Impact of North Brazil Current rings on air-sea CO2 flux variability in winter 2020

• Main Authors: Olivier, Léa, Boutin, Jacqueline, Reverdin, Gilles, Lefèvre, Nathalie, Landschützer, Peter, Speich, Sabrina, Karstensen, Johannes, Ritschel, Markus, Wanninkhof, Rik
• Format: Text
• Language: English
• Published: 2021
• Subjects: North Atlantic
• Online Access: https://doi.org/10.5194/bg-2021-269; https://bg.copernicus.org/preprints/bg-2021-269/

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 CO 2 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 EUREC 4 A-OA/ATOMIC campaign. In-situ surface fugacity of CO 2 (fCO 2 ), 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 CO 2 sink (−1.7 TgC.month −1 ), previously underestimated by a factor 10. The NBC rings transport saline and high fCO 2 water indicative of their equatorial origins and are a small source of CO 2 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 CO 2 . Spatial distribution of fCO 2 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.