Study On Refraction And Reconnection Of Internal Solitary Wave Behind The Dongsha Atoll By SAR Observation And Numerical Simulation

There are seasonal change in the vertical structure of sea water and drastic change in topography in the northern South China Sea(SCS).Therefore,SCS has become the active field of the internal solitary waves(ISWs).In general,a long ISW front propagating from the deep basin of the northern SCS splits into northern and southern branches when it passes the Dongsha Atoll(DSA).The northern and southern wave branches can refract and then reconnect behind the DSA,but the reconnection location varies.In this study,the refraction and reconnection of ISWs around the DSA in the northern SCS are investigated based on spaceborne synthetic aperture radar(SAR)observations and numerical simulations.The main contents and conclusions are as follows.A statistical study on the temporal and spatial distribution of ISWs around the DSA and the classification of the reconnection location of ISWs behind the DSA is firstly carried out by the wide-swath Envisat Advanced SAR(ASAR)images.The statistics of Envisat ASAR images from the year of 2003 to 2011 suggest that the ISWs mainly travel northwestward after passing the DSA and feature different shape and location patterns around the DSA.What is more,the seasonal variations of the ISWs from the ASAR images show that the occurrence of the ISWs is more frequent in summer than other seasons in the SCS.And it is evident that the refracted northern and southern ISW branches on the leeward side of the DSA mainly reconnect near the extension line of the long axis of Dongsha Island.So the reconnection of the northern and southern wave branches behind the DSA can be categorized into three classes depending on their locations relative to the extension line of Dongsha Island,i.e.,the locations are to the west,northwest and southwest of Dongsha Island.In addition,there are also cases where the northern and southern wave branches do not reconnect on the leeward side of the DSA.Then based on two consecutive TerraSAR-X(TS-X)images at the DSA within approximately 12 h,a previously developed nonlinear refraction model(NRM)is set up to simulate the refraction and reconnection of the ISWs behind the DSA.The simulation is performed on realistic environmental condition and defined as the reference experiment.The results of the realistic simulation agree well with consecutive TS-X images captured within 12 h of each other,which proves the validity of the NRM model around the DSA.Lastly,sensitivity experiments to verify which factors have significant impacts on ISW refraction and reconnection behind the DSA by separately changing the ocean stratification,background currents,initial wave amplitudes,locations,and shapes of the incoming wave branches at the DSA are conducted by NRM.Compared to the reference experiment,sensitivity simulations show that ocean stratification,background currents,and initial wave amplitudes all affect the phase speeds of wave branches and therefore shift their reconnection locations while shapes and locations of incoming wave branches upstream of the DSA profoundly influence the subsequent propagation paths.