The dynamic-thermodynamic sea ice model of Hibler (1979) is used to simulate northern hemisphere sea ice growth, drift, and decay over a 30-year period, 1951-1980. The model is run with a daily time step and is forced by interannually varying fields of geostrophic wind and temperature-derived thermodynamic fluxes. The results include documentation of the sensitivities to the source of the thermodynamic forcing data and to the number of thickness levels in the thermodynamic formulation. The fields of ice velocity and thickness show strong seasonal as well as interannual variability. The Pacific gyre is found to be well-developed in spring and autumn but less so in winter and summer. The mean drift pattern results in thicknesses of 4-5 m offshore of northern Canada and Greenland, while winter thicknesses of ~2 m are typical of the Alaskan and Eurasian coastal waters. Annual net growth ranges from 0.1 to 0.6 m over much of the Arctic Basin and Baffin Bay to over 1.0 m in the eastern Bering Sea. Annual net melt of 0.5-1.5 m occurs in the western Bering Sean and the North Atlantic marginal ice zone. No significant temporal trends are detectable in the total mass of simulated ice. The simulated interannual fluctuations in 20¿ longitudinal sectors correlate at 0.4-0.9 with corresponding observed fluctuations during 1961-1976. The correlations are smaller when dynamics are omitted from the model. The simulated velocities show no bias but considerable scatter relative to the drift of the Arctic buoys in 1979 and 1980. An analysis of the regional mass budgets shows that the normal seasonal cycle is controlled primarily by the thermodynamic processes but that the thickness anomalies in much of the Arctic are attributable primarily to dynamic processes during winter, spring, and autumn.

Thermodynamic processes contribute more strongly to summer anomalies and to anomalies near the ice edge. The tendency for ice anomalies to be advected by the pattern of mean drift is apparent in multiseason lag correlations involving subregions of the Arctic Basin and the peripheral seas. The simulated outflows of ice mass through the Fram and Bering straits vary by factors of 2-3 in successive years. The simulated outflow in the Fram Strait is 20-30% smaller when ocean currents are omitted from the model.