A large‐scale numerical model of sea ice

Work at the National Center for Atmospheric Research has resulted in the construction of a large‐scale sea ice model capable of coupling with atmospheric and oceanic models of comparable resolution. The sea ice model itself simulates the yearly cycle of ice in both the northern and the southern hemispheres. Horizontally, the resolution is approximately 200 km, while vertically the model includes four layers, ice, snow, ocean, and atmosphere. Both thermodynamic and dynamic processes are incorporated, the thermodynamics being based on energy balances at the various interfaces and the dynamics being based on the following five stresses: wind stress, water stress, Coriolis force, internal ice resistance, and the stress from the tilt of the sea surface. Although the ice within a given grid square is of uniform thickness, each square also has a variable percentage of its area assumed ice free. The model results produce a reasonable yearly cycle of sea ice thickness and extent in both the Arctic and the Antarctic. The arctic ice grows from a minimum in September, when the edge has retreated from most coastlines, to a maximum in March, when the ice has reached well into the Bering Sea, has blocked the north coast of Iceland, and has moved southward of the southernmost tip of Greenland. Maximum arctic thicknesses are close to 4 m. In the Antarctic the ice expands from a minimum in March to a maximum in late August, remaining close to the continent in the former month and extending northward of 60°S in the latter month. Maximum thicknesses are about 1.4 m. The distribution of modeled ice concentrations correctly reveals a more compact ice cover in the northern hemisphere than in the southern hemisphere. Modeled ice velocities obtain both the Beaufort Sea gyre and the Transpolar Drift Stream in the arctic summer as well as the Transpolar and East Greenland Drift streams in the winter. In the Antarctic, simulated velocities reveal predominantly westerly motion north of 58°S, with smaller‐scale cyclonic motions closer to the continent.