Detection of Ocean Internal Tide Source Oscillations on the Slope of Aogashima Island, Japan

Abstract

Barotropic tidal currents over bottom topography force density surfaces to oscillate vertically and thereby to act as quasi-stationary internal tide sources. Deploying a seafloor pressure gauge array with an aperture of 30 km for a year (2014–2015), we detected the low-mode semidiurnal internal tidal waves propagating with a horizontal phase speed of ~1 m/s in the onshore and offshore directions over the array along the eastern slope of Aogashima Island, south of Japan. The amplitudes of the offshore propagating waves were greater than those of the onshore propagating waves, and both were positively correlated with the amplitudes of the local semidiurnal tide, which peaked in September and March. A tide-resolving ocean circulation model (JCOPE-T) well reproduced the observed onshore and offshore internal tidal wave propagation. The model indicated a standing wave region on the slope, where offshore propagating waves interact with standing waves locally pinned to the slope. Along the same profile over a distance of 100 km, we conducted seismic-oceanographic analysis of the legacy multichannel seismic reflection data to retrieve vertical cross sections of the reflecting layers, which indicated sharp temperature changes in the ocean. Many of the slant reflecting layers were subparallel to the contour lines of the semidiurnal internal-tide-associated temperature anomalies in the JCOPE-T model, suggesting a causal link between the fine reflection layering structure and the semidiurnal low-mode internal tidal field.