Sea-ice melting processes are inferred from various summer sea-ice and upper ocean data obtained in the Ross Sea in January 1999. Using spatially (30 km) averaged continuous data, an ice concentration–water temperature plot (CT-plot) shows that the temperature at a depth of ∼7 m increases as ice concentration decreases in the ice interior region. The CT-plot is explained by a simple ice–upper ocean coupled model in which sea-ice melting is caused only by heat input through open water. The bulk heat transfer coefficient between ice and ocean (K _b ) is estimated to be 1.2 × 10⁻⁴ m s⁻¹. These findings are supported by the relationship between ice concentration and heat content in the mixed layer estimated from conductivity-temperature-depth (CTD) data. Salinity at a depth of ∼7 m, salt deficit in the mixed layer associated with sea-ice melt estimated from CTD data, and their relationships with ice concentration also suggest that melting is mainly caused by atmospheric heat input through open water and that local balance of salinity nearly hold. The time evolution of ice concentration calculated from the ice–upper ocean coupled model with the estimated K _b corresponds with that derived from the Special Sensor Microwave Imager (SSM/I), confirming the model's applicability and the value of K _b .