Variations and distributions of CO₂ in and over the equatorial Pacific during the period from the 1986/88 El Niño event to the 1988/89 La Niña event

Abstract

Measurements of pCO₂ in the background air and seawater of the equatorial Pacific were made during the period from January 1987 to February 1989 to study variations in CO₂ flux between the sea and the air. In January and February 1987, which was within the period of 1986/88 ENSO event, the surface water pCO₂ of the central equatorial Pacific decreased due to the reduction of upwelling, but less dramatically than during the 1982/83 ENSO event. In January and February 1989, the pCO₂ of the central equatorial Pacific increased with the SST decrease, due to the enhancement of upwelling during the La Niña event. Changes in surface water pCO₂ along the equator were well approximated by the effect on pCO₂ of the temperature, salinity, biological activity and the CO₂ exchange between the sea and the air. In January and February 1987, the CO₂ flux in the equatorial Pacific was estimated to be 0.4 Gt-C yr^-1, which was low compared to the non-ENSO periods, but remained positive. In January and February 1989, it increased to the level of 1 Gt-C yr^-1, mainly due to the increase of surface seawater pCO₂ during the La Niña event. The large interannual variation in CO₂ flux could affect the growth rate of atmospheric CO₂. For the period from January 31, 1987 to January 31, 1989, the concentration of atmospheric CO₂ has increased considerably (4.8 ppm). The δ¹³C of atmospheric CO₂ suggests the enhanced releases of CO₂ from the biosphere for the period between 1988 and 1987, and a relatively important role of air/sea CO₂ exchange between 1989 and 1988.