Arctic sea ice as a granular plastic

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Abstract
An important consideration in understanding sea ice mechanics is the integration of observed sea ice behavior on a floe neighborhood scale (1–10 km) into ice dynamics on a regional scale O(50 km). We investigate sea ice kinematics from October 1993 through April 1994 using relative motions from 13 drifting buoys with Global Positioning System navigation in a 20‐km array centered on the Sea Ice Mechanics Initiative ice camp, and we compare these motions to synthetic aperture radar (SAR)‐ derived ice velocities over a 100‐ by 500‐km region in the Beaufort Sea. There is excellent correspondence between the deformation of the buoy array and that from the SAR. Inferred ice dynamics from analysis of the two major northerly wind convergence events of the winter are consistent with a granular hardening plastic conceptual model for Beaufort sea ice. Under continued northerly winds the ice from the Alaskan shore to the camp failed in shear and convergence, in a progressive manner away from the coast. The continuum scale O(10 km) is an order of magnitude larger than the grain, i.e., floe, size O(1 km). The ice motion often forms aggregates of 20–200 km separated by narrow (<10 km) shear zones, similar to granular materials. At moderate forcing, i.e., wind stress multiplied by fetch, the ice appears to fail along slip lines that occur at an acute angle to each other and to the direction of the wind forcing, characteristic of a plastic material at critical state. With longer fetch the ice appears to fail in compression, perpendicular to the wind direction. Sea ice appeared to harden on a regional scale after the first event. During the second northerly wind event there was a sea ice breakout toward the west, apparently due to a lack of lateral confining stress. Our observations suggest that the ice floes advect through relatively stationary stress fields, created by the wind forcing and coastal boundaries. For example, while the SAR and advanced very high resolution radiometer images indicated the presence of the shear feature at the same geographic location for nearly a week, buoys would show shearing only for several days as they transited across the region of shear. There is a high correspondence between the major internal ice deformation events and persistent weather patterns on a 3‐ to 5‐day temporal scale. This implies that SAR data collection and analysis for regional sea ice dynamics should be consistent with the wind forcing and have a sampling of less than 3 days.