Study On The Oceanic Nonlinear Internal Waves South Of Hainan Island

The South China Sea(SCS)is one of the most active oceanic internal wave(IW)regions in the world and has been a hot spot for international oceanographic research.Previous studies mainly focus on the IWs in the northeastern SCS.Little attention is paid to the IWs in the northwestern SCS,especially the IWs in the south of Hainan Island(SHI).Through combing the Synthetic Aperture Radar(SAR)observation,theoretical analysis,MITgcm numerical simulation and field observation,the paper reveals the characteristics,sources and generation processes of the nonlinear internal waves(NLIWs)in the SHI.In addition,the paper proposes a new method to extract the vertical dynamical parameters of NLIWs from SAR images.Main contents and conclusions of the paper are as follows:(1)The spatial and temporal distribution characteristics of NLIWs in the SHI are analysed by using the SAR data in ten years.The results show that the NLIWs in the SHI are mostly distributed on the continental shelf and propagate onshore.Among the shoreward NLIWs on the shelf,two types of NLIWs are identified to occur most frequently.One type of NLIWs,named Type-N NLIWs,is mainly distributed in the northern shelf of SHI,and the other one is named Type-S NLIWs,mainly distributed in the southern shelf of SHI.The two types of NLIWs have the highest occurrence frequency in June and July,and the lowest occurrence frequency in January to February and October to December.Correlation analysis shows that the seasonal variation of NLIW occurrence observed by SAR is highly correlated with oceanic pycnocline depth and wind speed.(2)Using consecutive SAR data,a new method for retrieving the amplitude of NLIWs in the shallow water is proposed.The method solves the problem that the retrieved amplitudes of shallow-water NLIWs are influenced by the uncertainty of upper layer fluid thickness in the two-layer fluid model.In the proposed method,the SAR imaging theoretical model of NLIWs is firstly developed based on two-layer EKdV theory.Then,consecutive SAR images combined with oceanic stratification and water depth data are exploited to determine the theoretical model parameters.After that,by fitting the theoretical model with SAR observation,the optimum upper layer thickness is obtained to derive the NLIW amplitude.The case studies show that the proposed method can obtain reasonable results for NLIWs in the shallow water(including the shelf area of SHI),which is important for facilitating the study of amplitude retrieval of shallow-water NLIWs.(3)Based on the theoretical analysis,collaborative satellite-ocean observation experiment and two-dimensional MITgcm numerical simulation,the source and generation process of Type-N NLIWs are studied.The results show that the internal tides generated by the interaction between barotropic tide and local shelf break radiate most of its energy to the deep sea,and have little effect on the generation of the Type-N NLIWs.The Type-N NLIWs mainly originate from the Xisha Islands,which may have multiple source sites.The interaction between barotropic tide and Xisha Islands can generate diurnal internal tidal beams.Then the internal tidal beam can impinge on the pycnocline near the continental shelf region and give rise to an interfacial wave.As the interfacial wave propagating along the pycnocline approaches the shelf break,it can efficiently disintegrate into the Type-N NLIW.(4)Based on the theoretical analysis and two-dimensional MITgcm numerical simulation,the source and generation process of Type-S NLIWs are explored.The results show that unlike the Type-N NLIWs,the generation of Type-S NLIWs may be influenced by both the local shelf break and Xisha Islands.When only local shelf break is considered,the shelf break can excite a mode-1 diurnal internal tide,which can then disintegrate into the IWs that are basically consistent with the SAR-observed Type-S NLIWs;the background current has little effect on the generation of Type-S NLIWs,but can greatly change their nonlinear evolution.