans, 2009, 114(C5).\nThe coordinate axis changes with the background flow (examples)\n41. Line 119, with regards to heat fluxes the reader would benefit from a brief\ndescription of how these fluxes are calculated. Estimating heat transported by and\neddy, derived from a single point eulerian measurement is not a trivial. This is due\nto the fact that in the time mean, the velocity of an eddy advected past a mooring\nsite is zero (that is for a perfectly symmetric eddy). Some other non-trivial\naspects of the calculation would be over which length scale to integrate the single\npoint measurement.\nResponse: The poleward heat flux is calculated based on the background flow (v\ncomponent) and the potential temperature anomalies (v component). The\nbackground flow is taken as the 5-day low-pass velocity. We have added a brief\ndescription of the heat fluxes calculation in the methods section.\n42. Lines 127-128. It’s unclear what the reference to Subantarctic Mode Water is here\nfor. Consider removing.\nResponse: Agreed. As the response to point 2 above, we have compared our\nestimates with the heat transport of mCDW to the Amery ice shelf\n(Herraiz-Borreguero et al., 2015). Assuming that all of the eddy heat is available\nfor melting the Amery Ice Shelf, the poleward heat flux associated with eddy\nintrusions (0.79±0.15 Sv) would result in an average basal melt rate of 3.1±0.7 m\nof ice per year (see Methods). This is roughly 50% greater than the previous\nestimate of annual basal melt rate (2.0±0.5 m/year) driven by the mCDW flux\ninto the Amery Ice Shelf cavity (Herraiz-Borreguero et al., 2015).\n43. Line 191. Typo: replace describes by describe.\nResponse: We have changed the text accordingly.\n44. Lines 192-194. Can the authors elaborate on how exactly they observe the\n“passing” cyclones in Figure 5? I suspect the main signal in the SSH anomaly\nfield has a larger footprint than the small domain being plotted. My guess is that if\nthe authors replotted the same six panels, removing the local mean value, all\npanels would look much more similar. This large-scale signal is superimposed on\nthe eddy pattern could be the response to the large storm the authors show in the\nextended data figure 3.\nResponse: Thanks for your suggestion. Figure S4 shows the summer mean SSH\ndistributions as the large-scale pattern. It is clear to see that the regional\nlarge-scale circulation is dominated by the westward Antarctic Slope Current\n(ASC) and the Prydz Bay Gyre (PBG). The strong westward ASC along the\ncontinental slope (~66.6°E) is very clear and stable. Summer mean SSH shows\nstrong meridional gradient along the continental slope where the strong westward\nASC exists.\nThe large-scale summer mean SSH pattern can show us the large-scale\ncirculation. However, only the evolution of the SSH anomaly can show us the\neddy formation and intrusion process onto the continental shelf (Fig. 5a-f). The\nlocal mean value (with high range) will dampen the eddy signals (relatively small\ncompared to the high range mean value). We agree that the large-scale signal\ncould be in response to the large storm shown in the extended data figure 4. But\nthe eddy formation is just the subsequent response to the large-scale signal\nchanges. We focus on the warm eddy formation and intrusion process, so it is\nbetter to use the SSH anomaly evolution here.\nAs shown in Figure 5a-f, the SSH anomaly evolution shows a typical eddy\nintrusion process onto the continental shelf. The intensified cyclonic eddy\nupwelled the warm mCDW and moved southward into the Prydz channel. The\nwestward ASF near the continental shelf break bent southward, influenced by the\ntopographic troughs, and subsequently the eddy deformed and traversed the\nASF barrier to transfer relatively warm waters to the shelf.\nFigure S4. Summer mean SSH distributions\n45. Lines 198-200. The authors should consider rescaling panels a-f in figure 5. Due\nto the small size of the vectors, and the dark shades of blue, it is quite hard to see\nwhat the authors describe here as the ASF bending southward and the eddies\ntraversing the ASF.\nResponse: Thanks for your suggestion. We have tried our best to make Figure 5 as\nclear as possible. The submission system possibly reduced the resolution of\nfigures automatically when generating the combined PDF file that was available\nfor review. The original figures are clear to see.\n46. Line 204. Could the authors clarify the mechanism for how surface divergence\ntriggers cyclonic eddies?\nResponse: The consistent cyclonic winds caused rapid surface divergence and a\ndecrease in SSH. When the cyclonic winds disappear, the surrounding waters tend\nto flow back towards center to allow the SSH to recover/re-flatten. Due to the\nCoriolis force, the flow rotates to the left and thus forms a cyclonic eddy. Wang et\nal. (2016) observed a three-dimensional structure of ocean cooling and eddy\nformation induced by Pacific tropical cyclones.\nWang, G., Wu, L., Johnson, N., & Ling, Z. (2016). Observed\nthree-dimensional structure of ocean cooling induced by Pacific tropical cyclones.\nGeophysical Research Letters, 43(14), 7632-7638. doi:10.1002/2016gl069605.\n47. Lines 222. What does the 3-7 day lag in parenthesis refer to? And why is it\ndifferent than the 4-14 day?\nResponse: Regression between the zonal wind speed and warm events shows there\nis a statistically significant (above the 95% confidence level) impact on eddy\nevents with a lead-time of 4-14 days. However, regression between the wind stress\ncurl and warm events shows significant effect on eddy events with a shorter\nlead-time of 3-7 days.\n48. Line 284. Typo. Please remove “the” (in “the Antarctica”).\nResponse: We have changed the text accordingly.\n49. At the time of review the link to the mooring/CTD didn’t work.\nResponse: We have updated the data link in the ‘Data availability’ section.\n50. Can the authors please supply DOI’s for the complete, revised reference list?\nResponse: The reference list has been prepared in the format required by Nature\nCommunications journal.\n51. Figure 1\n- Could the authors use a different marker type to show in the map which exact stations are plotted in\nthe sections in panels b-e. Having some markers along the top of the sections illustrating where the\ndata actually is would also help pick up features that may have result from interpolation.\n- Figure 5, is quite important to the authors arguments. Instead of plotting the small area around the\nstations in the dashed blue line, they could plot the domain used in Figure 5. It could help providing\ncontext to Figure 5.\nResponse: As suggested, we have added markers along the top of the sections in\nFigure 1b-e illustrating where the data actually is.\n52. - Figure 5, is quite important to the authors arguments. Instead of plotting the small area around the\nstations in the dashed blue line, they could plot the domain used in Figure 5. It could help providing\ncontext to Figure 5.\nResponse: Thanks for your suggestion. The domain used in Figure 5a-f is similar\nto that used in Figure 5h. This helps to show the eddy evolution and intrusion\nprocess more clearly.\n53. Figure 5\n- Vectors in the southern part of panels a-f are not distinguishable from the background. Could the\nauthors clarify why are different averaging boxes used for the different variables.\n- And in line 558, please clarify what the “southern boundaries of the ASF” and how are they specified.\n- Please, add units to panel g.\nResponse: The SSH averaging box is the eddy formation and transport region\nwhere it is the most representative, thus we can see the variation of the SSH\nanomaly clearly. The warm events are relative to the zonal wind anomaly in a\nlarge domain (see Figure 6a). Therefore, the averaging boxes are different.\n54. - And in line 558, please clarify what the “southern boundaries of the ASF” and how are they specified.\nResponse: Here we use the southern boundaries of the Antarctic Slope Front (ASF)\nto show the ASF poleward shift between 2013 and 2015 in Figure 5h. We\nspecified the southern boundaries of the ASF where the meridional gradient in\npotential density is equal 0.1 kg/m3/latitude and the water temperature is over -1.7\nºC, which represents the ASF boundaries well. Then we use the mean depth of the\nsouthern boundaries to describe the changes of the ASF between 2013 and 2015.\n55. - Please, add units to panel g.\nResponse: Thanks for your suggestion. The zonal wind anomaly, wind stress curl\nanomaly and SSH anomaly use the same y-axis label in panel g. Therefore, we list\nthe units in the figure captions.\n56. Figure 6\n- Correlations values in figure 6b seem odd. In Panel b these range from [-1,1] why [-4,4] in panel b?\nAnd could the authors clarify what exactly is plotted in these two panels? Is it the maximum\ncorrelation from a lagged-correlation calculation? The authors briefly mention the lag in the\nmanuscript. If this is the case, it would be useful to provide the corresponding figure with the lags,\nat least as supplementary material.\n- Also, is the correlation analysis carried out only for the summer months? It is not described in the\ntext nor in the figure caption. If it was only calculated using summer months, how did the authors\nhandle the lag? Please clarify. If in the other hand, the correlation included all months, then can the\nauthors explained how can they rule out the role of the seasonal buoyancy loss in the correlation (\nI would expect seasonal winds would correlate well with seasonal winter heat loss).