Study finds hidden early warning indicators in the Gulf Stream could signal AMOC collapse

The Gulf Stream holds tantalizing clues about when other major Atlantic currents may disrupt due to climate change, a new study finds.

The Gulf Stream, which originates in the Gulf of Mexico and exits the U.S. East Coast near Cape Hatteras, North Carolina, is a tributary of the Atlantic Meridional Overturning Circulation (AMOC), a large ocean current system that brings heat, particularly to the Northern Hemisphere and Europe.

Increasing evidence suggests that the AMOC is weakening and accelerating towards a ‘tipping point’ that could completely change the climate in many parts of the world, including northwestern Europe and the tropical monsoon region. It’s unclear when the AMOC will collapse, but a new study shows that early warning signals hidden in the Gulf Stream could help scientists predict a collapse years before it happens.

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“First, there is a very gradual northward movement. [of the Gulf Stream]”This is related to a weakening of parts of the AMOC, but clearly we also have this spike when the AMOC becomes too weak, which is this early warning indicator,” study lead author Rene van Westen, a postdoctoral researcher in climate physics at Utrecht University in the Netherlands, told Live Science.

Unlike the other ocean currents that form the AMOC, the Gulf Stream is driven by winds. After passing through Florida, it travels north up the U.S. East Coast to Cape Hatteras, before turning east into the North Atlantic Ocean. Although the Gulf Stream is a surface current, its position is controlled by much deeper currents that also belong to the AMOC and generate tight eddies when interacting with the layers above. These eddies push the entire Gulf Stream southward, but as the AMOC weakens and the eddies relax, the Gulf Stream can begin to flow northward.

To further investigate these effects, van Westen and Henk Dijkstra, professor of physical oceanography at Utrecht University, simulated AMOC collapse in a very high-resolution ocean model over the Gulf Stream. In climate models commonly used in AMOC research, the Gulf Stream is “smoothed out so it has very few features and doesn’t capture the dynamics very well,” Van Westen said.

The researchers induced an AMOC collapse in their model, but the collapse was much more gradual than the collapse that humans could cause by heating the planet and accelerating the melting of Arctic ice, which prevents the formation of deep ocean currents. They looked at the Gulf Stream’s response in unprecedented detail and revealed for the first time that the current shifted northward quite abruptly 25 years before it began to collapse.

The results were published in the journal Communications Earth & Environmental on February 26th.

The researchers found that there were two phases to the Gulf Stream response, both measured off Cape Hatteras at 71.5 degrees west longitude. First, as the AMOC gradually weakened over the 392-year simulation, the Gulf Stream moved north by 83 miles (133 kilometers). Second, as the AMOC continued to weaken for two more years in the simulation, the Gulf Stream suddenly surged 136 miles (219 km) northward. This sudden change occurred just 25 years before AMOC collapse began, suggesting it could be used as an early warning signal to predict collapse.

While these two stages may be realistic, it is unlikely that the time lag between the sudden change in the Gulf Stream and AMOC collapse is actually 25 years, Van Westen said. That’s because the model did not take into account rising global temperatures, which are accelerating AMOC’s collapse. The model only simulated freshwater increase in the North Atlantic.

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The northward drift of the Gulf Stream currently affects marine ecosystems in the colder waters of the current’s north, but could soon reach the warmer waters of the south. Drifting could further exacerbate sea level rise along the East Coast, Van Westen said.

already in progress

The researchers then analyzed satellite data to determine whether the Gulf Stream had already started moving northward. “We found this relationship within our climate models. The next step was to see whether those results would show up in observations,” van Westen said. “What we’ve found in our observations is that the Gulf Stream has indeed been moving northward over the past 30 years.”

The discovery is further evidence that the AMOC is weakening and means it is in the first stages of responding to the Gulf Stream, Van Westen said. Natural climate and atmospheric fluctuations may be contributing to the slow drift, but their contribution is small compared to the weakening of the AMOC, he added.

It is unclear when the transition to the second phase of the Gulf Stream response will occur, but satellites are in place to detect when this switch occurs. The researchers’ next challenge is to figure out the actual time lag between this second stage and AMOC’s collapse so that it can serve as a reliable warning indicator, Van Westen said.

The study is the most detailed analysis yet of the potential impact of AMOC collapse on the Gulf Stream, but there are some caveats, said Maya Ben-Yami, a climate tipping point researcher at the Technical University of Munich and the Potsdam Institute for Climate Impact Research in Germany who was not involved in the study.

“This paper definitely points out that changes in the Gulf Stream could be a warning signal, but more work needs to be done to confirm that, for example by looking at different models,” Ben Yami told Live Science via email.

A sudden northward shift could occur without AMOC collapsing in the future, in which case it could be a response to weakening rather than an early warning indicator, he said.

Furthermore, the rate of AMOC weakening in this study is likely to be slower than expected under future warming conditions, Ben-Yami said, meaning the 25-year lag time may shrink to “almost zero” and be too slow to serve as a practical warning signal.

“Personally, I think it would be useful to have a signal that simply tells us that we have ‘crossed a tipping point’ without prior warning, but I think it remains to be seen which type of signal the changes in the Gulf Stream are,” she said.