Ocean Circulation Kinetic Energy: Reservoirs, Sources, and Sinks

TLDR

Ocean circulation spans scales from millimeters to over 10,000 km, with wind energy largely dissipated within the upper 100 m, while geostrophic eddies dominate kinetic energy and support both direct and inverse cascades, and high‑frequency internal waves near the Coriolis frequency drive wave breaking and mixing in the interior. The study focuses on the distinct kinetic‑energy reservoirs of the circulation, examining their maintenance, dissipation, and potential impact on small‑scale irreversible mixing. The authors analyze how the direction of the energy cascade governs dissipation of dominant geostrophic eddy kinetic energy, noting that quantification is hampered by observational challenges for wavelengths below ~100–200 km.

Abstract

The ocean circulation is a cause and consequence of fluid scale interactions ranging from millimeters to more than 10,000 km. Although the wind field produces a large energy input to the ocean, all but approximately 10% appears to be dissipated within about 100 m of the sea surface, rendering observations of the energy divergence necessary to maintain the full water-column flow difficult. Attention thus shifts to the physically different kinetic energy (KE) reservoirs of the circulation and their maintenance, dissipation, and possible influence on the very small scales representing irreversible molecular mixing. Oceanic KE is dominated by the geostrophic eddy field, and depending on the vertical structure (barotropic versus low-mode baroclinic), direct and inverse energy cascades are possible. The pathways toward dissipation of the dominant geostrophic eddy KE depend crucially on the direction of the cascade but are difficult to quantify because of serious observational difficulties for wavelengths shorter than approximately 100–200 km. At high frequencies, KE is dominated by internal waves with near-inertial frequencies (frequencies near the local Coriolis parameter), whose shears appear to be a major source of wave breaking and mixing in the ocean interior.