A special high-speed winch the researchers used to rapidly raise and lower the instruments to track the movements of the paint underwater. Credit: San Nguyen
Until now, the large-scale ocean circulation involving deep water rising to the surface had never been observed directly.
For the first time, researchers from the University of California, San Diego’s Scripps Institution of Oceanography have led an international team to directly measure the upwelling of cold, deep water through turbulent mixing along the slope of a submarine canyon in the Atlantic Ocean.
The growth rate the researchers observed was more than 10,000 times the global average rate predicted by the late noted 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 troubling mystery in oceanography and could eventually help improve humanity’s ability to predict climate change. The research was supported by grants from the Environmental Research Council and the National Science Foundation.
This barrel is filled with non-toxic fluorescent dye, which researchers released right onto the sea floor to answer a long-standing question in oceanography. Credit: San Nguyen
The world as we know it requires a large-scale ocean circulation, often called a conveyor belt circulation, in which seawater becomes cold and dense near the poles, sinks to the depths, and eventually rises back to the surface where it warms, starting cycle again. These broad patterns maintain a circulation of heat, nutrients, and carbon that supports global climate, marine ecosystems, and the ocean’s ability to mitigate human-induced climate change.
Despite the importance of the conveyor belt, however, 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 Advances
In 1966, Munk calculated a global average growth rate using the rate at which cold, deep water formed near Antarctica. He estimated the rate of rise at one centimeter per day. The volume of water transported by this rate of upwelling would be large, said Matthew Alford, professor of physical oceanography at Scripps and senior author of the study, “but spread throughout the global ocean, this flow is too slow to was measured directly.
Munk proposed that this increase occurred through turbulent mixing caused by the breaking of internal waves below the ocean’s surface. About 25 years ago, measurements began to reveal that underwater turbulence was highest near the seafloor, but that presented oceanographers with a paradox, Alford said.
Bethan Wynne-Cattanach and Matthew Alford observe operations aboard the research vessel during the experiment. Credit: San Nguyen
If turbulence is stronger near the bottom where the water is colder, then a given parcel of water would experience stronger mixing below it, where the water is colder. This would have the effect of making the waters below even colder and denser, pushing the water down rather than lifting it to the surface. This theoretical prediction, since confirmed by measurements, seems to contradict the observed fact that the deep ocean is not simply filled 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 had the potential to resolve this paradox. The idea was that steep slopes on the seafloor in places like the walls of underwater canyons could produce the right kind of turbulence to cause uplift.
Wynne-Cattanach, Alford and their collaborators set out to see if they could directly observe this phenomenon by conducting an experiment at sea with the help of a vat of a non-toxic, fluorescent green dye called fluorescein. Starting in 2021, researchers visited an underwater canyon roughly 2,000 meters deep in the Rockall Trough, about 370 kilometers (230 miles) northwest of Ireland.
“We chose this canyon out of about 9,500 that we know of in the oceans because this place is pretty extraordinary as deep sea canyons go,” Alford said. “The idea was to make it as typical as possible to make our results more generalizable.”
Floating above the submarine canyon in 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 ink for two and a half days until it dispersed using several instruments adapted in-house at Scripps to the experiment’s requirements. The researchers were able to track the movement of the paint in high resolution by slowly moving the craft up and down the canyon slope. The main measurements came from devices called fluorometers that are able to detect the presence of tiny amounts of 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 movements of the ink revealed turbulence-induced 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 uplift along the canyon’s slope, but that uplift was much faster than Munk’s 1966 calculations predicted.
Where Munk derived a global average of one centimeter per day, measurements in the Rockall Trough revealed that growth was continuing at 100 meters per day. Additionally, the team observed some ink migrating away from the canyon slope and toward its interior, suggesting that the physics of turbulent upwelling was more complex than Ferrari first theorized.
“We have seen growth that has never been measured before,” Wynne-Cattanach said. “The speed of this upwelling is also very fast, which, together with downwelling measurements elsewhere in the oceans, suggests that there are upwelling hotspots.”
Alford called the study’s findings “a call to arms for the physical oceanography community to much better understand ocean turbulence.”
Wynne-Cattanach said it was a great honor for her, as a graduate student, to lead a project that represents 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 to be invited to speak at the prestigious Gordon Research Conference on Ocean Mixing in 2022.
The next step will be to test whether there is similar growth in other submarine canyons around the world. Given the canyon’s unusual features, Alford said it seems reasonable to expect the phenomenon to be relatively common.
If the results hold true elsewhere, Alford said global climate simulations will have to begin explicitly accounting for this kind of turbulence-driven uplift in topographic features of the ocean floor. “This work is the first step in adding missing ocean physics to our climate models that 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, according to Alford. First, “we need to do 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 float.”
Researchers are already in the process of conducting a similar dye-release experiment off the coast of the Scripps campus in the La Jolla submarine canyon.
Reference: “Observations of diapycnal uplift within a steep submarine canyon” by Bethan L. Wynne-Cattanach, Nicole Couto, Henri F. Drake, Raffaele Ferrari, Arnaud Le Boyer, Herlé Mercier, Marie-José Messias, Xiaozhou. Spingys, Hans van Haren, Gunnar Voet, Kurt Polzin, Alberto C. Naveira Garabato, and Matthew H. Alford, 26 June 2024, Nature.
DOI: 10.1038/s41586-024-07411-2