Sizes and Shapes of Sea Ice Floes Broken by Waves–A Case Study From the East Antarctic Coast

Abstract

The floe size distribution (FSD) is an important characteristics of sea ice, influencing several physical processes that take place in the oceanic and atmospheric boundary layers under/over sea ice, as well as within sea ice itself. Through complex feedback loops involving those processes, FSD might modify the short-term and seasonal evolution of the sea ice cover, and therefore significant effort is undertaken by the scientific community to better understand FSD-related effects and to include them in sea ice models. An important part of that effort is analyzing the FSD properties and variability in different ice and forcing conditions, based on airborne and satellite imagery. In this work we analyze a very high resolution (pixel size: 0.3 m) satellite image of sea ice from a location off the East Antarctic coast (65.6°S, 101.9°E), acquired on February 16, 2019. Contrary to most previous studies, the ice floes in the image have angular, polygonal shapes and a narrow size distribution. We show that the observed FSD can be represented as a weighted sum of two probability distributions, a Gaussian and a tapered power law, with the Gaussian part clearly dominating in the size range of floes that contribute over 90% to the total sea ice surface area. Based on an analysis of the weather, wave and ice conditions in the period preceding the day in question, we discuss the most probable scenarios that led to the breakup of landfast ice into floes visible in the image. Finally, theoretical arguments backed up by a series of numerical simulations of wave propagation in sea ice performed with a scattering model based on the Matched Eigenfunction Expansion Method are used to show that the observed dominating floe size in the three different regions of the image (18, 13 and 51 m, respectively) agree with those expected as a result of wave-induced breaking of landfast ice.

Aerial observations of sea ice break-up by ship waves

“…To characterize the FSD, let's compute the number-based floe size distribution (NFSD) as it is done in most studies (Rothrock and Thorndike, 1984;Toyota et al, 2006;Herman et al, 2021). As said before, the metric we use for the floe size is the minor axis b in order to relate to a flexural break-up length.…”

“…A fragmented ice cover can also have a significantly different dynamical response to external forces, as discussed by Dumont et al (2011). Herman et al (2021) report with great detail, using high resolution satellite imagery, how waves broke up a very large ice floe into much smaller ones, and how easily they drifted and deformed in response to wind, waves and current. Floe size is also important for constraining wave propagation and attenuation.…”

“…Yet, there are no observations that directly relates the FSD to a given process in the natural environment. Most observations come from satellite or aerial imagery of Arctic and Antarctic MIZs where observable floes have an unknown history (Weeks et al, 1980;Rothrock and Thorndike, 1984;Holt and Martin, 2001;Toyota and Enomoto, 2002;Toyota et al, 2006Toyota et al, , 2011Lu et al, 2008;Herman, 2010;Alberello et al, 2019;Herman et al, 2021). After identifying the boundaries of individual ice floes, either manually or using autonomous image processing algorithms, a characteristic length scale such as the mean caliper diameter (e.g.…”

Theoretical framework for the emergent floe size distribution: the case for log-normality

“…It is unclear what would induce such a fragmentation pattern, however, as the random nature of the processes causing fracture events suggest the fragmentation process would be more random. Recent evidence from field measurements [19,9], laboratory experiments [18,31] and model simulations [16,29], further suggest that the FSD is not monotonically decreasing and instead has a clearly defined mode with two tails. The power-law is commonly used throughout science to describe heavy-tailed empirical relationships between quantities and statistical distributions [32], but upon close inspection few only seem to be statistically justified [46], while a good non-monotonic alternative is the lognormal distribution [7,32].…”

WIFF1.0: A hybrid machine-learning-based parameterization of Wave-Induced sea-ice Floe Fracture

“…Heuristic parameterizations have been developed to relate bulk properties of the ocean surface wave field to the FSD -generally assuming that fractured floes follow a power-law distribution (Williams et al, 2013;Zhang et al, 2015;Bateson et al, 2020). Yet there is conflicting evidence about whether power-law FSDs are observed in nature (Herman, 2013;Stern et al, 2018a, b;Horvat et al, 2019), and whether power-law size distributions are generated by the process of wave-induced floe fracture (Horvat et al, 2016;Herman et al, 2021).…”