Linking the evolution of floe sizes to the sea-ice deformation history using SAR imagery

Abstract. Arctic sea ice is a mosaic of ice floes whose distribution, sizes, and thicknesses greatly impact the interaction of sea ice with the atmosphere and the ocean. However, we are still lacking knowledge of the physics to describe the complex interplay of ice floes that are a key characteristic of sea ice. Here, we outline a framework to characterize sea-ice deformation at the floe-scale from observational data by studying the mechanical interactions of multiple identifiable floes. We use Sentinel SAR imagery acquired during the MOSAiC expedition to map ice floes in the larger area around Polarstern and describe the floe-size distribution with floe diameters from tens of kilometers down to tens of meters. We present a new chord-based mapping of the floe-size distribution using a new numerical statistical conversion between floe chord and size distribution based on observed floe geometries. With the repeated coverage of SAR imagery, ice motion is tracked and deformation estimates between consecutive SAR images are derived. By combining both floe-size estimates and deformation rates we provide insights into how the floe composition changes in regions that were exposed to deformation. To do so, we subdivide the SAR images into 50 × 50 km boxes and study how the floe-size distribution evolves as a function of divergence rates on a daily time scale. We find clear signatures of fracturing into smaller floes for both divergent and convergent ice regimes. The observed mechanical redistribution of floe sizes is described by a power-law that steepens with increasing divergence rate. Our observational-based process study is the first to show that internal ice fracture could be a primary driver of the power-law form of the floe-size distributions. Finally, we present a parameterization for the mechanical redistribution of floe sizes in fracture events for large-scale continuum sea-ice models with an interactive subgrid-scale floe-size distribution.