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

• Published in: Journal of Geophysical Research 2001-05, Vol.106 (C5), p.9241-9253
• Main Authors: Bakker, Dorothee C. E., Etcheto, Jacqueline, Boutin, Jacqueline, Merlivat, Liliane
• Format: Article
• Language: English
• Subjects: Earth Sciences; Earth, ocean, space; Exact sciences and technology; External geophysics; Geophysics; Physics; Physics of the oceans; Sciences of the Universe; Sea-air exchange processes
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/1999JC000275

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 and fCO2 reflected mixing between cold upwelled water with high fCO2 and warm tropical surface water with lower fCO2. The fCO2 maxima 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 water fCO2. While fCO2 was always above 400 μatm, 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 fCO2 with an amplitude up to 3.4 μatm 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°S, 20.1°W a sharp rise of temperature, a decrease of fCO2, 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 water fCO2 within large‐scale oceanic processes.