the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Feb 2025
Enhancement of near-inertial waves by cyclonic eddy in the northwestern South China Sea during spring 2022
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 6×10-10 m2/s3 at 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.
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Status: open (until 10 Apr 2025)
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RC1: 'Comment on egusphere-2025-283', Anonymous Referee #1, 14 Mar 2025
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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 -
RC2: 'Comment on egusphere-2025-283', Anonymous Referee #2, 17 Mar 2025
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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/s3 at 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
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