During the Antarctic ice-melting season, high-resolution sea ice data were collected with the video monitoring system aboard the icebreaker Shirase along with the monitoring of temperature and salinity in the upper ocean. On the basis of these data, relationships among sea ice concentration, temperature, and salinity are investigated. In the ice interior region away from the ice-free ocean, ice concentration is negatively correlated with temperature and positively correlated with salinity for the spatially averaged data, which suggests that the local balances of heat and salt nearly hold in a bulk area. At the ice margin, ice concentration is negatively correlated with both temperature and salinity, suggesting that the local balances are overwhelmed by the effects of ice advection. The expendable bathythermograph profiles at the ice margin also show that a considerable amount of sea ice was advected into the ice-free ocean and subsequently melted there. It is pointed out that a polynya works as an ice-melting factory in summer; it absorbs solar radiation during the period of opening, and then melts the ice advected there. From a heat budget analysis and ocean structure in the melting season, we propose a simple ice-upper ocean coupled model in which sea ice melts on the bottom and lateral faces with the heat source supplied to the open water area by solar radiation. The relations among ice concentration, temperature, and salinity derived from the model are consistent with the observed relations. The analytic solution for the no lateral melting case shows that the concentration-temperature relation converges to a certain asymptotic curve with time, which explains that the temperature-concentration plot shows a similar relation for any region. Dependence of the relations among ice concentration, temperature, and salinity on the spatial scale is also discussed. ¿ 1998 American Geophysical Union