Dynamics and Energetics of Trapped Diurnal Internal Kelvin Waves around a Midlatitude Island

Abstract

The generation of trapped and radiating internal tides around Izu-Oshima Island located off Sagami Bay, Japan, is investigated using the three-dimensional Stanford Unstructured Nonhydrostatic Terrain-following Adaptive Navier–Stokes Simulator (SUNTANS) that is validated with observations of isotherm displacements in shallow water. The model is forced by barotropic tides, which generate strong baroclinic internal tides in the study region. Model results showed that when diurnal K1 barotropic tides dominate, resonance of a trapped internal Kelvin wave leads to large-amplitude internal tides in shallow waters on the coast. This resonance produces diurnal motions that are much stronger than the semidiurnal motions. The weaker, freely propagating, semidiurnal internal tides are generated on the western side of the island, where the M2 internal tide beam angle matches the topographic slope. The internal wave energy flux due to the diurnal internal tides is much higher than that of the semidiurnal tides in the study region. Although the diurnal internal tide energy is trapped, this study shows that steepening of the Kelvin waves produces high-frequency internal tides that radiate from the island, thus acting as a mechanism to extract energy from the diurnal motions.

Dynamics of Internal Tides Over a Shallow Ridge Investigated With a High‐Resolution Downscaling Regional Ocean Model

“…In addition, this area is known as an important area for maintaining marine ecosystems in the Northwest Pacific Ocean, because of strong mass transport and mixing over the shallow ridge (e.g., Hasegawa et al, ). Masunaga, Fringer, et al () reported that tidal flows induce highly nonlinear internal tides in this area. It has also been reported that the intensity of internal tides over the ridge is much higher than it is along the Japanese mainland (Masunaga, Fringer, et al, ; Varlamov et al, ).…”

“…Masunaga, Fringer, et al () reported that tidal flows induce highly nonlinear internal tides in this area. It has also been reported that the intensity of internal tides over the ridge is much higher than it is along the Japanese mainland (Masunaga, Fringer, et al, ; Varlamov et al, ). Varlamov et al () conducted numerical modeling of M 2 internal tides with a horizontal resolution of 1/36° (~3 km), which may not be sufficiently fine to reveal details of the internal tide dynamics, particularly in coastal areas.…”

“…Baroclinic (BC) tides inclusive of internal tides are thus spontaneously computed in the L2 model with realistic large‐scale influences, such as those from the Kuroshio Current. The amplitudes and phases of the tidal forcing were corrected using Newton's method to improve the reproducibility of tidal signals (see details in the Appendix of Masunaga, Fringer, et al, ). The ocean surface is forced by hourly wind stress (from the Japan Meteorological Agency Grid Point Value Mesoscale Model) and monthly climatological surface heat, freshwater fluxes, and radiation fluxes (from the National Oceanic and Atmospheric Administration Comprehensive Ocean‐Atmosphere Data Set, NOAA‐COADS; Woodruff et al, ).…”

“…By contrast, numerical simulations are better suited to investigate these processes. Recent modeling studies have succeeded in examining shelf‐scale generation, propagation, and dissipation processes of internal tides (e.g., Kang & Fringer, ; Masunaga, Fringer, et al, ; Varlamov et al, ).…”

“…Varlamov et al () conducted numerical modeling of M 2 internal tides with a horizontal resolution of 1/36° (~3 km), which may not be sufficiently fine to reveal details of the internal tide dynamics, particularly in coastal areas. Thus, Masunaga, Fringer, et al () instead used a high‐resolution, triangle unstructured grid numerical model (SUNTANS) with greatly idealized forcings. High‐resolution models with realistic oceanic conditions are necessary for understanding the details and effects of internal tides over the ridge.…”