Formation of an aggregate scale in Arctic sea ice

Abstract

[1] The ice pack covering northern seas is a mixture of thick ridged and rafted ice, undeformed ice, and open water. Conventional Eulerian Arctic sea ice models use a plastic yield surface to characterize the constitutive behavior of the pack. An alternative is to adopt a discontinuous Lagrangian approach and explicitly model the formation of leads and pressure ridges. We use a Lagrangian ice model that consists of thousands of discrete polygonal floes 1-4 km in width. At the beginning of a simulation the ice floes are frozen together in a square domain. We apply a linearly varying wind stress that deforms the pack by stretching the viscous-elastic joints between adjacent floes. Fractures propagate along joints forming a crack pattern in the model ice pack. The crack pattern defines a system of large plates 10-100 km in width that are aggregates of many individual floes. The average size of the plates is determined by a competition between the rate of crack creation and the speed of the relaxation wave that travels outward from a newly broken joint and reduces stresses in the surrounding pack. Simulation results are used to characterize the formation of the aggregate structure and to determine how the rate of crack creation and the average area of the aggregate plates depends on tensile strength, the wind stress gradient, and the size of the individual floes. After the formation of the aggregate-scale plate structure, subsequent deformation occurs at the plate boundaries. Since the usual state of the ice pack is a state of failure, an interesting situation is created in which the initial wind-driven deformation creates the material conditions or aggregate structure under which subsequent deformation occurs.

Atomistic simulations of friction at an ice-ice interface

“…Examples include fracture of the Arctic sea ice cover [2][3][4][5][6][7][8][9][10][11][12], brittle compressive failure during interactions between natural ice features and engineered structures [13,14], and tectonic activity of ice-encrusted bodies within the outer solar system [15][16][17][18][19][20][21][22][23][24]. For most of these systems, it is the friction of ice sliding upon itself that dominates the mechanics and heat generated at the interface.…”

Effect of shear rupture on aggregate scale formation in sea ice

“…We accordingly extend the study of Hopkins et al [2004] by incorporating into the discrete element model a representation of shear rupture. We find that the modified discrete element model now reproduces the observed diamond-shaped blocks under typical wind stress patterns.…”

“…Here, we focus on the initial formation of the blocks, i.e., the fracturing of an ensemble of frozen-together floes into floe aggregates. We follow Hopkins et al [2004], who studied the fracture of a sea ice cover using a discrete element model that considered compressive and tensile failure of interfloe joints as floe move under an applied wind stress. When a crack starts to form, a relaxation wave travels outward and reduces stresses in the surrounding pack.…”

Scaling aspects of the sea-ice-drift dynamics and pack fracture

“…First of all, the plate-size distribution (number of ice pieces as a function of their dimension) follows the power law over several orders of magnitude (Matsushita, 1985;Korsnes et al, 2004) as well as the geometric proportions between the area and "coastline" of ice floes . This suggests that the plate dimension distribution in the ASIC is fractal (Hopkins et al, 2004). The deformation rate that determines the fracture pattern in the ASIC is also scale invariant (Marsan et al, 2004).…”

“…The ice-drift in the Arctic Ocean is driven, mainly, by the wind forcing (Lewis et al, 1994;Richter-Menge and Elder, 1998), and the variability of the weather pattern causes cycles of redistribution of stresses and deformations with ice breaking (Hopkins et al, 2004) followed by refreezing (Korsnes et al, 2004). On 10 February 2004, a basin-wide seaice fragmentation occurred, and it was detected in the satellite images.…”