Study On The Amplitude Of The Internal Tide Waves And Internal Solitary Waves In Northern Continental Slope Of The South China Sea

The northern shelf and slope area of the South China Sea(SCS)have active marine dynamic processes.In this aera,the topography changes dramatically,with internal tidal waves and internal solitary waves occurring frequently.After nearly 30 years of development,the research on internal waves in the SCS has made great progress.However,most of the previous studies on internal tidal waves focused on the changes of flow field caused by internal tidal waves,and few studies systematically discussed the changes of its amplitude.At the same time,theoretical studies have shown that the amplitude of internal tides(ITs)is an important factor affecting the generation of internal solitary waves by nonlinear polarization.However,due to the lack of observational data,the relationship between the amplitude of ITs and the generation of internal solitary waves has not been verified by observation.Therefore,based on the observation data collected by a set of submersible moorings deployed on the western slope of the Dongsha in the northern SCS and taking the amplitude of internal tides as the cutting-in point,this paper will conduct a systematic study on the internal tides and internal solitary waves in this area,and try to discuss the occurrence relationship between ITs and internal solitary waves from the perspective of observational statistics.The results of power spectrum and rotation spectrum analysis show that: there are strong diurnal component tides and relatively weak semidiurnal component tides in the whole water layer.The energy in clockwise direction is equivalent to the total energy between the nearinertial frequency f and the semidiurnal frequency S2,and the tidal current rotates mainly clockwise.By comparing the TMD(Tide Model Driver)based on the TPXO tide model data set to simulate the tide level of the observation station,and the Depth fluctuation of the bottom CTD(Conductivity Temperature and Depth),barotropic tide in this sea area is obtained.We found that the two methods share similar effectiveness.Extraction methods and statistical results of amplitude of ITs: Different from previous method of tidal tide amplitude extraction using velocity data,this paper uses two amplitude extraction methods to calculate and analyze the variation characteristics of maximum and average amplitude of semidiurnal tides,diurnal tides and mixed tides(superposition of frequency of semidiurnal tides and diurnal tides).The average amplitudes of semidiurnal tides,diurnal tides and mixed tides were less than 11.41 m,19.48 m,and 23.83 m respectively,and the maximum amplitudes were 26.51 m,41.28 m,and 36.46 m,respectively.The diurnal distribution of diurnal and mixed tide amplitudes presents a bimodal structure with a 14-day period,which is consistent with the 14-day tide period modulation in the vertical structure of diurnal tidal current velocity.The relation between the internal tides amplitude and the internal solitary waves:Calculated using the lunar day,the annual distribution of the number for the internal solitary wave have a bimodal structure and the two peaks of the internal solitary waves are approximately consistent with the double peaks of the lunar distribution of both diurnal tides and mixed tides.Within the distribution interval of diurnal tides and mixed tides,the frequency of internal solitary waves is positively correlated with the amplitude of diurnal and mixed tides.In addition,there is a certain correlation between the amplitude of internal solitary waves and the amplitude of internal tides.Discussion on the generation mechanism of internal solitary waves: gravity collapse method is used to generate internal solitary waves by using the internal waves theory of chromatography between two layers of fluid.Other parameters follow on the previous value.When the distance between both sides of the subsidence area is small,the nonlinear process is relatively simple,with larger amplitude and shorter time before the internal solitary waves are completely formed.With the remaining parameters unchanged,only the collapse depth is changed.The duration distance of the internal solitary waves nonlinear process remains basically unchanged,and the duration will changes accordingly.