The subsurface structures of internal solitary waves were observed over two days from 27 to 29 February 2019, by two bottom‐moored coastal acoustic tomography systems over a path length of 18.286 km in the Lombok Strait, Indonesia. One‐way travel times along the transmission path between the acoustic stations were determined for the first three arrival peaks. The resulting three travel times were used to execute a four‐layer (0–50, 150–300, 300–450, and 400–603 m) temperature inversion, constructing an underdetermined problem. The inverted four‐layer temperatures show three positive peaks and two negative peaks during the observation period, implying the sequential passage of internal solitary waves with depressed and elevated interfaces. The generation of these temperature peaks is synchronized with the diurnal tides, and the peak heights decrease with depth. The inverted four‐layer temperatures are in excellent agreement with conductivity‐temperature‐depth data obtained near the acoustic stations.
A reciprocal sound transmission experiment was carried out during June 10-13, 2015 along a cross-strait line in the Bali Strait with strong tidal currents to measure the vertical section structures of the range-averaged current and temperature at a 3 min interval. The five-layer structures of the range-averaged current and temperature in the vertical section were reconstructed by regularized inversion of the travel time data for two rays. The hourly-mean current showed the generation of nonlinear internal tides with amplitudes of 1.0-1.5 ms À1 and periods of 6 h superimposed on semidiurnal internal tides with amplitudes decreasing from the upper to lower layer. The hourly-mean temperature was characterized by variations with amplitudes of 1.0-1.5 C and periods of 6 and 8 h. The current variations showed an out-of-phase relation between the upper and lower layers while the temperature data varied in-phase for all five layers. The two-day-mean current and temperature showed a stratified structure, varying from À0:6 to À0:1 ms À1 and from 23.8 to 28.0 C, respectively. The five-layer current and temperature were significantly above the inversion errors.
A typhoon‐driven upwelling event was observed with coastal acoustic tomography in Hiroshima Bay during September 2013. The tomography data were used to obtain state estimates from an ocean model, employing the ensemble Kalman filter (EnKF) for data assimilation. Hiroshima Bay was represented as a two‐layer system with a fresh, near‐surface layer overlaying a more saline, lower layer. Wind from the typhoon forced the surface layer southward, thus drawing the lower layer northward and causing upwelling. After the upwelling, these currents reversed for a period. During the upwelling, the total volume transports for the upper and lower layers were well balanced, showing continuity of the two layers. However, the total upper layer volume transported northward during the reverse‐flow period was significantly smaller than the total upper layer volume transported southward during the upwelling, and significantly larger than the total lower layer volume transported southward during the reverse‐flow period. The downstream reductions of transported volume indicated mixing fractions of 24%–30%, with water mixing at the interface of the upper and lower layers. Errors in state estimates were less than the changes in current and salinity associated with the upwelling.
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