Low CO2 concentrations in the Gulf of Guinea during the upwelling season in 2006

The fugacity of CO2 (fCO2) was measured underway from 27 May to 5 July 2006 in the Gulf of Guinea when the upwelling conditions were taking place. The equatorial and the coastal upwellings are responsible for the large CO2 outgassing observed in the tropical Atlantic. The highest fCO2 (655 μatm) was measured in the eastern coastal upwelling. However, area of low fCO2 are also observed. Those occurred north of 2°N, in the Guinea Current, and near 6°S close to the coast due to the Congo River discharge. The decrease of salinity is a major factor explaining low fCO2. In the South Equatorial Counter Current region, south of 6°S near 10°W, low fCO2 are not related to a salinity effect as this region is subject to excess evaporation. A comparison with the meridional transects of the RMS St Helena made in 1995–1996 near 15°W suggests that these low fCO2 or even slight undersaturations can be observed from about 4°S to 10°S. It seems that they occur mainly in boreal summer. They could be caused by transport of water that has been in contact with the atmosphere long enough to come close to equilibrium. The low fCO2 area are not reproduced by the CO2 climatology probably because of the coarse resolution and the lack of data in this region. Despite the low fCO2 area observed in 2006, the ΔfCO2 in 2006 is higher than the 1995 climatology for the region 10°S–2°N, 10°W–10°E suggesting that fCO2 is increasing over time but from the few cruises available, it is difficult to definitely conclude.