For over a century, an unusually cold region of water south of Greenland has stood out against the general warming of the Atlantic Ocean. This persistent coolness has long sparked debate among scientists. Recent research suggests that the main cause of this phenomenon is a prolonged weakening of a major ocean circulation system.

Causes of the Anomalous Cooling

Researchers from the University of California, Riverside, have concluded that only one mechanism can simultaneously explain the observed temperature trends and changes in salinity: the slowdown of the Atlantic Meridional Overturning Circulation (AMOC). This large-scale system plays a crucial role in the global climate by transporting warm, salty surface waters northward and returning colder waters to the tropics at depth. When the AMOC weakens, less heat and salt reach the subpolar North Atlantic, leading to cooling and freshening of the waters south of Greenland.

Impact on Climate and Ecosystems

The slowdown of the AMOC reduces the transport of warm, salty water, resulting in colder and less saline surface conditions. Therefore, temperature and salinity data serve as important indicators of the strength of this circulation. Scientists analyzed about a century’s worth of such measurements, since direct observations of the AMOC have only been available for the past 20 years. Using these long-term records, researchers reconstructed past changes in circulation and compared the results with nearly a hundred climate models. Only those models that accounted for a weakening AMOC matched real-world data; simulations with a stronger current failed to reproduce the observed cooling.

The study also showed that the decrease in salinity in the region is linked to the AMOC slowdown, confirming the key role of reduced transport of warm, salty water. The consequences of this phenomenon extend far beyond the region itself: the anomaly south of Greenland is one of the most sensitive zones to changes in ocean circulation. Its cooling affects weather patterns in Europe, altering precipitation and influencing the jet stream—a powerful air current that steers storms and regulates temperatures in North America and Europe. Marine ecosystems may also be impacted, as changes in salinity and temperature shape habitats for various species.

Significance for Climate Forecasting

The findings help resolve a longstanding debate among climatologists about whether the cooling south of Greenland is primarily driven by ocean dynamics or by atmospheric factors such as aerosol pollution. Some modern models had suggested the latter and predicted a strengthening of the AMOC as aerosol emissions declined, but these models did not match the observed cooling pattern. The research shows that only models featuring a weakened AMOC produce accurate results, indicating that many current models are too sensitive to aerosol changes and less reliable for this region.

This work also demonstrates how scientists can draw meaningful conclusions from indirect data. Despite the limited direct measurements of the AMOC, temperature and salinity records provide reliable information about long-term changes in ocean circulation and help refine forecasts of future climate conditions. The study indicates that the AMOC has been weakening for over a century, and this trend is likely to continue if greenhouse gas emissions keep rising.

Outlook

As climate change progresses, the cold patch south of Greenland may become an increasingly important factor in shaping the future climate. Understanding its origins allows researchers to better prepare for upcoming changes. The method they used helps reveal how the system has evolved in the past and what the continued rise in greenhouse gas emissions could mean for the future.