the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Oct 2024
Turbulent erosion of a subducting intrusion in the Western Mediterranean Sea
Abstract. Frontal zones within the Western Alboran Gyre (WAG) are characterized by a density gradient resulting from the convergence of Atlantic and Mediterranean waters. Subduction along isopycnals at the WAG periphery can play a crucial role in upper ocean ventilation and influences its stratification and biogeochemical cycles. In 2019, physical parameters (comprising temperature, salinity, turbulent kinetic energy dissipation rates) and biogeochemical data (oxygen and chlorophyll-a) profiles were collected in transects along the northern edge of the WAG. Several intrusions of subducted water with elevated oxygen, chlorophyll-a and spice anomaly were identified towards the center of the anticyclone. These features had elevated kinetic energy dissipation rates on both their upper and lower boundaries. Analysis of the turbulent fluxes involving heat, salt, oxygen, and chlorophyll-a demonstrated a net flux of physical and biogeochemical properties from the intrusions to the surrounding ocean. Either the turbulent or diffusive convection mixing contributed to the observed dilution of the intrusion. Other factors (e.g., water column density stability, variability of the photic layer depth, and organic matter degradation) likely played a role in these dynamics. Enhanced comprehension of the persistence and extent of these features might lead to an improved quantitative parametrization of relevant physical and biogeochemical properties involved in subduction within the study zone.
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RC1: 'Comment on egusphere-2024-3294', Anonymous Referee #1, 03 Dec 2024
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This work discusses the turbulent dissipation rates and corresponding tracer fluxes associated with an interleaving feature observed within the Western Alboran Gyre. They used microstructure data and biochemical measurements to characterize the interleaving feature and describe how it can potentially erode within the water column. Using other parameters such as stratification and turner angle, they classify the water column and discuss possible mechanisms driving dissipation. The manuscript is well written and can contribute to further understanding the relationship between interleavings and dissipation and provide insight into the fluxes within the WAG. After further clarifying parts of the data analysis, this paper can be a good addition to the literature. Please see my comments below.
Specific comments:
In the methods sections, It was hard to follow what type of instruments were used in section 2.1. What are the profiles mentioned here? Are these UCTD profiles? I believe so after reading further, but I suggest that more information is provided about the instruments before quantifying profiles.
Further discussion about the assumption that all diffusivity values (Tt, Ks, Ko) are the same is needed. Even more, as the paper mentions, are double diffusion instabilities. What are the limitations of estimating epsilon in a DD regime and assuming a .2 gamma? Is there a relationship between elevated epsilon and shear in the data set? This last point might have been mentioned, but I could not find it; I suggest highlighting it and making it clearer. Even though Tu indicates DD, it might not be present in the area.
Lines 231-235: There seems to be an elevated Oxygen patch at 6 km, below 100 m depth. Can you elaborate a bit more about this subsurface Oxygen maximum?
Line 257-259: It was hard to follow what positive fluxes the authors are referring to here. From Figure 5, it appears that, for example, Heat flux is negative in both boundaries.
Line 324-327: The authors include an estimate of double diffusion dissipation here, but the explanation of how they did this is in the supplement material and very briefly described. The equations they use from Nagai et al. and Nakano et al are parameterizations using fine-scale values of Density ratio, but they did not discuss the limitations of these parameterizations, if the coefficients of this equation are valid in the Mediterranean Sea, or discuss other parameterizations (as mentioned in Nakano et al.). I think it is important to discuss the implication of the differences in fluxes between turbulent dissipation and DD, but if Double diffusion is included in the paper, I think there has to be more information in the paper about it, not only in the supplement material.
Technical corrections
Line 100: It might be helpful to clarify spice estimated from the TEOS-10 is known as "spiciness."
Line 192 and Figure 4: Is it supposed to be TKE dissipation? Not just TKE? Throughout the manuscript, there are several places where the word dissipation is missing, and only TKE is stated.
Line 202: I don’t understand what “ water column regimes” is referring to here
Line 221: I'm unsure what the starting sentence refers to. Can you elaborate?
Figure 7 supplement: It is hard to distinguish between the colors of Chlorofphul fluxes in this figure, when they are positive and when they are negative. The color bar is saturated.
Citation: https://doi.org/10.5194/egusphere-2024-3294-RC1 -
RC2: 'Comment on egusphere-2024-3294', Anonymous Referee #2, 19 Dec 2024
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The paper by Tesa et al presents an interesting data set of a subducting intrusion in the western mediterranean sea. Subducting intrusion play an important role in ocean ventilation and export of biogeochemical properties. The observations are interesting because they are based on high resolution hydrographic profiles and also present some turbulence profiles which allow quantification of turbulent fluxes of properties.
The paper will therefore be a valuable contribution afer some moderate revision regarding a clarification of the dynamical context and of some of the methods.
(I chose major, as revisions may be important for publication but they should be adressed quite easily I think)
The dynamical context needs further clarification, to cite the author “However, so far there has been limited research that specifically identifies occurrences of quasi-balanced subsurface vertical velocity and examines how turbulence responds to such instances.” This appears to be a motivation for the study, but the analysis is quite elusive regarding the dynamical conditions prevailing in the eddy (Rossby number, potential vorticity and symmetric instability potential, Richardson number for shear instabilities) these parameters can likely be estimated using the observations.
The definition of the subducting water is not very clear, I am not familiar with the spice concept, from your definition it is the temperature and salinity variability along isopycnals… so it mixes temperature and salinity, is it a normalized variability? can you provide an explicit expression?
Spice anomaly would then be an anomaly relative to a mean variability of salinity temperature along isopycnals, it is hard to me to relate this with subducting intrusion can you explain the relationship.
I have a few interrogations regarding fluxes and impact on dilution (see detailed comments)
I think it would be interesting to have panels of salinity and velocity in Fig.3 like in the supplementary material. In general why did you put so much stuff in supplementary instead of the main text (space restriction?)
Specific comments
L46-50 do you mean three dimensional turbulence here (~ mixing)
L104-106 Isopycnal strain, how is computed the mean density profile? Is it the mean density profile over the full section?
Figure 5 I am quite confused with the computation of the net fluxes. Do you try to get the net flux going into the intrusion, if so I can’t see a situation where two fluxes of the same sign could add up (one is necessarily exiting the layer) moreover it seems to me that computing a rate of change of properties (Delat of Flux/ intrusion width) would be more interesting as it could be more directly related to a time scale for the dilution of the intrusion properties
L320 321 “However, these diapycnal fluxes were too weak to induce a significant dilution of the intrusion, as daily fluxes (Supplementary Table 2) were orders of magnitude smaller than the mean property values within the intrusion”
It is difficult to compare fluxes and mean properties it has different units. Here again it seems to me that you should compute a flux divergence that will give you a rate of change per unit volume and then you can divide a variation of property along the intrusion to get a typical time scale of dilution of this property by turbulent fluxes
L329-330 I am not sure that solar radiation and evaporation transpiration have a significant impact below 50 m, not on salinity for instance, or if it is the case it is precisely with the help of turbulent mixing to connect surface water and intrusion, or maybe through convective instabilities but the intrusion is within stable water column…
Therefore you may rather insist on isopycnal mixing as a possible missing process to explain dilution
Citation: https://doi.org/10.5194/egusphere-2024-3294-RC2
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