We demonstrate the importance of heat entering the open water area from the atmosphere on sea ice decay in the Antarctic Ocean. The heat budget analyses, both from the European Center for Medium-Range Weather Forecasts and the in situ data, show that the net heat input at the water surface reaches 100--150 W m-2 in the active ice melting season due to large solar heating, while that at the ice surface is nearly zero because of the difference in surface albedo. Thus heat input to the ice-upper ocean system can be approximated as the product of the net heat at the water surface and the fraction of open water. Climatology data show that the total heat input to the upper ocean in the active melting season is comparable to the total latent heat required for sea ice melting in the whole Antarctic sea ice zone. The temporal variation of the heat input to the upper ocean corresponds well to the melting rate of sea ice, which is calculated from the Special Sensor Microwave Imager (SSM/I) data, in large ice extent sectors where the effect of advection is relatively small. These results suggest that melting of sea ice in the Antarctic Ocean is mostly accomplished by the heat input to the upper ocean through the open water area. On seasonal timescales the amount of heat supplied to the upper ocean is determined by the seasonal cycle of net heat input at the water surface, whereas the variability on shorter timescales and interannual differences are determined by the variation of the open water fraction. ¿ 2001 American Geophysical Union