Study On The Spatial Distributions And Temporal Variations Of Internal Solitary Waves In The South China Sea

Internal solitary wave (ISW), with characteristics of short period, large amplitudeand strong nonlinearity, has been frequently observed in the northern South China Sea(SCS), which plays a remarkable role in the ocean mixing, material transport and soundpropagation processes. It is generated by the astronomic tides flowing over the roughtopography in the Luzon Strait (LS), then radiates westward into the SCS and ultimatelybreaks over the continental shelf, with a propagation distance of more than600km. Thecomplex astronomic tides in the LS，as well as the fast-changing background conditions,make the generation and evolution processes of ISW quite complicated. In this study,based on a field experiment during two consecutive years, together with the numericalsimulations and satellite images，the spatial distributions and temporal variations of ISWin SCS are analyzed, and its generation mechanisms are further investigated.The synchronous observation using several moorings deployed along the ISWpropagation trajectory shows that, as the ISW propagated from the LS to the SCS, type-aISW usually begun to appear west of the LS. However, their existed two kinds of type-bISW according to its appear position is near the continental shelf or west of the LS.During the westward propagation, in most cases it became stronger, while the emergenceof a powerful warm eddy was considered to apparently weaken the strength of the ISW.The factors, which influenced the propagation speed of ISW, were analyzed and listed inthe order of effect as follows: water depth> powerful warm eddy>seasonal variation ofstratification Samplitude of ISW. Moreover, ISW in the SCS otfen manifested as ISWtrains, which made up of a group of solitons and arrived at the mooring in the order ofamplitude, and the observation shows that during the propagation the time intervalbetween solitons increased. Based on the arrival time and intensity of the observed ISWs，in combination withmeasurements near the Batan Island in the LS，the characteristics of ISW occurence timeand its temporal variation are analyzed. ISW in SCS was found to be most active fromlunar day1st to8th，and15th to22nd. On average, the ISW was observed1.19times perday in summer, while it was only observed0.64times per day in winter. Most of theobserved ISWs occurred during the strong semidiural tide periods. When both ofsemidiural and diurnal tides were strong, type-a ISW was strong while type-b ISW wasmuch weaker. However, when the semidiural tides were strong and diurnal tides wereweak, both of type-a and type-b ISW were strong. There existed significant inter-annualvariations of ISW occurences in two consecutive years, and the ISW was apparentlyweak in both winters.To understand the generation process and mechanism of ISW in the SCS, numericalsimulations using MITgcm were performed. It was shown that, when the semidiural anddiurnal tides in the LS were simultaneously strong, type-a ISW was produced by theeastward tidal current and evolved from the backslope of elevation wave west of the ridge,while type-b ISW was generated by the westward tidal current and arised from theinternal tide. When the semidiural tides were strong and diurnal tides were weak, both ofenergetic type-a and type-b ISWs were observed, which produced by the eastward tidalcurrent and evolved from the backslope of elevation wave west of the ridge. The strongerstratification was proved to favor the generation of ISW, while the deepening of mixedlayer depth were found to slightly increase its intensity. The occurrence of second verticalmode ISW, with counter-phase vertical displacement in the upper and lower layers, wasfirst observed in the deep basin of South China Sea，and a numerical simulationestablished that this wave was the product of the interaction of large-amplitude mode-1NIW with the western ridge in the LS.