Asymmetric Drifter Trajectories in an Anticyclonic Mesoscale Eddy

Round 1

Reviewer 1 Report

This article presents the Asymmetric drifter trajectories in an anticyclonic mesoscale eddy in south China sea.

This paper represents finding a better way of analyzing the mesoscale eddy. The paper is well organized and written as well. The analysis of the result is well described and it is enough for understanding the performance of each result. I would like to accept this paper present form but I request for authours to have an English correction before publish.

This article presents the Asymmetric drifter trajectories in an anticyclonic mesoscale eddy in south China sea.

This paper represents finding a better way of analyzing the mesoscale eddy. The paper is well organized and written as well. The analysis of the result is well described and it is enough for understanding the performance of each result. I would like to accept this paper present form but I request for authours to have an English correction before publish.

Author Response

Please see the attachment. 

We sincerely thank the editor and all the reviewers for generously dedicating their time to provide constructive remarks and valuable suggestions. Their insightful feedback has greatly enhanced the quality of the manuscript, allowing us to make significant improvements. Every suggested revision and reviewer comment was thoughtfully considered and diligently incorporated.

We have addressed each of the reviewers’ comments and provided point-by-point responses, along with the corresponding revisions that have been made. Furthermore, the entire manuscript has been revised carefully to ensure its accuracy and coherence. Please see it in the files.

Author Response File: Author Response.docx

Reviewer 2 Report

In this study, the authors compare drifters measured by drifters with geostrophic and Ekman currents interpolated to the drifter positions.  They show that an offset in the center of the looping drifters is likely due to a wind shift early in the measurements creating an Ekman-driven offset, and interpolate a number of synthetic drifters with different combinations of the velocity fields to compare against the observations.  An interesting result is the much lower retention of the synthetic drifters compared to the actual observations. 

My only major concern is that the gridded altimetry is, before interpolation by the authors, very coarse compared to the size of the eddies.  The authors should show that the center of the eddy in alongtrack altimetry was also offset ~30km from the looping trajectories, rather than this being an artifact of altimetry interpolation in the gridded product.

Specific comments:

Line 41: this is probably the Eulerian time scale, not the Lagrangian one, but this should be clarified.

Lines 76-87: the authors should state what the cross-sectional area of the drogue is to the cross-sectional area of the tether and surface float.  This has been shown in papers such as Niiler et al. (1995) to be an important nondimensional parameter describing how well the drifter follows the water, vs. being blown by the wind and pushed by waves.  GDP drifters have a ratio of 40:1. 

Reference: https://doi.org/10.1016/0967-0637(95)00076-3

Also, the authors should note the center depth and design of the drogue.  They note that the tether is 15 m long, but without the length of the drogue, it is impossible to know the center depth; this is very important for determining the direction of Ekman currents at the relevant depth.

This information may appear in reference 26 (lines 354-355) but should also appear here.

169: is the “distance offset” the distance between the eddy center determined from altimetry and the center of the looping trajectory?  That’s the most straightforward interpretation, but this should be clearly defined.  (This is subsequently defined on lines 176-177; the definition should be earlier in the text.)

194-206: The authors derive Ekman currents from ERA5 winds interpolated to their drifters, but do not specify what assumptions they make regarding the depth of wind energy penetration, rotation of the Ekman currents to the depth of the drogue, etc.  These details are needed.  Presumably they used the same Ralph&Niiler-based approach described in Section 3.2, but this isn’t stated here.  Are all the parameter choices following Ralph&Niiler, or have they been tuned to match the observations?  If the latter, this should be highlighted as it might be informative about the structure of vertical velocity shear in the upper ocean in the South China Sea.

Author Response

Please see the attachment. 

Note that we have carefully checked and improved the English writing in the revised manuscript with the help of professional English editing.

We sincerely thank the editor and all the reviewers for generously dedicating their time to provide constructive remarks and valuable suggestions. Their insightful feedback has greatly enhanced the quality of the manuscript, allowing us to make significant improvements. Every suggested revision and reviewer comment was thoughtfully considered and diligently incorporated.

