Virtual seafloor reduces internal wave generation by tidal flow

Author :Likun Zhang and Harry L. Swinney
Publication :Physical Review Letters
Publisher :APS
Volume :112
Pages :104502
Year :2014

Abstract: Our numerical simulations of tidal flow of a stratified fluid over periodic knife-edge ridges and random topography reveal that the time-averaged tidal energy converted into internal gravity wave radiation arises only from the section of a ridge above a virtual seafloor. The average radiated power is approximated by the power predicted by linear theory if the height of the ridge is measured relative to the virtual floor. The concept of a virtual floor can extend the applicability of linear theory to global predictions of the conversion of tidal energy into internal wave energy in the oceans.

Marked as ‘Editors’ Suggestion’? and featured for a Synopsis in Physics:

Editor’s Synopsis: Motion in the Ocean. The Earth’s climate is strongly affected by the ways in which energy moves into, out of, and around the oceans. One important component of energy flow is the conversion of tidal motion’changes in sea levels caused by gravitational effects of the Moon’into internal ocean waves. Such waves directly influence mixing of water from regions with different temperature and salinity, as well as overall circulation. In a paper in Physical Review Letters, Likun Zhang and Harry Swinney at the University of Texas at Austin, present numerical simulations of how tidal flow over seafloor ridges is transformed into wave energy. They find that only the topmost parts of seafloor topography contribute to wave generation, in effect creating a ‘virtual seafloor.’? It is only above it that tidal energy can be converted to wave energy.

The efficiency of tidal-to-wave energy conversion is difficult to calculate owing to the complex structure of the seafloor: When sea levels rise and fall, water moves up and down on top of underwater mountains and ridges; the vertical motion of the sea bounces off the slopes of these topographic features to create sideways oscillations that form a complex structure of internal ocean waves. Zhang and Swinney carried out simulations on both sinusoidal and random seafloor topographies and discovered that, some distance below the peaks of these structures, no conversion takes place. The reason is that wave interference cancels out tidal-to-internal energy conversion below this virtual seafloor. As a result, the authors suggest that future simulations of this important process can be simpler and more accurate because any topography below the virtual seafloor can be ignored. ‘ David Voss [American Physical Society: Physics Synopsis ‘ ‘Motion in the Ocean(Link)]