the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Dec 2024
Observations of near-inertial oscillations trapped at inclined front on continental shelf of the northwestern South China Sea
Abstract. Interactions of near-inertial oscillations (NIOs) with other oceanic processes have been broadly investigated in recent years. This study presents observed NIOs from a seabed-based platform, which are trapped at an inclined front with strong stratification on the continental shelf of the northern South China Sea (NSCS) during January–February 2021. The current observations reveal that four NIO events occurred induced by wind bursting. Under the same wind forcing, NIO currents in the third event increased from <0.05 m s−1 to about 0.08 m s−1. The mechanism analysis shows that the amplitude of NIOs was modulated by the shoaling depth of the mix-layer induced by the inclined front, and trapped in the upper layer. More important, the energy transferred from front to NIOs provides a new insight into the NIO dynamics. In addition, a remarkable non-linear interaction (fD1) between NIOs and diurnal spring tide occurred at the front zone owing to a strong vertical current shear at the depths of 20–50 m. The underlying physical phenomenon of this observation would be important for energy exchanges in the oceans.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3909', Anonymous Referee #1, 06 Mar 2025
General recommendations:
Dear Editor,
This study analyses data velocity data from an ADCP deployed in the South China Sea Northern shelf near a strong front. The front position is moving with time, and consequently, the ADCP measures the velocities on each side of the front at times. The authors study particularly the near inertial oscillations (NIOs) in this record. The authors also use satellite and ship-profiler data to estimate the density field and stratification to understand the variability of these NIOs.
The authors are making two points:
(i) The local stratification impact the amplitude of the NIOS
(ii) They claim to observe non-linear interactions fD1 in the velocity record.(i) It is not new to see an impact of the stratification on NIO. It has been known for decades that velocities scale with stratification. For that reason, the WKB method was developed in the 70s. The velocities in this study were not even WKBed. So, all claims about the vertical propagation of energy (done by simply looking at the amplitude of the velocities) are not backed by any logical evidence. Furthermore, the claim that the “stratification” is responsible for the observed variability misses the fact that, as shown in the paper, a submesocale feature is passing by, which can also generate NIOs while the local velocity field is adjusting.
(ii) The nonlinear interaction could be here, but it could also be an artifact of a strong internal tide heaving, creating the fD1 peak, which would disappear if the same study had been done with velocities projected on the isopycnal reference frame.
In general the study is poorly written and the logic is really difficult to floor and understand. A lot of statement are not backed up by evidence or clear justification and the reader is referred to poorly or even wrongly used literature.
I do not recommend that study for publication.
Detailed comments:
- citations with 2 names should not have a semi-column (;). It makes the whole document very difficult to read.
- It is customary to have the oldest references first when citing many references. Please modify the text accordingly
abstract -
L17-”More important, the energy transferred from front to NIOs provides a new insight into the NIO dynamics” : what new insight?Introduction -
L29: Your statement about Xing and Davies is missing a lot of what they say. It is the inclusion of a coastal boundary that led to the shift. See below their text:
“[66] The inclusion of a coastal boundary separated from the front by a homogeneous water column, lead to a barotropic current that was phase shifted by 180° from the surface current. This gave rise to a nonzero current below the thermocline on the eastern side of the front. The barotropic nature of this current meant that it did not substantially influence the distribution of energy at the (ωf + f) or (∣ωf − f∣) frequencies.”L31: “NIOs contribute most of the velocity shear to the mixed layer (Zhang et al. 2021a)”
This sounds like a global statement only backed up by a study on the Bay of Bengal. There are a lot of global NIO/NIW studies (see Global Observations of Rotary-with-Depth Shear Spectra). Also I do not find in Zhang2021a anything backing shear in the mixed layer. I am suspicious of the “exactness” of that statement because Zhang2021 used data from 40m to 440m. The mixed layer in the bay of benghal is shallower than this.L48-56: Not clear how this list of studies is helpful for the study. Please simply focus on the mechanism you’ll use or see and develop a lot more on the physics behind these interactions.
Fig1. Please add units on the colorbars. The y-axis clearly does not follow the isobath. I assume it is because the arrows matches the topography at SP. In which case say it in the captions and move the arrow to a better place.
L71- “ with a strong stratification on the continental shelf”: Define what a “strong” stratification means? compared to what? also change the color limit of your N2 colorbar it is all blue. We can not see the “strong” stratification.
equations 2,3 and 4 are really not necessary. just say barotropic is the vertical average and baroclinic is the difference between raw data and barotropic velocities.