\nResponse: Figure 6a shows the regression coefficients of warm events on zonal\nwind, and Figure 6b shows the regression coefficients of warm events with wind\nstress curl. Those are the maximum regression coefficients from a\nlagged-correlation calculation (wind leads warm events for 4-14 days). As\nsuggested, the square-mean correlation between lead levels in zonal wind and\nwarm events is shown in Figure S5.\nREDACTED\nFigure S5. Square-mean correlation between lead levels in zonal wind and warm\nevents (95% confidence level are shown in blue line)\n57. - Also, is the correlation analysis carried out only for the summer months? It is not described in the\ntext nor in the figure caption. If it was only calculated using summer months, how did the authors\nhandle the lag? Please clarify. If in the other hand, the correlation included all months, then can the\nauthors explained how can they rule out the role of the seasonal buoyancy loss in the correlation (\nI would expect seasonal winds would correlate well with seasonal winter heat loss).\nResponse: The warm events mainly occurred in the summer, so we calculated the\nregression for the summer months. We calculated the regression for each summer\nrespectively and then used the mean state to analyze the warm event processes.\nPoint-by-point response to Reviewer #3:\nReviewer #3 (Remarks to the Author):\nThis paper presents the observations of warm eddies with subsurface mooring near\nPrydz Bay. The eddy-induced heat transport is important to Antarctic ice shelf melt.\nThis is an interesting topic. This manuscript is well organized. However, further\nprocess studies are required to define the formation and recurrence of eddies on the\nmooring site and the characteristics of ASF flow and exchanges.\nComments:\n58. The eddies’characteristics are derived from the spiking patterns of temperature\ntime series and spiral velocity records. The salinity is also important to trace the\nwater mass of eddies. The T-S diagram might be useful to show the intrusion of\nwarm water and warm eddies.\nResponse: Thanks for the suggestion. We showed the hourly potential temperature\nat different depth levels (248m, 270m, 325m, 449m, and 520m) in figure 2a in our\nmanuscript. However, we only had one salinity sensor at 520m (SBE37). Figure\nS6 below shows the T-S diagram of the 520m CTD sensor during 2013-2015.\nAlthough the ‘warm event’ signal is small at the 520m level (near bottom), the\nT-S diagram still shows the intrusions of warm water clearly. Relatively warm\nmodified Circumpolar Deep Water (MCDW) (temperature > -1.87ºC, 34.49-34.57\npsu) intrusions occurred during the austral summer (November-April). During the\naustral winter (May-October), Winter Water (WW) (temperature < -1.9ºC,\n34.50-34.56 psu) reached the bottom layer due to the deep convection across the\nsea-ice growth season. Meanwhile, there were Shelf Water (SW) (temperature ~\n-1.89ºC, salinity > 34.58 psu) present in October. We have now added this T-S\ndiagram as one of the Extended Data Figures and updated the text in the\nmanuscript.\nFigure S6. T-S diagram of the 520m observations on the mooring during\n2013-2015. Colors represent calendar month. Black lines are the potential density\nsurface.\n59. Line 118-121: The poleward heat flux is calculated from the warm events. From\nFig.3c and Extended Data Fig.1c,d, the net cross-stream and meridional component of background velocity are\nvery small. It might be overestimated or underestimated without consideration of the path and\nmovement speed of eddies.\nResponse: We apologize for this typographical error. The units of heat flux we\ncalculated and the estimated heat gain from reference (Gao et al., 2018) should be\nw/m2 (not kw/m2).\n60. Line 133-134： the warm-eddy intrusion is assumed from the mCDW cross ASF.\nMore detailed numerical modeling will be needed to understand the exchanges\nbetween the eddy field and upwelling and cross-shelf transport. (Ref. 53 Liu et al’\ns model results are useful. )\nResponse: Based on an idealized eddy-resolving coupled ocean-ice shelf model,\nZhang et al. (2022) did a great job in investigating the characteristics of mesoscale\neddies generated over the continental slope in East Antarctica. In order to focus on\nmesoscale eddies generated by the ASC and exclude the sea surface influences,\ne.g., eddies generated by sea ice leads, the sea ice model and the atmospheric\nforcing are not included. Therefore, their idealized eddy-resolving coupled\nocean-ice shelf model maybe not suitable to revisit eddy intrusion driven by wind\nforcing.\nEven high-resolution eddy-resolving model analysis data cannot reproduce\neddy intrusion driven by wind forcing well. The simulated data cannot capture the\nupwelled warm eddy on to the continental shelf (Figure S7 and Figure S8), nor the\nsubsequent evolution of the SSH anomaly (Figure S9 compares with Figure 5a-f)\nwell. It is very challenging for numerical modeling to simulate detailed processes\naround the Antarctic coastal region, due to the commonality deviation in\nsimulations (e.g., sea ice, ice shelf changes, ice-sea-air interaction, and heat\ntransport). There is a critical need to implement eddy-resolution and associated\nprocesses into new-coupled modeling studies of the Southern Ocean.\nZhang L, Liu C, Sun W, Wang Z, Liang X, Li X and Cheng C (2022) Modeling\nMesoscale Eddies Generated Over the Continental Slope, East Antarctica. Front.\nEarth Sci. 10:916398. doi: 10.3389/feart.2022.916398.\nFigure S7. (a and b), In-situ potential temperature (shade) and salinity (white contours)\nfrom CTD transects in 2013 and 2015 along 73oE transect. (c and d) are similar to (a\nand b), but for eddy- and tide-resolving model HYCOM + NCODA Global 1/12°\nAnalysis data (GLBu0.08, https://www.hycom.org/dataserver/gofs-3pt0/analysis).\nFigure S8. (a and b), In-situ potential temperature (shade) and salinity (white contours)\nfrom CTD zonal transects in 2013 and 2015 along 67.25oS transect. (c and d) are\nsimilar to (a and b), but for eddy- and tide-resolving model HYCOM + NCODA\nGlobal 1/12° Analysis data.\nFigure 5a-f. Evolution of SSH anomaly (shade and contours, cm) and geostrophic flow\nanomaly (arrows) during 30-January~9-February in 2013. The mooring location\nis shown with red star. The 500m, 1000m, 2000m and 3000m bathymetric contours are\nshown in red.\nFigure S9. Similar to Figure 5a-f, but for the eddy- and tide-resolving model HYCOM\n+ NCODA Global 1/12° Analysis data.\n61. Line 188-195: Fig. 5a-f. The mooring is almost located at the center of the saddle\nshape of SSH. The rotation of the velocity at the mooring site was related to\nlarge-scale SSH oscillation. The mesoscale variability near mooring also could be\nfilaments.\nResponse: Figure S10 shows the summer mean current and temperature\ndistributions at the sea surface, 100m, 200m and 300m levels. It is clear to see that\nthe regional large-scale circulation is dominated by the westward Antarctic Slope\nCurrent (ASC) and the Prydz Bay Gyre (PBG). The strong westward ASC along the\ncontinental slope (~66.6°E) is very clear and stable at all depths shown.\nSummer mean SSH shows a strong meridional gradient along the continental\nslope where the strong westward ASC exists (Figure S4), but without the saddle\nshape of SSH mentioned by the reviewer. The saddle shaped SSH anomaly only\nexists during the warm-eddy events. As shown in Figure 5a-f, the SSH anomaly\nevolution shows a typical eddy intrusion process onto the continental shelf. The\nintensified cyclonic eddy upwelled the warm mCDW and moved southward into\nthe Prydz channel. The westward ASF near the continental shelf break bent southward,\ninfluenced by the topographic troughs, and subsequently the eddy deformed and\ntraversed the ASF barrier to transfer relatively warm waters to the shelf (Fig. 5a-f).\nSimilar to SSH, there are strong meridional gradients in temperature below\n100m along the continental slope (Figure S10, 200m and 300m figures). The ASF\nbends southward in the Prydz Channel and subsequent eddy formation can take\nthe warm water directly into Prydz Bay. However, there are no obvious zonal\ngradients in temperature below 100m near the mooring (Figure S10, 200m and\n300m figures), so that the large-scale east-west oscillation in SSH near the\nmooring cannot take warm water from the continental slope onto the continental\nshelf.\nFurthermore, in response to point 2 above, we have compared our estimates\nwith the heat transport of mCDW to the Amery ice shelf (Herraiz-Borreguero et\nal., 2015). The poleward heat flux associated with eddy intrusions (0.79±0.15 Sv)\nwould result in an average basal melt rate of 3.1±0.7 m of ice per year (see\nMethods). This is ~50% greater than the annual basal melt rate (2.0±0.5 m/year),\ndriven by mCDW flux into the Amery Ice Shelf cavity, estimated by previous\nstudy (Herraiz-Borreguero et al., 2015). This suggests that the eddy-induced heat\ntransport into Prydz Bay has an important impact on the mass balance of the\nAmery ice shelf.\nFigure S10. Summer mean current (arrows) and temperature (shade) distributions at\nsea surface, 100m, 200m, 300m levels from the eddy- and tide-resolving model\nHYCOM + NCODA Global 1/12° Analysis data. The mooring location is shown with\nred a star.\n62. Line 264-280: “We show that increasing and poleward shifting westerlies, in\nconjunction with the cyclonic wind stress curl, cause upward-tilting of the mCDW\nlayer that contribute to facilitating warm-eddy intrusions.” Both the cyclone and\nanticyclone eddies were observed. However, the mechanism of formation cyclone\nand anticyclone eddy is not clarified.