Arctic sea ice data from the 1953–77 period are digitized onto a set of 300 monthly grids covering the polar cap. Each grid contains 1648 ice concentration points at a spacing of 1° latitude (60 n mi). The synthesis of the regional ice data sets is described. The digitized data are used to evaluate quantitatively the normal seasonal cycle of ice extent, the 25 year extremes for winter and summer, and the longitudinal dependence of the variance and trend of ice extent. Interannual variations of ice extent exceeding 5° latitude are found at most longitudes. The time series of total Arctic ice extent shows a statistically significant positive trend and correlates negatively with recent high-latitude temperature fluctuations. Empirical orthogonal functions of longitude are used to identify the major spatial and temporal scales of ice fluctuations within the 25-year period. The dominant spatial mode is an asymmetric mode in which the North Atlantic anomaly is opposite in sign to the anomaly over the r... Abstract Arctic sea ice data from the 1953–77 period are digitized onto a set of 300 monthly grids covering the polar cap. Each grid contains 1648 ice concentration points at a spacing of 1° latitude (60 n mi). The synthesis of the regional ice data sets is described. The digitized data are used to evaluate quantitatively the normal seasonal cycle of ice extent, the 25 year extremes for winter and summer, and the longitudinal dependence of the variance and trend of ice extent. Interannual variations of ice extent exceeding 5° latitude are found at most longitudes. The time series of total Arctic ice extent shows a statistically significant positive trend and correlates negatively with recent high-latitude temperature fluctuations. Empirical orthogonal functions of longitude are used to identify the major spatial and temporal scales of ice fluctuations within the 25-year period. The dominant spatial mode is an asymmetric mode in which the North Atlantic anomaly is opposite in sign to the anomaly over the r...