The paper discusses seismic observations of shoaling internal tides and waves, and describes features the authors identify in the data. 

General comments:

It is hard to understand what the contribution of this manuscript is. The authors make it clear that neither the observational methos (seismic method) nor the phenomena  identified in the data are new (see the Introduction and Data and methods sections). The remaining potentially valuable contributions might be: the data set itself, and possible new phenomena the data set reveals.

The data set seems to be confined to the gray map shown in figure 2 (the "after" panel). Perhaps this data set is uniquely difficult to obtain and deserves a paper in itself. It seems doubtful (method not new). However, because this reviewer is not familiar with the seismic observational techniques, advantages and disadvantages, the quality of the data and its reliability will not be discusses here. 

The observations perhaps reveal something new and valuable, but after reading the manuscript, the features identified seem well known and understood. Perhaps just the fact they appear in that environment is a valuable piece of information? if so, a discussion is warranted (and in fact not found in the manuscript). 

The discussion of the data set is particularly disappointing. The general narrative seems to be this: the authors identify some features in the data (marked in some figures by lines); declare that those features have a very specific meaning; and apply some well-known formulations to provide some quantitative discussion. The analysis of observational data should be approached with a lot of skepticism, and should involve careful cross-checking and verification. in this manuscript, the authors declare they see something and then weave a story around it. They see "high-frequency internal waves, mode-2 internal waves, and shear instability", and "believe that strong nonlinear high-frequency internal wave packets are the result of the shoaling. Do they see all this just from seismic reflections? This seems problematic. Is the seismic data enough to fully characterize the flows identified? It's hard to believe that it's so, because the dynamics of internal waves are described by complicated equations that involve quite a number of state variables. Why is seismic data enough to characterize these complicated states?

Which raises another question that is completely ignored by the authors: what is the place of seismic data among other observation techniques? In other words - what should or should not be inferred based on seismic observations? How much can we trust these features. 

And then, what are these identified features? What does the gray map in figure 2 represent? What do the lines drawn on the gray levels mean? this might be clear to people familiar with seismic data, but this manuscript is, as the title seems to say, about internal waves, not seismic data. 

 Specific comments:

 

 The Introduction section is a general discussion about the observational method and known internal waves phenomena. It does not identify a question that would give the manuscript a purpose.

 

The part of the "Data analysis and methods" section concerned with the discussion of seismic data processing is incomprehensible for someone that is not familiar with the technique. The figure organization and captions are uninformative and incomprehensible. For example, in figure 2, the differences between the "before" and "after" gray maps seem insignificant. In figure 6, we are shown basically the same map 3 times, with no annotation that might help understand what part we are looking ate and why.

The manuscript is poorly constructed. The authors do not seem interested in explaining meaning and relevance of their data and observations. The "features" they discuss are not corroborated in any way, the reliability of their conclusions is not evaluated. Perhaps the authors are correct but we have no way to tell - only their word for this. For someone that is unfamiliar with seismic maps this is hard to swallow. 

In conclusion: the seismic observational approach is not new; its relevance and capabilities in relation to internal flows is not discussed; "feature" identification is superficial, based on the authors "impressions", rather than rigorous; we have no measure of the validity of these estimates. What exactly is the novel contribution of the manuscript?  

General Comments:

This paper presents a section of seismics reflection data from the South China Sea in which the detail of reflection variability is assessed. The presence of high-frequency and high mode internal waves, as well as KH billows are identified, and related to computed turbulent dissipation rates across the section.

There are some interesting features in the seismic data which are worth publication and this is one of the few techniques of mapping out the horizonal detail of such structures. The interpretation of the different features associated with internal wave processes is interesting, however I feel that some parts need to be presented as a little more speculative – particularly the high-mode internal solitary waves. In an ideal world, an acoustic response to a model of such features would be really needed to understand exactly what is shown in the data. To be of an ‘international standard’ this needs addressing.

A little more input around why the identification of these kind of features may be important (i.e. in the wider context) would be good, alongside what particularly the role of seismic data brings. This is needed to hit putting the ‘results into context’.

The paper is quite repetitive in places, especially around the internal wave dynamics in this environment and could be streamlined a little bit.

