A new modelling study indicates that a weakening Atlantic Meridional Overturning Circulation is causing the Gulf Stream to drift northwards, with satellite data showing a 50-kilometre shift over 30 years. Researchers suggest this gradual change could precede an abrupt move serving as an early warning for a potential AMOC collapse. Such a collapse might lead to drastic cooling in Europe, though timelines remain uncertain.
The Atlantic Meridional Overturning Circulation (AMOC) transports warm, salty surface water from the tropics to north-western Europe, where it cools, sinks, and returns south along the ocean floor. The Gulf Stream forms the northward segment along the US east coast up to North Carolina before veering east into the Atlantic.
Melting from the Greenland ice sheet is introducing fresh water into the north Atlantic, diluting the dense water and slowing the AMOC's sinking process. Reconstructions based on historic sea temperatures estimate a 15 per cent weakening since 1950, though direct monitoring via moored instruments began only in 2004.
A study by René van Westen and Henk Dijkstra at Utrecht University, published in Nature Communications Earth & Environment, used a high-resolution model with 10-kilometre pixels to simulate AMOC changes. It predicts that a weakening AMOC reduces the southward pull of the Deep Western Boundary Current, causing the Gulf Stream to extend further north along the US seaboard.
Satellite data confirms the Gulf Stream has already shifted about 50 kilometres northward in the past 30 years. "This is something we can measure," says van Westen. "So it is very likely that this reflects that the AMOC is indeed weakening."
In the model's simulation, after 392 years, the Gulf Stream abruptly jumps more than 200 kilometres north in just two years, followed by an AMOC collapse 25 years later. Previous research indicates such a collapse could cause severe cooling in Europe, with London experiencing -20°C snaps and Oslo reaching -48°C.
The study does not forecast a collapse in 400 years but proposes the abrupt Gulf Stream shift as a measurable early warning indicator. "There is now a very proper early warning indicator that actually goes off," says van Westen. "You can measure this very easily."
However, projections for AMOC shutdown vary from decades to centuries. Dan Seidov, a retired oceanographer from the US National Oceanic and Atmospheric Administration, notes that freshwater influx from Greenland might differ from model assumptions. "How, when and why it may or may not happen is the big question," he says. "If it happens as is prescribed in the model, then the Gulf Stream can be a precursor and provide a warning signal."
Stefan Rahmstorf at the University of Potsdam views the findings as further evidence of earlier-than-expected AMOC slowing. "This slowing is occurring 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."