the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Sep 2024
Climate influences on sea salt variability at Mount Brown South, East Antarctica
Abstract. The Mount Brown South (MBS) ice core in East Antarctica (69° S, 86° E) has produced records of sea salt concentration and snow accumulation for examining past climate. In a previous study, the sea salt concentration, but not snow accumulation, showed a significant, positive relationship with the El Niño-Southern Oscillation (ENSO) from June to November. Here, we use observations and reanalysis data to provide insights into the mechanisms modulating this previously identified relationship for the austral winter season (June-August). A teleconnection between the tropical Pacific and high-latitude winds in the vicinity of MBS is identified. Specifically, El Niño events are related to strengthened westerly winds ∼60° S, leading to more local sea ice via anomalous Ekman transport in an area to the northeast of the MBS site. Impacts from La Niña are less obvious, showing that there is a non-linear component to this relationship. MBS is a wet deposition site, and we show that sea salt is likely transported from northeast of MBS via synoptic-scale storms that accompany high precipitation events. These storms and their associated precipitation, show no substantial differences between years of high and low sea salt concentration, so we suggest it is the source of sea salt that differs, rather than the transport mechanism. El Niño-associated strengthened westerly winds in the MBS region could enhance sea salt availability by increasing ocean aerosol spray and/or by increasing sea ice formation, both of which can act as sources of sea salt. This may explain why sea salt concentration, rather than snow accumulation, is most closely related to ENSO variability in the ice core record. Identifying the mechanisms modulating key variables such as sea salts and snow accumulation at ice core sites provides further insights into what these valuable records can decipher about climate variability in the pre-instrumental period.
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RC1: 'Comment on egusphere-2024-2660', Anonymous Referee #1, 04 Nov 2024
The manuscript by Shea et al. presents an interesting interpretation of the mechanisms driving annual sea salt variability at Mount Brown South ice core site (MBS) and their potential relationship with ENSO. The paper is relevant as it provides new insights from a data-sparse region in East Antarctica. The results presented in this paper build upon previous work on MBS published by Crockart et al. (2021), Vance et al. (2024) and Jackson et al. (2023). This manuscript is interesting and relevant and should be considered for publication in Climate of the Past. However, some of the data presented in this study should be revised and/or clarified according to the following considerations, before acceptance for publication in Climate of the Past.
Main comments:
(1) One of the main datasets used in this manuscript is the log-transformed mean annual chloride (Cl-; here considered representative of sea salt concentration). In the absence of higher temporal resolution data, this annually resolved record is used as an austral winter representative. The manuscript highlights that previously, Crockart et al. (2021) used the mean annual sea salt concentration as representative of the June-November period. In this manuscript, the authors take a step beyond and use mean annual sea salt concentrations as June-August representatives. How representative is this, considering that Vance et al. (2024) (Fig 5) show that, during the satellite era, the sodium enhancements reached their maximum during September-October.
(2) Throughout the text there is no mention of testing the data (MBS sea salt concentration and ENSO) for the presence of outliers which could impact the results obtained from the Pearson’s linear correlation coefficient. Figures 1a and 1b show that there are some points which have considerable detours from the mean, potentially biasing the correlation results. An assessment of outliers will make the results more robust.
(3) Throughout the text it is highlighted that MBS is a “wet deposition site” and that annual precipitation is heavily influenced by extreme precipitation events. However, there is no information about the distribution of extreme precipitation events throughout the year. It is mentioned that “annual sea salt concentration is winter dominated (Vance et al., 2024) and can thus be considered a ‘polar winter’ record” (Lines 85-86), but there is no further information to know if this is because more extreme precipitation events occurring during winter. Knowing if the extreme precipitation events are bringing the sea salts during winter could contribute to the interpretation of the record.
(4) The composite maps presented in Figures 2-8 are a good way of representing the data that is discussed in the manuscript. However, it would be useful to know how consistent these maps are. Do all the years included in the quartiles converge into the same scenarios presented in the composites? Or the composites are biased by some years with a very strong signal/pattern?
