The Antarctic Bottom Water and the South Atlantic Meridional Overturning Circulation based on the analyses of observations and numerical models

Abstract

The meridional overturning circulation (MOC) is a primary mechanism for the transport and storage of heat, freshwater and carbon by the ocean and, therefore, has a large impact on climate variability and change. Alterations in the MOC have been linked to past climate variations, including abrupt changes caused by a slowing of the MOC. The Atlantic MOC (AMOC) consists of two vertical cells. The less studied lower cell is formed by the abyssal northward flow of the densest Antarctic Bottom Water (AABW) and a southward return flow underneath the lower limb of the upper cell. The AABW is a mass of cold and dense water that sinks around the Antarctic continent at selected locations. In the South Atlantic, the AABW slowly flows along the western continental slope as a Deep Western Boundary Current (DWBC) filling up the bottom of the Argentine Basin. From the Argentine to the Brazil Basin, due to bottom topography constraints, the AABW can only continue flowing to the north through abyssal channels and fractures in the submarine ridges connecting the deep oceanic basins. Most of the AABW that enters the Brazil Basin goes through the Vema Channel. Changes in the AABW properties may have significant reflex on sea level rise and on oceanic global heat and freshwater budgets. Despite this great importance, little is known of the inter-annual variability of the AABW in the South Atlantic. In January 2019, a current meter mooring equipped with a salinity and temperature sensors was deployed at the Vema Channel, as part of Project SAMBAR. Eventually, the time-series produced by this platform will be used in conjunction with other past and ongoing observation to enhance the knowledge on the meridional transport of volume, heat and salt by the AABW through the Vema sill.In parallel to the observational initiative, a concentrated effort will be carried out to better understand the climatological structure and the variability of the AMOC lower cell in the South Atlantic based on the analyses of models run at GEOMAR and in situ historical data. The observations will be compared with simulations from an existing hierarchy of global ocean/sea-ice model configurations at increasing model resolution (from non-eddying to eddy-rich) based on the ocean model NEMO3.6 (Nucleus for European Modeling of the Ocean).