[A-CG47] 海洋と大気の波動・渦・循環の力学

[ACG47-03] Direct and indirect energy pathways from tidal- to high-frequency internal waves on a continental slope

★Invited Papers

*Wei Yang¹、Zhiwu Chen²、Yohei Onuki³、Hao Wei¹、Shuqun Cai² (1.Tianjin University、2.South China Sea Institute of Oceanology, Chinese Academy of Sciences、3.Kyushu University)

Internal tides (ITs) are believed to be the greatest energy source for deep ocean mixing that sustains the overturning circulation and the marine ecosystem at both the regional and global scales. Even though the latest numerical ocean models are capable of reproducing the generation of ITs in the real ocean, their energy pathway toward turbulent dissipation remains an open question. Earlier work pointed out two major mechanisms responsible for the energy loss of ITs: the nonlinear steepening (NS) and the parametric subharmonic instability (PSI). Although these two phenomena have been addressed separately in existing studies, here we demonstrate that their interplay effectively assists in the dissipation of ITs on a continental slope. Based on mooring observations obtained at the East China Sea shelf slope, we first reveal the occurrence of PSI that transfer energy from ITs to near-inertial waves (NIWs). Moreover, the energy can be further transferred from the PSI-generated near-inertial waves to high-frequency internal waves (HFIWs) revealing the interplay between them. The physical mechanism is next investigated based on a set of numerical experiments. In our two-dimensional numerical model, an idealized IT wave train generated offshore approaches a shelf slope and experiences successive linear and nonlinear transformations. On a steep slope, the IT wavelength is shortened via near-critical reflection to create a beam-like structure and induce HFIWs due to the NS. Far above the slope, near-inertial waves arise to extract energy from the IT through the PSI. Then, HFIWs entering the near-inertial shear further extract energy from near-inertial waves and become strengthened, presumably in a manner akin to the time-dependent refraction.Such energy transfer widely exists for various latitudes and slope angles. The resulting intense HFIWs carries the transferred energy and will be dissipated elsewhere. This interaction, resembling the classical time-dependent wave refraction mechanism, is an overlooked process in the tide-induced mixing.