<p>Climate models project an Atlantic Meridional Overturning Circulation slowdown under anthropogenic greenhouse warming. Despite a ~1.5 °C increase in the global mean surface temperature, debate remains on whether and when this circulation has slowed. Here we identify a distinctive temperature fingerprint in the equatorial Atlantic that signals the Atlantic Meridional Overturning Circulation change through numerical ocean model experiments. The equatorial Atlantic is a crucial crossroads for the circulation anomalies to propagate to other oceans. A slowdown drives a mid-depth (1000–2000 m) warming in the equatorial Atlantic in a decade via baroclinic Kelvin waves. Analysis of climate models shows that this mid-depth temperature change is a better indicator for Atlantic Meridional Overturning Circulation change on decadal and longer timescales than other surface proxies. Observations reveal a robust mid-depth warming since 1960 that emerged from natural variability in the early 2000s, suggesting a slowdown that already started in the late 20^th century.</p>