Abstract
Fragmentation of the sea ice cover by ocean waves is an important mechanism impacting ice evolution. Fractured ice is more sensitive to melt, leading to a local reduction in ice concentration, facilitating wave propagation. A positive feedback loop, accelerating sea ice retreat, is then introduced. Despite recent efforts to incorporate this process and the resulting floe size distribution (FSD) into the sea ice components of global climate models (GCM), the physics governing ice breakup under wave action remains poorly understood, and its parametrisation highly simplified. We propose a two-dimensional numerical model of wave-induced sea ice breakup to estimate the FSD resulting from repeated fracture events. This model, based on linear water wave theory and viscoelastic sea ice rheology, solves for the scattering of an incoming time-harmonic wave by the ice cover and derives the corresponding strain field. Fracture occurs when the strain exceeds an empirical threshold. The geometry is then updated for the next iteration of the breakup procedure. The resulting FSD is analysed for both monochromatic and polychromatic forcings. For the latter results, FSDs obtained for discrete frequencies are combined appropriately following a prescribed wave spectrum. We find that under realistic wave forcing, lognormal FSDs emerge consistently in a large variety of model configurations. Care is taken to evaluate the statistical significance of this finding. This result contrasts with the power-law FSD behaviour often assumed by modellers. We discuss the properties of these modelled distributions, with respect to the ice rheological properties and the forcing waves. The projected output will be used to improve empirical parametrisations used to couple sea ice and ocean waves GCM components.
Highlights
We propose a two-dimensional numerical model of wave-induced sea ice breakup to estimate the floe size distribution (FSD) resulting from repeated fracture events
Even though we acknowledge that any parametric distribution is likely to be an inaccurate depiction of a real ice cover, they have the advantage of efficiently encoding the 105 informations to be exchanged between global climate models (GCM) components (Horvat and Tziperman, 2015)
405 The emergence of a lognormal FSD from repeated wave-induced breakup is the key outcome of this paper
Summary
Sea ice is a distinctive feature of both polar oceans and has a profound influence on our climate. Various models have implemented a breakup parametrisation, either to investigate the FSD (Montiel and Squire, 2017; Herman, 2017) or to evaluate the impact of its introduction on other quantities such as ice thickness or concentration (Roach et al, 2018) These parametrisations are usually based on either stress (Williams et al., 50 2017; Montiel and Squire, 2017) or strain (Kohout and Meylan, 2008; Williams et al, 2013; Horvat and Tziperman, 2015; Boutin et al, 2018) or a combination of both (Dumont et al, 2011). The authors included formulations for other FSD-rearranging processes, such as lead opening and ridging, but contrary to Horvat and Tziperman (2015) they make the FSD evolve alongside the ITD rather than considering the joint distribution They extended their approach to an implementation in the sea ice model PIOMAS (Zhang et al, 2016), proposing a functional for the participation factor. Even though we acknowledge that any parametric distribution is likely to be an inaccurate depiction of a real ice cover, they have the advantage of efficiently encoding the 105 informations to be exchanged between GCM components (Horvat and Tziperman, 2015)
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