the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jan 2026
Technical note: Heat-flash travel just above a deep Mediterranean seafloor
Abstract. The deep sea is weakly stratified in density but shows considerable variations in turbulent motions in all three directions. When registered by moored high-resolution temperature ‘T’-sensors, the motions cause variations of 0.01 °C or less and in time of minutes or less, which is much faster than hours or longer of internal waves. Occasionally, T-sensors close to the seafloor register minute-long flashes of 0.0005–0.001 °C warmer than the environment. When singular, such flashes may be artefacts. However, in a large mooring-array with 45 vertical lines at 9.5-m horizontal distances, near-seafloor heat flashes are seen to travel, most likely with internal-wave instabilities in overlying stratified waters. The instabilities seem to release the flashes from a geothermally heated seafloor of which turbulence convection is suppressed by warmer waters from above. The forms and turbulence intensity of these rare signals are compared with those induced by a Remotely Operated Vehicle working near the array. Other causes like unidentified marine mammal passing are hypothesized.
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RC1: 'Comment on egusphere-2026-195', Anonymous Referee #1, 08 Apr 2026
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AC1: 'Reply on RC1', Hans van Haren, 21 Apr 2026
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>>>I thank the reviewer for the time to comment my manuscript. Replies are behind >>>
Review of the manuscript
Heat-flash travel just above a deep Mediterranean seafloor by Hans van Haren
MS No.: egusphere-2026-195
MS type: Technical noteThe author presents an analysis of temperature data collected with a massive 3D array comprising 2,950 sensors located in the deep waters of the northwestern Mediterranean, in proximity to the KM3NeT/ORCA neutrino telescope. The main objective of the analysis is to discuss some temperature anomalies occasionally observed in adjacent sensor recordings, both in vertical and horizontal directions, and to try to identify their origin(s). These anomalies last a few minutes (1 to about 7 minutes) and have a magnitude of 0.0005-0.001 °C, difficult to appreciate by the sensors on the market and for this reason sensors specifically developed at NIOZ are used.
>>>Yes, that is correctly summarized, thank you.
The experimental approach is not very well described, although it is already extensively described in a previous paper (van Haren et al., 2021, DOI: 10.1175/JTECH-D-21-0045.1).
>>>That paper is referenced.
Data processing procedures are described only superficially or are completely absent. They are scattered throughout the sections, including an appendix. The one in the appendix is based on mathematical models that are discussed in a paper with submitted status, therefore not verifiable and which is not even listed in the reference list.
>>>No problem to list that in the references, but the appendix is removed now as it is not of primary importance here.
Despite the performance claims of the sensors used, it has not been evaluated how the post-processing performed impacts the final dataset and whether it is ultimately suitable for the specific application.
>>>Unclear what the reviewer means. I disagree that the post-processing is not adequate for the presented analysis. I wonder on what argumentation the reviewer bases this comment. The signals described in this paper are all well extending above instrumental noise level, as is evident from the presented figures.
In conclusion, the arguments presented and the final goal of the manuscript are weakened by the lack of clarity regarding the robustness of the data used and the methods applied.
>>>I am sorry but I disagree. The data are robust, as well as the methods applied. The reviewer should better indicate where and what he/she expects flaws are, rather than providing some general wording.
The reluctance to share data, which would also be expected from the journal's data policy, does not contribute to instilling reader confidence in the findings of this work.
>>>It is not a reluctance to share data, but it is impossble to share the rawdata from which each analysis is (differently) performed. These are genuine raw data in non-engineering units from custom made instrumentation, and therefore uite different from ‘raw’ data from off the shelf instruments. Current meter and CTD data are made available already, as well as the movie.
In light of the above, I believe that the manuscript in its current form is not suitable for publication in Ocean Science.
My additional specific comments are listed below.
Major Points
1) The study area is briefly introduced in the “Introduction”, but a hydrodynamic overview of the site is missing. It is located near to the area of Open-Ocean Deep Convection and in the path of the strong Liguro-Provençal current. It would be useful to add this information and comment on the analysis results accordingly.
>>>Fair enough to add such, noting that deep convection has not been observed during the 20 months of data taking. Local water flow amplitudes never exceeded 0.1 m/s, and have little effect on the described phenomena.
2) Materials and Methods. Since the experimental setup has already been described in depth by van Haren et al. (2021, DOI: 10.1175/JTECH-D-21-0045.1) I feel it is unnecessary to describe it again here, and I would find it more useful to focus this section on the data and the procedures applied for post-processing. However, I point out below some points that are not clear in this version.
>>>Correct, I agree, that is why that paper has been referenced, but some general info is needed to guide the reader. Although specific information was given on the post-processing procedures, more information is added now.
