New study links Atlantic “cold blob” to declining ocean heat transport

The region is unique in the modern climate record. While most of the world’s oceans have warmed since the late 19th century, the subpolar North Atlantic has cooled both at the surface and within the ocean itself. Scientists have debated whether the anomaly results from increased heat escaping from the ocean into the atmosphere or from a reduction in the amount of heat delivered by Atlantic currents.

Stefan Rahmstorf and colleagues examined ocean heat-content records together with multiple decades of observation-based reanalysis data to test those competing explanations. Their analysis found that the cooling extends far below the ocean surface and cannot be explained by changes in surface heat exchange alone.

Ocean heat-content data show that the region lost heat between 1955 and 2024 at an average rate of approximately 0.15 W/m², even as most of the global ocean accumulated heat over the same period. The greatest changes occurred within roughly the upper 1 000 m (3 281 feet) of the water column, corresponding to the northward-flowing upper branch of the AMOC. Below about 2 500 m (8 202 feet), the study found comparatively little long-term change.

The surface-flux evidence points in the opposite direction from what would be expected if atmospheric heat loss were driving the cooling. ERA5 reanalysis data show that heat loss from the ocean surface over the cold-blob region decreased rather than increased. If stronger heat loss were responsible for the cooling, the opposite trend would be expected. The authors argue that reduced surface heat loss is a consequence of less heat arriving in the region in the first place.

To quantify the mechanism, the researchers reconstructed the heat budget of the region using ocean heat-content measurements and surface-flux data. Decades-long variations in ocean heat content tracked changes in ocean heat transport more closely than changes in surface heat exchange. The study concludes that ocean circulation changes are the dominant driver of heat-content variability in the cold blob.

The findings are consistent with another well-known feature of a weakening AMOC. Satellite observations and Argo float measurements show a band of unusually strong warming along the U.S. East Coast north of Cape Hatteras, a pattern previously identified as an AMOC fingerprint. The study notes that observations indicate the Gulf Stream has shifted north since 2001, a change that climate researchers have linked to a weakening overturning circulation.

While the paper does not present direct measurements of an AMOC collapse, nor does it determine how close the circulation may be to a critical threshold, the authors argue that the cold blob remains one of several independent indicators pointing toward a long-term weakening trend.

They cite evidence from ocean salinity records, paleoclimate reconstructions, Gulf Stream observations, and changes in ocean density within the subpolar North Atlantic.

The AMOC transports vast amounts of heat northward through the Atlantic and plays an important role in climate patterns across Europe, North America, and the wider Atlantic basin. A substantial weakening would alter regional temperature, precipitation, and ocean circulation patterns.

Rahmstorf and colleagues note that uncertainty remains over how close the AMOC may be to a tipping point. They point to recent studies identifying early-warning signals associated with reduced circulation stability and note that a subset of CMIP6 climate-model simulations crosses an AMOC tipping threshold around the middle of this century. From a risk-management perspective, the authors conclude that the possibility warrants close scientific and policy attention.

References:

¹ Multidecadal Atlantic “Warming Hole” Heat Content Variations Are Caused by Ocean Heat Transport, Not by Surface Fluxes – Stefan Rahmstorf et al. – Geophysical Research Letters – May 28, 2026 – https://doi.org/10.1029/2025GL118383 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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