|This revised paper is improved and many details that were unclear in the previous revision are now improved. The detailed analysis of ENSO events with two different reference data sources highlights the discrepancies that any reconstruction will have but still supports the results of the current study. The discussion section begins more like a conclusion section. The discussion should be a comparison to other work, discussion of the strength and weakness of your results, and other interpretations, and implications of the study. |
This paper still has many grammar issues and unclear sentences. I suggest the authors get help to address these issues. I do not take the time to note all these issues in my detailed edits since there are many and this is a second review. I tried to focus on the science. But this paper needs to have the grammar addressed before publication. I use the free website Grammarly.com often and recommend my students use it as well.
Science items to address:
The wavelet coherence plots are interesting, but the authors do not take chronology error into account. Assuming the Wilson Nino reconstruction does not have chronology errors, if you shift the coral chronologies within the U-Th dating uncertainties, do the wavelet coherence plots change or find better coherence? With U-Th errors or ±2-3 years, 2d, you could shift by 6 years to see if the results are better or worse. These plots as they as presented are not showing the level of coherence I would expect if there was a strong ENSO signal in these records. I would like to see the Wavelet spectrums of the coral records to see what the ENSO periodicities look like, I would buy that more than the coherence with two reconstructions that both have chronology errors that could greatly impact these results, see Comboul 2014, doi:10.5194/cp-10-825-2014.
Specific items to address:
What the tense in your paragraph and sections and make sure the tense is consistent. There are still many grammar errors and typos in this paper that need to be corrected before publication.
Line 22 By one sample and two samples, do you mean coral or one measurement for this entire interval. Just say one coral and two corals.
Figure 1 What are numbers 640, 645, 650, 655 for on the right side of the map? Put the latitude and longitude degrees outside the box since the inset is covering part of the or move latitude to the right side of the map. It would be helpful if you map the mean SST of SST anomalies on this map to show the difference note in section 2.3.
Figure 2 The “Red” box for La Nina looks purple to me.
Figure 3 The blue line looks black to me.
Line 38 and 42 and elsewhere Comma is not needed after central “central tropical Indian Ocean”.
Line 44 The last phase of the sentence is confusing “…as these are phased-locked to the seasonal cycle and vary with the season” What does “these” refer to? The coral Sr/Ca, the ocean, climate phenomena, or something else. Perhaps clarify what “its” is in the same sentence. Revise to make the meaning clearer.
Line 46 Explain what you mean by “ENSO is centered”. Is this spatially centered? I think you mean “where ENSO occurs”. Same for Line 49. The central part of the tropical Pacific Ocean is not where ENSO occurs, ENSO occurs across the tropical Pacific Ocean — East, Central, and West. The central Pacific has the weakest climate response compared to the east and west Pacific. Additionally, there are different favors of ENSO, a central and eastern ENSO as well as a coastal. Therefore, using “centered” is confusing.
Line 46 Revise “Strong events associated with ENSO have occurred more frequently since the early 1980s relative…”
Line 48 and elsewhere - Do not use a “/” to replace the word “and”. This is a non-standard replacement, reserve “/” to mean “divide by” or to indicate a ratio like “Sr/Ca”. This is an informal usage.
Line 50 Use the adjective form “…oceanic-atmospheric parameters of the Indian Ocean…” Revise the “which” to a “that”. The same sentence, use the same tense for the verbs.
Line 52-53 Another confusing sentence, what is demonstrating? Revise sentence, a conjunction is needed. “Strong El Niño/La Niña events influence the tropical Indian Ocean thus establishing a SST-ENSO teleconnection between the Pacific and the Indian Ocean.” How you do know the teleconnection is “stationary” and for what time interval? Is this a question you can answer with your study or if others have shown this then say “previous studies have established a stationary SST-ENSO teleconnection…”.
Line 49 “While” is the incorrect word, you do not mean “at the same time as. If you mean to highlight contrasting relationship use “whereas”.
Line 64 Add for clarification the time interval you are referring to for “Indian Ocean warming”. Do you mean for the Little Ice Age or just the 20th century?
