Abstract. Recent years have seen a rapid reduction in the summer Arctic sea ice extent. To both understand this trend and project the future evolution of the summer Arctic sea ice, a better understanding of the physical processes that drive the seasonal loss of sea ice is required. The marginal ice zone, here defined as regions with between 15 % and 80 % sea ice cover, is the region separating pack ice from the open ocean. Accurate modelling of this region is important to understand the dominant mechanisms involved in seasonal sea ice loss. Evolution of the marginal ice zone is determined by complex interactions between the atmosphere, sea ice, ocean, and ocean surface waves. Therefore, this region presents a significant modelling challenge. Sea ice floes span a range of sizes but sea ice models within climate models assume they adopt a constant size. Floe size influences the lateral melt rate of sea ice and momentum transfer between atmosphere, sea ice, and ocean, all important processes within the marginal ice zone. In this study, the floe size distribution is represented as a power law defined by an upper floe size cut-off, lower floe size cut-off, and power-law exponent. This distribution is also defined by a new tracer that varies in response to lateral melting, wave-induced break-up, freezing conditions, and advection. This distribution is implemented within a sea ice model coupled to a prognostic ocean mixed-layer model. We present results to show that the use of a power-law floe size distribution has a spatially and temporally dependent impact on the sea ice, in particular increasing the role of the marginal ice zone in seasonal sea ice loss. This feature is important in correcting existing biases within sea ice models. In addition, we show a much stronger model sensitivity to floe size distribution parameters than other parameters used to calculate lateral melt, justifying the focus on floe size distribution in model development. We also find that the attenuation rate of waves propagating under the sea ice cover modulates the impact of wave break-up on the floe size distribution. It is finally concluded that the model approach presented here is a flexible tool for assessing the importance of a floe size distribution in the evolution of sea ice and is a useful stepping stone for future development of floe size modelling.

The Arctic region is a rapidly changing environment, characterized by increasing rates of summer sea ice decline, rising temperatures, and lengthening open-water seasons. Arctic coastlines are considered particularly vulnerable to these changing conditions, which pose a unique threat to biological systems, human communities, and infrastructure (Forbes, 2011). Indeed the erosion rates along the Arctic coast have been accelerating (Gibbs et al., 2015, 2019; Lantuit et al., 2012), and the length of the ice-free season appears to be directly related to the higher erosion rates (Barnhart et al., 2014). While warmer temperatures are an obvious driver of erosion, in particular in areas with permafrost bluffs, mechanical processes associated with wave activity are likewise considered a leading contribution (Overeem et al., 2011). The effect of coastal sea ice in dissipating large waves and decreasing the magnitude of storm surges is reduced as the open water season lengthens and extends further into the autumn period of increased storminess in the Alaskan Arctic

Arctic coastlines experience rapid rates of erosion, up to tens of meters per year Jones et al., 2009). The mean retreat rate for coastlines throughout the Arctic is 0.5 m/yr (Lantuit et al., 2012), with the highest rates reported in the Laptev (Günther et al., 2013;Nielsen et al., 2020) and Beaufort Seas (Gibbs et al., 2015;Obu et al., 2017). The ice-rich soils are particularly sensitive to thermal niching by seawater at the coastal interface, a process which promotes failure of large blocks of ground along ice wedges (Aré, 1988a(Aré, , 1988bHequette & Barnes, 1990;Günther et al., 2015). Incident wave energy and storm surges are considered dominant factors influencing the erosion rate, as these carry seawater into contact with ice-rich soils and mobilize nearshore sediments (Wobus et al., 2011). In recent decades, summertime pack ice extents in the Arctic have been declining, and the length of the open-water season has been increasing (Barnhart et al., 2016;Meier et al., 2013), a trend that is projected to continue. These changes have been linked to an increase in wave climate (

Abstract. Recent years have seen a rapid reduction in the summer Arctic sea ice extent. To both understand this trend and project the future evolution of the summer Arctic sea ice, a better understanding of the physical processes that drive the seasonal loss of sea ice is required. The marginal ice zone, here defined as regions with between 15 and 80 % sea ice cover, is the region separating pack ice from open ocean. Accurate modelling of this region is important to understand the dominant mechanisms involved in seasonal sea ice loss. Evolution of the marginal ice zone is determined by complex interactions between the atmosphere, sea ice, ocean, and ocean surface waves. Therefore, this region presents a significant modelling challenge. Sea ice floes span a range of sizes but climate sea ice models assume they adopt a constant size. Floe size influences the lateral melt rate of sea ice and momentum transfer between atmosphere, sea ice, and ocean, all important processes within the marginal ice zone. In this study, the floe size distribution is represented as a truncated power law defined by three key parameters: minimum floe size, maximum floe size, and power law exponent. This distribution is implemented within a sea ice model coupled to a prognostic ocean mixed layer model. We present results to show that the use of a power law derived floe size distribution has a spatially and temporally dependent impact on the sea ice, in particular increasing the role of the marginal ice zone in seasonal sea ice loss. This feature is important in correcting existing biases within sea ice models. In addition, we show a much stronger model sensitivity to floe size distribution parameters than other parameters used to calculate lateral melt, justifying the focus on floe size distribution in model development. It is finally concluded that the model approach presented here is a flexible tool for assessing the importance of a floe size distribution in the evolution of sea ice and is suitable for applications where a simple but realistic floe size distribution model is required.