New data for the Decades of Old Science challenge

Until now, large-scale ocean circulation involving deep water rising to the surface has never been directly observed.

Scientists from the University of California, San Diego’s Scripps Institution of Oceanography led an international team to directly measure the upwelling of cold deep water through turbulent mixing along the slope of an undersea canyon in the Atlantic Ocean.

The rate of upwelling the researchers observed was more than 10,000 times the global average rate predicted by the late renowned 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 are beginning to unravel a troubling mystery of 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.

The world as we know it requires a large-scale ocean circulation, often called conveyor belt circulation, in which seawater cools and condenses near the poles, sinks to the depths, and eventually rises back to the surface where it warms, starting the cycle. again. These broad patterns maintain the circulation of heat, nutrients, and carbon that underpins global climate, marine ecosystems, and the ocean’s ability to mitigate human-induced climate change.

However, despite the importance of the conveyor belt, a 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 by upwelling has been theorized and inferred but never directly measured.

Munk’s theories and recent advances

In 1966, Munk calculated the global average rate of upwelling using the rate at which cold, deep water formed near Antarctica. He estimated the rate of ascent at one centimeter per day. The volume of water transported at this upwelling rate would be enormous, said Matthew Alford, professor of physical oceanography at Scripps and lead author of the study, “but spread across the entire global ocean, this current is too slow to measure directly. “

Munk proposed that this upwelling occurred through turbulent mixing caused by the breaking of internal waves below the ocean surface. About 25 years ago, measurements began to reveal that undersea turbulence was higher near the sea floor, but that presented a paradox to oceanographers, Alford said.

If the turbulence is strongest near the bottom where the water is coldest, then that part of the water below where the water is cooler would experience stronger mixing. This would cause the groundwater to become even colder and denser, pushing the water down instead of lifting it to the surface. This theoretical prediction, which has been confirmed by measurements, seems to contradict the observed fact that the deep ocean did not simply fill with cold, dense water formed at the poles.

New theory and direct observation

In 2016, scientists 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 create the right kind of turbulence to cause upwelling.

Wynne-Cattanach, Alford and their collaborators set out to see if they could directly observe the phenomenon by conducting an experiment at sea using a barrel of a non-toxic, fluorescent green dye called fluorescein. Starting in 2021, researchers visited a roughly 2,000-meter-deep underwater canyon in the Rockall Trough, about 370 kilometers (230 mi) northwest of Ireland.

“We chose this canyon out of about 9,500 that we know of in the oceans because this place is pretty unremarkable in deep canyons,” Alford said. “The idea was to make it as typical as possible to make our results more generalizable.”

Hovering over the underwater canyon in a research vessel, the team lowered a 55-gallon drum of fluorescein to within 10 meters (32.8 feet) of the sea floor, 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 customized at Scripps for the experiment’s requirements. The scientists were able to track the movement of the dye in high resolution by slowly moving the ship up and down the canyon slope. The key measurements came from devices called fluorometers, which 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 movement of the dye revealed turbulence-driven uplift 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 the uplift was much faster than Munk’s 1966 calculations had predicted.

Where Munk inferred a global average of one centimeter per day, measurements at the Rockall Trough found that the rise was occurring at a rate of 100 meters per day. In addition, the team observed how certain dye migrated away from the canyon’s slope and toward its interior, suggesting that the physics of turbulent upwelling was more complex than Ferrari had originally theorized.

“We saw an increase that had never been directly measured before,” Wynne-Cattanach said. “The rate of this upwelling is also really fast, which, along with measurements of upwelling elsewhere in the oceans, suggests that there are foci of upwelling.”

Alford called the study’s findings “a call to arms for the physical oceanography community to much better understand ocean turbulence.”

Wynne-Cattanach said that as a graduate student, it was a huge honor for her to lead a project that represents the culmination of decades of work by scientists from across the field with such distinguished researchers as collaborators. Based on the team’s preliminary findings, Wynne-Cattanach has become the first student to be invited to present at the prestigious Gordon Research Conference on Ocean Mixing in 2022.

The next step will be to test whether similar uplift is occurring in other underwater 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 elsewhere, Alford said global climate simulations will need to begin explicitly accounting for this type of turbulence-driven upwelling in topographic features of the ocean floor. “This work is the first step in adding the missing ocean physics to our climate models, which will ultimately improve the ability of these models to predict climate change,” he said.

According to Alford, the path to improving scientific understanding of ocean turbulence is twofold. First, “we need to do more cutting-edge, 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.”

Scientists are already conducting a similar dye-release experiment just off the coast of Scripps’ campus in La Jolla’s underwater canyon.

Reference: “Observation 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, 26 Jun 2024, Nature.
DOI: 10.1038/s41586-024-07411-2