Wintertime process study of the North Brazil Current rings reveals the region as a larger sink for CO2 than expected

The North Brazil Current (NBC) flows northward across the Equator, passes the mouth of the Amazon River, and forms large oceanic eddies near 8° N. We investigate the processes driving the variability of air-sea CO2 fluxes at different scales in early 2020 in the region [50° W–59° W–5° N–16° N]. This region is a pathway between the equatorial and North Atlantic Ocean and was surveyed during the EUREC4A-OA/ATOMIC campaign. In-situ surface fugacity of CO2 (fCO2), salinity and temperature combined with maps of satellite salinity, chlorophyll-a and temperature highlight contrasting properties in the region. In February 2020, the area is a CO2 sink (−1.7 TgC.month−1), previously underestimated by a factor 10. The NBC rings transport saline and high fCO2 water indicative of their equatorial origins and are a small source of CO2 at regional scale. Their main impact on the variability of biogeochemical parameters is through the filaments they entrain into the open ocean. During the campaign, a nutrient-rich freshwater plume from the Amazon River is entrained from the shelf up to 12° N and caused a phytoplankton bloom leading to a significant carbon drawdown (~20 % of the total sink). On the other hand, saltier filaments of shelf water rich in detrital material act as strong local sources of CO2. Spatial distribution of fCO2 is therefore strongly influenced by ocean dynamics south of 12° N. The less variable North Atlantic subtropical water extends from Barbados northward. They represent ~60 % of the total sink due to their lower temperature associated with winter cooling and strong winds