Influence of winter pre-conditioning of Arctic sea ice due to atmospheric forcing associated with the North Atlantic Oscillation (NAO) on the reduction in summer sea ice extent is studied. It is found that winter sea ice is about 50 cm thinner in high-NAO index years than in low-NAO index years in the Eurasian coastal region mainly due to stronger wind-driven ice export. The thinner wintertime ice combined with strengthened southerlies in spring promotes an earlier break-up of the ice pack in the Eurasian coastal region, resulting in significant sea ice export. The higher ice efflux, in turn, further reduces the ice compactness, thus more solar radiation is absorbed by the oceans which enhances the summer melting process. Thus, winter and spring atmospheric anomalies associated with the positive phase of the NAO may underlie the reduction of summer sea ice extent observed during the 1980s and 1990s.