Enhancement of near-inertial waves by cyclonic eddy in the northwestern South China Sea during spring 2022

Chen, Qi'an; Xu, Hongzhou; Wang, Dongxiao; Hong, Bo; Liu, Chunlei; Zhang, Zheyang; Jiang, Huichang; Song, Wei; Long, Tong; Wang, Ling; Liu, Sumin; Chen, Rongjie

doi:https://doi.org/10.5194/egusphere-2025-283

Preprints

https://doi.org/10.5194/egusphere-2025-283

Preprints

13 Feb 2025

| 13 Feb 2025

Enhancement of near-inertial waves by cyclonic eddy in the northwestern South China Sea during spring 2022

Qi'an Chen, Hongzhou Xu, Dongxiao Wang, Bo Hong, Chunlei Liu, Zheyang Zhang, Huichang Jiang, Wei Song, Tong Long, Ling Wang, Sumin Liu, and Rongjie Chen

Abstract. By analyzing dataset derived from four moorings during spring 2022, this study provides direct evidence that near-inertial waves (NIWs) can be largely enhanced by a passing cyclonic eddy (CE) in the northwestern South China Sea. Results show that the enhancement of NIWs mainly occurred at north side of the CE due to asymmetry of eddy structure. In vertical, the enhancement concentrated at above 200 m and reached peaks at around 100 m. Significant energy transfer rates between the CE and NIWs appeared at the same depth of the enhancement that can reach up to 6×10^-10m²/s³at the CE’s edge. Under impact of the CE, power of the first five NIWs modes were promoted significantly and dominated by the second and third modes. Overall, the CE transferred energy to NIWs before near-inertial kinetic energy reaching its peaks, while NIWs gave energy back to the CE after the peaks.

Received: 21 Jan 2025 – Discussion started: 13 Feb 2025

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Journal article(s) based on this preprint

28 Oct 2025

Enhancement of near-inertial waves by cyclonic eddy in the northwestern South China Sea during spring 2022

Qi'an Chen, Hongzhou Xu, Dongxiao Wang, Bo Hong, Chunlei Liu, Zheyang Zhang, Huichang Jiang, Wei Song, Tong Long, Ling Wang, Sumin Liu, and Rongjie Chen

Ocean Sci., 21, 2631–2642, https://doi.org/10.5194/os-21-2631-2025,https://doi.org/10.5194/os-21-2631-2025, 2025

Short summary

Qi'an Chen, Hongzhou Xu, Dongxiao Wang, Bo Hong, Chunlei Liu, Zheyang Zhang, Huichang Jiang, Wei Song, Tong Long, Ling Wang, Sumin Liu, and Rongjie Chen

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-283', Anonymous Referee #1, 14 Mar 2025

