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

Abstract

Abstract. The key processes driving the air–sea CO2 fluxes in the western tropical Atlantic (WTA) in winter are poorly known. WTA is a highly dynamic oceanic region, expected to have a dominant role in the variability in CO2 air–sea fluxes. In early 2020 (February), this region was the site of a large in situ survey and studied in wider context through satellite measurements. The North Brazil Current (NBC) flows northward along the coast of South America, retroflects close to 8∘ N and pinches off the world's largest eddies, the NBC rings. The rings are formed to the north of the Amazon River mouth when freshwater discharge is still significant in winter (a time period of relatively low run-off). We show that in February 2020, the region (5–16∘ N, 50–59∘ W) is a CO2 sink from the atmosphere to the ocean (−1.7 Tg C per month), a factor of 10 greater than previously estimated. The spatial distribution of CO2 fugacity is strongly influenced by eddies south of 12∘ N. During the campaign, a nutrient-rich freshwater plume from the Amazon River is entrained by a ring from the shelf up to 12∘ N leading to high phytoplankton concentration and significant carbon drawdown (∼20 % of the total sink). In trapping equatorial waters, NBC rings are a small source of CO2. The less variable North Atlantic subtropical water extends from 12∘ N northward and represents ∼60 % of the total sink due to the lower temperature associated with winter cooling and strong winds. Our results, in identifying the key processes influencing the air–sea CO2 flux in the WTA, highlight the role of eddy interactions with the Amazon River plume. It sheds light on how a lack of data impeded a correct assessment of the flux in the past, as well as on the necessity of taking into account features at meso- and small scales.

Combining an Eddy Detection Algorithm with In-Situ Measurements to Study North Brazil Current Rings

“…However, Figure 4 reveals that a large fraction of the ring's upper layers is influenced by the presence of fresher water spreading across the eddy boundaries. This lower salinity water is very likely a remnant of the interaction of A2 with the Amazon River plume that has been advected around the eddy when A2 was younger and closer to the Amazon River mouth, very much as described by [18,19,57]. These fresher waters have been translating solidly with A2 northwestward.…”

“…[11] estimate that NBC rings transport 9.3 Sv (1 Sv = 10 6 m 3 s −1 ) on an annual average. They also contribute to the offshore advection and dispersal of fresh and nutrient-rich waters from the Amazon River (Figure 1) [7,10,18,19].…”

“…This article focuses on a shallow NBC ring, dubbed as A2 in the following (see also [19]), that was crossed by different RVs close to its center 4 times. A2 vertical properties were measured by a suite of different devices, providing a robust and relatively complete dynamical description of the eddy.…”

On the importance of the atmospheric coupling to the small-scale ocean in the modulation of latent heat flux

“…An existing rich literature shows that both mechanisms are important at different time-scales in different regions (Small et al (2008) and references therein). Following the metrics introduced in Meroni et al (2022), we test in this section which mechanism (DMM or PA) is taking place more prominently in the EUREC 4 A-OA region in order to improve our understanding of the SST-U 10m residual field coupling. If the PA mechanism is active, positive correlations between surface wind divergence (∂u s /∂s) and the SST field Laplacian (∂ 2 SST/∂s 2 ), both computed in the direction orthogonal to the large scale wind, are expected.…”

Sailing through the southern seas of air–sea CO 2 flux uncertainty

“…a state where the air loop is artificially put into a state of disequilibrium with the seawater, natural ‘shocks’, e.g. crossing steep fronts or mesoscale eddies, result in a slower response time, which needs to be considered when studying short-term and small-scale variations [29,30] compared with faster responding head-space equilibrator systems. A way forward to improve the representation of normalCO2 gradients and to reduce the measurement uncertainty would be to better quantify the adjustment time of the membrane, through laboratory comparisons with other, fast responding instruments.…”