Evidence for large-scale climate forcing of dense shelf water variability in the Ross Sea

Domain: earth_science
Published: 2024-09-18
Editorial outcome: minor_revision

Abstract
Antarctic Bottom Water (AABW), which supplies the lower limb of the thermohaline circulation, originates from dense shelf water (DSW) forming in Antarctic polynyas. Here, combining a long mooring record of DSW measurements with numerical simulations and satellite data, we show that significant correlation exists between interannual variability of DSW production in the Ross Sea polynyas, where DSW contributes between 20–40% of the global AABW production, and the Southern Annular Mode (SAM). The correlation is largest when the Amundsen Sea Low (ASL) is weakened and shifted east of the Ross Sea. During positive SAM phases, enhanced offshore winds and lower air temperatures over the western Ross Sea increase sea ice production and promote DSW formation, with the opposite response during negative SAM phases. These processes ultimately modulate AABW thickness in the open ocean. A projected positive shift of the SAM and eastward displacement of the ASL thus has implications for the future of DSW and AABW formation.

Introduction
Dense shelf water (DSW) formed by convective processes on Antarctic continental shelves, mainly from polynyas defined as low sea-ice-concentration areas, is the major source of the Antarctic Bottom Water (AABW)1–5, which feeds the lower limb of the global thermohaline circulation6–8. The formation and transport of DSW and AABW play an important role in transporting carbon from the biologically productive polynyas to the deep ocean9,10. Polynyas along the Antarctic coast are formed by cold katabatic winds blowing off the Antarctic continent toward the ocean, which drives sea ice transport offshore11–14. As the warm ocean is in direct contact with the cold air, massive heat loss leads to high sea ice production rates15,16 and drives the formation of the DSW via brine release and ocean convection5,17. The DSW is subsequently transported towards the shelf break and flows down the continental slope as gravity plumes18,19, entraining the Circumpolar Deep Water (CDW) to ultimately form AABW.

The Ross Sea is responsible for 20–40% of the total AABW production20,21, and the Terra Nova Bay polynya (TNBP) off the Victoria Land coast and the Ross Ice Shelf polynya (RISP) (Fig. 1a, b) are the two major formation sites of DSW in the Ross Sea22. The TNBP is opened by strong katabatic winds flowing eastward from the Transantarctic Mountains11, and RISP is maintained by offshore winds off the Ross Ice Shelf contributed by confluent drainage flows from West Antarctic ice sheets and the Transantarctic Mountains, synoptic-scale circulations and katabatic winds23,24. Earlier studies revealed the variability of DSW formation in the TNBP and surrounding regions on synoptic to seasonal timescales16,25,26, as well as the long-term trend of DSW salinity based on austral summer cruise observations27,28. In fact, a dramatic seasonal salinity increase in bottom water, which is a clear indication of the start of DSW formation, occurs in austral winter29. Up until now, there have been limited studies revealing the long-term variations of DSW formation and their connection to climate modes28,30, and studies discussing the relationship between wintertime DSW production and the dominant mode for climate variability in the southern hemisphere extratropical region—the Southern Annular Mode (SAM) are particularly scarce. Quantifying these interactions is essential to understanding the long-term variation of AABW and the meridional overturning circulation in the Southern Ocean.

SAM is characterized by opposing sea level pressure anomalies between the mid- and high-latitudes31–34. A shift of SAM toward its positive phase is associated with strengthening and poleward movement of the westerlies. While SAM plays a significant role in controlling infrequent opening of large-scale open-ocean sensible heat polynyas35,36, its linkage to oceanic variability on interannual and decadal timescales in the coastal latent heat polynyas, which are important DSW formation sites, has not been previously studied. In this study, combining long-term satellite data, a 7-year mooring record for measuring DSW characteristics in the Southern Ocean, and numerical simulations from a high-resolution coupled ice-ocean model that reproduces DSW variability in the Ross Sea reasonably well, we find (1) that the interannual variability of wintertime sea ice production and DSW formation in the TNBP is modulated by SAM via changes in offshore winds, and in the RISP via surface air temperatures, (2) that the response of polynya processes to SAM affects the DSW volume on the slope and ultimately the AABW volume in the open ocean of the Pacific sector, and (3) that these relations have decadal variations that are influenced by the strength and location of the largest Antarctic climatological low pressure system—the Amundsen Sea Low (ASL).

Results
The relationship between interannual variations of wintertime atmospheric fields over the TNBP and SAM
While earlier work indicates that a shift toward the positive SAM phase can enhance westerly winds and may potentially affect the offshore winds over the TNBP37, after examining the wind data from ERA5 and the SAM index over the past 30 years, we did not find a significant relationship between the interannual variations of zonal wind in this region and SAM (Fig. 1c) over the whole period. However, a significant and positive correlation between them is detected for the period 2000–2014 (R=0.71, P=0.003). In this period, the composite wind field for the 6 years with positive SAM (2003, 2004, 2008, 2010, 2012 and 2014) show notably stronger eastward (offshore) winds compared to that for the 9 years with negative SAM (2000, 2001, 2002, 2005, 2006, 2007, 2009, 2011 and 2013) (Fig. 1d, e), with the difference in average wind speed over the polynya being 0.6 ms−1 (10% of the mean winds, which is enough to significantly impact DSW formation rates38). The winter mean surface air temperature, another atmospheric quantity that affects sea ice production in the polynya, was significantly and negatively correlated to the winter SAM (R=−0.49, P=0.007; not shown) for the entire 1992–2020 period, while the correlation was weaker and insignificant for the period 2000–2014.

Mechanisms for a decadal relation between zonal winds over the TNBP and SAM
To find out why there exists a significant relationship between zonal wind over the TNBP and SAM for the period 2000–2014, we examined the role of other climate modes that can affect atmospheric circulations over the Ross Sea. Previous studies demonstrate that the ASL plays an important role in wind forcing over the this region39,40. In particular, a deepening and westward movement of the ASL enhances southerlies from the Ross Ice Shelf and decreases air temperatures over the Ross Sea. While the ASL and SAM are found to be interrelated, with a deepening of the ASL often coinciding with a shift toward a positive SAM phase39,41, they can also act as independent factors influencing the Ross Sea atmospheric circulations during different periods and seasons. For example, a significant correlation exists between the ASL longitude and SAM in austral autumn but not in other seasons42. An examination of the ASL indices reveal that from 2000, in austral winter, anomalies of the ASL central pressure relative to the climatological (1979–1999) mean remained positive until 2009 (Fig. 2a); a sharp decrease in pressure occurred in 2010, but the pressure rebounded quickly and oscillated around the climatological value between 2011–2014. Overall, comparing the period between 1979 and 1999 with that between 2000 and 2014, the ASL was weakened and its extent was noticeably reduced (Fig. 2c, d), while SAM shifted slightly toward its positive phase as indicated by an increase of the period-mean SAM index by 0.2. These results suggest that the coincidence of a deeper ASL with positive SAM derived by previous studies may not be robust for all timescales, and that SAM and the ASL can be independent climate forcings for regional atmospheric circulations over some periods. In 2000–2014, the contraction of the ASL reduces its influence on the wind field over the western Ross Sea. In addition, we found that there was a dramatic eastward shift of the ASL center over the latter portion of 2000–2014 compared to the climatology (Fig. 2b). This would further weaken the influence of the ASL on the atmospheric forcing over the TNBP and can result in an enhanced relationship between winds in this region and other climate forcings such as SAM. This is consistent with past work which showed that during winter, SAM and the ASL zonal location are uncorrelated. In fact, a significant correlation is detected between the variations of meridional wind over the TNBP and the ASL central longitude during the period 1979–1999 (R=−0.58, P=0.001; Fig. 2b), while such a relationship weakened during 2000–2014 (R=0.06). The weakened correlation, along with the strong correlation of zonal wind to SAM during 2000–2014, suggests a competition between these two climate factors in controlling winds over the western Ross Sea, and that a weakening in the ASL influence enables a strengthened SAM impact in this region.

The connection between winter sea ice production in the TNBP and SAM
Between 2000–2014, under the enhanced offshore winds in positive SAM conditions that would favor the opening of the polynya, the winter sea ice concentration (SIC) in TNBP during high SAM years was lower than during low SAM years (Fig. 1d, e), with a difference in the polynya-average SIC value of 3% (some areas having differences of more than 5%), indicating an increased polynya extent. Similar to the zonal wind, the interannual variation of SIC was not significantly correlated with SAM for 1992–2020, but the correlation improves with SAM for 2000–2014 (R=−0.47) and is significant at the 90% confidence level (Fig. 1f). The correlations of interannual variations of SIC to zonal winds and air temperature are −0.80 (P=0.0003) and 0.12 respectively, indicating that zonal winds are the major factor linking the SIC variability to SAM during 2000–2014 and that air temperatures do not play a significant role.

For the cumulative sea ice production (SIP) in June to August of each year during 2000–2013 available from satellite estimates, there was an overall trend of increasing SIP with increasing winter SAM (Fig. 3a–n). The average composite SIP in TNBP for the 6 high-SAM years is 0.7 m greater than that for the 9 low-SAM years. This period is also notable for a strong correlation between the interannual variations of winter SIP and SAM if data from 2007 are excluded (R=0.66, P=0.03; Fig. 3o), which year is featured by low SAM but not weak zonal wind and moderate SIC (Fig. 1c, f). In 2007, the density of cyclones that are centered near the Ross Ice Shelf reached the maximum between 2000 and 2014, and such cyclones can induce notable offshore wind over the TNBP region (Supplementary Fig. 1), which could have affected the relations between wind and SAM and between sea ice and SAM. Overall, the results above demonstrate that ice production was enhanced as strengthened offshore winds drove stronger offshore ice motion, enlarging the area for ocean-atmosphere heat losses to occur.

Enhancement in the DSW formation in positive SAM conditions in 2000–2014
Timeseries of the near-bottom salinity (~1050 m) from a mooring in the TNBP during freezing seasons (March to October) are shown for 7 years in 2000–2014, years when mooring data at consistent bottom depth are available (Fig. 4a–g). Earlier work suggested that while sea ice production starts in early March in this region, significant increases in bottom salinity do not appear until mid-June and last until October29. The late occurrence of salinity increase is possibly associated with advection of upstream ice shelf meltwater which decreases salinity in the TNBP area from March to July43, as well the time it takes for the newly formed DSW to replenish the bottom layer as it is continuously exported out. We found that for some years (2008 and 2010) salinity increases did not appear until mid-July. We then calculated the salinity variations from mid-July to mid-October (ΔS Oct−Jul), i.e., the time period characterized by the strongest signal of salinity increase, and found that ΔS Oct−Jul is positive (0.015–0.084) for all of the 4 years with positive SAM indices (2014, 2008, 2012 and 2010; Fig. 4h), and ΔS Oct−Jul is negative or close to 0 (−0.036–0.004) in all of the 3 years with negative SAM indices (2009, 2011 and 2013). The interannual variation of ΔS Oct−Jul has a significant, positive correlation with the variation of the winter SAM (R=0.83, P=0.02), and is significantly correlated with the interannual variability of winter SIP in TNBP in the same period (R=0.81, P=0.05). The significant correlation to SAM also exists if we consider the salinity change for an extended period, from mid-June to the end of November (R=0.79, P=0.04). Such a correlation results from increased sea ice production and brine rejection during a positive SAM phase, which induces stronger ocean convection and increases the bottom salinity. As salinity is the major factor determining the variation of density in the polynya, the interannual variability of bottom potential density change from mid-July to mid-October (Δσ Oct−Jul) is also significantly correlated with the winter SAM (R=0.83, P=0.02), indicating enhancement in the DSW formation during a positive SAM phase. Among the 7-year mooring data, hydrographic measurements at the middle of the water column (~500 m) are available for 5 years (2010, 2011, 2012, 2013, and 2014). The difference between potential density at the bottom and 500 m averaged over mid-July to mid-October, which indicates mean stratification during this period, was −0.04–0.02 kg m−3 for the 3 high-SAM years (2010, 2012, and 2014; Supplementary Fig. 2) and 0.04–0.05 kg m−3 for the 2 low-SAM years (2011 and 2013). This suggests weaker stratification and stronger convection in more positive SAM conditions that promote the DSW formation. The relationship of potential density change of bottom water with SAM is examined for a longer period, 2003–2014, using numerical simulations from a coupled high-resolution ocean-sea ice-ice shelf model that we developed for the Ross Sea and the Amundsen Sea, which can well capture the observed temporal variability of bottom water hydrography in the TNBP (see Methods). There exists significant correlation between the interannual variations of the modeled Δσ Oct−Jul and the winter SAM for this extended period, with R=0.65 and P=0.02 (Supplementary Fig. 3).

