Euphotic Zone Depth Anomaly in Global Mesoscale Eddies by Multi-Mission Fusion Data

Round 1

Reviewer 1 Report

Euphotic zone depth is an important parameter for marine ecosystem. This study investigated the variability of Zeu anomaly in response to global mesoscale eddies. Zeu was estimated from the chlorophyll a concentration with empirical algorithm. However, there have been similar studies about the Chla anomaly as affected by mesoscale eddies in regional or global scales, which might limit the novelty of this study. This point is critical which needs to be clarified in the introduction. 

I think there are much more space for this study to be improved. 

Main comments 

1)    The Zeu anomalies caused by eddies in this paper are largely in the range of 0~2 m. As far as I know, those anomaly signals may be same as the bias that estimated by the empirical algorithm. Further explanation and discussion are needed in main document.

2)    In the South Pacific subtropical gyre region, MLD and photoacclimation will affect the distribution of surface chlorophyll concentration. These regions are consistent with the negative anomaly of Zeu derived by using chlorophyll empirical algorithm. How to distinguish it from the effect of mesoscale eddies?

3)    Is it possible to compare the Kd data with that estimated with the IOP-based model (e.g. QAA method)? As we know that, the Zeu is result from the bulk attenuation of light in the upper layer. In subtropical gyre the low concentration of chlorophyll a might also result some bias for estimating Zeu. So further discussion is expected.

Specific points:

1)    The euphotic depth is a product provided by Globcolour, or is it calculated by yourself? How is it calculated? Is there an assessment of its accuracy? Detailed information about this data is important.

2)    Line 92. When selecting the research area, why is the depth of coastal ocean set at by 2000m? 

3)    Line 117-120. As we all know, the spatial coverage of ocean color data is limited. The method of anomaly calculation is definitely influenced by data coverage and how do consider about it?

4)    Line178-191. The description of the figure is not inserted in the text. And the description is too simple, and it is recommended to give further explains behind the phenomenon.

Author Response

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Reviewer 2 Report

Overall, this paper is interesting. It attempts to show the characteristic of depth euphotic based on eddy phenomena. The paper has used a related method to validate a relationship between Eddie in the euphotic zone with chlorophyll-a as a fertility variable in the waters. However, the paper needs some discussion about more phenomena related to the distribution of oceanic fertility to improve the point of discussion of the paper. Please get the attached file of comments for more details.

Comments for author File: Comments.pdf

Author Response

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Reviewer 3 Report

In general, the article has great innovation, which can combine remote sensing technology to monitor the euphotic zone depth caused by mesoscale eddies. But the whole article is mainly based on the analysis of remote sensing products. After reading the article, I put forward the following suggestions:

1. Line 43 of the article should mark the whole process when CHL appears for the first time;

2. The article introduces the current situation of ZEU in some parts that are lack of satellite observation at this stage. The retrieval method of ZEU data set used in the following article should be properly described;

3. In line 122, the title 3.1 ZEU’s in eddy global distribution should consider whether to change the title. The content in the figure shows the strength of ZEU, but the title is easily understood as frequency distribution.

4. The discussion part has a lot of content, which can be properly simplified. In addition, the article lacks the section of Conclusion and Prospects.

Author Response

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Reviewer 4 Report

The article studies the influence of mesoscale eddies on the variability of the euphotic zone depth on a global scale, which seems to be an important and relevant topic. Unfortunately, the work does not rely on field data to verify the conclusions. In addition, it is necessary to clearly explain which algorithms were used to calculate the ZEU parameter. The English language needs a serious help, some sentences are difficult to understand. Nevertheless, the findings of the work are undoubtedly of interest to other researchers.

Comments:

Line 34-36: An explanation of how "the hunting time of the olive ridley sea turtle" is connected with the contents of this paper is necessary. Perhaps, it is possible to find another, simpler example of ZEU depth effect on marine ecosystems.

L 67-68: Same as the one above, not very clear. It is possible to rephrase like that: "Research on eddy ecology has developed rapidly in the last decade, for exapmle, it was found that pelagic predator catches increased in anticyclonic eddies compared to cyclones and non-eddy [25]."

L 43: The algoritm in [8] is based on Kd and does not use CHL.
The algorithm in [10] is empirical, not semi-analytical.

L 72-73: "This study uses a new matching method to quantify the characteristics of eddy-induced ZEU and analyze the mechanisms involved." Probably, "eddy-indused ZEU anomalies".

In the Introduction it would be interesting to see what variations of the ZEU are observed in general and how strong are  the connections between ZEU variations and productivity.
    
L 76-77: Which water body did you study? Which color sensors were used? It is better to list them here.

