Observations of Reflected Internal Solitary Waves near the Continental Shelf of the Dongsha Atoll

Internal solitary waves (ISWs) near the Dongsha Atoll in the northern South China Sea (SCS) can be divided into incident, reflected, and refracted waves. Compared with the incident and refracted ISWs, the reflected ISWs are less likely to appear, but their impact on the ecological environment and marine activity should not be underestimated. In this work, field experiments were performed and moderate-resolution imaging spectroradiometer (MODIS) images were collected to analyze the reflected ISWs. Satellite observations showed that they were excited by the collision between the incident ISWs and the Dongsha Atoll and are often in the form of a wave packet composed of 2 to 5 solitons. During propagation, its spatial range gradually increases and interacts with the incoming waves generated by the next tidal cycle until it dissipates at approximately 117.5 degrees E. Eighty percent of the reflected ISWs occur from April to June. The length of the crest line is mainly between 50 and 150 km, and the average propagation speed is approximately 1.57 m/s, which is smaller than that of the incident ISWs. In situ observations showed that the amplitudes of the reflected ISWs were between 10 and 20 m, accounting for only 40% of the incident ISWs. Compared with the incident ISWs, the vertical, velocity, and zonal velocities of the reflected ISWs were all attenuated. The results of the depth-integrated horizontal energy flux calculation showed that the energy of the reflected ISWs was only 61% of that of the incident ISWs. The reflected ISWs accelerated the velocity of the surface flow field during propagation, and the maximum velocity on the sea surface was approximately 0.60 m/s. Strong tides are beneficial for the generation of reflected ISWs near the Dongsha Atoll. Incident and reflected ISWs are all first-mode ISWs.