Utilizing a mooring array, this study investigates the interaction between a cyclonic eddy (CE) and near-inertial waves (NIWs) in the northwestern South China Sea (SCS). Results demonstrate that NIWs were significantly amplified by the passage of a cyclonic eddy. Overall, the manuscript presents a well-structured analysis of an interesting topic. While the work appears suitable for publication in Ocean Science, a few minor clarifications are requested to strengthen the paper prior to final acceptance.
Specific Comments
Line 24: “power of the first five NIWs modes were promoted significantly and dominated by the second and third modes.” Use “was promoted” for subject-verb agreement.
Line 53: “Chen et al. (2011) suggest that eddies present 35%--60% of the time in the northern SCS...” Replace “present” with “occur” for clarity.
Line 60: “making this place as a seemly area for investigating NIWs...” “seemly” is incorrect; replace with “suitable” or “ideal.”
Line 81–82: “flow velocity compensation correction was applied to ADCP data, which was calculated based on depth change and flow direction. Clarify the exact method used for compensation (e.g., reference to a specific algorithm or equation).
Line 82: Does the lack of data in the upper tens of meters of the mooring data lead to errors in energy calculations and filtering?
Line 95: Since calculating the energy conversion rate requires gradients in the x and y directions, but the mooring array is a section. How is this considered when calculating the energy conversion rate?
Line 97: “moving average of 3 internal tide periods.” The analysis in the article focuses on near-inertial internal waves, not internal tides. Therefore, it should be 3 “internal wave periods”.
Question: Is this the only CE throughout the year? Or why was this particular CE selected for analysis?
Line 116: “near-inertial frequency with peak value of 0.616 cpd” Define “cpd” (cycles per day) upon first use.
Line 160: “Low modes rose from 46% (Q2) and 54.4% (Q3) during Period1 to 87.6% (Q2) and 79.5% (Q3) during Period2...” Clarify what “46%” refers to (e.g., percentage of total energy?).
Line 200–201: “The differences may be attributed to the strength of eddies, their rotation direction and the intensity of NIWs.” Expand on how rotation direction (cyclonic vs. anticyclonic) impacts energy transfer.
Line 206: “NIWs draw energy from background flow with small energy ratio between them, vice versa.” Rephrase for clarity (e.g., “NIWs draw energy from the background flow when the energy ratio is small, and vice versa”).
Line 229: “This study is helpful for us to understand multi-scale interaction...” Replace “for us” with “for understanding” to maintain objectivity.
Figure 1: Panel (c): Label the x-axis of the power spectra.
“The mooring dataset used for plotting Figure for this paper are available at”, the grammar is incorrect.
Ensure consistent use of hyphens (e.g., “near-inertial” vs. “near inertial”).
Use “cyclonic eddy (CE)” consistently after first introduction.

Citation: https://doi.org/10.5194/egusphere-2025-283-RC1
- AC2: 'Reply on RC1', Hongzhou Xu, 03 May 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-283/egusphere-2025-283-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-283-AC2
RC2:
'Comment on egusphere-2025-283', Anonymous Referee #2, 17 Mar 2025

This paper presents mooring ADCP observations in the northwestern SCS and interprets the variations in near-inertial energy based on analyses of satellite and reanalysis data. The energy exchange between mesoscale eddies and near-inertial waves is an interesting and important topic, and the authors provide observational evidence that near-inertial wave energy is enhanced in cyclonic eddies. I agree with the publication of this paper, but before it is published, I would like the authors to address some misleading statements and unclear descriptions.

Major comments:
L22-24: 'Significant energy transfer rates between the CE and NIWs appeared at the same depth of the enhancement that can reach up to m²/s³at the CE's edge.' : What percentage of near-inertial and eddy kinetic energy does this correspond to?
L56-59: 'Researchers ...' : This sentence is redundant and repeats information from L52-53. The authors should specifically present the findings of previous studies regarding mesoscale eddies in the SCS.
L86-94: This paragraph lists the datasets used in the analysis, but it is hard to follow because the authors do not specify the variables for each dataset. Both the CMEMS and AVISO datasets include SLA data, but it is unclear which part of the analysis each of the two SLAs is used in.
L96: 'Eddy-NIWs energy transfer rate': Do you mean 'energy transfer rate from mesoscale eddies to NIWs'? Specify the direction.
L97: The equation (1) seems different from the equation (3b) in Jing et al. (2017).
L119-122: Is this relative frequency shift consistent with the cyclonic eddy's relative vorticity at the mooring site?
L126: 'Meridional velocity wavelet power spectra': Why is only meridional velocity used? Near-inertial waves exhibit a circular hodograph, so why not include both components of velocity?
L153-154: It seems too speculative. I can't discern this from Fig.1a.
L166: 'due to vorticity asymmetry of the CE' : This seems too definitive. It is not clear from the figures whether the differences in near-inertial energy at each mooring site are attributable to the asymmetry in vorticity.
L169-170: 'the north side of the CE was merged with the western boundary current of the northwestern SCS (Fig. 1a)’: It's not clear from Fig. 1a.
L170-171: 'that generated strong shear and enhanced NIWs largely at this side': This is an inference, so the authors should avoid making definitive statements.
L180-181: What is this percentage to?
L185-186: 'suggesting prominent influence of eddy on near-inertial modes' : This is too vague.
L194: 'eddy-NIWs energy transfer efficiency is about 2%': the term 'eddy-NIWs energy transfer efficiency’ is unclear, so I don't understand what the '2%' refers to.
L200: 'rather than surface and mixing layers' : What is the mixed layer depth at the mooring sites?
L207-208: Which terms in equation (1) explain the high rate at Q1? Please provide a similar discussion as for Q4.
L209-210: 'with small energy ratio between them': I don't understand what the authors mean.
Fig. 9c: How about showing the cumulative energy transfer from the CE to NIWs? It is hard to tell the net energy transfer from the current figure.
L228: 'due to asymmetry of eddy structure and strong shear' : This is too vague. The authors would be able to specify which terms in equation (1) explain the result.

