[A-OS16] Frontiers of Ocean Mixing Research

[AOS16-P01] Efficient turbulent mixing hotspots potentially associated with convective breaking of internal waves

*Takashi Ijichi¹, Louis St. Laurent² (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 2.Applied Physics Laboratory, University of Washington)

Turbulent mixing in the ocean interior has been thought to be induced by the breaking of internal waves largely via Kelvin-Helmholtz type shear instability. On this basis, turbulent mixing efficiency (i.e., the fraction of the available turbulent energy that is irreversibly used to raise the background gravitational potential energy due to mixing) has been usually treated as a global constant of about 20%, a typical value for shear-driven turbulence. In contrast to shear-driven turbulence, convectively-driven turbulence is characterized by much more “efficient” mixing with about 50% efficiency. It remains unclear which type of instability is responsible for each of a variety of breaking internal waves in the ocean, and hence how mixing efficiency should be treated. Using several microstructure datasets obtained in distinct mixing hotspots, we examine bulk (or dataset-averaged) efficiency characteristics. While the conventional 20% efficiency is supported in a tidal mixing hotspot over rough bathymetry, significantly higher efficiency, up to 50%, is suggested in a near-sill overflow site as well as in a prominent internal-tide generation site. These efficient mixing hotspots are suggested to be associated with convective breaking of large-amplitude internal waves.