Seasonal Evolution of Chlorophyll-a in the North Indian Ocean Associated with the Indian Ocean Dipole and Two Types of El Niño Events
Round 1
Reviewer 1 Report
The paper aims to show the seasonal evolution of interannual Chla variations in the North Indian Ocean, and their responses to such climatic variables as IOD and El Niño indices, SST, sea level anomaly, wind and precipitation. Authors present the three significant orthogonal functions describing the spatial pattenrs of chlorophyll-a distribution.
Unfortunately, the purpose of this study is not made clear. The effect of the climatic variables was not discussed, only the regressions (not supported by numbers) were shown.
Authors should answer the question "Why is this work important?", which will help to improve the Introduction section, and discuss in details or at least provide some hypothesis about the physical mechanisms causing the observed relations between S-EOFs and climatic variables.
The paper draws no conclusion, the result of this work and its future use are not shown. The Discussion and Conclusions sections need to be rewritten, Discussion now just repeats the observations made in the Results section.
The language and style require further improvement. It is recommended to cite more of the current references (only 4 of 20 are within the last 5 years now). The figures are of poor quality, blurred and unclear.
Notes:
It is better not to use acronims in the title. IOD-> Indian Ocean Dipole
The title says "North Indian Ocean", but figures 5 - 7 show Indian and Pacific Oceans.
Line 21 - regressed anomalies of ocean dynamic variables with the dominant modes have confirmed the bio-physical responses to climate change
El Nino and IOD are not "climate change". The paper did not confirm the response, it just has shown the correlation.
Line 35 - However, recent studies suggest that there exist two types of El Niño events
The cited reference was published in 2009. It is hardly "recent".
Line 63 - Finally, each main mode contains four consecutive seasonal evolution patterns, which are linked by the same time series of principal component.
"JJA(0), SON(0), D(0)JF(1) and MAM(1)" and "seasonal evolution patterns" are the same thing or not? The terms used in Methods section require clarificaton, the section has to be expanded. Also, apparently the notifications "JJA(0), SON(0), D(0)JF(1) and MAM(1)" follows the previous studies, but do you really need such complicated acronims within this paper? They need to be explained at least.
Line 91 - when typical IOD or ENSO events occur.
This statement requires a reference.
Figures 1 - 4 are slightly blurred.
Figures 5 - 7 are not readable at all. What are the numbers on the scales? What quantity is shown in these figures? Anomaly? Or the correlation coefficient? It is impossible to make any comments on section 3.3 because of this.
Line 103 - ...represent PC of Chla lags, leads one year and synchronizes to the climate index, respectively.
Do you suggest that Chla anomalies can lead to an El Nino? Otherwise why would you calculate "leads one year" corelation? Is it justified by some observations or is it just doing statistics?
Line 148 - Results suggest that IOD and two types of El Niño are the primarily factors for interannual Chla variations
and
Line 152 - Regressed anomalies of dynamic variables (SST, SLA, Rain and Wind) with the dominate modes have explained the major processes involved in the Chla variability under climate change, though the detailed underlying mechanisms still need to be investigated and clarified in the future work.
Correlation does not mean causation. "After" does not mean "because of". So you have shown that Chla variation have some connection with IOD and El Nino, but not that it is caused by IOD and El Nino.
No major processes were explaned, without discussion of the "detailed underlying mechanisms"this work can not be concluded. Also, climate change was not discussed, despite the statement "processes involved in the Chla variability under climate change".
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Authors provide a statistical analysis of annual to interannual satellite chlorophyll regressed against other environmental indices in the Indo-Tropical Pacific Ocean.
2 main areas the paper needs improvement:
1) The figures are of very low quality and in some cases, such as Figure 4, are very difficult to read. Authors need to make the figures larger and improve the resolution/quality.
2) The discussion is too light. The main finding here appears to be PC modes are most notably correlated to the IOD, with major patterns evident off the coast of Sumatra and in the southern/central portion of the Indian Ocean basin (see in figure 1, panels (a) and (b), and Discussion section lines 140-143). Authors would do well to return from this statistical abstraction and discuss how the IOD impacts factors that would influence surface phytoplankton, such as light, mixed layer depth, seasonal upwelling, et cetera. Authors then need to be much more thorough on how exactly the “different types” of ENSO are impacting the IOD, and how this, in turn, is impacting phytoplankton. This is briefly touched on in lines 120-123, but it needs to be expanded upon there or in the discussion.
In summary, authors need to put as much effort into explaining what their results mean as they did into performing the statistical analysis.
Some errors:
Line 66 “What’ more”
Line 116 Figure(s) – plural if more than one
Line 133 “which is not favor” – poor English usage
Author Response
Authors provide a statistical analysis of annual to interannual satellite chlorophyll regressed against other environmental indices in the Indo-Tropical Pacific Ocean.
2 main areas the paper needs improvement:
Point 1: The figures are of very low quality and in some cases, such as Figure 4, are very difficult to read. Authors need to make the figures larger and improve the resolution/quality.
Response 1: Thank you for your advice. We have improved Figures in the full paper for people to read.
Point 2: The discussion is too light. The main finding here appears to be PC modes are most notably correlated to the IOD, with major patterns evident off the coast of Sumatra and in the southern/central portion of the Indian Ocean basin (see in figure 1, panels (a) and (b), and Discussion section lines 140-143). Authors would do well to return from this statistical abstraction and discuss how the IOD impacts factors that would influence surface phytoplankton, such as light, mixed layer depth, seasonal upwelling, et cetera. Authors then need to be much more thorough on how exactly the “different types” of ENSO are impacting the IOD, and how this, in turn, is impacting phytoplankton. This is briefly touched on in lines 120-123, but it needs to be expanded upon there or in the discussion.
Response 2: Thank you for your advice. We have added sufficient discussion about how the IOD may impact phytoplankton in Section 3.3.
In summary, authors need to put as much effort into explaining what their results mean as they did into performing the statistical analysis.
Some errors:
Point 3: Line 66 “What’ more”
Line 116 Figure(s) – plural if more than one
Line 133 “which is not favor” – poor English usage
Response 3: Thank you very much. We have revised the expression of our article, including the language. Thank you again!
Round 2
Reviewer 1 Report
1. The aim of the paper is claimed to be "to identify the dominant seasonal evolutions of interannual Chla variation in the North Indian Ocean during 1998-2016, and to investigate their responses to climate variability of IOD and two types of El Niño as well as ocean dynamic anomalies of SST, sea surface height anomaly (SLA), sea surface rainfall (Rain) and wind field (Wind)."
The Conclusions should support the aim. What exactly is "their responses to climate variability"? You should reformulate the Conclusions based on what you discussed in sections 3.3 and Discussion.
2. Line 300: "The correlation between the main modes of interannual variation of Chla and the dynamic parameters of tropical Indo-Pacific ocean, including SST, SLA, Rain and Wind, is studied. The results of regression coefficients are shown in Figure 7-9."
Regression coefficients are, for example, A and B in y=Ax+B, if y is the regression equation. In the Responce to the first review you have mentioned that Figures 7-9 show correlation coefficients. If so, why there are numbers like +5 or +2 on the scale?
3. In Discussion section, the effects of SLA, SST, Wind and Rain are not discussed. You shoul add that. The explanation of the effect of El Nino and IOD on circulation, mixed layer depth, chlorophyll distribution is still too short and not supported by any references.
4. Line 450: "The S-EOF1 and S-EOF2 modes show significant variation period of 2~2.5 years similar with IOD and ENSO, whereas the S-EOF3 exists insignificant period of 40 months. "
40 months is above 3 years, how it is insignificant compared to 2~2.5 years?
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