Minor comments:
L33: 'microscale turbulent mixing' -> 'microscale turbulence'
L39: 'energy transfer between ...' -> 'energy exchange between ...'
L41: 'affect the vertical transport' : Do you mean 'affect the energy vertical transfer' or 'affect the material vertical transport'?
L42-43: 'Moreover, ...' : I don't understand the sentence. Could you rephrase it for better clarity?
L44: 'permanent energy can be transferred from eddies to NIWs ...' -> 'there is a permanent energy transfer from eddies to NIWs ...'
L82-83: 'the remaining were linearly interpolated vertically' : Does this mean that the raw ADCP vertical profiles are vertically interpolated? If so, to which vertical grid? And what is the reasoning behind this?
L83-85: What types of corrections are applied to ADCP data?
L96: 'qualitatively': Do you mean 'quantitatively'?
L99: 'the reanalysis data' : Do you mean 'the 1/12 CMEMS three-dimensional products'? It is unclear and hard to follow.
L101: Is near-inertial velocity derived from mooring data or CMEMS products? Unclear.
L104: Is near-inertial velocity at sea surface derived from mooring data or CMEMS products? ADCP data do not cover the surface layer.
L107-108: Specify which dataset each variable is derived from.
L112: Clarify the equations for shear and strain deformation and relative vorticity, as is done for the normal strain and shear strain in L98.
L132: 'The absences of NIKE' -> Do you mean 'Missing data'?
Figure 5a: Y-axis label: 'Energy' -> This is not energy. near-inertial wind work.
L202: 'in which ...' : Did Jing et al. (2018) and Chen et al. (2023) target the same area in the northwestern SCS? If not, the location information should be described.

Typos and grammatical correction:
L64: 'a' Teledyne RD
L67: 'observing results' -> 'the observation results' or 'the observed results'.
L72: 'up-looking' -> 'upward-looking' (Same for other parts)
L74: 'averaged at above 200 m' -> 'averaged above 200 m'
L88: 'during the observing period' -> 'during the observation period'
L110: 'method' -> 'parameter'
L118-119: 'While a significant ...': The sentence is incomplete.
L127: 'To qualify' -> 'To quantify'?
L136: 'asymmetry in spatial' -> 'asymmetry in space'
L151: 'will be qualified' -> 'will be quantified'?
L165: 'In spatial' -> 'In space'
L198: 'was' -> 'is' or 'will be' would be appropriate.
L207: 'were relative weak' -> 'were relatively weak'
L209: 'NIWS' -> 'NIWs'

Citation: https://doi.org/10.5194/egusphere-2025-283-RC2
- AC1: 'Reply on RC2', Hongzhou Xu, 03 May 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-283/egusphere-2025-283-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-283-AC1
EC1:
'Comment on egusphere-2025-283', Sjoerd Groeskamp, 15 Apr 2025

I thank the reviewers for their effort reviewing this manuscript.
Both reports provide positive feedback on this paper and consider it worthy for publication, after additional revisions and improvements are made. I therefore invite the authors to provide detailed point by point response to the reviewers comments and upload a revised version of their manuscript for consideration for publication in Ocean Sciences.

