Climate change might not slow ocean circulation as much as thought

New findings from an international ocean observing network are calling into question the long-standing idea that global warming might slow down a big chunk of the ocean’s “conveyor belt.” The first 21 months of data from sensors moored across much of the North Atlantic are giving new insight into what controls the strength of the Atlantic Meridional Overturning Circulation, a system of currents that redistributes heat around much of the Western Hemisphere.

Researchers had thought the strength of that circulation, known by the acronym AMOC, was largely influenced by the sinking of cold freshwater in the Labrador Sea, between Greenland and Canada. And climate simulations suggest that the sea’s deepwater formation might slow as the world continues to warm — which also could slow down the entire Atlantic current system and possibly make temperatures on land in the northeastern United States and the United Kingdom plunge. That concept inspired the (otherwise unrealistic) 2004 climate apocalypse film The Day After Tomorrow.

But, the data collected over those 21 months show that the Labrador Sea’s influence on the AMOC paled in comparison with that of another North Atlantic ocean region, just east of Greenland. How the intensity of deepwater formation in that area changed with time accounted for 88 percent of the observed variability in the entire AMOC, physical oceanographer Susan Lozier of Duke University and colleagues report in the Feb. 1 Science.

The results provide “an unprecedented insight into how the modern North Atlantic operates,” says paleoceaonographer David Thornalley of University College London, who was not involved in the study.

Atlantic Ocean circulation is driven by differences in water density related to freshness and temperature: Warm, salty water (including the Gulf Stream) flows north at the ocean surface, delivering heat to the northeastern United States and the British Isles. Near Greenland, the current splits, with one arm heading for the Labrador Sea west of Greenland and the other toward the Nordic Sea to the east. There, the waters become both colder and fresher, thanks to meltwater from land. The colder water then sinks and travels south again along the ocean floor.

Many studies have suggested that the Labrador Sea regulates AMOC’s strength, but those are largely based on climate simulations, Lozier says. “We need to ground-truth the simulations,” she says. “This is where we really need observations.”

Previously, the only AMOC measurements came from the RAPID-AMOC array deployed in 2004. But that array monitors the current system much farther south, in the subtropics. To understand how deepwater formation in the north might control the current’s strength, Lozier and other scientists in 2014 launched OSNAP, short for Overturning in the Subpolar North Atlantic Program, an international consortium tasked with providing a continuous record of salinity, temperature and current velocity throughout the full water column.

Data recovery

As part of the OSNAP program, long tethers of instruments topped with yellow floats are anchored to the seafloor in carefully chosen arrays, continuously recording temperature, salinity, current velocity and other data from the surface of the ocean all the way down to the seafloor. In July 2018, four years after this array of such moorings was deployed in the Irminger Sea east of Greenland, OSNAP scientists reeled the instruments back in to retrieve the data; the team then redeployed the moorings.

The group set up more than 55 moorings, or lines of sensors tethered to the seafloor along two main transects — one stretching west from Greenland across the Labrador Sea, and one stretching east to Scotland.

The AMOC doesn’t only redistribute heat, Lozier notes: It also helps to regulate how much atmospheric carbon dioxide the ocean can absorb. Earth’s oceans have already absorbed about 30 percent of the carbon dioxide emitted by humans since the Industrial Revolution, she says. “Half of that is now in the deep North Atlantic Ocean due to the overturning circulation.” That means that continued circulation of large currents such as the AMOC will also moderate the ocean’s future ability to help mitigate global warming.

The first reported results from analyzing the sensor data may be a surprise to many scientists, Lozier says, as they go against the prevailing wisdom.

But what ultimately controls the AMOC in the long-term is far from settled. The OSNAP data in the study cover only two years, and may not reflect circulation over longer timescales, such as decades, Thornalley notes. 

And several recent studies, including two published in Nature last year, have suggested that the AMOC has shown signs of slowing down. One of the studies, led by Thornalley, reported that the AMOC has been relatively weak over the past 150 years, compared with the previous 1,500 years. Thornalley’s team also reported that Labrador Sea circulation was very weak during that time.

One thing that all the researchers agree on is that OSNAP’s continued monitoring will be essential to solving this puzzle.