Secular variation in the M2 tide, resonant amplification, and energy dissipation in a semi-enclosed sea (Ariake sea, Japan) connected to the East China sea

In this study, we analyzed decreasing trends in the amplitude (− 0.185 m century − 1 , or − 12.1% century − 1 relative to the mean) and phase (− 7.69° century − 1 ) of M 2 tidal elevation over the period 1970–2024 in the semi-enclosed Ariake Sea located on the east coast of the East China Sea (ECS). We used a one-dimensional quarter-wavelength bay resonance model to identify the contributions of the incoming M 2 tide from the ECS, tidal amplification due to the quarter-wavelength bay resonance, and damping caused by bottom friction. In addition to the previously reported secular variation in the incoming M 2 tide, this study quantified the secular variations in the intrinsic resonance frequency and linear bottom friction coefficient, as well as their impact on tidal amplification in the Ariake Sea. A less resonant condition for the M 2 tide was detected after 1997, mainly due to the partial enclosure of Isahaya Bay, a sub-embayment of the Ariake Sea, by a dike. Furthermore, we found that bottom friction and M 2 energy dissipation decreased, especially after the 2000s, resulting in an increase in the amplitude and a decrease in the phase lag of the M 2 elevation. We discuss the impacts of the less resonant condition after 1997 and the decreasing incoming M 2 tide on the recent decrease in energy dissipation. These results have implications for mixing and material transport in the Ariake Sea and should be useful for understanding past environmental change and for predicting future changes in tides in semi-enclosed coastal seas.