\nResponse: Thanks for your suggestion. We have done the composition analysis of\nall warm events with anticyclonic eddies (Extended Data Fig. 6). Similar to\ncyclonic eddies (Extended Data Fig. 5), warm events with anticyclonic eddies that\ncarry heat onto the continental shelf are also associated with wind forcing. The\nwarm anticyclonic eddy events usually accompany cyclonic eddies (Extended\nData Fig. 6a). Strong cyclones (negative wind stress curl anomaly) passed north of\nPrydz Bay at the beginning of the observations (Extended Data Fig.6b), driving\nsurface water divergence by Ekman transport and resulting in a continuous\ndecrease in SSH and increase in cyclonic eddies formation near the continental\nslope. The intensified cyclonic eddy upwelled the warm mCDW and moved\nsouthward into the Prydz channel.\nThe westward ASF near the continental shelf break bent southward,\ninfluenced by the topographic troughs. It is worth noting that while the eddy scale\nis usually smaller than 10km (figure 4d), Prydz Bay channel is wider than 100km.\nSo there can be more than one meander when the ASF turns southward.\nSometimes both cyclonic and anticyclonic eddies form on different sides of the\nmeander (Figure S11). The persistent positive westerly wind (Extended Data\nFig.6c) increased the offshore Ekman transport and associated upward Ekman\npumping near the continental slope, which drove the eddies onto the continental\nshelf and overcame the ASF barrier. This analysis suggests that the increasing and\npoleward shifting westerlies, together with the resulting cyclonic wind stress\ncurl, are responsible for the warm-eddy (both cyclonic and anticyclonic eddies)\nintrusions (Extended Data Fig.5 and Extended Data Fig.6).\nExtended Data Fig. 6 | Composition analysis of warm events with anticyclonic\neddies. a, Evolution of SSH anomaly (averaged in the 70-74oE, 65.7-67.1oS\nregion, black line, unit: cm). b, Evolution of wind stress curl anomaly (averaged\nin the 65-85oE, 56-66oS region, red line, unit: 210-10 m·s2). c, Evolution of zonal\nwind anomaly (averaged in the 65-85oE, 56-66oS region, cyan line, unit: 3 m·s-1).\nRed lines are the ensemble mean. X-axis is days.\nFigure S11. In-situ temperature distribution at 200m observed during the summer\ncruise in 2013.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nReview of revised manuscript \"Persistent warm-eddy transport to Antarctic ice shelves driven by\nenhanced summer westerlies\", by Libao Gao, Xiaojun Yuan, Wenju Cai, Guijun Guo, Weidong Yu, Jiuxin\nShi, Fangli Qiao, Zexun Wei, and Guy D. Williams.\nGeneral Comments\nI thank the authors for their efforts in response to my earlier comments, including putting the heat\nflux from the eddies in a broader context with the new estimate of the Amery ice shelf melting due to\nheat transport by the eddies. However, I do have some questions about this estimate:\n1) Are the authors sure the listed area of mCDW (line 343) is correct? This looks too small compared\nto the extent of the black line in Figure 1d and if the area actually is 4.95e5 m^2, then the poleward\nmCDW flux (assuming a velocity of 1.6 cm/s) is actually << 0.79 Sv. Could the exponent be a typo\nand the area actually is 4.95e7 m^2? That looks closer to the extent in Figure 1d and would give a\nflux of 0.79 Sv.\n2) If the melt rate calculated is the extra melt due to extra heat from the eddies beyond an ambient\ntemperature of -1.8, then I do not understand the (T_f - T) term, where T \"is the mean temperature\nabove -1.8C of the mCDW\", in equation 3. Shouldn't that term just be -T (i.e. the excess temperature\nabove -1.8)? As written (and with this definition of T), I think this equation would be significantly off.\n3) If I'm wrong about point 2 above and T_f really does need to be in the calculation, then I think\nmore needs to be explained why a pressure freezing temperature at 345 dbar was chosen (as was\ndone in Herraiz-Borrequero et al. where this equation was introduced).\n4) As in Herraiz-Borreguero et al., the area used for the melt rate is not the entire Amery ice shelf,\nbut rather is the portion of the ice shelf where H-B et al. thought mCDW can potentially drive basal melt.\nThat is briefly mentioned here in the Methods section (lines 357-358), but should be mentioned in the\nmain text (lines 140-145) where the melt from the eddies is presented and compared to the H-B et al.\nestimate.\nI have several other new questions/comments, but the vast majority of these are very minor. I still\nthink some minor revision is necessary before publication, but I believe all these comments can be\neasily addressed by the authors.\nSpecific Comments\nLine 49: Do the authors mean that there are significant differences between various regions in East\nAntarctica or that the East Antarctic regions \"differ significantly\" compared to the West Antarctic\nregions?\nLine 50: Should \"warm\" be defined?\nLine 51: Suggest adding \"waters on the\" before \"shelves are cold and dense\".\nLines 64-66: Are all these references (36-41) appropriate for this sentence? While these studies do\nshow mesoscale eddies transferring CDW poleward, most of these are not eddy resolving over the\nAntarctic continental shelf and I think Stewart and Thompson (apologies if I'm wrong) is the only one\nthat could show eddies \"overcoming the barrier imposed by the ASF\" and transferring \"mCDW\ntowards the Antarctic ice shelves\".\nLine 107: \"enable\" should be \"enables\".\nLines 140-143: See comment above about what portion of the ice shelf this calculation is for. Perhaps\njust add something like \"over the portion of the ice shelf thought to be subject to melting from\nmCDW\" right after \"ice per year\".\nLine 144: Need to mention here as well that the 2.0 m/year melt estimate is only over a portion of the\nAmery Ice Shelf (same area as for the authors' calculation).\nLine 179: Can these chord sizes be used to come up with an estimate of the full size of the eddies and\nthen compare that to what is seen in the altimetry?\nLine 182: The authors answered my previous question about the Rossby deformation radius in this\narea in their Response letter (Item 16), but I didn't see that the value (3-4 km) was added to the\nmanuscript anywhere.\nLine 219: Suggest changing \"help understand\" to \"help explain\".\nLines 236-247 and Extended Data Figures 5 and 6: I just wanted to thank the authors for these\nadditions: I really like the new figures. Have the authors thought about doing one like Extended Data\nFigure 6 for the warm events with cyclonic eddies?\nLines 263-264: I think the authors can change \"is likely to have occurred\" to \"has occurred\". Don't\nFigure 6d and Extended Data Figures 8 and 9 show that it did actually happen?\nLine 300: \"infer\" should be \"imply\".\nLines 348-350: Suggest a slight rewrite of the first part of sentence to something like:\nFollowing a previous study(^54), the annual basal melt rate of the portion of the Amery Ice Shelf\ndirectly impacted by mCDW inflow due to the eddy heat transport is...\"\nFigure 1 caption (lines 591-592): Suggest moving the last sentence about how the\npolynyas are shown to just after \"cover the Prydz Channel\" (line 587).\nFigure 3 caption (line 607): Suggest adding \"(x-axis in a-f)\" just after \"The time\".\nFigure 4 caption (line 614): Should the denominator for the units for temperature and salinity be \"deg\nlatitude\" instead of just \"latitude\"?\nFigure 4 caption (line 615): Should \"across\" be \"along\" here? I don't know if the authors agree, but I\ntend to think of this channel being more in the north-south direction and thus a latitudinal\ndisplacement being \"along\" channel.\nFigure 6: I think the information the authors provided in their Response letter (Item 19) to my\nprevious question about how the regressions in 6a and 6b are plotted should be added somewhere\n(either in the Figure caption or the text).\nReviewer #2 (Remarks to the Author):\nFirst of all, I would like to thank the authors for their very detailed response to my earlier comments.\nI appreciate they have put a significant amount of effort in addressing the issues I raised. In general\nI’m quite happy with their responses, and by all means, I would like to see their manuscript published.\nHowever, there are still a number of lingering issues, I think need to be addressed before the paper\ncan be accepted. These are aspects of the paper that in my opinion remain highly speculative, and the\nauthors would need to provide further evidence supporting them if they were to remain in the\nmanuscript.\n1. First is the eddy formation mechanism proposed by the authors. Mesoscales eddies being formed in\nresponse to large negative vorticity imparted by the passing of a storm is a new concept, and it needs\nto be backed by more evidence. The reference the authors provide in the response to my previous\ncomments (Wang et al., 2016), does not describe the eddy formation mechanism they propose, it\nsimply illustrates the effects of a tropical cyclone in the temperature distribution within the mixed\nlayer (a quick search of the word “eddy” in that paper yielded 0 matches, and all the observational\ndata in the paper is restricted to the top 50m of the water column at most). The eddies the authors\nare describing in this paper don’t appear to have much of a surface signature, instead they show as\nanomalies in the subsurface properties. There is ample literature on the subject of tropical cyclones\ninteracting with mesoscale eddies, but the formation of the latter is always attributed to baroclinic\ninstability. And note, that the mechanism they proposed is also inconsistent with Extended Data figure\n5, where they show SSH continues to decrease in the region (panel a), for approximately 3 weeks\nafter the storm has passed (panel b).