A seasonal tropical sink for atmospheric CO2 in the Atlantic ocean: the role of the Amazon River discharge

In the western equatorial Atlantic ocean, near-surface observations show that during summertime, the low-salinity oceanic water, arising from mixing with the Amazon River discharge at the equator, has low CO2 fugacity levels. Near the coast of South America where the salinities are the lowest (S<20), the fugacity of oceanic CO2 decreases down to 150 μatm and the shelf area acts as a significant sink for atmospheric CO2. The dilution effect by low-salinity water only partly accounts for the decrease in CO2, and the biological production in the Amazon Plume water enriched in nutrients lowers dissolved inorganic carbon and decreases the fCO2 by nearly 30%. The low-salinity Amazon water tongue spreads northwestwards along the coast by the North Brazil Current (NBC) and is deflected eastwards north of 5°N in the NBC retroflection in summer. Consequently, the low-salinity and oceanic fCO2 (below the atmospheric fCO2 level) signatures may extend more than 2000 km eastwards. The impact of the river outflow on the air–sea CO2 exchanges in the western region is demonstrated by using the climatologies of the sea surface salinity (SSS) to estimate the magnitude of the annual net CO2 flux in the western part of the equatorial Atlantic. This is in contrast with the central and eastern parts that are sources for atmospheric CO2.