Investigation of the Source of Iceland Basin Freshening: Virtual Particle Tracking with Satellite-Derived Geostrophic Surface Velocities
Round 1
Reviewer 1 Report (Previous Reviewer 2)
Comments and Suggestions for AuthorsThe work presented is very relevant to various aspects of oceanography.
Despite the fact that, as in the introduction, they cite several works in which model outputs are analysed, in this work they reveal that with lower resolution and satellite data it is possible to explain quite well the advection processes of water masses.
It should be noted that the paper reads fluently even though one could consider it lengthy, which is correct in this case considering that it is important to highlight the importance of certain processes described in the paper. Likewise, the results and figures are well supported, giving strength to the discussion and conclusions. Finally, although it is a work that I do not consider to be conclusive, it is motivating and gives rise to further analysis of the processes that contribute to or affect the formation of water masses.
Author Response
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Author Response File: 
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Reviewer 2 Report (New Reviewer)
Comments and Suggestions for AuthorsGeneral Comments:
In this manuscript, a virtual particle tracking with satellite-derived geostrophic surface velocities is used to Investigate the source of Iceland Basin freshening. The authors asserted they had observed a distinct regime change in 2009, similar to the timing found in the previous modeling papers. These spatial shifts were accompanied by faster transit times along the pathways which led to along-stream convergence and more particles arriving to the eastern subpolar gyre. They also suggested these findings support the hypothesis that a diversion of relatively fresh Labrador Current waters eastward from the Grand Banks can explain the unprecedented freshening in the Iceland Basin. It is well written and the evaluation of the data seems robust, leading to distinct conclusions. I only have some minor conclusions, as seen below.
Detailed Comments:
1. Suggest to emphasize the significance of the study in the part of Abstract.
2. The keyword "particle tracking" appears only three times in this article, in the title, abstract, and keyword. The author should explain this concept further.
3. Introduction is too long and wide-ranging. Please focus on “virtual particle tracking with satellite-derived geostrophic surface velocities” but on “Investigation of the source of Iceland Basin freshening”.
4. In “2. Materials and Methods”, please describe virtual particle tracking more detail including including methods of particle deployment and observations.
5. For “ Results” and “Discussion”, recommend select some relative contents from introduction and integrate to sectors of results and discussion.
Author Response
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Author Response File: Author Response.docx
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsReview on “Investigation of the source of Iceland Basin freshening: virtual particle tracking with satellite-derived geostrophic surface velocities”
The manuscript investigates the pathways of water mass in the North Atlantic by using a particle tracking method with an altimetry-derived geostrophic velocity field. Although some previous work revealed the pathways in the study region by using high-resolution reanalysis products, this work is significant for the readers since the observational velocity field was used. Further, this study clarified the interannual variability of different pathways of the freshwater going into the Iceland Basin. The results would be of great importance for us to investigate the recent climate change modulated by the overturning circulation in the Atlantic Ocean. The results are shown clearly and logically. However, I think there are a few points the authors should clarify. I would suggest a minor revision for this manuscript.
Major comments:
The aim of this study is to interpret the transport of fresh water (dissolved material?) in the study area. The particle tracking, to a great extent, represents the transport of particle materials, such as biological larvae and sediments in the ocean. The diffusion/transport equation for fresh water and particles should also be different since the former contains a diffusivity term. I suggest the authors add some interpretation to overcome the mismatch between the two concepts.
Minor comments:
Figure 1a: Please note Irminger Sea since the name was mentioned in the text.
Line 225: The details of Ocean Parcels should be described. Is the random walk method included in the trajectories calculation to simulate the horizontal mixing?
Line 238: What do the “circuitous routes” mean? Are there any criteria to eliminate these particles?
Line 257: Are the 163 drifters the GDP drifters or your simulated particles? You mentioned that 161 surface drifters were chosen for verification of the particle tracking.
Figure 5b. Add a legend for the two curves.
Line 417: Figure 7 should be Figure 7a.
Figure 7a: Add a legend.
Line 551: Another shortage by using altimetry-derived velocity might be that the resolution is not sufficient to resolve mesoscale eddies in the ocean. It might be worth to note this point in section 4.2.
Author Response
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Author Response File: 
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Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper is interesting and I find the result in figure 6 a very interesting result. However, the paper is very long and boring to read. Both the introduction and the results seem to me confusing at times, so I strongly recommend to rewrite it again. I therefore recommend a major revision.
As suggestions I propose:
Not to talk about Ekman+geostrophic velocities, it generates doubts and noise, honestly simple geostrophy has been shown to work well.
Use figure 6 more, it is a very interesting and important result.
1.- As major revisions, I suggest to show if there is no difference between using 1 hour time step or every 15 min or less time. Numerical drift should be considered when integrating for such a long time. At least mention it.
2.- Section 3.2 is confusing and long, it seems to me that it is more important to count those that arrive to the polygon and those that don't (blue and red, nothing else), secondly it doesn't make much sense to focus on those that take the west route since it can be observed that it is a very shallow region where they are released and the warning data are not the most optimal for that.
3.- It is suggested to show at least the mean circulation, anomalies, to show or evidence changes in the currents, and thus give more robustness to the work.
4.- It is suggested to use other approaches such as lyapunov exponents either finite scale or finite time, to show more clearly the consistency of their results. Try methodologies such as those shown in Gough et al. 2019 or Maslo et al. 2020, doing the calculations with 2000-2010, 2010 to 2017, and 2000-2017. But first try simpler things like mean currents, anomalies etc.
5.- Show the seasons in which most arrivals occur, not just the years, it may be more important to show the seasons. In fact, if you show the average circulation for one year it may not differ much from another year, for example 2000 vs 2010, but if you evaluate the spring 2000 vs 2010, these changes may stand out.
Finally, I think it is a job well done, but I would like to see it rewritten so that the information that has been generated can have a greater impact.
Gough, M. K., Beron-Vera, F. J., Olascoaga, M. J., Sheinbaum, J., Jouanno, J., & Duran, R. (2019). Persistent Lagrangian transport patterns in the northwestern Gulf of Mexico. Journal of Physical Oceanography, 49(2), 353-367.
Maslo, A., de Souza, J. M. A. C., Andrade-Canto, F., & Outerelo, J. R. (2020). Connectivity of deep waters in the Gulf of Mexico. Journal of Marine Systems, 203, 103267.
d'Ovidio, F., Fernández, V., Hernández‐García, E., & López, C. (2004). Mixing structures in the Mediterranean Sea from finite‐size Lyapunov exponents. Geophysical Research Letters, 31(17)
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2004GL020328
Haller, G. (2015). Lagrangian coherent structures. Annual review of fluid mechanics, 47, 137-162.
Mathur, M., Haller, G., Peacock, T., Ruppert-Felsot, J. E., & Swinney, H. L. (2007). Uncovering the Lagrangian skeleton of turbulence. Physical Review Letters, 98(14), 144502.
http://georgehaller.com/reprints/annurev-fluid-010313-141322.pdf
Onu, K., Huhn, F., & Haller, G. (2015). LCS Tool: A computational platform for Lagrangian coherent structures. Journal of Computational Science, 7, 26-36.
https://www.sciencedirect.com/science/article/abs/pii/S187775031400163X
Author Response
Pls see the attachment.
Author Response File: Author Response.pdf
