the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Apr 2024
A sub-fossil coral Sr/Ca record documents meridional variability of the Intertropical Convergence Zone in the eastern Indian Ocean
Abstract. Sea surface temperature (SST) variability in the south-eastern tropical Indian Ocean is crucial for rainfall variability in Indian Ocean rim countries. A large body of literature has focused on zonal variability associated with the Indian Ocean Dipole (IOD), but it is unclear whether meridional shifts in the position of the Intertropical Convergence Zone (ITCZ), which at present co-vary with the IOD, may also occur independently. We have developed a new, monthly resolved Sr/Ca record from a sub-fossil coral cored at Enggano Island (Indonesia, 5° S, 102° E). Core sections containing diagenetic phases are omitted from the SST reconstruction. U/Th dating shows that the Sr/Ca-based SST record extends from 1917–1868 and from 1861–1823 with a relative age uncertainty of ±2.4 years (2σ). At Enggano Island, coastal upwelling and cooling in austral spring is coupled to the position of the ITCZ, and impacts SST seasonality. The sub-fossil coral indicates an increase in SST seasonality due to enhanced austral spring cooling between 1917 and 1855, which we attribute to stronger SE winds and a northward shift in the position of the ITCZ in austral spring. A nearby sediment core indicates SST cooling and a shallowing of the thermocline prior to ~1930. These results are consistent with an increase in the North-South SST gradient in the eastern Indian Ocean calculated from historical temperature data, that is not seen in the zonal SST gradient. We conclude that the relationship between meridional and zonal variability in the eastern Indian Ocean is non-stationary and influenced by long-term temperature trends.
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RC1: 'Comment on cp-2024-25', Anonymous Referee #1, 18 Apr 2024
This study aims to understand how the meridional SST gradient in the eastern equatorial Indian Ocean has changed over the past ~200 years. To do so, the author developed a Sr/Ca record using fossil corals collected from Enggano Island for analysis. The authors found that there was an increase in seasonality strength and an earlier seasonal SST maximum between 1855-1917 compared to the modern period and 1823-1854 CE and attributed the change to an earlier onset of austral spring and strengthened SE winds, which consequently imply a northward shift of ITCZ and a stronger meridional SST gradient. On the other hand, there is no conclusive evidence that changes in zonal SST gradient (e.g., IOD) played a role in changing SST in Enggano Island and meridional gradient despite previous suggestions. Therefore, they concluded that the meridional SST gradient and zonal SST gradient are not always coupled and require more analyses on the mechanisms that drive the meridional SST gradient.
I think this new record is a useful contribution to better understand climate variability in the Indian Ocean and complement existing records in nearby areas. I also think the analysis of meridional SST gradient in the NE Indian Ocean is interesting. The screening for coral quality/diagenesis is also extensive. That said, I have some suggestions/comments, that I hope would help improve the manuscript. There are also several inconsistencies within the text and figures/tables, which needs to be corrected. Otherwise, it is difficult to judge the results and conclusion of this study.
Overall comments:
- I find referencing the years in reversed chronological order (e.g., 2008 to 1930 instead of 1930 to 2008) confusing. There are also several instances where the years are referred in chronological order instead, for instance L317 “Between 1854 and 1923…”. I suggest making this consistent throughout the text, and preferably in chronological order (i.e., older to younger).
- Within the manuscript, there are multiple instances where statistical significance is mentioned (e.g., 99% confidence levels). However, in most cases, it is unclear how this was determined. Even when a Monte Carlo approach was used, it was also unclear how it was carried out. It would be helpful if this can be clarified.
- The results of this study hinge on an accurate chronology and constraints on the annual cycle. While there are multiple instances within the manuscript where dating uncertainty was mentioned, as far as I can tell, there are no testing of how robust the results were against dating uncertainties. It would be nice to see sensitivity tests to check this. Furthermore, it is currently unclear how the annual cycle was derived (see the comment L181-183 below for more details), which makes me unsure of the results. Additionally, the chronology is derived based on the assumption that the internal chronology and U/Th ages have a 1:1 relationship. While in Figure A6, it shows that they correspond to each other fairly well, there are also instances where the U/Th diverges from the 1:1 line (the 2sigma of KNFa(8/11), KNFa(5/11) do not line up with the regression line). So, I wonder how accurate this assumption is in this case? Moreover, it would be great to provide the estimated extension rates of these corals, just so we can make sure it is indeed possible to estimate monthly SST changes.
- Given that the change in SE wind strength, and the shift in ITCZ are supposed to correspond to changes in the South Asian monsoon, I wonder if it will also be helpful if you can show the South Asian monsoon also changed concurrently with these changes.
Specific comments:
L31-32: A ‘reference period’ needed to compare with in order to claim ‘an increase in SST seasonality due to enhanced austral spring cooling’.
Figure 1: In most cases, the figure is referred in the text when discussing about anomalies during IOD events. So, I wonder if it would be better to change the subplots b-e into OLR and SST anomalies so that they can better serve their purposes?
L111-L112: It is unclear to me how Aug-Oct temperature and symmetry is inferred based on Figs 2-3 – both do not show mean Aug-Oct temperatures.
Figure 3: I suggest checking the consistencies between this figure, the caption, and the main text on which months are mentioned and used for analyses. In the text and the caption, Aug-Oct was mentioned, whereas the figure label suggests Sept-Nov. Additionally, the vertical dashed lines don’t seem to be located at the same months for the plot of each location in subplot c.
L154-155: I think the sentence is missing a verb (e.g., were carried out’).
L181-183: This is actually *inconsistent* with Cahyarini et al. (2021) and Pfeiffer et al. (2021). Both studies tied September to Sr/Ca maxima and May to Sr/Ca minima. But here, it suggests only September was tied to Sr/Ca maxima. Please clarify which way it was done.
L188-195: Fig A6 should probably be referred somewhere here so the readers can go to that figure to get a better sense how this was done.
L233: ‘were’ -> where
L237: I think the Appendix/Supplementary figures referenced here are incorrect.
L311: repeated ‘(Fig. A8)’.
L312-320: Should ‘1854 and 1923’ be 1854-1823 instead? Otherwise, this will be referring to the same overlapping period as the previous sentence (1917-1855). Additionally, I would like more quantification on the comparisons between the distributions instead of simply relying on visualizations. Tests such as a Kolmogorov Smirnov test (or its variant) would be helpful here.
Figure 6: I only see one type of line in subplot (a) with two green and blue lines. I do not see red solid and dashed lines.
L350-354: I wonder if there’s a more objective way to ‘separate’ these time periods? Right now, it seems a bit arbitrary and relies on visualization. One suggestion perhaps would be to analyze the wavelet power of annual cycle and identify periods that are weaker to a ‘reference period’. This can be pulled out from the wavelet spectra.
Figure 7: Why is the seasonal cycle shown here span from July to January 2 years later? There are several mistakes in the caption (e.g., ‘dashed green lines’ for core PB, no explanation of ‘dark grey and dashed lines’ in subplot a). Additionally, which ‘modern’ (coral or observation) is used in (d) and (f)?
