FY 1999

Influence of El Niño on the equatorial Pacific contribution of atmospheric CO₂ accumulation

Feely, R.A., R. Wanninkhof, T. Takahashi, and P. Tans

Nature, 398, 597–601, doi: 10.1038/19273 (1999)

The equatorial oceans are the dominant oceanic source of CO₂ to the atmosphere, annually amounting to a net flux of 0.7-1.5 Pg (10¹⁵ g) of carbon, up to 72% of which emanates from the equatorial Pacific Ocean. Limited observations indicate that the size of the equatorial Pacific source is significantly influenced by El Niño events, 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 equatorial Pacific region. During the 1991-94 El Niño period, the derived net annual sea-to-air flux of CO₂ was 0.3 Pg C from autumn 1991 to autumn 1992, 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-El-Niño year. The total reduction of the regional sea-to-air CO₂ flux during the 1991-95 El Niño 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.