We have addressed each of the reviewers’ comments and provided point-by-point responses, along with the corresponding revisions that have been made. Furthermore, the entire manuscript has been revised carefully to ensure its accuracy and coherence. Please see it in the files.

Author Response File: Author Response.docx

Reviewer 3 Report

Considering the numerous issues present in this paper that prevent it from meeting the requirements of this journal, it is recommended to consider submitting it to another journal.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Note that we have carefully checked and improved the English writing in the revised manuscript with the help of professional English editing. 

We sincerely thank the editor and all the reviewers for generously dedicating their time to provide constructive remarks and valuable suggestions. Their insightful feedback has greatly enhanced the quality of the manuscript, allowing us to make significant improvements. Every suggested revision and reviewer comment was thoughtfully considered and diligently incorporated.

We have addressed each of the reviewers’ comments and provided point-by-point responses, along with the corresponding revisions that have been made. Furthermore, the entire manuscript has been revised carefully to ensure its accuracy and coherence. Please see it in the files.

Author Response File: Author Response.docx

Reviewer 4 Report

This paper is focused on the influence of wind forcing on inner currents within a mesoscale eddy. The authors performed an experiment with 10 drifters in the South China Sea and analyzed their motion within an eddy. They highlight the important role of wind-driven ageostrophic currents in formation of asymmetry of currents within the eddy. The study is well developed and provides an important contribution to our understanding of surface ocean circulation. I have a number of comments and suggestions for the authors, which are given below.

General comments

Lines 93-96: What are the limitations of linear interpolation of SLA data from 1/4°×1/4° grid to 1/20°×1/20° grid? Despite the importance of this interpolation to detect small eddies, does it affects accuracy of eddy identification, in particular, overestimation of the real number of small eddies?

Line 98: Ocean reanalysis is mentioned in this line, but no details are given, only atmospheric reanalysis ERA5 is described below. Please provide the necessary information about ocean reanalysis, its resolution, accuracy and its usage in the current study.

Lines 101-104: Please give more information about the “sea surface wind datasets from Remote Sensing Systems”

Figure 1 and the related text below: How was the trajectory of the eddy center calculated? Please, specify it.

Figure 2b,c and the related text below: How were the Ekman speed and direction calculated? Please, specify it.

Lines 208-209: The important question is the possible influence of windage effect on drifters’ motion, i.e., did drifters elevate above the sea surface to be directly affected by wind stress? Please provide more detailed information about the construction of drifters and a photo/image of a drifter in the sea to solve this concern.

Minor comments

Line 94: Please change “0.25°” to “1/4°” to be consistent with other grid descriptions in the text.

Line 119. The caption of Table 1 is inappropriate, it should provide the exact information about what is shown in the table.

Lines 154-155: The black line with red dots denotes the trajectory of the eddy’s CENTER, please make the related correction to the caption

Lines 268-271: Please specify the difference between the top and bottom panels (before and after 8 May).

Line 281: Make a subscript for “-2”

Line 317: The figure caption should start from capital letter.

Line 351: Remove “in the future work”, as you have already done it.

Line 358: Remove “mass”.

Line 384: Rename the section title to “Summary and Conclusions” or “Discussion and Conclusions”.

References: please remove yellow coloring of references 4 and 11.

Author Response

Please see the attachment. 

Note that we have carefully checked and improved the English writing in the revised manuscript with the help of professional English editing.

We sincerely thank the editor and all the reviewers for generously dedicating their time to provide constructive remarks and valuable suggestions. Their insightful feedback has greatly enhanced the quality of the manuscript, allowing us to make significant improvements. Every suggested revision and reviewer comment was thoughtfully considered and diligently incorporated.

We have addressed each of the reviewers’ comments and provided point-by-point responses, along with the corresponding revisions that have been made. Furthermore, the entire manuscript has been revised carefully to ensure its accuracy and coherence. Please see it in the files.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

I noticed that the authors have made certain modifications to the content of the paper based on the review comments, and the paper now meets the requirements for publication.

Reviewer 4 Report

Everything is fine now, the article could be published as is