New Study Links North Atlantic Cold Blob to Weakening of AMOC

A recent *Geophysical Research Letters* paper reports a persistent region of unusually cold water at depths up to 1,000 m southeast of Greenland, cooling faster than surrounding oceans. The authors link the deep-water cooling to reduced northward heat transport by the Atlantic Meridional Overturning Circulation (AMOC), not to surface atmospheric processes.

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The AMOC is a large-scale ocean conveyor that moves warm surface water northward and returns colder, denser water southward at depth, redistributing heat and influencing climate across the North Atlantic basin, Europe, the tropics, and the U.S. East Coast.

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The study was led by oceanographer Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, Germany. Commentary was provided by David Thornalley, climate scientist at University College London.

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Temperature records from 1995-2024, combined with climate reanalysis, show that surface heat loss to the atmosphere has declined since 1955, contradicting wind-driven cooling hypotheses, while deep-water temperatures (~1,000 m) exhibit a marked downward trend, indicating reduced warm-water delivery by the AMOC.

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If the AMOC continues to weaken, models project colder European winters, disrupted tropical rainfall threatening agriculture, and higher sea levels along the U.S. East Coast due to regional ocean dynamics.

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Rahmstorf notes that surface atmospheric forcing explains only a modest part of the cooling, and the data point to oceanic mechanisms. Thornalley cautions that the findings are not a definitive resolution of the cold-blob debate and call for further study.

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Some researchers still explore atmospheric explanations such as shifting wind patterns for surface heat loss. The authors argue these mechanisms cannot fully account for the deep-water temperature decline, highlighting a split in the scientific community over the primary driver.

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Reduced surface heat loss challenges earlier atmospheric-only models, yet the magnitude of AMOC weakening remains uncertain. Direct AMOC measurements are limited, creating a gap between modeled trends and observed ocean changes.

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> “only explain a modest fraction of the warming hole,” — Stefan Rahmstorf, Oceanologist

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> “Even if, in some modelling approaches, it seems possible that the cold blob is caused by the atmosphere, in fact, the data show it is caused by the ocean,” — Stefan Rahmstorf, Oceanologist

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> “It won’t be the final word,” — David Thornalley, Climate Scientist

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The authors recommend expanding deep-ocean monitoring and refining coupled ocean-atmosphere models to better quantify AMOC variability. Thornalley’s comment underscores the need for interdisciplinary studies before consensus on the cold blob’s origin can be reached.

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