Review
Turbulent dissipation from AMAZOMIX off the Amazon shelf along internal tides paths
Fabius Kouogang et al.
The measurements include single microstructure profiles at a variety of stations inside and outside of a modelled tidal beam on the slope near the mouth of the Amazon River. There are also 25 CTD stations and ship-based ADCP. The authors have tried to examine the difference in turbulence levels in- and outside of this tidal beam. They have mainly examined their ship-based sections with a few stations per section. In my view, the sparse data makes their task very difficult. Many statements in the manuscript are unsupported by the figures: e.g., mixing is higher in the tidal beams, mixing is higher where tidal energy is higher, mixing is higher with low Ri, mixing is higher where internal tide rays cross, and so on. While it is possible these are true statements, the chosen approach has not delivered a clear result.
Perhaps a way forward is to do some more averaging and establish 5 averaging areas. These are in the tidal beam: (1) slope, (2) offshore, and (3) internal solitary wave station. And outside the tidal beam: (4) slope and (5) offshore. So rather than make sections, the authors should make scatter plots like Fig 7 but using data from these groups and outside the surface and bottom boundary layers (SBL and BBL) and see if they can find something that is actually related to the dissipation. For example, find all the stations in the beam and calculate mean epsilon. Is this higher than stations out of the beam based on the model? Maybe compare (1) and (4) and also compare (2) and (5). Compare (1, 2, 3) vs (4-5). And so on. Scatter plots could be based on observed tidal amplitudes, current speed, lateral gradients of density or velocity. This is a complicated region with a strong mean flow, fronts, eddies, strong river outflow, and strong tides. So it will be a difficult task to come up with a simple explanation based on 25 stations over a wide area. In other words, summary Fig 9 is not well supported but could be true. So there could be 5 groups of stations with sufficient averaging, arranged in a logical manner as opposed to 25 stations somehwat randomly named on 5 transects again without fully logical naming with limited statistics. I have suggested one approach above but the authors could come up with something else.
There is a lot of extraneous material in this manuscript. Basically everything in the paper that does not support Fig 9 should be eliminated from the paper. Fig 9 should be presented much earlier. Perhaps as Fig 1b. The current Fig 1b-c could be moved later.
If the manuscript emphasizes internal tide-related mixing, then the focus should likely be on mid water. BBL and SBL mixing could be related to other processes and will cloud relationships between internal tide shear/strain and dissipation. Strain = d(displacement)/dz. Shear and strain are related to turbulence as shown by Gregg (1989) and papers that follow.
In this revision, the microstructure methods have been updated nicely and closely follow community standards.
The summary compares the observed turbulence to a wide variety of measurements around the world. I would be interested in a more narrow view. There are other papers related to this AmazonMix project. Do the results in this manuscript help us to understand any of the previous finding from previous AmazonMix papers?
The appendix is 23 figures and zero text. This is one relatively major example, which leads me to believe that the first author has not received appropriate guidance from the senior authors of this manuscript. There are further examples. The senior authors should see such obvious organizational issues and get them fixed before submitting a revised manuscript to a journal. I raised a similar point in the previous review with seemingly limited success.
Figures and captions on the same page are usual.
The first lines in paragraphs on adjacent lines have an indent. Or skip a line and paragraphs can be left justified.
Maybe try running the text through some AI grammar checker. It's ok as is, but could be better.
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Comment by line
Fig 1c - tidal height of what and where?
16- reserve "significant" for statistically significant and just drop significant and your sentence is still ok
30 - there is not further mention of parameterizations in this manuscript. Delete this sentence.
68 - units: 30% not 30[space]%. Still some funny units here and there- e.g., m.s-1 Check each figure.
73 - The introduction is still a bit diffuse. There's a lot of info which seems good to know but it's unclear where the manuscript is going with all this info. I would favor a more narrow focus on this region and previous AmzonMix papers. Some more specific information on what was found in Bertrand (2021), what gaps that left, and how this manuscript fills those gaps. This can be short but it seems to be missing or maybe not clearly stated. Maybe it's something like: "The Amazon plume is highly variable with boundary currents, eddies, and internal tides. Bertrand showed something about mixing. However, he didn't really talk about internal tides. We show internal tides and nonlinear solitary waves produceda lot of mixing. So there are implications for some other things." There are other AmazonMix papers. How does this paper expand on/explain gaps in thoise papers?
75 - "Direct microstructure measurements of temperature, salinity and velocity were
conducted..." Does this mean the instrument was equipped with fast thermistors, microconductivity probes, and shear probes? And are you going to talk about all these?
