We analyze model results from a regional coupled ice/ocean model forced with NCEP Reanalysis geostrophic winds and surface air temperatures over the 56-year period 1948 - 2003. Ice concentration and velocity observations are assimilated to improve the ice thickness estimates. Two prominent maxima in the mean ice thickness occurred during the period, one in 1966 and the other in 1987. Since 1987 the basin-wide thickness has thinned by 1.24 m or 41%. The thinning was greatest along the coast in the sector from the Chukchi Sea to the Beaufort Sea to Greenland.

Our analysis of the model forcings and model results show that: 1) the winter air temperature has gradually warmed over the entire 56-year period, leading to a reduced equilibrium ice thickness; 2) a sudden temporary shift in two principle atmospheric indexes, the Arctic Oscillation and the Pacific Decadal Oscillation, caused a flushing of some of the older and thicker ice and caused the creation of increased summer open water by shifting the strength and the location of the Beaufort Gyre; 3) the increasing amounts of summer open water allowed for greatly increasing absorbtion of solar radiative fluxes; 4) the large extents of summer open water caused thinner first-year ice to be created because of the additional heat absorbed by the ocean during the summer; and finally 5) the thinner first-year ice is often entirely melted by the end of the subsequent summer. We find that thermodynamic changes related to the ice-albedo feedback dominated the thinning processes.

This activity is part of the Arctic Sea Ice-Ocean Reanalysis (ASOR) project which seeks to determine the historical states of the ice/ocean system using a range of modeling and data assimilation methods.