A main objective is to simulate the seasonal variations in a carbon cycle during the mixed layer development from a fall through winer over the northern North Pacific. A bulk mixed layer model is employed, which is initialized with the historical data and forced by the observed atmospheric data. The model retains the following mechanisms: cooling and freshwater flux through the sea surface, kinetic energy input due to wind stresses, and air-sea CO2 flux as well as turbulent vertical mixing of heat, salt, total CO2, and carbonate alkalinity at the bottom of the mixed layer. Total CO2, carbonate alkalinity, and pCO2 are related to each other under the assumption of chemical equilibrium. Since biological processes are secondary for the pCO2 evolution during winter, they are omitted. The evolution of pCO2 in the model mixed layer shows a rapid increase of pCO2 in the subpolar region due to mixing of deep water rich in CO2 before exceeding the atmospheric pCO2, and then the CO2 flux stays almost constant during the winter. In the subtropical region the CO2 is transported always from the atmosphere to the ocean, as the pCO2 decreases from cooling. The ocean is a CO2 sink (-0.18 GtC half-year-1) over the entire model domain north of 30 ¿N. However, in the subpolar region the ocean is a CO2 source (0.04 GtC half-year-1). The CO2 flux varies from year to year, while there is no qualitative difference in the flux distribution. ¿ 2000 American Geophysical Union