This paper studies the shoaling internal solitary waves near the Dongsha Atoll of the South China Sea by seismic oceanography method. We improve the scheme of seismic data processing and use common-offset gathers to obtain time series images of shoaling internal solitary wave. In this paper, three aspects of shoaling internal solitary wave near the Dongsha Atoll are studied, including the vertical structure, waveform evolution and induced mixing.

We calculate the vertical structures of the 11 ISWs observed in seismic sections, and compared them with two theories. We find that three ISWs conform to the linear vertical structure function, four conform to the first-order nonlinear vertical structure function, and the remaining conform to neither function. The results show that the vertical structure is mainly determined by nonlinearity, which is an important dynamic characteristic of ISW. In addition, topography and background flow affect the vertical structure.

We study the shoaling processes of a mode-1 depression internal solitary wave on the gentle slope by using our improved seismic data scheme. We find that the change in shape of waveform varies at different depths. The results showed that the waveform in deep water deforms before that in shallow water and the waveform in shallow water deforms to a greater degree. We measure four parameters of the six waveforms during the shoaling including propagation speed, amplitude, wavelength, and slopes of leading and trailing edge. The propagation speed and amplitudes of waveforms in shallow water increase, the wavelengths decrease, and the slopes of trailing edge gradually become larger than that of the leading edge, while the amplitudes of the deep waveforms do not change significantly and the propagation speed decrease.

We studied three ISWs in reversing polarity and calculated their spatial distribution of diapycnal diffusivity. Our results show that the average diffusivities along three survey lines are 2 orders of magnitude larger than the open-ocean value. The average diffusivity in internal solitary waves with reversing polarity is 3 times that of the non-polarity reversal region. The diapycnal diffusivity is higher at the front of one internal solitary wave and gradually decreases from shallow to deep water in the vertical direction. Our results also indicate that the enhanced diapycnal diffusivity is related to reflection seismic events. Besides, the convective instability and shear instability may both contribute to the enhanced diapycnal mixing in the polarity-reversing process.