FY 1996

Surface water fCO₂ in the eastern equatorial Pacific during the 1992–1993 El Niño

Wanninkhof, R., R.A. Feely, H. Chen, C. Cosca, and P.P. Murphy

J. Geophys. Res., 101(C7), 16,333–16,343, doi: 10.1029/96JC01348 (1996)

The fugacity of CO₂ in surface water (fCO_2w) was measured in the eastern equatorial Pacific (EEP) during the boreal spring and fall of 1992 and in the spring of 1993. A prolonged El Niño occurred during this period with anomalously warm sea surface temperatures (SST) during the spring of 1992 and 1993. Correspondingly, the fCO_2w values were lower than historical non-El Niño values at the equator. However, the fCO_2w in the spring of 1993 was up to 50 µatm higher than in the spring of 1992, despite similar SSTs. The trend is attributed to the slower response times of factors causing fCO_2w decrease compared to rapid increase of fCO_2w by upwelling of cold water with high carbon content and subsequent heating. During the fall of 1992, SSTs south of the equator were 5°C cooler than in the spring, which is indicative of vigorous upwelling of water with high CO₂ content from below the thermocline. Decreases in fCO_2w due to net biological productivity and gas exchange take of the order of months, causing the fCO_2w levels during the spring of 1993 to be elevated compared to the spring of the previous year. Our data and data obtained in 1986 and 1989 along 110°W suggest that fCO₂ maxima in the equatorial Pacific can be either associated with temperature minima or temperature maxima. Despite the multitude of factors which influence fCO_2w, most of the variance can be accounted for with changes in nitrate and SST. A multilinear regression of fCO_2w with SST and nitrate for the 1992 data has a standard error in predicted fCO_2w of 10 µatm. Air-sea fluxes of CO₂ in the EEP were estimated to be 30% higher in the spring of 1993 and 10% higher in the fall of 1992 than in the spring of 1992.