Impact of Floe-Size Dependent Processes on Sea Ice Evolution

The region of sea ice between the open ocean and start of continuous sea ice cover can be extensive. In the summer, over 50% of observed Antarctic sea ice extent can be classified as belonging to this marginal ice zone (Stroeve et al., 2016). In this region of heterogeneous sea ice cover, floes vary in size from centimetres to kilometres and significant heat flux exchanges can occur between ice, ocean and atmosphere. Variation in floe size determines the importance of lateral melt (Steele, 1992), which is significant for floes up to 50km in size (Horvat et al., 2016), as well as the response of ice to mechanical stresses (Feltham, 2005). It also determines the attenuation of ocean surface waves (Doble & Bidlot, 2013) which can fracture sea ice hundreds of kilometres away from the ice edge during storms (Kohout et al., 2013) and accelerate ice retreat (Thomson & Rogers, 2014). We therefore hypothesize that floe-size dependent processes are a key factor in determining sea ice evolution during the melt season and the location of the ice edge.

The sea ice components of global climate models are currently unable to represent lateral floe sizes, describing only the distribution of ice into thickness categories (Thorndike et al., 1975, Hunke et al., 2010). As a result they represent the marginal ice zone primarily as decreasing concentration due to thermodynamic effects. Thus, to test our hypothesis, we have developed a framework to model the distribution of sea ice in lateral floe size categories for a global-scale sea model.

We adapt the mathematical formulation of a fully prognostic joint floe size and thickness distribution presented in Horvat & Tziperman (2015) for the Los Alamos sea ice model, CICE. New floe-size dependent melt and growth are integrated with the existing numerical scheme. We include a parametrization for ocean surface waves that are attenuated according to sea ice properties and determine floe break-up. Unlike previous studies (Williams et al., 2013; Tsamados et al., 2015; Dumont et al., 2011), we make no assumptions as to the shape of the floe size distribution; it is an emergent property arising from thermodynamics and wave fracture only. Preliminary results suggest that inclusion of floe-size dependent processes accelerates the rate of melt, in support of our hypothesis.

References

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