L115: I want to see the shape of your filters in the spectral domain and how much they overlap.
L132: I sure hope k_2 is the wavenumber cut off before the instrument noise kicks in and that you correct for the missing variance between k_2 and the kolmogorov wavenumber (Lueck2024)L150: Please provide the error bars for your spectra, especially to show that your fD1 peak is not lower than to statistical spectral error. To do it correctly add in the method section how you compute your spectra (e.g., degrees of freedom)
L151: please make sure to mention that you are plotting KE spectra from the baroclinic velocities
L152: “frequency shifts” I truly do not no what the author are talking about? What frequency shift where? I do not see anything special around 0.024 and 0.03 h^{-1}
L156: How are D2 ,D3,D4 evident? It is really difficult to see either in Fig2.d or even a,b. There might be something (but not evident) on Fig2.c
L157: “The barotropic tidal current (BT, black curve)” : What black curve on which panel?
I am getting frustrated because the text and figures of that section are poorly written and described; there are no error bars, and the text mentions features that are not visible. I am going to spend less time on all those details to cut to the chase and try to understand the main point of the paper, but that does not mean the rest of the document is not full of these frustrating issues.
L207-208: “These features as shown in Figure 3 imply that the stratification of sea water in E3 is different compared to the other events.” This could very well be a mesoscale or submesocale feature (common in the region) passing by. So yes it could modify the stratification but it also comes with its own KE
L239-240: Another example among many of weak logic: The mention of the Ch-a and CDOM could justify your logic regarding the SP position relative to the front during E3 but the text should be modified to highligh the fact that you do not have multiple periods measuring these parameters on each side of the front.
Fig4: We do not know on what MODIS point you computed your data on Fig4f.
I am incapable of understanding Fig4h What does the arrow direction mean?L258: How do we know the temperature and salinity “have been impacted by the GDCC”? A citation is not enough. The authors needs to explain it here.
L310-315 Should be in Methods
L356: “The wind momentum rapidly diffused throughout the water column” This statement can not be made because you not have applied WKB to your velocities. The velocies needs to be normalized by the local stratification to be compared at depth.
L371: “remaining and time-lag energy” what is that?
L372 What is “energy transmission” and transmission between what?
L373-381: This paragraph makes no sense. I do not understand any of what is said here. What is the link between the doppler shift of mesoscale vorticity (Kunze1985) and whatever “energy transmission” mean ?Section 5.2: fD1 can also artificially occurs inside Eulerian measurement (fixed depth) when the amplitude of the internal tide is strong. This is very much the case in this region (LeBoyer2020)
Citation: https://doi.org/10.5194/egusphere-2024-3909-RC1 -
RC2: 'Comment on egusphere-2024-3909', Anonymous Referee #2, 08 Mar 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3909/egusphere-2024-3909-RC2-supplement.pdf
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RC3: 'Comment on egusphere-2024-3909', Anonymous Referee #3, 12 Mar 2025
The manuscript titled 'Observations of near-inertial oscillations trapped at inclined front on continental shelf of the northwestern South China Sea' presents observations of near-inertial oscillations on a continental shelf and describes its dynamics in reference to a moving ocean front. Generally, the manuscript is well written, but some sections and descriptions are lacking. The authors were restricted by their lack of density observations that covered the same time period as their velocity data, requiring a significant number of assumptions that are not always transparent in the manuscript. I am unsure of the purpose of some of the analysis, like the modal analysis. The manuscript also lacks a clear description of what exactly this manuscript contributes to the literature.
Major comments:
1. The manuscript requires further clarity throughout. Further explanations on terminology is required, and below I emphasize the need for consistent use of terminology. As much as possible, information should be explicit in its meaning and not hinted at or left for the readers interpretation. I have many examples given in the minor comments, but I encourage the authors to find further examples where they could be clearer.
An example of this lack of clarity and explicit detail is the abstract. Text like ‘provides new insight’ (line 17) does not summarise the findings. Also, it is stated there are ‘remarkable non-linear interaction’ (line 18), but what this interaction is was not described. Then ‘the underlying physical phenomenon of this observation’ (line 20) is described as important, but has yet to be described.
Many terms require defining as its usage in the manuscript is not well established. These terms include ‘velocity shear’ (line 31), ‘strain variance’ (line 34), ‘self-advection’ (line 35), ‘nonlinear momentum terms in the front zone’ (line 51), ‘thicker layer over the front’ (line 53), ‘inclined front’ (line 79). I do not understand what these terms mean in the context used. Please look for additional examples and correct them throughout the manuscript.
