Different Types of Near-Inertial Internal Waves Observed by Lander in the Intermediate-Deep Layers of the South China Sea and Their Generation Mechanisms
Round 1
Reviewer 1 Report
The manuscript reports on an extensive set of data on Near Inertial Waves (NIW) accumulated in measurements using an ADCP positioned on a Lander moored deep in water. As such, the results are important and certainly deserve publication. However, the paper is not written well, to the extent that it is nearly incomprehensible to anyone not involved directly in NIW research. While the missing necessary information can be found e.g. in refs. [1,2], the paper should be reasonably self-contained. The Introduction should include the basic concepts and definitions regarding NIW. Moreover, in the present version there are many unclear statements. For example, what exactly is meant by the “maximum wind speed radius” (line 47), and what is the reference for the 10% increase (in which velocity?) in the same line. In general, the Introduction is not focused; it contains a broad collection of references, some of them apparently not directly relevant to the present study. The final part of the Introduction needs additional work; it should formulate the goals of the study and stress the novelty of approach.
The description of the algorithm is unclear; the advantages of curve fitting over Fourier analysis should be substantiated by providing appropriate examples based on real data processing. On the other hand, the authors attempt to use biocoherence intrinsically based on the Fourier analysis that presumably does not work for the recorded signals. There seems to be an inconsistency in the approach here.
The presentation of data also suffers from numerous drawbacks, so that it is often difficult to grasp the meaning of the results. Caption of Fig. 1 does not contain all necessary information regarding the experiment. As for the data processing, I may suggest starting with plotting the records of two velocity components which presumably may help to understand the approach applied. No reference for phase determination is specified. In the present from, it is difficult to understand the data processing presented in paragraph starting on line 261, the applied method is not substantiated well enough. There are apparent dominant frequencies in the panels of the left column of Fig. 3, but I could not find the discussion of those frequencies and their values; it is difficult to estimate them from the small plots presented, the value of f in line 269 is not specified.
Eventually I lost the authors totally. In my opinion, the results deserve a much better presentation. Only after the manuscript is completely rewritten and made “user-friendly”, it will become possible to assess its value.
Author Response
We would like to take this opportunity to thank the kindly comments from the Reviewer, and to thank the efficient work of the Editor on our manuscript. We have carefully revised the manuscript following the Reviewer’s comments, detailed revision and response are listed below.
1-The manuscript reports on an extensive set of data on Near Inertial Waves (NIW) accumulated in measurements using an ADCP positioned on a Lander moored deep in water. As such, the results are important and certainly deserve publication. However, the paper is not written well, to the extent that it is nearly incomprehensible to anyone not involved directly in NIW research. While the missing necessary information can be found e.g. in refs. [1,2], the paper should be reasonably self-contained. The Introduction should include the basic concepts and definitions regarding NIW.
Response: We thank for the reviewer’s kindly comments. We have rewritten the introduction following the reviewer’s suggestion. The revised text has included the definition of NIW, the importance of the NIW in the ocean, the different generation mechanisms of the NIWs, and the NIWs in the South China Sea. Detailed revision please find in the revised manuscript.
2-Moreover, in the present version there are many unclear statements. For example, what exactly is meant by the “maximum wind speed radius” (line 47), and what is the reference for the 10% increase (in which velocity?) in the same line.
Response: Thank you for this comment and we are sorry for misleading. Maximum wind speed radius means the distance from the center of a cyclone to its strongest wind band. The horizontal scale of typhoon-forced NIWs is at least hundreds of kilometers, the maximum near-inertial wavelength can be in an order of 1000 km and the horizontal structures of NIWs may vary with wind stress, location, geostrophic shear and so on. The maximum wind speed radius has been changed to radius of maximum wind speed to better clarify the ambiguous statement. The sentence was revised as:
“Further investigation shows that the energy of the NIW is associated with the strength of the typhoon. For example, the corresponding near-inertial kinetic energy (NIKE) would increase by 18%, 38% and 19%, if the radius of maximum wind speed, maximum wind speed, or moving velocity of typhoon was increased by 10%, respectively [17].”
3-In general, the Introduction is not focused; it contains a broad collection of references, some of them apparently not directly relevant to the present study. The final part of the Introduction needs additional work; it should formulate the goals of the study and stress the novelty of approach.
Response: We thank for the reviewer’s kindly comments. So far, most of these studies are focus on the upper layer. Meanwhile, these NIWs are generally case studies that occurred in different locations. On July 25th 2017, we have deployed a Lander equipped with an upward looking Acoustic Doppler Current Profiler (ADCP) to observe the current velocity profile in the northwest of the Luzon Strait. A total of 124 days continuous time series was obtained. The observation has captured 5 NIWs with different generation mechanism at the same location, which has not been reported yet. In this study, the features and generation mechanisms of the pronounced 5 NIWs are investigated and compared.