Line 64 Revise “We develop coral Sr/Ca records… to reconstruct past SST variability.”
Line 99 Revise this confusing incomplete sentence.
Line 108 What corals are you referring to? The previous sentence refers to two studies. Perhaps you mean “Those corals reconstructions revealed a few strong IOD events…” After reading the following sentence, perhaps you are referring to your own reconstruction that has not been presented yet.
Line 109-112 “However, neither in 1675 nor in 1961 a positive anomaly can be found in our coral SST records.” This is a result and the following sentences are how you intend to interpret your results, it does not belong in the introduction, but in the methods or results. Additionally, this sentence is not properly written and thus confusing.
Line 113-114 How is “28.1±0.9°C for the open ocean and reef and 28.5±0.6°” different from each other? Did you do a statistical test of the mean difference? What is ±0.9ºC? the standard deviation of the mean or standard error of the mean?
This is a poorly constructed sentence and all abbreviations should be defined at first use. Revise “Analysis of SST determined from the Advanced Very High-Resolution Radiometer (AVHRR) satellite SST product (Casey et al., 2010) for the varying grid areas in Chagos (open ocean (give area in degrees) and lagoon (give area in degrees) reveals differences in SST means and seasonality at Chagos depending on the reef setting. This analysis finds 28.1±0.9°C for the open ocean and reef and 28.5±0.6°C for the lagoon setting averaged over the interval from 1997–2012 (Fig. 3). Not sure what Leupold et al., 2019 is a reference for from the sentence construction. If this figure and analysis is from the study Leupold et al., 2019, this should be made clearer and referenced in Figure 3.
Line 119-120 Revise for improper use of /, use a hyphen. “such as El Niño in 1997-1998 or La Niña in 2010-2011”.
Line 119-120 What do you mean by “Both anomaly records are not significantly different (t-value = 0.34; p-value = 0.37)”? Is this a statistical test for the means, variance, or something else? You can also look at correlation to describe co-variance, which is more interesting for an SSTA time series looking at ENSO than if the means and or variance are the same or different.
Line 121 The reader does not know anything your coals or about the location your corals yet in the text, revise the text as needed, and refer to Fig. 1 where you have the coral locations.
Line 125 Revise “We therefore use various ENSO indices for comparison with our coral data…”
Line 128-129 “TexMex” is not the correct geographical term, use “Texas-Mexico”. Define USA abbreviation at first use. Revise “and other locations in the Tropics”. Revise “The annually-resolved El Niño Index Niño3.4…reconstructs past El Niño and La Niña events back to 1607.” Why is Niño3.4 in italics in this usage? Do not use italics to make it appear different, give it a name different from Niño3.4, which is a defined index used by climatologists to determine ENSO, see https://climatedataguide.ucar.edu/climate-data/nino-sst-indices-nino-12-3-34-4-oni-and-tni. This is not what you mean. Wilson reconstructed the Nino3.4 index in their study but it is not the Nino3.4 index. Refer to it as Wilson Nino3.4 or reconstructed Nino3.4 to make it clear you are referring to the reconstruction and not the average of instrumental SST for 5N-5S, 170W-120W. Even better revision to this sentence “The study of Wilson et al. (2010) reconstructs an annually-resolved Niño3.4 index …of past El Niño and La Niña events back to 1607 beyond the instrumental era, which we will refer to as the “Wilson Niño Index.”
Line 129-130 Delete “We use the Wilson Niño Index for comparison with our coral SST records performing Wavelet Coherence Analysis in the time domain (see section 4.4).” This does not need to be stated here but in your results.
Line 135 Explain what you mean by “it should be relatively independent from statistical biases”. Historical records interpreted with quantitative or qualitative methods do have a bias, just different biases from a coral or proxy biased reconstruction. See Paleoclimatology textbook by Bradley 2015 and the chapter on Historical reconstructions and Garcia-Herrera, R., Konnen, G., Wheeler, D., Prieto, M., Jones, P. & Koek, F. 2005: CLIWOC: A Climatological Database for the World's Oceans 1750-1854. Climatic Change 73, 1. and Ingram, M. J., Underhill, D. J. & Wigley, T. M. L. 1978: Historical climatology. Nature 276, 329-334, http://dx.doi.org/10.1038/276329a0.