2. The introduction was overall difficult to understand and follow and has some incorrect information. For example, it is incorrect to state NIOs are only created by winter storms or tropical cyclones (line 25). Any wind burst can create these oscillations (e.g., Schlosser et al., 2019) and indeed the manuscript suggests NIOs are generated from strong winds. I have used Schlosser et al. (2019) as an example paper throughout my comments, but cited papers, review papers, etc., should also be checked.
The introduction left me confused and trying to figure out how all the information cited is related. I encourage the authors to rewrite the introduction so that the information from difference papers is seemlessly meshed together. One metaphor I like from a writing workshop is the 'stepping stone' approach (see https://medium.com/an-idea/how-to-strategically-plant-stepping-stones-for-powerful-writing-1b5a1762d0a). In this metaphor, each sentence is like a strategically placed stepping stones to guide the reader. At all stages, the reader has some idea of what is to come and there are no 'jumps' in reasoning.
3. Line 78, I encourage you to try to motivate your study beyond a ‘regional study’. Do we really need to know about trapped NIOs on the shelf in this region specifically? Or is there a general lack of knowledge of the interaction between coastal NIOs and fronts? This point also needs more clarification in the Discussion, Conclusions, and Abstract.
4. The manuscript lacks distinction between near-inertial oscillations and waves that vertically propagate. One lines 214-216, I am not convinced that NIOs are trapped in the upper layer. The authors should rule out the generation of waves and the vertical propagation of energy, or at least highlight it as a possibility.
5. The authors should be clearer on what was observed and what is assumed throughout. For example, Figure 6 and calculations related to Equation 8, how do you validate using the buoyancy frequency on Jan 15 when elsewhere in the manuscript you emphasize that the front location is moving and hence the stratification is changing? This section (lines 316 to 321) needs further explanation and validation.
You must also explain how you are estimating mixed layer depth and what you mean by setting it at 24 m (line 322). You are also stating it is above 20 m, and so it sounds like you are contradicting yourself. Then on line 327 you state the upper layer as 50 m, so is it 24 m or not?
6. In section 4.3 and 4.4, analysis techniques are being used but not why? What is the purpose? What exactly are you trying to understand by using these techniques? Given you lack stratification data and must use the shipboard data as best you can, why go to these lengths? If you could briefly state that before introducing the techniques, it would assist the reader in understanding where the analysis is going. As it is, I am unsure of the purpose, and it makes it harder to follow. The modal analysis in particular seems redundent. This point also relates to major comment 3. After emphasizing what the novel contribution of this study is, please re-assess how each component of your analysis contributes.
Similarly, what are the limitations of your analysis techniques? What uncertainties do you have in your analysis?
For example, on lines 378-381 I don’t agree with this argument. It has not been established that NIOs are generated at the point of peak wind stress. What reason is there to explain that propagation could not occur from other regions? Is the slab model even valid for your circumstances, with a nearby coastline, relatively shallow depth, and the possibility of non-geostrophic currents associated with the front?
Section 4.5. The authors seem to be placing a significant amount of focus on Figure 7 and the differences between observed and simulated NIO velocities using the slab model, but the difference is only ~0.01 m s-1. NIO velocities were obtained using band-passing, with potential interference from other nearby frequencies. Have the authors considered the accuracy of the band-passing? For such a small difference in velocities, are you sure it is ‘real’ or significant?
7. Lines 392-398, I am not convinced. Firstly, the authors need to clearly explain their reasoning. How do you explain that the ‘contribution of gradient of cross-shelf current term is larger than the shear terms’? How does Figure 8 show this? Again, you must clearly explain your working and be explicit. A large number of assumptions is going into Figure 8 and this reasoning and you must be clear about the impact these assumptions may have on your interpretation. It should be immediately clear to the reader which sections of the manuscript are from observations and what is being approximated.
8. Line 432, do you really have the evidence to conclude this? Is there not other explanations you can explore? Can you estimate vorticity yourself? Have you considered the impact of stratification, the Richardson number, and the minimum wave frequency (see Schlosser et al. 2019 for example)?
Minor comments:
1. Is the frequency super- or sub-inertial on line 39? What about the so-called ‘new signal’ in the prior line?
2. Line 42, I do not think this statement is correct. The wind forcing still has a strong control on the NIO amplitude (again see Schlosser et al., 2019 and cited references).
3. Line 48 and 50, what do you mean by ‘the front’? I think you mean just ‘fronts’. Perhaps clarify you mean sub-mesoscale fronts when first introduced?
