For more than a century, a patch of cold water south of Greenland has resisted the Atlantic Ocean's overall warming, fueling debate among scientists. A new study from the University of California, Riverside pinpoints the reason: the long-term weakening of the Atlantic Meridional Overturning Circulation (AMOC).
The AMOC is a massive current system that helps regulate climate by moving warm, salty water northward and cooler water southward at depth. Scientists often describe it as a giant conveyor belt, delivering heat and salt from the tropics to the North Atlantic. When the AMOC slows, less heat and salt reach the North Atlantic, leading to cooler, fresher surface waters. A slowdown means less warm, salty water reaches the sub-polar region, resulting in the cooling and freshening observed south of Greenland.
Wei Liu, a climate scientist at the University of California, Riverside, and doctoral student Kai-Yuan Li led the study. Liu said, “People have been asking why this cold spot exists. We found the most likely answer is a weakening AMOC.” Li added, “It’s a very robust correlation. 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.”
Direct AMOC observations go back only about 20 years, so Liu and Li analyzed a century’s worth of temperature and salinity data. From those long-term records they reconstructed changes in the AMOC and compared the results with nearly 100 different climate models. The paper, published in Communications Earth & Environment, shows that only the models simulating a weakened AMOC matched the real-world data. Models that assumed a stronger AMOC did not match the observed cooling. “Our results show that only the models with a weakening AMOC get it right. That means many of the recent models are too sensitive to aerosol changes, and less accurate for this region,” Liu said.
The study found that the weakening of the AMOC correlates with decreased salinity. Decreased salinity is another clear sign that less warm, salty water is being transported northward. Salinity and temperature data offer a valuable alternative for detecting long-term change, especially with limited direct AMOC measurements. “We don’t have direct observations going back a century, but the temperature and salinity data let us see the past clearly,” Li said. He also noted, “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.”
The South Greenland anomaly matters not just because it is unusual, but because it is one of the most sensitive regions to changes in ocean circulation. The anomaly affects weather patterns across Europe, altering rainfall and shifting the jet stream. The jet stream is a high-altitude air current that steers weather systems and helps regulate temperatures across North America and Europe. A slowdown of the AMOC may disturb marine ecosystems, as changes in salinity and temperature influence where species can live.
The study may help settle a dispute among climate modelers about whether the South Greenland cooling is driven primarily by ocean dynamics or by atmospheric factors such as aerosol pollution. Many newer models suggested that atmospheric factors were driving the cooling, predicting a strengthened AMOC due to declining aerosol emissions. Those newer models failed to recreate the actual, observed cooling south of Greenland. By resolving the mismatch between models and observations, the research strengthens future climate forecasts, especially those concerning Europe.
As the climate system shifts, the South Greenland cold spot may grow in influence. “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. The hope is that by unlocking the origins of the South Greenland cold spot, scientists can better prepare societies for what lies ahead.
Written with the help of a news-analysis system.