the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Aug 2024
Turbulent dissipation from AMAZOMIX off the Amazon shelf along internal tides paths
Abstract. The Amazon shelf-break is a key region of the ocean where strong internal tides (ITs) are generated, which may have a key role to play on both Climate and Ecosystem, via its vertical mixing. AMAZOMIX survey (2021) collected microstructure and hydrographic (ADCP/CTD-O2) profiles to quantify mixing, associated processes and their impact on marine ecosystems. Measurements are obtained over M2 tidal period (12 h) inside and outside of both the ITs generation sites and propagation beams, respectively at mode-1 distances (90 km and 210 km) from the shelf-break to evaluate the IT impact on mixing.
Hydrography analysis showed strong step-like characteristics (~20–40 m thick) and vertical displacements (20–60 m) triggered by ITs, as well as the signatures of high modes up to 5–6 on generation sites and IT pathways.
The results of the microstructure analysis coupled with those of the hydrography revealed important mixing associated with a competition of processes between the semidiurnal shear of ITs and the baroclinic shear of the mean current (BC). Closer to the generation sites, mixing is stronger within [10-6,10-4] W.kg-1, with a greater contribution (~65 %) from ITs shear than BC shear. It is reduced but nevertheless considerable between [10-8, 10-6] W.kg-1 along the IT pathways, owing to equal contributions from ITs and BC shear. At a distance of ~225 km, mixing was still higher within [10-7,10-6] W.kg-1 because of the increased contribution (~65 %) of ITs shear, where IT beams may intersect and interact with background circulation. Mixing in no-tidal fields was fairly minimal ([10-8,10-7] W.kg-1), owing to a minor contribution (~50.4 %) of BC shear from the North Brazil Current.
Finally, the nutrient flux estimations showed that ITs mixing could reach the surface (by a large tidal diffusivity of [10-4,10-1] m-2.s-1). This resulted in high vertical fluxes of nitrate ([10-2, 10-0] mmol N m-2.s-1) and phosphate ([10-3, 10-1] mmol P m-2.s-1), which can stimulate chlorophyll production, biodiversity and cool surface water, so influencing the whole ecosystem and climate in this river-ocean continuum region. This study provides a guide for the mixing parameterization in future numerical simulation (e.g., in physical-biogeochemical coupled models) in the Amazon region in order to include the impact of the IT turbulence on the whole ecosystem (i.e., from physics to biological production).
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RC1: 'Comment on egusphere-2024-2548', Anonymous Referee #1, 10 Sep 2024
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Review
Turbulent dissipation from AMAZOMIX off the Amazon shelf along internal tides paths
Kouogang et al.
Summary
This manuscript describes measurements of currents, hydrography, and turbulence at numerous sites near the Amazon outflow, where internal tides are also generated at the slope. The dataset appears suited to study the questions posed by the authors. Do internal tides affect the mixing? Does the mixing affect nutrient fluxes?
However, I have some concerns about the analysis listed below. Since many of the details of the analysis are not presented, it is difficult for me to evaluate what has been done. Some of the more prominent ones include: tidal decomposition of the currents, use/decomposition of shear, description/applicability of the finescale parameterizations used, and shaded color plots of SADCP velocity that include transits and times on station.
There are also presentation/stylistic problems that make the manuscript very difficult to read. Many of them are major, but also getting down to such points as the labelling of figures and sections. Some examples are listed below. It is well beyond the scope of a review to point all of these out. Furthermore, the cumulative difficulty of numerous minor things (let alone the more major ones) turns into a major distraction to the detriment of the hard work done and the science. The senior authors must make considerably more effort in this regard.
Major comments
1. SI units should be written W kg-1 not W.kg-1 and 90 km not 90km and so on.
2. 50.4% on line 29. Please state the error and adjust the significant figures accordingly.
3. line 34 - A “guide for the mixing parameterization in future numerical simulation” is mentioned. What is the guidance?
4. Section 2.2- some of the microstructure processing steps should be shown in either the main text, appendix, or supplementary material. In particular, I would like to see some examples of the spectral fits.
5. Semidiurnal currents are obtained by removing the mean current over a tidal period from the baroclinic velocities. This will include other frequencies. In fact Figure 4 has everything: 6 hrs, 12 hrs, … It would be better to make a least squares fit of sines and cosines to the various tidal periods. With short time series of 1 day, the frequency resolution is 1 cycle/day. So you could try fitting diurnal and semidiurnal, but recognizing that they are not formally distinct. Also the inertial period here is at least 5 days and so cannot be determined in your dataset. This is a key limitation that should be pointed out. At the equator every frequency just about is in the internal wave band. How or even whether the currents should be separated in frequency seems to be a major question given the limitations of the data and the desire to examine dissipation.
6. Shear. Based on the calculations for barotropic u, baroclinic u, and tidal u. I do not see why shear is different for baroclinic and tidal u. Tidal u = baroclinic u - lowpassed u. It seems like it should be almost the same as the shear in baroclinic u. I’m confused. It would be helpful to see more of the processing steps and their justifications for these steps. Perhaps there is a problem with the short time series or large tidal isopycnal displacements. I suggest plotting some profiles of u and shear for barotropic, baroclinic, and tidal u to make sure these calculations are correct. Also you could try doing these calculations on an isopycnal since it looks like the velocity structure is being heaved up and down by the internal waves. That could lead to artifacts in the calculations on depth surfaces. For example in Figure 4a1, on sigma = 26, current is negative. At 120 m, current switches sign multiple times. As far as I can tell, there are no plots of shear. Also I do not understand why only one component of the current is shown. Using both components is needed to understand propagation of internal tides either vertically or horizontally (alongshelf vs cross-shelf). Section 3.2.2 uses a lot of words that could be instead expressed concisely in a map with current vectors. Different colours could indicate different depths.
