A special high-speed winch that the researchers used to quickly raise and lower instruments to track the movements of the dye underwater. Credit: San Nguyen
Until now, no large-scale ocean circulation in which deep water rises to the surface had ever been observed.
For the first time, researchers from 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 an undersea rift in the Atlantic Ocean.
The researchers observed an upwelling speed more than 10,000 times higher than the average speed predicted by famed oceanographer Walter Munk in the 1960s.
The results appear in a new study led by Scripps postdoctoral fellow Bethan Wynne-Cattanach and published in the journal Nature. The findings begin to unravel a thorny mystery in oceanography and could ultimately help improve humanity’s ability to predict climate change. The research was supported by grants from the Natural Environment Research Council and the National Science Foundation.
This vessel is filled with 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 into the depths, and eventually rises back to the surface where it warms, starting the cycle again. These broad patterns maintain a turnover rate of heat, nutrients, and carbon that underlies global climate, marine ecosystems, and the ocean’s ability to mitigate human-induced climate change.
Despite the importance of the conveyor belt, one component of it, 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 through upwelling has been theorized and inferred, but never directly measured.
Munk’s theories and recent developments
In 1966, Munk calculated a global average upwelling rate, using the rate at which cold, deep water formed near Antarctica. He estimated the upwelling rate at one centimeter per day. The amount of water transported by this upwelling rate would be enormous, says Matthew Alford, a professor of physical oceanography at Scripps and lead author of the study, “but across the entire global ocean, the current is too slow to measure directly.”
Munk proposed that this upwelling occurred via turbulent mixing caused by the breaking of internal waves beneath the ocean surface. About 25 years ago, measurements began to reveal that the submarine turbulence was higher near the seafloor, but this presented oceanographers with a paradox, Alford said.
Bethan Wynne-Cattanach and Matthew Alford observe activities aboard the research vessel during the experiment. Credit: San Nguyen
If turbulence is strongest near the bottom where the water is coldest, then a given parcel of water below will experience greater mixing where the water is colder. This would have the effect of making the bottom water even colder and denser, pushing the water downward instead of lifting it to the surface. This theoretical prediction, which has since been confirmed by measurements, appears to contradict the observed fact that the deep ocean is not simply filled with the cold, dense water that 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 had the potential to resolve this paradox. The idea was that steep slopes on the seafloor, such as the walls of underwater canyons, could produce the right kind of turbulence to cause upwelling.
Wynne-Cattanach, Alford and their collaborators set out to see if they could directly observe this phenomenon by conducting an experiment at sea using a vessel containing a non-toxic, fluorescent green dye called fluorescein. Starting in 2021, the researchers visited an approximately 2,000-meter-deep submarine canyon in the Rockall Trough, about 370 kilometers (230 miles) northwest of Ireland.
“We selected this canyon from the approximately 9,500 we know of in the oceans because this spot is quite unremarkable for deep-sea canyons,” Alford said. “The idea was to make it as typical as possible to make our results more generalizable.”
Hovering above the undersea canyon in a research vessel, the team lowered a 55-gallon drum of fluorescein to 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 disappeared, using a variety of instruments customized in-house at Scripps to meet the needs of the experiment. The researchers were able to track the dye’s movement at high resolution by slowly rocking the ship up and down the canyon slope. The key measurements came from devices called fluorometers, which can detect the presence of tiny amounts of the fluorescent dye—down to less than 1 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 Ferrari’s proposed solution to the paradox with direct observations for the first time. Not only did the team measure upwelling along the canyon slope, it was also much faster than Munk’s 1966 calculations predicted.
Where Munk derived a global average of one centimeter per day, measurements at Rockall Trough showed upwelling at 100 meters per day. In addition, the team saw some dye moving away from the canyon slope and into its interior, suggesting that the physics of the turbulent upwelling was more complex than Ferrari had originally thought.
“We have observed an upwelling that has never been measured directly before,” Wynne-Cattanach says. “The speed of that upwelling is also very fast, which, together with measurements of upwelling elsewhere in the ocean, suggests that there are hot spots of upwelling.”
Alford called the study’s findings “a call to arms for the physical oceanographic 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 is the culmination of decades of work by scientists across the field, with such distinguished researchers as collaborators. Based on the team’s preliminary findings, Wynne-Cattanach became the first student 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 inconspicuous features, Alford said it seems reasonable to expect the phenomenon to be relatively common.
If the results hold elsewhere, Alford said global climate simulations will need to explicitly account for this type of turbulence-driven upwelling at topographic features on the ocean floor. “This work is the first step toward adding missing ocean physics to our climate models, which will ultimately improve the ability of those models to predict climate change,” he said.
The path to improving scientific understanding of ocean turbulence is twofold, Alford said. First, “we need to conduct more high-tech, high-resolution experiments like this in key parts of the ocean to better understand the physical processes.” Second, he said, “we need to measure turbulence in as many different places as possible with autonomous instruments like the Argo floats.”
The researchers are already conducting a similar dye-release experiment just off the coast of the Scripps campus in the La Jolla submarine canyon.
Reference: “Observations of diapycnal upwelling in 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 file: 10.1038/s41586-024-07411-2