Variability of surface waterfCO2during seasonal upwelling in the equatorial Atlantic Ocean as observed by a drifting buoy

The fugacity of carbon dioxide (fCO2) 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°S, 7.5°W to 0.2°N, 12.5°W (June 20 to July 3). An inverse trend between temperature andfCO2reflected mixing between cold upwelled water with highfCO2and warm tropical surface water with lowerfCO2. ThefCO2maxima reflected the strength of the upwelling. Subsequently, the buoy crossed the cold tongue toward the southwest from 0.2°N, 12.5°W to 4.5°S, 20.1°W (July 3 to August 7). During this crossing, warming increased surface waterfCO2. WhilefCO2was always above 400 μatm, the air‐sea CO2flux was highest in the southern part of the cold tongue as a result of the spatial distribution of the CO2exchange coefficient. A variable diel cycle of surface‐waterfCO2with an amplitude up to 3.4 μatm was attributed to the combined effects of diel changes in temperature and stratification, biological activity, and oceanic CO2release from a shallow daytime mixing layer. At 4.5°S, 20.1°W a sharp rise of temperature, a decrease offCO2, and a maximum fluorescence marked the exit of the region with a strong upwelling signature. Finally, the buoy drifted westward from 4.5°S, 20.1°W to 2.8°S, 25.0°W (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 waterfCO2within large‐scale oceanic processes.