A key ocean current is slowing in places around the Atlantic

Buoy measurements show that the Atlantic Southern Overturning Circulation, which moderates Europe’s climate, is weakening at four different latitudes, the strongest evidence yet that this ocean current system is slowing and could be headed for collapse.

The AMOC is part of the oceanic conveyor belt of currents circling the globe and brings warm, salty water from the Gulf of Mexico into the North Atlantic, keeping temperatures in western Europe milder than in Canada or Russia. The dense water then cools and sinks, moving south on the sea floor along the western side of the Atlantic.

An analysis of old ocean temperature data suggests the AMOC has weakened by 15 percent since 1950, and some computer modeling has warned it could shut down within decades. But scientists have only measured it directly for about two decades, which is not long enough to draw definite conclusions.

Now, a study in the western Atlantic has shown more conclusively that the AMOC is slowing.

“The Atlantic circulation is weakening at the western boundary, and we use several latitudes of basin data to confirm that such a signal from the western boundary is consistent in the broader North Atlantic,” he says. Qianjiang Xing at the University of Miami in Florida, who led the study.

In 2004, the University of Miami and other institutions installed a series of moorings from the Bahamas to the Canary Islands called RAPID-MOCHA. By measuring the field’s temperature, salinity and velocity, scientists estimate the pressure, or “how much water is effectively piled up” on both sides of the Atlantic, a team member said. Shane Elipotalso at the University of Miami.

Water flows from areas of high pressure to areas of low pressure, but is deflected to the right by the Earth’s counter-clockwise rotation, driving an overturning circulation. Changes in pressure can therefore indicate changes in the strength of the AMOC.

The study’s analysis of the latest RAPID-MOCHA data shows that AMOC flow is declining by about 90,000 cubic meters of water per second each year, faster than previously observed. This means that between 2004 and 2023, the AMOC weakened by about 10 percent.

But the range of uncertainty of this flux change is almost as large as the change itself. For this reason, Xin’s study also analyzes pressure changes at three moorings that have been installed since 2004 in the Western Atlantic off the West Indies, the US East Coast and Nova Scotia, Canada. There, they find an even greater weakening of the AMOC with much less uncertainty.

“It’s the strongest direct observational evidence yet” that the AMOC is weakening, as models have long shown, he says Stefan Rahmstorf at the University of Potsdam, Germany, who was not involved in the research.

Scientists believe that freshwater from the melting of the Greenland ice sheet dilutes the dense, salty AMOC water so that it sinks more slowly and weakens the southward flow along the western Atlantic floor. The downward trend observed by the study at four latitudes in the western Atlantic suggests that this is indeed happening.

“We expect to see this in the deep western frontier,” says a team member David Smeed at the UK National Oceanographic Centre. “It gives us confidence that this interpretation is correct.”

“They show for the first time, that I’m aware of, that there is this very coherent picture of a deep westerly overturning weakening for all different kinds of latitudes,” he says. René van Westen at Utrecht University in the Netherlands, who was not part of the research.

The findings highlight the need for more observations to try to understand whether the AMOC is headed for collapse, according to Elipot. A collapse would cause dramatically colder winters in Europe and could disrupt the Asian and African monsoons.

“The trend would be consistent with a transition to a tipping point,” he says.

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