The response of DSW on the continental slope and AABW in the open ocean to the SAM variability
DSW formed in the TNBP is transported through the Drygalski Trough (Fig. 1a) toward the continental slope of the Ross Sea, where it mixes with Southern Ocean waters to form AABW that replenishes the Pacific sector and the Australian Antarctic Basin21,44. As DSW mainly flows on the western side of troughs due to the Coriolis force45, a box region at the slope abutting the western section of the Drygalski Trough (Fig. 5a) is selected to examine the potential linkage of DSW properties on the slope to the TNBP processes and SAM. Mooring observations in the Drygalski Trough just south of the box region reveal that a seasonal increase in the bottom water neutral density starts in September and lasts until the following April46,47, lagging the density change in the TNBP by a couple of months due to the water transport time. Given this fact, we computed the change of DSW (defined as neutral density greater than 28.27 kg m−3) layer thickness from early September of each year to the next April in the box region in 2003–2014 (Fig. 5a–l), using numerical simulations from the coupled ocean-sea ice-ice shelf model that can also well capture the bottom water characteristics on the Ross Sea slope (Methods). Compared to the DSW thickness changes in years with low SAM below or near 0, the changes in years with high SAM above 1.0 (in particular 2008, 2004 and 2010) are notably larger and can reach 80 m or above. Strong positive correlation exists between the interannual variations of DSW thickness change averaged over the box region and the density change Δσ Oct−Jul in the TNBP (R=0.66 and P=0.05; Fig. 5m). The relationship between interannual variations of DSW thickness change and winter SAM is also significant (R=0.54 and P=0.07; Fig. 5n). These results suggest that the variation of SAM can exert dramatic influence on the DSW volume in the slope area of the Ross Sea via changes in the coupled atmosphere-ice-ocean processes in the TNBP, which may potentially impact the AABW properties in the Pacific and Indian sectors of the Southern Ocean.

To verify the significant influence of the SAM variation on the DSW formation in the Ross Sea, including changes in both the TNBP and the Ross Ice Shelf polynya (RISP, Fig. 1a), and to further quantify the SAM-induced DSW changes as well as AABW changes in the open ocean, two numerical sensitivity experiments using the coupled ocean-sea ice-ice shelf model are conducted. In Experiments SAMP and SAMN, anomalies of atmospheric variables associated with one-standard-deviation (1-std) increase and 1-std decrease in the SAM index are added to the forcing fields of the realistic model simulation in 2003–2014 (named CTRL), respectively (see Methods). It can be seen that a shift toward the positive SAM phase can induce significant enhancement in the westerlies over most of the Ross Sea including the TNBP (Fig. 6a), and is also associated with decreases in surface air temperatures (SAT) over most of the Ross Sea that are most prominent near the Ross Ice Shelf (Fig. 6b), as a result of adiabatic cooling near the Antarctic continent27. No significant changes in the meridional wind are found over the Ross Sea (not shown). The enhanced offshore wind over the TNBP and the cold air temperature over the RISP lead to increases in annual accumulative ice production (averaged over all simulation years) in these two polynyas in SAMP compared to CTRL (Fig. 6c), which can reach 1.0 m and above. Conversely, ice production in polynyas in SAMN is notably reduced compared to CTRL (Fig. 6d). The promotion and inhibition in ice production in SAMP and SAMN, respectively, result in increases and decreases of neutral density in the model bottom layer in the next February (the last month when DSW can be significantly affected by accumulative ice production from the previous freezing seasons, as new ice production begins in March) by more than 0.02 kg m−3 on the shelf and slope and more than 0.01 kg m−3 in the open ocean relative to those in CTRL (Fig. 6e, f). Ultimately, compared to CTRL, in SAMP the DSW thicknesses increase by 20–50 m on the shelf and slope and the AABW thicknesses increase by 100–200 m in the open ocean of the Pacific sector (Fig. 6g), and in SAMN the thicknesses of DSW and AABW both show decreases (Fig. 6h). Compared to the role of offshore winds in polynya activity and the DSW formation, the role of SAT is discussed less often. In a model sensitivity experiment analyzing the effects of future climate change on the Ross Sea water masses, it is found that a decrease of SAT by 1 °C over the Ross Sea will lead to a 7% increase of ice production over the shelf, 6% increase in the HSSW volume, and 9% increase in the HSSW flux off the Ross Sea shelf48. To quantify the effects of SAT on the DSW formation in our study, sensitivity experiments were conducted in which SAT anomalies associated with a 1-std increase and decrease in SAM are added to the atmospheric forcing fields in a year with otherwise neutral SAM conditions. The results (Supplementary Fig. 4) show that the decrease of SAT during the positive SAM phase results in more DSW formation on the shelf (Supplementary Fig. 4b compared to 4a), and the increase in DSW thickness can reach 50 m and above, which is comparable to the increase of DSW thickness in Fig. 6g over the central and eastern Ross Sea that is mainly influenced by the RISP processes. These results further demonstrate that the effects of air temperature changes related to SAM on the RISP ice production and DSW production are significant.

Methods
Atmospheric data
The atmospheric fields over the TNBP were derived from monthly data of the ERA5 reanalysis product. Surface winds at 10 m, air temperature at 2 m and the mean sea-level pressure are used. Comparisons between wind from ERA5 and wind from measurements at 6 automatic weather stations in the Ross Sea area show that the correlation coefficient between the variability of ERA5 wind speed and observed wind speed reaches 0.77 and above58.

Sea ice concentration and production
SIC from satellite data is used to indicate the polynya extent in this work following earlier studies59, as the retrieval of ice concentration does not depend on surface heat fluxes from reanalysis products as used in the retrieval of ice thickness and production. SIC data are taken from Version 2 of the Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data, available at the National Snow & Ice Data Center (NSIDC) with a horizontal resolution of 25 km. This dataset is generated from the brightness temperature data. Sea ice production data are obtained from heat- and salt-fluxes and the sea ice production analysis product is provided by the Institute of Low Temperature Science, Hokkaido University. Sea ice production is estimated by the heat flux calculation using thin-ice thickness (less than 0.2 m) derived from the SSM/I passive microwave data and atmospheric reanalysis products including ERA-40, ERA-interim and NCEP260. This dataset is available for 1992–2013.

Mooring observations
A mooring (mooring D) was deployed in the TNBP (75°06.10′S, 164°13.04′E) in February 1995 (Fig. 1b) by the MORsea Project of the Italian National Research Antarctic Program. The mooring was recovered and re-deployed each year up to the present. The mooring was equipped with SBE16 temperature/conductivity recorders, Aanderaa RCM7/9 current meters, sediment traps and turbidity meters at different depths, and the depths varied over the years. We used data from the SBE16 recorders located near the ocean bottom (~50 m above the seafloor) from 2008 to 2014, a timespan within the critical period of 2000–2014 when the SBE16 recorders were maintained at nearly the same depths. For analyzing the stratification, we also used data from the SBE16 recorders installed at the depth ~500 m from 2010 to 2014 and calculated the difference in potential density between the middle and bottom depths.

Climate indices
The seasonal SAM indices are provided by the British Antarctic Survey (https://legacy.bas.ac.uk/met/gjma/sam.html), which are calculated by the zonal pressure difference between the latitudes of 40 and 65°S based on station observations31. The ASL indices, including the central pressure and central longitude, are available at http://scotthosking.com/asl_index49 as monthly data. These indices are derived from the ERA5 reanalysis product, and are temporally averaged to obtain the seasonal indices.

Numerical simulation data
In this study, we developed a high-resolution coupled ocean-sea ice-ice shelf model that covers the Ross Sea, the Amundsen Sea and the surrounding open ocean (see the model grids in Supplementary Fig. 5a). The model is an implementation of the Regional Ocean Modeling System (ROMS), which is a primitive-equation, finite-volume model with a terrain-following vertical coordinate system61,62. The model horizontal resolution varies from ~2 km in the coastal areas to ~6 km in the open ocean, and over the TNBP area is 2–2.7 km. The model includes 32 vertical layers with variable thicknesses that depend on water column depth and are smaller at the surface and bottom. The model bathymetry and ice shelf draft are interpolated from BedMachine-Antarctica-v2.0, which has a spatial resolution of 500 m on the Antarctic Polar Stereographic projection. The sea ice module63 is based on two-layer ice thermodynamics and a molecular sublayer beneath the sea ice64,65, and elastic–viscous–plastic rheology for ice dynamics66. A snow layer is included, and snow is converted into ice when the snow–ice interface is below sea level. The sea-ice model also includes a simple estimate of frazil ice production67. The ice shelves in the model are static and there is no mass change of the ice shelf or iceberg calving. The thermodynamic and mechanical interactions between the ice shelf base and the water cavity underneath47 are included as model processes. Vertical mixing of momentum and tracers are computed using the K-profile parameterization mixing scheme68. Atmospheric forcing fields for the model are derived from the ERA5 reanalysis product. Temperature, salinity, sea surface height and depth-averaged velocity for the open boundaries are obtained from daily mean data of the Met Office Global Seasonal forecasting system version 5 (Glosea5)69. Daily SIC boundary conditions are obtained from the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) and Advanced Microwave Scanning Radiometer 2 (AMSR2) datasets provided by the University of Bremen using the ARTIST sea ice algorithm70. Tidal forcing is derived from the global tidal solution TPXO971. The model was initialized in January 1998 and integrated until April 2015; the simulations from 1998 to 2002 are treated as spin-up results. This model simulation is considered control experiment (CTRL) in this study.

The simulated accumulative sea ice production in austral winter are compared with satellite estimates in Supplementary Fig. 5. It can be seen that the model can well capture the SIP spatial patterns in the Ross Sea polynyas (a and b). The simulated sea ice production rates are higher than the satellite estimates. This may be partly due to inadequate representation of sea ice thermodynamic and dynamic processes in the model, and partly due to the satellite estimates not including oceanic heat fluxes; such fluxes would increase the sea ice production based on observed vertical temperature profiles in the Terra Nova Bay13. The interannual variations of winter accumulative ice production from the model are significantly correlated with those from satellite estimates for both the TNBP (c) and RISP (not shown; R=0.52 and P=0.1). The simulated salinity in the TNBP is compared with mooring observations at the bottom and middle depths in Supplementary Fig. 6, and correlations between the modeled and observed salinity variations are significant for both depth levels. The simulated neutral density in the slope area of the Ross Sea is validated by bottom-water measurements from two moorings near Cape Adare (Supplementary Fig. 7), revealing significant correlations between the modeled and observed variations of bottom-water neutral density. The model is biased high for both salinity and density in the TNBP and slope regions, which could result from the overestimate of sea ice production and inadequate representation of ice shelf basal melting in the Amundsen Sea (about 140–190 Gt yr−1 lower than satellite estimates) that leads to less fresh meltwater transported to the Ross Sea.