Figure 8: in subplot c, there is a discrepancy on which years were used for analysis: it was labelled 1917-1869 in the figure whereas in the caption 1917-1855 was referred.
Figure 9: I don’t quite understand what “not the large 95% confidence levels of the SiZer test” means. It would also be helpful to explain what the horizontal lines associated with the change point indicators mean. Additionally, there needs to be clarification on how the SiZer analysis was carried out. As far as I recall, SiZer applies a range of Gaussian filters with varying bandwidths and calculate the trends based on those bandwidths. But in the caption, it only mentions about a 21 year running average to show the data and not any other information related to SiZer.
L440: I am not sure if age uncertainty is the main issue here. By just looking through the KNFa record, there is no ‘major’ anomalies (<-4C) between 1869-1917. Given that the absolute dating uncertainties are almost <10 years, I don’t think age uncertainty is an issue here. In fact, the remaining paragraph does not discuss about age uncertainty. So, I suggest modifying the first sentence of this paragraph.
L444-446: I don’t think this is an accurate statement. My understanding is that the Enggano record is better in capturing meridional SST gradient compared to previous Mentawai records. So, it is logical that the Enggano record might not detect IOD changes. That said, a comparison between meridional and zonal SST gradients can be achieved by making use of the Enggano record for meridional changes and the recently southern Mentawai record (Abram et al., 2021 Nature) that is supposed to record IOD changes.
L456: hereinafter, ‘foraminifers’ should be ‘foraminifera’.
L461: Would (p>0.05) mean it is not significantly different?
L461-462: I think it would be helpful to test statistically if the relative abundance of thermocline dwelling forams changed significantly.
L464: ‘linkedn’ -> linked
Figure 11: I wonder if the abundance of mixed layer forams timeseries is needed, since it was never discussed in the text?
Table A1: What do those asterisks next to KNFa(3/11) and KNFa(7/11) mean?
Figure A6: I don’t think the equation displayed here is correct. Plugging in any years prior to 1934.1 will result a negative y, which isn’t supposed to happen here.
L601: As far as I can tell, I don’t see any U/Th ages that correspond to 1823 both in the figure and in table A1.
L619: ‘(a)’ should be (b) here.
Citation: https://doi.org/10.5194/cp-2024-25-RC1 -
AC1: 'Reply on RC1', Miriam Pfeiffer, 19 Jul 2024
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2024-25/cp-2024-25-AC1-supplement.pdf
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RC2: 'Comment on cp-2024-25', Anonymous Referee #2, 13 May 2024
Overall comments;
This study aims to contribute to the growing number of reconstructions in the Southeastern Indian Ocean region and document how the meridional SST gradient has changed in the past 200 years. To achieve this the authors, develop a sub-fossil coral Sr/Ca record collected from Enggano Island, an island previously used to reconstruct Indian Ocean Dipole variability. The authors found the sub-fossil exhibits an enhanced seasonal cycle between 1855-1917 compared to the modern equivalent and the later 1823-1854 periods. This enhanced seasonal cycle was attributed to an earlier onset of austral spring, and increased SE winds during July-October. This was then attributed to a northward shift in the mean position of the ITCZ and a stronger meridional SST gradient in the eastern Indian Ocean. They conclude that this is unlikely to be associated with enhanced IOD variability as spectral analysis suggests that variability associated with the Enggano coral is mostly interannual, and IOD variability would only impact the IOD season rather than the full seasonal cycle. Based on historical SST products they additionally conclude that the zonal (IOD) and meridional components controlling the shift in the ITCZ are not linearly coupled in the past.
This coral reconstruction is a useful contribution to the regional understanding of the Indian Ocean variability due to the sparse observational record in the region. This record complements previous records in the region, particularly adding to a large reconstruction effort of the IOD. Some of the methodological aspects of this study are excellent, particularly the diagenesis screening which is an excellent example of how to address issues with sub-fossil usage. I do have some concerns with the study, which I believe would improve the manuscript and better align with the knowledge of the community.
Overall concerns;
- I have an issue with the definition of the ITCZ region. In this region, the Maritime Continent, the definition of the traditional ITCZ does not typically apply. Due to the numerous monsoonal systems that operate in the region, the system should be defined as a ‘Tropical Convergence Zone (TCZ) or Tropical Rainfall Belt’, as outlined in Geen et al., 2020 (https://doi.org/10.1029/2020RG000700). The transition of the monsoons (as the author here is describing) is commonly associated with the global monsoon transition. I would encourage the authors to think about this definition and which they should be using. Additionally, due to the width of the TCZ in this region, I would encourage the authors to instead state this as the Southern Boundary of the TCZ.
- The main finding of this paper is based on the difference between the seasonal range in the sub-fossil and the modern. There are some issues with this methodology. Firstly, the description of how the age model was constructed is inconsistent, with different explanations of how the age model was constructed. In the modern coral in Pfeiffer et al., 2022 tie points are constructed on both minima and maxima which may make a difference when comparing the seasonal cycle, as according to this paper the maxima temperature is not constrained.
- I feel that the modern comparison is not sufficiently explored in this paper, particularly towards the detection of IOD events, the authors state that the change in the seasonal variability is not linked to IOD events however state that there are IOD-like events in the coral (which are mentioned in the methods). Additionally, as the main finding is centred around the difference between Mentawai and Enggano a more in-depth comparison would be appropriate.
Specific comments;
Line 43/44 – I believe that the suite of Abram et al., papers should be included here or in the next line of referencing. Particularly the paleoclimate perspectives paper
Line 49 - As I have stated above I question the use of the word ITCZ as the proper definition of this region should be classified as a Tropical Rainfall Belt (TRB) or Tropical Convergence Zone (TCZ). The Weller and Cai., 2014 paper refers to the ITCZ the author has described there as the Oceanic Tropical Convergence zone (OTCZ), and monsoonal papers refer to this region at the TRB as do other paleo papers.
Figure 1 – in the caption the author brings to attention the 27.5 and 28°C isotherms however doesn’t highlight them in the figure. This would be helpful as this could be a key component of the paper. Additionally, as the Weller et al., 2014 paper states the 27°C isotherm location is very similar to the North-south gradient this could be a good point of comparison.
Figure 3 – Panel C would be helpful to have the little icons on the figures as well as the location of each, so it is intuitively easier to determine which location is which. Additionally, it would be more intuitive if the plots were ordered from West to East (i.e. Northern Mentawai should be first) as this would better connect to panel a.
Line 104 – This line suggests that the SST reaches the entire Mentawai Islands in October, however, the cool temperatures reach the Southern Mentawai Islands earlier allowing for the capture of the full pIOD associated upwelling. At South Pagai the full spectrum of moderate and positive IOD events are captured appropriately, South Pagai and Enggango should be very similar.
Line 109 – the phrase Meridional gradients in SST are particularly steep is confusing. Meridional gradients in SST suggest that the author is talking about the difference between two locations within the region, however, it seems that they are simply talking about the location of Enggano and that the seasonal variability is drastic as there is a steep/speedy transition in temperature?