(5) Winds are mentioned as a potential mechanism enhancing sea spray production; however, their potential contribution is always presented relative to the wind anomaly. Previous studies have reported that there is a strong relationship between wind strength and sea spray production, with wind strengths above certain thresholds generating considerably more sea spray. To assess the potential effects of the anomalies over sea spray production, I suggest this manuscript should include a reference to mean wind strength in the region from where sea salts could have been removed. Are the winter regional winds strong enough to produce a considerable enhancement in the MBS sea salt concentration?
Minor comments:
Lines 22-29: I suggest changing the order of these lines:
“(1)The effects of the El Niño Southern Oscillation (ENSO) on West Antarctica have been thoroughly investigated (Harangozo, 2000; Genthon and Cosme, 2003; Turner, 2004; Clem and Fogt, 2013; Etourneau et al., 2013; Clem et al., 2016). (2) However, East Antarctica has comparatively fewer ice core records despite comprising the majority of ice mass of the continent and thus the influence of ENSO on East Antarctica is less well understood. (3) The Mount Brown South (MBS) ice core (69.111◦S, 86.312◦E) was drilled in 2017/2018 to fill a spatial gap of data for East Antarctica (Crockart et al.,2021). (4) The climate impacts of ENSO vary across the Antarctic continent; (5) therefore it is important to have a well–dispersed network of ice cores that can be used to understand the long term and region–specific links to climate drivers of Antarctic climate (Li et al., 2021b).”
I suggest the following order: (4) à (1) à (2) à (5) à (3)
Line 26: consider adding “austral summer” to make it “…was drilled in austral summer 2017/2018 to fill…”
Line 31: I think the text will be benefit if the R and p values related to the “significant positive correlation” obtained by Crockart et al. (2021) are included here.
Line 31: I suggest specifying that the correlation found by Crockart et al. (2021) is between the June-November ENSO and the annual sea salt concentration, as previously mentioned in the abstract (line 3).
Line 36: It seems that this line is repeated (see line 31).
Line 51: please, specify that it is Antarctic sea ice
Line 53-54: I believe there is a typo in this sentence which prevents me from understanding the sentence in full. I thought “It” at the beginning of the sentence referred to the Antarctic Dipole, however, it doesn’t seem to be the case. It makes sense when removing the words “Antarctic Dipole produce the”. I suggest revising this line.
Line 60: please consider replacing the word “to” after “insights”, for the word “into”.
Line 66: please consider adding the word “pattern” or “configuration” after the words “weather and climate”.
Line 71: There is a mismatch with the section number which propagates throughout the text. The subsection currently levelled as 3, should be 2.1. All section numbers and in-text reference to sections have to be re-assessed from this point onwards.
Line 71: change the word “cite” for “site”.
Line 72: Please consider including the full decimal coordinates here
Line 90: please specify which months will be used for austral winter estimates (e.g. JJA; JAS; JJAS)
Line 238-241: Figure 5 shows that there is an easterly flow along the coast triggered by the low-pressure field. However, this easterly flow seems to be limited to the coastal region. Wind vectors near MBS seem to be northerly, pushing air from the interior of Antarctica, potentially preventing the access of air from the coast. Could you please comment on this?
Lines 243-301: It will be useful to know which are the years that correspond to the upper and the lower ENSO3.4 quartiles. Maybe a figure in the supplementary, similar to Fig 1 where the upper and lower ENSO quartiles are highlighted. From the scatter plot is clear that MBS and ENSO quartiles are not formed by the same years, but there is no clarity about which years form the ENSO3.4 quartiles
Figure 9: I suggest color-coding the numbers in the schematic to better link the graphics with the explanation
Citation: https://doi.org/10.5194/egusphere-2024-2660-RC1 -
RC2: 'Comment on egusphere-2024-2660', Anonymous Referee #2, 23 Dec 2024
Summary
This paper provides a climatological calibration of the annual sea salt record (log-transformed chloride concentration) from the Mount Brown South (MBS) ice core from 1979-2016. The MBS site is in a data-poor region of the East Antarctic ice sheet in the Indian Ocean sector, with a ~1,000 year-long record that could provide valuable paleoclimate information about sea ice extent, sea level pressure, zonal winds, and even El Nino-Southern Oscillation behavior. In general, I believe this manuscript will be a valuable addition to the literature, assuming that the analysis herein will benefit the interpretation of the longer record. This manuscript also helps to elucidate some of the dynamical controls on sea-salt concentrations in Antarctic ice cores, which is beneficial for the interpretation of other Antarctic ice core sites. However, I have several suggestions for improving the manuscript that I recommend be addressed by the authors prior to publication.
