A special high-speed winch that the researchers used to quickly raise and lower the instruments to track the movements of the dye underwater. Credit: San Nguyen
Until now, no large-scale ocean circulation involving deep waters rising to the surface had ever been directly observed.
For the first time, researchers at the Scripps Institution of Oceanography at the University of California, San Diego, have led an international team to directly measure the upwelling of cold, deep water by turbulent mixing along the slope of a submarine canyon in the Atlantic Ocean.
The rate of rise observed by the researchers was more than 10,000 times higher than the global average rate predicted by the late oceanographer Walter Munk in the 1960s.
The findings appear in a new study led by Scripps postdoctoral fellow Bethan Wynne-Cattanach and published in the journal Nature. These results begin to unravel a complex mystery in oceanography and could eventually help improve humanity’s ability to predict climate change. The research was funded by grants from the Natural Environment Research Council and the National Science Foundation.
This barrel is filled with a non-toxic fluorescent dye, which researchers released just above the seafloor to answer a long-standing question in oceanography. Credit: San Nguyen
The world as we know it requires large-scale ocean circulation, often called conveyor belt circulation, in which seawater becomes cold and dense near the poles, sinks to the depths, and eventually rises to the surface where it warms, starting the cycle again. These general patterns maintain a turnover of heat, nutrients, and carbon that underpins the global climate, marine ecosystems, and the oceans’ ability to mitigate human-induced climate change.
Despite the importance of the meridional circulation, one of its components, known as the meridional overturning circulation (MOC), has proven difficult to observe. In particular, the return of cold water from the deep ocean to the surface by upwelling has been theorized and inferred, but never directly measured.
Munk’s theories and recent advances
In 1966, Munk calculated a global average rate of upwelling using the rate at which cold, deep water formed near Antarctica. He estimated the rate of water rise at one centimeter per day. The volume of water carried by this upwelling rate would be enormous, said Matthew Alford, a professor of physical oceanography at Scripps and lead author of the study, “but spread across the entire global ocean, this flow is too slow to be measured directly. »
Munk proposed that this upwelling occurred via turbulent mixing caused by internal waves breaking beneath the ocean surface. About 25 years ago, measurements began to reveal that underwater turbulence was higher near the seafloor, but that presented a paradox to oceanographers, Alford said.
Bethan Wynne-Cattanach and Matthew Alford observe operations aboard the research vessel during the experiment. Credit: San Nguyen
If turbulence is greatest near the bottom, where the water is coldest, then some water would be mixed more strongly below, where the water is colder. This would have the effect of making the bottom waters even colder and denser, pushing the water down instead of lifting it to the surface. This theoretical prediction, since confirmed by measurements, seems to contradict the observed fact that the deep oceans did not simply fill with cold, dense water formed at the poles.
New theory and direct observations
In 2016, researchers including Raffaele Ferrari, an oceanographer at the Massachusetts Institute of Technology and co-author of the current study, proposed a new theory that could resolve this paradox. The idea was that steep slopes of the sea floor, particularly on the walls of underwater canyons, could produce the type of turbulence conducive to upwelling.
Wynne-Cattanach, Alford and their collaborators investigated whether they could directly observe this phenomenon by conducting an experiment at sea using a barrel of a non-toxic fluorescent green dye called fluorescein. Starting in 2021, researchers visited an underwater canyon about 2,000 meters deep in the Rockall Trough, about 370 kilometers (230 miles) northwest of Ireland.
“We chose this canyon from the approximately 9,500 we know of in the oceans because this location is pretty unremarkable for an underwater canyon,” Alford said. “The idea was to make it as typical as possible so that our results would be more generalizable. »
Floating above the underwater canyon aboard a research vessel, the team lowered a 55-gallon drum of fluorescein 10 meters (32.8 feet) above the seafloor and then remotely triggered the release of the dye.
The team then tracked the dye for two and a half days until it dissipated using several instruments adapted in-house at Scripps for the requirements of the experiment. The researchers were able to track the movement of the dye at high resolution by slowly moving the ship up and down the canyon slope. The key measurements were made using devices called fluorometers, which can detect the presence of trace amounts of fluorescent dye – down to less than one part per billion – but other instruments also measured changes in water temperature and turbulence.
Implications and future research
Tracking the dye’s movements revealed turbulence-driven upwelling along the canyon slope, confirming for the first time Ferrari’s resolution of the paradox through direct observations. Not only did the team measure upwelling along the canyon slope, it was also much faster than Munk’s 1966 calculations had predicted.
While Munk had deduced an overall average of one centimeter per day, measurements at Rockall Trough revealed that the upwelling was occurring at a rate of 100 meters per day. Additionally, the team observed a migration of dyes from the slope of the canyon into its interior, suggesting that the physics of turbulent upwelling was more complex than Ferrari initially theorized.
“We observed upwelling that had never been directly measured before,” Wynne-Cattanach said. “The rate of this upwelling is also very rapid, which, together with measurements of downwelling elsewhere in the oceans, suggests that there are upwelling hotspots.”
Alford called the study’s results “a call to arms for the physical oceanography community to better understand ocean turbulence.”
Wynne-Cattanach said it was a tremendous honor for her, as a graduate student, to lead a project that represents the culmination of decades of work by scientists across fields with such distinguished researchers as collaborators. Based on the team’s preliminary findings, Wynne-Cattanach became the first student to be invited to speak at the prestigious Gordon Research Conference on Ocean Mixing in 2022.
The next step will be to test whether similar upwelling occurs in other submarine canyons around the world. Given the canyon’s unremarkable features, Alford said it seems reasonable to expect the phenomenon to be relatively common.
If the results hold up elsewhere, Alford said, global climate simulations will need to start explicitly accounting for this type of turbulence-driven upwelling at the topographic features of the ocean floor. “This work is the first step in adding the missing elements of ocean physics to our climate models that will ultimately improve the ability of these models to predict climate change,” he said.
According to Alford, there are two steps to improving the scientific understanding of ocean turbulence. First, “we need to do more high-tech, high-resolution experiments like this in key areas of the ocean to better understand the physical processes.” Second, he added, “we need to measure turbulence in as many different places as possible with autonomous instruments like the Argo floats.”
Researchers are already conducting a similar dye-release experiment off the coast of the Scripps campus, in the La Jolla underwater canyon.
Reference: “Observations of diapycnal upwelling within a sloping submarine canyon” by Bethan L. Wynne-Cattanach, Nicole Couto, Henri F. Drake, Raffaele Ferrari, Arnaud Le Boyer, Herlé Mercier, Marie-José Messias , Xiaozhou Ruan, Carl P. Spingys, Hans van Haren, Gunnar Voet, Kurt Polzin, Alberto C. Naveira Garabato and Matthew H. Alford, June 26, 2024, Nature.
DOI: 10.1038/s41586-024-07411-2