Eddies encourage the ocean’s absorption of carbon dioxide from the atmosphere and help regulate the planet’s climate. Now, scientists have more details about how these ephemeral ocean features die.
Eddies are circular currents that wander around the ocean like spinning tops, ranging from tens to hundreds of kilometers in diameter. They mimic weather systems in the atmosphere and serve as a feeding grounds for sharks, turtles, and fish. Eddies often spin off major ocean currents and typically die within a matter of months.
Some fundamental questions in physical oceanography center around the life cycle of eddies: What gives rise to them, and how do they die? “It’s a big puzzle that’s been long-standing in the community,” said fluid dynamicist Hussein Aluie from the University of Rochester, N.Y.
Aluie and his colleagues found that when it comes to eddy killing, the planet’s winds are partly to blame.
Their innovative analysis of satellite data suggests that wind sucks energy out of the ocean from features smaller than 260 kilometers—features that include most eddies. Wind continually extracts about 50 gigawatts of energy from eddies around the world. The team published their research in Science Advances in July.
“Fifty gigawatts is equivalent to detonating a Hiroshima nuclear bomb every 20 minutes, year-round,” said first author Shikhar Rai, a doctoral student at the University of Rochester. “It is equivalent to operating 50 million microwave ovens continuously throughout the year.”
Although it’s long been suspected that wind zaps eddies of their spin, the latest study provides a seasonal signal and an estimate of wind power loss in major currents. Although wind may be a killer of eddies, it supercharges larger-scale ocean circulation. Wind adds about 970 gigawatts of energy to features larger than 260 kilometers, the recent research found.
Eddies boost ocean heat intake, ocean mixing at the surface, and the exchange of gases with the atmosphere, so calculating these processes relies on accurate depictions of eddies in computer models.
Blowing in the Wind
Eddies likely form from interconnected physical forces in the ocean that include density-driven motion from water of different temperatures or salinities.
Wind destroys ocean eddies by applying stress to the ocean’s surface and slowing eddies’ spin to the point of extinguishing them. Because wind stress hinges on the difference between the speed and direction of wind compared with the speed and direction of the ocean’s surface flow, wind categorically slows eddies rather than quickening them.
Eddy killing happens year-round, but the effects are particularly strong in winter, when winds grow stronger because of storms, according to the new study.
Most eddies come from western boundary currents like the Gulf Stream in the Atlantic and the Kuroshio in the Pacific, and the latest results reveal just how much energy relative to the total input wind removes from these currents’ eddies: 50% from the Gulf Stream and a whopping 90% from the Kuroshio.
“The movement of the ocean is critical in regulating the climate of the Earth,” Aluie said. Eddies can affect the trajectories of major currents: For example, eddies are widely believed to play a crucial role in causing the warm waters of the Gulf Stream to curve away from the eastern United States, keeping the climate of Canada, Greenland, and the Labrador Sea cold.
The research adds to the building evidence that wind stifles eddies. Chris Hughes, a professor of sea level science at the University of Liverpool and author of a 2008 study that found that wind sucked 60 gigawatts of energy from the ocean, said, “It’s nice to see this confirmed independently and some new diagnostics shown.”
A Blurred Photograph
The research team used an emerging method in physical oceanography to conduct the new work. Typically, researchers study how the ocean changes over time. But in the latest analysis, the scientists looked at differences over space, not time.
The latest study “represents a novel application of the newly developed coarse-grain method,” said physical oceanographer Xiaoming Zhai of the University of East Anglia, who was not involved in the research.
Coarse-graining analysis can be explained with a simple example, said Aluie. Imagine a flower in a photograph. If you blur the photograph, you can’t see the texture of the flower’s petals, the grains of pollen on its anthers, or the edges of the sepals. If you now take the unblurred photo and subtract the blurry one from it, you get only the fine details of the flower.
The new study used measurements taken between 1999 and 2007 from NASA’s QuikSCAT satellite scatterometer. By “blurring” the satellite information, Rai and his colleagues used coarse-graining analysis to see the details of small-scale ocean flow, which included eddies. The method allowed them to pinpoint the 260-kilometer cutoff.
Sadly, QuikSCAT died in 2009, but an upcoming NASA mission, Surface Water and Ocean Topography (SWOT), along with wind data from other satellite missions could provide Rai and others with higher-quality data soon.
The team will continue to use spatial techniques like coarse-grain analysis in future work, which will include a look into the other side of an eddy’s life cycle: its birth.
—Jenessa Duncombe (@jrdscience), Staff Writer
Duncombe, J. (2021), Eddy killing in the ocean, Eos, 102, https://doi.org/10.1029/2021EO161292. Published on 29 July 2021.
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