- Line 61: The “eighteen steel tubes 12 m long and 0.61 m in diameter are not identified in Figure 1.>>>Provided in the caption now, with ’12 m’ indicated in the figure. This was information is given in van Haren et al (2021).
- Lines 65-67: I can't visualize the eight small rings and the imaginary (?) intersects.>>>This is also treated in van Haren et al (2021). The small rings are visible in Fig. 1. Numbers were explicitly given in the figure caption to guide the reader.
- A 3D diagram representing the system in its final position would help the reader visualize it better.3) In the text, temperature is generally referred to, but in the figures Conservative Temperature is represented and reference is made to (IOC et al., 2010) in the caption of Fig. 1.
>>>Such a diagram is given in Fig. 55d of van Haren et al. (2021). I would have expected the quasi-3D cube of Fig. 2 representing it reasonably well. This is better explained now. A sentence is added referring to the use of ‘temperature’ in short for ‘Conservative Temperature’.
The Materials and Methods section does not provide information on how this property was calculated in the absence of salinity measurements contemporaneous with the temperature measurements.
>>>This is indicated now, using value from the CTD profile observations. Salinity does not vary much to affect Theta.
What kind of approximation was used for salinity, and what is the order of magnitude of the error it introduces on the Conservative Temperature value? Is it small enough to allow us to appreciate anomalies of the order of magnitude discussed in this work? What would have changed if in situ temperatures had been used instead?>>>See above, salinity data from the CTD profile are used, as indicated now (also in Fig. 1). The error is small: to change one decimal of 0.0001 degr C one has to modify AS by 0.005 g/kg. In situ temperature cannot be used, as the dynamical impact of the adiabatic lapse rate (0.00017 degr C/m) is much larger. This is indicated now.
4) I'm not entirely clear about the electronic drift correction (lines 87-91). This procedure needs to be better described.
>>>Although I am not sure what the reviewer means, the sentences have been clarified after careful reading.
Specifically, are the vertical profiles perhaps the 65 simultaneous measurements acquired by the sensors installed on a single vertical mooring line? What exactly is the smooth polynomial used and how does it work? What is “an arbitrary single mean value”, what is its value?>>>Yes, that is correct. The reader is referred to extensive descriptions of this method in van Haren and Gostiaux (2012) and van Haren (2018). In this case a third order polynomial was used, after fitting to data from a truely homogeneous period. The value is 12.825 degr C from the CTD-observations, as indicated and shown now.
5) It's unclear whether the depth correction mentioned in Appendix A was applied systematically to all mooring lines, and if so, how. However, this is a topic that should be covered in the Materials and Methods section. Please also note that this part of the post-processing is based on mathematical models that are discussed in a paper with a submitted status and which is not verifiable and is not even listed in the reference list.
>>>Fair enough to list it, but it is still not published and under submission elsewhere. Here it merely indicates the different heights of the lines above the seafloor, rather that correcting for it, as the described phenomena are observed at the lowest T-sensors mainly. Considering this, the Appendix is removed now.
6) Data sharing. Ocean sciences data policy mandates that data, metadata, and derived products be made publicly accessible upon publication. Author’s claim that the data is unshareable due to extensive post-processing, including manual checks, is untenable. At least post-processed data should be shared without hesitation when the steps of converting raw data into them are clearly explained in the manuscript.
>>>Post-processed can be made available once the paper is accepted for publication.
Minor Points
1) Line 51: 0.0005-0.005 day corresponds precisely to 43.2-432 s.
>>>Yes, corrected now.
2) Line 56: I would add the reference (Van Haren, 2018), or reference to another related work, after 'NIOZ4' T-sensors.>>>OK, done now.
3) Lines 57-58: I would add a map of the area with the locations of the 'large-ring' mooring and the neutrino telescope indicated.>>>Such a map is available in van Haren et al. (2021, 2026). An extra figure is added here (as Fig. 1).
4) To contextualize the results, it would be helpful to have a figure showing the CT and SA profiles acquired during the implementation period, or derived from climatological data.>>>CT and sigma-2 profiles are given in van Haren et al. (2026). Here now, CT and AS profiles are presented as Fig. 2b,c.
5) Lines 284 and 377 – An error needs to be corrected in the year of the two references. According to the caption in Figure A1, it should be van Haren (2026b). Note that this reference is missing from the reference list.>>>The b in l.284 should be a 6. That manuscript is still under submission elsewhere. It will be put in the reference list once accepted for publication, no problem.