L 78: Please give references to the algorithm you have used. Is it the algorithm that you referenced as [8, 9]?  The algoritm in [8] is regional. Did you use the dependency between CHL and Kd from [9] and put it into the equations from [8]? You need to point this out and to explain how you have dealt with the regionality. Why didn't you use the other algorithms [10, 11] for comparison?

L 82-83: What is the spatial resolution of the eddy data?

L 84-85: The abbreviations AE, CE need to be explained first time they appear (lines 58 and 60).

L 88-89: The meaning of this sentence is not clear enough, you need to check your English. Also, what type of interpolation do you use that it can generate eddy-like structures?

L 90-91: " 7-day interpolation time" means that you use a median filter, right? Could you please explain how can it "increase the amount of data matching the eddy"?
    
Table 1: What is "valid data"? It would be better to evaluate the percentage of the valid data for the whole study range and represent it as a temporal graph.

L 101-108: Is "Radial radius" a correct term? You nedd to clarify the terms such as relative radius and eddy radius to make a distinction between R and r. Now it looks like the radius of the eddy depends on the dictance D from the center to the ZEU measurement point. From the Figure 1, R is the same as r. From equation (1), r = R/D.

L 116: Is "eddy periphery" defined for all eddies in the studied area or is it for every separate eddy?

L 117: Why do you call it "climatological" value, if it is a daily value?

Figure 2: How did you obtain two distributions for AE and CE? How the distinction between AE and CE is made judjing from the SLA field only?
Does the white color in the plot mean "no ZEU anomaly" or "no eddy"?
Panels a) and b) are not referenced in the text.

L 128: What does "opposite to the general ZEU' " mean?

L 132: "However, the ZEU' of some regions is opposite globally..." Again, what do you mean by "opposite"?

Figure 3: Is the ZEU' averaged over longitude or is it some cross-section?

L 156-157: "maximum value was +2.29 m (-2.05 m). " But in Figure 2 maximum values reach 5m, also in Line 133: "The eddy-induced ZEU varies by more than 5 m in the boundary currents. " Can you explain this?

Table 2: Mean values need standard deviations. SD can prove that eddies behavior in the lower and middle latitudes is really different.

L 169-173: Please check the description carefully. Check for the negative/positive,  Northern/Southern, and months of the maximum.

L 187-189: "The eddy has a dipole phenomenon in the low-latitude region. The coupling direction is consistent with the rotation direction of different polar eddies in the Northern and Southern Hemispheres. " If it is a dipole, which direction is a "coupling direction" (same in the Discussion)?

Figure 6: It would be interesting to see the AE graph behavior for r>2 in SL. Now it looks like it starts to grow.

Figure 7: What is "eddy amplitude"? You need to explain how was it calculated.

I suggest you add a Conclusion part as well, it will help to state the main findings of this paper more clearly.

Author Response

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Round 2

Reviewer 1 Report

I can see the improvements of this revised version. However, I still want to suggest authors to fully consider some points as follows:

1.    The background of related research could be improved rather than simply listing some results (Line 54-73). 

2.    Line 40-41，As shown in recent researches, compensation depth in biology is not equal to the Zeu (depth at which 1% of surface PAR value). Please refer to Wu Jinghui et al. (2021)( Wu, J., Lee, Z., Xie, Y., Goes, J., Shang, S., Marra, J. F., et al. (2021). Reconciling between optical and biological determinants of the euphotic zone depth. Journal of Geophysical Research: Oceans, 126, e2020JC016874. https://doi.org/10.1029/2020JC016874)

3.    Line 72-73, “This study uses a new matching method……” Does it mean the method shown in Section 2.4. How new? It’s a new method developed in this study, or some method quote from other research?

4.    Section 2.1, I guess the daily Zeu data were download from the Globcolour, right? If so, please add the website of this dataset. 

5.    What’s the meaning of Table 1? Why there is comparison between MODIS and Multi-sensor fusion? I could not find any other descriptions about it.

6.    It’s still hard to understand the ZEU anomaly just being around 2m, which means to some extent, the eddy has no obvious on the depth of euphotic zone. 

7.    Could you add more description about the calculation of seasonal data shown in Figure 4? All daily data were used to do the calculation? 

8.    Only means and maximum data were shown in Table 2. A range about the variation of Zeu anomaly in AE or CE could be shown for comparison.

9.    1.4 trillion Zeu’data was used in this study. You mean daily raw data? How many eddies were discriminated and discussed in this study? 

10. Discussion about the mechanism for the variability of Zeu anomaly over different areas or seasonal timescales are relatively superficial. I still suggest to do more explanation about the physical forcing effect.

11. The description in Lines 332-333 is not robust enough. Eddy may modulate Zeu by changing the water constitute (e.g. Chl) which could affect the light transmittance. However, this process is related with both the physical forcing and biological forcing. I think this sentence is not logical enough.

Author Response

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