Injection of High Chlorophyll-a Waters by a Branch of Kuroshio Current into the Nutrient-Poor North Pacific Subtropical Gyre
Round 1
Reviewer 1 Report
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This is my second review on the manuscript *Injection of high-chlorophyll-a waters by a Kuroshio branch into the nutrient-poor North Pacific Subtropical Gyre* by Chun Hoe Chow and colleagues. In the previous round I was labelled as reviewer 1.
I have carefully read the updated version as well as the response letters to my own comments and to those contributed by the other three reviewers. Attending to the comments, it is clear that the manuscript raises wide interest and that the topic is exciting. I appreciate authors' effort to clarify all doubts and suggestions, and for the detailed response letter.
After revisiting the manuscript, I still have two major comments;
1. To clarify my previous inquiry about the nature of the CHL signal, the authors repeatedly refer to teh injection of high chlorophyl waters. Considering that phytoplankton have a very short lifespan [a day or a few days], it is much more likely that there is an injection of nutrients, or that the enhanced front enahnces local upwelling and thus enhanced production.
2. In my opinion the authors need to make clear whether they are describing a new current or just a connection between Kuroshio Current and the STCC, or just an enahncement of the STCC. Comments by other reviewers about whether similar Chl/SST signals are observed in other years may help here, though perhaps that is beyond the current communication. I would appreciate nonetheless clearly stating in the title, abstract, summary, etc., whether the authors observe a new current or an enhanced STCC. [To be entirely honest, my impression is that it is the latter]
I also included some minor suggestions below. Taker together, all these aspects let me recommend a minor revision. I hope the comments provided help the authors while preparing a revised version of the manuscript.
#### *Minor comments*
Page and line numbers; P#L#
*Title*
One reviewer commented on the need to refer to the 'Kuroshio current' instead than just 'Kuroshio'; what about 'by a branch of Kuroshio current'?
*Abstract*
L12 - 'about 21ºN' instead of 'at around'?
L12 - as revealed by unusual high chlorophyll? Note that chlorophyll is not conservative, the front is most likely enhancing local growth rather than advecting chl from the coast ... another question is whether the branch advects nutrients or enhances local upwelling
L19 - cooled waters through local upwelling? please clarify
L21 - perhaps you just observed an strengthened eastward flow [the subtropical countercurrent]
*Introduction*
L80 - Terra not Aqua [it is *almost* the same, but not the sam]
*Data and Methods*
L90ff - effort to improve data description is much appreciated
L145 - there are altimetry products that already include Ekman circulation [http://www.globcurrent.org/, also at NOAA AOML but couldn't find the product https://www.aoml.noaa.gov/]
*Results*
L181 - wHere advection? Again, note that these local anomalies most likely reflect local phytoplankton growth enhancement rathern than direct advection (plankton are short lived organisms)
*Summary*
L352 - again, please be careful about chl advection
L370 - if the mechanism is the same as STCC, it shares location, then, perhaps it is the same?
L376 - again, 'injection of high chl waters'?
L378 - indeed, it is a hotspot of high productivity
*Figures*
Figure 1 - Enhancements are much appreciated. Perhaps you can also add labels for geographic features cited in teh text [*e.g.* Luzon Strait].
BTW, I think the chl field you show was captured by Terra (not Aqua) MODIS [TERRA_MODIS.20100704T015507.L2]
Figures 3 and 4 - The trajectories of Argo floats and drifters are perhaps [or at least to me] the most important piece of information demonstrating the proposed path. Please make sure it can be properly seen [perhas another color or thicker line, a larger map, arrowheads to incidate dates, etc] I would put them together instead than in two separate figures.
Comments for author File: 
Comments.pdf
Author Response
Please see the attached file for our point-to-point response.
Author Response File: Author Response.pdf
Reviewer 2 Report
Please see attached.
Comments for author File: Comments.pdf
Author Response
Please see the attached file for our point-to-point response.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript has improved nicely from the edits done by the authors. I still have a couple of comments though:
- I'm still missing a discussion about the relevance of the study. The authors responded to my comment in their rebuttal, but that won't help a reader of the published paper.
- All of the reviewers had comments about how the uniqueness of event should be quantified, and I don't think that the authors addressed this sufficiently. It seems a bit premature to publish the study without including the 2013 analysis, but that's for the editor to decide.
Author Response
Please see the attached file for our point-to-point response.
Author Response File: Author Response.pdf
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
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### remotesensing-1560774
The manuscript *Injection of high-chlorophyll-a waters by a Kuroshio branch into the nutrient-poor North Pacific Subtropical Gyre* by Chun Hoe Chow and colleagues describes an eastward flowing filament extruding from Kuroshio into the North Pacific Subtropical Gyre [NPSG]. The manuscript builds the case through the combination of reanalyses of satellite ocean color and thermal images, and satellite altimetry, and from drifter, float and CTD data. Then they analyze current fields, drifter trajectories, Ekman forcing, thermal winds and vorticity estimates to assess the physical drivers of such anomaly.
