What it is
Climate models have long projected that the Atlantic Meridional Overturning Circulation (AMOC) will weaken under greenhouse warming, but observations have struggled to confirm whether or when it has begun. This study identifies a distinctive temperature fingerprint: a slowdown drives warming at 1000 to 2000 m depth in the equatorial Atlantic within a decade, carried by baroclinic Kelvin waves. Analyzing observations back to 1960, the authors find a robust mid-depth warming that emerged from natural variability in the early 2000s. They argue this signal is a better decadal indicator of AMOC change than the surface proxies used before.
Why it matters
The AMOC is one of the climate system's major tipping elements, and its state governs heat transport, regional weather, and sea level across the Atlantic. Despite roughly 1.5 degrees C of global surface warming, debate has persisted over whether the circulation has actually weakened. By reading a deep-ocean signal rather than a noisy surface one, this work provides independent evidence that the slowdown started in the late 20th century, sharpening the case that a critical circulation is already changing.
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Filed underAMOC, ocean circulation, climate tipping points, Atlantic Ocean, ocean observations