Line 138-140 This sentence is redundant and not needed here but in the methods or results section, Delete or move “We use both indices by Quinn (1993) and Brönnimann et al. (2007) for identifying past warm and cold events in each coral record and we use these events to compile composites (see section 4.5).”
Line 150 Check this exposure time for the x-ray process, this is a really long time. It usually takes a fraction of a second unless this is an ancient machine. If you used digital plates for the X-ray, I doubt the exposure time is that long.
Line 200 The reference Groth and Ghil (2015) is for a Monte Carlo Singular Spectrum Analysis (SSA) not wavelet coherence. Please use the correct reference for Wavelet coherence, Grinsted et al. (2004) doi:10.5194/npg-11-561-2004 is the one I use.
Line 210 You mean “section” not “chapter”. This is not a book or dissertation.
Line 212-213 Eq. 1 The equation should be divided by the square root of the degrees of freedom. Data with sinusoidal cycles is not independent and violates the assumptions of many statistical tests. Your “n” values should be adjusted for degrees of freedom. It takes only two terms to define a sinusoid, the wave height, and length (or period), thus two degrees of freedom. If you have 15 years of data with a seasonal cycle, the degrees of freedom should be about 30. There are quantitative methods to determine your degrees of freedom, Runs test is the simplest dof test for a single data series and it is non-parametric. This adjustment is important because the larger your n values, the smaller your error, therefore, if you use unadjusted n values, your analysis could result in false positives.
Line 217 Revise for incorrect usage of which “…coral samples, that show a good…”
Section 4.2 This is a short section with 9 lines of text, the two subsections are not needed.
Table 2 Explain what median RSD% is, is this from your ICP-OES analysis? That is the only other place RSD has been mention up to line 226.
Line 230 The number of samples appears twice in the sentence, same for the other sentences for the other corals in this section. The two subsections basically put what is in table 2 into text. The reader can read the table. Tell the reader something else about your results, such as means and medians are the same, thus these corals are not biased towards one season. B8 has a different mean, this one reason we removed the mean from the coral Sr/Ca records. The ranges vary among the corals with E5 having the greatest range and E3 the smallest. Rewrite this whole section to be more informative. Table 2 is for your raw Sr/Ca data, do these statistics change after linear interpolation? That is more interesting to me.
Line 241 Revise “Such decadal variability in the Indian Ocean is described in previous studies”. Change tense and redundant in word usage.
Line 245 How were the coral Sr/Ca records detrended? Linear trend, polynomial?
Line 247 Are the mean annual cycles determine from the detrended data? make this clear. Include a figure with the detrended coral Sr/Ca series so the reader can see how you detrended the data. “Mean annual cycles” is confusing, do you mean annual values or seasonal cycles. In line 248 the authors use “seasonal amplitudes in coral SST” that is clearer.
Line 250 I could not find in the supplementary material and Fig. S6 where the “26-32% of the coral-SST variance” is explained. The section 8 in the supplementary material notes the SSA is done with monthly anomalies, so the seasonal cycle is already removed. To figure out the % variance in seasonal cycle. Take the variance of the monthly Sr/Ca (before detrending) – variance of the monthly Sr/Ca anomalies with the seasonal cycle removed. That is your % variance due to the seasonal cycle. Can do the same with before and after detrending.
Line 271 State explicitly what is meant by “in recent periods”.
Line 269 How is an “anomaly events” events defined? >0.5ºC or something else? Give explicit details.
Line 290 Fig S7 is the detrending plot with the anomalies. Move this figure to the main paper.
Line 299 Revise “These sub-periods were selected because…”
Line 400-411 “In summary…” this should be in the conclusion section.
The conclusion basically repeats lines 400-411. Conclusions should not have numerical results, that would be in the results or discussion section. I suggest just moving lines 400-411 to the conclusion and delete most of the present conclusion sentences.