Numerical sensitivity experiments
Using monthly data of atmospheric forcing fields for the model and the SAM index in 2003–2014 and by performing linear regression, monthly anomalies of forcing variables including u- and v-component wind, surface air temperature, sea level pressure, humidity, precipitation and cloudiness associated with one-standard-deviation increase in the SAM index are derived. These anomalies are then added to and substracted from the atmospheric forcing fields (repeated over all time steps in a month) of the CTRL simulation over the entire 2003–2014 period to perform the SAMP and SAMN experiments, respectively.

Statistical analysis
The relationship between two quantities is quantified by the Pearson correlation coefficient (R) and p value (P) that is determined by the Wald Test with t-distribution of the test statistic; P≤0.1 is considered statistically significant in this work. Composite analysis is performed to compare the patterns of winter surface wind and SIC over the TNBP under high- and low-SAM conditions in 2000–2014, and the composite fields are derived by averaging the quantities over 6 years with positive SAM indices and 9 years with negative SAM indices. Statistics for atmospheric variables and sea ice concentration for the TNBP are obtained over the region 75.4–74.6°S, 163–166°E.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "[Unconditional Reject]\nAuthors tried to show the impact of SAM on the coastal polynya production and the ensuing dense\nwater formation in the Ross Sea. However, I cannot recommend this paper to be published in this\nhigh-profile journal. I cannot understand and follow the authors’ arguments at all. Detailed reasons\nare listed below:\nLine 19 [a clerical error] transports -> transport\nLine 19 Remove ‘also’. It is unnecessary. I don’t understand what this stands for\nLines 19-20 ‘… play important roles …’ -> ‘… play an important role …’\nLine 20 ‘highly productive’ for what? Clarify it.\nLine 21 [a clerical error] Antarctica -> Antarctic\nLine 22 the Antarctic continent over the ocean? What on earth does this mean? How can the\ncontinent be located over the ocean?\nLine 24 Add ‘The’ in front of ‘DSW’. DSW is -> The DSW is\nLine 25 Add ‘such’ between ‘slope’ and ‘as’. the continental slope as -> the continental slope such\nas\nLines 26-27 ‘entraining other water … (CDW) and Antarctic Surface Water to ultimately form AABW’\nAfter the DSW forms, how does the DSW entrain the Antarctic Surface Water as it flows down the\ncontinental slope? I don’t understand. Clarify.\nLine 28 ‘The Ross Sea produces … production’ What is this supposed to mean? Replace ‘produces’\nwith ‘covers’.\nLine 31 ‘cascading’ seems to be wrong choice of word. Replace ‘cascading’ with ‘flowing’.\nLine 34 ‘a dramatic salinity seasonal increase’ I don’t understand what this means. It needs\ngrammar check.\nLine 34 ‘a dramatic salinity seasonal increase in bottom water, a clear indication of the start of DSW\nformation …’ This sentence has two subjects without any relative pronoun. Modify as follows. ‘a\ndramatic salinity seasonal increase in bottom water, which is a clear indication of the start of DSW\nformation, …’\nLine 36 [a clerical error] no studies -> no study\nLine 41 Add ‘the’ in front of ‘SAM’. A shift of SAM -> A shift of the SAM\nLine 42 Add ‘the’ in front of ‘SAM’. While SAM -> While the SAM\nLine 43 Remove ‘the’ in front of ‘infrequent opening’. the infrequent opening -> infrequent opening\nLine 44 ‘low-frequency variability’: In comparison with which phenomenon, on which time scale,\nwhat is this supposed to mean? Clarify.\nLine 45 ‘relatively small’: In comparison with what? Are you saying that an impact of the SAM-\ninduced coastal polynyas on the AABW formation is weaker than that of the SAM-induced open-\nocean polynya? A major site of the AABW formation is the Weddell Sea coastline. Since you are just\ntalking about the DSW, coastal polynyas and AABW formation in the Weddell Sea should be\nincluded. Then, it cannot be ‘relatively small’. The whole sentence ‘While SAM plays … has not been\nrevealed’ are very weird. Clarify.\nLines 59-63 [Self-contradiction] ‘after examining the wind data … over the whole period. However, a\nsignificant and positive … 2000-2014 (R=0.72, P=0.006).’ If you did not find a significant\nrelationship between the interannual variations of zonal wind in this region and SAM for the whole\nperiod over the past 30 years, what is the scientific meaning for the positive correlation between them just\nduring a shorter period. I think that this analysis for the shorter period is scientifically meaningless.\nLines 67-70 ‘The winter mean surface … was insignificant for 2000-2014’ O.K. If you are correctly\narguing, the positive SAM anomaly leads to lower-than-normal surface air temperature, eventually\nleading to higher-than-normal sea-ice production. Then, What is your point here? It is too obvious.\nClarify what you really want to express.", "reviewer_2": "Reivew of “Evidence for large-scale climate forcing of dense shelf water variability in the Ross Sea”,\nby Zhang et al.\nThis study primarily uses a long mooring record over the continental shelf of the Ross Sea and\nnumerical simulations and satellite data to investigate relationship between interannual variability\nof DSW formation and SAM index. The authors find that when the ASL is weakened and further away\nfrom the Ross Sea, positive SAM phases enhance the offshore winds in the western Ross Sea,\npromote the sea ice production and DSW formation in the TNBP polynya, and ultimately increase\nthe AABW volume in the deep ocean off the Ross Sea.\nBecause the variability of DSW formation is critical to the overturning circulation and global climate\nsystem, I believe these findings presented in this manuscript will be of interest to the Nature\nCommunication readers. The authors have done great jobs illustrating the evidence for the\nmodulation of DSW variability by SAM phase changes. But I think more could be done to make the\nconclusions more robust before publication. Detailed comments are provided below.\nMajor Comments\n1. The authors select 7 years of near bottom salinity time series to evidence the enhancement of\nthe DSW formation in positive SAM conditions (lines 95-100). They explain that the observed near\nbottom depths at the mooring are consistent in these 7 years. However, vertical differences of the\nsalinity beneath 800 m in TNBP are generally less than 0.01, much lower than the interannual\nvariability. I suggest that more observed time series of salinity near the bottom could be shown to\nprovide a robust support for the connection between the DSW formation and SAM phase changes.\n2. The simulated sea ice production and salinity are correlated well with the observed time series.\nBut the absolute values of the simulated SIP and salinity are obviously higher than the observed\nvalues in the Extended Data Fig 5 and 6. I suggest the authors explain how this large difference\noccurs and whether this difference will influence the sensitivity experiments.\n3. Given that the ASL will move eastward in winter and the positive trend of SAM will be\nstrengthened in the future in CMIP5 and CMIP6 models, the authors argue that both trends of ASL\nand SAM would enhance sea ice production, DSW formation and AABW volume in the deep ocean\n(lines 218-228). However, positive phase of SAM is generally believed to suppress the DSW\nformation and its density. And CMIP6 models also show significant warming of the Antarctic Shelf\nbottom water due to positive SAM trends and this would contribute negatively to the AABW volume\nin the Southern Ocean (Purich et al., 2021, GRL). I understand this study focus on a different\nsubject, but I think more discussions can be added in this manuscript.\nMinor comments\n1. Line 5. Make sure the “longest” is correct.\n2. Line 35-37. I think there are many previous studies that have investigate the connections of DSW\nformation to climate forcings (may be in different perspectives), e.g., Silvano et al., 2020 (NG) and\nZhou et al., 2023 (NCC).\n3. Lines 73-82 I am a little confused why the authors do the correlation analysis between SIC and\nSAM index. Do the authors intent to prove that lower SIC during high SAM years means larger\npolynya extent and stronger polynya activity? If so, I suggest to use the sea ice thickness to define\nthe polynya extent, like Tamura et al., 2008 and 2016 (sea ice thickness<0.2 m). Because the\npolynya is generally covered by thin ice, and sea ice concentration is not appropriate to represent\nthe strength of polynya activity.\n4. Lines 104-107 The salinity variations from mid-July to mid-Oct are calculated. If the salinity\nincrease starts earlier or later than mid-July, will this parameter underestimate or overestimate the\npolynya activity?\n5. Fig 1c. Error bars should be added to winter-mean zonal wind anomaly.", "reviewer_3": "Review of “Evidence for large-scale climate forcing of dense shelf water variability in the Ross Sea”\nBy Prof. Adrian McDonald (University of Canterbury)\nThis work examines a mixture of atmospheric reanalyses, satellite sea ice data, mooring data in the\nvicinity of Terra Nova bay polynya and output from the ROMS model to examine the influence of\nlarge scale climate forcing on DSW and AABW and infer future change. The ideas presented are\nnew, build off previous research and put together an impressive amount of work to form\nconclusions. Fundamentally, this is a strong piece of research that should be reported. But, the\ncurrent presentations lacks clarity and I question whether some of the conclusions need further\nsupport before this work should be published. I would suggest significant reworking of the\nmanuscript and clarification, but extra analysis may not be necessary.\nMajor comments:\nClarity of presentation: The text is written well, but throughout the text I was constantly wondering\nabout the reasoning behind the selection of the 2000-2014 period for the analysis which seemed\nlike an odd choice. I also wondered about the robustness of composites based on even shorter\nsubsets of data within this period. The authors reasoning for this selection was only clarified near\nthe end of the article on Line 209 which identified that this period related to a time when the\ninfluence of the Amundsen Sea Low (ASL) would be expected to be weaker on the polynya regions\nin the Ross Sea. If the paper was presented as a study of the influence of the Southern Annular\nMode on polynya, DSW and AABW during periods of weak ASL influence the work would have made\nmuch more sense to me earlier and would have been more convincing to me. It would also have put\nthe work into a more consistent context given previous work that has shown that the ASL strongly\nmodulates sea ice in the Ross Sea region, which would in turn impact DSW and AABW. I would thus\nsuggest rewriting to highlight this key point much earlier in the document. I would still question the\nrobustness of composites and that robustness could be further tested. But, it would at least\nprovide guidance on the choice of the period selected to aid the reader. I also note that at around\nLine 90 I questioned the interpretation in terms of SAM because I did not have the ASL\ncontext. So I think some reordering and highlighting the weak ASL state (due to its eastward location) would\nmake a huge amount of sense.\nDiscussion of RISP: Some of the data available is specific to the Terra Nova Bay Polynya (TNBP)\nregion and the analysis is thus rightly focussed on this region. However, given the size of the Ross\nIce Shelf Polynya (RISP) relative to TNBP and the RISP’s well known influences, I am a little\nconcerned that the strong evidence reported for the TNBP is extrapolated to the RISP region and\nthe wider Ross Sea region. For example, while the literature supports the fact that the TNBP is\nstrongly impacted by primarily zonal katabatic flows the RISP is much more strongly controlled by\nmeridional flows associated with the Ross Ice Shelf Airstream – a local low-level jet. Thus, the\nidentification that the model analysis shows “No significant changes in the meridonal wind …\nover the Ross Sea” on Line 169 for SAM variations makes me wonder whether a deeper examination of\nthe RISP and its contributions to DSW and AABW is really required to make the broader statements\nthat are inferred. I do acknowledge the authors identification of the temperature anomalies\nassociated with SAM over the entire Ross Sea region – but I am sceptical that this has a major\ninfluence. Fundamentally, I believe the polynya our more responsive to the winds based on\nprevious literature. Can some identification of the contributions to DSW and AABW from the RISP\nand TNBP be done or is there something that I missing in my interpretation of Figure 5. I also note\nthat the very broad patterns seen in Figure 5 e-h make me wonder about the wider influence of RISP.