This map, created from satellite data by Shikhar Rai, shows in blue areas where eddy kill results in net loss kinetic energy. Black areas represent land masses and regions that have permanent or seasonal ice cover. Credit: Hussein Aluie
Our climate is greatly influenced by ocean currents that are propelled by wind kinetic energy. They help to make our planet habitable by transferring heat from the poles to the equator.
Yet, scientists have failed to correctly account for wind's impact on the ocean's most energy components, which are the swirling, mesoscale, eddies. These circular currents of water between 50 and 500 km in diameter are crucial to determining the trajectory for larger ocean currents such as the Gulf Stream.
Researchers from the University of Rochester, Los Alamos National Laboratory, and the University of Rochester have published their first paper in Science Advances. They show how wind propels larger currents but has the opposite effect on smaller eddies, resulting in "eddy killing."
These measurements also give the first direct measurement on the overall impact of this Eddy Killing: An ongoing loss of 50 gigawatts kinetic energyequivalent of the Hiroshima nuclear blast every 20 minutes year-round.
Satellite observations allow for better analysis
Hussein Aluie (associate professor of mechanical engineering at Rochester) says that "for the first time, we are able to unravel Eddy Killing by direct measurement using satellite observations, with minimal assumptions."
Shikhar Rai (a Ph.D student in Aluie’s Turbulence and Complex Flow Group) and Matthew Maltrud, Los Alamos National Laboratory scientists, applied a coarse-graining method to satellite imagery. This allowed them to distinguish the multi-scale, complex structures of ocean currents from eddies that were embedded within each other.
Aluie states that this method allows for a more thorough spatial analysis than the ones used by most oceanographers. They focus on temporal fluctuations. These methods do not account for eddy killing and provide widely varying estimates. Aluie states that "the numbers have been all over"
Aluie thanked Rai, a fifth year Ph.D. student for "all the heavy lifting" in preparing the paper. Aluie states that there were technical problems, but Rai persevered and was capable of figuring them out. He has a lot potential and talent."
The ocean's currents are determined by eddies, which are water currents that flow in circular directions. They can be seen here as light blue and green swirling patterns of phytoplankton flowers. Credit: NASA/Goddard Space Flight Center Ocean Color
A new method for studying ocean currents could change the tide
Aluie claims that scientists have known about eddy death since the 1980s, when they studied idealized models.
It is easy to see the basic idea. An eddy can be described as a circle that rotates clockwise or counterclockwise. However, any wind that flows over an eddy will only move in one direction and "help" the circle in at least part in the same direction as the other.
Imagine riding your bicycle in the same direction as a car. It's like wind blowing over the area of the eddy that is moving in the opposite direction. A bike ride past a car going in the opposite direction will result in a difference in velocity that is proportionately less than if you are riding in the same direction. This is similar to the wind pushing against the side of an eddy. This difference in velocity is what causes the net "killing effect" on the eddy. The wind then extracts energy.
"On one hand, the wind makes the ocean move and on the other it kills the energy-rich part. It is therefore counterintuitive, and it was not directly measured before because people used the wrong tools," Aluie said.
He says a better tool is needed because there are many questions about other factors that can influence eddy death, as well as about the importance of eddies for other aspects of the ocean's currents and heat flow.
Aluie states that oceanographers will be able to adapt the method in this paper to help "unravel" these mysteries.
Continue reading The proposed set of conservation laws finds order in chaos of turbulence
Shikhar Rai and colleagues, Scale of oceanic-eddy killing from wind using global satellite observations, Science Advances (2021). Information from Science Advances Shikhar Ra et al., Scale of Oceanic Eddy Killing by Wind From Global Satellite Observations, (2021). DOI: 10.1126/sciadv.abf4920