Citation: https://doi.org/10.5194/egusphere-2025-283-EC1
- AC3: 'Reply on EC1', Hongzhou Xu, 03 May 2025
  
  Dear Editor,
  We thank you for your patience and help. We addressed all comments from two reviewers and submitted the revised version for next step. Hope this version can meet the request of publication in your journal.
  Best regards,
  Hongzhou Xu
  
  Citation: https://doi.org/10.5194/egusphere-2025-283-AC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-283', Anonymous Referee #1, 14 Mar 2025

Utilizing a mooring array, this study investigates the interaction between a cyclonic eddy (CE) and near-inertial waves (NIWs) in the northwestern South China Sea (SCS). Results demonstrate that NIWs were significantly amplified by the passage of a cyclonic eddy. Overall, the manuscript presents a well-structured analysis of an interesting topic. While the work appears suitable for publication in Ocean Science, a few minor clarifications are requested to strengthen the paper prior to final acceptance.
Specific Comments
Line 24: “power of the first five NIWs modes were promoted significantly and dominated by the second and third modes.” Use “was promoted” for subject-verb agreement.
Line 53: “Chen et al. (2011) suggest that eddies present 35%--60% of the time in the northern SCS...” Replace “present” with “occur” for clarity.
Line 60: “making this place as a seemly area for investigating NIWs...” “seemly” is incorrect; replace with “suitable” or “ideal.”
Line 81–82: “flow velocity compensation correction was applied to ADCP data, which was calculated based on depth change and flow direction. Clarify the exact method used for compensation (e.g., reference to a specific algorithm or equation).
Line 82: Does the lack of data in the upper tens of meters of the mooring data lead to errors in energy calculations and filtering?
Line 95: Since calculating the energy conversion rate requires gradients in the x and y directions, but the mooring array is a section. How is this considered when calculating the energy conversion rate?
Line 97: “moving average of 3 internal tide periods.” The analysis in the article focuses on near-inertial internal waves, not internal tides. Therefore, it should be 3 “internal wave periods”.
Question: Is this the only CE throughout the year? Or why was this particular CE selected for analysis?
Line 116: “near-inertial frequency with peak value of 0.616 cpd” Define “cpd” (cycles per day) upon first use.
Line 160: “Low modes rose from 46% (Q2) and 54.4% (Q3) during Period1 to 87.6% (Q2) and 79.5% (Q3) during Period2...” Clarify what “46%” refers to (e.g., percentage of total energy?).
Line 200–201: “The differences may be attributed to the strength of eddies, their rotation direction and the intensity of NIWs.” Expand on how rotation direction (cyclonic vs. anticyclonic) impacts energy transfer.
Line 206: “NIWs draw energy from background flow with small energy ratio between them, vice versa.” Rephrase for clarity (e.g., “NIWs draw energy from the background flow when the energy ratio is small, and vice versa”).
Line 229: “This study is helpful for us to understand multi-scale interaction...” Replace “for us” with “for understanding” to maintain objectivity.
Figure 1: Panel (c): Label the x-axis of the power spectra.
“The mooring dataset used for plotting Figure for this paper are available at”, the grammar is incorrect.
Ensure consistent use of hyphens (e.g., “near-inertial” vs. “near inertial”).
Use “cyclonic eddy (CE)” consistently after first introduction.

Citation: https://doi.org/10.5194/egusphere-2025-283-RC1
- AC2: 'Reply on RC1', Hongzhou Xu, 03 May 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-283/egusphere-2025-283-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-283-AC2
RC2:
'Comment on egusphere-2025-283', Anonymous Referee #2, 17 Mar 2025

This paper presents mooring ADCP observations in the northwestern SCS and interprets the variations in near-inertial energy based on analyses of satellite and reanalysis data. The energy exchange between mesoscale eddies and near-inertial waves is an interesting and important topic, and the authors provide observational evidence that near-inertial wave energy is enhanced in cyclonic eddies. I agree with the publication of this paper, but before it is published, I would like the authors to address some misleading statements and unclear descriptions.