\nSpecific comments:\nLine 22: I think the current citation is probably not great for the influence of katabatic winds on\npolynya. One of the references below might be more relevant and also talks about the influence of\nother winds on polynya in the Ross Sea region.\nBlanchard-Wrigglesworth, E., Roach, L. A., Donohoe, A., & Ding, Q. (2021). Impact of Winds and\nSouthern Ocean SSTs on Antarctic Sea Ice Trends and Variability. Journal of Climate, 34(3), 949-\n965. https://doi.org/https://doi.org/10.1175/JCLI-D-20-0386.1\nDale, E. R., McDonald, A. J., Coggins, J. H. J., & Rack, W. (2017, Jan). Atmospheric forcing of sea ice\nanomalies in the Ross Sea polynya region [Article]. Cryosphere, 11(1), 266-280.\nhttps://doi.org/10.5194/tc-11-267-2017\nFarooq, U., Rack, W., McDonald, A., & Howell, S. (2023). Representation of sea ice regimes in the\nWestern Ross Sea, Antarctica, based on satellite imagery and AMPS wind data. Climate Dynamics,\n60(1), 227-238. https://doi.org/10.1007/s00382-022-06319-9\nHollands, T., & Dierking, W. (2016). Dynamics of the Terra Nova Bay Polynya: The potential of multi-\nsensor satellite observations. Remote Sensing of Environment, 187, 30-48.\nhttps://doi.org/https://doi.org/10.1016/j.rse.2016.10.003\nKwok, R., Pang, S. S., & Kacimi, S. (2017). Sea ice drift in the Southern Ocean: Regional patterns,\nvariability, and trends. Elementa: Science of the Anthropocene, 5.\nhttps://doi.org/10.1525/elementa.226\nThompson, L., Smith, M., Thomson, J., Stammerjohn, S., Ackley, S., & Loose, B. (2020). Frazil ice\ngrowth and production during katabatic wind events in the Ross Sea, Antarctica. The Cryosphere,\n14(10), 3329-3347. https://doi.org/10.5194/tc-14-3329-2020\nLine 30: This sentence is correct, but the Ross Ice Shelf Polynya (RISP) is not connected directly to\nkatabatic flows which would have no forcing over the Ross Ice Shelf. So the influence of katabatic\nflows and barrier winds due to synoptic systems instead leads to strong low-level jets (such as the\nRoss Ice Shelf airstream) which impact the RSP. See the following papers:\nCoggins, J. H. J., McDonald, A. J., & Jolly, B. (2014). Synoptic climatology of the Ross Ice Shelf and\nRoss Sea region of Antarctica: k-means clustering and validation. International Journal of\nClimatology, 34(7), 2330-2348. https://doi.org/10.1002/joc.3842\nDuVivier, A. K., Molina, M. J., Deppenmeier, A. L., Holland, M. M., Landrum, L., Krumhardt, K., &\nJenouvrier, S. (2024). Projections of winter polynyas and their biophysical impacts in the Ross Sea\nAntarctica [Article]. Climate Dynamics, 62(2), 989-1012. https://doi.org/10.1007/s00382-023-\n06951-z\nFarooq, U., Rack, W., McDonald, A., & Howell, S. (2023). Representation of sea ice regimes in the\nWestern Ross Sea, Antarctica, based on satellite imagery and AMPS wind data. Climate Dynamics,\n60(1), 227-238. https://doi.org/10.1007/s00382-022-06319-9\nParish, T. R., Cassano, J. J., & Seefeldt, M. W. (2006). Characteristics of the Ross Ice Shelf air stream\nas depicted in Antarctic Mesoscale Prediction System simulations [Article]. Journal of Geophysical\nResearch-Atmospheres, 111(D12), Article D12109. https://doi.org/10.1029/2005jd006185\nSeefeldt, M. W., & Cassano, J. J. (2008, Nov). An Analysis of Low-Level Jets in the Greater Ross Ice\nShelf Region Based on Numerical Simulations [Article]. Monthly Weather Review, 136(11), 4188-\n4205. https://doi.org/10.1175/2008mwr2455.1\nSentence staring on Line 42: You are correct that SAM influence on coastal latent heat polynya and\ndownstream impact on DSW information has not been revealed. But, the SAM is known to\nmodulate the ASL and a few studies have suggested that ASL modulates winds and sea ice. The\nfollowing papers are relevant, but Raphael et al. (2019) is clearly the most relevant.\nCoggins, J. H. J., & McDonald, A. J. (2015). The influence of the Amundsen Sea Low on the winds in\nthe Ross Sea and surroundings: Insights from a synoptic climatology [Article]. Journal of\nGeophysical Research-Atmospheres, 120(6), 2167-2189. https://doi.org/10.1002/2014jd022830\nDuVivier, A. K., Molina, M. J., Deppenmeier, A. L., Holland, M. M., Landrum, L., Krumhardt, K., &\nJenouvrier, S. (2024). Projections of winter polynyas and their biophysical impacts in the Ross Sea\nAntarctica [Article]. Climate Dynamics, 62(2), 989-1012. https://doi.org/10.1007/s00382-023-\n06951-z\nRaphael, M. N., Holland, M. M., Landrum, L., & Hobbs, W. R. (2019). Links between the Amundsen\nSea Low and sea ice in the Ross Sea: seasonal and interannual relationships [Article]. Climate\nDynamics, 52(3-4), 2333-2349. https://doi.org/10.1007/s00382-018-4258-4\nSentence starting on Line 101: I wonder whether the presence of ice shelf water plumes, which\nhave also been reported in the general vicinity might impact these results.\nRobinson, N. J., Williams, M. J. M., Stevens, C. L., Langhorne, P. J., & Haskell, T. G. (2014). Evolution\nof a supercooled Ice Shelf Water plume with an actively growing subice platelet matrix [Article].\nJournal of Geophysical Research: Oceans, 119(6), 3425-3446.\nhttps://doi.org/10.1002/2013JC009399\nLine 187: Made me wonder whether you should be identifying that ASL and SAM are not\nindependent in general, but maybe for this specific period?\nSentence starting on Line 211: Connecting to my earlier major comment. This sentence seems to\nsuggest that the SAM influence is strong in periods when ASL is weak. I don't know if there are\ncompeting effects, but I think there would be for RISP. Given its larger size it could be more\nimportant for DSW and AABW all things being equal. This sentence made me wonder whether it\npossible to run the model with a ASL strong/weak configuration for positive and negative SAM to\nstrengthen the work. But, this might beyond the scope of the current study.", "reviewer_author_response": "Response to Review Comments\nWe thank the editor and reviewers for their efforts in making constructive remarks and suggestions,\nwhich have significantly improved the quality of our manuscript. Below you can find point-by-\npoint replies to the major and minor comments (font in Italic) and the corresponding revisions to\nthe manuscript. In the revised manuscript, revisions are highlighted by light-blue color. We hope\nthat all the editor’s and reviewers’ concerns have been addressed adequately.\nReviewer #1 (Remarks to the Author):\n[Unconditional Reject]\nAuthors tried to show the impact of SAM on the coastal polynya production and the ensuing dense\nwater formation in the Ross Sea. However, I cannot recommend this paper to be published in this\nhigh-profile journal. I cannot understand and follow the authors’ arguments at all. Detailed\nreasons are listed below:\nWe thank the reviewer for making efforts in reviewing this manuscript. We noticed that all the\nreview comments are limited to text before Line 70 and are mostly related to grammar issues. We\nthink our concepts and arguments may be better clarified if the reviewer has comments for the\nlatter part of the manuscript, providing a chance for us to further interact with the reviewer. Below\nare our point-to-point response to the review comments, and hope we have clarified the issues that\nthe reviewer is concerned about in the response.\nLine 19 [a clerical error] transports -> transport\nRevised accordingly (Line 19 in the new version).\nLine 19 Remove ‘also’. It is unnecessary. I don’t understand what this stands for\n“Also” is removed in the new version (Line 19).\nLines 19-20 ‘… play important roles …’ -> ‘… play an important role …’\nRevised accordingly (Line 20).\nLine 20 ‘highly productive’ for what? Clarify it.\nThis sentence has been revised as “… from the biologically productive polynyas to …” (Line 20)\nLine 21 [a clerical error] Antarctica -> Antarctic\nRevised accordingly (Line 21).\nLine 22 the Antarctic continent over the ocean? What on earth does this mean? How can the\ncontinent be located over the ocean?\nFor clarity, this sentence is revised to “…katabatic winds blowing off the Antarctic continent\ntoward the ocean” (Line 22).\nLine 24 Add ‘The’ in front of ‘DSW’. DSW is -> The DSW is\nWe added “the” in front of “DSW” in the two places (Line 24).\nLine 25 Add ‘such’ between ‘slope’ and ‘as’. the continental slope as -> the continental slope\nsuch as\nHere “as” means “appearing to be” or “in the form of”, not “such as”.\nLines 26-27 ‘entraining other water … (CDW) and Antarctic Surface Water to ultimately form\nAABW’ After the DSW forms, how does the DSW entrain the Antarctic Surface Water as it flows\ndown the continental slope? I don’t understand. Clarify.\nThe Antarctic surface water can be mixed into the CDW, and then entrained into the DSW. As the\nDSW only directly entrains the CDW, we revised this sentence to read: “entraining the\nCircumpolar Deep Water (CDW) to ultimately form AABW” (Lines 26–27).\nLine 28 ‘The Ross Sea produces … production’ What is this supposed to mean? Replace\n‘produces’ with ‘covers’.\nWe think “is responsible for” may be more appropriate, and thus replaces “produces” with “is\nresponsible for” (Line 28).\nLine 31 ‘cascading’ seems to be wrong choice of word. Replace ‘cascading’ with ‘flowing’.\nSeveral past papers use the word “cascade” when describing the katabatic wind. For example,\nMathiot et al. (2012) mentions “The TNB polynya is opened by the strong katabatic winds\ncascading eastward down from the Transantarctic Mountains …”. Nonetheless we took the\nreviewer’s suggestion and changed “cascading” to “flowing” (Line 41).\nReference\nMathiot, P., Jourdain, N. C., Barnier, B., Gallée, H., Molines, J. M., Le Sommer, J. & Penduff, T.\nSensitivity of coastal polynyas and high-salinity shelf water production in the Ross Sea, Antarc-\ntica, to the atmospheric forcing. Ocean Dyna. 62, 701–723 (2012).\nLine 34 ‘a dramatic salinity seasonal increase’ I don’t understand what this means. It needs\ngrammar check.\n“a dramatic salinity seasonal increase” is revised as “a dramatic seasonal salinity increase” (Line\n37).\nLine 34 ‘a dramatic salinity seasonal increase in bottom water, a clear indication of the start of\nDSW formation …’ This sentence has two subjects without any relative pronoun. Modify as\nfollows. ‘a dramatic salinity seasonal increase in bottom water, which is a clear indication of the\nstart of DSW formation, …’\nRevised accordingly (Line 37).\nLine 36 [a clerical error] no studies -> no study\nConsidering the comments of Reviewer #2, this sentence has been revised as “there have been\nlimited studies …” (Line 38).\nLine 41 Add ‘the’ in front of ‘SAM’. A shift of SAM -> A shift of the SAM\n“The” is added before “SAM” (Line 45).\nLine 42 Add ‘the’ in front of ‘SAM’. While SAM -> While the SAM\nRevised (Line 47).\nLine 43 Remove ‘the’ in front of ‘infrequent opening’. the infrequent opening -> infrequent\nopening\n“The” is removed (Line 47).\nLine 44 ‘low-frequency variability’: In comparison with which phenomenon, on which time\nscale, what is this supposed to mean? Clarify.\nWe revised this sentence to now read: “its linkage to variability on interannual and decadal\ntimescales in the coastal latent heat polynyas” (Lines 48–49).\nLine 45 ‘relatively small’: In comparison with what? Are you saying that an impact of the SAM-\ninduced coastal polynyas on the AABW formation is weaker than that of the SAM-induced open-\nocean polynya? A major site of the AABW formation is the Weddell Sea coastline. Since you are\njust talking about the DSW, coastal polynyas and AABW formation in the Weddell Sea should be\nincluded. Then, it cannot be ‘relatively small’. The whole sentence ‘While SAM plays … has not\nbeen revealed’ are very weird. Clarify.\nThe sentence before this one mentions the sensible heat polynyas, and this sentence is “… in the\ncoastal latent heat polynyas, which are relatively small in size but exhibit dramatic DSW\nformation”. So “relatively small” here means that the size of coastal latent heat polynyas are\nnormally relatively small compared to the sensible heat polynyas, particularly for the Terra Nova\nBay polynya that this study is focused on. It is true that some coastal polynyas, such as the Ross\nIce Shelf polynya and the polynyas along the Weddell Sea coast are large, so we revised this\nsentence to “… in the coastal latent heat polynyas, which are important DSW formation sites”.\nThe whole sentence has been rewritten following the reviewer’s comments above (Line 49–50).