Citation: https://doi.org/10.5194/egusphere-2026-195-AC1
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AC1: 'Reply on RC1', Hans van Haren, 21 Apr 2026
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- 1
Review of the manuscript
Heat-flash travel just above a deep Mediterranean seafloor by Hans van Haren
MS No.: egusphere-2026-195
MS type: Technical note
The author presents an analysis of temperature data collected with a massive 3D array comprising 2,950 sensors located in the deep waters of the northwestern Mediterranean, in proximity to the KM3NeT/ORCA neutrino telescope. The main objective of the analysis is to discuss some temperature anomalies occasionally observed in adjacent sensor recordings, both in vertical and horizontal directions, and to try to identify their origin(s). These anomalies last a few minutes (1 to about 7 minutes) and have a magnitude of 0.0005-0.001 °C, difficult to appreciate by the sensors on the market and for this reason sensors specifically developed at NIOZ are used.
The experimental approach is not very well described, although it is already extensively described in a previous paper (van Haren et al., 2021, DOI: 10.1175/JTECH-D-21-0045.1).
Data processing procedures are described only superficially or are completely absent. They are scattered throughout the sections, including an appendix. The one in the appendix is based on mathematical models that are discussed in a paper with submitted status, therefore not verifiable and which is not even listed in the reference list.
Despite the performance claims of the sensors used, it has not been evaluated how the post-processing performed impacts the final dataset and whether it is ultimately suitable for the specific application.
In conclusion, the arguments presented and the final goal of the manuscript are weakened by the lack of clarity regarding the robustness of the data used and the methods applied.
The reluctance to share data, which would also be expected from the journal's data policy, does not contribute to instilling reader confidence in the findings of this work.
In light of the above, I believe that the manuscript in its current form is not suitable for publication in Ocean Science.
My additional specific comments are listed below.
Major Points
1) The study area is briefly introduced in the “Introduction”, but a hydrodynamic overview of the site is missing. It is located near to the area of Open-Ocean Deep Convection and in the path of the strong Liguro-Provençal current. It would be useful to add this information and comment on the analysis results accordingly.
2) Materials and Methods. Since the experimental setup has already been described in depth by van Haren et al. (2021, DOI: 10.1175/JTECH-D-21-0045.1) I feel it is unnecessary to describe it again here, and I would find it more useful to focus this section on the data and the procedures applied for post-processing. However, I point out below some points that are not clear in this version.
- Line 61: The “eighteen steel tubes 12 m long and 0.61 m in diameter are not identified in Figure 1.
- Lines 65-67: I can't visualize the eight small rings and the imaginary (?) intersects.
- A 3D diagram representing the system in its final position would help the reader visualize it better.
3) In the text, temperature is generally referred to, but in the figures Conservative Temperature is represented and reference is made to (IOC et al., 2010) in the caption of Fig. 1.
The Materials and Methods section does not provide information on how this property was calculated in the absence of salinity measurements contemporaneous with the temperature measurements.
What kind of approximation was used for salinity, and what is the order of magnitude of the error it introduces on the Conservative Temperature value? Is it small enough to allow us to appreciate anomalies of the order of magnitude discussed in this work? What would have changed if in situ temperatures had been used instead?
4) I'm not entirely clear about the electronic drift correction (lines 87-91). This procedure needs to be better described.
Specifically, are the vertical profiles perhaps the 65 simultaneous measurements acquired by the sensors installed on a single vertical mooring line? What exactly is the smooth polynomial used and how does it work? What is “an arbitrary single mean value”, what is its value?
5) It's unclear whether the depth correction mentioned in Appendix A was applied systematically to all mooring lines, and if so, how. However, this is a topic that should be covered in the Materials and Methods section. Please also note that this part of the post-processing is based on mathematical models that are discussed in a paper with a submitted status and which is not verifiable and is not even listed in the reference list.
6) Data sharing. Ocean sciences data policy mandates that data, metadata, and derived products be made publicly accessible upon publication. Author’s claim that the data is unshareable due to extensive post-processing, including manual checks, is untenable. At least post-processed data should be shared without hesitation when the steps of converting raw data into them are clearly explained in the manuscript.
Minor Points
1) Line 51: 0.0005-0.005 day corresponds precisely to 43.2-432 s.
2) Line 56: I would add the reference (Van Haren, 2018), or reference to another related work, after 'NIOZ4' T-sensors.
3) Lines 57-58: I would add a map of the area with the locations of the 'large-ring' mooring and the neutrino telescope indicated.
4) To contextualize the results, it would be helpful to have a figure showing the CT and SA profiles acquired during the implementation period, or derived from climatological data.
5) Lines 284 and 377 – An error needs to be corrected in the year of the two references. According to the caption in Figure A1, it should be van Haren (2026b). Note that this reference is missing from the reference list.