Internal Solitary Wave Reflection Near Dongsha Atoll, the South China Sea

Abstract

Internal solitary wave (ISW) reflection is rarely observed in satellite images, even in the South China Sea (SCS), where the strongest and most energetic ISWs in the world have been observed. Compared to the large number of satellite images showing shoaling ISW in the SCS, fewer than 10 satellite images have been reported showing ISW reflection. In this study, we collect recent satellite images and implement a numerical model to analyze ISW reflection near Dongsha Atoll, in the SCS. Satellite observations show that the reflection appears to be associated with the large-amplitude ISWs generated by strong tidal currents in Luzon Strait. Numerical simulations show that ISWs break when reaching the sloping bottom. Part of ISW energy is reflected by mode-1 waves and their trailing mode-2 waves. The mode-1 waves have two types: long inertia-gravity waves and breaking ISW-induced short waves. They propagate quickly but induce weak vertical velocity and surface imprints. Mode-2 waves induce strong vertical velocity, showing visible signature in satellites. Horizontal distribution of the energy indicates that a maximum energy of about 2% of the incident energy is contained in a single reflected wave. This could explain why the reflected waves are rarely observed, because reflected waves must be sufficiently large to be detected in satellite images. Although individual wave's energy is small, in total up to 20% of the incident wave is reflected by the groups of mode-1 and mode-2 waves. This suggests that ISW reflection has a significant impact on energy distribution over the continental slopes.