We have revised the last paragraph of the introduction by adding this information and giving a short description of the organization of the paper.
4-The description of the algorithm is unclear; the advantages of curve fitting over Fourier analysis should be substantiated by providing appropriate examples based on real data processing.
Response: Thank you for your review comment. The frequency corresponding to the abscissa of the spectral analysis is discrete. Its corresponding value is affected by the sampling frequency and the number of samples. The specific formula is as follows.
For the data studied in the manuscript, the time interval was 20 min and the average sampling time per event was approximately 12 days. In this case, only three discrete frequencies can be obtained near the inertial frequency, which is disadvantageous for accurately calculating the frequency of NIWs. So we took the approach of fitting equations. This method can more accurately determine the frequency of each NIWs event.
5-On the other hand, the authors attempt to use bicoherence intrinsically based on the Fourier analysis that presumably does not work for the recorded signals. There seems to be an inconsistency in the approach here.
Response: Thanks for your comment. There is a description of the Bicoherence method, which is a measure of the correlation of one signal with two other signals. The PSI mechanism is that two kinds of waves interact nonlinearly to generate a third wave. Observing the strongest M2 internal waves in the sea area can sometimes generate internal waves with two frequencies of M2-f and f at the same time. These three waves are coherent because they form each other. When this phenomenon occurs, the bispectral correlation exceeds 90%. The Fourier transform in this method is to calculate the double correlation between the two waves.
6-The presentation of data also suffers from numerous drawbacks, so that it is often difficult to grasp the meaning of the results. Caption of Fig. 1 does not contain all necessary information regarding the experiment.
Response: We thank for the reviewer’s kindly comments. We have revised the description of the data used in this study. The resolution and time interval have been introduced. The passage of the typhoons that may related to the NIWs have also been included.
7-As for the data processing, I may suggest starting with plotting the records of two velocity components which presumably may help to understand the approach applied. No reference for phase determination is specified. In the present from, it is difficult to understand the data processing presented in paragraph start.
Response: We thank for the reviewer’s kindly comments. We totally agree with the reviewer that the record of two velocity components should be shown here for better understanding. Figure S1 shows the initial records of the u- and v-component of the current velocity. We can see that multiple signals are involved in the figure, including seasonal-like variation in the end of October (negative/positive u before/after November 1st), tidal-like signal with typical period of half or one day, as well as barotropic and baroclinic component. It is hardly to see any NIWs signature from the initial records of the current velocity. Therefore, we have added a new figure (Figure 2 in the revised manuscript), showing the bass-pass filtered u- and v-component, instead of the initial records, in order to achieve better understanding. The results generally corroborate with that of NIKE as shown in Figure 3.
Figure S1. The initial records of the u- and v-component of the current velocity.
Figure S2 (Figure 2 in the revised manuscript). Near-inertial current velocity profiles. (a) and (b) are the U and V components, respectively. The bandpass range used is from 0.8 f0 to 1.2 f0. The gray vertical lines in the figure mark the extent of each NIWs event.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The five NIW events were observed at depths of 600 m to 1100 m in the northern South China Sea from July to November 2017. The largest NIW event during this observation was revealed to have been caused by the combination of the attenuation of land shelf waves and the monsoon. A component which was supposed to have been reflected from the seabed was also detected, although this was not clarified.
Since it is difficult to take the meaning of the sentence on the 792th to 794th lines, the sentence should be corrected.
Author Response
We would like to take this opportunity to thank the kindly comments from the Reviewer, and to thank the efficient work of the Editor on our manuscript. We have carefully revised the manuscript following the Reviewer’s comments, detailed revision and response are listed below.
Point 1: The five NIW events were observed at depths of 600 m to 1100 m in the northern South China Sea from July to November 2017. The largest NIW event during this observation was revealed to have been caused by the combination of the attenuation of land shelf waves and the monsoon. A component which was supposed to have been reflected from the seabed was also detected, although this was not clarified. Since it is difficult to take the meaning of the sentence on the 792th to 794th lines, the sentence should be corrected.
Response: We thank for the reviewer’s kindly comments. We have polished the whole manuscript to make it easier to follow, and detailed revision please find in the revised manuscript. For the line 792 to 794, We have revised it as follows:
The study found that there is a time interval of 18 days between the ND and the strong sub-inertial flow event, coincide with the arrival time of the NIWs as suggested in previous studies. However, the source of the NIKE of ND cannot be identified, as the propagation direction is not capable of capturing by a single station measurement of current profile.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The manuscript has been improved substantially in response to my comments. Nevertheless, some editing of the text is still desirable, in particular in the parts added/modified in the revision.
