Influence of El Nino on the equatorial Pacific contribution to atmosphericCO2 accumulation

The equatorial oceans are the dominant oceanic source of CO2 to the atmosph ere, annually amounting to a net flux of 0.7-1.5 Pg (10(15) g) of carbon, u p to 72% of which emanates from the equatorial Pacific Ocean(1-3). Limited observations indicate that the size of the equatorial Pacific source is sig nificantly influenced by El Nino events(4-10), but the effect has not been well quantified. Here we report spring and autumn multiannual measurements of the partial pressure of CO2 in the surface ocean and atmosphere in the e quatorial Pacific region. During the 1991-94 El Nino period, the derived ne t annual sea-to-air flux of CO2 was 0.3 Pg C from autumn 1991 to autumn 1.9 92., 0.6 Pg C in 1993, and 0.7 Pg C in 1994. These annual fluxes are 30-80% of that of 1996, a non-EG Nino year. The total reduction of the regional s ea-to-air CO2 nux during the 1991-94 El Nino period is estimated to account for up to one-third of the atmospheric anomaly (the difference between the annual and long-term-average increases in global atmospheric CO2 content) observed over the same period.