Possible dynamic and thermal causes for the recent decrease in sea ice in the Arctic Basin

A dynamic-thermodynamic sea ice model with 50-km spatial and 24-hour temporal resolution was used to investigate the spatial and temporal variability of the sea ice cover and the surface energy exchange in the Arctic Basin. The sea ice cover in the model is described by three categories: flat ice, which undergoes thermodynamic growth or melting due to heat exchange with the atmosphere and ocean; ridges with fixed thickness, whose formation and destruction are determined by ice convergence, lateral melting, and prescribed bottom melting; and leads, which form by ice divergence and are subject to thermodynamic lateral melting or surface freezing. The underlying ocean is treated as a well-mixed layer with fixed depth and is heated due to absorption of solar radiation penetrating through leads and is cooled by turbulent exchange with the bottom of the sea ice. Daily surface pressure and surface-level air temperature data from NCEP for 1958-1997 together with climatic data for cloudiness, relative humidity, snow precipitation, and the heat flux from the deep ocean in the Greenland and Barents Seas are used as external forcing. The model is integrated using the method of large particles (Belotserkovskii, 1984).

The model satisfactorily reproduces the seasonal and interannual variability of the main characteristics of the sea ice, the sea ice exchange through straits, and the surface heat balance for different parts of the Arctic Basin. In particular, estimates of the difference from year-to-year between mean sea ice thickness in September show good agreement with the essential thinning of sea ice in the Canadian Basin that Rothrock et al. (1999) found. We also found that most of the decrease in sea ice thickness is caused by a decrease in ridge concentration and an increase in the area occupied by flat ice. Numerical experiments shows that only 20% of ice decrease can be explained by an increase in surface-level air temperature and sensible heat flux directed to the surface in summer.

Comparing model results for points near the North Pole and the SHEBA ice camp as well as for the Canadian and Eurasian Basins as a whole allows us to determine the main trends in the temporal variability of ice thickness and surface energy balance in these regions. The total sea ice volume in the Canadian Basin and the mean ice thickness at the SHEBA camp correlate with cyclonic and anticyclonic regimes of the wind-driven ice motion (Proshutinsky and Johnson, 1997). The sharp decrease in ice volume in the Canadian Basin after 1990 also coincides with a significant increase in the mean vorticity index for the central Arctic Ocean (Walsh et al., 1996).