Researchers tackle ocean paradox with 55 liters of fluorescent dye

For the first time, researchers from UC San Diego’s Scripps Institution of Oceanography led an international team that directly measured cold, deep water upwelling via turbulent mixing along the slope of a submarine rift in the Atlantic Ocean.

The upwelling rate the researchers observed was more than 10,000 times the global average predicted by now-famous 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 difficult 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.

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 create a turnover 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 part of it, known as the meridional overturning circulation (MOC), is 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 a global average upwelling rate, using the rate at which cold, deep water was formed near Antarctica. He estimated the rate of upwelling at one centimeter per day. The amount of water transported by this rate of upwelling would be enormous, says Matthew Alford, professor of physical oceanography at Scripps and lead author of the study, “but across the entire global ocean, that flow 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’s surface. About 25 years ago, measurements began to reveal that submarine turbulence was higher near the seafloor, but this presented oceanographers with a paradox, Alford said.

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.

In 2016, researchers including Raffaele Ferrari, 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 create 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 inconspicuous compared to deep-sea canyons,” says Alford. “The idea was that it would be as typical as possible to make our results more generalizable.”

Hovering above the undersea canyon in a research vessel, the team lowered a 208-liter barrel of fluorescein to 10 meters above the seabed 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 various instruments customized internally at Scripps to the requirements of the experiment. The researchers were able to track the movement of the dye at high resolution by moving the ship slowly up and down the slope of the canyon.

The main measurements came from devices called fluorometers, which can detect the presence of small amounts of the fluorescent dye (down to less than 1 part per billion), but other instruments also measured changes in water temperature and turbulence.

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

While Munk derived a global average of one centimeter per day, measurements at Rockall Trough showed that upwelling was progressing at 100 meters per day. In addition, the team observed dye migrating away from the canyon slope toward the interior, suggesting that the physics of the turbulent upwelling was more complex than Ferrari had originally theorized.

“We observed an upwelling that has never been directly measured before,” says Wynne-Cattanach. “The rate of that upwelling is also very fast, which, together with measurements of downwell elsewhere in the oceans, indicates 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 result of decades of work by scientists from across the field, with such leading 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 upwellings exist in other submarine canyons around the world. Given the canyon’s inconspicuous features, Alford says it seems reasonable to expect the phenomenon to be relatively common.

If the results apply elsewhere, Alford says global climate simulations will have to explicitly account for this kind of turbulence-induced upwelling of 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 those models’ ability to predict climate change,” he said.

The route 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 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.