Major comments:
L22-24: 'Significant energy transfer rates between the CE and NIWs appeared at the same depth of the enhancement that can reach up to m²/s³at the CE's edge.' : What percentage of near-inertial and eddy kinetic energy does this correspond to?
L56-59: 'Researchers ...' : This sentence is redundant and repeats information from L52-53. The authors should specifically present the findings of previous studies regarding mesoscale eddies in the SCS.
L86-94: This paragraph lists the datasets used in the analysis, but it is hard to follow because the authors do not specify the variables for each dataset. Both the CMEMS and AVISO datasets include SLA data, but it is unclear which part of the analysis each of the two SLAs is used in.
L96: 'Eddy-NIWs energy transfer rate': Do you mean 'energy transfer rate from mesoscale eddies to NIWs'? Specify the direction.
L97: The equation (1) seems different from the equation (3b) in Jing et al. (2017).
L119-122: Is this relative frequency shift consistent with the cyclonic eddy's relative vorticity at the mooring site?
L126: 'Meridional velocity wavelet power spectra': Why is only meridional velocity used? Near-inertial waves exhibit a circular hodograph, so why not include both components of velocity?
L153-154: It seems too speculative. I can't discern this from Fig.1a.
L166: 'due to vorticity asymmetry of the CE' : This seems too definitive. It is not clear from the figures whether the differences in near-inertial energy at each mooring site are attributable to the asymmetry in vorticity.
L169-170: 'the north side of the CE was merged with the western boundary current of the northwestern SCS (Fig. 1a)’: It's not clear from Fig. 1a.
L170-171: 'that generated strong shear and enhanced NIWs largely at this side': This is an inference, so the authors should avoid making definitive statements.
L180-181: What is this percentage to?
L185-186: 'suggesting prominent influence of eddy on near-inertial modes' : This is too vague.
L194: 'eddy-NIWs energy transfer efficiency is about 2%': the term 'eddy-NIWs energy transfer efficiency’ is unclear, so I don't understand what the '2%' refers to.
L200: 'rather than surface and mixing layers' : What is the mixed layer depth at the mooring sites?
L207-208: Which terms in equation (1) explain the high rate at Q1? Please provide a similar discussion as for Q4.
L209-210: 'with small energy ratio between them': I don't understand what the authors mean.
Fig. 9c: How about showing the cumulative energy transfer from the CE to NIWs? It is hard to tell the net energy transfer from the current figure.
L228: 'due to asymmetry of eddy structure and strong shear' : This is too vague. The authors would be able to specify which terms in equation (1) explain the result.

Minor comments:
L33: 'microscale turbulent mixing' -> 'microscale turbulence'
L39: 'energy transfer between ...' -> 'energy exchange between ...'
L41: 'affect the vertical transport' : Do you mean 'affect the energy vertical transfer' or 'affect the material vertical transport'?
L42-43: 'Moreover, ...' : I don't understand the sentence. Could you rephrase it for better clarity?
L44: 'permanent energy can be transferred from eddies to NIWs ...' -> 'there is a permanent energy transfer from eddies to NIWs ...'
L82-83: 'the remaining were linearly interpolated vertically' : Does this mean that the raw ADCP vertical profiles are vertically interpolated? If so, to which vertical grid? And what is the reasoning behind this?
L83-85: What types of corrections are applied to ADCP data?
L96: 'qualitatively': Do you mean 'quantitatively'?
L99: 'the reanalysis data' : Do you mean 'the 1/12 CMEMS three-dimensional products'? It is unclear and hard to follow.
L101: Is near-inertial velocity derived from mooring data or CMEMS products? Unclear.
L104: Is near-inertial velocity at sea surface derived from mooring data or CMEMS products? ADCP data do not cover the surface layer.
L107-108: Specify which dataset each variable is derived from.
L112: Clarify the equations for shear and strain deformation and relative vorticity, as is done for the normal strain and shear strain in L98.
L132: 'The absences of NIKE' -> Do you mean 'Missing data'?
Figure 5a: Y-axis label: 'Energy' -> This is not energy. near-inertial wind work.
L202: 'in which ...' : Did Jing et al. (2018) and Chen et al. (2023) target the same area in the northwestern SCS? If not, the location information should be described.