\n2. The second aspect I remained unconvinced is the fact that the eddy-like features the authors show\nin the snapshots of SSH anomalies have length scales that are one order of magnitude larger\n(O(100km)) than the length scales the authors estimate from the mooring record (O(10km)). To me\nthis scale difference suggests the oscillations captured by the mooring record are different in nature to\nthose observed offshore, and likely the result of different processes. The connection between these\ntwo types of eddies relies, according to the authors, relies on the offshore eddies being advected by\nthe Ekman transport. Which is something that again, I don’t feel they provide strong evidence or a\nsound dynamical reasoning for.\n3. And lastly, with regards to the discussion on the long-term changes in the wind field, the authors\nprovide convincing evidence of how storms passing over the region immediately offshore of the Prydz\nBay channel can uplift isopycnals and drive intrusions of warm water across the shelf break. I can’t\nhowever see how the predicted strengthening and shift in the westerlies alone could result in\nenhanced CDW intrusions, unless these long-term changes in the westerlies are also\naccompanied by either stronger or more frequent/persistent storms in the region. Note that the features\ncapture in panel b in Extended figure 5 have typical duration of ~5days. While the data in Extended Data Fig 9 is\ngiven as annual means, and hence unable to capture a possible increase in \"storminess\".\nWe have revised our manuscript accordingly with changes to the revised manuscript\nshown in green italics.\nPoint-by-point response to Reviewer #1:\nReviewer #1 (Remarks to the Author):\nReview of re-revised manuscript \"Persistent warm-eddy transport to Antarctic ice\nshelves driven by enhanced summer westerlies\", by Libao Gao, Xiaojun Yuan, Wenju\nCai, Guijun Guo, Weidong Yu, Jiuxin Shi, Fangli Qiao, Zexun Wei, and Guy D.\nWilliams.\nGeneral Comments\nI thank the authors for their efforts in response to my earlier comments, including\nputting the heat flux from the eddies in a broader context with the new estimate of the\nAmery ice shelf melting due to heat transport by the eddies. However, I do have some\nquestions about this estimate:\n1. Are the authors sure the listed area of mCDW (line 343) is correct? This looks too\nsmall compared to the extent of the black line in Figure 1d and if the area actually\nis 4.95e5 m^2, then the poleward mCDW flux (assuming a velocity of 1.6 cm/s) is\nactually << 0.79 Sv. Could the exponent be a typo and the area actually is 4.95e7\nm^2? That looks closer to the extent in Figure 1d and would give a flux of 0.79\nSv.\nResponse: Yes, the area of mCDW intrusion into the Prydz Channel should be\n~4.95107 m2. We apologize for this typographical error.\n2. If the melt rate calculated is the extra melt due to extra heat from the eddies\nbeyond an ambient temperature of -1.8, then I do not understand the (T_f - T) term,\nwhere T \"is the mean temperature above -1.8C of the mCDW\", in equation 3.\nShouldn't that term just be -T (i.e. the excess temperature above -1.8)? As written\n(and with this definition of T), I think this equation would be significantly off.\nResponse: To clarify, we have changed the text to \"T is the mean temperature of\nthe mCDW (waters with temperatures over -1.8 ºC)\". In another words, we\ndefined the waters with temperatures over -1.8 ºC to be the mCDW intrusions by\neddies. And we assume that all of those intrusions can be transported into the\nAmery Ice Shelf cavity for basal melt (i.e. mCDW interacts with the base of the\nAmery Ice Shelf). Thus, the T-T term is the temperature difference between the\nf\npressure freezing temperature at 345 dbar and the temperature of the mCDW\ninflow, which supplies extra heat for basal melt. Thereafter equation 3 is used to\ncalculate the annual basal melt rate of the portion of the Amery Ice Shelf directly\nimpacted by mCDW inflow due to the eddy heat transport. The same equation was\nused in Herraiz-Borrequero et al. (2015).\n3. If I'm wrong about point 2 above and T_f really does need to be in the calculation,\nthen I think more needs to be explained why a pressure freezing temperature at\n345 dbar was chosen (as was done in Herraiz-Borrequero et al. where this\nequation was introduced).\nResponse: Agreed. Following Herraiz-Borrequero et al. (2015), we have explained\nwhy a pressure freezing temperature at 345 dbar was chosen. The text has been\nupdated as \"T is the pressure freezing temperature (-2.14 ºC) at 345 dbar (the\nf\nmean ice shelf draft for the area of the ice shelf base where mCDW can\npotentially drive basal melt)\".\n4. As in Herraiz-Borrequero et al., the area used for the melt rate is not the entire\nAmery ice shelf, but rather is the portion of the ice shelf where H-B et al. thought\nmCDW can potentially drive basal melt. That is briefly mentioned here in the\nMethods section (lines 357-358), but should be mentioned in the main text (lines\n140-145) where the melt from the eddies is presented and compared to the H-B et\nal. estimate.\nResponse: Agreed. The main text has been updated as \"Assuming that all of the\neddy heat is available for basal melting of the area beneath the Amery Ice Shelf\nbasal melt that the mCDW can potentially access, the poleward heat flux\nassociated with the eddy intrusions (0.79±0.15 Sv) would result in an average\nbasal melt rate of 3.1±0.7 m of ice per year over this section of the ice shelf (see\nMethods)\".\nI have several other new questions/comments, but the vast majority of these are very\nminor. I still think some minor revision is necessary before publication, but I believe\nall these comments can be easily addressed by the authors.\nSpecific Comments\n5. Line 49: Do the authors mean that there are significant differences between\nvarious regions in East Antarctica or that the East Antarctic regions \"differ\nsignificantly\" compared to the West Antarctic regions?\nResponse: We mean that the East Antarctic regions differ significantly compared\nto the West Antarctic regions. To clarify, we have changed the sentence to ‘It is\nimportant to note that the characteristics of the continental shelf regions\nsurrounding East Antarctica differ significantly from the West Antarctic regions’.\n6. Line 50: Should \"warm\" be defined?\nResponse: Agreed. Following Thompson et al. (2018), the warm shelves are\ndefined where the seafloor of the continental shelf is occupied by waters with\ntemperatures exceeding 0 ºC. We have changed the text as ‘In the western areas,\nthere are primarily warm shelves (bathed in waters with temperatures exceeding 0\nºC) …’ Thompson et al. (2018) is in our references list.\n7. Line 51: Suggest adding \"waters on the\" before \"shelves are cold and dense\".\nResponse: We have changed the text accordingly.\n8. Lines 64-66: Are all these references (36-41) appropriate for this sentence? While\nthese studies do show mesoscale eddies transferring CDW poleward, most of\nthese are not eddy resolving over the Antarctic continental shelf and I think\nStewart and Thompson (apologies if I'm wrong) is the only one that could show\neddies \"overcoming the barrier imposed by the ASF\" and transferring \"mCDW\ntowards the Antarctic ice shelves\".\nResponse: Yes, Stewart and Thompson (2015) is the only reference that refers to\neddies \"overcoming the barrier imposed by the ASF\" and transferring \"mCDW\ntowards the Antarctic ice shelves\". Other references cited here show mesoscale\neddies transferring CDW poleward in the southern open ocean, but do not mention\neddies overcoming the barrier imposed by the ASF and transferring mCDW\ntowards the Antarctic ice shelves. We only keep the Stewart and Thompson (2015)\npaper cited here as suggested.\n9. Line 107: \"enable\" should be \"enables\".\nResponse: Agreed.\n10. Lines 140-143: See comment above about what portion of the ice shelf this\ncalculation is for. Perhaps just add something like \"over the portion of the ice\nshelf thought to be subject to melting from mCDW\" right after \"ice per year\".\nResponse: We have changed the text accordingly.\n11. Line 144: Need to mention here as well that the 2.0 m/year melt estimate is only\nover a portion of the Amery Ice Shelf (same area as for the authors' calculation).\nResponse: Agreed. We have indicated that both calculations are for the same area\nof the Amery Ice Shelf in the revised manuscript.\n12. Line 179: Can these chord sizes be used to come up with an estimate of the full\nsize of the eddies and then compare that to what is seen in the altimetry?\nResponse: It is difficult to estimate the full size of eddies based on the chord sizes,\nbecause it is hard for us to confirm whether the chords cross through the centers of\neddies or not (assuming eddies are circular in the horizontal plane). The size of an\neddy must be greater than or equal to its chords, but it is not related to its chords.\nFor a given chord, the eddy sizes can be very different (see Figure S1 below). It is\nbetter to see the full size of eddies from altimetry data.