The South Atlantic Circulation Between 34.5°S, 24°S and Above the Mid‐Atlantic Ridge From an Inverse Box Model

Abstract

The South Atlantic Ocean plays a key role in the heat exchange of the climate system, as it hosts the returning flow of the Atlantic Meridional Overturning Circulation (AMOC). To gain insights on this role, using data from three hydrographic cruises conducted in the South Atlantic Subtropical gyre at 34.5°S, 24°S, and 10°W, we identify water masses and compute absolute geostrophic circulation using inverse modeling. In the upper layers, the currents describe the South Atlantic anticyclonic gyre with the northwest flowing Benguela Current (26.3 ± 2.0 Sv at 34.5°S, and 21.2 ± 1.8 Sv at 24°S) flowing above the Mid‐Atlantic Ridge (MAR) between 22.4°S and 28.4°S (−19.2 ± 1.4 Sv), and the southward flowing Brazil Current (−16.5 ± 1.3 Sv at 34.5°S, and −7.3 ± 0.9 Sv at 24°S); the deep layers feature the southward transports of Deep Western Boundary Current (−13.9 ± 3.0 Sv at 34.5°S, and −8.7 ± 3.8 Sv at 24°S) and Deep Eastern Boundary Current (−15.1 ± 3.5 Sv at 34.5°S, and −16.3 ± 4.7 Sv at 24°S), with the interbasin west‐to‐east flow close to 24°S (7.5 ± 4.4 Sv); the abyssal waters present northward mass transports through the Argentina Basin (5.6 ± 1.1 Sv at 34.5°S, and 5.8 ± 1.5 Sv at 24°S) and Cape Basin (8.6 ± 3.5 Sv at 34.5°S–3.0 ± 0.8 Sv at 24°S) before returning southward (−2.2 ± 0.7 Sv at 24°S to −7.9 ± 3.6 Sv at 34.5°S), without any interbasin exchange across the MAR. In addition, we compute the upper AMOC strength (14.8 ± 1.0 and 17.5 ± 0.9 Sv), the equatorward heat transport (0.30 ± 0.05 and 0.80 ± 0.05 PW), and the freshwater flux (0.18 ± 0.02 and −0.07 ± 0.02 Sv) at 34.5°S and 24°S, respectively.

A Multi‐Data Set Analysis of the Freshwater Transport by the Atlantic Meridional Overturning Circulation at Nominally 34.5°S

“…However, our AX18‐XBT mean result shows a stronger MOC than the 14.8 ± 1.0 Sv estimated by Arumí‐Planas et al. (2023).…”

“…Our estimate of the strength of MOC at nominally 34.5°S using AX18-XBT data is also consistent with previous observational and model estimates of[14.7-20.7 Sv] byManta et al (2021),Kersalé et al (2021),Meinen et al (2018),Perez et al (2011),Dong et al (2011Dong et al ( , 2015,Majumder et al (2016), andGarzoli et al (2013), as well asWeijer et al (2020). However, our AX18-XBT mean result shows a stronger MOC than the 14.8 ± 1.0 Sv estimated byArumí-Planas et al (2023).…”

“…The South Atlantic Ocean has historically been less studied than the North Atlantic, as a result, model estimates of the M ov have been poorly constrained in this basin (Garzoli et al., 2013; Weijer et al., 2019). Nevertheless, the repeated high‐density eXpendable BathyThermograph (XBT) lines (trans‐basin AX18 line from South America to Cape Town; AX22 and AX25 lines across the Drake Passage and the Agulhas retroflection, respectively), together with the Argo program collecting a large number of temperature and salinity profiles (Roemmich & Owens, 2000), and the quasi‐decadal occupation of trans‐basin hydrographic lines (Arhan et al., 2003; Arumí‐Planas et al., 2023; Bryden et al., 2011; Hernández‐Guerra et al., 2019; Manta et al., 2021; McDonagh & King, 2005; Saunders & King, 1995) have increased data coverage in the South Atlantic region. Consequently, observed and simulated estimates of mean AMOC and Meridional Heat Transport (MHT) in the South Atlantic have become more consistent over the last decade (Baringer & Garzoli, 2007; Chidichimo et al., 2023; Dong et al., 2009, 2011; Kersalé et al., 2021; Pita et al., 2024; Sitz et al., 2015; Weijer et al., 2020), with fairly good agreement in terms of AMOC and MHT variability between models and some observations in more recent studies (Baker et al., 2023; Biastoch et al., 2021; Dong et al., 2021).…”

An ARGO and XBT Observing System for the Atlantic Meridional Overturning Circulation and Meridional Heat Transport (AXMOC) at 22.5°S

“…This method is a classical approach to estimate AMOC and MHT (e.g., Dong et al, 2015Dong et al, , 2021Goes et al, 2015Goes et al, , 2020. However, inverse models are also widely used to calculate the AMOC (e.g., Arumí-Planas, et al, 2023;Caínzos et al, 2022;Hernández-Guerra et al, 2010;Lux et al, 2001). The geostrophic AMOC stream function is estimated from the adjusted velocities (v, Equations 2 and 3), and its strength is defined as the maximum value of the stream function at each timestep.…”