Internal-wave characteristics of strong bottom currents at the sand-wave zone of the northern South China Sea and its role in sand-wave motion
南海北部沙波区海底强流的内波特征及其对沙波运动的影响

From the 6th March to the 9th April in 2008, serial bottom-current observations were carried out at the sand-wave zone of the continental shelf and slope in the northern South China Sea (SCS). The measurements show that the tidal current and ocean current are weak at the observation sites, but bottom currents as strong as 30-77 cm.s-1 occur frequently. The directions of the strong bottom currents are related to the northwestward propagation of the internal waves in the region. Most of the strong currents are nearly northwestward or nearly southeastward, and the southeastward currents are stronger than the northwestward ones, which corresponds to the feature of downslope flows stronger than upslope ones when internal waves propagating onshore. Rotary power spectral analysis is carried out for the time series of current velocity. The results show that most of the rotary components higher than the frequency of M2 tide have the characteristics of internal-wave flows, further demonstrating that the frequent, strong bottom currents are induced by internal waves. The measurements are used to calculate the motion speed of sand waves. The results also show that the strong currents can initiate the sea bottom sediments, and due to downslope flows stronger than upslope ones, the sand waves move southeastwards at a low speed less than 1.6 m.yr-1. In addition, the authors model the coupled tide-storm bottom current fields in the northern SCS. From the modeling, the bottom currents, driven by strong typhoon and tide, can move the sand waves, but only move them a very short distance during each typhoon process. The direction of the movement is subject to the typhoon route, which is not always southeastward. Therefore, the coupled tide-storm currents are not the major factor to move the sand waves.