Coastal Acoustic Tomography of the Neko-Seto Channel with a Focus on the Generation of Nonlinear Tidal Currents-Revisiting the First Experiment

The first coastal acoustic tomography (CAT) experiment site of the Neko-Seto Channel was revisited to elucidate the propagation and generation characteristics of the M-2 and M-4 tidal currents with a second CAT experiment, which was conducted from 3-6 April 2018 (local time). Two-dimensional flow fields of the M-2 and M-4 tidal currents and the residual current were reconstructed using a coast-fitting inversion model with the reciprocal travel-time data of five acoustic stations. The M-2 tidal current is a progressive-type wave that propagates eastward at a speed of 0.7 ms(-1), much slower than expected for free progressive tides in this region (19 ms(-1)). The M-4 nonlinear current constructed an out-of-phase relationship between the western and eastern halves of the tomography domain, implying the generation of standing-type waves. Such nonlinear processes led to flood- and ebb-dominant tidal current asymmetries for the western and eastern halves of the model domain, respectively. The two-day mean residual currents constructed a northeastward current with a maximum speed of 0.3 ms(-1) in the western half of the model domain and a clockwise rotation in the eastern half. The averaged inversion errors were 0.03 ms(-1), significantly smaller than the amplitude of the aforementioned currents.