A 1-D thermodynamic sea ice model, forced with North Pole Drift Station observations from 1954--91, is used to study the effect of changing atmospheric forcing on multi-year Arctic sea ice. From 1954--70, most seasons show positive trends in calculated sea ice thickness over much of the Arctic. A dip in calculated ice thickness takes place between 1971--77 over most of the Arctic. Following the North Pacific regime shift in 1976--1977, the period 1978--91 reveals large negative trends in calculated sea ice thickness in all seasons. The results indicate that an important part of the variability and trends in Arctic sea ice thickness is thermodynamically-driven. Of the total variance in multi-year sea ice thickness, 10 to 20% is explained by variations in the Arctic Oscillation and Pacific North American patterns. The multi-year ice thickness response to a positive wintertime Arctic Oscillation anomaly occurs the following summer and persists for more than a year.