The cooling effect on buoyancy-driven current is studied in the coastal embayment of Ise Bay, Japan, by applying knowledge derived from studies of convection in a rotating fluid. This convection process is highly sensitive to the presence of inhomogeneities in the preexisting stratification and to differential buoyancy loss. On freshwater input the bay water is divided into two regions that differ in their response to the cooling from the surface. One region represents a river plume, which, in effect, shields the deeper regions from the cooler surface. In the other region, convective mixing is active. Although the mixed water forms and becomes extensive in this convection region, the mixed water hardly develops in the river plume region. The subsequent differential deepening of the mixed water causes gravitational circulation and yields a mixed water lens, which has a reversed dome shape and rotates anticyclonically in a thermal wind balance. In the mixed water lends, nonvanishing stratification remains because of the rotational effect. The location of the mixed water lens and associated anticyclonic circulation determines the basin-wide circulation during cooling periods.