Major Comments
(1) I think the paper would benefit from a slight re-framing in the abstract and introduction. The hypothesis of Crockart et al. that ENSO significantly influences MBS sea salt concentrations is centered as the motivation from the second sentence of the abstract. However, the title of the manuscript and – in my opinion – the strength of this paper lies with your analysis of the regional sea ice concentration, sea level pressure, and wind drivers of high and low MBS sea salt concentrations. So, I find the ENSO framing early in the paper to be somewhat distracting from the real point of the paper: “what are the controls on MBS sea salt concentration?” While there is potentially an ENSO influence on local sea ice, pressure and winds (as you show), the majority of the variability (~>75%) of those climate metrics is unrelated to ENSO. It seems to me that the strongest motivation for this paper is not that Crockart hypothesized an ENSO connection, but rather that it is a potential ~1000-year record of climate variability from a data-poor region of the continent. The length of the full core is not mentioned until nearly the end of the paper. To me, that’s a central part of the motivation for this work.
(2) I think an updated review of the literature on the sources of sea salt in Antarctica would further strengthen the manuscript. In particular, there are many references to the need for “new sea ice” or “sea ice formation” to promote salt emissions, which stems from the focus on salt flowers. Salt flowers were a popular topic in the literature in the early-mid 2000s, but more recent work shows that they may not be as important a sea-salt source as previously thought. There is no mention of blowing snow on sea ice in this paper, despite citing the Huang and Jaegle 2017 paper that emphasizes the importance of that sea salt source. Thinking about wind speed over sea ice – not just new sea ice formation – might lead to a richer interpretation of your zonal wind analysis. The evidence from chemical transport models like GEOSChem, building on the H&J17 work, can help clarify the local partitioning and relative importance of various sea salt sources. While it would be ideal to include something like GEOSChem output in this manuscript to directly inform your interpretations (similar to Winski et al., 2021), it is not absolutely necessary. But including the published work in this area on blowing snow from sea ice, and other recent sea salt source partitioning work, would strengthen this manuscript. One additional figure that would benefit this analysis would be the correlation between SIC within the MBS anomaly region (the Fig. 3 fuchsia box) and 850 mb zonal winds. You’ll see a strong positive correlation, with stronger westerly winds associate with higher SIC in that box (I made the figure for my own interest). This figure would support several of your arguments, especially if you add a discussion on blowing snow on sea ice.
(3) All the anomaly figures and related discussion/interpretation would be strengthened if you tested the anomalies for statistical significance. I have concerns that some of the SLP anomalies, for example, are not statistically significant. I think the sea ice and wind anomalies are significant, so I don’t think it will change anything major in the take-home results, but it’s an essential test to include in all of your figures.
(4) The synoptic/daily analysis shown in Figure 5 would be strengthened by using more than one low sea salt year (1991) and one high sea salt year (2014), and by using more than five high-precip days in each of those years using ERA5 precipitation data. This is particularly the case because the ERA5 precipitation data is known to have significant biases, especially in data-poor Antarctica (see Roussel, Marie-Laure, et al. "Brief communication: Evaluating Antarctic precipitation in ERA5 and CMIP6 against CloudSat observations." The Cryosphere 14.8 (2020): 2715-2727). I’m left wondering if the differences between Figure 5a and b are real and important (e.g. the different positions of the lows), or simply a result of having such a small “n”. This is relevant given the potential nonlinear relationships with ENSO discussed herein, and also relevant given the Pinatubo eruption in 1991. As mentioned in the “Limitations” section, that makes 1991 difficult to interpret on its own.
(5) Given that the dynamical link between the tropical Pacific/ENSO and Antarctica is via atmospheric teleconnections, I think it would be informative to investigate the ENSO indices that include atmospheric components, or even the SOI itself. If correlations between MBS and MEI or BEST or SOI are weaker than those with Nino3.4, then it seems likely to me that the relationship between ENSO and MBS is weaker than it appears using Nino3.4. In other words, stronger r values with Nino3.4 may simply be due to chance as you are correlating two rather noisy, and rather short, time series. I can’t think of a dynamical reason why tropical SSTs would have a stronger influence on MBS than the atmospheric anomalies that those SSTs induce, given that the atmospheric anomalies are a step closer to MBS in the hypothesized cause-effect progression.
