The 2-decade records of the partial pressure of CO2 in surface waters (pCO2sea) and the resulting air-sea CO2 flux in the extensive subtropical to equatorial area along 137¿E in the western North Pacific in winter exhibited significant interannual variations that differed in different regions. The pCO2sea varied largely in the equatorial region of 3¿N to 6¿N, depending on the oceanographic conditions related to the El Ni¿o--Southern Oscillation (ENSO) events. The magnitude of the variations differed year by year, corresponding to the different degrees of compensation between the sea surface temperature (SST) and dissolved inorganic carbon (DIC) effects on pCO2sea during the events. Small pCO2sea variations in the subtropical gyre north of 23¿N were due to highly counteracting effects between anticorrelated SST and DIC anomalies through the entrainment process. In contrast, a low negative correlation existed between SST and DIC, associated with the lateral advection in the region restricted around 15¿N to 18¿N in the North Equatorial Current, resulting in a large amplitude of variations of pCO2sea and hence CO2 influx. The interannual variations of CO2 flux depended predominantly on the difference in pCO2 between air and sea south of 18¿N, but on wind speed in the northern region. It is important to monitor how the contributions of different properties to the CO2 flux could change in the western North Pacific, responding to possible shifts of the ENSO conditions and the future progress of global warming.