Tomographic measurement of tidal current and associated 3-h oscillation in Bali Strait

A coastal acoustic tomography experiment involving four acoustic stations was conducted from June 1–3, 2016 in the northern part of the Bali Strait. Range-average currents obtained along four transmission paths for a 30-h observation period were investigated in two-period bands: (a) > 6 h and (b) 10 min–6 h. The dominant phenomena for period bands (a) and (b) were semidiurnal tides with periods of approximately 12 h and internal modes with periods of approximately 3 h, respectively. The phase of the semidiurnal tidal currents was significantly deviated from a sea surface height-current relation expected for the progressive waves in the observation site. The 3-h oscillation had an amplitude comparable to that of the semidiurnal tidal current (with amplitudes of 1.5 ms−1) and was embedded in a semidiurnal envelope oscillation. In relation to the semidiurnal sea surface variations, the 3-h oscillation reached a peak at low water and diminished at high water. The velocity error of 0.07 ms−1 was estimated from the differential travel-time variations in the high-frequency band (10 min–1 h). The error was significantly smaller than the amplitudes of the semidiurnal tidal currents and the 3-h oscillation. Finally, it was suggested that the 3-h oscillation is an internal seiche synchronized with the semidiurnal tides.