Line 181 – Some clarification here would be good, the methods in Cahyarini et al., 2021 and what is stated here differ. If the core is tied only to September, it would be appropriate to remove this reference here, if the coral is also tied to the minima values then this would negate some of the discussion later on about the change in the timing of the offset. Additionally in Pfeiffer et al., 2022 (where the original modern corals are published) it is stated that the maxima are tied to May which if only the sub-fossils are not tied would potentially influence the interpretation. Could the author comment on this?
Line 297 – why would this be misleading?
Line 300 – the definition of extreme pIOD events here is different to that stated elsewhere, i.e. in Abram et al., 2015 where the pIOD events of 1963, and 1967 are defined as moderate events. Another inconsistency is the lack of picking up the 1982 moderate event in the Engagno coral.
Line 307 – this should be ‘likely’ due to vital effect, or by the intercolonial differences in Porites
Line 311 – there are two (Fig. A8) here.
Line 314 – As stated in line 313 above, the author lists the timing of the strong positive skewness during extreme pIOD events period, this would be helpful in the example without extreme positive IOD events to allow the reader to immediately compare.
Lines 310-320 – this whole region of the test there are inconsistencies in the periods instances where the period stated ranges from (oldest – youngest or youngest- oldest) It is more intuitive to have all periods stated from oldest – youngest;. i.e. line 316 should be 1855-1917.
Line 315-310 – there is some confusion in this section, only like states a larger spread between 1917-1855, and the next line states that between 1854-1923 the spread reduces. Perhaps the author meant 1854-1823 otherwise we would be talking about the same period.
Line 324 – I believe as the basis of this paper it would be pertinent that this section be expanded on. Firstly in Figure A9; If this analysis is based on the Weller et al., 2014 papers, the Meridional SST gradient boxes are different to those displayed in this figure and also in Figure 9. If the relationship is based on this relationship and the occurrence of extreme TCZ shifts in the future it would be good to adjust the boxes to match. Additionally, extending the relationship between the North-south gradient and local SST at Enggano between the periods of 2005-2020 would further allow for comparison and give a longer period of ‘modern testing’, if the coral is strongly related to SST this would be a fine comparison. Perhaps this could be added as a panel in the figure?
- Additionally in Figure 9, the Authors have calculated the North-South gradient using the HadCRUT5. It would be interesting to see if the relationship shown in Figure A9 holds up with this extended period, i.e. how does this compare to the coral?
Line 350-355 – This analysis is very interesting; my major question is whether the choice of period changes the analysis. Particularly with the distribution analysis in Figure 8. Figures 8a and b are based on 73 years’ worth of data, while figure 8c is based on 48 years of data. If these could be compared on the same number of years this would be more comparable. Additionally, is there a way to improve the statistical comparison of these, rather than relying on the visual comparison?
Figure 7/8 – to allow for better comparison could the author set up the figures so that the figures in Figure 8 are in the same layout as Figure 7. Additionally, the periods are confusing. KNFa changes from 1917-1855 to 1917-1869 in Figure 7 and Figure 8 respectively.
Line 395 – I believe from reading the various Weller papers that there are instances in the recent period where the occurrence of a pIOD event is not associated with a northward shift in the TCZ, for example in 1982 where the meridional temperatures did not change significantly and thus the TCZ was not classified as an extreme northward shift.
Section 5.5 – Comparing the records in the SE tropical Indian Ocean would suggest the authors would like to encompass other records of coral-based variability in the region. Additionally including reconstruction of both extreme IOD and pIOD. In Abram et al., 2020 – Coupling of Indo-Pacific climate variability over the last millennium – there are several pIOD events suggested between 1850-1900 which are picked up in the Mentawai coral reconstruction. As the periods are the same it would be interesting to know if the Enggano coral also picks up these events. Additionally, to make the same comparison between Mentawai and Enggano I would suggest the authors do a similar mean seasonal cycle analysis as done in Figure 7 for Mentawai. This could simply be an appendix figure to show there is no significant difference between the Mentawai periods if this is true.
Line 440-445 – the first sentence in this paragraph does not connect to the remaining paragraph. The age uncertainty is probably not the issue here and as such if the authors are trying to state that the extreme positive event is not seen because of the higher SST variability (likely reflecting variability closer to 7°S as stated earlier and the extreme pIOD is more similar to regular cooling) that should be emphasized. However, I am confused by this as earlier in the text the authors state that there are pIOD-like events (ones that match the magnitude of 2006, in line 330) so if this is true these events should not be picked up in the cores.
Line 461 – the t-test indicates this is not significant, perhaps the author meant less than.
Line 465 – the use of fully in this line is unnecessary.
Citation: https://doi.org/10.5194/cp-2024-25-RC2 -
AC2: 'Reply on RC2', Miriam Pfeiffer, 19 Jul 2024
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2024-25/cp-2024-25-AC2-supplement.pdf
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RC3: 'Comment on cp-2024-25', Anonymous Referee #3, 13 May 2024
The manuscript by Pfeiffer et al. presents a monthly coral Sr/Ca record from the eastern Indian Ocean from a fossil coral colony spanning portions of the 19th and 20th centuries. The authors compare this record with previously published coral Sr/Ca records from modern corals at the same site (Enggano Island) and with a published d18O record from farther north at Mentawai to examine past changes in meridional SST gradients related to ITCZ-induced upwelling. The authors find that there was an increase in SST seasonality and an earlier onset of maximum SSTs from 1917-1855 at the fossil coral site, which they conclude is related to stronger SE winds due to a northward shift in the ITCZ that results in stronger seasonal upwelling. They argue that this stronger seasonality is not present in the published Mentawai coral record farther north, concluding that the ITCZ does not shift beyond the Mentawai site. The authors conclude that the lack of seasonality at Mentawai allows for a stronger response to interannual IOD-related upwelling events and results in larger meridional SST gradients between the two sites from 1917-1855.
Overall, the manuscript provides an important new record in the eastern Indian Ocean that allows a more complete examination of meridional SST gradients in a crucial upwelling region. The authors are also very rigorous with their assessment of diagenesis and secondary calcification. However, there are instances where the authors need to improve clarity in their methodology, writing, and figures to allow for full assessment of the manuscript.
General Comments
Methodology:
- The authors say they developed the age model using 1 tie point following Cahyarini et al. (2021), but that paper used 2 tie points. Using 2 tie points seems important given the focus on seasonal variability.
- The strength of coral Sr/Ca is unclear since the modern coral records used for comparison have weak monthly calibrations with r2 values of only 0.45 and 0.5. No calibration comparisons were provided in this manuscript, or in the original publication (Pfeiffer et al., 2022) to assess the Sr/Ca proxy and determine its reliability across months and seasons.
- More description of methodology is also needed to assess the gradient calculations, such as the spatial areas averaged for use in the calculations.