\nLines 59-63 [Self-contradiction] ‘after examining the wind data … over the whole period. However,\na significant and positive … 2000-2014 (R=0.72, P=0.006).’ If you did not find a significant\nrelationship between the interannual variations of zonal wind in this region and SAM for the whole\nperiod over the past 30 years, what is the scientific meaning for the positive correlation between\nthem just during a shorter period. I think that this analysis for the shorter period is scientifically\nmeaningless.\nWe do not agree with this point. Regional processes in the Southern Ocean can be affected by\nseveral climate forcings (climate modes or systems), and it is important for us to know why one\nclimate forcing can dominant the variability of atmospheric, oceanic and sea ice processes in\ncertain periods, and also by what mechanism(s) the climate forcing modulates these processes.\nThis knowledge can improve our ability to predict changes in Southern Ocean processes when we\nknow the nature of future trends in climate forcings; e.g. a positive trend in the SAM. In particular,\nin the last section of the manuscript we describe that as the SAM tends to a more positive phase,\nand the ASL tends to move eastward in the future, that the variation of Ross Sea polynya processes\nand DSW/AABW formation might become dominated by the SAM. We can also use SAM\nprojections from CMIP models to project future changes of polynyas and DSW formation\nprocesses in this region.\nLines 67-70 ‘The winter mean surface … was insignificant for 2000-2014’ O.K. If you are correctly\narguing, the positive SAM anomaly leads to lower-than-normal surface air temperature, eventually\nleading to higher-than-normal sea-ice production. Then, What is your point here? It is too obvious.\nClarify what you really want to express.\nThe role of lower surface air temperature in sea ice production in the Ross Sea polynyas is not so\nobvious as people might expect. For ice production in the Ross Ice Shelf polynya (RISP), most\nearlier studies address the role of offshore winds from the Ross Ice Shelf air stream and synoptic-\nscale events, and few studies have mentioned the effects of surface air temperature. This is why\nReviewer #3 suggests that we provide more evidence on the importance of air temperature in sea\nice production in the RISP and the DSW formation, and we have designed numerical sensitivity\nexperiments to demonstrate this. The discussions are provided in Lines 228–242 of the revised\nmanuscript.\nReviewer #2 (Remarks to the Author):\nReview of “Evidence for large-scale climate forcing of dense shelf water variability in the Ross\nSea”, by Zhang et al.\nThis study primarily uses a long mooring record over the continental shelf of the Ross Sea and\nnumerical simulations and satellite data to investigate relationship between interannual\nvariability of DSW formation and SAM index. The authors find that when the ASL is weakened\nand further away from the Ross Sea, positive SAM phases enhance the offshore winds in the\nwestern Ross Sea, promote the sea ice production and DSW formation in the TNBP polynya, and\nultimately increase the AABW volume in the deep ocean off the Ross Sea.\nBecause the variability of DSW formation is critical to the overturning circulation and global\nclimate system, I believe these findings presented in this manuscript will be of interest to the Nature\nCommunication readers. The authors have done great jobs illustrating the evidence for the\nmodulation of DSW variability by SAM phase changes. But I think more could be done to make the\nconclusions more robust before publication. Detailed comments are provided below.\nWe thank the reviewer for the constructive comments on this work, which help to improve the\nquality of the manuscript. We acknowledge that some discussions need to be elaborated to provide\nstronger support for the arguments of our study. Please see our detailed response to the specific\ncomments below.\nMajor Comments\n1. The authors select 7 years of near bottom salinity time series to evidence the enhancement of\nthe DSW formation in positive SAM conditions (lines 95-100). They explain that the observed near\nbottom depths at the mooring are consistent in these 7 years. However, vertical differences of the\nsalinity beneath 800 m in TNBP are generally less than 0.01, much lower than the interannual\nvariability. I suggest that more observed time series of salinity near the bottom could be shown to\nprovide a robust support for the connection between the DSW formation and SAM phase changes.\nTo respond to this point, first we checked the mooring data from 1995 to 2014 obtained by the\nItalian CLIMA project (which actually changed to the MORsea project after 2009, and we made\nthis revision in the revised manuscript), to see if there are years when salinity data from more than\n2 depth levels below 800 m are available, such that we can know the vertical range of salinity in\nthe bottom layer, but there are no such data. We then examined CTD observations in the Terra\nNova Bay region from the World Ocean Database, and plotted the vertical profiles of salinity in\nthe depth range of 800–1000 m for different months (Fig. R1). It can be seen that in austral winter\nand spring, the seasons focused on in this work, the vertical salinity variation in this depth range\ncan often exceed 0.01 PSU and reach as high as 0.03 PSU, which is comparable to or greater than\nthe interannual variation of bottom layer salinity in this region (Fig. 4h in the manuscript). In this\ncase, we may not be able to use salinity data at other depth levels below 800 m to analyze\ninterannual variations of bottom salinity. We understand the reviewer’s concern about using a\nlimited period (7 years) of mooring data to obtain the DSW-SAM relation. In fact, we considered\nthis point and demonstrated the robustness of such a relation using 12-year model simulation which\nis shown in Extended Data Fig. 3 in the original and revised manuscript. The result supports the\nconclusion drawn from mooring observations that the interannual variation of DSW is significantly\ncorrelated with SAM in the Terra Nova Bay polynya.\nFig. R1. Vertical profiles of salinity in the depth range of 800–1000 m in the Terra Nova Bay\nregion based on data from the World Ocean Database.\n2. The simulated sea ice production and salinity are correlated well with the observed time series.\nBut the absolute values of the simulated SIP and salinity are obviously higher than the observed\nvalues in the Extended Data Fig 5 and 6. I suggest the authors explain how this large difference\noccurs and whether this difference will influence the sensitivity experiments.\nIt is true that the model overestimates sea ice production (SIP) in the Terra Nova Bay and Ross Ice\nShelf polynyas compared to satellite estimates. The overestimate of SIP in Antarctic polynyas\nseems to be a common issue for Southern Ocean or global high-resolution coupled ocean-ice\nmodels, such as the Hamburg Ocean Primitive Equation Model for the Southern Ocean in Zhang\net al. (2015), the coupled ocean-sea ice-ice shelf model in Kusahara et al. (2017), and the Finite\nElement Sea-Ice Ocean Model in Wang et al. (2021). On the one hand, we acknowledge that there\nmight be some missing or unresolved processes in the model that lead to such an SIP overestimate,\nsuch as missing eddy heat fluxes and tidal effects. On the other hand, the satellite product cannot\nbe treated as pure observations and may also have biases related to the retrieval algorithms. For\nexample, the SIP product used in this study (as well as most other satellite products) does not\ninclude oceanic heat fluxes. Based on Thompson et al. (2020), in the Terra Nova Bay during winter\nthe ocean temperature is colder in the subsurface layer (below 10 m) than in the surface layer (see\ntheir Fig. 4). As such, if oceanic heat fluxes are considered, the surface ocean should be cooled\nmore by vertical ocean heat exchange, which can result in greater SIP than the estimation in the\nsatellite product, and in such a case our model simulations would match better with the satellite\nSIP. Such discussions are provided in Lines 367–371 of the revised version.\nFor the overestimate of DSW salinity by the model, in our recent study it is found that this is partly\nassociated with the underestimate of ice shelf melting rates in the Amundsen Sea, which results in\nreduced fresh meltwater transported to the Ross Sea resulting in higher DSW salinity. We then\nundertook sensitivity experiments in which the ice shelf melting rates are artificially increased by\nmodulating the heat and salt transfer coefficients at the ice shelf base, and in these experiments the\nhigh-salinity bias in the model is significantly reduced (see Fig. R2). It should be noted that while\nthe DSW salinity is overestimated in the model, we are focused on the interannual variations of\nDSW salinity in this work. The model simulations are mainly used for examining the relation\nbetween variability of DSW on the slope and SAM (as long-term observations are missing), and\nfor the sensitivity experiments. Extended Data Fig. 6 in the manuscript shows that the modelled\nDSW temporal variation on the slope is significantly and highly correlated with observations\nduring mooring periods (R>0.8 and P<0.001). As such, we believe that the model revealed\nrelationship between DSW in the slope region and SAM should be robust, and the model-produced\nchange of DSW associated with the SAM variation is valid. The relevant discussions have been\nadded to Lines 379–383 of the revised version.\nFig. R2. (a) Time series of 5-day-average neutral density at 1735 m from the model CTRL\nsimulation (red solid line), the model simulation with increased ice shelf melting rates in the\nAmundsen Sea (red dashed line), and observations (blue line) at the location of mooring CA1\ndeployed in the U.S. CALM program. (b) Same as (a) but for neutral density at 1929 m at the CA2\nmooring location. The correlation coefficients between the simulated neutral density from the\nCTRL run and observed density (R) and p-values (P) are provided.\nReferences\nZhang, Z., Vihma, T., Stössel, A. & Uotila, P. The role of wind forcing from operational analyses\nfor the model represen- tation of Antarctic coastal sea ice. Ocean Model. 94, 95–111 (2015).\nKusahara, K., Williams, G. D., Tamura, T., Massom, R. & Ha- sumi, H. Dense shelf water\nspreading from Antarctic coastal polynyas to the deep Southern Ocean: A regional circumpo- lar\nmodel study. J. Geophys. Res. Oceans. 122, 6238–6253 (2017).\nWang, X., Zhang, Z., Dinniman, M.S., Uotila, P., Li, X & Zhou, M. The response of sea ice and\nhigh-salinity shelf water in the Ross Ice Shelf Polynya to cyclonic atmosphere circulations. The\nCryosphere 17, 1107–1126 (2023).\nThompson, L., Smith, M., Thomson, J., Stammerjohn, S., Ackley, S. & Loose, B. Frazil ice growth\nand production during katabatic wind events in the Ross Sea, Antarctica. The Cryosphere 14,\n3329–3347 (2020).\n3. Given that the ASL will move eastward in winter and the positive trend of SAM will be\nstrengthened in the future in CMIP5 and CMIP6 models, the authors argue that both trends of ASL\nand SAM would enhance sea ice production, DSW formation and AABW volume in the deep ocean\n(lines 218-228). However, positive phase of SAM is generally believed to suppress the DSW\nformation and its density. And CMIP6 models also show significant warming of the Antarctic Shelf\nbottom water due to positive SAM trends and this would contribute negatively to the AABW volume\nin the Southern Ocean (Purich et al., 2021, GRL). I understand this study focus on a different\nsubject, but I think more discussions can be added in this manuscript.\nThe reviewer raised a good point. In relation to the “positive phase of SAM is generally believed\nto suppress the DSW formation and its density”, we did not find the relevant literature, but we\nguess it might be due to the fact that positive SAM phase results in enhancement and poleward\nshift of the subpolar westerlies, which can promote upwelling and on-shelf intrusion of the\nCircumpolar Deep Water (CDW) that causes more ice shelf basal melting and more meltwater\nvolume, suppressing the formation of dense DSW. The effects of increased meltwater transport to\nthe Ross Sea from the Amundsen Sea are included in the discussions of the original manuscript as\n“This may help lessen the observed freshening trend of bottom water in the Ross Sea and could\npartially counteract the impacts that increased ice shelf melting in the West Antarctica are\nprojected to have on reducing Ross Sea DSW salinity and in turn reducing AABW production”. For\nthe reference Purich et al. (2021, GRL), we assume the reviewer is referring to Purich and England\n(2021, GRL), who show a notable warming trend of Antarctic shelf bottom water based on\nprojections from CMIP6 models, and such a trend would also inhibit the formation of dense DSW;\ndiscussions of this point have been added to the revised version as “it could also balance the effect\nof warming of Antarctic shelf water associated with a positive shift of the SAM that leads to\npoleward shift of westerlies and intensification of CDW upwelling onto the shelf, which can\nsuppress DSW formation55” (Lines 294–297).