Strange cold spot in the Atlantic

 Strange Atlantic cold spot could signal big trouble for climate around the world 

For more than a century, the North Atlantic has hosted an anomaly. South of Greenland lies a patch of cool ocean water. This region, called the North Atlantic Warming Hole (NAWH), defies global warming. A patch of cold water south of Greenland has resisted the Atlantic Ocean's overall warming, fuelling debate among scientists. A new study identifies the cause as the long-term weakening of a major ocean circulation system. Researchers from the University of California, Riverside show that only one explanation fits both observed ocean temperatures and salinity patterns: the Atlantic Meridional Overturning Circulation, or AMOC, is slowing down. This massive current system helps regulate climate by moving warm, salty water northward and cooler water southward at depth. Scientists long debated its cause. A new study now shows that the weakening of a vital ocean current, the Atlantic Meridional Overturning Circulation (AMOC), explains it best. “People have been asking why this cold spot exists,” said 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. This is why salinity and temperature data can be used to understand the strength of the AMOC. This system shapes climate across continents. Liu and Kai-Yuan Li from University of California Riverside studied temperature and salinity data spanning a century. Direct AMOC measurements only cover 20 years. But using six long-term sea surface temperature (SST) datasets and three salinity datasets, the researchers built a much clearer picture. They also tested 94 global climate models. Only models simulating a slowing AMOC recreated the observed North Atlantic cooling. These models showed trends of up to −0.3 °C per century (about −0.54 °F per century), matching the real-world pattern known as the NAWH. From these long-term records, they reconstructed changes in the circulation system and compared those with nearly 100 different climate models. As the paper published, only the models simulating a weakened AMOC matched the real-world data. Models that assumed a stronger circulation didn't come close. "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."

The team defined AMOC fingerprints using a dipole pattern: cooling in the sub polar gyre (south of Greenland) and warming near the Gulf Stream. This contrast, visible in SST and salinity (SSS), offers a reliable way to track AMOC strength. From the observations, they estimated that the AMOC weakened by −1.01 to −2.97 Sverdrup's per century from 1900 to 2005. A single Sverdrup equals one million cubic meters of water per second. That’s a massive loss of energy transport across the Atlantic. The study also found that the weakening of the AMOC correlates with decreased salinity. This is another clear sign that less warm, salty water is being transported northward. The consequences are broad. The South Greenland anomaly matters not just because it's unusual, but because it's one of the most sensitive regions to changes in ocean circulation. It affects weather patterns across Europe, altering rainfall and shifting the jet stream, which is a high-altitude air current which steers weather systems and helps regulate temperatures across North America and Europe.

The cooling and freshening do not stop at the surface. The study reveals that subsurface waters down to 3,000 meters also exhibit clear signs of this trend. The vertical profile shows cooling and freshening at depth, confirming that AMOC’s slowdown affects the entire ocean column. Model comparisons between weakened and strengthened AMOC simulations highlighted another key point. A slower AMOC causes a heat transport gap, less warm water moves northward. This creates a cold zone between 40°N and 65°N, which matches the NAWH location. The slowdown may also disturb marine ecosystems, as changes in salinity and temperature influence where species can live. This result may help settle a dispute among climate modellers about whether the South Greenland cooling is driven primarily by ocean dynamics or by atmospheric factors such as aerosol pollution. Many newer models suggested the latter, predicting a strengthened AMOC due to declining aerosol emissions. But those models failed to recreate the actual, observed cooling. Salt transport in the North Atlantic also changes. A weakened AMOC means more freshwater sits at the surface. This freshening was strongest across the NAWH and extended to the Labrador Sea. While observations showed some salinity increase in the Labrador Sea, models still closely captured the broad trend. Both the SST- and SSS-based fingerprint indices (FPISST and FPISSS) showed strong correlation with AMOC weakening. These patterns help researchers verify the slowdown, even without direct current measurements.

"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 resolving that mismatch, the study strengthens future climate forecasts, especially those concerning Europe, where the influence of the AMOC is most pronounced. The study also highlights the ability to draw clear conclusions from indirect evidence. With limited direct data on the AMOC, temperature and salinity records provide a valuable alternative for detecting long-term change, and for helping to predict future climate scenarios. Some past theories suggested the NAWH came from changes in wind patterns or reduced air pollution. These theories were tested in the new study. In experiments with slab-ocean models (which isolate atmospheric effects), the cooling did not appear. Only in fully coupled models, with real ocean circulation, did the NAWH form. “We tested CO₂ experiments in slab-ocean and fully coupled models,” the authors wrote. “Only the fully coupled models with a weakening AMOC showed the cooling feature.” The South Greenland anomaly in the North Atlantic affects more than just temperature. It reshapes the jet stream and alters weather across Europe and North America.

The cold zone also threatens marine ecosystems. Many fish species rely on certain salinity and temperature levels. Disruptions could shift where they live, breed and migrate. This study helps improve climate forecasts by identifying the most realistic models. It also highlights the value of indirect evidence. Fingerprints built from ocean temperature and salinity can track changes even when direct measurements are sparse. “We don’t have direct observations going back a century, but the temperature and salinity data let us see the past clearly,” Li noted. If greenhouse gas emissions keep rising, the AMOC may continue to weaken. That means more pronounced cooling near Greenland and stronger climate impacts across Europe. "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." Li said. As the climate system shifts, the South Greenland cold spot may grow in influence. The hope is that by unlocking its origins, scientists can better prepare societies for what lies ahead. By understanding this, scientists gain a clearer view of Earth’s climate system. The North Atlantic Warming Hole is not a mystery anymore. It’s a sign, quiet, persistent and measurable, of a changing ocean current that affects us all. "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.