A shift in the Gulf Stream could signal a collapse of the ocean current

The gradual northward shift of the Gulf Stream provided further evidence that the current system that keeps Europe warm is weakening. Additionally, modeling suggests that any sudden shift in the Gulf Stream could signal an impending catastrophic collapse of the ocean current.

The Atlantic Meridional Overturning Circulation (AMOC) is the flow of warm, salty surface water from the tropics to northwestern Europe, where it cools and sinks, returning southward along the ocean floor. The part of this circulation that runs from the Gulf of Mexico along the east coast of the US to North Carolina, where it turns east into the Atlantic, is called the Gulf Stream.

The AMOC is expected to weaken as the melting of the Greenland ice sheet releases fresh water into the North Atlantic, diluting the dense, salty AMOC water and slowing the rate at which it sinks and flows south. Some research suggests this is already happening, but scientists don’t have direct proof.

Now a modeling study René van Westen and Henk Dijkstraboth at Utrecht University in the Netherlands, showed that a weakening AMOC would shift the path of the Gulf Stream to follow the US coast further north before turning into the Atlantic.

In addition, the study found that the Gulf Stream has moved northward by about 50 kilometers in 30 years, according to satellite data.

“That’s something we can measure,” says van Westen. “So it’s very likely that this reflects that the AMOC is indeed weakening.”

Reconstructions that estimate AMOC flow based on historical sea temperatures suggest it has weakened by 15 percent since 1950. However, its actual flow has only been monitored by moored instruments since 2004, which is not long enough to say whether the observed changes are natural fluctuations or a trend.

“That’s why we’re trying to come up with some alternative approaches, like the Gulf Stream pathway,” says van Westen.

The model in the study represents the world in 10-kilometer pixels rather than the typical 100-kilometer pixels, allowing researchers to track the bulge where the Gulf Stream carries masses of water.

The path of the bulge changes due to the Deep Western Boundary Current, one of the arms of the AMOC, which carries cold, salty water southward along the seafloor. This current normally flows along the coast of North America under the Gulf Stream and pulls it south. As the AMOC weakens, the deep western boundary current also weakens and the curve of the Gulf Stream gradually shifts northward.

However, 392 years into the future of the simulation, the Gulf Stream jumps more than 200 kilometers north in just two years. Twenty-five years later, the AMOC collapses. Previous research has shown that such a collapse would be drastic cool Europe; London could see cold temperatures of -20 °C (-4 °F) and Norway’s Oslo could reach -48 °C (-54 °F).

The modeling is an idealized scenario that does not suggest that the AMOC will collapse in 400 years. But it does suggest a sudden shift in the Gulf Stream that could serve as an early warning of an impending AMOC shutdown, the only such prior indicator we know of. While by then it may be too late to prevent the AMOC from collapsing, Europe could prepare by warming homes and finding more southerly places to grow food.

“Now there is a very accurate early warning indicator that actually goes off,” says van Westen. “You can measure that very easily.

But in the real world, it’s not clear how long after the Gulf Stream shifts, the AMOC might collapse. And projections for when the AMOC may shut down range from decades to centuries.

Dan Seidov, a retired oceanographer with the U.S. National Oceanic and Atmospheric Administration, warns that freshwater from Greenland could “hose” the AMOC at a different speed and location than the model predicts.

“How, when and why this may or may not happen is the big question,” he says. “If this happens as predicted in the model, then the Gulf Stream can be a precursor and provide a warning signal.”

While the link between the sudden shift and the collapse of the AMOC will need to be confirmed by other models, this study provides more evidence that the AMOC is already slowing down, he says Stefan Rahmstorf at the University of Potsdam, Germany.

“This slowdown occurs earlier than in global warming scenarios,” he says. “Climate models appear to be underestimating the problem, and thus potentially how soon the AMOC tipping point will be reached.”

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