Atlantic cold spot shows AMOC has 'been slowing for a century'

For months – if not years – debate has raged among scientists over the general health of an ocean current system critical to regulating Earth’s climate – arguing over whether or not the Atlantic Meridional Overturning Circulation (or AMOC) is slowing down.

This week, researchers looking into the root cause of a centuries-old patch of cold water south of Greenland and its resistance to the overall warming trend of the Atlantic Ocean, has come to one simple conclusion. That it is.

Landing on only one explanation for the observed ocean temperatures and patterns in salinity across the region, researchers from the University of California, Riverside concluded that the AMOC – a massive current system responsible for moving warm, salty water northward and cooler water southward at depth, is indeed weakening.  

“People have been asking why this cold spot exists,” said University of California, Riverside climate scientist Wei Liu, who led the study with doctoral student Kai-Yuan Li. “We found the most likely answer is a weakening AMOC.”

The AMOC acts like a giant conveyor belt, delivering heat and salt from the tropics to the North Atlantic. A slowdown in this system means less warm, salty water reaches the sub-polar region, resulting in the cooling and freshening observed south of Greenland. 

When the current slows, less heat and salt reach the North Atlantic, leading to cooler, fresher surface waters. This is why salinity and temperature data can be used to reconstruct the strength of the AMOC.

Together, Li and Liu analysed a century’s worth of this data, as direct AMOC observations go back only as far as 20 years. From these long term records, they reconstructed changes in the circulation system and compared those with nearly 100 different climate models.

Their paper – published this week in Communications Earth & Environment – shows that only in the models simulating a weakened AMOC were outcomes generally matched to the real-world data. Models that assumed a stronger AMOC didn’t come close.