LAUSR

Field measurements of second-mode internal solitary waves (mode 2 ISWs) during the winter on the upper continental slope of the northern South China Sea were reported in Yang et al. (J Geophys Res 114, 2009), but their generation mechanism remains elusive. We investigated this issue with a multi-modal evolution model and theoretical analysis, which suggest that the observed mode 2 ISWs were generated by a shoaling mode 2 semidiurnal internal tide (IT). The results show that two groups of mode 2 ISWs usually appear within one semidiurnal tidal period, successively riding on expanded and subsequently compressed pycnoclines. The number of wave groups largely depends on the amplitudes of the ITs, and a larger IT produces larger and more mode 2 ISWs. Furthermore, intermodal coupling dominates the evolution of a mode 1 IT, highlighting the importance of considering mode scattering in the propagation of low-mode ITs. Finally, the barotropic body force is calculated, and we deduce that the mode 2 semidiurnal IT originates from the Luzon Strait.