I like the topic pursued by the manuscript, and the arguments posed by the authors are convincing. The injection of nutrient rich waters into oligotrophic gyres thorugh filaments has great potential interest but it is an area largelly unexplored (except perhaps around permanent upwelling areas). My background prevents me to reassure whether this is the first time that the proposed circulation pattern is described, but in my opinion that does not rest interest to this contribution. Besides, the paper is well written and, except for some comments and suggestions listed below, easy to follow.
I have, however, some concerns, regarding the need to better frame this contribution in the context of past research, and to improve some formal aspects. Specifically,
1. I strongly recommend the authors to include an schematic of the Kuroshio circulation system including its recirculations to help readers. Something in the line of Fig 10.3 of [Talley et al 2011](https://doi.org/10.1016/B978-0-7506-4552-2.10010-1), but highlight the proposed new path.
Such map could also include the location of in-situ observations. I also missed some reference or comparison with other western boundary current systems. Such filaments are common in upwelling systems. Have anybody found similar recirculation patterns associated with western internsification?
2. The manuscript conveys, probably inadvertently, the message that chlorophyll is advected by the filement into the gyre. Though reported current speeds are quite impressive, I would appreciate some comment and discussion on whether (or why) the authors think that the chlorophyll signal corresponds to advection of high cholophyll waters instead of the result of a perturbation of local conditions [say, upwelling injecting nutrients through the thermocline or on the contrary a deepening of the mixed layer and photoacclimation]. Note that these population and physiological responses are fast in phytoplankton [~days].
3. A quick search through Ocean Color Website [L2 search engine](https://oceancolor.gsfc.nasa.gov/cgi/browse.pl?sen=amod) revealed a extraordinarily low coverage in July 2010 in the study region [*i.e.* no chlorophyll retrievals due to cloudy conditions]. Considering the that CMEMS chlorophyll data is a reanalysis product, it would be worth checking whether the apparent connection about 125E20N in figure 1a might be an artifact. I think it is worth to include raw data [an L2 image] or thermal images [say, from AVHRR or similar, although OSTIA would probably be reliable here] supporting the proposed branch.
4. From the trajectory of the drifter and satellite altimetry data, perhaps an alternative explanation might be that there is an eddie east to Luzon strait which interacts and reinforces the southward branch of the STCC. I think the manuscript would benefit from clarifying whether such an option is feasible. I also wonder whether the authors have considered ot explored the possibility of conducting particle tracking experiments.
5. Finally, the framing of the manuscript around the discovery of the filament as an ocean color anomaly is nice, but considering the potential limitations of the CMEMS data for this particular application, perhaps starting with the drifter would be a better option.
Taker together, all these aspects let me recommend a major revision. I hope the comments provided help the authors while preparing a revised version of the manuscript.
#### *Minor comments*
Page and line numbers; P#L#
*Title*
*Abstract*
*Introduction*
P1L27 - perhaps point readers to the recent monography about Kuroshio edited by [Nagal et al 2019](https://www.wiley.com/en-us/Kuroshio+Current%3A+Physical%2C+Biogeochemical%2C+and+Ecosystem+Dynamics-p-9781119428312)?
P1L35 - via from [choose one?]
P2L64ff - As commented above, the manuscript conveys, probably inadvertently, the message that chlorophyll is advected by the filement. The speeds reported are quite impressive, but I would appreciate some comment on whether
P3L70 - I would be more cautious about the lack of previous reports, just in case [especially considering the vastness of the scientific literature and potential publications in languages other than english]
*Data and Methods*
P3L80ff - please note the comments above with respect ot CMEMS data. It might be worth to detailing a bit the methodology employed by CMEMS to interpolate chlorophyll. Besides, most of these data sets have a DOI associated. Please cite and acknowledge them properly.
P4L108ff - please detail here floar and profiler number codes
P4L119 - nowadays there might be online MLD estimates available; do they compare well to your H0?
*Results*
P5L134 -- why not just saying the factor above normal conditions (based on a log anomaly)? As a lognormally distributed variable, it is important to detail whetehr you are working with log trnasformed data or not ...
P5L139 - the reference to Fig 3a is potentially confusing, might seem you refere to gradients of 1.4-1.5 meters instead than to specific contour levels
Fig 4c -- units of color bar vs units of the contours ...
Comments for author File: Comments.pdf
Author Response
Please see the attachment docx file for the point-to-point response (marked in red). Thank you.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Introduction
- Please note that before showing any figure you have to refer to it first in the text and not the other way around, as you do in several figures.
- Also note that on all figure legends you need to specify everything as much as you can, independently of what is explained in the main text. This means that you need to inform for example in Figure 1 what is the image spatial resolution, which sensor/satellite. Also need to write out in full what STCC means, even if this is already explained in the main text. In fact, the reader should never need to read the main text to understand the figure (or table)
- Please make sure you use always the same units (mg.m-3 or mg/mg3) everywhere. What you cannot have is mg.m-3 in the image color bar and then in the text something that is actually incorrect (mg/m-3) (see line 64).