(6) I strongly suggest the authors use a winter-spring seasonal average rather than a winter average only for the analyses in this manuscript. Other Antarctic ice cores and aerosol monitoring stations show that sea salt deposition peaks through September and even October. This is, likely, related to the importance of sea ice on sea salt deposition combined with the seasonal lag of sea ice. The ideal months to include would perhaps be July-October, but JJASON would also be better than JJA. The fact that Crockart et al. also use a winter-spring seasonality for their analysis of this same MBS core is further motivation to use that seasonality in this paper.
Minor Comments:
The captions for Figures 2, 3, 4, 6, 7, 8 should include the time period (years) represented by the data in each figure.
Line 28: “climate drivers of Antarctic climate” delete the first “climate”
Line 33: missing comma after “site”
Line 36: Same sentence largely repeated from line 31.
Line 39: delete “that” after “found”
Line 44: Blowing snow from sea ice should be added to this sentence as an important sea salt source. It’s probably more important than frost flowers, and seasonally more important than the open ocean. See the paper you cite by Huang and Jaegle, 2017 (and note the title of their paper).
Line 71: Section “3” is really section 2.1, correct?
Lines 72-76: The section on Mount Brown South site characteristics requires more details for the reader. Please add mean annual temperature and mean annual snow accumulation of the site; the accumulation is particularly important for readers to put the repeated references to a “wet deposition site” into context. Also, please explicitly state for the reader that the Mount Brown South ice core “site average” sea salt record discussed here represents the average of three ice cores collected within ~100 m of one another. This is important context for the reader. I would also suggest you state that the three ice core sea salt records correlate with one another with an r value between 0.43 to 0.59, and they each correlate with the site-averaged record with an r value >0.8 (and cite Crockart). Again, this is helpful context for the reader to evaluate your correlation coefficients. Also, please clarify if you are using the raw annual or Gaussian smoothed annual site-averaged sea salt record from Crockart in this analysis. Also include the time length of the full MBS record here. You mention that it is 1,137 years long only towards the end of the manuscript.
Line 83: “annually not seasonally averaged”. Perhaps “annually rather than seasonally”?
Line 83-85: These consecutive sentences are a bit at odds with one another. The first says you used the annual averaged data because that was all that was available. The next says that you “elected” to use the annual averaged data because of the episodic nature of the snow accumulation. Which is it?
Line 89: “presence or absence of sea ice”. Only the presence or absence (e.g., 0 or 1), or the varying concentration of sea ice? I’m sure it’s the latter.
Line 91 (and 96, and elsewhere): Which months constitute the “austral winter mean”? Please be specific about which months are included (I’m assuming JJA), and if that varies at all within this analysis.
Lines 93-95: This SST section should clarify – are you analyzing SSTs in the Southern Ocean or in the tropical Pacific? I think the later (based on the next page), but it should be clarified here.
Line 93: “Defined in section 2.2” – I don’t know what this is referring to, as there is no section 2.2 But this section would be section 2.2 “Data” if it were numbered correctly, I think. Perhaps this means section 2.3 “Analysis”?
Line 93: Define the acronym “HadISST” the first time you use it, and it needs a reference.
Line 99: Define the acronym “ERA5” the first time you use it, and it needs a reference.
Line 106: Should be “investigate possible mechanisms”
Lines 123-125: This sentence should be revised for clarity and flow – semicolon should be a comma, “correlation analysis” should be made plural, and the sentence structure is awkward.
Line 133-134: “high years and low years” should be “highest quartile and lowest quartile”. For which geographic region(s) are you creating these composite maps?
Line 139: “events” should be “event”
Line 141: should be “was calculated”
Line 147: Double check the section numbering throughout the manuscript.
Line 151: Should be “we first consider correlations between the annual MBS Cl- concentration (MBS Cl-) and different ENSO indices (Table 1)”
Line 152: missing comma after “this study”.
