[A-OS14] Coastal physical processes associated with mixing, eddies, internal waves

[AOS14-P03] Secular variations of M₂ tide, its resonant amplification, and the associated energy dissipation in a semi-enclosed sea connected to the East China Sea (Ariake Sea, Japan), 1980–2019.

*Eisuke Tsutsumi¹, Matsuno Takeshi², Sachihiko Itoh¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Research Institute for Applied Mechanics, Kyushu University)

Keywords:Changing tides, Ariake Sea, Bottom drag, Tidal energy dissipation

After 2000, decrease in M₂ tidal elevation amplitude have been obverved in and around a semi-enclosed shallow embayment connected to the eastern East China Sea, Ariake Sea. In this study, we present an updated analysis of the secular variation of M₂ tide in Ariake Sea by introducing the bottom friction and its nonlinearity, which have not been explicitly considered in the tidal studies of the Ariake Sea. Over the last five decades, M₂ tidal elevation amplitude has decreased at an average rate of ~−11 % century⁻¹ both inside and outside of the sea. The result of the analysis indicated that linear bottom drag coefficient had decreased at ~−10 % century⁻¹, suggesting that if the quadratic bottom drag coefficient was unchanged, mean M₂ velocity in the sea also had decreased at the same rate. Comparing 1997 and 2015, when the lunar nodal factor for M₂ tide was nearly at its maximum, we found that the energy dissipation of M₂ tide decreased by ~11 % in 2015. The possible causes of these decreasing trends were changes in the frictional resonance properties of the sea and change in the incoming M₂ tide from the East China Sea.