\nFigure S1. Possible eddy sizes for a chord\n13. Line 182: The authors answered my previous question about the Rossby\ndeformation radius in this area in their Response letter (Item 16), but I didn't see\nthat the value (3-4 km) was added to the manuscript anywhere.\nResponse: We have added the values in the new revised manuscript.\n14. Line 219: Suggest changing \"help understand\" to \"help explain\".\nResponse: We have changed the text accordingly.\n15. Lines 236-247 and Extended Data Figures 5 and 6: I just wanted to thank the\nauthors for these additions: I really like the new figures. Have the authors thought\nabout doing one like Extended Data Figure 6 for the warm events with cyclonic\neddies?\nResponse: Great idea. We have done this before. The figure for the warm events\nwith cyclonic eddies is quite similar to the figure for all warm events (Extended\nData Figures 5), in that most of the warm events are cyclonic eddies. Therefore, it\nmight be enough to keep Extended Data Figures 5 and 6 here.\n16. Lines 263-264: I think the authors can change \"is likely to have occurred\" to \"has\noccurred\". Don't Figure 6d and Extended Data Figures 8 and 9 show that it did\nactually happen?\nResponse: Agreed. Those figures show that it did occur. We have changed the text\naccordingly.\n17. Line 300: \"infer\" should be \"imply\".\nResponse: Agreed.\n18. Lines 348-350: Suggest a slight rewrite of the first part of sentence to something\nlike: Following a previous study(^54), the annual basal melt rate of the portion of\nthe Amery Ice Shelf directly impacted by mCDW inflow due to the eddy heat\ntransport is...\"\nResponse: Agreed. We have changed the text accordingly.\n19. Figure 1 caption (lines 591-592): Suggest moving the last sentence about how the\npolynyas are shown to just after \"cover the Prydz Channel\" (line 587).\nResponse: Agreed. We have changed the text accordingly.\n20. Figure 3 caption (line 607): Suggest adding \"(x-axis in a-f)\" just after \"The time\".\nResponse: Agreed. We have changed the text accordingly.\n21. Figure 4 caption (line 614): Should the denominator for the units for temperature\nand salinity be \"deg latitude\" instead of just \"latitude\"?\nResponse: Agreed. We have changed the units to \"ºC/degree latitude\" and\n\"psu/degree latitude\".\n22. Figure 4 caption (line 615): Should \"across\" be \"along\" here? I don't know if the\nauthors agree, but I tend to think of this channel being more in the north-south\ndirection and thus a latitudinal displacement being \"along\" channel.\nResponse: Agreed. We have changed the text accordingly.\n23. Figure 6: I think the information the authors provided in their Response letter\n(Item 19) to my previous question about how the regressions in 6a and 6b are\nplotted should be added somewhere (either in the Figure caption or the text).\nResponse: Agreed. We have added a sentence in the text to clarify how the\nregressions are plotted.\nPoint-by-point response to Reviewer #2:\nReviewer #2 (Remarks to the Author):\nReviewer #2 (Remarks to the Author):\nThe manuscript by Dr. Gao and collaborators presents evidence of seasonal warm\nwater intrusions in the Prydz Bay channel based on temperature observations from a\n2-year moored record and hydrographic sections collected in the vicinity of the\nmooring 2 years apart. Con-current velocity observations from the mooring further\nshow warm intrusions to be associated with either cyclonic and anticyclonic eddies.\nThe authors explained the presence/absence of this warm intrusions is linked to two\nfactors: (1) local westerly winds and the associated negative wind stress curl\nupwelling lifting CDW onto the continental shelf, (2) the seasonal cycle of surface\nbuoyancy loss eroding the warm temperature signal despite the eddies being present\nyear-round.\nThe authors have done a great job extracting information from the in-situ record and\ncombining their results with the remote sensing analysis. I believe results presented in\nthis paper will be of interest to the Nature Communications readership. But the\nmanuscript still needs a fair bit of work before it can be published. The introduction\nand discussion lack focus, there are a number of arguments that are unclear or\nincomplete, and there is missing information throughout the manuscript. Detailed\ncomments are provided below.\nMajor comments\n24. First is the eddy formation mechanism proposed by the authors. Mesoscales\neddies being formed in response to large negative vorticity imparted by the\npassing of a storm is a new concept, and it needs to be backed by more evidence.\nThe reference the authors provide in the response to my previous comments\n(Wang et al., 2016), does not describe the eddy formation mechanism they propose,\nit simply illustrates the effects of a tropical cyclone in the temperature\ndistribution within the mixed layer (a quick search of the word “eddy” in that\npaper yielded 0 matches, and all the observational data in the paper is restricted to\nthe top 50m of the water column at most). The eddies the authors are describing in\nthis paper don’t appear to have much of a surface signature, instead they show as\nanomalies in the subsurface properties. There is ample literature on the subject of\ntropical cyclones interacting with mesoscale eddies, but the formation of the latter\nis always attributed to baroclinic instability. And note, that the mechanism they\nproposed is also inconsistent with Extended Data figure 5, where they show SSH\ncontinues to decrease in the region (panel a), for approximately 3 weeks after the\nstorm has passed (panel b).\nResponse: Based on classical Ekman layer theory, a simple analytical solution of a\nmesoscale eddy induced by a stationary hurricane in a homogenous ocean was\ndiscussed in a previous study (e.g., Lu and Huang, 2010). They described the eddy\nformation mechanism in response to large negative vorticity imparted by the\npassing of a storm. In the upper ocean, strong wind stress curl drives radial\noutward Ekman transport, resulting in a SSH decrease. Because of the continuity\nof mass, there must be a strong upwelling underneath the center of the hurricane\nto supply this mass flux. In the subsurface layer immediately below the surface\nlayer, the frictional force is assumed to be negligible, and the flow is primarily\ncyclo-geostrophic, thus a cyclonic mesoscale eddy formed in the subsurface layer\n(see Figure S2 below). We have now cited Lu and Huang (2010) in the revised\nmanuscript.\nAbout Extended Data figure 5, please note that the evolution of the SSH\nanomaly (panel a) is averaged in the region (70-74oE, 65.7-67.1oS) over the\ncontinental shelf break and the Prydz Bay channel where eddies intrude onto the\ncontinental shelf. And the evolution of the wind stress curl anomaly (panel b) is\naveraged in the region (65-85oE, 56-66oS) further north to the open ocean where\nstorms usually passed. Both the eddy formation and southward transport need a\nfew days’ time. The storm-induced eddy forms in the open ocean where the storm\npassed through, and it is driven southward to the continental shelf break by the\nwesterly winds. The intensified cyclonic eddy upwells the warm mCDW and it\nmoves southward into the Prydz channel. Then the westward ASF near the\ncontinental shelf break bent southward, influenced by the topographic troughs,\nand subsequently the eddy deformed and traversed the ASF barrier onto the shelf\n(Figures 5a-f). Figures 5a-f and Extended Data Figure 4 show that two strong\ncyclones passed eastwards, north of Prydz Bay, between 26-30 January 2013 (5\ndays). Thereafter the eddy formed, intensified and move southward between 30\nJanuary-5 February 2013 (6 days). The eddy then deformed with the ASF,\nbending into the topographic troughs between 6-9 February 2013 (4 days). The\nentire process for this eddy formation and transport takes more than two weeks.\nMoreover, in a manner similar to Figures 5a-f and Extended Data Figure 4,\nFigure S3 and Figure S4 show another case of eddy formation and intrusion onto\nthe continental shelf driven by wind forcing, which also took approximately two\nweeks. In addition, new storms may occur again before the eddy transportation is\nfinished, so the SSH can continue to decrease for much longer time.\nLu Z. & Huang R. The three-dimensional steady circulation in a homogenous\nocean induced by a stationary hurricane. J. Phys. Oceanogr. 40(7), 1441-1457\n(2010).\nREDACTED\nFigure S3. Evolution of SSH anomaly (shade and contours, cm) and geostrophic flow\nanomaly (arrows) during 20-January~28-January in 2014. The mooring location is\nshown with red star. The 500m, 1000m, 2000m and 3000m bathymetric contours are\nshown in red.\nFigure S4. Evolution of wind forcing anomaly during 16-January~27-January in 2014.\n(wind stress curl anomaly, shade, unit: 10-10 m·s2; wind anomaly, arrows; Sea Level\nPressure anomaly, contours).\n25. The second aspect I remained unconvinced is the fact that the eddy-like features\nthe authors show in the snapshots of SSH anomalies have length scales that are\none order of magnitude larger (O(100km)) than the length scales the authors\nestimate from the mooring record (O(10km)). To me this scale difference suggests\nthe oscillations captured by the mooring record are different in nature to those\nobserved offshore, and likely the result of different processes. The connection\nbetween these two types of eddies relies, according to the authors, relies on the\noffshore eddies being advected by the Ekman transport. Which is something that\nagain, I don’t feel they provide strong evidence or a sound dynamical reasoning\nfor.