Observed Variability of the Atlantic Meridional Overturning Circulation and the Deep Western Boundary Current along 34.5°S

Abstract

The Atlantic meridional overturning circulation (AMOC) is a vitally important component of the global ocean circulation because of its impact on the environment, weather, and ecosystems. The South Atlantic is a key gateway for water mass exchanges between the Atlantic and other basins as southward overturning freshwater transport at 34.5°S increases the likelihood of an AMOC collapse in the future. In two-thirds of state-of-the-art coupled climate models, the overturning freshwater transport at 34.5°S is northward and AMOC is monostable, whereas most observations find that freshwater transport is southward suggesting AMOC is bistable. The upper limb of the AMOC and Deep Western Boundary Current (DWBC), a major element of AMOC’s lower limb, control freshwater transport at 34.5°S. It is therefore crucial to observe the daily strength of both of these circulation systems and use these observations to validate numerical models. We examine AMOC and DWBC variability from over fourteen years of South Atlantic MOC Basin-wide Array (SAMBA) measurements between South America and South Africa along 34.5°S . These observational records enable concurrent examination of the temporal variations of the upper and lower limbs of AMOC. During 2009-2022, the AMOC volume transport weakened by -0.6 Sv/yr, but this trend is obscured by significant higher frequency variability (± 10 Sv standard deviation with respect to the 18.6 Sv long-term mean) and a 3-year data gap on the eastern boundary during 2010-2013. The inclusion of more years of data has shifted the AMOC seasonal cycle from semi-annual to quasi-annual, and has improved agreement with Argo-altimetry based estimates on seasonal timescales. SAMBA transports are more energetic than Argo-altimetry on intraseasonal and interannual time scales, with the largest differences occurring when SAMBA density-driven variations are strong. The SAMBA DWBC has a mean southward transport of -17 Sv and a standard deviation of 22 Sv, with a significant negative trend of -0.3 Sv/year (DWBC increasing in strength). AMOC and DWBC variations are modestly correlated along 34.5°S on monthly and longer timescales, such that a weaker AMOC corresponds to stronger DWBC anomalies. This covariability will be explored further to better establish the connectivity between AMOC and the DWBC in the South Atlantic.