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

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 degrees 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 degrees 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 degrees 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 degrees 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 degrees 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. The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents responsible for the north-south movement of water, heat, and other properties, which influences climate, weather, sea level, and ecosystems. Because direct measurements of this circulation are limited in space and time, especially in the South Atlantic, most observational estimates of the overturn here are based on statistical relationships between satellite and in situ measurements. This study presents a new method to estimate the AMOC at 22.5 degrees S based only on available in situ measurements and optimized with the help of satellite altimetry data. This method resolves the energetic Brazil Current (BC) near the western boundary, westward propagating signals, and coastal sea level variability. It is shown that the overturning circulation is stronger during the austral fall/winter and weaker in spring, and its interannual variability is mostly driven by changes in the BC. The deep southward transport at 22.5 degrees S is dominated by two flows along the western and eastern boundaries near 2,500 m depth. Our analysis demonstrates the importance of sustained in situ observations to monitor the state of the AMOC. First in situ estimates of the Atlantic Meridional Overturning Circulation and Meridional Heat Transport time series at 22.5 degrees SMapping method developed here captures the coastal sea level and westward propagating featuresInterannual variability of the overturning at 22.5 degrees S is dominated by western boundary currents, followed by eastern boundary currents