Line 156-157: I’m curious about why you would limit your analysis to the austral winter (presumably JJA, although not defined) given that a) Crockart focused on winter-spring, and more importantly b) sea salt seasonal cycles generally peak in winter-spring, not just the winter. This can be seen in any number of papers on Antarctic sea salt seasonality in both ice cores and aerosol stations; for example, in Winski et al., 2021, cited herein. This winter-spring seasonality reflects the importance of sea ice as a sea salt source.
Line 167: extra comma. Should be “…winds around MBS to investigate…”
Line 171: “high-pressure anomaly above MBS in the MBS upper quartile”. The word “above” in this sentence is confusing. Do you mean “north of MBS” in the southern Indian Ocean? Or do you mean in the atmosphere on top of the MBS site? I think you mean the former since on top of the MBS site in the upper quartile plot is a low-pressure anomaly.
Figure 1: Caption should indicate that the red and blue years in panel (a) x axis represent the MBS upper and lower quartile years.
Figure 2: It is important to indicate which pressure and SST anomalies are statistically significant given the variability of the respective datasets. I doubt, for example, that the ~0.8 hPa high pressure anomaly north of MBS in Figure 2a is statistically significant. The H pressure anomaly ~55 S, 130 W of Figure 2a probably is significant, however, along with the tropical SST anomalies. I similarly doubt the low-pressure anomaly NE of MBS in Figure 2b is statistically significant.
Line 180-181: It’s not just the “presence or absence” of sea ice, but rather changes in sea ice concentration. Further, sea salt aerosol does not only come from “newly formed sea ice”, but from blowing snow on sea ice, as described nicely in Huang and Jaegle, 2017.
Line 184-185: The sentence, “Figure 3 shows the high sea…” is unnecessary, as it repeats info from the prior few sentences.
Line 185-186: “The MBS northeast coast box…” sentence is awkward (please revise), and it should mention that this box encompasses not just the positive SIC anomaly but also the negative SIC anomaly.
Line189: Missing comma. Should be “…topical Pacific, however”
Line 191: You don’t know that there is a “variance in the source of sea salt”, only that there is variability in the MBS sea salt annual concentrations. Separately (grammatically), I recommend moving “wind anomalies near the MBS region were analyzed” to the start of the sentence.
Line 196: Check verb tenses throughout. You generally use present tense (which is recommended) in the manuscript, but here and in some other places you write that the maps “showed” things. Should be “show”.
Line 199-200: I suggest something like, “The direction of these zonal wind anomalies is consistent with the mean sea level pressure anomalies associated with the highest and lowest qu6artiles shown in Fig. 2.”
Lines 238-241: I think the sentence that begins “Figure 5 shows...” is based on an incorrect premise. Note that Figure 4 shows the meridional (and zonal) wind anomalies. They do not show the actual direction of those winds. It is possible (likely?) that, due to katabatics, the meridional winds are going from MBS towards the coast on a monthly average basis. But certainly during the storms that deposit snow and sea salt on MBS the meridional winds would be onshore as you suggest. But to be clear, Fig. 4 does not show offshore meridional winds – it shows offshore wind anomalies. So the text needs to be revised.
Lines 265-270: In this section, make sure it is clear that easterly wind anomalies in the westerly wind belt means (on a monthly basis) weaker westerly winds, not true easterly winds.
Lines 275-277: This section should also mention blowing snow on sea ice as a major source of sea salt, in addition to frost flowers.
Line 287: “The high MSLP anomaly above MBS…”. Echoing a previous comment, I believe this means “north of MBS”, rather than over MBS.
Line 299: Figure 7a does not support the idea that “El Nino is related to ….new sea ice formation”, as there is essentially no SIC anomaly in the MBS sea ice source region associated with El Nino.
Citation: https://doi.org/10.5194/egusphere-2024-2660-RC2 -
RC3: 'Comment on egusphere-2024-2660', Anonymous Referee #3, 03 Jan 2025
Shea et al. provide an analysis of possible mechanisms linking tropical sea-surface temperature variability (Nino 3.4) to Mount Brown South-regional sea ice concentration as recorded by the MBS ice core. This atmosphere-transmitted teleconnection between the tropical Pacific and Antarctica is well known, and is a process that would benefit from ice core paleoclimate reconstruction if indeed a strong enough modern proxy relationship can be demonstrated.As the authors point out in section 5.1, the correlation is demonstrated but the mechanism linking to sea ice concentration is not made clear by this analysis. A broader analysis including other seasons or combinations of months would be beneficial. It is very important to understand proxy relationships for the MBS core, as that millennial length record is indeed in a region of very limited recent or paleoclimate information. I think the manuscript needs revisions given the concerns of reviewers 1 and 2, but after these considerations would be appropriate to publish in CP.
