Numerical Simulations Of Internal Tides In The Luzon Strait

Internal tide, which is a weakly nonlinear tide-generated internal waves, plays animportant role in abyssal mixing, nutrient redistribution, etc. In this paper, we presentthe spatial-temporal characteristics and energetics of internal tides in the Luzon Straitusing a fully nonlinear nonhydrostatic three-dimensional model driven by eightprincipal tidal constituents together. And the differences of internal tides betweenspring and neap tide and in different seasons are discussed. We also conducted a set ofexperiments to examine the role of bottom topography.The internal tides near island slopes is stronger than the other areas and themaximum amplitude there is about35m in summer. The internal tides are generated inthe Luzon Strait and propagate into South China Sea(7.7GW) and West Pacific（4.7GW）with the amplitude decreasing. The internal tide propagated into SCSwhich is most semidiurnal tide is obviously stronger than that propagated into WestPacific which is most diurnal tide.The internal tide propagated into South China Sea has two different branches.One of them is generated in the island slope areas in the northwest of Itbayat islandand the southwest of Batan islands and travels to the South China Sea shelf like abeam. Continental shelf and slopes have a important function of energy dissipation.The other branch is generated in the northwest of Babuyan islands and directly turnsinto the sea basin which finally propagate into the centre of South China Sea. As thepart of Western Pacific adjacent to the Luzon Strait is quite deep, the internal tidepropagated into Western Pacific recedes sharply in the direction of northeast. Afterpasses124°E, it turns into southeast. According to the cross section analysis in20°Nand21°N, the first mode internal tide energy flux propagates westward in SouthChina Sea at the speed of about3.5m/s, while it propagates eastward in West Pacific at the speed of about4.0m/s. There is also a second mode internal tide energy fluxthat propagates westward at the speed of1.2m/s. The speeds decreases dramaticallywhen the internal tides reach the continental slope.The internal tides most generated in the Luzon Strait which is also the maindissipating area of internal tides. The phenomena of barotropic tide converting intobaroclinc tide and baroclinic tide converting into barotropic tide take place in thestrait at the same time. When the barotropic tide flows over the ridges in the LuzonStrait, it converts into baroclinic tide, some of which converts back into barotropictide before it propagates far away.The energy flux of internal tides vary a lot in spring and neap tide. During theperiod of neap tide, the energy flux is much weaker than that during the period ofspring tide. In South China Sea, the energy flux most travels to northwest and little tothe sea basin.5.5GW(4.2GW) baroclinic tidal energy propagated into South ChinaSea(Western Pacific) which is28.6%(10%) smaller than that during the period ofspring tide. The cycles of the internal tides propagated into South China Sea(WesternPacific) are also different.The internal tide energy flux also shows pronounced seasonal variability. Theenergy flux propagated into South China Sea is27.2%larger in autumn and winterthan that in spring and summer. And the energy flux propagated into Western Pacificis largest in autumn(55.3%larger than that in summer) and smallest in summer.Different barotropic tides in different seasons play a vital role in this situation whilethe changes of stratification affect little.The east ridge plays a vital role to the generation of internal tide in the LuzonStrait, while the west ridge affacts the internal tide branch propogated into the SCSbasin greatly. The depth difference of the two ridge varies from north to south, whichcauses the difference of the internal tide energy flux for north to south in the LuzonStrait.