The South Atlantic Meridional Overturning Circulation and Mesoscale Eddies in the First GO‐SHIP Section at 34.5°S

Abstract

The variability of the Atlantic meridional overturning circulation (AMOC) has considerable impacts on the global climate system. Past studies have shown that changes in the South Atlantic control the stability of the AMOC and drive an important part of its variability. That is why significant resources have been invested in a South (S)AMOC observing system. In January 2017, the RV Maria S. Merian conducted the first GO-SHIP hydrographic transect along the SAMOC-Basin Wide Array (SAMBA) line at 34.5°S in the South Atlantic. This paper presents estimates of meridional volume, freshwater (MFT), and heat (MHT) transports through the line using the slow varying geostrophic density field and direct velocity observations. An upper and an abyssal overturning cell are identified with a strength of 15.64 ± 1.39 Sv and 2.4 ± 1.6 Sv, respectively. The net northward MHT is 0.27 ± 0.10 PW, increasing by 0.12 PW when we remove the observed mesoscale eddies with a climatology derived from the Argo floats data set. We attribute this change to an anomalous predominance of cold core eddies during the cruise period. The highest velocities are observed in the western boundary, within the Brazil and the Deep Western Boundary currents. These currents appear as a continuous deep jet located 150 km off the slope squeezed between two cyclonic eddies. The zonal changes in water masses properties and velocity denote the imprint of exchange pathways with both the Southern and the Indian oceans. Plain Language Summary The Atlantic meridional overturning circulation (AMOC) is a crucial element of the global ocean circulation and climate. It connects the Southern Ocean to the northern North Atlantic, and is responsible for the interhemispheric northward transport of heat and freshwater. The South Atlantic is a crossroad for water masses from the Southern, the Indian and the North Atlantic oceans. This paper analyzes the first full-ocean-depth trans-basin measurements of the southernmost enclosed section of the Atlantic between South Africa and Brazil along 34.5°S. Our results confirm a northward transport of heat at this latitude. We also found a complex water mass structure and dynamics, characterized by intense boundary currents and mesoscale eddies. It is the sum of these elements that is not only crucial for the Atlantic but also for the global ocean circulation and climate. MANTA ET AL.

Thirty Years of GOSHIP and WOCE Data: Atlantic Overturning of Mass, Heat, and Freshwater Transport

“…The relationship between temperature and AMOC enables the use of satellite sea surface temperature as a proxy for long‐term reconstructions of AMOC strength (Caesar et al., 2018; Manta et al., 2021), which can be compared to the estimations obtained with global climate numerical models (Caesar et al., 2018; Fraser & Cunningham, 2021). Longer term reconstructions and projections from proxies and high‐resolution climate models display a decline in the strength of the AMOC by 15% (Caesar et al., 2018) and 30% (Rahmstorf et al., 2015) since the 1950s, in the frame of a consistent weak AMOC for the last 150 years (Thornalley et al., 2018).…”

Ocean Acidification and Long‐Term Changes in the Carbonate System Properties of the South Atlantic Ocean

“…However, the central and intermediate waters were the focus of this study, as significant decadal C ant changes have been observed mainly in these waters (Wanninkhof et al., 2010; Woosley et al., 2016). Guided by earlier water mass definitions (Hernández‐Guerra et al., 2019; Manta et al., 2021; Stramma & England, 1999) the first 1100 m of the water column were split into two layers (hereafter referred to as central and intermediate water layers) using the neutral density (γ n ) values extracted from the GLODAPv2‐2021 database (calculated following Jackett & McDougall, 1997). The central water layer was defined by γ n boundaries between 26.2 kg m −3 and 27.1 kg m −3 , which goes from ∼150 to ∼750 m, and is mainly composed of the South Atlantic Central Water (SACW).…”

Transport of Antarctic Bottom Water Entering the Brazil Basin in a Planetary Geostrophic Inverse Model

“…However, in other studies, such as Manta et al. (2021), Lower Circumpolar Deep Water (LCDW) occupies the range 28.1–28.27 γ n while AABW is more narrowly defined as water denser than 28.27 γ n . By that definition there is no AABW east of 30°W.…”