\nResponse: Firstly, the length scales estimated from the mooring record are the\neddy chords (assuming eddies are circular in the horizontal plane). Somewhat\nsimilar to point 12 raised by Reviewer #1, it is hard for us to confirm whether the\nchords cross through the centers of the eddies or not. Indeed, the overwhelming\nmajority of eddy chords do not. So the length scales estimated from the mooring\nrecord must be smaller than the length scales seen from the SSH anomalies.\nSecondly, the mooring record used to estimate the eddy chord length is\nobserved by a current meter located at 270m (Figure 2b and Figure 3), so the\nlength scales estimated from the mooring record are the eddy chords at the 270m\ndepth layer. Since the three-dimensional structure of eddies are vertical-taper\nstructures (Figure 3d-f), the length scales estimated at 270m must be much smaller\nthan the length scales at the sea surface seen from the SSH anomalies.\nThirdly, there are multiple troughs in the topography of the Prydz Bay channel\n(see Figure 1a). The westward ASF near the continental shelf break bends\nsouthward, influenced by these troughs. There can be multiple meanders when the\nASF turns southward, with both cyclonic and anticyclonic eddies formed on\ndifferent sides of the meanders. So the length scales of the eddies estimated from\nthe mooring record could be much smaller than the length scales seen from the\nSSH anomalies offshore.\nFourthly, the mooring is located at 67.18oS, approximately 100km from the\ncontinental shelf break. Intruding eddies could move eastward with the Prydz Bay\nGyre before the entire eddy passes the mooring location. This scenario may also\nreduce the length scales of the eddies estimated from the mooring record.\nFinally, Figures 5a-h show the typical process of eddy formation and intrusion\nonto the continental shelf driven by the wind forcing, by examining the SSH\nanomaly, wind forcing, and hydrological section observations across the same\ntime period. The agreement of these independent observations supports our\nfindings. Further examination of this process will require more situ observations\nand eddy-resolution coupled modeling studies of the Southern Ocean that\nincorporate Antarctic ice shelves.\n26. And lastly, with regards to the discussion on the long-term changes in the wind\nfield, the authors provide convincing evidence of how storms passing over the\nregion immediately offshore of the Prydz Bay channel can uplift isopycnals and\ndrive intrusions of warm water across the shelf break. I can’t however see how the\npredicted strengthening and shift in the westerlies alone could result in enhanced\nCDW intrusions, unless these long-term changes in the westerlies are also\naccompanied by either stronger or more frequent/persistent storms in the region.\nNote that the features capture in panel b in Extended figure 5 have typical duration\nof ~5days. While the data in Extended Data Fig 9 is given as annual means, and\nhence unable to capture a possible increase in \"storminess\".\nResponse: To clarify, all of the associated wind forcing plots (Figures 6a-d,\nExtended Data Figure 4, Extended Data Figure 8 and Extended Data Figure 9) are\ncalculated basing on the daily wind data from ERA-interim products, hence it is\nable to capture variations in \"storminess\". We have updated the text as \"All of the\nassociated wind-forcing analyses were based on the daily wind data from the\nEuropean Centre for Medium-Range Weather Forecasts (ECMWF)\nReanalysis-interim (ERA-interim) products\" in the Data section of the revised\nmanuscript.\nFor example, we firstly calculated the wind stress curl for each day across the\n1979-2018 periods, then calculated the austral summer (including all the daily\ndata for each year’ DJF season) wind stress curl trend based on those daily wind\nstress curls data. This provided the distributions of wind stress curl trend during\nthe 1979-2018 periods (Extended Data Figure 8b). The same method is used in all\nof the long-term trend calculations for zonal wind and wind stress curl in our\nstudy (Figures 6c-d, Extended Data Figure 8 and Extended Data Figure 9). To\nsubtract the high frequency variability in the variation of austral summer wind\nduring 1979-2018 periods, we calculated the mean values of zonal wind and wind\nstress curl for each austral summer (including all the daily data for each year’ DJF\nseason), then we calculated the long-term trend for the time series (Extended Data\nFigure 9). To subtract the high frequency variability in the variation of austral\nsummer wind across 1979-2018, we calculated the mean values of zonal wind and\nwind stress curl for each austral summer (including all daily data for each years’\nDJF season), and then we calculated the long-term trend for the time series\n(Extended Data Figure 9). In reality, the long-term patterns remain relatively\nconsistent, regardless of whether the high-frequency variability is retained or not.\nFigures 6c-d, Extended Data Figure 8 and Extended Data Figure 9 were\ncalculated based on the daily wind data. These figures show that both the austral\nsummer (DJF) westerly winds in the 50oS-67oS region, together with the coastal\neasterly winds south of 67oS, have strengthened and thus more cyclonic wind\nstress curl has occurred over the continental shelf break of the Prydz channel.\nSuch changes would drive more warm eddies towards the front of the Amery Ice\nShelf (north part), where thinning/mass loss is occurring at a significant rate.\nReviewer #1 also agreed that the long-term changes of westerly wind and\nwind stress curl did actually happen basing on Figures 6c-d, Extended Data Figure\n8 and Extended Data Figure 9 (see point 16 raised by Reviewer #1). Auger et al.\n(2022) shows the EOF modes 1 spatial pattern of westerly wind and wind stress\ncurl (see Figure S5), which are consistent with our results. The strengthening and\nshift in the westerlies can increase the meridional shear of zonal wind, resulting in\nstronger cyclonic wind stress curl near the coastal region (Figures 6c-d, Extended\nData Figure 8, and Figure S5). So we suggest that \"Given the prediction that\nSouthern Ocean westerlies will continue to strengthen and shift poleward57-58,\nwarm-eddy intrusions onto the continental shelves are also likely to increase, with\nramifications for ice shelf melting and global sea level rise\".\nIn the end, we agree that it is better to concentrate on the processes observed at\na regional scale. Indeed, we mainly focus on the eddy formation and transport\nonto the Prydz Bay continental shelf driven by regional wind forcing, based on\nsubsurface mooring and hydrological section data from 2013-2015. Figure 5,\nExtended Data Fig. 5 and Figures 6a-b show the influence of zonal winds as a\npotential physical mechanism for the warm-eddy intrusions into the Prydz Bay\nregion. Figures 6c-d, Extended Data Figure 8 and a few additional\nthought-provoking sentences in our manuscript are provided to put forward the\nhypothesis that the regional changes in westerly winds in past decades could have\ndriven more warm eddies towards the front of the Amery Ice Shelf. We prefer to\nkeep these sentences in the manuscript with a view to prompting further work to\naddress this topical concept.\nAuger, M., Sallée, J.-B., Prandi, P., & Naveira Garabato, A. C. (2022).\nSubpolar Southern Ocean seasonal variability of the geostrophic circulation from\nmulti-mission satellite altimetry. Journal of Geophysical Research: Oceans, 127,\ne2021JC018096. https://doi.org/10.1029/2021JC018096\nREDACTED\nFigure S5. EOF modes 1 spatial pattern of Wind Stress Curl (left) and Zonal Wind\nStress (right) during 2013-2019. (From Auger et al., 2022)\nPoint-by-point response to Reviewer #3:\nReviewer #3 (Remarks to the Author):\nThe authors have taken care of my previous concerns. The manuscript has improved\nsignificantly. I recommend accepting it for publication.\nResponse: Thanks.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nReview of re-revised manuscript \"Persistent warm-eddy transport to Antarctic ice shelves driven by\nenhanced summer westerlies\", by Libao Gao, Xiaojun Yuan, Wenju Cai, Guijun Guo, Weidong Yu, Jiuxin\nShi, Fangli Qiao, Zexun Wei, and Guy D. Williams.\nGeneral Comments\nI again thank the authors for their efforts in response to my previous comments. My only comment\nabout any of their reply is with respect to item 12 about chord sizes. I agree that for a given chord\nlength the eddy sizes can be very different. However, the authors do have a distribution of chord\nlengths. What if one assumes that there is an isotropic distribution of the eddies down the trough with\nrespect to the mooring position (the authors would know better than I if that is a reasonable\nassumption) and that the eddies are not widely different in size? If so, wouldn't the\nmaximum chord lengths from the distribution, even if still an under-estimate, be at\nleast representative of the eddy diameters? Lilly and Rhines, who the authors cite (62) in the methods section with respect to computing chord lengths, note that it is a chord being estimated this way, and thus under-represents the diameter. However, they also do still use the chord lengths to characterize the size of their\nobserved eddies. I do agree with the authors on some of their points to Reviewer #2 regarding eddy\nsizes. However, I think it's worth the effort trying to at least get an estimate from the mooring data on\nthe size of the eddies over the mooring, even if there are caveats, to compare to the satellite\nestimates.