84 / 2023-07-28 21:52:39

Mechanisms of reappeared period shifts of internal solitary waves in the northern South China Sea

Internal soltitary wave,South China Sea,reappeared period shifts

Mooring observations have revealed that the reappeared periods of internal solitary waves (ISWs) in the northern South China Sea (SCS) manifest systematic shifts. This study aims to reveal the dynamic mechanisms behind this phenomenon using theories of modulation of shear flow to the internal wave propagation. A physics model of two-dimensional laminar jet is adopted to simulate the tidal shear flow injecting into the northern SCS through the Luzon Strait. Approximate solutions for the frequency modulation function (FMF) and the amplitude modulation function (AMF) of internal diurnal tide waves are derived and validated by the mooring array measurements on the northern SCS continental shelf in 2019 and 2020. The correlation coefficients reach 0.92 for FMF and 0.96 for AMF. Namely, the mechanisms of the ISW reappeared period shift (RPS) and amplitude variation are attributed to modulation of fortnightly lunar tide (Mf) jet flow to internal diurnal tide waves. The FMF is induced by the vorticity of background flow and the Doppler effect. Thus, the sign of the FMF value may be positive (blue shift or C-type waves) and negative (red shift or B-type waves), depending on the algebraic summation of the two terms. The estimates of FMF and AMF from CMEMS surface velocity are also consistent with that from mooring observations, further confirming the modulation effects of background current on internal tides and ISWs.