\nMinor comments\n1. Line 5. Make sure the “longest” is correct.\nThe mooring deployed by the Italian CLIMA project in the Terra Nova Bay spans the period of\n1995 to now, and to the best of our knowledge there are no other moorings spanning a comparable\nor longer period. That said, as this study mainly uses the mooring data from 2008 to 2014, we\nchanged “the longest mooring record of DSW measurements …” to “a long mooring record of DSW\nmeasurements …” in the revised version (Line 5).\n2. Line 35-37. I think there are many previous studies that have investigated the connections of\nDSW formation to climate forcings (may be in different perspectives), e.g., Silvano et al., 2020\n(NG) and Zhou et al., 2023 (NCC).\nThe statement in the original manuscript is “In fact, a dramatic salinity seasonal increase in bottom\nwater, a clear indication of the start of DSW formation occurs in austral winter. Up until now, there\nhave been no studies revealing the variations of winter DSW formation or their connection to\nclimate modes …”. It is true that Silvano et al. (2020) and Zhou et al. (2023) discussed the\nconnections of DSW formation with climate forcings, while in this work we address the DSW\nproduction in austral winter, i.e. the season when DSW is intensely formed, and we mainly discuss\nthe interaction between winter DSW formation and the SAM. To be more accurate, we have revised\nthis sentence to read: “Up until now, there have been limited studies revealing the long-term\nvariations of DSW formation and their connection to climate modes28,30, and studies discussing\nthe relationship between wintertime DSW production and SAM are particularly scarce” (Lines 38–\n41). References 28 and 30 denote Silvano et al. (2020) and Zhou et al. (2023), respectively.\n3. Lines 73-82 I am a little confused why the authors do the correlation analysis between SIC and\nSAM index. Do the authors intent to prove that lower SIC during high SAM years means larger\npolynya extent and stronger polynya activity? If so, I suggest to use the sea ice thickness to define\nthe polynya extent, like Tamura et al., 2008 and 2016 (sea ice thickness<0.2 m). Because the\npolynya is generally covered by thin ice, and sea ice concentration is not appropriate to represent\nthe strength of polynya activity.\nFor the indication of the polynya extent and activities, we followed a couple of studies that use sea\nice concentration as the criterion, such as Wenta and Cassano (2020), Ding et al. (2020) and Haid\nand Timmerman (2013; this study uses SIC<70% or ice thickness < 0.2 to indicate polynya area).\nLower sea ice concentration indicates more open-water area and larger polynya extent, which is\nexplained in Line 129 of the revised manuscript. Many studies developed thin ice thickness\nalgorithms for the Southern Ocean and identified areas with ice thickness lower than 0.2 m, using\nthis to indicate polynya areas and then estimating sea ice production based on the ice thickness\nand surface heat fluxes (e.g., Tamura et al., 2008; 2016; Nihashi and Ohshima, 2015; Nihashi et\nal., 2017). Here we chose to use sea ice concentration to indicate the polynya extent for two reasons.\nFirst, the signal of ice thickness < 0.2 m is actually included in the sea ice production data, which\nare examined in Fig. 3 (revised version). Second, the estimate of thin-ice-thickness depends on the\naccuracy of surface heat flux datasets, which normally come from atmospheric reanalysis. Nihashi\net al. (2024) demonstrates notable differences between the ice thickness estimates using ERA5 and\nERA-Interim. Inaccuracy in heat fluxes data can affect both the estimates of sea ice thickness and\nproduction rates. Considering this, we chose to use sea ice concentration, the estimate of which is\nindependent from reanalysis products, as an independent parameter for measuring the polynya\nextent. This is now explained in Lines 309–312 in the revised version.\nReferences\nDing, Y. et al. Specific Relationship between the Surface Air Temperature and the Area of the Terra\nNova Bay Polynya, Antarctica. Adv. Atmos. Sci. 37, 532–544 (2020).\nHaid, V. & Timmermann, R. Simulated heat flux and sea ice production at coastal polynyas in the\nsouthwestern Weddell Sea. JGR Oceans 118, 2640–2652 (2013).\nNihashi, S. & Ohshima, K. I. Circumpolar Mapping of Antarctic Coastal Polynyas and Landfast\nSea Ice: Relationship and Variability. JOURNAL OF CLIMATE 28, (2015).\nNihashi, S., Ohshima, K. I. & Tamura, T. Sea-Ice Production in Antarctic Coastal Polynyas\nEstimated From AMSR2 Data and Its Validation Using AMSR-E and SSM/I-SSMIS Data. IEEE\nJournal of Selected Topics in Applied Earth Observations and Remote Sensing 10, (2017).\nNihashi, S., Ohshima, K. I. & Tamura, T. Reconstruct the AMSR-E/2 thin ice thickness algorithm\nto create a long-term time series of sea-ice production in Antarctic coastal polynyas. Polar Science\n(2024).\nTamura, T., Ohshima, K. I. & Nihashi, S. Mapping of sea ice production for Antarctic coastal\npolynyas. Geophysical Research Letters 35, 2007GL032903 (2008).\nTamura, T., Ohshima, K. I., Fraser, A. D. & Williams, G. D. Sea ice production variability in\nAntarctic coastal polynyas. JGR Oceans 121, 2967–2979 (2016).\nWenta, M. & Cassano, J. J. The Atmospheric Boundary Layer and Surface Conditions during\nKatabatic Wind Events over the Terra Nova Bay Polynya. Remote Sensing 12, 4160 (2020).\n4. Lines 104-107 The salinity variations from mid-July to mid-Oct are calculated. If the salinity\nincrease starts earlier or later than mid-July, will this parameter underestimate or overestimate\nthe polynya activity?\nDuring our investigations for this work, we also analyzed the correlation for salinity variations\nover a longer period, from mid-June to the end of November, and there still exists significant\ncorrelation between the interannual variations of ∆𝑆 and SAM (R=0.79, P=0.04). We have added\nthis information in Lines 156–158 of the revised manuscript.\n5. Fig 1c. Error bars should be added to winter-mean zonal wind anomaly.\nGood suggestion. We have calculated the standard deviation of the anomaly timeseries and added\nthis as errors bars (indicated by green shading) in the revised Fig. 1c.\nReviewer #3 (Remarks to the Author):\nReview of “Evidence for large-scale climate forcing of dense shelf water variability in the Ross\nSea”\nBy Prof. Adrian McDonald (University of Canterbury)\nThis work examines a mixture of atmospheric reanalyses, satellite sea ice data, mooring data in\nthe vicinity of Terra Nova bay polynya and output from the ROMS model to examine the influence\nof large scale climate forcing on DSW and AABW and infer future change. The ideas presented\nare new, build off previous research and put together an impressive amount of work to form\nconclusions. Fundamentally, this is a strong piece of research that should be reported. But, the\ncurrent presentations lacks clarity and I question whether some of the conclusions need further\nsupport before this work should be published. I would suggest significant reworking of the\nmanuscript and clarification, but extra analysis may not be necessary.\nWe thank the reviewer for the positive comments on the overall quality of this work. We\nacknowledge that some parts of the manuscript need to be reorganized for clarity, particularly for\nthe relation between SAM and the ASL, and more analysis should be done to strengthen our\narguments and conclusions. Please see our response to the specific comments below.\nMajor comments:\nClarity of presentation: The text is written well, but throughout the text I was constantly wondering\nabout the reasoning behind the selection of the 2000-2014 period for the analysis which seemed\nlike an odd choice. I also wondered about the robustness of composites based on even shorter\nsubsets of data within this period. The authors reasoning for this selection was only clarified near\nthe end of the article on Line 209 which identified that this period related to a time when the\ninfluence of the Amundsen Sea Low (ASL) would be expected to be weaker on the polynya regions\nin the Ross Sea. If the paper was presented as a study of the influence of the Southern Annular\nMode on polynya, DSW and AABW during periods of weak ASL influence the work would have\nmade much more sense to me earlier and would have been more convincing to me. It would also\nhave put the work into a more consistent context given previous work that has shown that the ASL\nstrongly modulates sea ice in the Ross Sea region, which would in turn impact DSW and AABW.\nI would thus suggest rewriting to highlight this key point much earlier in the document. I would still\nquestion the robustness of composites and that robustness could be further tested. But, it would at\nleast provide guidance on the choice of the period selected to aid the reader. I also note that at\naround Line 90 I questioned the interpretation in terms of SAM because I did not have the ASL\ncontext. So I think some reordering and highlighting the weak ASL state (due to its eastward location)\nwould make a huge amount of sense.\nWe think the reviewer’s suggestion about earlier appearance of the discussion about SAM-ASL\nrelation is important for improving the flow of the manuscript. We have adopted this suggestion\nand moved the section “Mechanisms for a decadal relation between zonal winds over TNBP and\nSAM” (the last section prior to conclusions in the old version) to appear earlier in the revised\nmanuscript, right after the section “The relationship between interannual variations of wintertime\natmospheric fields over the TNBP and SAM”. In this section and the introduction, we provided\nmore thorough discussions about the relations between SAM and ASL, in order to explain why\nthey can be treated as independent factors affecting polynyas processes in the Ross Sea. Please\nfind these discussions in our response to the specific comments below.\nFor the composite analysis for zonal wind and sea ice concentration in positive and negative SAM\nphases, in the old version we selected 4 years in positive SAM phase and 4 years in negative SAM\nphase during the period 2000–2014. To test the robustness of the composites, in the revised version\nwe used all data in this 15-year period and selected 6 years with positive SAM indices and 9 years\nwith negative SAM indices. The updated results are shown in Fig. 1d and e of the revised version.\nThe difference in the area-mean zonal wind speed over the TNBP between the positive and\nnegative SAM composites is 0.6 m s-1, lower than the value between the original composites (1.0\nm s-1); such difference is 10% of the mean zonal winds, which is enough to significantly impact\nDSW formation rates as suggested by previous work. The difference in the area-mean sea ice\nconcentration over the TNBP is 3% (slightly greater than the value between the original composites,\nwhich was 2%). The text is now updated at Lines 69–73, Line 119 and Line 130 in the revised\nversion.\nDiscussion of RISP: Some of the data available is specific to the Terra Nova Bay Polynya (TNBP)\nregion and the analysis is thus rightly focused on this region. However, given the size of the Ross\nIce Shelf Polynya (RISP) relative to TNBP and the RISP’s well known influences, I am a little\nconcerned that the strong evidence reported for the TNBP is extrapolated to the RISP region and\nthe wider Ross Sea region. For example, while the literature supports the fact that the TNBP is\nstrongly impacted by primarily zonal katabatic flows the RISP is much more strongly controlled\nby meridional flows associated with the Ross Ice Shelf Airstream – a local low-level jet. Thus, the\nidentification that the model analysis shows “No significant changes in the meridional wind …\nover the Ross Sea” on Line 169 for SAM variations makes me wonder whether a deeper\nexamination of the RISP and its contributions to DSW and AABW is really required to make the\nbroader statements that are inferred. I do acknowledge the authors identification of the\ntemperature anomalies associated with SAM over the entire Ross Sea region – but I am skeptical\nthat this has a major influence. Fundamentally, I believe the polynyas are more responsive to the\nwinds based on previous literature. Can some identification of the contributions to DSW and\nAABW from the RISP and TNBP be done or is there something that I missing in my interpretation\nof Figure 5. I also note that the very broad patterns seen in Figure 5 e-h make me wonder about\nthe wider influence of RISP.