Numerical Experiments On Second Mode Internal Solitary Waves

Internal solitary waves whose vertical displacement possesses two extremevalues in the vertical direction are also called second mode internal solitary waves. Onthe continental shelf of the South China Sea in the summer,most of the second modeinternal solitary waves follow the first mode internal solitary waves,which implicitesthat the two types of internal solitary waves have close relations with eachother.Acccording to this fact,Liu assumes that the energy source of the second modeinternal solitary waves is the first mode internal solitary waves and when the firstmode internal solitary wave propagating from the deep basin arrives at the shelf break,which can interact with the background surroundings and generate second modeinternal solitary wave in proper environment particularly when the buoyancyfrequency profile could be divided into three layers because of the seasonalpycnocline presenting in the upper ocean.Based on the above assumptions,Liuproposed the corresponding modal decomposition theory.By means of this theroy, anumber of numerical experiments are carried out.The numerical resuts are listed asfollows..First, in a three-layer system,when the thickness of surface-mixed layer isaround the critical value,a bigger possibility of generation of second mode inernalsolitary waves arises from a bigger thicnkess of the upper layer comparing to that ofthe middle layer..Second，there is a critical value for the total depth of shallow waterwhere generation of second mode internal solitary wave is most probably.The twoconditions should be satisfied if the deep first mode internal solitary wave generatesthe shallow second mode internal solitary wave by the greatest extentThere are two reasons contributing to the rare detection of the second modeinternal solitary wave.One is that their energy is small.The other is that the disperiveeffect is stronger that the nonlinear effect and the turbulent dissipation is intense nearthe shelf break. Consequently, even if they are generated,their survival time is so shortthat it is difficult to observe them.