Technical corrections:

Include how often was velocity picking conducted?

‘Further denoising’ do you mean trace smoothing as this can impact displacement spectra

Line 47 – around here it would be good to include a couple of sentences explaining what seismic oceanography is (how it works), exactly what it captures, and its resolution. i.e. move lines 79 to 83 in here.

Lines 83-85: Have you looked at any nearby hydrographic data to verify the relative temp:salinity contribution, and if seismic reflectors likely track isopycnals?

Data and methods – include a paragraph on the hydrographic data used i.e. when collected

Line 164: this range will vary depending on dissipation rates – is it ok to just use a set upper and lower wavenumber bound?

Fig 4- there seems to be some odd jumps in the black lines between reflectors e.g. distance 25 km and distance ~ 46 km, depth ~400 m.

Fig4 – what are the slopes of the fitted red lines? A pdf would be good to check they align with Batchelor. I don’t understand what the lower wavenumber, negative slope is fitted to – it does not appear to match the spectra at all?

Fig 2 – it is difficult to assess differences in the stacks – maybe you could show some zoomed regions and highlight some key areas of improvement? Also, what do you mean by before and after processing? i.e. data in a has been processed to a degree?

Fig 7: thermocline in XBT 7 looks deeper than XBT5 and 6? Line 204 – needs more clarification about how you identify the thermocline and what you mean by a significant change?

Line 244 – ok but a little bit of an assumption. Could also be lenses? Especially considering your comment in line 260. I think you need to clearer that this is one of other possible interpretations. Noted you come back to this discussion in 4.1 – but I think some kind of modelling is needed to really interpret these structures. I think you need to be more tentative in the identification here e.g. line 395

Line 275-280. I’m finding it hard to distinguish between symmetric and asymmetric – appears very qualitative. Also – have you investigated how migration approaches impact this analysis?

Fig 12 – does this account for all the errors noted or just the spectral fitting ones?

More generally – does the internal wave part of the spectra tell you anything about the character of these waves to support your conclusions?

Line 403 – the increased mixing in fig 10 does need seem to strongly coincide with the region of KH billows identified? A pdf of mixing rates for billow vs non-billow region would be good to see.

Typing errors:

Remove ‘And’ in line 16

Fig 10 – check this colour bar is ok with colour blindness

Comments on Meng et al., “Seismic observations of high-frequency internal waves, high-mode nonlinear waves, and K-H billows on the South China Sea’s shelf revealed by marine seismic observation.”

 General comments: This paper is an attempt to use seismic reflection data over the continental slope (note I would call this region the slope, not the shelf) in the SCS to calculate the horizontal wavenumber and then the TKE and diapycnal diffusivity for the “high frequency” thermal displacements observed.  The technique is sound, as has been previously documented in Holbrook (2003, 2013) and the nice how-to by Ruddick et al., 2009 in Oceanography (22).  It’s an attractive way, perhaps the only way, of estimating these quantities efficiently over a large spatial area.  The numbers are probably solid and the distributions interesting.

I struggle however, with the scientific interpretation regarding what is being observed in this section and where/how this variance originated. 

• The largest, can’t-miss-it oceanographic signal by far in this part of the world is the mode-1 NLIWs with amplitudes from 100-150m, as described by many previous authors. Why are they not observed in this data set?  Note there may be explanations for this, for instance observations made during neap tide in the Luzon Strrait, but the authors need to address this. 
• Nobody has ever seen mode-2 NLIW this large, especially in the absence of mode-1 waves, so I have a hard time believing this interpretation. Are these “mode-2” waves perhaps misinterpreted mode-1 waves?
• Small amplitude (10-15m) random internal waves are everywhere in the SCS. They did not necessarily form by shoaling/breaking.  In fact, all the literature dedicated to shoaling (Sinnett et al. 2022 and similar) are talking about the very large mode-1 NLIWs which occupy a significant fraction of the water column.  The fission waves are always seen trailing the mode-1 soliton. There is no reason to believe the small linear IWs described here feel the bottom at all at these depths.  I think the internal waves referred to here are just a snapshot of the random IW field in the SCS. 
• Upslope bores are limited to very shallow (less than 50 m) water. I wouldn’t call anything I can see here a “bore.” 
• Likewise, I can’t see anything in this data I would convincingly call a KH billow. The authors need to somehow highlight these better, and more convincingly. 