4. Line 54, what do you mean by ‘NIOs originates from the front’? Do you mean there is energy exchange from sub-mesoscale fronts to NIOs? This needs to be explained in more detail.
5. Line 63, the way it is written, it sounds like water mixing in winter induces NIOs, but I think you mean the other way around? Please clarify. In general, this paragraph was well written and was much easier to follow.
6. Figure 1, please rewrite so you first introduce along-shelf direction, and then define the symbol in brackets. Essentially, you should rarely begin a sentence with a symbol. Is the thermocline really the thermocline, and not the location of the front?
7. Line 82, what do you mean by ‘partition of the mixed layer’? Do you mean you use this data to define the mixed layer depth?
8. Line 94, please me more specific about using the ADCP data at 8 m carefully. Do you mean the observations were carefully quality controlled?
9. Line 114, has the inertial frequency been previously given? If not been supply here. Also please be consistent in your units. Please convert all to d-1 so they can be easily compared.
10. Line 135, CMEMS data must be correctly referenced. There should be a unique DOI for each data product. Check CMEMS for how to reference and if acknowledgements are needed.
11. Line 142, also correctly reference ocean color data.
12. Line 156, delete ‘evident’ here. Just state there are peaks at specific frequencies. Similarly on line 178, remove ‘one can see’ and just state there are four peaks. Line 202, remove ‘It looks like’ and ‘We notice that’. Other examples throughout the manuscript.
13. Line 159, you say barotropic current here but the figure says ‘total’. Please make consistent as ‘total’ suggests barotropic and baroclinic. Also what does ‘primary component’ mean here? Are you suggesting the barotropic component is mostly composed of tidal energy? Please clarify.
14. Line 162, should this be a new paragraph with a single line? Perhaps join with earlier paragraph.
15. Figure 2, please clarify if this figure shows barotropic or baroclinic currents.
16. Line 182, what do you mean by ‘show a different increase’? Are you saying the magnitudes are different. Please clarify.
17. The authors should consider whether near-inertial energy may carry over between wind events in their analysis around line 207.
18. Figure 3, it is very difficult to compare the panels given the different x-axis limits. At the minimum, panels a and d should be the same. Can dashed lines be added to panel a or d to show the x-axis limits of panels b and c?
19. Line 218, what is meant by no hydrological data? Do you mean shipboard data?
20. Line 237, if SST has decreased by 2, then no negative should be included.
21. Lines 234-240, the authors should establish the importance of Chl-a and CDOM and how it relates to the front position. Please make clearer when the front switches position relative to SP and the wind events.
22. Line 252, here the front is labelled as unstable, but on line 232 and 288 it is defined as stable. Please clarify and make sure this section is consistent. Also, how is the generation of NIOs at a front unique? Did you not cite in the introduction studies that also do this? I think you mean to emphasize that you have observations of NIO generation at different relevant positions of the front?
23. Line 325, again need to clarify and use the appropriate descriptive terms. What does ‘front occurred before E3’ mean here? If you are discussing the front moving its location, then please use these terms. Be more specific throughout in how you describe the results, while avoiding terms like ‘obvious’, ‘you can see’, etc.
24. Figure 7, here and elsewhere please be consistent with terminology. Are you going to use upper layer or mixed layer depth? Please check if there is a space in ‘wind stress’ in panel a.
25. Line 355, again need to explain. How would events being ‘too close’ mean the phase is off?
26. Line 359, was the mix layer actually 24 m or just your best guess? Do not overstate your results and be transparent on the uncertainty in your interpretation.
27. Line 364, here and throughout I would be careful how you use the word ‘unique’. I would argue that a moving front is not ‘unique’ as it must happen all the time. I don’t think emphasizing this point really contributes to your study either. Instead, I would emphasize that your dataset is relatively unique because you observed NIOs on both sides of the front and can compare and contrast.
28. Line 412, what study area? Where exactly are the waves trapped? Are you implying the coastal side of the front?
29. Line 415, ‘other area’??
30. Line 427, ’blue-shifted’? Again, use the same terminology. You explain above as frequencies shifted above or below f, but not how this relates to red or blue-shifts.
31. Line 448, how does this show the spring tide, when that is related to barotropic currents?
32. Lines 467-473, the authors need to be careful in how they present others work and their own results here. It seems like the authors are suggesting there is strong mixing where vertical shear is enhanced, but this was not observed. Be clear this is likely based on other studies. And how does this relate to SST increasing?
Citation: https://doi.org/10.5194/egusphere-2024-3909-RC3
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