Origin of CO2 undersaturation in the western tropical Atlantic

Underway fCO₂ has been measured from two merchant ships sailing from France to French Guyana and France to Brazil, and during two zonal cruises from Africa to French Guyana. In the western Tropical Atlantic, the strongest undersaturation is associated with the Amazon discharge near 55°W. In the 5°S–10°N, 65–35°W region, the carbon system is strongly correlated to salinity and robust empirical relationships could be determined. This region is a sink of CO₂ in May–June during the high-flow period of the Amazon river. The eastward propagation of Amazon waters is observed when the retroflection of the North Brazil Current takes place. In August 2008, freshwater is observed as far as 40°W when the North Equatorial Counter Current is quite strong. The Amazon plume, defined as salinities less than 34.9, is a sink of CO₂ of 0.96 mmol m_-2 d^-1. Further east, near 27°W, CO₂ undersaturation is recorded thoughout the year between 5°N and 8°N. This is caused by the high precipitation associated with the presence of the intertropical convergence zone (ITCZ). Removing the temperature effect leads to low (high) fCO₂ associated with low (high) salinities in boreal summer (winter), which is consistent with the seasonal migration of the ITCZ.