Wave Attenuation Through an Arctic Marginal Ice Zone on 12 October 2015: 2. Numerical Modeling of Waves and Associated Ice Breakup

Abstract

Many processes that affect ocean surface gravity waves in sea ice give rise to attenuation rates that vary with both wave frequency and amplitude. Here we particularly test the possible effects of basal friction, scattering by ice floes, and dissipation in the ice layer due to dislocations, and ice breakup by the waves. The possible influence of these processes is evaluated in the marginal ice zone of the Beaufort Sea, where extensive wave measurements were performed. The wave data includes in situ measurements and the first kilometer‐scale map of wave heights provided by Sentinel‐1 SAR imagery on 12 October 2015, up to 400 km into the ice. We find that viscous friction at the base of an ice layer gives a dissipation rate that may be too large near the ice edge, where ice is mostly in the form of pancakes. Further into the ice, where larger floes are present, basal friction is not sufficient to account for the observed attenuation. In both regions, the observed narrow directional wave spectra are consistent with a parameterization that gives a weak effect of wave scattering by ice floes. For this particular event, with a dominant wave period around 10 s, we propose that wave attenuation is caused by ice flexure combined with basal friction that is reduced when the ice layer is not continuous. This combination gives realistic wave heights, associated with a 100–200 km wide region over which the ice is broken by waves, as observed in SAR imagery.

Overview of the Arctic Sea State and Boundary Layer Physics Program

“…Stopa et al (2018) have also determined attenuation further into the ice pack during Wave Experiment 3, using a larger domain thanks to wave heights derived from Sentinel 1 SAR imagery. The associated processes appear very different from what is found in pancake ice and is described by Boutin et al (2018) and discussed by Ardhuin et al (2018). Montiel et al (2018) further analyze wave attenuation and directional spreading during the large wave event of Wave Experiment 3.…”

Wave Attenuation Through an Arctic Marginal Ice Zone on 12 October 2015: 1. Measurement of Wave Spectra and Ice Features From Sentinel 1A

“…We do not observe larger waves deep into the ice pack so we expect that the sea ice conditions are controlling the observed wave decay and not dispersion. Using a numerical model in Part 2, we will compare various attenuation parameterizations to these observations, taking into account the complex fetch geometry and non‐stationarity of the wavefield (Ardhuin et al, ).…”

“…The difficulty is that there may still be large floes where we see no leads, and the backscatter of the image shows some differences between km. These questions will be taken up again in Part 2 (Ardhuin et al, ).…”

“…We expect the more dramatic changes in directions are effects of refraction (at the ice edge or due to a change in the ice thickness) (e.g., Shen et al, ). The overall pattern of the wave directions can be explained by the angular spreading of wave energy and can be captured by a numerical wave model without including such a refraction (see Part 2: Figure 9a, Ardhuin et al, ). Deeper into the ice Y > 600 km the ice features distort the wave directions but in general the quality subimages have wave directions propagating to the North (355°).…”

Observations of Surface Wave Dispersion in the Marginal Ice Zone

“…A change in wave number manifests itself through the shoaling and refraction, analogous to waves approaching shallow water or gradients in surface currents. While attenuation has been well studied (e.g., Stopa et al, 2018;Wadhams et al, 1988), relatively little attention has been paid to the change in wave number outside of theoretical models. Thus, data from the Arctic Sea State field experiment represent an opportunity to address this gap in the literature.…”