Cold blob over Atlantic points to AMOC weakening

A long-running patch of colder water in the North Atlantic—known as the “cold blob” or “warming hole”—has become the latest flashpoint in debates over whether the Atlantic Meridional Overturning Circulation (AMOC) is weakening. New work using climate reanalyse

For decades. one patch of the North Atlantic has stubbornly refused to keep up with the rest of the planet’s warming. It sits south-east of Greenland. cooled by as much as 1°C relative to the 1951–80 average. and it shows up so consistently in data visuals that it has been given two names with an uneasy familiarity: the “warming hole” and the “cold blob.”.

The stakes are larger than the label. The cold blob matters because it lines up with concerns about the Atlantic Meridional Overturning Circulation. or AMOC—the vast system of currents that carries warm. salty water from the Gulf of Mexico northwards toward the North Atlantic. There the water cools and sinks. then flows back south along the ocean floor. redistributing heat and helping shape regional climate.

A weakening AMOC is what many scientists now suspect is behind the cold spot. Freshwater from Greenland’s melting ice could be diluting that warm, salty water, reducing its density and making it sink more slowly. That shift could weaken the overturning circulation itself.

Some research has even raised the possibility of a tipping point within decades—an outcome that would lock in a future collapse. The consequences. if such a collapse came. would be severe: Europe could freeze. and monsoon rains crucial for agriculture in Africa and Asia could be disrupted. Yet even that possibility runs into a hard reality. There are only 22 years of direct observation of AMOC strength, not enough to tease out a clear trend.

What has kept the cold blob from becoming a settled story is that scientists have never agreed on the mechanism. Climate modelling has suggested that a slowing AMOC would deliver less warm water to the North Atlantic. producing the cooler patch. But other modelling placed much of the blame on the atmosphere.

A 2022 study by Chengfei He at Northeastern University in Boston and colleagues added fuel to the atmospheric side of the debate. They argued that rapid Arctic warming reduces the temperature difference between the pole and the tropics. shifting the jet stream northwards into the cold blob region. When the strong westerly winds arrive, they force more evaporation and churn the water, drawing heat out of the ocean.

Another study pointed to clouds as a further driver: greater evaporation can lead to more clouds, shading the cold blob from the sun’s warmth.

Now. a new effort reframes the argument by looking more directly at what the ocean is doing. rather than what the atmosphere might be doing to it. Stefan Rahmstorf at the Potsdam Institute for Climate Impact Research in Germany and his colleagues investigated the cold blob using climate reanalyses—datasets built from direct weather observations from satellites. buoys and ships. rather than climate modelling alone.

Their findings cut against the idea that winds and clouds are the main story. They found that heat loss from the ocean surface has decreased in the cold blob since 1955. The cooling, they say, isn’t limited to the surface. The ocean has been cooling not just at the top layer, but also down to 1,000 metres. Taken together, that points toward the ocean transporting less heat, not winds stripping heat away.

Rahmstorf put the view sharply: “Winds and clouds ‘only explain a modest fraction of the warming hole’.” He added that even if some modelling makes it look possible for the atmosphere to be responsible, “in fact, the data show it is caused by the ocean.”

For him, the implication goes beyond explaining a puzzle on a map. The discovery suggests that Atlantic Ocean circulation has already been changing for decades. That’s where the human stakes creep in—quietly at first, and then with a growing sense of urgency.

Rahmstorf also warned about what could happen to the broader circulation system. He said concerns extend not only to a collapse of the AMOC. but also to the subpolar gyre. a massive swirl of currents around the cold blob. The subpolar gyre helps bring in salty surface water that feeds the sinking of cold. dense water that drives the AMOC. If it shuts down. it could reduce temperatures in the UK and nearby countries more quickly than a full AMOC collapse.

He pointed to timing, too. “The subpolar gyre passing this tipping point could already lead to serious climate impacts in western Europe as early as in the 2040s,” he said.

But even with the ocean-centric conclusion, the study doesn’t settle the debate completely. One limitation is unavoidable: the ocean surface heat flux hasn’t been directly measured. Instead, Rahmstorf’s team had to infer it using modelling. And even earlier work complicates the picture.

In 2021. a study based on some of the same reanalyses as Rahmstorf’s found that stronger winds accounted for most of the cold blob. He—who is also central to the earlier 2022 Arctic-jetstream argument—described the challenge in trying to reconstruct energy flows from reanalyses: “It’s challenging to try to use reanalysis to infer the energy budget in the cold blob.”.

That uncertainty is echoed by David Thornalley at University College London, who called the new work useful, but not definitive. “It won’t be the final word” on what is causing the cold blob, he said.

Others caution that the data still leave room for alternative explanations. Neil Fraser at the Scottish Association for Marine Science said the cold blob can’t entirely be pinned down to one cause yet. One possibility. he pointed out. is that a branch of the AMOC known as the Norwegian current may be strengthening. transporting more heat out of the cold blob area.

Fraser’s summary is careful, but it lands on the same tension felt by everyone watching the anomaly form and persist. “The cold blob is consistent with a weakening AMOC,” he said. “But it is not conclusive evidence.”

The cold blob is not just a curiosity in the ocean’s temperature record. It has become a test case for competing explanations of how climate systems shift—and how quickly those shifts could start to matter. With only limited direct measurements of AMOC strength. the debate is still anchored to what can be reconstructed from decades of observations and model-based inference.

What’s changed with Rahmstorf’s team is the direction of the argument: the cold spot is cooling through the ocean. and the best fit to the observed heat loss since 1955 points away from the atmosphere as the dominant driver. Whether that means the AMOC is already weakening—or whether other parts of the current system are rearranging the heat in ways that mimic the same signal—remains the central question.

For now, the cold blob persists, and so does the unease that it may be more than a local anomaly—an early marker of a larger circulation turning under pressure.

cold blob warming hole AMOC weakening Atlantic Meridional Overturning Circulation Greenland melting ice climate reanalyses ocean heat loss subpolar gyre jet stream Norwegian current