the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Nov 2024
Buoy measurements of strong waves in ice amplitude modulation: a signature of complex physics governing waves in ice attenuation
Abstract. The Marginal Ice Zone (MIZ) forms a critical transition region between the ocean and sea ice cover as it protects the close ice further in from the effect of the steepest and most energetic open ocean waves. As waves propagate through the MIZ, they get exponentially attenuated. Unfortunately, the associated attenuation coefficient is difficult to accurately estimate and model, and there are still large uncertainties around which attenuation mechanisms dominate depending on the conditions. This makes it challenging to predict waves in ice attenuation, as well as sea ice breakup and dynamics. Here, we report in-situ observations of strongly modulated waves-in-ice amplitude, with a modulation period of around 12 hours. We show that simple explanations, such as changes in the incoming open water waves, or the effect of tides and currents and bathymetry, cannot explain for the observed modulation. Therefore, the significant wave height modulation observed in the ice most likely comes from a modulation of the waves-in-ice attenuation coefficient. To explain this, we conjecture that one or several waves-in-ice attenuation mechanisms are periodically modulated and switched on and off in the area of interest. We gather evidence that sea ice convergence and divergence is likely the factor driving this change in the waves in ice attenuation mechanisms and attenuation coefficient, for example by modulating the intensity of floe-floe interaction mechanisms.
Status: open (until 26 Dec 2024)
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RC1: 'Comment on egusphere-2024-2619', Anonymous Referee #1, 09 Dec 2024
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General comments:
This manuscript presents unique observations of ocean wave attenuation in sea ice, in which the attenuation is modulated on a 12-hour cycle. The increased attenuation coincides with sea convergence driven by tidal and/or inertial currents. The observations are clearly documented, including contextual information on sea ice and oceanographic conditions. The manuscript carefully presents and evaluates different hypotheses for the underlying mechanisms, before concluding [cautiously] that the increased attenuation arises from floe-floe interactions during ice convergence.
I find the analysis herein to be rigorous, and I am fully convinced that the observed changes in the waves are related to changes in the sea ice. That said, I have recommendations for major reorganization of the work and reframing of the results. I think the attenuation estimates should be the very first part of the results (and thereby more central to the paper). I think the wave-current analysis and “extra” hypotheses should be evaluated after the sea ice mechanism is evaluated, possibly as appendices or discussion material. It is important to retain this material and show that currents cannot explain the observed modulation, but in the present form the ‘reader-fatigue’ from this material undermines that impact of the attenuation results.
Once the attenuation estimates are more central in the paper, the results can be reframed to acknowledge that small changes in attenuation rate make big differences in wave observations over long propagation distances. Thus, the observed factor of 10 modulation in significant wave height (SWH) from 0.03 to 0.33 m arises from a mere factor of 2 modulation in the attenuation rate. In the context of prior waves-in-ice studies, this a very modest change in the attenuation rate. For example, Rogers et al 2016 (DOI:10.1002/2016JC012251) find similar changes that occur simply from differences in the shape and maturity of pancake ice floes. Other studies find that a factor of 2 change in the attenuation rate can occur between the compact edge of the marginal ice zone and the more diffuse interior (Hosekova et al 2020, DOI: 10.1029/2020JC016746). With this in mind, I disagree that with the interpretation that the convergence of the sea ice "switches on" a new mechanism related to floe-floe interactions (collisions, etc). Rather, I think that convergence of sea ice causes subtle changes in sea ice concentration and/or thickness (through volume conservation), and that causes an increase the attenuation rate. The increases might be reasonably well-described by existing parameterizations (Meylan et al, 2018, DOI: 10.1002/2018JC013776; Rogers et al 2018, NRL/MR/7320--18-9786). Those existing parameterizations have tuning parameters that are not tested here, so we cannot say whether new formulations are required.
Specific comments:
The introduction could be a bit more more careful not to overstate the ongoing buoy revolution. Certainly, more and more buoys are being developed and deployed (and this is great). Of the nine OMBs deployed for this study, only a few ended up in the analysis. We should humbly remember that works like Doble et 2006 (DOI: 10.3189/172756406781811303) deployed almost as many buoys 20 years ago, with similar capabilities.
Fig 2 shows clear the modulation of the wave spectra within the sea ice. The next logical step to compare with prior studies is to calculate the spectral attenuation rates (and then explore how this is modulated on the 12-hr cycle). In present form, that does not occur until page 31, and even then it is only a bulk attenuation rate. A spectral calculation might reveal more physics, including exploring the power laws described by Meylan et al 2018. If the goal of the paper is to show the cyclic convergence of sea ice changes wave attenuation, then please calculate the attenuation!
The work to show that currents and other non-ice mechanisms are insufficient to explain observations is very thorough, but it is almost a distraction. It's pretty clear from buoy 200913 that there is not modulation near the ice edge. So it’s not the incident wave field that is changing. In particular, the historic/statical open-water wave analysis would be better placed in an appendix.
Bottom of p 27: the statement that the CICE model used as input to the wave-current-ice ("WCI") model results reproduces the "time dynamics of the sea ice cover" is not supported in a quantitative way. Does CICE reproduce the convergence and divergence calculated from the buoys (Figure 3)? More broadly, the tone of this section is “well, the WCI model does not show the modulation, so a new mechanism must be needed”. My alternate interpretation is that the WCI model has ice damping parameters that could be tuned for convergence of sea ice (increasing concentration, thickness, or both). Figure 5 shows that sea ice concentration is very high at buoy 19648, but it is not 100%. Convergence could cause it to increase.
Bottom of p29: the literature is pretty clear that ice floes do not follow the waves in "synchronization" but rather they slide down-slope on the face of the waves and have 'added mass' that introduces phase changes. Thus, they definitely collide. See Shen et al 1987 (DOI: 10.1029/JC092iC07p07085) and also Herman et al (JGR, 2018). Also the Smith and Thomson 2020 stereo work (already cited in the intro).
Technical corrections:
The lack of line numbers in the PDF is frustrating.
The usage of a hyphenated ‘waves-in-ice’ or simply ‘waves in ice’ is not very consistent. I suggest the convention of hyphenation when the phrase is used as an adjective (e.g., “waves-in-ice physics are estimated”) and no hyphen when used as a noun (e.g., “waves in ice are measured…”)
Top of p3: it is the *gradients* in wave radiation stress that transfer momentum to the ice and water. Without gradients, the radiation stress is simply an ongoing flux of momentum (but no transfer).
Top p3: another recent fetch study is Brenner and Horvat (2024): https://doi.org/10.1029/2024JC021629
P 11: The inclusion of possible temperature modulation (and associated changes in sea ice rheology) is a good point, but evaluating with ERA5 seems like a poor match to the task. ERA5 would probably only show temperature changes if it also had modulation sea ice, which it does not. Surely the CICE model employed herein has temperatures?
Citation: https://doi.org/10.5194/egusphere-2024-2619-RC1
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