Variability of surface water fCO(2) 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 (CARI OCA) buoy from June to September 1997 during strong seasonal equatorial upw elling. The buoy drifted along the northern side of the equatorial cold ton gue 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 mixi ng between cold upwelled water with high fCO(2) and warm tropical surface w ater with lower fCO(2). The fCO(2) maxima reflected the strength of the upw elling. 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). W hile fCO(2) was always above 400 mu atm, the air-sea CO2 flux was highest i n the southern part of the cold tongue as a result of the spatial distribut ion 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 e ffects of diel changes in temperature and stratification, biological activi ty, and oceanic CO2 release from a shallow daytime mixing layer. At 4.5 deg reesS, 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 upwelli ng signature. Finally, the buoy drifted westward from 4.5 degreesS, 20.1 de greesW to 2.8 degreesS, 25.0 degreesW (August 7 to September 15). This stud y has demonstrated the potential of autonomous CARIOCA buoys to monitor the evolution and high-frequency variability of surface water fCO(2) within la rge-scale oceanic processes.