Latitudinal distribution of atmospheric CO₂ sources and sinks inferred by δ¹³C measurements from 1985 to 1991

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 Niño 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.