[Paper Review] Buoy measurements of strong waves in ice amplitude modulation: a signature of complex physics governing waves in ice attenuation

Jean Rabault, Trygve Halsne+26 more|arXiv (Cornell University)|Jan 15, 2024

Arctic and Antarctic ice dynamicsEarth and Planetary Sciences3 citations

TL;DR

This study presents in-situ observations of 12-hour modulated wave amplitude in sea ice, indicating periodic changes in wave attenuation likely driven by tidal-induced sea ice convergence and divergence. The modulation cannot be explained by open-ocean wave variability or tides alone, suggesting dynamic, periodically switching attenuation mechanisms in the Marginal Ice Zone.

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.

Motivation & Objective

• Investigate the cause of strong 12-hour amplitude modulation in waves propagating through sea ice in the Marginal Ice Zone (MIZ).
• Determine whether observed wave modulation arises from changes in incoming open-ocean waves, tides, currents, or bathymetry.
• Identify the physical mechanisms responsible for variable wave attenuation in sea ice, particularly those that may switch on and off periodically.
• Assess the role of sea ice dynamics—especially convergence and divergence—on modulating wave attenuation coefficients.
• Provide a foundation for future field campaigns to measure floe-floe interactions and attenuation dynamics in detail.

Proposed method

• Deployed oceanographic buoys (OMBs) with high temporal resolution to record wave height and ice motion in the MIZ near Svalbard.
• Analyzed in-situ wave amplitude time series to detect periodic modulation with a dominant 12-hour period.
• Ruled out external drivers such as tidal currents, bathymetry, and open-ocean wave variability through comparative analysis.
• Used satellite SAR data and wave model outputs (MFWAM) to validate buoy observations and assess spatial wave patterns.
• Applied ray-tracing and spectral wave analysis to reconstruct wave propagation and attenuation behavior.
• Evaluated the role of sea ice dynamics via correlation with tidal forcing and ice motion data to infer modulation mechanisms.

Figure 1 : Overview of the buoy deployment during the modulated significant wave height event. A total of 9 buoys are clustered in the same area at the South East of Svalbard around 2021-03-01. The overview map subfigure (left) presents the sea ice concentration (SIC, data source: internal product a

Experimental results

Research questions

• RQ1 What causes the observed 12-hour periodic modulation in wave amplitude within sea ice?
• RQ2 Can the modulation be explained by changes in incoming open-ocean waves, tides, or bathymetry?
• RQ3 Is the wave attenuation coefficient in the MIZ subject to periodic modulation, and if so, what physical processes drive it?
• RQ4 To what extent do sea ice convergence and divergence events modulate wave attenuation mechanisms?
• RQ5 How can future measurement campaigns better capture floe-floe interactions and their impact on wave energy dissipation?

Key findings

• A strong 12-hour modulation in wave amplitude was observed in sea ice, with significant wave height varying by up to 50% over the cycle.
• The modulation cannot be attributed to changes in incoming open-ocean waves, tides, currents, or bathymetry, as confirmed by multi-source data analysis.
• The observed modulation is most plausibly explained by a periodically varying wave attenuation coefficient in the ice-covered region.
• Tidal forcing is likely responsible for modulating sea ice convergence and divergence, which in turn may switch on or off floe-floe interaction mechanisms that control wave energy dissipation.
• The South-East Svalbard region is identified as a prime location for future long-term studies due to reliable ice dynamics and favorable deployment conditions.
• The study calls for future field campaigns to include enhanced buoy instrumentation to record collision statistics and response amplitude operators of ice floes.

Figure 2 : 1-dimensional wave spectrum for the 3 instruments furthest in the MIZ that are equipped with wave measurement capability (referred to as the Buoys Of Interest (BOIs) with IDs 19648, 200905, 13319), obtained during the time of the modulated SWH wave event. The instrument ID 19648 is a v202