7. Dissipation is estimated using parameterizations based on shear in the tidal current. Actual dissipation depends on the total shear or strain.
8. Some more explanation of the applicability of MacKinnon-Gregg parameterization vs those based on Gregg (1989) is needed. The former is related to the limited bandwidth of internal waves (in shallow water for example), while the latter is for a typical open ocean environment away from generation sites. How does this apply to your study sites?
9. Naming of the sites and transects. Maybe these are what are used on the cruise but they should be rearranged in a logical order to help the reader. It is completely confusing as it is. Consider another system, such as A B C D … for generation sites. If there’s a transect near site A, it’s transect A. If there are 3 stations on transect A, they are sites A1 A2 A3 and counting higher offshore. Or labelled by isobath, A500, A1000, A2000… There may not be a generation site for each transect. For example lines 265-267 read: “The highest baroclinic tidal current velocities were observed (between 25-48 cm.s-1) at sites Aa and Ab along T1-T2. Whereas lower tidal velocities (< 25 cm.s-1) are found in site F along T4 (e.g., at S20 and S21) compared to OUT-ITs stations (e.g., at S24).” So I have to use the map. This could read instead as: “The highest baroclinic tidal current velocities were observed (between 25-48 cm.s-1) at sites D and E along transects D and E. Whereas lower tidal velocities (< 25 cm.s-1) are found in site A along transect A (e.g., at A2 and A3) compared to OUT-ITs stations (e.g., at site B1).”
10. Mixing is invoked as the explanation for increased nutrients. How about coastal upwelling? Where is the euphotic zone?
11. Were adjustments to the shear parameterization based on the Gregg (2003), where it was shown that internal wave interactions may lead to little mixing at the equator?
Minor comments
1. Line 107- While the bin size of the LADCP may be 8 m, its resolution will be about 50 m. Examine where the vertical wavenumber spectra fall off
2. I would recommend a native english speaker help with some of the grammar, if possible. It’s ok as is but could be much better. It will be less distracting for the reader. Then the reader will pay more attention to the science.
3. In sections 2.1 and 2.2, there is some jumping around from instrument to instrument. Please collect into sections for each instrument.
4. MacKinnon and Gregg (2003) provided validation in their paper. There needs to be some explanation of the advantages/conditions for this parameterization over those following from Gregg 1989. Gregg (2003) describe how latitude affects these mixing parameterizations. I saw no mention of this.
5. It is unclear how mixing layer depth is chosen. There is a statement about choosing a minimum but the profiles show a lot of variance.
6. It is stated the mixing layer depth is always less than the mixed layer depth. This seems like it cannot be true by definition- you need mixing to make a mixed layer. Maybe the mixed layer depth criterion is too small. How about 0.1 kg m-3?
7. Figure caption does not correspond to what is plotted. Shear is missing? Figure labelling: how about (a), (b), … instead 4.a.1? There are also faint grey lines around the figures. All the dots in the shear vs N figures are hard to see. Consider binning the results.
8. There are lots of seemingly correct facts and figures in the text but I am unclear what is the point of them. Take section 3.2.2, for example. Currents are going this way and that at various depths and locations. True. I would suggest a format for each paragraph as follows. “In this paragraph, we examine the flow patterns over the slope to show something [topic sentence]. Here are the relevant facts and skip ones not immediately related to the topic sentence. In summary, we have shown flow is in this direction here which is important for something. Something is described in the next paragraph.”
Comments by line number
19 - twelve hours is the semidiurnal period but not the M2 period = 12.42 hrs
129 - fft_length, etc. Perhaps you could just give these a symbol if they come up more than once, e.g., L or n. Otherwise skip the variable names in a manuscript.
136 - what is the high pass? and the low pass?
150 - mixing layer depth
170 - I don’t understand. Please rephrase
172 - what is H?
193 - references? Also strong internal tides propagating upward and impinging on the thermocline make ISW. No need for bottom reflections.
200- You have cross-shore measurements of N. Are those not enough to make a horizontally varying N?
207- there has to be some minimal description of amazon36 here even if it is referenced
210- sensitivity not sensibility test, although the latter could also be used
263- 3-5 not 03-05
267-272 - Shear obviously varies with N because if shear were bigger than it would have lower Ri and be unstable. It would be more instructive to consider Ri or reduced shear = S2 - 4N2
You could use some mean N or interpolated N and see what it looks like. If the text talks a lot about shear, it would be helpful to plot shear. In fact the baroclinic vs tidal shear is discussed butthere are no plots.
Fig 4- caption and figure all on same page please. Add tick marks to the x axis to show when your CTD/VMP casts took place. Also this figure seems to include transits and time on station. In Fig 4b1 there are some strange looking changes in the current. Perhaps it would be better to do your analysis station by station. Or are there artifacts of the SADCP processing going from on station to transit?
301- what sort of of mean? time, depth, etc
312-314- More explanation is needed here.
Fig 5- the tidal rays look to have shallower slope than the topography. It would be helpful to plot ray slope vs topographic slope to verify. The dissipation seems unrelated to the ray paths as far as I can tell. Perhaps it is not 2D. There appear to be 2 sources at least, which can constructively interfere. Rays may propagate at an angle to the coast.
384 - “for the first time” How sure are you of this? It would be conservative to add: to the best of our knowledge
Citation: https://doi.org/10.5194/egusphere-2024-2548-RC1
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