Modeling a Variable Thickness Sea Ice Cover

Abstract

A numerical framework suitable for simulating a variable thickness sea ice cover over a seasonal cycle is presented. This framework is largely based on the ice thickness distribution model developed by Thorndike et al. (1975). However, the numerical scheme is more general and certain additional developments are included. Namely, a fixed depth mixed-layer formulation including open water heat absorption and lateral melting terms is added, and a mechanical redistribution function consistent with hypothesized and observed physics of the ridging process is proposed. The numerical scheme is formulated in a fixed Eulerian grid and allows an arbitrary number of irregularly spaced thickness levels to be considered. Using this numerical framework in conjunction with a previously developed dynamical model (Hibler, 1979) and a thermodynamic model similar to that of Semtner (1975), a seasonal equilibrium simulation of the Arctic Basin ice cover is carried out. This simulation is performed by doing a 5-year numerical integration at 1-day time steps. Shorter sensitivity simulations are also carried out using the fourth year equilibrium simulation results as initial conditions. Input fields include mean monthly air temperatures and dew points for the heat budget computations together with time-varying observed winds over a 1-year period. The equilibrium simulation produces realistic geographical ice thickness variations with April ice thicknesses in excess of 7 m along the Canadian Archipelago and thicknesses of ∼2 m along the Alaskan North Slope. These spatial variations are in good agreement with submarine sonar estimates. In summer a substantial ice-free region forms off the Alaskan and Siberian coasts. While the geographical shape of this edge is in agreement with observations, the amounts of open water (750 km off the Alaskan North Slope in mid August) were excessive. However, only a small (10%) change in the ice albedo under melting conditions yields a much more realistic ice edge in a sensitivity simulation. Geographical changes in the ice thickness characteristics took about 3 years to fully develop. This evolution included the formation of a “shear zone” of higher annual ridge production ∼400 km off the Canadian Archipelago in agreement with ice roughness observations. Simulated mass balance characteristics are significantly affected by ice deformation and ridging. Over 40% of the basin-averaged ice production for January is due to growth over thin ice (<1 m) largely produced by deformation. Over an annual cycle an equivalent thickness of more than 0.5 m of ice is transferred by ridging to categories thicker than 5.17 m. Spatial variations in ridging and open water creation cause a net annual ice ablation of over 1 m of ice off the Canadian Archipelago concomitant with a net annual growth of over 1.8 m off the Alaskan coast. Ice velocity fields were realistic in shape but ∼25% too large in magnitude when compared to net ice station drift over a year. This velocity discrepancy is probably due to rather small central basin ice strengths (∼1.5×104 N m−1 in January). However, increasing the frictional losses assumed in the ridging process by an order of magnitude yields unrealistically small ice velocities in a sensitivity study and results in an almost total stoppage of ice flow in April and May. The sensitivity simulation also has a reduced average annual ice export rate of ∼0.04 Sv as compared to ∼0.09 Sv for the equilibrium simulation.