the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Jan 2024
High-frequency Internal Waves, High-mode Nonlinear Waves and K-H Billows on the South China Sea's Shelf Revealed by Marine Seismic Observation
Abstract. From July to September 2009, a set of multi-channel seismic data was collected in the northern shelf area of the South China Sea. After the data was processed, we observed a series of shoaling events on one of the survey lines, including high-frequency internal waves, high-mode nonlinear internal waves, and shear instability. Using theoretical results from previous numerical simulations and field observations, coupled with local temperature and salinity data, we analyzed their depth distribution, waveform characteristics, and formation mechanisms, and discussed the influence of seafloor topography and stratification on the shoaling of solitary internal waves. We estimated the mixing parameters of seawater using a parameterization scheme based on hydrographic data and seismic data, respectively. And we found that the diapycnal mixing caused by these shoaling events in the shelf area were about 3.5 times greater than those on the slope. Consequently, the fission of internal solitary waves and the induced shear instability serve as significant mechanisms for the energy dissipation of internal solitary waves at the slope and shelf of the South China Sea. Additionally, the high-frequency internal waves generated during shoaling might also have a crucial role in this process.
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RC1: 'Comment on egusphere-2024-92', Anonymous Referee #1, 29 Mar 2024
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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?Citation: https://doi.org/10.5194/egusphere-2024-92-RC1
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