Parametric Subharmonic Instability of the M2 Internal Tides in the Tokara Strait

The Tokara Strait is a mixing hotspot due to the coexistence of complex bottom topographies and strong composite flow including both the Kuroshio and tidal currents. Although previous studies have revealed several mechanisms from the view of Kuroshio-Topography interaction, the role of tides in driving mixing is still not clear. Given that it is located at the M₂ critical latitude (29°N), parametric subharmonic instability (PSI) is expected as an important process responsible for the mixing. Here, we study PSI of the M₂ internal tides in the Tokara Strait based on a high-resolution model. Our model results indicate that intense near-inertial waves are generated via PSI, which exhibit a horizontally layered structure and have much larger vertical wavenumbers than the M₂ internal tides. Energy is transferred from the M₂ internal tides to the near-inertial waves around the generation sites, and most of the near-inertial energy is dissipated locally. The dissipation rates of near-inertial waves are comparable to those of the M₂ internal tides. Simulations with and without the Kuroshio Current revealed the suppression of PSI along the Kuroshio path, which could be attributed to two mechanisms. First, the Kuroshio Current modifies the local minimum internal wave frequency by its horizontal and vertical shear, making the condition for PSI not satisfied. Second, the Kuroshio Current advects the near-inertial waves downstream in the Okinawa Trough, which inhibits the accumulation of near-inertial energy there. However, in most of the areas outside the Kuroshio path, PSI majorly contributes to mixing in and around the Tokara Strait.