\nWe understand the reviewer’s concern about the role of surface air temperature (SAT) on sea ice\nproduction in the Ross Ice Shelf polynya (RISP) and thus the formation of DSW and AABW, as\nmost previous studies address the role of meridional wind from the Ross Ice Shelf air stream and\nsynoptic-scale events on the opening of the polynya and sea ice production. To examine the role\nof air temperature, we conducted two sensitivity experiments in which SAT anomalies associated\nwith 1-std increase and decrease in SAM are added to the atmospheric forcing fields in 2009, a\nyear with neutral SAM state, while all other atmospheric forcing variables including winds are not\nperturbed. As shown in Fig. R3, the decrease of SAT during positive SAM phase (as shown in Fig.\n6 of the manuscript) does result in more DSW formation on the shelf (Fig. R3b compared to 3a),\nand the increase in the DSW thickness can reach 50 m and above. While there is no notable signal\nof increased AABW in the open ocean, this is highly likely due to the fact that we only integrated\nthe model for one year, and the response would be more obvious when the model is integrated\nlonger. Conversely, the increase of SAT associated with negative SAM results in less DSW\nformation on the shelf (Fig. R3c compared to 3a). These results demonstrate that the SAT change\nassociated with SAM variations does play a role in the DSW changes and should not be ignored.\nRelevant discussion has been added to Lines 228–242 of the revised manuscript, and Fig. R3 has\nbeen added to the Supplementary Information as Extended Data Fig. 4.\nThe separation of contributions from TNBP and RISP to the total changes in DSW/AABW requires\nadding DSW tracers respectively originating from the TNBP and RISP, which means that we have\nto rerun the two 12-year sensitivity experiments (SAMP and SAMN) which would take a\nsubstantial amount of computational resources and time. As an alternative solution, we put DSW\ntracers originating separately from TNBP and RISP and traced their evolution for 5 years in the\nCTRL simulation, and the results are shown in Fig. R4. We can see that the TNBP mainly affects\nDSW in the western Ross Sea (in the Drygalski Trough), while the RISP affects DSW over wider\nregions in the central and eastern Ross Sea (the Joides Trough and Glomar Challenger Trough).\nThese results further suggest that changes in DSW over the central and eastern Ross Sea in the\nSAM-perturbation experiments (Fig. 6) are mainly related to process changes in the RISP, which\nare in turn associated with surface air temperature changes.\nFig. R3. Results from sensitivity experiments in which SAT anomalies associated with 1-std\nincrease and decrease in SAM are added to the atmospheric forcing fields in a neutral-SAM year\n(2009), which are named as SAMP (T ) and SAMN (T ) respectively. (a) DSW/AABW thickness\nair air\nin the CTRL simulation, and (b) and (c) changes in DSW/AABW thickness in the two sensitivity\nexperiments compared to CTRL.\nFig. R4. Concentrations of DSW tracers originating from (a) the Ross Ice Shelf polynya and (b)\nthe Terra Nova Bay polynya in the CTRL simulation.\nSpecific comments:\n1. Line 22: I think the current citation is probably not great for the influence of katabatic winds on\npolynya. One of the references below might be more relevant and also talks about the influence of\nother winds on polynya in the Ross Sea region.\nBlanchard-Wrigglesworth, E., Roach, L. A., Donohoe, A., & Ding, Q. (2021). Impact of Winds and\nSouthern Ocean SSTs on Antarctic Sea Ice Trends and Variability. Journal of Climate, 34(3), 949-\n965. https://doi.org/https://doi.org/10.1175/JCLI-D-20-0386.1\nDale, E. R., McDonald, A. J., Coggins, J. H. J., & Rack, W. (2017, Jan). Atmospheric forcing of\nsea ice anomalies in the Ross Sea polynya region [Article]. Cryosphere, 11(1), 266-280.\nhttps://doi.org/10.5194/tc-11-267-2017\nFarooq, U., Rack, W., McDonald, A., & Howell, S. (2023). Representation of sea ice regimes in\nthe Western Ross Sea, Antarctica, based on satellite imagery and AMPS wind data. Climate\nDynamics, 60(1), 227-238. https://doi.org/10.1007/s00382-022-06319-9\nHollands, T., & Dierking, W. (2016). Dynamics of the Terra Nova Bay Polynya: The potential of\nmulti-sensor satellite observations. Remote Sensing of Environment, 187, 30-48.\nhttps://doi.org/https://doi.org/10.1016/j.rse.2016.10.003\nKwok, R., Pang, S. S., & Kacimi, S. (2017). Sea ice drift in the Southern Ocean: Regional patterns,\nvariability, and trends. Elementa: Science of the Anthropocene, 5.\nhttps://doi.org/10.1525/elementa.226\nThompson, L., Smith, M., Thomson, J., Stammerjohn, S., Ackley, S., & Loose, B. (2020). Frazil ice\ngrowth and production during katabatic wind events in the Ross Sea, Antarctica. The Cryosphere,\n14(10), 3329-3347. https://doi.org/10.5194/tc-14-3329-2020\nWe thank the reviewer for providing this literature, covering more recent work addressing the\ninfluence of katabatic winds on Ross Sea polynyas. We selected 3 references (Dale et al., 2017;\nThompson et al., 2020; Farooq et al., 2023) and added them to the revised manuscript.\n2. Line 30: This sentence is correct, but the Ross Ice Shelf Polynya (RISP) is not connected directly\nto katabatic flows which would have no forcing over the Ross Ice Shelf. So the influence of katabatic\nflows and barrier winds due to synoptic systems instead leads to strong low-level jets (such as the\nRoss Ice Shelf airstream) which impact the RSP. See the following papers:\nCoggins, J. H. J., McDonald, A. J., & Jolly, B. (2014). Synoptic climatology of the Ross Ice Shelf\nand Ross Sea region of Antarctica: k-means clustering and validation. International Journal of\nClimatology, 34(7), 2330-2348. https://doi.org/10.1002/joc.3842\nDuVivier, A. K., Molina, M. J., Deppenmeier, A. L., Holland, M. M., Landrum, L., Krumhardt, K.,\n& Jenouvrier, S. (2024). Projections of winter polynyas and their biophysical impacts in the Ross\nSea Antarctica [Article]. Climate Dynamics, 62(2), 989-1012. https://doi.org/10.1007/s00382-\n023-06951-z\nFarooq, U., Rack, W., McDonald, A., & Howell, S. (2023). Representation of sea ice regimes in\nthe Western Ross Sea, Antarctica, based on satellite imagery and AMPS wind data. Climate\nDynamics, 60(1), 227-238. https://doi.org/10.1007/s00382-022-06319-9\nParish, T. R., Cassano, J. J., & Seefeldt, M. W. (2006). Characteristics of the Ross Ice Shelf air\nstream as depicted in Antarctic Mesoscale Prediction System simulations [Article]. Journal of\nGeophysical Research-Atmospheres, 111(D12), Article D12109.\nhttps://doi.org/10.1029/2005jd006185\nSeefeldt, M. W., & Cassano, J. J. (2008, Nov). An Analysis of Low-Level Jets in the Greater Ross\nIce Shelf Region Based on Numerical Simulations [Article]. Monthly Weather Review, 136(11),\n4188-4205. https://doi.org/10.1175/2008mwr2455.1\nThe references recommended by the reviewer indeed suggest that while katabatic winds are a\ncomponent of the offshore air flow over the Ross Ice shelf polynya, they are not the major one,\nand the confluence drainage flows from the West Antarctic Ice Sheet and Transantarctic Mountains\nand synoptic-scale circulations play more important roles. Thus we revised the sentence in Line\n30 to “TNBP is opened by strong katabatic winds flowing eastward from the Transantarctic\nMountains11, and RISP is maintained by offshore winds off the Ross Ice Shelf contributed by\nconfluent drainage flows from West Antarctic ice sheets and the Transantarctic Mountains,\nsynoptic-scale circulations and katabatic winds23,24” (Lines 30–34 in the revised version). We\nadded Coggins et al. (2014) and Parish et al. (2006) as the references here.\n3. Sentence starting on Line 42: You are correct that SAM influence on coastal latent heat polynya\nand downstream impact on DSW information has not been revealed. But, the SAM is known to\nmodulate the ASL and a few studies have suggested that ASL modulates winds and sea ice. The\nfollowing papers are relevant, but Raphael et al. (2019) is clearly the most relevant.\nCoggins, J. H. J., & McDonald, A. J. (2015). The influence of the Amundsen Sea Low on the winds\nin the Ross Sea and surroundings: Insights from a synoptic climatology [Article]. Journal of\nGeophysical Research-Atmospheres, 120(6), 2167-2189. https://doi.org/10.1002/2014jd022830\nDuVivier, A. K., Molina, M. J., Deppenmeier, A. L., Holland, M. M., Landrum, L., Krumhardt, K.,\n& Jenouvrier, S. (2024). Projections of winter polynyas and their biophysical impacts in the Ross\nSea Antarctica [Article]. Climate Dynamics, 62(2), 989-1012. https://doi.org/10.1007/s00382-\n023-06951-z\nRaphael, M. N., Holland, M. M., Landrum, L., & Hobbs, W. R. (2019). Links between the Amundsen\nSea Low and sea ice in the Ross Sea: seasonal and interannual relationships [Article]. Climate\nDynamics, 52(3-4), 2333-2349. https://doi.org/10.1007/s00382-018-4258-4\nThis comment, along with Comments 5 and 6 below, involves the complex relationship between\nthe ASL and SAM, and we feel that such a relationship needs further clarification and discussion.\nTo address this, we reviewed several references (including those recommended by the reviewer)\nthat discussed the relationship between the SAM and different ASL indices (depth, latitude and\nlongitude) and that discussed the impacts of the ASL on wind and sea ice fields in the Ross Sea.\nWe organized conclusions from these studies as follows.\n1) Relationship between ASL and SAM\n• Significant correlation exists between SAM and the ASL depth (central pressure), with ASL\nbecoming deeper when SAM shifts toward its positive phase (e.g., Turner et al., 2013; Coggins\nand McDonald, 2015).\n• There is significant negative correlation between SAM and the ASL latitude in austral summer,\nautumn and winter months (Clem et al., 2017; Turner et al., 2013), such that ASL moves\nsouthward during a positive SAM phase.\n• The ASL longitude is not significantly connected with SAM in the annual mean sense (Turner et\nal., 2013), while in austral autumn it is significantly correlated with the SAM (Clem et al., 2017).\nNote that our study is focused on austral winter, and we examined the correlation between SAM\nand the ASL longitude and found that they are not significantly correlated in this season,\nconsistent with the conclusion from Clem et al. (2017).\n2) The influence of the ASL on Ross Sea wind and sea ice\n• Raphael et al. (2019) showed that the ASL can significantly modulate meridional wind in the\nRoss Sea. As our study is focused on the austral winter, we paid particular attention to their\nresults for the ice-advance season. As shown in Fig. 6 of Raphael et al. (2019) (displayed below),\nonly the actual central pressure (ACP) of the ASL has significant impacts on the meridional wind\nin the Ross Sea, and as for the polynya areas, there is only weak response of the meridional winds\nover the Ross Ice Shelf polynya. As stated in the manuscript (Lines 171–172 in the original\nversion), we found that the meridional wind over the Ross Sea is not significantly influenced by\nthe SAM. Therefore, in terms of wind, the influences of SAM and ASL seem to be independent.\nSubplots (a), (e) and (g) of Fig. 6 in Raphael et al. (2019), which show regression of the ACP,\nlatitude and longitude of the ASL against the 10 m meridional wind during ice-advance seasons.\nBlack contour outlines the region that is significant at 0.05 confidence level.\n• For sea ice in the Ross Sea, Raphael et al. (2019) showed that during the ice-advance season,\nthere are significant impacts of the ASL ACP and ASL longitude on sea ice (concentration) in\nthe northern Ross Sea (see their Fig. 9. as shown below), while ice concentration in the polynya\nareas show insignificant response to changes in the ASL indices. Our correlation analysis and\nsensitivity experiments suggest that the SAM can significantly modulate sea ice and oceanic\nprocesses in the Ross Sea polynyas, and in this sense, SAM will not influence the polynya\nprocesses via interactions with ASL.