Hydrography-constrained AMOC reconstruction shows freshwater-driven decline and fading thermal compensation

The historical evolution of the Atlantic Meridional Overturning Circulation (AMOC) remains debated due to limited continuous observations. To bridge this gap, we leverage historical hydrographic data spanning 1955 to 2022 to constrain a high-resolution global ocean model. Here, we show an overall weakening of the AMOC relative to a 1955–1964 baseline, interrupted by a transient recovery during 1995–2004. The regional footprint of this weakening shifts over time. While earlier reductions (~ 7% during 1965–1994) are driven by suppressed dense water formation in the subpolar North Atlantic, steeper recent declines (~ 11% during 2005–2022) stem from combined reductions across the subpolar North Atlantic and Nordic Seas, despite partially compensating increases in the Arctic Ocean. Sensitivity experiments reveal that the AMOC weakening is overwhelmingly driven by a long-term freshening regime, manifested by repeated episodes of high-latitude salinity anomalies—occurring during the 1970s–1990s and again in recent years—that are broadly tied to Arctic export and Greenland ice melt. Finally, while a relatively cold thermal state during 1975–2004 partially buffers the freshwater-driven decline, this thermal compensation is absent during 2005–2022 due to anomalous warming of the Nordic Seas and the Labrador Current, acting alongside the freshwater forcing to drive the steeper modern decline of the AMOC.