Figures:
- Many of the time periods shown in the figures do not correspond with the years discussed in the main text
- Often figure captions reference lines or data that is not shown on the figures
- Specific months that define austral spring are often not defined
Data Interpretation:
- Some of the conclusions made by the authors seem to be based on a visual assessment rather than statistical analysis (for example the discussion of multi-decadal variability in temperature gradients). It would be good for the authors to provide quantitative support for all analyses of the records.
- One of the authors’ primary conclusions is that the lower seasonality of the Mentawai record compared with Enggano from 1855-1917 indicates a northward shift in the ITCZ. The way this conclusion is discussed throughout the manuscript would benefit from improved clarity. At first this conclusion was unclear to me given that the ITCZ migrates to the northern hemisphere annually, moving northward of the Mentawai site. I now realize that the authors are discussing the southern margin of the ITCZ shifting northward, strengthening winds and increasing upwelling, which would impact Enggano more strongly than Mentawai. The authors should clarify this point throughout the manuscript to make sure the reader understand how this mechanism differently impacts the two sites. I also suggest comparisons to monsoon wind strength and ITCZ position, and a schematic to visualize the proposed mechanism. This would improve clarity and understanding for the reader.
Detailed Comments
28: provide more specific GPS coordinates for coral sites (at least two decimal places)
43: change to “a zonal mode characterized by a reversal”
47-48: Define the months you are referring to when you say “austral spring”. Make sure this is defined throughout manuscript.
47-48: I’m a little confused about the reference to Figure 1 here. It seems that the sentence is talking about seasonal northward shifts of the ITCZ driving changes in SST gradients, but Figure 1 is related to changes in SST gradients induced by IOD+ events. Either the text should be modified to more clearly discuss the Figure, or the Figure should be modified to show the seasonal ITCZ shifts. If the authors are trying to use Figure 1 to demonstrate the influence of strong positive IOD events on the meridional SST gradient, I suggest more clearly discussing the differences between panel a compared with b-e.
56: change to “which may shift the ITCZ position”
59: I suggest outlining the ITCZ region in all panels to better show the northward shift
81: In Figure 1 caption, define “Austral spring”
84: Label the contour lines for OLR <240 M/m2, and for the 27.5 and 28ºC contours. Also label the latitude of the ITCZ
90: In Figure 2 state months that define “Austral fall”
93: In Figure 2 say “AVHRR OI SST” to be consistent with other figure captions.
97: State lat/lon to two decimal places
107: state months for austral spring
117: Which panels of Figure 1 are you referring to? There does not appear to be cooling to 25ºC (mostly down to 25.5ºC), nor does the cooling seem to consistently extend to the equator.
178: How many samples were run to determine the RSD%? State n value
181: 1 tie-point in September is not consistent with Cahyarini et al. (2021) who used two tie points per year (one in September and one in May). Did you use one or two tie points?
204: The modern coral calibration of the Enggano site from Pfeiffer et al. (2022) yield an average monthly slope of -0.047, which is considerably shallower than the slope used in this manuscript for calibration purposes. The authors should use the modern coral slopes from their coral sites.
I also wonder how reliably the Enggano site can resolve SST variability using the Sr/Ca proxy at sub-annual timescales. The monthly calibrations presented in Pfeiffer et al. (2022) only have r2 values of 0.45 to 0.5, which is low for a monthly calibration. The authors should show the calibration data from the modern coral records (Sr/Ca vs. OISST in scatter plots and timeseries) to discuss the strength of the Sr/Ca proxy on monthly timescales at this site. This comparison was not available in the original publication and may help identify whether certain months are more strongly reflecting SST variability than others.
233: change “were” to “where”
237: This sentence references Figures S5 and S7 which were not provided with this manuscript and do not seem to correspond with A5 and A7, making it difficult to evaluate the manuscript here.
245-246: Did you re-drill/re-analyze this section to ensure that the signal is replicable?
289-290: This sentence implies that this manuscript demonstrated a strong Sr/Ca and SST calibration in the KN2 and PB records. Though Pfeiffer et al. (2022) is cited at the end, the sentence is long and this statement is far removed from the in-text citation. I suggest the authors re-write this sentence to more clearly indicate this conclusion is based on published work.
334: How are you assessing that the seasonal variability is weaker than interannual variability? It seems that in the modern records, there are more periods with significant seasonal variance compared with interannual. In addition, the magnitude of shading looks similar between the significant seasonal and interannual periods.
342: red lines are not shown in figure
355-356: Which of the two modern records was used to calculate the difference in mean seasonal cycles? PB or KN2? Make sure to clarify here and in the Figure 7 caption.
366-372: A schematic would be useful to visualize the mechanism you are proposing. The text should also be modified to make clear that the authors are discussion a northward shift in the southern margin of the ITCZ. For example, the northern edge of the ITCZ migrates to northern latitudes annually in July-September. It should move northward of the Mentawai records. The authors should clarify the focus on the southern portion of the ITCZ when discussing their proposed mechanism for a northward shift that does not reach Mentawai while impacting Enggano.
379: It would be helpful to add a comparison to the Mentawai record in Figure 7 or in the supplementary material to support the discussion in section 5.3. Also make sure to keep the y-axis scaling consistent across all panels to facilitate easier comparison of the magnitude of the seasonal cycles. Currently the difference plots have different scaling that exaggerates the seasonality. I found this confusing because the difference plots at first glance look as if the magnitude is larger than the variability depicted in panels a-c.
389: For panel 8c, why is the time period depicted 1917-1869 rather than 1917-1855 as is discussed in the text and figure caption? Make sure to be consistent. Also make sure to explain the lat/lon differences between panels in the figure caption.
398-400: State the spatial domains (lat/lon) used to calculate the meridional and zonal gradients. It would also be good to show these regions on a map (Fig 1 or 2).
405-406: Did you conduct any quantitative assessments of the variability, such as spectral analysis or other spectral methodologies? Currently, the evaluation of multi-decadal variability seems visual, making it difficult to assess.
406-408: It’s difficult to assess the changes in the NE vs. SE Indian Ocean zonal gradient given that the spatial domains used to calculate the gradients were not provided. It’s unclear to me whether the authors took averages of the entire northern and southern Indian ocean, or focused specifically on the NE and SE Indian Ocean.
409: Here, you suggest that your results indicate a warmer eastern Indian Ocean relative to the western Indian Ocean, but in lines 403-405 you said that your data suggest that the western Indian Ocean is warming faster than the eastern Indian Ocean. The positive values in the west-east gradient in Figure 9 from the 1980s-2000s would suggest that the western Indian Ocean is warmer than the eastern Indian Ocean. Is there a specific time period you are focused on that you can discuss more clearly?
410-411: It only seems possible to have a stronger north-south meridional SST gradient in the east in the earliest portion of the record (1970-1910). Is this the time period you mean to discuss? Make sure to be specific in the main text to clarify at which time periods each process is occurring. It would also be helpful to add a comparison of the Lenssen et al. (2019) results to Figure 9 to compare with your findings and support your conclusions.
