How internal waves transport energy thousands of miles across the ocean

How internal waves transport energy thousands of miles across the ocean

By Nathaniel Scharping, American Geophysical Union

Winds and tides serve as significant sources of energy for the oceans, with internal waves—massive underwater waves—facilitating the transport of that energy over vast distances, reaching thousands of miles between ocean basins. Accurately quantifying the energy conveyed by these internal waves and understanding their dynamics presents challenges due to their underwater location and enormous scale. Nevertheless, investigating this phenomenon is vital, as internal wave dynamics have a profound impact on global climate systems and marine ecosystems by affecting currents, ocean mixing, and various other processes.

In their recent study, Youran Li and collaborators employed the LLC4320 configuration of the MIT General Circulation Model to analyze meridional internal wave fluxes in and through the Southern Ocean. They categorized internal wave energy into distinct bands, including tidal, wind, and general background motions, and evaluated fluxes across latitudes ranging from 35°S to 65°S. This modeling enabled the researchers to estimate the volume of internal wave energy, quantified in gigawatts, that enters and exits the Southern Ocean, as well as to investigate how different bathymetric and hydrographic features influence internal wave fluxes. Their comprehensive findings are documented in the Journal of Geophysical Research: Oceans.

The study indicates that the net internal wave flux moves poleward, reaching approximately 15 gigawatts at both 35°S and 55°S, and about 7 gigawatts at 45°S. A significant portion—over 80%—of this internal wave flux is attributed to tidal energy. Conversely, energy from wind-driven waves constitutes merely 1% to 3% of the total, moving in an equatorward direction and partially counterbalancing the overall poleward flux. Additionally, other background motions, including higher tidal harmonics, lee waves, and nonlinear interactions, contribute to the energy balance. Notably, tidal energy diminishes sharply at 65°S, accounting for less than half of the total flux in that region.

Upon further analysis, the researchers examined the contributions of key high-energy areas within the Southern Ocean, such as the Drake Passage and the Macquarie Ridge, which are linked to poleward and equatorward fluxes, respectively. Their results revealed that while tidal forces result in a net export of energy from the Southern Ocean between latitudes 45°S and 55°S, they are net imported in the 35°S–45°S and 55°S–56°S segments.

This pivotal research, marking the first quantification of the internal wave energy flux structure in the Southern Ocean, has the potential to serve as a critical reference for future observational investigations. However, the authors acknowledge certain limitations of their model that warrant attention in subsequent studies. They noted that the duration of the model simulation was inadequate to capture the seasonal to interannual variability of internal wave fluxes and that distinguishing between wave modes or identifying wave interference remained unresolved. Future efforts may benefit from incorporating modal decomposition and wave-fitting techniques to enhance the analysis of these dynamics.