Typos and grammatical correction:
L64: 'a' Teledyne RD
L67: 'observing results' -> 'the observation results' or 'the observed results'.
L72: 'up-looking' -> 'upward-looking' (Same for other parts)
L74: 'averaged at above 200 m' -> 'averaged above 200 m'
L88: 'during the observing period' -> 'during the observation period'
L110: 'method' -> 'parameter'
L118-119: 'While a significant ...': The sentence is incomplete.
L127: 'To qualify' -> 'To quantify'?
L136: 'asymmetry in spatial' -> 'asymmetry in space'
L151: 'will be qualified' -> 'will be quantified'?
L165: 'In spatial' -> 'In space'
L198: 'was' -> 'is' or 'will be' would be appropriate.
L207: 'were relative weak' -> 'were relatively weak'
L209: 'NIWS' -> 'NIWs'

Citation: https://doi.org/10.5194/egusphere-2025-283-RC2
- AC1: 'Reply on RC2', Hongzhou Xu, 03 May 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-283/egusphere-2025-283-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-283-AC1
EC1:
'Comment on egusphere-2025-283', Sjoerd Groeskamp, 15 Apr 2025

I thank the reviewers for their effort reviewing this manuscript.
Both reports provide positive feedback on this paper and consider it worthy for publication, after additional revisions and improvements are made. I therefore invite the authors to provide detailed point by point response to the reviewers comments and upload a revised version of their manuscript for consideration for publication in Ocean Sciences.

Citation: https://doi.org/10.5194/egusphere-2025-283-EC1
- AC3: 'Reply on EC1', Hongzhou Xu, 03 May 2025
  
  Dear Editor,
  We thank you for your patience and help. We addressed all comments from two reviewers and submitted the revised version for next step. Hope this version can meet the request of publication in your journal.
  Best regards,
  Hongzhou Xu
  
  Citation: https://doi.org/10.5194/egusphere-2025-283-AC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Hongzhou Xu on behalf of the Authors (03 May 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (09 May 2025) by Sjoerd Groeskamp

RR by Anonymous Referee #1 (16 May 2025)

RR by Anonymous Referee #2 (31 May 2025)

Suggestions for revision or reasons for rejection

I appreciate the improvements made in the revised manuscript. While many of the comments have been addressed appropriately, it appears that some of my previous comments may have been misunderstood. I suggest that the authors revisit those specific points and provide further clarification.

On my earlier comments:

2. L22-24: 'Significant energy transfer rates between the CE and NIWs appeared at the same depth of the enhancement that can reach up to 6 x 10^-10 m2/s3 at the CE's edge.' : What percentage of near-inertial and eddy kinetic energy does this correspond to?

Authors' Response:
Thank you for your comments. We calculated the percentage changes in the averages of NIKE and EKE for two time periods: before February 16th and from February 16th to March 8th, which is 115.2% and -71.6% respectively. We have added the percentage changes in the averages of NIKE and EKE in section 3.2: “NIWs were enhanced gradually accompanied by weakening EKE during Period1 when wind-input NIKE were relatively stable and minor. The percentage changes in the averages of NIKE and EKE for two time periods, before February 16th and from February 16th to March 8th, were 115.2% and -71.6%, respectively. Results suggest a vivid energy transfer from Eddy to NIWs during this period that will be quantified and discussed in Section 4.”

Reviewer's Response:
The paragraph newly added in response to my previous comment is unclear and does not appear to fully address the concern I raised. I would recommend removing it.

What I intended is that the expression “significant energy transfer” is vague, and I am concerned that the estimated value of 6 x 10^-10 m^2/s^3 may be too small to represent a meaningful contribution to near-inertial energy. This paper assumes that changes in near-inertial energy are due to energy transfer between cyclonic eddies and near-inertial waves. To support this, it is necessary to compare the estimated energy transfer rate with the changes in near-inertial energy.

