Investigation Of The Propagation And Evolution Processes Of The Internal Solitary Waves In The Northern South China Sea Based On The Variable Coefficients KdV-type Theoretical Models

The research on oceanic internal solitary waves (ISWs) has a significant value of practicalapplication for military and ocean engineering. In the recent two decades, owing to the reportedissues that ocean oil materials frequently suffer damages, researchers have drawn their attention tothe South China Sea (SCS). Numerous remote sensing images and in situ measure-ments haveshown that northern SCS (NSCS) is one of the most active regions for oceanic ISWs everobserved around the world. In the past decade, the research on the generation, propagation andevolution of ISWs in the NSCS is still now one of the hot topics.In the density stratified fluids, the Korteweg-de Vries (KdV) theoretical model based on theweakly nonlinear assumption is a reduced model which describes the propagation and evolution ofoceanic ISW. This model represents the dynamical balance between nonlinearity and non-hydrostatic dispersion. This dissertation has, for the first time, systematically incorporated thevariable coefficient KdV type (KdV-type) theoretical model into the situation of pratical oceanicbottom topography, continuous stratification and background currents, and discussed the differenteffects of high-order nonlinearity and the Earth’s rotation. Meanwhile, using the in situobservational data of temperature fluctuation and current, we have verified the feasibility of thetheoretical model in the research of propagation of oceanic ISWs. The variations of environ-mental parameters of ISW, to a certian extent, present the essential characteristics of ISWs in theinterested area, which is rarely studied in previous work. In this dissertation, the spatialdistribution and seasonal variation of the internal wave environmental parameters in the NSCS,such as long wave phase speed c, nonlinear parameter, high-order nonlinear parameter1anddispersion parameter, and the influence of background current on the internal wave modalstructure and environmental parameters, are investigated using a climatological monthly meantemperature and salinity data and a current dataset from the ouput of a general circulation modelas a background field.It is shown that the spatial distribution and seasonal variation of the internal waveenvironmental parameters in the NSCS is mainly due to the vaule of water depth and the variation of the strength of stratification and background current. The values of c and are mainlydetermined by water depth, and they will both gradually turn smaller as the water depth becomesshallower. Seasonal variation of stratification in the deep water is not obvious, and thecorresponding variation of c and is fairly small. However, in the shallow region, seasonalvariation of stratification is relatively obvious, and the values of c and will significantly change.The values of in the deep water are basically negative, and turns to positive in the shallow water,where there exists a dividing line which separates the negative and positive regions and variesseasonally. The emergence and disappearance of the dividing line is closely associated with thestrength of stratification. Stratification is relatively weak in autumn and winter, and thecorresponding dividing line is distinct, approximately consistent with the200-m isobath, while inspring, as stratification turns stronger, the dividing line will gradually disappear, and in summer,this line completely vanish. The values of1are mostly positive in the deep water, and there is anobvious dividing line across which the values change from positive to negative, existingthroughout the year. The position of this dividing line is more deeper than that of, and shiftsseason by season, which is shallower in summer and deeper in winter. The background current hasa larger impact on the modal structure of higher mode internal wave, and this impact is moreprominent in the continent shelf regions where the current is relatively larger. In the region ofstrong tidal current, flood and ebb tide mainly affect the value of1, which may differ by an orderof magnitude during different phases of tide.Numerical simulations of KdV-type models show that the variation of nonlinearity anddispersion is the main mechanism governing the propagation and evolution processes of ISWs.The wave amplitude will increase when the nonlinearity grows and decrease when dispersioneffect exists. According to the relative strength between nonlinearity and dispersion, thepropagation region of ISWs could be divided into two different areas, the balance area in whichnonlinearity balances with dispersion and the steepening area in which nonlinearity dominates.This dividing technique could be applied to illustrate the distribution features of ISWs in theNSCS observed by Synthetic Aperture Radar (SAR) images. The high-order nonlinearity primarilyaffects the deformation and nonlinear disintegration of large amplitude ISWs. Consideringhigh-order nonlinearity term in the theoretical model, the enhancement of nonlinearity breaks thebalance between nonlinearity and dispersion in the KdV model, and therefore the amplitude ofISWs increases and wave profile narrows. Accordingly, the induced velocity by ISWs prominently increases. In contrast, high-order nonlinearity has little effect on the propagation and disinte-gration of small amplitude internal tide. The KdV-type theoretical models could simulate thedecay and reemergence processes during the long-distance propagation of ISW under theinfluence of the Earth’s rotation, which is consistent with the conclusion obtained by Helfrich(2007) under the fully nonlinear framework. Moreover, high-order nonlinearity will slow downthe decay processes of ISW. The Earth’s rotation has little effect on a single ISW propagatingalong the simulating cross section, but evidently inhibites the nonlinear disintegration of aninternal tide. The difference is determined by the value of Ostrovsky number which represents therelative magnitude between nonlinearity and rotation. For a single ISW, the Ostrovsky number isfar more than O(1), and thus nonlinearity dominates. But for the internal tide, the Ostrovskynumber in the simulating area is relatively small, therefore the effect of rotation could beprominent.The wave profile and current velocity simulated by the variable coefficients KdV-typetheoretical model is compared with the in situ measurements, which indicate that the theoreticalmodel is capable of illustrating the weakly nonlinear and even some highly nonlinear ISWs, but iscertainly limited when illustrating the much larger amplitude and much higher nonlinear ISWs.