Smack dab between eastern Canada’s Misery Point and Greenland's Cape Desolation is a place where the thrashing of the Atlantic Ocean’s churn sounds about as friendly as the nearby place names. This stretch of water, the Labrador Sea, has long been considered a critical junction in the global circulatory system of the world's oceans. By pumping warm water north and cool water south, the system regulates the planet’s climate.
For decades, scientists have turned to the Labrador Sea to understand how ocean processes there may be affecting the strength of a massive oceanic conveyor belt known as the Atlantic Meridional Overturning Circulation. Many researchers have found that the system is weak, which could spell trouble for changing climate conditions in the future. But data emerging from new suites of ocean-monitoring instruments suggests this narrative is headed for a twist.
The idea of a faltering AMOC entered public consciousness years ago with the popular but exaggerated catastrophe flick The Day After Tomorrow. In it, the AMOC grinds to a halt, causing superstorms to ravage entire cities and mega hurricanes to suck frozen air down from space.
That’s not going to happen. But if the AMOC continues to lose strength, or even temporarily shuts down (as some studies have suggested), it could mean cooler winters and summers in Europe and other regions around the North Atlantic coast. Great Britain might see crop production plummet, according to one study, and the ocean could end up sequestering less carbon, which would leave more heat-trapping CO2 in the atmosphere and trigger faster warming elsewhere.
Given the direct impacts the AMOC can have on our climate, scientists have been trying to assess its circulatory strength mainly through the use of computer models, a generally reliable tool in climate science. But with the AMOC specifically, models often don’t work as well. They fall short, scientists say, when trying to simulate important but small-scale ocean processes. For example, ocean eddies—swirling currents that can bring deep waters to the surface and strengthen the system’s overturning (the “O” in AMOC)—are difficult to simulate accurately. Martha Buckley, a climate scientist at George Mason University, says it’s mainly a problem of scale. “Certain ocean processes cannot be explicitly represented by models due to resolution constraints,” she says.
An underlying issue is that the ability to measure what’s happening in the ocean—not simulate it—has been extremely limited. “Observations of the AMOC are sparse, and scientific knowledge is mainly based on model simulations,” wrote Monika Rhein, a scientist at the University of Bremen, in Germany, in the journal Science last year.
Scientists are increasingly looking to sensor-based instruments that monitor the ocean at various depths to take the pulse of the AMOC. And in some cases, these instruments' observations are serving up dramatic twists to the reigning AMOC narrative. “We’re seeing signs through direct ocean observations that the AMOC is not as weak as some scientists are suggesting,” says Igor Yashayaev, a marine scientist at the Bedford Institute of Oceanography.
As part of an ongoing ocean monitoring program, he and his colleagues observed that during a series of colder-than-average winters between 2010 and 2018, surface waters in the Labrador Sea became denser and more voluminous as they surrendered their heat to the atmosphere. These heavier surface waters then sank deeper into the ocean, pushing up less-dense waters from below. It was as if the ocean was turning upside down. As a result, the conveyor belt’s throttle was notched higher.
Yashayaev and his team based these observations on data collected with instruments known as Argo floats. The floats, which look like human-sized yellow syringes, autonomously nose-dive thousands of meters into the abyss and then resurface, measuring water temperature and salinity along the way. The measurements provide snapshots in time that allow him and his team see how far surface waters end up sinking, “and thus infer overturning strength,” he says.
Yashayaev’s finding that the AMOC may be ramping up is just one counterpoint. But recent ocean data has also surfaced other discrepancies.
Penny Holliday, a scientist at the National Oceanography Center, in the UK, also relies on direct ocean observations to study the AMOC. She’s been focused on a different set of instruments deployed several hundred miles east of the Labrador Sea. There, more than 50 sensor-equipped instruments called profiling moorings are continuously monitoring the ocean. The moorings, which were purpose-built for AMOC studies, were deployed in 2014 as part of a $35 million, multinational project called OSNAP (short for Overturning in the Subpolar North Atlantic Program) aimed at answering fundamental questions about what’s driving its strength. “We weren’t sure how strong the overturning was in the North Atlantic, or how much it varied on timescales of months to years,” says Holliday. “The only way to know the strength is by these series of moorings.”
She says it’s too early to tell if the AMOC is showing a clear strengthening or weakening trend based on only a few years of data. But the array has changed some of the fundamental thinking about what’s driving the AMOC. A 2019 study reviewing the array’s initial data sets suggests the waters between Greenland and Scotland are primarily responsible for driving the AMOC—not the Labrador Sea.
He’s not convinced that he and other scientists have been focusing on the wrong corner of the Atlantic. But the study raises another question: Are the mechanisms that control the strength of AMOC a moving target, like climate change itself? Researchers at Yale University recently reported that the Indian Ocean may be a key driver of the AMOC, which could see an estimated 30 percent increase in strength from a 1 degree increase in surface warming.
To try to resolve some of these mysteries, the modeling community is now working more closely with ocean observers and their data. “Both are key to understanding how the future is going to change,” says Jon Robson, a climate modeler at the University of Reading, in the UK.
Modelers are even starting to incorporate ocean data into their models nearly in real-time. This particular technique, known as ocean reanalysis, is “what atmospheric scientists have been doing for years in weather forecasting,” says Holliday, but ocean observers lacked the data to follow suit. “It keeps models closer to reality as they’re running.”
More collaborative use of technology may help, but the complex and nuanced nature of the AMOC almost ensures that scientific conflicts will continue to bubble up. In fact, a modeling study published last month by researchers at Utrecht University, in The Netherlands, suggested there’s a chance the AMOC may temporarily collapse in the next 100 years. The reason? The flow of freshwater from Greenland’s (simulated) melting ice sheets into the North Atlantic could dilute the ocean’s salty surface water and make it lighter and more buoyant, impairing its ability to sink.
Neither Holliday nor Yashayaev thinks the effect of the freshwater will be quite so dramatic based on what they know from direct ocean observations. “Although we know there is extra fresh water coming into the Labrador Sea, we have not seen any evidence that it’s disrupting the complexion,” says Holliday.
When it comes to the AMOC, technology may be creating more questions than answers. But that’s how good science often happens.
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