Researchers tackle ocean paradox with 55 gallons of fluorescent dye

For the first time, researchers from the Scripps Institution of Oceanography at UC San Diego led an international team that directly measured the upwelling of cold, deep water via turbulent mixing along the slope of a canyon beneath -marine 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 researcher Bethan Wynne-Cattanach and published in the journal Nature.

The findings begin to unravel a vexing 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.

The world as we know it is the result of 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 rise to the surface where it heats up, thus starting the cycle again. These general patterns maintain a turnover of heat, nutrients, and carbon that underpins the global climate, marine ecosystems, and the ocean’s ability to mitigate human-caused climate change.

Despite the importance of the conveyor belt, a component 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.

In 1966, Munk calculated the average global rate of upwelling based on the rate at which cold, deep water was forming near Antarctica. He estimated the upwelling rate at about 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 the study’s lead author, “but spread across the entire global ocean, this flow is too slow to measure directly.”

Munk suggested that this upwelling occurred through 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 posed a paradox for oceanographers, Alford said.

If turbulence is greatest near the bottom, where the water is coldest, then a given portion of water would experience greater mixing below, where the water is colder. This would have the effect of making the bottom water 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 ocean depths did not simply fill with cold, dense water formed at the poles.

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, in places like 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 barrel of non-toxic fluorescent green dye called fluorescein. Starting in 2021, researchers visited an approximately 2,000 meter deep underwater canyon in the Rockall Trough, approximately 370 kilometers northwest of Ireland.

“We chose this canyon out of about 9,500 that we know of in the oceans because it’s a fairly commonplace location for submarine canyons,” 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 (208-liter) 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 came from devices called fluorometers, which could detect the presence of tiny amounts of fluorescent dye, down to less than one part per billion, but other instruments also measured changes in temperature and turbulence of the ‘water.

Tracking the dye’s movements revealed upwelling caused by turbulence along the canyon slope, confirming for the first time Ferrari’s proposed resolution of the paradox with direct observations. Not only did the team measure the upwelling of water along the canyon slope, but it was also much faster than Munk’s 1966 calculations predicted.

Where Munk deduced a global average of one centimeter per day, measurements at Rockall Trough revealed an upwelling of 100 meters per day. Additionally, the team observed some migration of the dye from the canyon slope inward, suggesting that the physics of turbulent upwelling was more complex than Ferrari had 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, combined with measurements of upwelling 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 has become 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 see if a similar phenomenon occurs in other submarine canyons around the world. Given the canyon’s unremarkable features, Alford says it seems reasonable to expect the phenomenon to be relatively common.

If the results prove valid elsewhere, Alford said global climate simulations will need to start explicitly accounting for this type of turbulence-driven upwelling in topographic features of the ocean floor. “This work is the first step in adding 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 forward to improving the scientific understanding of ocean turbulence is twofold.

First, “we need to do more high-tech, high-resolution experiments like this in key areas of the ocean to better understand 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 underwater La Jolla Canyon.