Detection of Ocean Internal Tide Source Oscillations on the Slope of Aogashima Island, Japan. Issue 7 (18th July 2019)

• Record Type: Journal Article
• Authors: Fukao, Y.; Miyama, T.; Tono, Y.; Sugioka, H.; Ito, A.; Shiobara, H.; Yamashita, M.; Varlamov, S.; Furue, R.; Miyazawa, Y.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 7(2019)
• Page Start: 4918
• Page End: 4933
• Publication Date: 2019-07-18
• Subjects: ocean internal tide -- ocean circulation model -- ocean bottom observation -- seismic oceanography -- pressure gauge; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC014997
• Languages: English
• ISSNs: 2169-9275

Abstract

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. Plain Language Summary: Internal tidal waves are generated in the ocean by interactions between surface tides and the bottom topography. Their amplitudes are less than a few centimeters at the sea surface but can be more than a few tens of meters within the ocean. These waves play important roles in tidal dissipation, ocean mixing, hydroacoustic transmission, and submersible technology. Thus far, observations of internal tides have mainly been made from above by satellite altimetry and through ocean columns by cable mooring. In this report, we show that a pressure gauge array at the bottom of the ocean is a promising tool for detecting internal tides and constraining their generation mechanisms. We incorporated a tide‐resolving ocean circulation model that could simulate bottom pressure variations at the same locations in the same period as that during which the observations were made so that the same analyses could be performed for both the observed and simulated data. Remarkable agreement was found between the observed and simulated internal tidal signals. Concurrent simulation of ocean circulation with tides is useful for interpreting not only the array records of ocean bottom pressure gauges but also vertical cross‐sectional images of ocean acoustic reflections obtained using a seismic–oceanographic technique. Key Points: We detected partly standing semidiurnal internal tides on the eastern slope of the Izu‐Bonin ridge using a seafloor pressure gauge array A tide‐resolving ocean circulation model (JCOPE‐T) well reproduced the observed nature of internal tides, including seasonal variations The temperature anomaly contours of the modeled internal tides tended to be subparallel to the acoustically detected reflecting layers