Mapping the Ocean’s Motion Energy – GWC Mag

Earth’s various ocean currents regulate our climate, moving Sun-heated water from the equator to the poles. Tracking how motion energy flows between different scales of circulation—from massive jets and gyres to localized high-turbulence regions, and vice versa—has been a long-standing scientific challenge.

“It gives us a framework to understand how climate change on a global scale influences regional patterns, including specific weather events.”

Now, in a study published in Science Advances, a team of oceanographers has developed a global map of energy transfer in the ocean, describing for the first time how gyres that span thousands of kilometers interact with much smaller and short-lived eddies.

“I believe that what we have done lays a promising blueprint toward deterministic climate attribution,” said Hussein Aluie, a fluid dynamicist at the University of Rochester. “It gives us a framework to understand how climate change on a global scale influences regional patterns, including specific weather events.”

To this end, Aluie said the team is keen to connect with scientists studying atmospheric rivers or hurricane systems.

Massive Gyres and Turbulent Eddies

Ocean gyres are vortex-like circulations that spiral within each of the world’s major ocean basins. Whipped along by wind and the rotation of the planet, these systems shape long-term climate conditions in coastal regions by transporting heat and steering storms. They are rimmed by large, permanent currents such as the Gulf Stream along the East Coast of the United States.

Hundreds of thousands of swirling eddies are also distributed across the oceans. They form as small parcels of water within currents and are the oceans’ equivalent of localized weather systems on land.

Unlike weather on land, which usually changes within a few days, these eddies typically last a few weeks or months and can travel hundreds of thousands of kilometers before dissipating, patterns that make coming up with the mathematical formulas to describe them challenging.

“Scientists have long speculated that these ubiquitous and seemingly random eddies communicate with the climate-scale gyres, but it wasn’t clear how to disentangle this complex system to measure their interactions,” Aluie said.

The team developed a new approach to describing interactions between gyres and eddies to determine the amount of kinetic energy transfer between them.

“This is opening up a wonderful picture on how the ocean works in a way we’ve been wanting to have for generations.”

“This is opening up a wonderful picture on how the ocean works in a way we’ve been wanting to have for generations,” said Stephen Griffies, a physicist at NOAA and a coauthor of the study. “It allows us to visualize the way motion energy flows across scales, from the planetary scale down to the very small scale, as small as you can observe.”

At certain latitudes, the gyres are energizing the eddies. Elsewhere, they are weakening or extracting energy from them. And these patterns mirror what happens in the atmosphere.

The researchers found that ocean patterns of energy flow are mediated by the atmosphere and match the three main planetary-scale atmospheric circulations: low-latitude Hadley cells, in which air rises at the equator and sinks at roughly 30° latitude; Ferrel cells in midlatitudes; and polar cells. Where the Hadley cells meet, in a band known as the Intertropical Convergence Zone, the energy transfer from the gyres generates the most intensely turbulent eddies.

This atmospheric influence hasn’t been conceptualized before, and this is why this research represents a “step change in our understanding of the ocean’s kinetic energy cycle,” according to Dhruv Balwada, a physical oceanographer at the Lamont-Doherty Earth Observatory who was not involved in the study. “Before this [study], the understanding of how kinetic energy moves across scales was limited to local regions and scales as large as a few hundred kilometers. This methodology allows for the first truly global estimate of these transfers.”

Mapping Motion Energy

The team’s new approach relies on a filtering technique known as coarse graining. It’s like an optometrist using a series of lenses to make a person’s vision progressively clearer, only in reverse, said Benjamin Storer, an applied mathematician specializing in ocean processes at the University of Rochester. “We slowly blur the oceans, bit by bit, and keep track of what changes at each step.” The analysis allowed the researchers to calculate how much energy is involved at each scale of resolution.

Clearer understanding of energy transfers is key to understanding ocean dynamics, said physical oceanographer Sarah Gille at the Scripps Institution of Oceanography. Climate models often neglect eddy-scale processes, but the study’s “tour de force global assessment” makes a strong case for resolving them because they play an intricate role in global energy balances, she said.

Most global Earth system models represent the ocean at a resolution of roughly 1° latitude × 1° longitude—not high enough to capture ocean eddies. Regional downscaled models are currently used to incorporate eddies, but new “eddy-rich” global models are being developed, said oceanographer Joe O’Callaghan from Oceanly, who cochaired a United Nations Ocean Decade white paper on global ocean observing systems.

One of the team’s most important findings is that many ocean eddies penetrate the entire water column, O’Callaghan said. “As an observational oceanographer, [I think] this highlights the need for multiscale observations from the surface to the deep ocean to characterize heat, material fluxes, or productivity at depth.”

Aluie is planning to apply the technique to map decadal-scale variability of oscillations such as the Southern Annular Mode, which describes the north-south movement of a westerly wind belt that circles Antarctica.

—Veronika Meduna (@VeronikaMeduna), Science Writer

Citation: Meduna, V. (2024), Mapping the ocean’s motion energy, Eos, 105, https://doi.org/10.1029/2024EO240103. Published on 5 March 2024.

Text © 2024. The authors. CC BY-NC-ND 3.0
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