- Line 64 - change "On July 4, 2010, to simply "On 4 July 2010," or "On 4 July, 2010".
- Lines 64 and 65. This sentence is written in the passive voice. Please consider writing in the active voice.
- Line 68 - circling "around" may be redundant. I would suggest simplifying to "circling 123..."
- Summer is only July? Why do you refer to a summer mean when you show in Figure 1b the 10 years climatological mean of only one summer month (July?). It would be better to show the climatological mean of all summer months if you want to talk about a summer climatological mean.
- Propose to change lines 75 to 76 as: "This study designates the unusual eastward flow as the eastward cross-shore Kuroshio branch (ECKB)."
Methods
- Line 80 - Please do not initiate a sentence with an abbreviation so rather than "Chl concentrations" write "Chlorophyll a concentrations". You have to be specific about the satellite product. There are many types of chlorophyll as you know, but the one you are deriving should be chlorophyll a. What you can say is something like: "Chlorophyll a (hereafter abbreviated to Chla) concentrations ..."
- Line 82 - regardless of whether you have abbreviated a word before, whenever it starts a sentence later, it must always be written out in full again. Only in the middle of a text will it be abbreviated.
- Lines 82-84: I suggest correcting to: "Chl concentrations are “cloud-free” merged products from multiple satellite sensors at daily intervals and about 1/24° spatial resolution".
- Line 116 - please notes what was said previously, either you use kg/m or kg.m-3 but keep always the same way to write the units
- Line 129 why do you choose 0.06 mg.m-3 when in figure 1 you refer to concentrations above 0.07 mg.m-3?
- Figure 3 - I would suggest smoothing the background colours such as to be able to see better the red curves.
Results
- Line 116 - please notes what was said previously, either you use kg/m or kg.m-3 but keep always the same way to write the units
- Line 135 - You do not refer to Figure 3 for the first time in the Results chapter this figure appears in the Methods chapter! Once again, please correct this problem throughout the paper. Figures should be mentioned in the main text before they actually appear.
- Lines 137-140 - I do not understand what is your point here. This is what is expected by using a monthly climatology of 10 years versus a daily image! In Figure 3 b you can also see an anticyclonic vortice around 125E with ADT of about 1.5m. But this is smoothed to the east, and it is not surprising.
- Line 153 - again, be careful with units and units spaces. Have to be the same throughout the text.
- Figure 4 is interesting and makes me think that it would be interesting in the beginning, rather than presenting the July climatology you should incorporate either June or June and July. These results even suggest more that you should not call in figures 1 and 3, summer climatology when it is really just July.
- Line 169 - above "the" mixed-layer...
- Figure 5 should have the time reference. I am a bit confused but where is May to July (Figures 5a-5c) and then the minimum salinity in June and July (also in the same Figures). I cannot understand the x-axis (date) in these figures. Care to explain these?
- I believe that the TS diagrams could be improved if lines were less thick. It gets more confusing and harder to fully interpret.
- You never call Figure 6 d in the text.
- Please put figure 7 after line 216!
- Please put figure 8 after line 226!
- Please put figure 9 after line 258 (or less but never before 246)!
Note: It would be very interesting to check if this "unusual" eastward flow is in fact unusual, by performing a similar study but for every year and month. It may be that it is not that unusual but it was just never found or reported before.
Author Response
Please see the attachment docx file for the point-to-point response (marked in red). Thank you.
Author Response File: Author Response.docx
Reviewer 3 Report
Please see attached document.
Comments for author File: Comments.pdf
Author Response
Please see the attachment docx file for the point-to-point response (marked in red). Thank you.
Author Response File: Author Response.docx
Reviewer 4 Report
The manuscript describes an eastward flow that branched out out from the Kuroshio near Taiwan in the summer of 2010, carrying high chlorophyll-a (Chl) waters for over 1,000 km from about 125°E into the North Pacific subtropical gyre. It’s a neat study of a possibly rare event but I think that the manuscript needs some improvement before publication:
1. The text needs to be proofread and edited for clarity. There are plenty of small grammatical errors that makes it hard to follow along.
2. I would like to see a more comprehensive analysis about how rare the event is. Maybe frequency plots and/or hofmøller diagrams from 1998 to now of Chl and SST at the 135°E line. This would show how much of an anomaly it is.
3. While the event is interesting and the description of the mechanism is informative, I’m left with a feeling of “why is this important?”. Would it be possible to discuss the generality of the mechanisms in section 3.3 or possibly try to identify other occurrences in the satellite record? The authors could also add a a discussion about how the event can affect different ecosystems or increase connectivity of phytoplankton or larvae. Is it possible to make a simple calculation of how much extra biomass that were injected into the gyre?
4. Please describe the Chl product used in more detail.
Author Response
Please see the attachment docx file for the point-to-point response (marked in red). Thank you.
Author Response File: Author Response.docx