428-434: I’m still unclear how the results presented in this manuscript indicate a shift in the ITCZ. The ITCZ migrates to the northern hemisphere seasonally. Are you specifically discussing the southern margin of the ITCZ?
442: Enggano is located at 5ºS, so it is not south of 7ºS latitudes.
459: Do you mean between 1823 and 1854 as is indicated in Figure 7? Make sure you are consistent in your time periods across all figures and text.
464: change to “linked”
475: It would be helpful to add a panel to this figure where you examine the change in SST gradient between the Mentawai and Enggano coral records to support the discussion in Section 5.5
476: 10-year running averages are not shown in panel a
Citation: https://doi.org/10.5194/cp-2024-25-RC3 -
AC3: 'Reply on RC3', Miriam Pfeiffer, 19 Jul 2024
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2024-25/cp-2024-25-AC3-supplement.pdf
Status: closed
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RC1: 'Comment on cp-2024-25', Anonymous Referee #1, 18 Apr 2024
This study aims to understand how the meridional SST gradient in the eastern equatorial Indian Ocean has changed over the past ~200 years. To do so, the author developed a Sr/Ca record using fossil corals collected from Enggano Island for analysis. The authors found that there was an increase in seasonality strength and an earlier seasonal SST maximum between 1855-1917 compared to the modern period and 1823-1854 CE and attributed the change to an earlier onset of austral spring and strengthened SE winds, which consequently imply a northward shift of ITCZ and a stronger meridional SST gradient. On the other hand, there is no conclusive evidence that changes in zonal SST gradient (e.g., IOD) played a role in changing SST in Enggano Island and meridional gradient despite previous suggestions. Therefore, they concluded that the meridional SST gradient and zonal SST gradient are not always coupled and require more analyses on the mechanisms that drive the meridional SST gradient.
I think this new record is a useful contribution to better understand climate variability in the Indian Ocean and complement existing records in nearby areas. I also think the analysis of meridional SST gradient in the NE Indian Ocean is interesting. The screening for coral quality/diagenesis is also extensive. That said, I have some suggestions/comments, that I hope would help improve the manuscript. There are also several inconsistencies within the text and figures/tables, which needs to be corrected. Otherwise, it is difficult to judge the results and conclusion of this study.
Overall comments:
- I find referencing the years in reversed chronological order (e.g., 2008 to 1930 instead of 1930 to 2008) confusing. There are also several instances where the years are referred in chronological order instead, for instance L317 “Between 1854 and 1923…”. I suggest making this consistent throughout the text, and preferably in chronological order (i.e., older to younger).
- Within the manuscript, there are multiple instances where statistical significance is mentioned (e.g., 99% confidence levels). However, in most cases, it is unclear how this was determined. Even when a Monte Carlo approach was used, it was also unclear how it was carried out. It would be helpful if this can be clarified.
- The results of this study hinge on an accurate chronology and constraints on the annual cycle. While there are multiple instances within the manuscript where dating uncertainty was mentioned, as far as I can tell, there are no testing of how robust the results were against dating uncertainties. It would be nice to see sensitivity tests to check this. Furthermore, it is currently unclear how the annual cycle was derived (see the comment L181-183 below for more details), which makes me unsure of the results. Additionally, the chronology is derived based on the assumption that the internal chronology and U/Th ages have a 1:1 relationship. While in Figure A6, it shows that they correspond to each other fairly well, there are also instances where the U/Th diverges from the 1:1 line (the 2sigma of KNFa(8/11), KNFa(5/11) do not line up with the regression line). So, I wonder how accurate this assumption is in this case? Moreover, it would be great to provide the estimated extension rates of these corals, just so we can make sure it is indeed possible to estimate monthly SST changes.
- Given that the change in SE wind strength, and the shift in ITCZ are supposed to correspond to changes in the South Asian monsoon, I wonder if it will also be helpful if you can show the South Asian monsoon also changed concurrently with these changes.
Specific comments:
L31-32: A ‘reference period’ needed to compare with in order to claim ‘an increase in SST seasonality due to enhanced austral spring cooling’.
Figure 1: In most cases, the figure is referred in the text when discussing about anomalies during IOD events. So, I wonder if it would be better to change the subplots b-e into OLR and SST anomalies so that they can better serve their purposes?
L111-L112: It is unclear to me how Aug-Oct temperature and symmetry is inferred based on Figs 2-3 – both do not show mean Aug-Oct temperatures.
Figure 3: I suggest checking the consistencies between this figure, the caption, and the main text on which months are mentioned and used for analyses. In the text and the caption, Aug-Oct was mentioned, whereas the figure label suggests Sept-Nov. Additionally, the vertical dashed lines don’t seem to be located at the same months for the plot of each location in subplot c.
L154-155: I think the sentence is missing a verb (e.g., were carried out’).
L181-183: This is actually *inconsistent* with Cahyarini et al. (2021) and Pfeiffer et al. (2021). Both studies tied September to Sr/Ca maxima and May to Sr/Ca minima. But here, it suggests only September was tied to Sr/Ca maxima. Please clarify which way it was done.
L188-195: Fig A6 should probably be referred somewhere here so the readers can go to that figure to get a better sense how this was done.
L233: ‘were’ -> where
L237: I think the Appendix/Supplementary figures referenced here are incorrect.
L311: repeated ‘(Fig. A8)’.
L312-320: Should ‘1854 and 1923’ be 1854-1823 instead? Otherwise, this will be referring to the same overlapping period as the previous sentence (1917-1855). Additionally, I would like more quantification on the comparisons between the distributions instead of simply relying on visualizations. Tests such as a Kolmogorov Smirnov test (or its variant) would be helpful here.
Figure 6: I only see one type of line in subplot (a) with two green and blue lines. I do not see red solid and dashed lines.
L350-354: I wonder if there’s a more objective way to ‘separate’ these time periods? Right now, it seems a bit arbitrary and relies on visualization. One suggestion perhaps would be to analyze the wavelet power of annual cycle and identify periods that are weaker to a ‘reference period’. This can be pulled out from the wavelet spectra.
Figure 7: Why is the seasonal cycle shown here span from July to January 2 years later? There are several mistakes in the caption (e.g., ‘dashed green lines’ for core PB, no explanation of ‘dark grey and dashed lines’ in subplot a). Additionally, which ‘modern’ (coral or observation) is used in (d) and (f)?
Figure 8: in subplot c, there is a discrepancy on which years were used for analysis: it was labelled 1917-1869 in the figure whereas in the caption 1917-1855 was referred.
Figure 9: I don’t quite understand what “not the large 95% confidence levels of the SiZer test” means. It would also be helpful to explain what the horizontal lines associated with the change point indicators mean. Additionally, there needs to be clarification on how the SiZer analysis was carried out. As far as I recall, SiZer applies a range of Gaussian filters with varying bandwidths and calculate the trends based on those bandwidths. But in the caption, it only mentions about a 21 year running average to show the data and not any other information related to SiZer.
