For more than 100 years, an unusually cold pool of water south of Greenland has stood out against the overall warming of the Atlantic Ocean. This persistent chill has prompted long-running scientific debate. A new study now points to a long-term weakening of a major ocean circulation system as the underlying cause.
Researchers at the University of California, Riverside report that only one mechanism can simultaneously account for observed temperature trends and changing salinity levels: a slowdown in the Atlantic Meridional Overturning Circulation, or AMOC. This large-scale system influences global climate by carrying warm, salty surface waters northward and sending cooler waters back toward the tropics at depth.
"People have been asking why this cold spot exists," said UCR climate scientist Wei Liu, who led the study with doctoral student Kai-Yuan Li. "We found the most likely answer is a weakening AMOC."
Because the AMOC functions like a climate-regulating conveyor belt, any reduction in its flow means that less heat and salt reach the sub-polar North Atlantic. The result is the cooling and freshening that researchers observe south of Greenland.
Century of Data Reveals a Long-Term Trend
A slowing AMOC reduces the transport of warm, salty water, leading to cooler and less saline surface conditions. For this reason, temperature and salinity records are valuable indicators of the circulation's strength.
Liu and Li examined approximately a century of these measurements, since direct AMOC monitoring only began about 20 years ago. Using these long-term records, they reconstructed past changes in the circulation and compared the results to nearly 100 climate model simulations.
Their analysis, published in Communications Earth & Environment, shows that only the models reflecting a weakened AMOC are consistent with the real-world data. Simulations that assumed a stronger current did not reproduce the observed cooling.
"It's a very robust correlation," Li said. "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."
Cooling, Salinity Decline, and Broad Climate Impacts
The study also found that lower salinity in the region aligns with a slowing AMOC, reinforcing the conclusion that reduced transport of warm, salty water is driving the trend.
The implications reach far beyond the immediate area. The South Greenland anomaly is one of the most sensitive regions to shifts in ocean circulation. Its cooling influences weather patterns across Europe by altering rainfall and affecting the jet stream, a high-altitude air current that helps guide storm systems and regulate temperatures throughout North America and Europe.
Marine ecosystems may also be affected, as variations in salinity and temperature shape the environments where species can survive.
Clarifying a Debate in Climate Modeling
The findings help resolve a disagreement among climate modelers about whether the South Greenland cooling is primarily caused by ocean dynamics or by atmospheric influences such as aerosol pollution. Some newer models argued for the latter and predicted a strengthening AMOC as aerosol emissions declined. However, those models did not match the observed cooling pattern.
"Our results show that only the models with a weakening AMOC get it right," Liu said. "That means many of the recent models are too sensitive to aerosol changes, and less accurate for this region."
By addressing that discrepancy, the study improves confidence in climate projections, especially those involving Europe, where AMOC-related shifts have significant influence.
Using Indirect Clues to Reconstruct Ocean History
The research also demonstrates how scientists can draw meaningful conclusions from indirect evidence. Although direct AMOC measurements are limited, temperature and salinity data offer a reliable window into long-term ocean circulation changes and can help refine predictions for future climate conditions.
"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."
Growing Influence of the South Greenland Cold Spot
As the climate continues to shift, the cold spot south of Greenland may become an increasingly important factor in future climate behavior. By identifying its origins, researchers hope to provide better preparation for the changes ahead.
"The technique we used is a powerful way to understand how the system has changed, and where it is likely headed if greenhouse gases keep rising," Li said.
