Variability of surface water fCO2 during seasonal upwelling in the equatorial Atlantic Ocean as observed by a drifting buoy

The fugacity of carbon dioxide (fCO(2)) in tropical Atlantic surface waters was hourly monitored by a drifting carbon interface ocean atmosphere (CARIOCA) buoy from June to September 1997 during strong seasonal equatorial upwelling. The buoy drifted along the northern side of the equatorial cold tongue from 0.2 degreesS, 7.5 degreesW to 0.2 degreesN, 12.5 degreesW (June 20 to July 3). An inverse trend between temperature and fCO(2) reflected mixing between cold upwelled water with high fCO(2) and warm tropical surface water with lower fCO(2). The fCO(2) maxima reflected the strength of the upwelling. Subsequently, the buoy crossed the cold tongue toward the southwest from 0.2 degreesN, 12.5 degreesW to 4.5 degreesS, 20.1 degreesW (July 3 to August 7). During this crossing, warming increased surface water fCO(2). While fCO(2) was always above 400 mu atm, the air-sea CO2 flux was highest in the southern part of the cold tongue as a result of the spatial distribution of the CO2 exchange coefficient. A variable diel cycle of surface-water fCO(2) with an amplitude up to 3.4 mu atm was attributed to the combined effects of diel changes in temperature and stratification, biological activity, and oceanic CO2 release from a shallow daytime mixing layer. At 4.5 degreesS, 20.1 degreesW a sharp rise of temperature, a decrease of fCO(2), and a maximum fluorescence marked the exit of the region with a strong upwelling signature. Finally, the buoy drifted westward from 4.5 degreesS, 20.1 degreesW to 2.8 degreesS, 25.0 degreesW (August 7 to September 15). This study has demonstrated the potential of autonomous CARIOCA buoys to monitor the evolution and high-frequency variability of surface water fCO(2) within large-scale oceanic processes.