L440: I am not sure if age uncertainty is the main issue here. By just looking through the KNFa record, there is no ‘major’ anomalies (<-4C) between 1869-1917. Given that the absolute dating uncertainties are almost <10 years, I don’t think age uncertainty is an issue here. In fact, the remaining paragraph does not discuss about age uncertainty. So, I suggest modifying the first sentence of this paragraph.
L444-446: I don’t think this is an accurate statement. My understanding is that the Enggano record is better in capturing meridional SST gradient compared to previous Mentawai records. So, it is logical that the Enggano record might not detect IOD changes. That said, a comparison between meridional and zonal SST gradients can be achieved by making use of the Enggano record for meridional changes and the recently southern Mentawai record (Abram et al., 2021 Nature) that is supposed to record IOD changes.
L456: hereinafter, ‘foraminifers’ should be ‘foraminifera’.
L461: Would (p>0.05) mean it is not significantly different?
L461-462: I think it would be helpful to test statistically if the relative abundance of thermocline dwelling forams changed significantly.
L464: ‘linkedn’ -> linked
Figure 11: I wonder if the abundance of mixed layer forams timeseries is needed, since it was never discussed in the text?
Table A1: What do those asterisks next to KNFa(3/11) and KNFa(7/11) mean?
Figure A6: I don’t think the equation displayed here is correct. Plugging in any years prior to 1934.1 will result a negative y, which isn’t supposed to happen here.
L601: As far as I can tell, I don’t see any U/Th ages that correspond to 1823 both in the figure and in table A1.
L619: ‘(a)’ should be (b) here.
Citation: https://doi.org/10.5194/cp-2024-25-RC1 -
AC1: 'Reply on RC1', Miriam Pfeiffer, 19 Jul 2024
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2024-25/cp-2024-25-AC1-supplement.pdf
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RC2: 'Comment on cp-2024-25', Anonymous Referee #2, 13 May 2024
Overall comments;
This study aims to contribute to the growing number of reconstructions in the Southeastern Indian Ocean region and document how the meridional SST gradient has changed in the past 200 years. To achieve this the authors, develop a sub-fossil coral Sr/Ca record collected from Enggano Island, an island previously used to reconstruct Indian Ocean Dipole variability. The authors found the sub-fossil exhibits an enhanced seasonal cycle between 1855-1917 compared to the modern equivalent and the later 1823-1854 periods. This enhanced seasonal cycle was attributed to an earlier onset of austral spring, and increased SE winds during July-October. This was then attributed to a northward shift in the mean position of the ITCZ and a stronger meridional SST gradient in the eastern Indian Ocean. They conclude that this is unlikely to be associated with enhanced IOD variability as spectral analysis suggests that variability associated with the Enggano coral is mostly interannual, and IOD variability would only impact the IOD season rather than the full seasonal cycle. Based on historical SST products they additionally conclude that the zonal (IOD) and meridional components controlling the shift in the ITCZ are not linearly coupled in the past.
This coral reconstruction is a useful contribution to the regional understanding of the Indian Ocean variability due to the sparse observational record in the region. This record complements previous records in the region, particularly adding to a large reconstruction effort of the IOD. Some of the methodological aspects of this study are excellent, particularly the diagenesis screening which is an excellent example of how to address issues with sub-fossil usage. I do have some concerns with the study, which I believe would improve the manuscript and better align with the knowledge of the community.
Overall concerns;
- I have an issue with the definition of the ITCZ region. In this region, the Maritime Continent, the definition of the traditional ITCZ does not typically apply. Due to the numerous monsoonal systems that operate in the region, the system should be defined as a ‘Tropical Convergence Zone (TCZ) or Tropical Rainfall Belt’, as outlined in Geen et al., 2020 (https://doi.org/10.1029/2020RG000700). The transition of the monsoons (as the author here is describing) is commonly associated with the global monsoon transition. I would encourage the authors to think about this definition and which they should be using. Additionally, due to the width of the TCZ in this region, I would encourage the authors to instead state this as the Southern Boundary of the TCZ.
- The main finding of this paper is based on the difference between the seasonal range in the sub-fossil and the modern. There are some issues with this methodology. Firstly, the description of how the age model was constructed is inconsistent, with different explanations of how the age model was constructed. In the modern coral in Pfeiffer et al., 2022 tie points are constructed on both minima and maxima which may make a difference when comparing the seasonal cycle, as according to this paper the maxima temperature is not constrained.
- I feel that the modern comparison is not sufficiently explored in this paper, particularly towards the detection of IOD events, the authors state that the change in the seasonal variability is not linked to IOD events however state that there are IOD-like events in the coral (which are mentioned in the methods). Additionally, as the main finding is centred around the difference between Mentawai and Enggano a more in-depth comparison would be appropriate.
Specific comments;
Line 43/44 – I believe that the suite of Abram et al., papers should be included here or in the next line of referencing. Particularly the paleoclimate perspectives paper
Line 49 - As I have stated above I question the use of the word ITCZ as the proper definition of this region should be classified as a Tropical Rainfall Belt (TRB) or Tropical Convergence Zone (TCZ). The Weller and Cai., 2014 paper refers to the ITCZ the author has described there as the Oceanic Tropical Convergence zone (OTCZ), and monsoonal papers refer to this region at the TRB as do other paleo papers.
Figure 1 – in the caption the author brings to attention the 27.5 and 28°C isotherms however doesn’t highlight them in the figure. This would be helpful as this could be a key component of the paper. Additionally, as the Weller et al., 2014 paper states the 27°C isotherm location is very similar to the North-south gradient this could be a good point of comparison.
Figure 3 – Panel C would be helpful to have the little icons on the figures as well as the location of each, so it is intuitively easier to determine which location is which. Additionally, it would be more intuitive if the plots were ordered from West to East (i.e. Northern Mentawai should be first) as this would better connect to panel a.
Line 104 – This line suggests that the SST reaches the entire Mentawai Islands in October, however, the cool temperatures reach the Southern Mentawai Islands earlier allowing for the capture of the full pIOD associated upwelling. At South Pagai the full spectrum of moderate and positive IOD events are captured appropriately, South Pagai and Enggango should be very similar.
Line 109 – the phrase Meridional gradients in SST are particularly steep is confusing. Meridional gradients in SST suggest that the author is talking about the difference between two locations within the region, however, it seems that they are simply talking about the location of Enggano and that the seasonal variability is drastic as there is a steep/speedy transition in temperature?
Line 181 – Some clarification here would be good, the methods in Cahyarini et al., 2021 and what is stated here differ. If the core is tied only to September, it would be appropriate to remove this reference here, if the coral is also tied to the minima values then this would negate some of the discussion later on about the change in the timing of the offset. Additionally in Pfeiffer et al., 2022 (where the original modern corals are published) it is stated that the maxima are tied to May which if only the sub-fossils are not tied would potentially influence the interpretation. Could the author comment on this?
Line 297 – why would this be misleading?
Line 300 – the definition of extreme pIOD events here is different to that stated elsewhere, i.e. in Abram et al., 2015 where the pIOD events of 1963, and 1967 are defined as moderate events. Another inconsistency is the lack of picking up the 1982 moderate event in the Engagno coral.
