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
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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
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