|This substantial revision now presents many additional results that bring added insight to interpret cable voltages. The authors have taken much effort to rework their presentation, and it shows in a strong article. I have a few minor comments that would make a stronger article still.|
I have 5 substantive comments:
(1) I urge the authors to explicitly state that ocean state estimates like ECCO do not represent with 100% fidelity the true ocean that was sampled by the cables. Additional source of error include atmospheric and magnetospheric noise which is not modelled, and might not be fully removed from the data. This is why I recommended earlier a perfect model experiment, to avoid such issues. Their chosen approach to present their results does stand on its own, but now the correlations should serve as a lower bound.
I am not aware of published results that analyze how global ocean models compare with direct velocity measurements - mostly because negative results are not published. One recent article does discuss this in passing, however. See Szuts et al., 2019.
With minor effort they could add more insight into their ECCO-based predictions: calculate the correlation between the estimated cable voltage and the net transport across the cable. With entirely numerical results, there are no sources of noise to decrease this correlation, and so such a correlation could be considered an upper bound to what is achievable with measurements.
(2) line 50: Be careful: before making such a bold claim in an interdisciplinary field a thourough literature search is required. I found a half dozen articles that already cover this topic, most of which cite publications listed in their bibliography. I'm also including a number of references that treat short submarine cables or related topics, for further background.
(3) The discussion would be more complete and thourough by adding a few sentences that discuss how the middle and bottom subplots of Figures 4, 5, and 6 agree or disagree.
(4) For an instantaneous process like motional induction, lagged correlation analysis does not hae any useful interpretation to my knowledge, which suggests removing figure 7. What would be useful, however, would be to provide confidence limits on the correlations (at zero lag). This would give insight into how significant such comparisons are, though my previous comment about the low fidelity of ECCO circulation to the real ocean means that these correlations are lower bounds. In doing these statistics, don't forget that the time-series are red and thus their degrees of freedom are much reduced than the number of data points in the time-series. So, divide the duration of the time-series by the separation of the splines's knot separation (e.g. 30.5 days).
(5) In terms of the correlation values, I find it hard to believe that the HAW1S and HAW1N time-series, which are very similar, have correlations as different as 0.23 and 0.04. Have you de-trending all three time-series prior to calculating correlations? Unremoved trends could easily account for the difference in their correlations.
line 6: "numerical predictions of the electric field induced by ocean circulation" doesn't make clear that the ocean circulation itself is an estimate.
line 7-8: "correlation between cable voltage data and numerical predictions strongly depends on both the strength and coherence of the velocities flowing across the cable," surely the correlation also depends on whether the numerical models accurately reproduce the true ocean signals (velocity and the resulting EM fields) sampled by the cable? Unless you can prove that the ocean models accurately predict ocean velocities - and there are far too few velocity observations to make such an estimate - then lack of correlation between cables voltages and numerical predictions is merely a representation issue that prevents answered the research question posed.
line 19-20: To my knowledge, Faraday's experiment was inconclusive and "unsuccessful"; "not very successful" suggests some level of sucess.
Figures 4, 5, and 6: These color contour plots really make my eyes hurt. Especially for the sake of color-blind individuals and black-and-white printouts, please use a simple 2-toned colormap like in Figure 3 instead. I find I can extend the dynamic range of colors by transitioning from red/blue to black at the end of the positive/negative range, which does not hinder the ability to visually separate positive regions from negative regions. See Light and Bartlein (2004), and updates to this (e.g. https://betterfigures.org/2014/11/18/end-of-the-rainbow/ )
Figures 4, 5, and 6: How do you calculate transport? In oceanography, transport is volume per time, or m3/s. The legend for the bottom subplot says m2/s, however, which is transport per unit width. Please make consistent.
Figure 4: Can you explain the relation between the voltage and transport plots in a little more detail? why there is strong transport from 250 km to 600 km, but instead the voltage induced in the cable is only positive around 150-250 km and 250-540 km? Why is the negative transport from 700-1200 km not reciprocated in voltage? Why does the weak positive transport from 0-150 km have a negative induced voltage?
Figure 4: For the top subplot, the caption refers to a brown line, but none is visible.
Figures 5 and 6: There is closer correspondance betwen the transport and voltage in these two examples compared with Figure 4. I would be interested to know why.