Line 307 – this should be ‘likely’ due to vital effect, or by the intercolonial differences in Porites
Line 311 – there are two (Fig. A8) here.
Line 314 – As stated in line 313 above, the author lists the timing of the strong positive skewness during extreme pIOD events period, this would be helpful in the example without extreme positive IOD events to allow the reader to immediately compare.
Lines 310-320 – this whole region of the test there are inconsistencies in the periods instances where the period stated ranges from (oldest – youngest or youngest- oldest) It is more intuitive to have all periods stated from oldest – youngest;. i.e. line 316 should be 1855-1917.
Line 315-310 – there is some confusion in this section, only like states a larger spread between 1917-1855, and the next line states that between 1854-1923 the spread reduces. Perhaps the author meant 1854-1823 otherwise we would be talking about the same period.
Line 324 – I believe as the basis of this paper it would be pertinent that this section be expanded on. Firstly in Figure A9; If this analysis is based on the Weller et al., 2014 papers, the Meridional SST gradient boxes are different to those displayed in this figure and also in Figure 9. If the relationship is based on this relationship and the occurrence of extreme TCZ shifts in the future it would be good to adjust the boxes to match. Additionally, extending the relationship between the North-south gradient and local SST at Enggano between the periods of 2005-2020 would further allow for comparison and give a longer period of ‘modern testing’, if the coral is strongly related to SST this would be a fine comparison. Perhaps this could be added as a panel in the figure?
- Additionally in Figure 9, the Authors have calculated the North-South gradient using the HadCRUT5. It would be interesting to see if the relationship shown in Figure A9 holds up with this extended period, i.e. how does this compare to the coral?
Line 350-355 – This analysis is very interesting; my major question is whether the choice of period changes the analysis. Particularly with the distribution analysis in Figure 8. Figures 8a and b are based on 73 years’ worth of data, while figure 8c is based on 48 years of data. If these could be compared on the same number of years this would be more comparable. Additionally, is there a way to improve the statistical comparison of these, rather than relying on the visual comparison?
Figure 7/8 – to allow for better comparison could the author set up the figures so that the figures in Figure 8 are in the same layout as Figure 7. Additionally, the periods are confusing. KNFa changes from 1917-1855 to 1917-1869 in Figure 7 and Figure 8 respectively.
Line 395 – I believe from reading the various Weller papers that there are instances in the recent period where the occurrence of a pIOD event is not associated with a northward shift in the TCZ, for example in 1982 where the meridional temperatures did not change significantly and thus the TCZ was not classified as an extreme northward shift.
Section 5.5 – Comparing the records in the SE tropical Indian Ocean would suggest the authors would like to encompass other records of coral-based variability in the region. Additionally including reconstruction of both extreme IOD and pIOD. In Abram et al., 2020 – Coupling of Indo-Pacific climate variability over the last millennium – there are several pIOD events suggested between 1850-1900 which are picked up in the Mentawai coral reconstruction. As the periods are the same it would be interesting to know if the Enggano coral also picks up these events. Additionally, to make the same comparison between Mentawai and Enggano I would suggest the authors do a similar mean seasonal cycle analysis as done in Figure 7 for Mentawai. This could simply be an appendix figure to show there is no significant difference between the Mentawai periods if this is true.
Line 440-445 – the first sentence in this paragraph does not connect to the remaining paragraph. The age uncertainty is probably not the issue here and as such if the authors are trying to state that the extreme positive event is not seen because of the higher SST variability (likely reflecting variability closer to 7°S as stated earlier and the extreme pIOD is more similar to regular cooling) that should be emphasized. However, I am confused by this as earlier in the text the authors state that there are pIOD-like events (ones that match the magnitude of 2006, in line 330) so if this is true these events should not be picked up in the cores.
Line 461 – the t-test indicates this is not significant, perhaps the author meant less than.
Line 465 – the use of fully in this line is unnecessary.
Citation: https://doi.org/10.5194/cp-2024-25-RC2 -
AC2: 'Reply on RC2', Miriam Pfeiffer, 19 Jul 2024
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2024-25/cp-2024-25-AC2-supplement.pdf
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RC3: 'Comment on cp-2024-25', Anonymous Referee #3, 13 May 2024
The manuscript by Pfeiffer et al. presents a monthly coral Sr/Ca record from the eastern Indian Ocean from a fossil coral colony spanning portions of the 19th and 20th centuries. The authors compare this record with previously published coral Sr/Ca records from modern corals at the same site (Enggano Island) and with a published d18O record from farther north at Mentawai to examine past changes in meridional SST gradients related to ITCZ-induced upwelling. The authors find that there was an increase in SST seasonality and an earlier onset of maximum SSTs from 1917-1855 at the fossil coral site, which they conclude is related to stronger SE winds due to a northward shift in the ITCZ that results in stronger seasonal upwelling. They argue that this stronger seasonality is not present in the published Mentawai coral record farther north, concluding that the ITCZ does not shift beyond the Mentawai site. The authors conclude that the lack of seasonality at Mentawai allows for a stronger response to interannual IOD-related upwelling events and results in larger meridional SST gradients between the two sites from 1917-1855.
Overall, the manuscript provides an important new record in the eastern Indian Ocean that allows a more complete examination of meridional SST gradients in a crucial upwelling region. The authors are also very rigorous with their assessment of diagenesis and secondary calcification. However, there are instances where the authors need to improve clarity in their methodology, writing, and figures to allow for full assessment of the manuscript.
General Comments
Methodology:
- The authors say they developed the age model using 1 tie point following Cahyarini et al. (2021), but that paper used 2 tie points. Using 2 tie points seems important given the focus on seasonal variability.
- The strength of coral Sr/Ca is unclear since the modern coral records used for comparison have weak monthly calibrations with r2 values of only 0.45 and 0.5. No calibration comparisons were provided in this manuscript, or in the original publication (Pfeiffer et al., 2022) to assess the Sr/Ca proxy and determine its reliability across months and seasons.
- More description of methodology is also needed to assess the gradient calculations, such as the spatial areas averaged for use in the calculations.
Figures:
- Many of the time periods shown in the figures do not correspond with the years discussed in the main text
- Often figure captions reference lines or data that is not shown on the figures
- Specific months that define austral spring are often not defined
Data Interpretation:
- Some of the conclusions made by the authors seem to be based on a visual assessment rather than statistical analysis (for example the discussion of multi-decadal variability in temperature gradients). It would be good for the authors to provide quantitative support for all analyses of the records.
- One of the authors’ primary conclusions is that the lower seasonality of the Mentawai record compared with Enggano from 1855-1917 indicates a northward shift in the ITCZ. The way this conclusion is discussed throughout the manuscript would benefit from improved clarity. At first this conclusion was unclear to me given that the ITCZ migrates to the northern hemisphere annually, moving northward of the Mentawai site. I now realize that the authors are discussing the southern margin of the ITCZ shifting northward, strengthening winds and increasing upwelling, which would impact Enggano more strongly than Mentawai. The authors should clarify this point throughout the manuscript to make sure the reader understand how this mechanism differently impacts the two sites. I also suggest comparisons to monsoon wind strength and ITCZ position, and a schematic to visualize the proposed mechanism. This would improve clarity and understanding for the reader.
