Scientists at the University of California, Riverside have identified a weakening Atlantic Meridional Overturning Circulation as the cause of a persistent cold water patch south of Greenland, observed for over a century. This slowdown in the ocean's climate-regulating conveyor belt explains the region's cooling and freshening trends. The findings, based on a century of temperature and salinity data, improve climate model accuracy and highlight ongoing shifts.
For more than 100 years, an unusually cold pool of water south of Greenland has puzzled researchers amid the broader warming of the Atlantic Ocean. A new study led by climate scientist Wei Liu and doctoral student Kai-Yuan Li at the University of California, Riverside attributes this anomaly to a long-term slowdown in the Atlantic Meridional Overturning Circulation (AMOC).
The AMOC acts as a global conveyor belt, transporting warm, salty surface waters northward and returning cooler, denser waters southward. Any reduction in its flow diminishes heat and salt delivery to the sub-polar North Atlantic, resulting in cooler temperatures and lower salinity south of Greenland. "People have been asking why this cold spot exists," Liu said. "We found the most likely answer is a weakening AMOC."
To reach this conclusion, the researchers analyzed about a century of temperature and salinity measurements, as direct AMOC monitoring started only around 20 years ago. They reconstructed historical circulation changes and compared them to nearly 100 climate model simulations. Only models simulating a weakened AMOC matched the observed cooling and salinity decline. "It's a very robust correlation," Li noted. "If you look at the observations and compare them with all the simulations, only the weakened-AMOC scenario reproduces the cooling in this one region."
This discovery resolves a debate among modelers, who had debated whether atmospheric factors like aerosol pollution or ocean dynamics were responsible. Recent models suggesting AMOC strengthening due to declining aerosols failed to replicate the cooling. "Our results show that only the models with a weakening AMOC get it right," Liu added. "That means many of the recent models are too sensitive to aerosol changes, and less accurate for this region."
The implications extend across the Northern Hemisphere. The cooling influences Europe's weather patterns, rainfall, and the jet stream, which guides storms and temperatures in North America and Europe. Marine ecosystems also face changes, as temperature and salinity variations affect species habitats. Although direct long-term AMOC data is scarce, these indirect indicators confirm a century-long weakening trend, likely to persist with rising greenhouse gases. "We don't have direct observations going back a century, but the temperature and salinity data let us see the past clearly," Li said. "This work shows the AMOC has been weakening for more than a century, and that trend is likely to continue if greenhouse gases keep rising."
Published in Communications Earth & Environment, the study enhances climate projections, particularly for AMOC-sensitive regions like Europe.