In the middle of the rapidly warming Atlantic Ocean, a persistent cold patch just south of Greenland is bucking global climate trends—and raising alarm. Known as the North Atlantic Warming Hole, this anomaly has puzzled scientists for years. Now, a new international study has traced its likely cause to a slow but steady weakening of the Atlantic Meridional Overturning Circulation (AMOC), a crucial system of ocean currents that helps balance Earth’s climate.
Published in Nature Communications Earth & Environment, the study analyzed over a century of observational ocean data alongside 94 state-of-the-art climate models. The conclusion is striking: only simulations that include a weakening AMOC reproduce the cold anomaly seen in real-world data. Models that assume a stable or strengthening current miss the mark entirely.
This finding adds weight to long-standing concerns about the AMOC, often likened to a vast conveyor belt moving warm water northward and cold water south. A slowdown could dramatically reshape global weather patterns—from rainfall across Europe to storm tracks over North America.
The Ocean’s Hidden Fingerprint
Between 1900 and 2005, global sea surface temperatures (SST) rose almost everywhere—except south of Greenland, where a distinct zone cooled by as much as 0.3°C per century. This phenomenon, visible across multiple independent datasets, had been difficult to reconcile with broader climate models.
The new study, led by Wei Liu of the University of California, Riverside, dives deep into this discrepancy. By comparing SST and sea surface salinity (SSS) trends with the output of dozens of models from the CMIP5 and CMIP6 archives, the team identified a defining pattern. Only models simulating a weakened AMOC produced both the surface cooling and freshening observed in the North Atlantic.
The authors also show that this pattern—colder waters near Greenland and warmer waters near the Gulf Stream—is not just a surface phenomenon. Cooling and freshening trends extend throughout the water column, even down to 3,000 meters. These changes are consistent with a reduced vertical overturning in the ocean, a hallmark of a slowing AMOC.
Importantly, the models that fail to simulate this—especially many from CMIP6—also tend to overestimate the AMOC’s strength in the 20th century. This raises concerns about the accuracy of some of the latest Intergovernmental Panel on Climate Change (IPCC) projections.
Implications Far Beyond the Atlantic
The Atlantic circulation system doesn’t just regulate temperatures locally. It plays a central role in global climate stability, affecting rainfall across Western Europe, storm activity in the Atlantic Basin, and even the strength of the South Asian monsoon.
A continued AMOC slowdown could shift the position of the jet stream, leading to more extreme and erratic weather. Winters in Europe could become colder and wetter. Summers could turn drier in some areas, wetter in others. In the tropics, a weaker AMOC could increase hurricane intensity and disrupt the rhythm of seasonal rains.
Marine ecosystems are also at risk. The cold anomaly is already altering salinity and temperature gradients that dictate the distribution of fish species and other marine life. According to the Techno-Science report, this could lead to significant habitat transformations, impacting biodiversity and commercial fisheries across the North Atlantic.
The freshening of the region, driven by melting Arctic and Greenlandic ice, adds another layer of complexity. Freshwater is lighter than salty water and resists sinking—disrupting the downward flow that drives the AMOC.
Climate Models Diverge, but the Data Doesn’t
One of the most revealing aspects of the study is its critique of current climate models. While most CMIP5 models simulate an AMOC that weakens over the 20th century—consistent with historical data—many CMIP6 models do the opposite, suggesting a strengthening current.
To resolve this inconsistency, Liu’s team created a novel AMOC fingerprint index based on SST and SSS differences between two key regions: the subpolar gyre (north) and the Gulf Stream zone (south). This dipole pattern proved strongly correlated with AMOC strength across model runs. When applied to actual SST and SSS records, it suggested a decline in AMOC strength of up to 2.97 Sverdrups (Sv) per century between 1900 and 2005.
The study also found that simplified “slab ocean” models, which don’t include full ocean circulation, failed to reproduce the warming hole altogether. Only fully coupled models that explicitly simulate AMOC dynamics matched the historical data—reinforcing the link between the warming hole and circulation slowdown.