Detailed Comments
28: provide more specific GPS coordinates for coral sites (at least two decimal places)
43: change to “a zonal mode characterized by a reversal”
47-48: Define the months you are referring to when you say “austral spring”. Make sure this is defined throughout manuscript.
47-48: I’m a little confused about the reference to Figure 1 here. It seems that the sentence is talking about seasonal northward shifts of the ITCZ driving changes in SST gradients, but Figure 1 is related to changes in SST gradients induced by IOD+ events. Either the text should be modified to more clearly discuss the Figure, or the Figure should be modified to show the seasonal ITCZ shifts. If the authors are trying to use Figure 1 to demonstrate the influence of strong positive IOD events on the meridional SST gradient, I suggest more clearly discussing the differences between panel a compared with b-e.
56: change to “which may shift the ITCZ position”
59: I suggest outlining the ITCZ region in all panels to better show the northward shift
81: In Figure 1 caption, define “Austral spring”
84: Label the contour lines for OLR <240 M/m2, and for the 27.5 and 28ºC contours. Also label the latitude of the ITCZ
90: In Figure 2 state months that define “Austral fall”
93: In Figure 2 say “AVHRR OI SST” to be consistent with other figure captions.
97: State lat/lon to two decimal places
107: state months for austral spring
117: Which panels of Figure 1 are you referring to? There does not appear to be cooling to 25ºC (mostly down to 25.5ºC), nor does the cooling seem to consistently extend to the equator.
178: How many samples were run to determine the RSD%? State n value
181: 1 tie-point in September is not consistent with Cahyarini et al. (2021) who used two tie points per year (one in September and one in May). Did you use one or two tie points?
204: The modern coral calibration of the Enggano site from Pfeiffer et al. (2022) yield an average monthly slope of -0.047, which is considerably shallower than the slope used in this manuscript for calibration purposes. The authors should use the modern coral slopes from their coral sites.
I also wonder how reliably the Enggano site can resolve SST variability using the Sr/Ca proxy at sub-annual timescales. The monthly calibrations presented in Pfeiffer et al. (2022) only have r2 values of 0.45 to 0.5, which is low for a monthly calibration. The authors should show the calibration data from the modern coral records (Sr/Ca vs. OISST in scatter plots and timeseries) to discuss the strength of the Sr/Ca proxy on monthly timescales at this site. This comparison was not available in the original publication and may help identify whether certain months are more strongly reflecting SST variability than others.
233: change “were” to “where”
237: This sentence references Figures S5 and S7 which were not provided with this manuscript and do not seem to correspond with A5 and A7, making it difficult to evaluate the manuscript here.
245-246: Did you re-drill/re-analyze this section to ensure that the signal is replicable?
289-290: This sentence implies that this manuscript demonstrated a strong Sr/Ca and SST calibration in the KN2 and PB records. Though Pfeiffer et al. (2022) is cited at the end, the sentence is long and this statement is far removed from the in-text citation. I suggest the authors re-write this sentence to more clearly indicate this conclusion is based on published work.
334: How are you assessing that the seasonal variability is weaker than interannual variability? It seems that in the modern records, there are more periods with significant seasonal variance compared with interannual. In addition, the magnitude of shading looks similar between the significant seasonal and interannual periods.
342: red lines are not shown in figure
355-356: Which of the two modern records was used to calculate the difference in mean seasonal cycles? PB or KN2? Make sure to clarify here and in the Figure 7 caption.
366-372: A schematic would be useful to visualize the mechanism you are proposing. The text should also be modified to make clear that the authors are discussion a northward shift in the southern margin of the ITCZ. For example, the northern edge of the ITCZ migrates to northern latitudes annually in July-September. It should move northward of the Mentawai records. The authors should clarify the focus on the southern portion of the ITCZ when discussing their proposed mechanism for a northward shift that does not reach Mentawai while impacting Enggano.
379: It would be helpful to add a comparison to the Mentawai record in Figure 7 or in the supplementary material to support the discussion in section 5.3. Also make sure to keep the y-axis scaling consistent across all panels to facilitate easier comparison of the magnitude of the seasonal cycles. Currently the difference plots have different scaling that exaggerates the seasonality. I found this confusing because the difference plots at first glance look as if the magnitude is larger than the variability depicted in panels a-c.
389: For panel 8c, why is the time period depicted 1917-1869 rather than 1917-1855 as is discussed in the text and figure caption? Make sure to be consistent. Also make sure to explain the lat/lon differences between panels in the figure caption.
398-400: State the spatial domains (lat/lon) used to calculate the meridional and zonal gradients. It would also be good to show these regions on a map (Fig 1 or 2).
405-406: Did you conduct any quantitative assessments of the variability, such as spectral analysis or other spectral methodologies? Currently, the evaluation of multi-decadal variability seems visual, making it difficult to assess.
406-408: It’s difficult to assess the changes in the NE vs. SE Indian Ocean zonal gradient given that the spatial domains used to calculate the gradients were not provided. It’s unclear to me whether the authors took averages of the entire northern and southern Indian ocean, or focused specifically on the NE and SE Indian Ocean.
409: Here, you suggest that your results indicate a warmer eastern Indian Ocean relative to the western Indian Ocean, but in lines 403-405 you said that your data suggest that the western Indian Ocean is warming faster than the eastern Indian Ocean. The positive values in the west-east gradient in Figure 9 from the 1980s-2000s would suggest that the western Indian Ocean is warmer than the eastern Indian Ocean. Is there a specific time period you are focused on that you can discuss more clearly?
410-411: It only seems possible to have a stronger north-south meridional SST gradient in the east in the earliest portion of the record (1970-1910). Is this the time period you mean to discuss? Make sure to be specific in the main text to clarify at which time periods each process is occurring. It would also be helpful to add a comparison of the Lenssen et al. (2019) results to Figure 9 to compare with your findings and support your conclusions.
428-434: I’m still unclear how the results presented in this manuscript indicate a shift in the ITCZ. The ITCZ migrates to the northern hemisphere seasonally. Are you specifically discussing the southern margin of the ITCZ?
442: Enggano is located at 5ºS, so it is not south of 7ºS latitudes.
459: Do you mean between 1823 and 1854 as is indicated in Figure 7? Make sure you are consistent in your time periods across all figures and text.
464: change to “linked”
475: It would be helpful to add a panel to this figure where you examine the change in SST gradient between the Mentawai and Enggano coral records to support the discussion in Section 5.5
476: 10-year running averages are not shown in panel a
Citation: https://doi.org/10.5194/cp-2024-25-RC3 -
AC3: 'Reply on RC3', Miriam Pfeiffer, 19 Jul 2024
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2024-25/cp-2024-25-AC3-supplement.pdf
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