The Gulf Stream carries warm water from the Gulf of Mexico up the US East Coast
NASA’s Scientific Visualization Studio/SCIENCE PHOTO LIBRARY
A gradual northward shift in the Gulf Stream has provided more evidence that the current system that keeps Europe warm is weakening. Moreover, modeling suggests that any abrupt shift in the Gulf Stream could signal an imminent, catastrophic collapse in 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 south along the ocean floor. The part of this circulation that runs from the Gulf of Mexico up the US east coast to North Carolina, where it turns east into the Atlantic Ocean, is called the Gulf Stream.
The AMOC is expected to lose strength as the melting of the Greenland Ice Sheet dumps 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 that this is already happening, but researchers do not have direct evidence.
Now a modeling study by René van Westen and Henk Dijkstra, both at Utrecht University in the Netherlands, has shown that a weakening of the AMOC would shift the path of the Gulf Stream so that it follows the US coast further north before turning into the Atlantic.
Furthermore, the study finds that the Gulf Stream has already shifted northwards by approximately 50 kilometers in 30 years, according to satellite data.
“This is something we can measure,” says van Westen. “So it’s very likely that this reflects that the AMOC is actually weakening.”
Reconstructions that estimate the AMOC flow rate based on historical ocean temperatures suggest it has weakened by 15 percent since 1950. But the actual flow has only been monitored by moored instruments since 2004, not long enough to say whether observed changes are natural fluctuations or a trend.
“That’s why we’re trying to come up with some alternative approaches, such as the Gulf Stream,” says van Westen.
The model in the study represents the world in 10-kilometer pixels instead of the typical 100-kilometer pixels, so the researchers can trace 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 that carries cold, salty water southward along the ocean floor. This current normally flows down the coast of North America under the Gulf Stream, pulling 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 simulation’s future, the Gulf Stream jumps more than 200 kilometers to the north in just two years. 25 years after that, the AMOC collapses. Previous research has shown that such a collapse would drastically cool Europe; London could see temperatures of -20°C (-4°F) and Oslo, Norway, 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 that an abrupt shift in the Gulf Stream could serve as an early warning of an impending AMOC shutdown, the only such earlier indicator we know of. While it may be too late by then to avoid AMOC collapse, Europe can prepare by insulating houses and finding more southerly places to grow food.
“There is now a very real early warning indicator that actually goes off,” says van Westen. “You can measure this very easily.”
But it is unclear in the real world how long after a Gulf Stream shift the AMOC can collapse. And estimates of when the AMOC might shut down vary from decades to centuries.
Dan Seidov, an oceanographer retired from the US National Oceanic and Atmospheric Administration, warns that freshwater from Greenland could “snake” the AMOC at a different speed and in a different location than the model assumes.
“How, when and why it may or may not happen is the big question,” he says. “If it happens as prescribed in the model, the Gulf Stream could be a precursor and give a warning signal.”
While the link between the abrupt shift and the AMOC collapse needs to be confirmed by other models, this study provides more evidence that the AMOC is already slowing down, says Stefan Rahmstorf of the University of Potsdam in Germany.
“This decline is happening earlier than in the global warming scenarios,” he says. “Climate models appear to underestimate the problem and thus potentially how soon an AMOC tipping point will be reached.”
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