Interaction between internal solitary waves and an isolated atoll in the Northern South China Sea

Substantial temperature drops in the Dongsha Atoll with diurnal period have been reported and considered to be the result of an internal solitary wave (ISW). The cold water event occurs daily with duration of a few hours, a time scale much longer than that of the ISW. To further understand the physical process, a detailed numerical simulation on the interaction of the Dongsha Atoll and the ISW is carried out using the Princeton Ocean Model. Internal solitary waves are found to merge, become asymmetric, and break in the shoaling process. Then, a violent tidal bore forms and runs up the slope as a bolus, bringing up cold deep water to the shallows. The simulated temperature variation and flow field in November are very similar to the largest temperature drop in the field measurement where the flow field is dominated by the ISW. In the merging process, three ISWs become one longer wave lengthening the time scale of the cooling process compared with that of a single ISW. The step around the atoll also contributes substantially by increasing the exposure time of the local ecosystem to the cold deep water. The same calculation in the absence of the step yields a much shorter time for the cold water cycle.