Sea surface temperature change from 1880 to 2023. Nearly every ocean has warmed (yellow to red), with one stark exception: the blue-purple “cold blob” southeast of Greenland, the only major patch to cool.
NASA/Columbia University
Nearly every corner of the global ocean has warmed over the past century.
But one patch of the North Atlantic, located southeast of Greenland and Iceland, has spent decades doing the opposite. While most of the world’s oceans have heated up, this region has cooled.
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Scientists call it the Atlantic “cold blob,” and a new study suggests it may offer one of the clearest clues yet about changes underway in a major ocean circulation system that helps regulate climate across the Northern Hemisphere.
Over roughly the past 150 years, while nearly every other stretch of ocean has warmed, this patch just south of Greenland and Iceland has cooled by as much as 1 degree Celsius.
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The cold blob sits thousands of miles from California, but scientists are paying close attention because it may reveal changes in the Atlantic Meridional Overturning Circulation, a vast ocean current system that helps redistribute heat around the planet.
While the AMOC operates in the Atlantic, climate researchers have found that changes to the system can ripple through the atmosphere and influence Pacific climate patterns that ultimately help shape California’s wet and dry seasons.
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So why is the region cooling while much of the rest of the world warms?
Scientists have long debated what’s causing the unusual cold anomaly. One explanation was that the region was simply losing more heat to the atmosphere because of natural climate variability.
A study out this month in Geophysical Research Letters, led by Potsdam Institute climate scientist Stefan Rahmstorf, points to a different explanation.
How fast each patch of the North Atlantic has warmed or cooled compared with the global ocean, 1993-2021. Blue marks the “cold blob” southeast of Greenland, the rare stretch bucking the worldwide warming trend.
Stefan Rahmstorf/Potsdam Institute for Climate
Working from real ocean observations instead of climate models alone, the research team found that the cooling extends through the full depth of the water column. They also found that the region losing heat at the surface does not fully overlap with the region experiencing the strongest cooling.
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Their conclusion: The cold blob appears to be cooling because less ocean heat is arriving. The finding strengthens evidence that the Atlantic Meridional Overturning Circulation has weakened over time.
Scientists continue to debate how much additional weakening may occur this century and what the long-term consequences could be. But the new study adds to a growing body of research suggesting the North Atlantic cold blob is linked to broader changes in the ocean circulation system rather than a simple seasonal exchange of heat with the atmosphere.
The AMOC functions like a giant ocean conveyor belt. It carries warm tropical water northward near the ocean surface while returning colder water southward at depth. Along the way, it releases heat into the atmosphere that helps influence climate across parts of Europe and eastern North America.
The cold blob sits near one of the key regions where that heat exchange occurs. If less warm water is arriving there, scientists would expect the region to cool.
The new study does not settle every question surrounding the cold blob or the future of the AMOC. Direct observations of the circulation are relatively recent, and scientists continue to debate how quickly the system may change in a warming climate.
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But the findings add to growing evidence that one of the planet’s most unusual ocean temperature patterns may be revealing something important about how Earth’s climate system is evolving.
The cold patch itself is an Atlantic story. The larger question is whether it signals a continuing slowdown of the AMOC and how that might affect climate elsewhere.
For California, the connection is indirect and runs through the atmosphere rather than the ocean. Climate model experiments have shown that a substantially weaker AMOC can alter tropical rainfall patterns, strengthen Pacific trade winds and reinforce the Walker circulation, parts of the same Pacific climate machinery that influences El Niño and La Niña.
Because those Pacific patterns help steer winter storm tracks and atmospheric rivers, researchers are increasingly interested in whether changes unfolding in the North Atlantic could eventually have drastic influence on weather conditions on the West Coast.
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