Figure 7: Unless you discuss the lagged correlations, this figure is unnecessary.
Szuts ZB, Bower AS, Donohue KA, Girton JB, Hummon JM, Katsumata K, Lumpkin R, Ortner PB, Phillips HE, Rossby HT, Shay LK, Sun C and Todd RE (2019) The Scientific and Societal Uses of Global Measurements of Subsurface Velocity. Front. Mar. Sci. 6:358. doi: 10.3389/fmars.2019.00358
Light, A., and P. J. Bartlein (2004), The end of the rainbow? Color schemes for improved data graphics, Eos, 85(40), doi:10.1029/2004EO400002. Published on 5 October 2004.
Articles discussing ocean circulation and voltages from long cables:
 A. CHAVE, D. LUTHER, L. LANZEROTTI, and L. MEDFORD. GEOELECTRIC FIELD- MEASUREMENTS ON A PLANETARY SCALE - OCEANOGRAPHIC AND GEOPHYSICAL AP- PLICATIONS. GEOPHYSICAL RESEARCH LETTERS, 19(13):1411–1414, JUL 6 1992.
 A. Flosadottir, J. Larsen, and J. Smith. Motional induction in North Atlantic circulation models. JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS, 102(C5):10353–10372, MAY 15 1997.
 A. Flosadottir, J. Larsen, and J. Smith. The relation of seafloor voltages to ocean transports in North Atlantic circulation models: Model results and practical considerations for transport monitoring. JOUR- NAL OF PHYSICAL OCEANOGRAPHY, 27(8):1547–1565, AUG 1997.
 I. Fujii and A. Chave. Motional induction effect on the planetary-scale geoelectric potential in the eastern North Pacific. JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS, 104(C1):1343–1359, JAN 15 1999.
 K. Kim, Y. B. Kim, J. J. Park, S. Nam, K.-A. Park, and K.-I. Chang. Long-term and Real-time Monitoring System of the East/Japan Sea. OCEAN SCIENCE JOURNAL, 40(1):25–44, MAR 2005.
 K. Kim, S. Lyu, Y. Kim, B. Choi, K. Taira, H. Perkins, W. Teague, and J. Book. Monitoring volume transport through measurement of cable voltage across the Korea Strait. JOURNAL OF ATMO- SPHERIC AND OCEANIC TECHNOLOGY, 21(4):671–682, APR 2004.
 T. Minami. Motional Induction by Tsunamis and Ocean Tides: 10 Years of Progress. SURVEYS IN GEOPHYSICS, 38(5, SI):1097–1132, SEP 2017.
 J. A. U. Nilsson, P. Sigray, and R. H. Tyler. Geoelectric monitoring of wind-driven barotropic transports in the Baltic sea. JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY, 24(9):1655–1664, SEP 2007.
 N. Pal’shin, L. Vanyan, R. Medzhitov, G. Shapiro, M. Evdoshenko, H. Utada, H. Shimizu, and Y. Tanaka. Use of the Nakhodka-Naoetsu submarine cable for studying the temporal variability of the integral water transport in the Sea of Japan. OCEANOLOGY, 41(3):447–453, MAY-JUN 2001.
 O. Pankratov, D. Avdeev, A. Kuvshinov, V. Shneyer, and I. Trofimov. Numerically modelling the ratio of cross-strait voltage to water transport for the Bering Strait. EARTH PLANETS AND SPACE, 50(2):165–169, 1998.
 F. Santos, A. Soares, L. Trindade, R. Nolasco, H. Rodrigues, and I.-D. team. Voltage measurements over the CAM-1 submarine cable between Madeira Island and Portugal mainland. EARTH PLANETS AND SPACE, 54(4):393–398, 2002.
 P. Sigray, P. Lundberg, and K. Doos. Observations of transport variability in the Baltic Sea by par- asitic use of a fiber-optic cable. JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY, 21(7):1112–1120, JUL 2004.
 D. THOMSON, L. LANZEROTTI, C. MACLENNAN, and L. MEDFORD. OCEAN CABLE MEA- SUREMENTS OF THE TSUNAMI SIGNAL FROM THE 1992 CAPE-MENDOCINO EARTH- QUAKE. PURE AND APPLIED GEOPHYSICS, 144(3-4):427–440, AUG 1995.