Main comments:
Many references to other MBS work (Jackson, Udy, Vance, Crockart) but less of broader evidence from the literature. What similarities do the West Antarctic ITASE cores have, with respect to the proposed mechanisms affecting sea salt concentrations (work of Dixon and others)? A deeper analysis of the literature would improve the manuscript.
There is a conflation of El Niño and ENSO throughout the paper that should be reconsidered. It would seem that if only SST correlations are examined, as they show the strongest significant correlation, the authors need to limit their interpretations to El Niño albeit knowing that the teleconnection between tropical Pacific SST is transmitted through the atmosphere.
Line-by-line comments:
L24-25: none of the West Antarctic references use ice cores to understand ENSO effects. Following statement then about East Ant. lack of ice cores is unrelated to understanding ENSO linkages
L28 change to “… links to drivers of Antarctic climate”
L 48 reference to Abrams et al., 2013
L81 remove redundant “remove long term trends and”
L85 add a little more justification from the Vance 2024 reference to make clear why the annual sea salt conc is winter dominated as this is not obvious. Just in one additional sentence, perhaps.
L90 what months are taken for austral winter?
L111 only selecting two relatively anomalous years also risks them being outliers…
L242 are these “high precipitation storms” atmospheric rivers? I suspect so, in many cases and encourage looking into the work of Wille, Gorodetskaya and others on the subject
L307 Change “stem” to something like “originate”
L325 It would have been useful to see a reanalysis-based examination of this MBS - Law Dome disagreement to see if mechanisms could be teased out. Perhaps beyond the scope of the paper but a lingering limitation as they indicate.
L 332 add more specifics about these additional studies (locations in more detail…) as they indeed provide good broader context.
L 333 Drive this home more. These findings are comparable, so what are the commonalities across the sites? Is it generally similar that high SSC upwind of ice core sites during heavy snowfall events is conducive to greater SSC at ice core sites? Presumably so.
L 340 indeed why do the authors not explore other seasons? This should be a relatively minor change to code to plot summer rather than winter.
L 341 yes ENSO is not the same as El Niño and the authors should be more careful and consistent with their references to these phenomena. I’d argue it’s not appropriate to discuss ENSO at all if only Nino indices are used.
L345-347 seems like a superfluous statement and much work on flavors of Pacific SST and their influence on Antarctica has been done (work of Ding, Steig, others).
Figure 9 is a little too abstract… what is the red blob and blue blob generated from? Presumably correlation maps presented earlier, and if so that should be made clear in the figure caption. I do understand the diagram and it is actually fairly clear, but could perhaps be refined for clarity. Some authors choose to contract an artist or graphic designer for this work, which does have some cost but improves clarity.
Citation: https://doi.org/10.5194/egusphere-2024-2660-RC3 -
EC1: 'Comment on egusphere-2024-2660', Eric Wolff, 07 Jan 2025
You will see that you have comments from three reviewers. They all consider this a useful study, particularly because of the way it opens up the prospect of learning from the 1000 year record that is to come. However they do make important comments about the framing of the paper, and other aspects of the work. Please make sure to respond to all comments when you submit your author comments - only when that has been done can I give a final editorial decision and invite a revised manuscript. From my own viewpoint, i think it is important for you to make clear how this paper adds to what was in the published Crockart paper. From this viewpoint I agree that you may want to emphasise in your framing and abstract a little more the local controls (sea ice, wind patterns) on sea salt, and then the role of ENSO in controlling those as a secondary aspect (this probably requires only minor changes, but is an important distinction because one can imagine the 1000 year record being used as an indicator of past sea ice or local pressure anomalies for example, but i doubt you would suggest we use the long record as an ENSO proxy). Please also clarify the questions about seasonality, which have been raised by two reviewers. I look forward to seeing your responses.
Citation: https://doi.org/10.5194/egusphere-2024-2660-EC1
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