Atlantic’s Mysterious Cold Mass: Implications for AMOC Weakening

The Earth’s surface has experienced warming over the last 150 years, with the exception of certain regions in the North Atlantic Ocean. This area, particularly southeast of Greenland, is known as the “warming hole” or “cold blob” and exhibits a temperature drop of up to 1°C.

Experts remain divided on the cause of this phenomenon, with recent findings indicating that a weakening of the Atlantic Meridional Circulation (AMOC) could be a significant factor. The AMOC is a critical ocean current system responsible for transporting warmth from tropical regions to Europe.

The AMOC conveys warm, salty water from the Gulf of Mexico to the North Atlantic, where it cools, sinks, and circulates back south along the ocean floor. Concerns arise that freshwater influx from Greenland’s melting ice may reduce the salinity levels, disrupting this vital ocean current and weakening the overall circulation system.

Research has suggested that the AMOC could approach a tipping point within the next few decades, potentially leading to a collapse that could freeze parts of Europe and disrupt monsoon patterns crucial for agriculture in Africa and Asia. However, only 22 years of direct AMOC strength observations are available, which limits our understanding of clear trends.

Climate modeling indicates that the AMOC’s slowdown results in decreased warm water flow to the North Atlantic, contributing to the formation of this cold mass. Alternatively, some studies attribute the phenomenon to atmospheric changes.

In a 2022 study, He Chengfei and colleagues from Northeastern University found that rapid Arctic warming has diminished the temperature gradient between polar and tropical regions, pushing the jet stream northward into the cold blob area. This shift generates strong westerly winds that intensify evaporation, pulling heat away from the ocean.

Increased evaporation leads to more cloud formation, which may also contribute to cooling in this region, while other studies suggest solar activity plays a significant role.

Researchers such as Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research are using climate reanalysis—derived from direct observations via satellites, buoys, and ships—rather than solely on modeling to investigate this cold mass. Their findings indicate a decrease in heat loss from the ocean surface in the cold blob since 1955. Notably, cooling is observed not just at the surface but also at depths of 1,000 meters, suggesting a decline in heat transport by the AMOC rather than increased heat loss due to atmospheric factors.

According to Rahmstorf, “Wind and clouds can only explain a small part of the warming hole. While some modeling appears to attribute the cold blob to atmospheric effects, empirical data point toward an oceanic origin.”

This research signals that the Atlantic circulation has been changing for many years, raising alarms not only about the potential collapse of the AMOC but also about the subpolar circulation—a massive eddy of currents surrounding the cold mass. A shutdown of this circulation could result in rapid temperature drops in the UK and neighboring nations, potentially more severe than an outright AMOC collapse.

“The subarctic circulation nearing a tipping point could invoke drastic climate changes in Western Europe as soon as the 2040s,” warns Rahmstorf.

However, directly measuring ocean surface heat fluxes remains challenging, leading to estimates based on modeling approaches. A 2021 study, utilizing similar reanalysis data, found that winds account for the majority of cold mass dynamics.

He commented, “Inferring the energy budget of a cold mass from reanalysis is complicated.”

While new research offers valuable insights, David Thornalley from University College London suggests that findings are “not definitive” regarding the causes of cold blobs. Limited data means alternate explanations cannot be entirely dismissed. Neil Fraser at the Scottish Marine Science Society proposes that a branch of the AMOC—the Norwegian Current—might be gaining strength, redirecting heat away from colder regions.

“The cold mass aligns with a weakening AMOC,” Fraser asserts, “but conclusive evidence is still elusive.”