Fig 1- this is really about the worst naming of CTD stations I have ever seen in decades as a scientist. It is slightly better than completely random. Some stations increase onshore and some offshore. Sometimes they do both on the same transect. The letters do not all increase northward.
If you feel the need to retain this because you are attached to the cruise naming convention and previous papers, maybe you could add a supplement with this station naming and provide something logical so that a reasonable reader can follow along without having to look at Fig 1a literally all the time. This constant work is extremely distracting and seriously detracts from the manuscript. See line 258, for example, where XLD is noted as deeper than MLD except at stations S8, S10, and S25. S8 and S10 are neighboring stations near the slope in the south and S25 is way off to the north in deep water. Every sentence requires similar parsing. This is a simple example.
Here is a longer example from the discussion: "Stations were located in the most energetic regions of IT, specifically at sites Aa, Ab, and D, covering stations S2 to S14, as
identified in previous studies (Magalhaes et al., 2016; Tchilibou et al., 2022; Assene et al., 2024). Stations S19 to S21 were
positioned in less energetic IT generation areas at site E, while stations S24 and S25 were located outside the influence of the
IT fields (site G). Stations were distributed across different areas, including the shelf (e.g., S4, S9, and S19), the shelf-break
(e.g., S3, S6, and S10), and the open ocean (e.g., S14, S24, and S20)." It's complicated and does not have to be.
105 - what was done with the pre- and post-cruise calibrations?
106 - means?
107 - lag effects- does this refer to S spikes from mismathed times of T and C?
123 - Great
151- tracer variance not energy
152-153 - unclear
161 - Gregg (2003) showed mixing is dramatcially reduced with decreasing latitude for a given internal wave energy level compared to higher latitudes.
170 - "XLD is specified as the depth where ε drops from its first minimum value." I don't understand this definition.
186 - define along-/across-shore
188 - alternately?
200 - energy is E, dissipation is epsilon
204 - "measured bathymetry from CTD-O2"- what?
206 - In this ratio the N's cancel- I don't understand this. Why N*S? Is S either S or S^2? See line 208.
225 - combined not glued. Has some ffort been made to avoid discontinuities in the profiles?
226 - there needs to be some minimal description of this model
Fig 3- What is plotted? Two selected profiles about 6 hrs apart to emphasize displacements at each station? Up- and downcasts? This is not necessarily an indication of mixing. A linear wave can displace isopycnals. Displacements can be calculated as (rho - mean rho)/(vertical density gradient) and plotted as a figure with shaded colors. Then you can also calculate strain = d(displacement)/dz which will highlight finer scales and maybe show internal wave propagation along the section
252-253- Maybe plot these on a different scale than the others.
256 - diffusivity not mixing coeff
258 - "It is important to note that the XLD is typically deeper than the MLD at all stations" but then there is no further mention of XLD or MLD in ths subsection.
Fig 4a - what do the symbols mean? Also plotting the max epsilon seemds like it will produce widely varying results. Better to use a depth- and time-mean value over some depth or density ranges.
270 - Some stations are plotted in the Appendix and some in the main text. This means after reading this sentence, I have to go to 2 widely separated figures.
Fig A5 - With a 2-day record, the frequency resolution is 1/2 cycles per day. The individual semidiurnal constituents cannot be distinguished and neither can the individual diurnal constituents. A fit of 1 cpd and 2 cpd and 4 cpd is sufficient- my previous review comment was unclear on this point.
305 - 3-5 tidal modes - are these vertical normal modes? If vertical modes, no info has been provided on the mdal decomposition.
Fig 6- S^2'' in m s-1?
Fig 7 - arrange panels better.
366 and Fig 7- At S10, turbulence appears unrelated to Ri calculated from N^2 and S^2''
Fig 8a - the entire slope appears critical. So up- or downward and offshore beams could be emanating from any location on the slope in this figure. Beams if present would be visible not in the mean velocity (because tides are oscillating) but in the variance. Dissipation appears largest in the upper 100 m where currents are strong. Possibly this is a front at S6 and S10.
413- I think there is little evidence showing elevated turbulence along ray paths. I can't see where interference/interaction between waves along these ray paths occurs.
553- "The most relevant finding of this study was the relative increase in mixing within the pycnocline layer, observed at S14 in the
open ocean, far from the IT generation sites." Agreed. This is a possible focus of the manuscript. Everything in the paper should support this statement. Everything that is unrelated to this statement can be removed.
561 - I don't understand? - "...with large-amplitude ISWs
exceeding 100 m clearly visible in satellite records..." |