Variability of fCO₂ in the Eastern Tropical Atlantic from a moored buoy

A fCO 2 sensor, based on a colorimetric method used for the CARIOCA buoys, has been installed on a Pilot Research Moored Array in the Tropical Atlantic (PIRATA) mooring at 6°S, 10°W, in the gulf of Guinea, in June 2006 during the EGEE 3 cruise. Hourly fCO 2 data recorded from June to December 2006 are presented. An alkalinity‐salinity relationship has been determined using data from different cruises, which allows the calculation of dissolved inorganic carbon. Although the tropical Atlantic is an important source of CO 2 , an unexpected area of low CO 2 concentrations is observed in the South Equatorial Counter Current with fCO 2 values close to equilibrium conditions or even slightly undersaturated with respect to the atmospheric fCO 2 value of 367.7 μ atm measured during the cruise. At the end of June, an increase of seawater fCO 2 to 400 μ atm is consistent with the beginning of the upwelling season occurring from July to September. Although the mooring is not located within the upwelling area, the spreading of the cold tongue explains the large CO 2 outgassing. The monthly CO 2 flux ranges from 1.19 mmol m −2 d −1 in June to a maximum of 8.37 mmol m −2 d −1 in October, when high fCO 2 values above 420 μ atm are maintained by the warming of surface water. Most of the fCO 2 distribution can be explained by physical processes and a strong relationship between fCO 2 and SST is determined for the upwelling season. From mid‐September, diurnal cycles can be detected. Using a dissolved inorganic carbon budget, periods where net community production or diurnal warming and cooling dominates are observed.