\nI have a couple of other very minor editorial comments, but in general I'm quite happy with the\nchanges made. I recommend acceptance after the comments about chord length (and the other trivial\ncomments below) are considered.\nSpecific Comments\nLines 50-51: I think this sentence should have a reference at the end. The Thompson et al. reference\n(21) used for the next sentence would fit this one as well.\nLine 142: Suggest removing \"basal melt\" from between \"Amery Ice Shelf\" and \"that the mCDW\".\nLines 211-213 and reference 54: I think it would help the reader if the authors added a little more to\nthe text summarizing the results of the Lu and Huang study that are relevant here. Perhaps something\nsimilar to the first paragraph of their reply to Reviewer #2 in their new Response Letter?\nLines 251-254 and Extended Data Figures 7 a+b: What are the correlation coefficients for the\nregressions shown in the figures?\nEquation 3: Now that I understand the T_f - T term better, should T_f and T be switched? As written,\nwouldn't this give a negative melt rate?\nFigure 6 caption (also an issue in the captions for Extended Data Figures 4, 5, 6, 8, and 9): The units\nfor wind stress curl are correct in the text for 6b, but there's a typo in the text for 6d (should be s^(-2), not s^2).\n___________\nComment on re-revised manuscript \"Persistent warm-eddy transport to Antarctic ice shelves driven by\nenhanced summer westerlies\", with respect to the authors reply and revisions to comments from\nReviewer #2.\nI am not Reviewer #2, but have been asked by the editor my thoughts on the authors response to\nthose comments.\nReviewer comment about eddy formation mechanism: I have to admit to being more familiar with\npapers either discussing the impacts of ocean eddies on atmospheric storms (especially tropical\ncyclones) or where wind stress weakens ocean mesoscale eddies (e.g. Xu et al., 2016 and the many\nreferences cited there), than ones where storms induce eddies in the ocean. However, there are\nexamples in the literature where it is shown that wind stress curl can be a mechanism for eddy\ngeneration (e.g. Stammer et al. 2001, Wang et al., 2008). So, while I don't totally agree with reviewer\n#2 that \"eddies being formed in response to large negative vorticity imparted by the passing of a\nstorm is a new concept\", I do think it's an idea that is still an open question and thus deserves more\ndiscussion in the main text than just a simple reference to the Lu and Huang paper. I mentioned in\nmy earlier review (I am reviewer #1) that I thought it would help the reader if the authors added a little\nmore to the text summarizing the results of the Lu and Huang study along the lines of what they put\nin their reply to Reviewer #2. I think perhaps a couple more references about this idea (see examples\nabove) may also help.\nStammer et al., 2001. The role of variable wind forcing in generating eddy energy in the North\nAtlantic. Progress in Oceanography.\nWang et al., 2008. Winter Eddy Genesis in the Eastern South China Sea due to Orographic Wind Jets.\nJournal of Physical Oceanography.\nXu et al., 2016. Work done by atmospheric winds on mesoscale ocean eddies.\nGeophysical Research Letters.\nReviewer comment about length scale of eddy features: I did touch on something similar in my earlier\ncomment about the chord lengths. I agree with the authors point about how \"the length scales\nestimated from the mooring record must be smaller than the length scales seen from the SSH\nanomalies\", but I don't think it should be a huge difference, and I think (given the assumptions I\nmentioned about circular eddies and an isotropic distribution) a good estimate can be made (or an\nargument made about why those assumptions are way off).\nI can see how the length scales estimated from a mooring at 270 m could be smaller than those\nestimated from SSH anomalies, but I don't see how it could be \"much smaller\" (and the ratio could be\nestimated by the authors based on Figures 3d-f). I'm not sure I understand the physics involved in the\nauthors third (eddies onshelf are smaller than offshelf eddies because of multiple meanders in the\nASF?) and fourth (eddies become smaller while transiting through the Pyrdz Bay Gyre)\nreplies to this point. I do agree with the authors that Figure 5 is illustrative of the\neddy formation and intrusion onto the shelf. I still think more discussion in the\nmanuscript is necessary in response to this point.\nReviewer comment about the relationship between the westerlies and CDW intrusions: I think the\nauthors have quite nicely answered reviewer 2's concerns on this point. My only suggestion is that\nthere are several examples of modeling studies showing increasing intrusion of CDW and heat onto the\nAntarctic continental shelf with strengthening and poleward shifted westerlies that could be cited\n(perhaps starting with Spence et al, 2014).\nSpence et al., 2014. Rapid subsurface warming and circulation changes of Antarctic coastal waters by\npoleward shifting winds. Geophysical Research Letters.\nReviewer #2 (Remarks to the Author):\nI will again start by thanking the authors for the efforts. In my last review, I raised three points where\nI still had significant doubts. With regards to the second and third points, on the subject of the scale of\nthe eddies onshore/offshore and on the subject of future trends in wind stress and curl, I am happy\nwith the response the authors have provided to me. However, only minor modifications have been\nmade to the actual manuscript to clarify these points and they could still lead to confusion. For\ninstance, instead of simply stating that the wind analysis is based on daily data, the authors could be\nmore explicit about why the they think their wind stress analysis is able to capture changes in\nstorminess. Similarly, a sentence to reconcile the order of magnitude difference between the length\nscales observed by the mooring and those inferred from altimetry would be useful.\nWith regards to the formation mechanism of the offshore eddy I strongly encourage the authors to\nsoften the language and present it as speculation. The alternative explanation being that the observed\neddies are part of the background eddy field, maintained through baroclinic instability, and can\ninteract with the passing storms. My reasons for questioning the validity of this argument again are\nthe following. The new reference the authors bring up in response to my comment, Lu and Huang\n(2010), is quite an interesting highly idealized theoretical study. As the authors of the study admit, it\nshould be only considered as a limit, given that hurricanes are very rarely stationary (slow moving at\nmost) and the ocean is stratified. But even then, if one was to accept the assumption of stationarity\nand homogenous ocean, the mechanism proposed in the study can only explain the\nformation of cyclones, not the anticyclones. And there is the additional problem of scales, where the\nscale of the oceanic eddy is an order of magnitude smaller than the scale of the storm that triggers it,\nwhen the theory predicts comparable amplitude.\nLastly advection of a subsurface feature by the Ekman flow, something the authors reiterate in their\nresponse, without further work to demonstrate it, is in my view speculation.\nWe have revised our manuscript accordingly with changes to the revised manuscript\nshown in green italics.\nPoint-by-point response to Reviewers\nReviewer #1 (Remarks to the Author):\nReview of re-re-revised manuscript \"Persistent warm-eddy transport to Antarctic ice\nshelves driven by enhanced summer westerlies\", by Libao Gao, Xiaojun Yuan,\nWenjuCai, GuijunGuo, Weidong Yu, Jiuxin Shi, FangliQiao, Zexun Wei, and Guy D.\nWilliams.\nGeneral Comments\nI again thank the authors for their efforts in response to my previous comments. All\nmy previous concerns have been addressed and while I have a few new comments\nbelow, they are all very minor editorial issues. I'm happy to recommend acceptance\nafter the comments below are considered.\nSpecific Comments\n1. Lines 142-143: I think it would help the reader if they knew approximately what\nportion of the Amery Ice Shelf is subject to melting from mCDW. The authors\ngive the area later in the manuscript (line 393: 12,733 km^2), but I think it would\nhelp if the percentage (~12,800/62,000) was added after the word \"access\" on line\n143 such as \"(about 20% of the total area)\".\nResponse: Agreed. We have changed the text accordingly.\n2. Line 176: Typo, \"antcyclonic\" should be \"anticyclonic\".\nResponse: Thanks. We have changed the text accordingly.\n3. Lines 189-199: Since the AVISO altimetry is provided on a 1/4 degree grid (lines\n356-358), could another reason the chords from the mooring record are smaller\nthan the eddy size estimate from the altimetry be that the smaller eddies just are\nnot resolved by the altimetry data product?\nResponse: Agreed. We have added new text in the manuscript as follows: \"Finally,\nsince the merged SSH data is provided on a 0.25° grid, the smaller eddies just are\nnot resolved by the altimetry product \".\n4. Line 225: Since the example storm in the reference is stationary, suggest changing\n\"passing of a storm\" to \"presence of a storm\".\nResponse: Agreed. We have changed the text accordingly.