Characteristics And Mechanisms Of The Internal Solitary Waves Over Northwestern South China Sea Shelf

The study of internal solitary waves (ISWs) has become an international hotspot in the northern South China Sea (SCS). Most study of ISWs has been focused on northeastern SCS, while there has been few or no field study of ISWs over northwestern SCS. Field observations were made from April to October 2005 at Wenchang Station on the northwestern shelf of SCS by Institute of Oceanology, Chinese Academic of Science. A complete set of current and temperature data of mooring measurements has been acquired to investigate the characteristics and mechanism of internal solitary waves. Some important scientific achievement has been acquired and summarized as followed:Internal solitary waves were active in this area. ISWs with large amplitude appeared frequently from April to June while less internal solitary waves occurred from July to September and their amplitude was smaller than that from April to June. The polarity of ISWs also displayed apparent seasonal features. The seasonality of ISWs is closely related to the depth and strength of the thermocline.A sequence of large internal solitary wave packets was observed in September 2005, propagating mainly northwest. Both elevation and depression ISWs were observed at the same mooring location in different days during this period. Results indicated that they were probably generated locally from the shelf break by the evolution of the internal tides due to nonlinear and dispersive effects. To our knowledge, this is the first report to confirm the locally generation mechanism of the ISWs based on in-situ measurements in the northwestern South China Sea.We also found mode-2 internal solitary waves accompanied by the mode-2 internal tides during our experiment. Two mode-2 internal solitary waves occurring roughly during the same time in consecutive days suggests that tides are the origin of the waves.Long-time strong current phenomenon was recorded from 22 April to 28 April. This period is during the ebb tides of the mooring site. The current mainly moved along the coastline. The analysis suggests that the strong currents are induced by the internal tide currents.Based on observed current and temperature data, the response of the internal wave to the typhoon and internal wave spectrum were analyzed. The temperature and current field were significantly affected by the typhoon.The new discoveries and knowledge of ISWs over the northwestern SCS have important scientific values for the study of the generation mechanism and technical application for the ISWs.