Uniformity in upper-ocean high-frequency dynamics

Despite differences in local conditions, internal waves of the ocean follow a universal energy spectrum known as the Garrett-Munk (GM) spectrum. Recent research on how non-linear eddy-wave interactions, like mesoscale eddies and near-inertial waves, influence the GM spectrum has prompted the question of whether the GM spectrum also describes upper-ocean high-frequency motions — after all, eddy-wave interactions are intensified on the ocean surface.

To answer this question, Zhang et al. characterized the high-frequency kinetic energy (HFKE) spectrum in the surface mixed layer. Using GPS-tracked surface drifters combined with independent Eulerian observations at Ocean Station Papa, they found that HFKE of the upper ocean closely followed a GM-like spectral structure.

“This result is unexpected because the mixed layer is directly forced by winds and strongly turbulent, unlike the quieter ocean interior where the classical GM theory was developed,” said author Qian Zhang.

Additionally, the team found that mesoscale eddies modulated the slope of the HFKE spectrum, whereas near-inertial waves predominantly governed the spectrum’s energy levels across diverse surface mixed-layer dynamics. Seasonal forcing and latitude exerted very little influence. Combined, these observations point to a universal scaling in the GM-like HFKE spectra, ultimately opening new possibilities to understanding the high-frequency motions of the upper ocean.

The team next looks to investigate the conditions under which the surface kinetic energy spectrum departs from its universal scaling behavior.

“Even though we found an averaged universal spectrum, local eddies, near-inertial waves, and internal tides can reshape this spectrum in different regions or time periods,” said Zhang. “Understanding these regional variations will provide a more complete view of the surface ocean dynamics and the changing marine environments.”

Source: “Quasi-universal high-frequency kinetic energy spectrum in the surface mixed layer,” by Qian Zhang, Zhiwu Chen, Gengbin Liu, Huaihao Lu, Zifei Chen, Beilei Hu, Jiexin Xu, Yankun Gong, and Shuqun Cai, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0318049 .