Fragmentation and melting of the seasonal sea ice cover

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Recent years have seen a rapid reduction in the summer extent of Arctic sea ice, renewing interest in the physical processes responsible for summer sea ice retreat. Sea ice components of atmosphere-ocean coupled Global Climate Models contain a simplified representation of physical processes at the edge of the sea ice cover. In particular sea ice climate models treat the floe size as a constant whereas observations show there to be a distribution of floe sizes, with significant spatial and temporal variability, especially in the seasonal and marginal ice zones. Smaller floes have an enhanced lateral melt, so providing a feedback when ocean the ocean is warm, potentially accelerating summer sea ice retreat. This PhD project will examine the impact of the observed heterogeneity in floe size on the summer melt and retreat of sea ice.The seasonal reduction of the sea ice cover is driven by a combination of mechanical floe break up, and melting at the floe edges and upper and lower surfaces. Mechanical floe breakup occurs due to brittle failure in response to heterogeneity in imposed wind stresses, confining stresses, and, especially near the ice edge, by failure in flexure in response to incoming ocean waves and tidal motions, e.g. Squire[2007]. As floes break up, the total perimeter, or edge length, of a given area of sea ice increases. The reduction in floe size affects the response of the ice cover to the incoming wave field and promotes lateral melting. The process of floe breakup is currently absent from climate models: new, physics-based model development will allow its contribution to ice-climate feedbacks, relative to surface melt, to be evaluated for the first time. This PhD project will:(1)Examine factors controlling the floe size distribution in the seasonal and marginal ice zone.Observations show that the floe size distribution(FSD)in the marginal ice zone follows a power law distribution [Toyota et al, 2011]. New evidence from ICESAT and forthcoming Marginal Ice Zone campaigns (Office of Naval Research funded Arctic campaign, 2014) will be examined. A model of FSD evolution will be developed including fragmentation and flocculation (freezing together of floes[extending Dumont et al, 2011]. At the sea ice edge in summer, the lack of floe refreezing, and rapid response to storm-driven incoming ocean waves,will allow us to diagnose the FSD from the gross variables included in existing climate models. This FSD model will be ported to the sea ice model CICE.(2)Examine lateral melt in CICE model.The CICE model will be used to examine the impact of FSD on lateral melt at floe perimeters. The model will be driven with summer melt season atmospheric forcing and used to examine solar heat distribution into surface melt of the ice (and snow),warming of the mixed layer, and lateral melt. As solar heat is absorbed in the mixed layer and as ice melt occurs, the mixed layer is expected to shallow enabling concentration of heat in the mixed layer. The sensitivity of ice mass balance (concentration and thickness) to the FSD,formulation of lateral melt, and forcing conditions will be explored.(3)Examine the impact of lateral melting in a coupled climate model The HadGEM3 climate model will be modified to include the new version of CICE, containing the FSD model, and used to examine Arctic sea ice retreat in the seasonal and marginal ice zones (e.g. Barent's Sea, Fram Strait etc) in historical simulations to identify any enhanced sea ice - climate feedback. ReferencesDumont D,A Kohout, and L Bertino (2011), A wave-based model for the marginal ice zone including a floe breaking parameterization, J.Geophys.Res.,116, doi:10.1029/2010JC006682.Squire, VA, Of ocean waves and sea-ice revisited,Cold Reg.Sci.&Tech.,49,110-133, 2007.Toyota T,C Haas, and T Tamura (2011),Size distribution and shape properties of relatively small sea-ice floes in the Antarctic marginal ice zone in later winter,Deep Sea Res. II, 58, 1182-1193.