Wintertime process study of the North Brazil Current rings reveals the region as a larger sink for CO 2 than expected

[摘要] The key processes driving the air–sea CO 2 fluxes in the westerntropical Atlantic (WTA) in winter are poorly known. WTA is a highly dynamicoceanic region, expected to have a dominant role in the variability in CO 2 air–sea fluxes. In early 2020 (February), this region was the siteof a large in situ survey and studied in wider context through satellitemeasurements. The North Brazil Current (NBC) flows northward along the coastof South America, retroflects close to 8 ∘  N and pinches off theworld's largest eddies, the NBC rings. The rings are formed to the north ofthe Amazon River mouth when freshwater discharge is still significant inwinter (a time period of relatively low run-off). We show that in February2020, the region (5–16 ∘  N, 50–59 ∘  W) is a CO 2 sink from the atmosphere to the ocean ( −1.7   Tg C per month), a factor of 10 greater than previously estimated. Thespatial distribution of CO 2 fugacity is strongly influenced by eddiessouth of 12 ∘  N. During the campaign, a nutrient-rich freshwaterplume from the Amazon River is entrained by a ring from the shelf up to12 ∘  N leading to high phytoplankton concentration and significantcarbon drawdown ( ∼20  % of the total sink). In trappingequatorial waters, NBC rings are a small source of CO 2 . The lessvariable North Atlantic subtropical water extends from 12 ∘  Nnorthward and represents ∼60  % of the total sink due tothe lower temperature associated with winter cooling and strong winds. Ourresults, in identifying the key processes influencing the air–sea CO 2 flux in the WTA, highlight the role of eddy interactions with the AmazonRiver plume. It sheds light on how a lack of data impeded a correct assessmentof the flux in the past, as well as on the necessity of taking into accountfeatures at meso- and small scales.

[学科分类] 大气科学