Seamounts cause currents critical to Earth’s climate

(ORDO NEWS) — Few forces are as fundamental to climate as the rotating circulations in the oceans. These “conveyor belts,” as oceanographers call them, pull tropical surface water toward the poles, where it warms high latitudes, then cools and sinks miles below, carrying residual heat and dissolved carbon dioxide with it.

But the final stage of the pipeline is mysterious. For circulation to continue, deep water must rise back to the surface, and oceanographers cannot explain how this happens.

Now, the results of a study by the British research vessel RRS Discovery seem to support a radical new look at how water rises in the deep ocean. Measurements of tracers rising over uneven seafloor topography show that deep water is not slowly rising over much of the ocean, as previously thought.

Instead, it rises in concentrated bursts as a result of turbulence generated by seamounts, including volcanic mid-ocean ridges and seamounts.

“The shape of the seafloor is closely related to the structure of the ocean,” says Trevor McDougall, an ocean physicist at the University of New South Wales who helped lay the theoretical foundation for the discovery. “This is a new look at the depths of the ocean.”

The discovery, which was announced earlier this month at an ocean sciences meeting, could have wide-ranging implications. Deep waters, instead of being mothballed for hundreds or thousands of years, could quickly return, accelerating climate change by releasing the carbon they store.

Rising water can also lead to sea level rise in some places. The new picture may force oceanographers to rethink the behavior of the oceans in the past, when the shape of the seabed was different from today.

Attempts to unravel the mystery of upwelling have been made for decades, beginning with renowned oceanographer Walter Munch’s seminal 1966 work, The Abyssal Recipes. He suggested that internal waves that form along the boundaries between ocean layers of different density sometimes break, like waves on a coast.

This turbulence, if widespread, could slowly mix deep heavy waters and send them up. After reaching a level of 2 kilometers below the surface, the waters will rush into the Southern Ocean, where fierce winds will pull them to the surface.

However, when free-falling probes began measuring turbulence in the deep ocean a few decades ago, they found that much of the ocean was calm too calm.

“People went out and searched ad infinitum and couldn’t find any 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 increase with depth. Like a spoon stirring milk into coffee, it moved the water down instead of up, says Raffaele Ferrari, a physical oceanographer at the Massachusetts Institute of Technology and head of the Discovery campaign.

“The mixing was happening in the opposite direction than predicted by Walter Munch.” Water fell not only at the poles, but throughout the ocean, and twice as much,

In 2016, two teams of researchers, including a team led by Ferrari, pieced together a picture that could explain how deep water was rising despite the downward movement. They suggested that near the seabed, breaking waves could no longer push the water down.

Instead, if there were seamounts nearby, the turbulence drove the water up the sides of the mountains, mixing with the lighter waters above. The water could rise to depths of up to 2 kilometers, where the Southern Ocean pump could take over.

The idea was met with skepticism surely such large upwellings could have been detected before? But oceanographers have made few measurements near the seabed to test this idea. “It’s a good way to break your instrument,” says Ferrari.

His team decided to fill that gap on two trips last year to the Rockall Trench, a rugged terrain northwest of Ireland. The researchers fired non-toxic tracers 1,800 meters down at the base of the jagged canyon wall and monitored the water with jetties and free falling turbulence profilers.

One tracer will allow researchers to document the long-term evolution of water when they return to Discovery in the summer. Another short lived fluorescent dye can be monitored in real time. For three days he climbed 100 meters a day. “It was very interesting,” says Alford. “You could watch the water rise up.”

The first results are “pretty cool,” says Sarah Purky, a Scripps physical oceanographer not involved with the project. “It feels like we’ve been talking about this day for a long time.”

The upwelling rate seems to be in line with the theory, she said. Now the question is whether it is possible to extrapolate the processes taking place in this one place. “How can we extend this to the whole ocean?”

Upwelling and turbulence measurements taken 10 years ago in the Drake Passage, a rough seafloor channel between Chile and Antarctica, are about to be published and are basically the same, says Ali Mashayehi, an environmental hydrodynamicist at Imperial College London. “So there are some indications that what they find is of general significance.”

Findings from the Discovery study also indicate that the story is not as simple as Ferrari and others first thought, says Sonia Legg, a physical oceanographer at Princeton University.

The tides seem to be affecting the currents, not just the turbulence. And it remains to be seen what the fate of the rising water will be. It may have been carried away and dispersed by ocean eddies.

But Ferrari is encouraged by the results and says they help to understand some of the features of the ocean. For example, there is no significant inverted circulation in the North Pacific. But it also has few volcanic seamounts or ridges, and without these supporting elements, water cannot move upward.

The results also mean that the currents in the oceans of the past could be fundamentally different depending on the volcanic activity of the Earth and how uneven it made the seabed. “It’s not just about where the continents are,” he says. “You also need to know the structure of the seabed.”


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