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

Abstract

Changes in the Atlantic Meridional Overturning Circulation (AMOC) and associated Meridional Heat Transport (MHT) can affect climate and weather patterns, regional sea levels, and ecosystems. Direct observations of the AMOC are still limited, particularly in the South Atlantic. This study establishes a cost‐effective trans‐basin section to estimate for the first time the AMOC and MHT at 22.5°S, using only sustained ocean observations. For this, an optimal mapping method that minimizes the difference between surface in situ dynamic height and satellite altimetry was developed to retrieve monthly temperature and salinity profiles from Argo and XBT data along the 22.5°S section. The mean states, as well as the interannual and seasonal changes of the obtained AMOC and MHT were compared with other products. The mean AMOC and MHT for 22.5°S are 16.3 ± 3.2 Sv and 0.7 ± 0.2 PW, respectively, showing stronger transports during austral fall/winter and weaker in spring. The high‐density XBT data available at the western boundary were vital for capturing the highly variable Brazil Current (BC), whose mean and variability was improved compared to other products. At 22.5°S, the North Atlantic Deep Water is divided into two cores that flow along both the western and the eastern boundaries near 2,500 m depth. Our results (a) suggest a greater influence of the western boundary current system on the AMOC variability at 22.5°S, (b) highlight the importance of high‐density in situ data for AMOC estimates, and (c) contribute to a better understanding of the AMOC and MHT variability in the South Atlantic.

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

“…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 Chidichimo et al, 2023;Dong et al, 2009Dong et al, , 2011Kersalé 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). However, previous estimations of freshwater transport from observations are limited in the South Atlantic, and models and observations have shown disagreement about the freshwater transport direction in this region (e.g., Bryden et al, 2011;Caínzos et al, 2022;de Vries & Weber, 2005;Dijkstra, 2007;Drijfhout et al, 2011;Garzoli et al, 2013;Huisman et al, 2010;Liu et al, , 2017Weijer & Dijkstra, 2003).…”