\nGiven the discussions above, we wish to avoid any confusion about the effects of SAM and ASL\non the Ross Sea. In response we have now added more discussion about the relationship between\nASL and SAM in the revised section “Mechanisms for a decadal relation between zonal winds\nover the TNBP and SAM” (Lines 96–100), and addressed the dominance of the SAM during the\nperiod 2000–2014, when the ASL was shifted away from the Ross Sea.\nSubplots (a), (e) and (g) of Fig. 9 in Raphael et al. (2019), which show regression of the ACP,\nlatitude and longitude of the ASL against sea ice concentration during the ice-advance season.\nBlack contour outlines the region that is significant at 0.05 confidence level.\nReferences\nTurner, J., Phillips, T., Hosking, J. S., Marshall, G. J. & Orr, A. The Amundsen Sea low.\nInt. J. Climatol. 33, 1818–1829 (2013).\nClem, K., Renwick, J. A. & McGregor, J. Large-Scale Forcing of the Amundsen Sea Low and\nIts Influence on Sea Ice and West Antarctic Temperature. J. Clim. 30(20), 8405–8424 (2017).\n4. Sentence starting on Line 101: I wonder whether the presence of ice shelf water plumes, which\nhave also been reported in the general vicinity might impact these results.\nRobinson, N. J., Williams, M. J. M., Stevens, C. L., Langhorne, P. J., & Haskell, T. G. (2014).\nEvolution of a supercooled Ice Shelf Water plume with an actively growing subice platelet matrix\n[Article]. Journal of Geophysical Research: Oceans, 119(6), 3425-3446.\nhttps://doi.org/10.1002/2013JC009399\nAt Line 101, we mentioned that “The late occurrence of salinity increase is possibly associated\nwith advection of upstream fresher water …”. In fact the fresh water we mentioned here is mainly\nice shelf meltwater (or ice shelf water plumes). To be clearer, we revised the sentence to “The late\noccurrence of salinity increase is possibly associated with advection of upstream ice shelf\nmeltwater …” (Line 146). The paper by Robinson et al. (2014) recommended by the reviewer talks\nabout the McMurdo-Ross Ice Shelf, which can be included into “the upstream ice shelf”. This\nreference is not separately cited in the manuscript due the current number of references has reached\nthe upper limited required by Nature Communications.\n5. Line 187: Made me wonder whether you should be identifying that ASL and SAM are not\nindependent in general, but maybe for this specific period?\nIn our response to Comment #3, the independence of ASL and SAM and their impacts on the Ross\nSea atmospheric and sea ice fields have been clarified, combining the conclusions from existing\nstudies and findings from our work. Here we tested the variations of SAM and ASL over long\nperiods to see if their variations are interacted as what earlier studies suggested or not. As shown\nin Fig. R5, comparing the periods between 1979 and 1999 with 2000 and 2014, while the ASL is\nweakened as indicated by increasing central pressure (b), SAM slightly shifted toward its positive\nphase indicated by an increase of the SAM index by 0.2. These results suggest that the coincidence\nof deeper ASL with positive SAM derived by previous studies may not always work (comparing\nthe periods of 2000–2014 and 2015–2020 yields similar conclusions), in particular for the decadal\nvariability as shown here. As such, SAM and ASL can act as independent climate forcings for\nregional atmospheric circulation over separate periods, such as 2000–2014 which is the focus of\nthis study. Relevant discussions are provided in Lines 95–101 of the revised manuscript.\nFig. R5. Bar plots of austral-winter (a) SAM index, (b) the ASL central pressure and (c) the ASL\ncentral longitude from 1980 to 2000. Red dashed lines in each subplot indicate the mean values\nof the variable over the periods 1979–1999, 2000–2014 and 2015–2020.\n6. Sentence starting on Line 211: Connecting to my earlier major comment. This sentence seems\nto suggest that the SAM influence is strong in periods when ASL is weak. I don't know if there are\ncompeting effects, but I think there would be for RISP. Given its larger size it could be more\nimportant for DSW and AABW all things being equal. This sentence made me wonder whether it\npossible to run the model with a ASL strong/weak configuration for positive and negative SAM to\nstrengthen the work. But, this might beyond the scope of the current study.\nThis is a good suggestion for further demonstrating the competing effects of SAM and ASL on the\nRoss Sea polynya processes and DSW/AABW formation. But as the reviewer points out, such\nanalysis requires a new set of sensitivity experiments (4 or more) as we also need to consider both\nchanges in the ASL depth and longitude, which is beyond the scope of this study. We think this\nsuggestion is a good idea for a follow-up study of the current work, which we are planning to work\non, and thus do not include such analysis in the current manuscript.\nREVIEWERS' COMMENTS\nReviewer #2 (Remarks to the Author):\nI appreciate the authors for their very detailed response to my earlier comments and I would like to\nsee their manuscript published. My only concern about their reply is with respect to the third major\ncomment I raised about the negative contribution of positive SAM to AABW formation. Is the\nsimulated increase in Antarctic Bottom Water (AABW) production described in the paper caused\nsolely by the changes in ice production in polynyas under the influence of positive SAM, or is it the\ncomposite result of both changes in ice production in polynyas and ice shelf meltwater under\npositive SAM regulation? Typically, a positive SAM phase leads to accelerated ice shelf melting,\nwhich reduces AABW formation. However, the paper states that a positive SAM phase leads to\nincreased ice production in polynyas, thereby increasing AABW production. Did the numerical\nsimulation in the paper account for the response of ice shelf melting to the positive SAM phase?\nReviewer #3 (Remarks to the Author):\nSecond Review of “Evidence for large-scale climate forcing of dense shelf water variability in the Ross\nSea”\nBy Prof. Adrian McDonald (University of Canterbury)\nThis work examines a mixture of atmospheric reanalyses, satellite sea ice data, mooring data in\nthe vicinity of Terra Nova bay polynya and output from the ROMS model to examine the influence\nof large scale climate modes on DSW and AABW and infer future change. The updated presentation\nsignificantly strengthens the clarity of this work, in particular highlighting that the relationships\nidentified are strongest when the ASL is weak and is shifted to the east is an important clarification\nof the context. The authors have completed extra work to ensure that their analysis is robust, in\nparticular lengthening the periods used in positive and negative SAM phases gives me more\nconfidence in the results presented. Their arguments around relationships between SAM and ASL\nare also convincing. I am thus happy that this draft is now publishable. I identify a few minor\ntypographical/stylistic points on the updated document below that the authors should consider.\nMinor Comments:\nLine 81: Replace ‘factors’ with ‘modes’.\nLine 101: Replace ‘would lead’ with ‘leads’\nLine 229: Replace “much less discussed’ with ‘discussed less often’\nSentence starting on Line 274: This point has been made in previous work detailed in reference 39\nand may be useful to add this citation here.\nSentence starting on Line 291: The addition is important, but this sentence is also very long now. I\nwould suggest rewording to make this into two shorter sentences to aid clarity.\nResponse to Review Comments\nWe appreciate the efforts made by the editor and reviewers for 2nd round of review, and please\nfind our pointwise replies to review comments below.\nREVIEWERS' COMMENTS\nReviewer #2 (Remarks to the Author):\nI appreciate the authors for their very detailed response to my earlier comments and I would like\nto see their manuscript published. My only concern about their reply is with respect to the third\nmajor comment I raised about the negative contribution of positive SAM to AABW formation. Is\nthe simulated increase in Antarctic Bottom Water (AABW) production described in the paper\ncaused solely by the changes in ice production in polynyas under the influence of positive SAM,\nor is it the composite result of both changes in ice production in polynyas and ice shelf meltwater\nunder positive SAM regulation? Typically, a positive SAM phase leads to accelerated ice shelf\nmelting, which reduces AABW formation. However, the paper states that a positive SAM phase\nleads to increased ice production in polynyas, thereby increasing AABW production. Did the\nnumerical simulation in the paper account for the response of ice shelf melting to the positive SAM\nphase?\nYes – the numerical simulations include the response of ice shelf melting to the forcing anomalies.\nThe simulated increase in DSW and AABW production under positive SAM conditions is the\ncombined result of changes in both ice production in Ross Sea polynyas and ice shelf meltwater,\nalthough the two effects act in an opposite sense in terms of DSW/AABW formation. As described\nin the manuscript, in the sensitivity experiments we applied the SAM-induced changes in\natmospheric forcings over the entire model domain, not just over the polynyas. Therefore, the\neffects of SAM variations on the on-shelf intrusion of Circumpolar Deep Water and ice shelf\nmelting are both resolved in the model simulations. Our results demonstrate that the effect of\npositive contributions from positive SAM to DSW/AABW formation (by enhancing sea ice\nproduction in polynyas) overwhelms the negative effect of positive SAM on DSW/AABW\nformation (by increasing ice shelf melting and reducing salinity), ultimately resulting in\nenhancement in DSW/AABW production under positive SAM conditions. We have now added\ndiscussion about this point in the revised manuscript (Lines 269–277), as “It is worth noting that\na positive shift of the SAM leads to an intensification of CDW upwelling onto the shelf due to a\npoleward shift of the westerlies, causing warming of Antarctic shelf water and an increase in ice\nshelf melting rates56,57, which can suppress DSW formation. This process is included in our\nsensitivity experiments as the SAM-induced atmospheric changes are applied over the entire\nmodel domain including the shelf and open ocean regions. The results of our study suggest that\nthe effect of positive SAM on enhancing DSW/AABW production via changes in sea-ice\nformation in polynyas overwhelms the effect of inhibiting water mass formation via changes in\nshelf water temperature and ice shelf melting.”\nReviewer #3 (Remarks to the Author):\nSecond Review of “Evidence for large-scale climate forcing of dense shelf water variability in\nthe Ross Sea”\nBy Prof. Adrian McDonald (University of Canterbury)\nThis work examines a mixture of atmospheric reanalyses, satellite sea ice data, mooring data in\nthe vicinity of Terra Nova bay polynya and output from the ROMS model to examine the influence\nof large scale climate modes on DSW and AABW and infer future change. The updated\npresentation significantly strengthens the clarity of this work, in particular highlighting that the\nrelationships identified are strongest when the ASL is weak and is shifted to the east is an important\nclarification of the context. The authors have completed extra work to ensure that their analysis is\nrobust, in particular lengthening the periods used in positive and negative SAM phases gives me\nmore confidence in the results presented. Their arguments around relationships between SAM and\nASL are also convincing. I am thus happy that this draft is now publishable. I identify a few minor\ntypographical/stylistic points on the updated document below that the authors should consider.\nMinor Comments:\nLine 81: Replace ‘factors’ with ‘modes’.\nRevised.\nLine 101: Replace ‘would lead’ with ‘leads’\nRevised.\nLine 229: Replace “much less discussed’ with ‘discussed less often’\nRevised.\nSentence starting on Line 274: This point has been made in previous work detailed in reference\n39 and may be useful to add this citation here.\nWe added the citation of reference 39 here.\nSentence starting on Line 291: The addition is important, but this sentence is also very long now.\nI would suggest rewording to make this into two shorter sentences to aid clarity.\nThis sentence is indeed long. We revised the sentence to now read: “This may help weaken the\nobserved freshening trend of bottom water in the Ross Sea52 and downstream in the Australian\nAntarctic Basin53. It could also partially counteract potential reduction in the Ross Sea DSW\nsalinity54 and AABW production55 resulting from projected increases in ice shelf melting in West\nAntarctica”."}, "author_response": ""}]

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