Sea ice extent from January 1973 through August 1990 was digitized from weekly operational sea ice charts produced by the Navy/NOAA Joint Ice Center. Charts were digitized by hand using a 1 degree latitude x 2.5 degrees longitude grid. The grid boxes were then summed in 1 degree latitude x 10 degrees longitude slices, and the ice covered area computed. The data represent the extent of sea ice at the end of each month, given for 36 10-degree longitudinal sectors in each hemisphere. Data were provided by the NOAA National Weather Service National Meteorological Center (NMC) Climate Analyses Center (CAC).