Multi-decadal weakening of the Atlantic Overturning from a physics and observation-based reconstruction

The Atlantic meridional overturning circulation (AMOC) is a key component of the global ocean circulation, transporting heat, salt, nutrients and carbon between hemispheres. Observational proxies and models suggest an AMOC slowdown over the 20th and 21st Centuries ^1,2,3,4,5 , yet its historical variability remains poorly constrained, and there are inconsistencies across studies and methodologies ^6,7,8 . Here we reconstruct AMOC strength and variability since 1940 using a physically based water-mass transformation framework ⁹ applied to multiple observation-based ocean datasets and atmospheric reanalysis fields. This approach correctly estimates the overturning streamfunction in density space across the Atlantic by incorporating air-sea buoyancy fluxes, subsurface mixing, and volume tendencies. The reconstructed AMOC strength closely matches direct 21st Century measurements from the RAPID, OSNAP, and SAMBA arrays. Moreover, the reconstruction reveals a long-term AMOC weakening of 13.7, 8.3, and 7.3 Sverdrups (1 Sverdrup = 10 ⁶ m ³ s ^-1 ) per century since 1960 at 34.5°S, 26°N, and in the subpolar cell, respectively. While air-sea fluxes account for ~80% of the time-mean AMOC strength at 26°N, the long-term decline is primarily driven by reductions in subsurface mixing and a decline in the upper-limb volume. These results provide a new practical framework for evaluating AMOC evolution and emphasize the importance of sustained ocean observations for tracking modern day AMOC changes.