Friday, April 1, 2022

Undersea mountains stir up currents critical to Earth's climate

Extinct volcanoes like the Pao Pao seamount (right) in the south Pacific Ocean may help deep waters rise, a function critical to ocean conveyor belts.

From Science by Paul Voosen
Seafloor topography plays outsize role in circulation sequestering carbon and heat

Few forces are as fundamental to the climate as the overturning circulations in the world’s oceans.
These “conveyor belts,” as oceanographers call them, drag tropical surface waters toward the poles, where they warm the high latitudes before cooling and sinking to the abyss kilometers below, taking residual heat and dissolved carbon dioxide with them. 
But the last leg of the conveyor is mysterious.
To keep the circulation going, those deep waters have to rise back to the surface—and oceanographers can’t quite explain how it happens.

Now, results from a campaign by the RRS Discovery, a U.K. research ship, seem to confirm a radical new view for how deep-ocean water rises. 
Its measurements of tracers rising above rough seafloor topography suggest that deep water does not well up slowly across most of the ocean, as once thought. 
Instead, it is shunted upward in concentrated bursts by turbulence created by undersea mountains, including volcanic midocean ridges and seamounts
“The shape of the sea floor is intimately tied to the structure of the ocean,” says Trevor McDougall, an ocean physicist at the University of New South Wales who helped lay the theoretical framework for the discovery. 
“This is a new way of looking at the deep ocean.”

The finding, reported earlier this month at the Ocean Sciences Meeting, could have broad implications. Deep waters, rather than remaining sequestered for hundreds or thousands of years, may return quickly—speeding climate change by releasing the carbon they store. 
The upwellings could also add to sea level rise in some locations. And the new picture could force oceanographers to rethink the behavior of past oceans, when the contours of the sea floor differed from today’s.

Efforts to solve the puzzle of upwelling go back decades, to a seminal 1966 paper by famed oceanographer Walter Munk titled “Abyssal Recipes.” 
He proposed that internal waves forming along boundaries between ocean layers of different densities occasionally break, much like waves on a shore. 
This turbulence, if widely distributed, could slowly mix deep heavy waters and send them upward. Once they reached a level 2 kilometers below the surface, the waters would flow to the Southern Ocean, where fierce winds pull the waters to the surface.

When free-falling probes began to measure deep-ocean turbulence several decades ago, however, they found that much of the ocean was calm—too calm. 
“People went out and looked forever and ever and couldn’t find [turbulence],” says Matthew Alford, a physical oceanographer at the Scripps Institution of Oceanography and co-investigator of the new campaign. 
The turbulence that was found tended to grow with depth. 
Like a spoon stirring milk into coffee, it was driving water down, not up, says Raffaele Ferrari, a physical oceanographer at the Massachusetts Institute of Technology and leader of the Discovery campaign. 
“The mixing was doing the opposite of what Walter Munk had predicted.” 
Water was sinking not only at the poles, but also throughout the ocean, twice as much as previously thought.

Undersea mountains cause disruptions in ocean currents that create oceanic paradises on their peaks.
In 2016, two teams of researchers, including one led by Ferrari, pieced together a picture that could explain how the deep water rose in spite of the downward push. 
Close to the sea floor, they proposed, the breaking waves could no longer propel water downward.
Instead, if there were any undersea mountains nearby, the turbulence would drive the waters up mountain slopes, mixing with lighter waters above. 
Water could surge all the way up to a depth of 2 kilometers, where the pump of the Southern Ocean could take over.

The idea met skepticism—surely such large upwellings would have been detected before?
But oceanographers had made few measurements near the sea floor to test the idea. 
“It’s a good way to break your instrument,” Ferrari says.

His team set out to fill the gap in two visits last year to the Rockall Trough, a rough terrain northwest of Ireland. 
The researchers released nontoxic tracers 1800 meters down, at the base of a jagged canyon wall, and monitored the water with moorings and free-falling turbulence profilers. 
One tracer will allow the researchers to document the long-term evolution of the water when they return on the Discovery in the summer. 
Another short-lived fluorescent dye could be followed in real time. It zipped upward 100 meters per day over 3 days. 
“That was quite exciting,” Alford says. 
“You could watch the water upwell.”

The initial results are “pretty cool,” says Sarah Purkey, a physical oceanographer at Scripps who is unaffiliated with the project. 
“It feels like we’ve been talking about this day for a long time.” 
The rate of upwelling seems to match the theory, she says. 
The question now is whether the processes at this one location can be extrapolated. 
“How do we scale this to the entire ocean?”

Soon-to-be-published measurements of upwelling and turbulence from a survey conducted 10 years ago in the Drake Passage, a bumpy seafloor channel between Chile and Antarctica, agree in principle, says Ali Mashayekhi, an environmental fluid dynamicist at Imperial College London. 
“So there is some indication that what they’re finding is of generic importance.”

The Discovery results also suggest the story won’t be as simple as Ferrari and others first indicated, says Sonya Legg, a physical oceanographer at Princeton University. 
Tides seem to influence the flows, not just turbulence.
And the longer term fate of the upwelling water remains to be seen. 
It’s possible it was whisked away and dissipated by ocean eddies.

But Ferrari is emboldened by the results and says they help make sense of certain ocean idiosyncrasies. For example, the north Pacific Ocean lacks much of an overturning circulation. 
But it also has few volcanic seamounts or ridges—and without those enablers, the water can’t move upward. 
The findings also mean the currents of past oceans could have been fundamentally different depending on Earth’s volcanic activity—and how bumpy it made the sea floor. 
“It’s not just a matter of where the continents are,” he says. 
“You also need to know the structure of the sea floor.”

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