A few small details.  Pardon me for being a bit repetitive.

Line 16 – delete “and”

Line 90 figure 2:  I really can’t see any difference between a) and c).

Line 115 - Seems like M-H Chang more recent papers near Dongsha should be referenced.

Figure 6.  Aren’t b) and c) the same as a)?  Suggest drawing boxes around the region of interest.  I am hard pressed to see any mode-2 waves or KH billows here. 

Line 207:  These IWs are quite small, compared to previous literature.  Where are the mode-1 solitons?  These should be the most obvious signal. 

Lines 212-214:  Three lines of text on “dunes.”  There are entire papers on this phenomenon. The authors need to strengthen this argument considerably or just leave it out. What’s the data source of the bottom profile?  Why does it matter for this paper?

Line 230:  There are many other references on the impact of the bottom slope on wave evolution. 

Line 258:  Nobody has ever seen mode-2 NLIW this large.  Are these perhaps misinterpreted mode-1 waves? 

Figure 9.  I can’t see the difference between symmetric vs. asymmetric billows.  We need more description of what is meant by this. 

Summary:  The scientific interpretation of the fluctuations observed in the seismic data needs to be improved a lot to be publishable in NPG. 

General Comments:

This has the potential to be a very interesting study adding to the growing literature of seismic oceanography. However, I have several concerns about the observations and interpretation, and the lack of clarity on the overall contribution.  

Specific Comments:

My main concern is that the study starts with the assumption that nonlinear internal waves are being detected. However, I am not convinced. I agree that the seismic reflection methodology is well-established, however, the detection of internal waves using this method is not. For example, the authors state that seismic oceanography has been extensively used in the study of internal solitary waves, but only provide one reference to back up this claim. Further, the Tang et al. (2014) reference provided observes two much clearer examples of internal solitary waves of depression that are then corroborated by a satellite image. These waves are convincingly leading waves in a packet of Mode-1 ISWs. The proposed internal wave detections here are not as convincing and should be corroborated with other data before they are interpreted.

I am also confused as to why the proposed high-frequency internal waves are classified as mode-2 waves. Where are the mode-1 waves, which should be the largest signal, and were detected by Tang et al. (2014)? I realize this survey line is a snapshot in time – do the authors propose that the mode-1 wave has already passed? If so, please discuss further.

I also think corroborating evidence such as satellite images or a tidal model indicating whether ISWs are anticipated to have been generated several days before at the Luzon Strait (i.e., phase of the spring-neap tidal cycle) is necessary. I am not convinced that the observations are not background, ubiquitous internal waves. Were the other survey lines similarly analyzed? If the other lines are analyzed, are these signals common (i.e., are they background noise)? I recommend using the other lines to form a background noise baseline and show whether the signals observed here actually deviate significantly from the background.

Minor Comments:

Please add more discussion of the data collection and survey lines. For example, when is this specific line from? Where is it at in the spring-neap tidal cycle at the Luzon Strait where these waves presumably have been generated? Why was this the only line used? Was it the only line analyzed, or were other lines analyzed but there were no ISW signals?

The seismic data processing discussion is convoluted and should be understandable to a broader audience; please simplify it.

I think Sections 2.2 - 2.3 should be moved to the discussion instead of Data and methods. A lot of time is spent discussing different types of waves, which really is not relevant until the discussion and detracts from the methodology. The figures in these sections also might not be necessary and are a distraction from the observations. 

Summary:

• Needs more work to verify that these are ISWs from the Luzon Strait either with corroborating data (such as satellite images, tidal information) and/or comparing to other seismic reflection lines to verify what the background noise is and that Line 25 deviates significantly from the background. If it cannot be convincingly shown that these are internal waves, the claims in this study should be treated with far more skepticism.
• Add in more of a discussion of why this study is important. I do think it has the potential to contribute to seismic oceanography, but please emphasize this more and add more context. For example, if more work is done to demonstrate that these are ISWS, emphasize that. I do not agree that this is well-established and that alone would be an important result.