In Section 4 (Discussion), the authors cite Jing et al. (2017) and Liu et al. (2023), who report that the energy transfer efficiency from mesoscale eddies to near-inertial waves can be 2 % or 13 % of the wind input. Alternatively, the authors might consider estimating what fraction of the observed near-inertial energy during Period 1 (or 2) could be attributed to the energy transferred from the eddy (or to the eddy). Would it be possible to evaluate the magnitude of the energy transfer in such a quantitative manner?

7. L119-122: Is this relative frequency shift consistent with the cyclonic eddy's relative vorticity at the mooring site?

Authors' Response: Yes, the frequency shift of the spectral peak is generally consistent with the results of eddy vorticity input. The power spectra in Figure 1c are averaged across four moorings for depths above 200 m. By combining the relative vorticity from Figure 6 with the local inertial frequency, we calculated the average frequency shift during the eddy period under the same conditions, which is approximately 0.69 cpd. There are some error sources: the tilted vertical structure of eddies and the temporal variations in mooring positions relative to the eddy which introduce discrepancies in frequency shift calculations. We conclude that this result aligns with the observed near-inertial frequency shift, despite potential calculation discrepancies.

Reviewer's Response: Thank you for your response. I think the description of the frequency peak shift being consistent with the cyclonic eddy’s relative vorticity should be included in the manuscript.

8. L126: 'Meridional velocity wavelet power spectra': Why is only meridional velocity used? Near-inertial waves exhibit a circular hodograph, so why not include both components of velocity?

Authors' Response: Considering the circularly polarized nature of near-inertial internal waves, the wavelet analysis results of zonal velocity (U) are similar to those of meridional velocity (V). They exhibit consistent variations during "Period 1" and "Period 2" (Figure R1). Since presenting an additional component wouldn’t provide more conclusive insights for data analysis, and considering subplot quantity and layout, we only displayed the wavelet analysis of one velocity component. The wavelet analysis results for both velocity components are shown below.

Reviewer's Response: Unless there is a specific reason to use only the meridional component, I think it would be more natural to show the wavelet spectrum of horizontal velocity by combining the power spectral densities of the zonal and meridional components.

18. L209-210: 'with small energy ratio between them': I don't understand what the authors mean.

Authors' Response: Sorry for the confusion. We have rewritten the sentence to “NIWs draw energy from the background flow when the energy ratio between them is small, and vice versa”

Reviewer's Response: This sentence is still unclear. Do you mean that NIWs draw energy from the background flow when NIKE is small compared to the background flow?

19. Fig. 9c: How about showing the cumulative energy transfer from the CE to NIWs? It is hard to tell the net energy transfer from the current figure.

Authors' Response: Thank you for your comment. We have added the cumulation bar during the two time periods in Fig.9c (Figure R2c).

Reviewer's Response: My suggestion was to plot the net flux at each location (Q1, Q2, Q3 and Q4), rather than separately averaging the positive and negative fluxes. The cumulation bars may not be necessary because adding the results from the four locations does not appear to be meaningful in this context.

New comments:

L66: Please check for typos.

L121 Equation (6): I missed this point in the first review. Near-inertial wind work refers to the dot product of the near-inertial component of the wind stress and the near-inertial component of the surface current velocity (e.g., Voet et al. 2024). Please verify this in your analysis.

Voet, G., A.F. Waterhouse, A. Savage, E. Kunze, J.A. MacKinnon, M.H. Alford, J.A. Colosi, H.L. Simmons, T. Klenz, S.M. Kelly, J.N. Moum, C.B. Whalen, R.-C. Lien, and J.B. Girton. 2024. Near-inertial energy variability in a strong mesoscale eddy field in the Iceland Basin. Oceanography 37(4):34-47, https://doi.org/10.5670/oceanog.2024.302.