\n5. Line 231: Suggest changing \"also examples\" to \"also other examples\".\nResponse: Agreed. We have changed the text accordingly.\n6. Line 233: Should \"caused\" be \"and caused\" or \"causing\"?\nResponse: Agreed. We use \"and caused\" here.\n7. Lines 269-272, Extended Data Figures 7a+b, and Response to Reviewers #5: I\nthank the authors for computing the correlation coefficients and putting them in\ntheir reply. I think these values (0.63 for 7a, 0.75 for 7b) should either be added to\nthe main text or the Extended Data Figure captions.\nResponse: The Extended Data Figures 7a+b show the diagrams of warm signals to\nzonal wind and wind stress curl. We just plot two regression lines in the plots, so i\nthink it is not necessary to add the correlation coefficients to the main text.\nMoreover, the readers can get the information from the rebuttal files.\n8. Line 304: Are there other alternative explanations? If so, should \"The alternative\"\nbe \"An alternative\"?\nResponse: Agreed. We have changed the text accordingly.\n9. Line 342: I think \"new-coupled\" should be \"new coupled\".\nResponse: Agreed. We have changed the text accordingly.\n10. Line 385, and Response to Reviewers #6: Yes, you use the same sign convention\nas Herraiz-Borreguero et al. (2015). However, negative melt rate (at least to me)\nmeans freezing and increases in ice. If you're going to have negative melt rate\nmean that the volume of ice is decreasing, then I think it should be explicitly\nstated (I felt the same way about how it was used in Herraiz-Borreguero et al.).\nResponse: Agreed. We have added new text in the manuscript as follows:\n\"Negative melt rate means the volume of ice is decreasing \".\n11. Lines 396-401: Where are the mooring data? Apologies if I missed it, but I did not\nsee it in any of the listed websites in this section.\nResponse: We have updated the data link which can find the data directly.\nReviewer #2 (Remarks to the Author):\nThe authors have done a great job at responding to my comments, and am looking\nforward to see their manuscript published.\nResponse: Thanks.\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nReview of re-re-revised manuscript \"Persistent warm-eddy transport to Antarctic ice shelves driven by\nenhanced summer westerlies\", by Libao Gao, Xiaojun Yuan, Wenju Cai, Guijun Guo, Weidong Yu, Jiuxin\nShi, Fangli Qiao, Zexun Wei, and Guy D. Williams.\nGeneral Comments\nI again thank the authors for their efforts in response to my previous comments. All my previous\nconcerns have been addressed and while I have a few new comments below, they are all very minor\neditorial issues. I'm happy to recommend acceptance after the comments below are considered.\nSpecific Comments\nLines 142-143: I think it would help the reader if they knew approximately what portion of the Amery\nIce Shelf is subject to melting from mCDW. The authors give the area later in the manuscript (line\n393: 12,733 km^2), but I think it would help if the percentage (~12,800/62,000) was added after the\nword \"access\" on line 143 such as \"(about 20% of the total area)\".\nLine 176: Typo, \"antcyclonic\" should be \"anticyclonic\".\nLines 189-199: Since the AVISO altimetry is provided on a 1/4 degree grid (lines 356-358), could\nanother reason the chords from the mooring record are smaller than the eddy size estimate from the\naltimetry be that the smaller eddies just are not resolved by the altimetry data product?\nLine 225: Since the example storm in the reference is stationary, suggest changing \"passing of a\nstorm\" to \"presence of a storm\".\nLine 231: Suggest changing \"also examples\" to \"also other examples\".\nLine 233: Should \"caused\" be \"and caused\" or \"causing\"?\nLines 269-272, Extended Data Figures 7a+b, and Response to Reviewers #5: I thank the authors for\ncomputing the correlation coefficients and putting them in their reply. I think these values (0.63 for\n7a, 0.75 for 7b) should either be added to the main text or the Extended Data Figure captions.\nLine 304: Are there other alternative explanations? If so, should \"The alternative\" be \"An alternative\"?\nLine 342: I think \"new-coupled\" should be \"new coupled\".\nLine 385, and Response to Reviewers #6: Yes, you use the same sign convention as Herraiz-\nBorreguero et al. (2015). However, negative melt rate (at least to me) means freezing and increases\nin ice. If you're going to have negative melt rate mean that the volume of ice is decreasing, then I\nthink it should be explicitly stated (I felt the same way about how it was used in Herraiz-Borreguero et\nal.).\nLines 396-401: Where are the mooring data? Apologies if I missed it, but I did not see it in any of the\nlisted websites in this section.\nReviewer #2 (Remarks to the Author):\nThe authors have done a great job at responding to my comments, and am looking forward to see\ntheir manuscript published.\nWe have revised our manuscript accordingly with changes to the revised manuscript\nshown in green italics.\nPoint-by-point response to Reviewers\nReviewer #1 (Remarks to the Author):\nReview of re-re-revised manuscript \"Persistent warm-eddy transport to Antarctic ice\nshelves driven by enhanced summer westerlies\", by Libao Gao, Xiaojun Yuan,\nWenjuCai, GuijunGuo, Weidong Yu, Jiuxin Shi, FangliQiao, Zexun Wei, and Guy D.\nWilliams.\nGeneral Comments\nI again thank the authors for their efforts in response to my previous comments. All\nmy previous concerns have been addressed and while I have a few new comments\nbelow, they are all very minor editorial issues. I'm happy to recommend acceptance\nafter the comments below are considered.\nSpecific Comments\n1. Lines 142-143: I think it would help the reader if they knew approximately what\nportion of the Amery Ice Shelf is subject to melting from mCDW. The authors\ngive the area later in the manuscript (line 393: 12,733 km^2), but I think it would\nhelp if the percentage (~12,800/62,000) was added after the word \"access\" on line\n143 such as \"(about 20% of the total area)\".\nResponse: Agreed. We have changed the text accordingly.\n2. Line 176: Typo, \"antcyclonic\" should be \"anticyclonic\".\nResponse: Thanks. We have changed the text accordingly.\n3. Lines 189-199: Since the AVISO altimetry is provided on a 1/4 degree grid (lines\n356-358), could another reason the chords from the mooring record are smaller\nthan the eddy size estimate from the altimetry be that the smaller eddies just are\nnot resolved by the altimetry data product?\nResponse: Agreed. We have added new text in the manuscript as follows: \"Finally,\nsince the merged SSH data is provided on a 0.25° grid, the smaller eddies just are\nnot resolved by the altimetry product \".\n4. Line 225: Since the example storm in the reference is stationary, suggest changing\n\"passing of a storm\" to \"presence of a storm\".\nResponse: Agreed. We have changed the text accordingly.\n5. Line 231: Suggest changing \"also examples\" to \"also other examples\".\nResponse: Agreed. We have changed the text accordingly.\n6. Line 233: Should \"caused\" be \"and caused\" or \"causing\"?\nResponse: Agreed. We use \"and caused\" here.\n7. Lines 269-272, Extended Data Figures 7a+b, and Response to Reviewers #5: I\nthank the authors for computing the correlation coefficients and putting them in\ntheir reply. I think these values (0.63 for 7a, 0.75 for 7b) should either be added to\nthe main text or the Extended Data Figure captions.\nResponse: The Extended Data Figures 7a+b show the diagrams of warm signals to\nzonal wind and wind stress curl. We just plot two regression lines in the plots, so i\nthink it is not necessary to add the correlation coefficients to the main text.\nMoreover, the readers can get the information from the rebuttal files.\n8. Line 304: Are there other alternative explanations? If so, should \"The alternative\"\nbe \"An alternative\"?\nResponse: Agreed. We have changed the text accordingly.\n9. Line 342: I think \"new-coupled\" should be \"new coupled\".\nResponse: Agreed. We have changed the text accordingly.\n10. Line 385, and Response to Reviewers #6: Yes, you use the same sign convention\nas Herraiz-Borreguero et al. (2015). However, negative melt rate (at least to me)\nmeans freezing and increases in ice. If you're going to have negative melt rate\nmean that the volume of ice is decreasing, then I think it should be explicitly\nstated (I felt the same way about how it was used in Herraiz-Borreguero et al.).\nResponse: Agreed. We have added new text in the manuscript as follows:\n\"Negative melt rate means the volume of ice is decreasing \".\n11. Lines 396-401: Where are the mooring data? Apologies if I missed it, but I did not\nsee it in any of the listed websites in this section.\nResponse: We have updated the data link which can find the data directly.\nReviewer #2 (Remarks to the Author):\nThe authors have done a great job at responding to my comments, and am looking\nforward to see their manuscript published.\nResponse: Thanks."}]

Decision letter
Editorial Decision Letter
Dear Dr. Gao,
We are pleased to inform you that your manuscript, "Persistent warm-eddy transport to Antarctic ice shelves driven by enhanced summer westerlies", has been accepted for publication in Nature Communications.
Thank you for your patience and for the thorough revisions you have made in response to the reviewers' comments. We believe the manuscript is now in a strong position and will be of significant interest to our readership.
Best regards,
Nature Communications Editorial Team