Net CO2 fluxes between the atmosphere and the ocean, and between the atmosphere and the terrestrial biosphere, were estimated by constraining a two-dimensional atmospheric transport model with the CO2 concentration and its Δ13C data obtained from our shipboard measurements in the western Pacific region during the period April 1984 to December 1991. The results obtained for the non-El Ni¿o and Southern Oscillation (ENSO) years during this time period showed that the ocean acted as a CO2 sink in middle latitudes of both hemispheres and a CO2 source around the equator. The results also suggested that during the 1984--1991 period there were biospheric CO2 sources in southern low and northern middle latitudes, and significant biospheric sinks in northern high latitudes. By comparing the latitudinal distributions of CO2 source/sink for ENSO years with those for non-ENSO years, it was found that a considerable amount of CO2 was released from the terrestrial biosphere in low latitudes in association with the 1986--1987 ENSO event. While the oceanic region from the equator to 30 ¿N acted as an excess CO2 sink of about 1.0 Gt C yr-1 during 1987 and 1988, the oceanic contributions to the CO2 anomaly in the atmosphere were less pronounced in the northern high latitudes and in the southern hemisphere. Also, compared with the estimated interannual fluctuations in the atmosphere-biosphere CO2 flux, the net CO2 flux between the atmosphere and the ocean showed relatively less interannual variability. ¿ 2000 American Geophysical Union