Sea water fugacity of CO₂ at the PIRATA mooring at 6°S, 10°W

In order to better understand the variability of surface CO₂ in the Tropical Atlantic, a CARIOCA sensor has been installed on a PIRATA mooring at 6°S, 10°W in June 2006. The fugacity of CO₂ (fCO₂) is recorded hourly from 7 June 2006 to 30 October 2009 with two important data gaps. From July to September, an upwelling develops and a decrease in sea surface temperature (SST) is observed, associated with an fCO₂ increase. However, the highest fCO₂ is observed in October, after the upwelling season, due to the warming of surface waters. The region is a net source of CO₂ to the atmosphere of 2.10 ± 0.69 mol m_-2 yr^-1 in 2007. The monthly flux is maximum (3.21 ± 0.8mol m^-2 yr^-1) in November (averaged over 2006 and 2008). High frequency variability is observed throughout the time series but is particularly pronounced after the upwelling season. Biological and thermodynamic processes explain the diurnal variability. Dissolved inorganic carbon (DIC) is calculated from (alkalinity) TA and fCO₂ using an empirical TA—salinity relationship determined for the eastern equatorial Atlantic. Net community production (NCP) is calculated from DIC daily changes and ranges from 9 to 41 mmol m^-2 d^-1, which is consistent with previous measurements in this region.