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ED: Publish subject to minor revisions (review by editor) (11 Jun 2025) by Sjoerd Groeskamp

AR by Hongzhou Xu on behalf of the Authors (28 Jun 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (15 Jul 2025) by Sjoerd Groeskamp

RR by Anonymous Referee #2 (22 Jul 2025)

Suggestions for revision or reasons for rejection

1. Regarding the first response from the authors,
(authors’ comments) In addition, we found that the data of energy transfer rate was excessively time-smoothed which leaded too small values of 10-10 m2/s3, so we recalculated the results which can reach about 10-8 m2/s3. We revised the old Figures 8 and 9 accordingly (see Figures R1 and R2).

Did the authors apply a moving average to the energy transfer rate estimates? The moving-average window should be substantially shorter than 7 days, as longer windows may affect the integrated energy transfer rate value. In either case, if any kind of temporal averaging was applied, the authors should explicitly describe the method and clearly state the window length in the manuscript.

2. Thank you for considering the revision. However, the revised sentences do not convey the intended meaning. I recommend rephrasing as follows:

Original:
Meanwhile, we calculated the relative vorticity based on surface geostrophic currents and the local inertial frequency, and found that the average spectral peak frequency reached about 0.69 cpd during the eddy period. This value is very close to our observed ωp.

Suggested revision:
The observed near-inertial spectral peak, ω_p=0.667 cpd, in the CE is consistent with the effective inertial frequency, f_eff=f+ζ≈0.69 cpd, estimated from surface geostrophic currents and the local inertial frequency.

7. The authors appear to have misunderstood my earlier concern.
Near-inertial wind work (NIWW) refers to the rate of energy transfer from wind stress to near-inertial motions, and it should be computed as:
NIWW=τ_i⋅u_i,
where both the wind stress τ_i and surface current u_i are band-pass filtered to isolate near-inertial frequencies.
However, in Equation (6) (and accompanying text in lines 120-126), it seems that the wind stress is not filtered to retain only the near-inertial components. This means the resulting calculation does not represent near-inertial wind work.
I recommend that the authors carefully check their calculation, ensure appropriate filtering is applied to the wind stress, and revise the corresponding interpretation accordingly.

Typos:
L226: qualified -> quantified
L237: Q1 -> Q1, Q2?

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ED: Publish subject to minor revisions (review by editor) (08 Aug 2025) by Sjoerd Groeskamp

AR by Hongzhou Xu on behalf of the Authors (11 Aug 2025) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (25 Aug 2025) by Sjoerd Groeskamp

AR by Hongzhou Xu on behalf of the Authors (28 Aug 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (28 Aug 2025) by Sjoerd Groeskamp

AR by Hongzhou Xu on behalf of the Authors (30 Aug 2025) Manuscript

Journal article(s) based on this preprint

28 Oct 2025

Enhancement of near-inertial waves by cyclonic eddy in the northwestern South China Sea during spring 2022

Qi'an Chen, Hongzhou Xu, Dongxiao Wang, Bo Hong, Chunlei Liu, Zheyang Zhang, Huichang Jiang, Wei Song, Tong Long, Ling Wang, Sumin Liu, and Rongjie Chen

Ocean Sci., 21, 2631–2642, https://doi.org/10.5194/os-21-2631-2025,https://doi.org/10.5194/os-21-2631-2025, 2025

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Qi'an Chen, Hongzhou Xu, Dongxiao Wang, Bo Hong, Chunlei Liu, Zheyang Zhang, Huichang Jiang, Wei Song, Tong Long, Ling Wang, Sumin Liu, and Rongjie Chen

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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NIWs have horizontal spatial scales of 10–100 km and slow group velocities, making them to interact strongly with mesoscale eddies. In the northwestern SCS, large portion of eddy propagate westward and may interact with local NIWs. In this study, four moorings were deployed in this area and they captured a vivid case of interaction between a cyclonic eddy and NIWs. Our paper provides direct evidence of strong interaction between eddy and NIWs in the northwestern SCS.


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