Global energy spectrum of the general oceanic circulation

University of Rochester researchers have for the first time quantified the energy of ocean currents larger than 1,000 kilometers. In doing so, she and her colleagues discovered that the Antarctic Circumpolar Current, with a diameter of around 9,000 kilometers, is the most energetic.

The team, led by associate professor of mechanical engineering Hussein Aluie, used the same coarse-grained technique developed by his lab to previously document energy transfer at the other end of the scale during “vortex killing” that occurs when wind interacts with temporary, circular water currents less than 260 kilometers in size.

These new results, reported in nature communicationshow how the coarse-graining technique can provide a new window to understanding oceanic circulation in all its multiscale complexity, says lead author Benjamin Storer, research associate in Aluie’s Turbulence and Complex Flow Group. This gives researchers the opportunity to better understand how ocean currents work as a key moderator of Earth’s climate system.

The team also includes researchers from the University of Rome Tor Vergata, the University of Liverpool and Princeton University.

Traditionally, researchers interested in climate and oceanography have plucked boxes ranging in size from 500 to 1,000 square kilometers in the ocean. Those box regions, thought to represent the global ocean, were then analyzed using a technique called Fourier analysis, Aluie says.

“The problem is, when you pick a box, you’re already limiting yourself to analyzing what’s in that box,” says Aluie. “You miss everything on a larger scale.”

“What we’re saying is we don’t need a box; we can think outside the box.”

For example, when the researchers use the coarse-grain technique to “blur” satellite images of global circulation patterns, they find that “we gain more by settling for less,” says Aluie. “It allows us to systematically unravel different sized structures of ocean currents.”

He draws an analogy of taking off your glasses and then looking at a very sharp, detailed image. It seems blurry. But if you look through a series of increasingly powerful lenses, you’ll often be able to discern different patterns at every step that would otherwise be hidden in the details.

Essentially, the coarse granularity allows researchers to do just that: quantify different structures in ocean currents and their energy “from the smallest, finest to the largest scales,” says Aluie.

Aluie credits Storer with further developing and refining the code; It was published for other researchers to use.