CO2 distributions in the equatorial Pacific during the 1991–1992 ENSO event

As part of the U.S. JGOFS Equatorial Pacific Process Study, measurements of CO2 species concentrations were made in the atmosphere and in the surface waters of the central and eastern equatorial Pacific during the boreal spring and autumn of 1992. Surface water fCO2 data indicate significant differences between the springtime El Niño conditions and the autumn post-El Niño conditions. The autumn fugacity (ΔfCO2) maxima were approximately 15–55 μatm higher than in the spring. The lower surface ΔfCO2 values in the spring data set were the result of: (i) advection of CO2-depleted water from the west at the equator near 170°W; and (ii) reduced upwelling and lower ΔfCO2 distributions as consequence of lighter zonal winds in the eastern Pacific from 140°W to 110°W. Assuming the springtime data are representative of the El Niño conditions and the autumn data are representative of the post-El Niño conditions, it is estimated that the net annual C02 flux during the 1991–1992 ENSO period is 0.3 Gt C. Over 60% of this flux occurred during the 4-month period in the autumn when ΔfCO2 values were close to normal. The net annual reduction of the ocean-atmosphere CO2 flux during the 1991–1992 El Niño is estimated to be on the order of 0.5–0.7 Gt C.