Extensive drill hole and electromagnetic induction measurements of sea ice thickness in the Siberian and central Arctic Seas in the summers of 1993, 1995, and 1996 reveal significant interannual variability. In the Laptev Sea, minimum and maximum modal first-year ice thicknesses amounted to 1.25 and 1.85 m in 1995 and 1996, respectively. Ice thickness correlates with ice extent, which reached a record minimum in August 1995 and was well above average in 1996. These differences are explained by the strength and location of a summer cyclonic atmospheric circulation pattern affecting both ice advection and surface melt. From drifting buoys deployed in 1995 and satellite radar backscatter data, first- and second-year ice regimes are delineated. Differences in first-year ice backscatter coefficients between 1993, 1995, and 1996 are explained by differences in level ice surface roughness. The Lagrangian evolution of ice thickness between 1995 and 1996 is studied. While the shape of the thickness distribution does not change significantly, the mean (modal) ice thickness of the ice field increases from 1.80 m (1.25 m) in 1995 to 2.86 m (2.25 m) in 1996. The thickness distribution of second-year ice in 1996 closely agrees with that of level multiyear ice downstream in the Transpolar Drift obtained in 1991. In 1996, mean level ice thickness increases at 0.23 and 0.16 m deg-1 with latitude in the Kara and Laptev Sea sectors of the Arctic Ocean, respectively. Copyright 2001 by the American Geophysical Union.