Radiation And Dissipation Of Internal Tides In The Northwestern Pacific

Internal tides(baroclinic tides)are internal waves within the tidal frequency band and they are generated by barotropic tidal currents flowing over rough topographic feature such as such as mid-ocean ridges,seamounts and trenches.The diapycanal mixing caused by internal tide is a key driving factor for the maintenance and variation of the oceanic meridional circulation.The northwestern Pacific Ocean has the most typical ridge-trench-arc-basin topography features in the world oceans.It is also one of the strongest internal tide occurrence areas.The study of the energetics and dynamics of internal tides could lead to the in-depth understanding of the multi-scale energy cascade in this area.Based on high-resolution numerical simulation,this paper systematically studies the generation,radiation and dissipation processes of tidal energy in the northwestern Pacific Ocean(mainly in the Philippine Sea).Long-range radiation and interference of M2 internal tides from multiple sources in the Northwestern Pacific are examined by driving a high-resolution numerical model.The M2 internal tides are effectively generated around the boundary area of the Philippine Sea basin,which includes the Luzon Strait,Ryukyu Island chain,Bonin Ridge,Mariana Arc and Izu Ridge,favouring the occurrence of complex interference patterns.The mode-1 and mode-2 M2 tidal beams from boundary sources radiate a long distance into the basin but exhibit different interference-modulated geography variations.Two notable interference cases are investigated: 1)the superposition of internal tides from Luzon Strait and Miyako Strait bifurcates into several southeastward beams,consistent with previous numerical simulations and altimeter measurements,and 2)the interference between Tokara Strait and Bonin Ridge exhibits a multi-scale spatial pattern,which is modulated by the local generated energy and bathymetry features.Energetic dissipation occurs both near the boundary sources and in the basin.Enhanced dissipation is found to coincide closely with the interference-modulated flux field in the deep basin.We further study the diurnal internal tide radiation process under subtidal backgrounds circulation in the Northwestern Pacific.It is found that the distribution of generation sites of the diurnal internal tide is significantly different from the semi-diurnal tide.The Luzon Strait and the Santos Strait are the two dominant source of the Philippine Sea,while the tide generation in the Bonin ridge-Mariana Island Arc is weak.There are also relatively significant diurnal tide signals in the Izu ridge and the Tokara Strait which exceed the critical latitude of the diurnal internal tide.The diurnal internal tide energy from the Luzon Strait can propagate over 2500 kilometers,even crossing over the Mariana Island arc.Due to the effects of earth rotation,the diurnal internal tides tend to bend equatorward notably during the propagation.The diurnal internal tide from Luzon Strait and Santos Strait encounter in the basin of the western Mariana basin and interfered with each other,forming southeastwardly branches of energy flux.The enhanced branches also correspond to strong local dissipation,further illustrating the important role of the interference process in the multi-band internal tide energy dissipation distribution.The theoretical line-source model is established,and the multi-source interference pattern of semidiurnal internal tide is successfully reproduced.The earth rotation effect is then added into the line-source model.The interference process and bending equatorward phenomenon of the diurnal internal tide are also well reproduced.The important roles of interference process and the Coriolis effect in the internal tide energy propagation are therefore theoretically explained.The modulation effect of the background circulation field on the diurnal internal tide radiation path is studied by conducting the ray tracing theory model.It is found that the Kuroshio,the northern equatorial current system and the Mindanao eddy can significantly affect the transmission path of the diurnal internal tide.