In order to quantitatively investigate the role of leads and sea-ice in air-mass modification, aircraft observations were conducted over the partially ice-covered Sea of Okhotsk. We investigated two cold-air outbreak events with different sea-ice concentrations. In both cases, the difference between the temperatures of surface air and the sea surface (Delta T) dropped rapidly with the accumulated fetch-width of leads up to about 35-40 km, and then decreased very slowly. The surface sensible heat flux originating from open water was about 300 W m(-2) within a few kilometres from the coast and decreased with increasing accumulated fetch-width. The sensible heat flux was about 100 W m(-2) stop on average. These results indicate that the downwind air-mass modification depends mainly on the total (accumulated) extent of open water. The total buoyancy flux (w'T-v') calculated by the joint frequency distribution method correlated very well with ice concentration. Such a relationship was not clear in the case of the moisture flux (w'q'). The ratio between rising thermals (w(+)T(v)(+)) and cold downdrafts (w(-)T(v)(-)) differed significantly between upwind and downwind regions; that is, the buoyancy flux was dominated by w(+)T(v)(+) in the developing stage of the boundary layer, while w(-)T(v)(-) also became important after the development of the boundary layer.