Acoustic Measurement of Multisubtidal Internal Modes Generated in Hiroshima Bay, Japan

A coastal acoustic tomography (CAT) experiment with seven acoustic stations was successfully carried out to measure multisubtidal internal modes, generated in Hiroshima Bay, Japan, in spite of severe data gaps due to many oyster aquaculture rafts, accompanied by an array of vertical wire strings for growing oysters and distributed widely over the bay. The travel-time data acquired along five transmission lines with better data acquisition rates in one-way or reciprocal directions are analyzed, focusing on subtidal variations of sound speed (proportional to temperature). Significant multisubtidal internal modes are observed in the period range of 1.0–4.0 h. To specify the source region of internal modes and to estimate their propagation speed and direction, the power-spectral and cross-spectral analyses are applied to the time-series data of range-averaged sound speed along the successful transmission lines. Waves of period ranges 3.8–4.2, 2.0–2.3, and 1.4–1.5 h are identified as the second, third, and fourth modes of long internal waves, respectively, and the first internal mode is attributed to the semidiurnal tide. It is suggested that the source region of internal modes is located in the narrow channel at the northeastern corner of the tomography site and the modes radiate out from the source region, satisfying the propagation parameters (period, phase speed, and propagation direction) determined by this study. The propagation parameters of the subtidal internal modes can also be measured by point temperature sensors, attached to aquaculture rafts with an appropriate arrangement. However, it should be noted that the tomographic technique can confirm temperature variations through a spatial average along each ray path.