Three-dimensional Numerical Simulation And Analysis Of Internal Tides In The Northern South China Sea

Internal tides, as a ubiquitous phenomenon in the ocean, play an important rolein regulating tidal energy dissipation, abyssal mixing, nutrient redistribution and themeridional overturning circulation.In this paper, we present the spatial-temporal characteristics and energetics ofinternal tides in the northern South China Sea (SCS) using a fully nonlinearnonhydrostatic three-dimensional (3D) model driven by four principal tidalconstituents (M2, S2, K1, and O1) individually or together. We also conducted a set ofexperiments to examine the roles of the Kuroshio and wind forcing.The two ridges are the main sites of internal tide generation, and the OrchidRidge is much more efficient (71.2%) than the Heng-Chun Ridge (28.2%). There areseveral hot spots in LS and most of them are in the east slopes of the two ridges at adepth of80–3000m. The spots found in the west slopes of the ridges are mainlylocated in the southwest of Sabtang Island, the northwest of Calayan Island and thewest of Barit Island, where are supposed to be the source regions of the internalsolitary waves (ISWs) in the northeastern SCS.The model results show that, during spring (neap) tide, about64(47) GW ofbarotropic tidal energy is consumed in Luzon Strait (LS), of which59.0%(50.5%) isconverted to baroclinic tides. About22(11) GW of the derived baroclinic energy fluxcan subsequently emanates out of LS, among which50.9%(54.3%) is westward intothe South China Sea (SCS), and45.0%(39.7%) eastward into the Pacific Ocean, andthe remnant16(13) GW is lost locally due to dissipation and convection. What ismore,1.6(0.9) GW of the westward baroclinic energy would be converted into theISWs. The internal tides show more mode-2characterics in spring tide/northern LSthan that in neap tide/southern LS, so did the meridional baroclinic velocity comparedto the zonal velocity.It is revealed that generation areas of internal tides vary with the spring and neaptide, indicating different source areas for the ISWs in the northern SCS. The regionaround Batan Islands is the most important generation region of internal tides duringboth spring and neap tides. In addition, the baroclinic tidal energy shows pronouncedseasonal variability. Both the total energy transferred from barotropic tides tobaroclinic tides and the baroclinic energy flux flowed out of LS are largest in summerand weakest in winter. The internal tides show more mode-2characterics in summerthan that in other seasons, and the amplitude of mode-2internal tides in summer is largest.The baroclinic constituents M2and K1are more prominent than S2and O1. Theconstituent S2is so especially weak that could be ignored. Compared to the east ridgethe west ridge is more efficient in regulating M2baroclinic tide than K1baroclinic tide.Although the diurnal tides dominate in LS, the nonlinear features in the SCS arepredominantly semidiurnal because diurnal internal tides exhibit excessive rotationaldispersion that inhibits nonlinear steepening.The results of four numerical simulations reveal that the energetics of LS internaltides show a well-defined and pronounced seasonal cycle. Both the barotropic-to-bar-oclinic tidal energy transformation and the baroclinic energy flux through LS arelargest in summer and smallest in winter, which mainly reflects the variations ofdensity stratification in LS. Compared to the winter run, about12.4%more barotropictidal energy is converted to baroclinic energy, and westward baroclinic energy fluxfrom LS into the northern SCS is increased by about15.8%, providing explanation forthe elevated ISWs occurrence in remote sensing observations.Our results show that the Kuroshio would strengthen both the generation anddissipation of internal tides in LS, so is the westward baroclinic energy flux, whichhas close connection to the generation of ISWs in the northern SCS. The twotyphoons are selected for their typical paths around LS. The Hagubit passed throughthe LS at the south of the Luzon Island and moved northwestward towards toSouthern China. The Kalmaegi moved northward along the eastern coast of Luzon,then turned northwest and made a landfall at the northern Taiwan Island. The twotyphoons both weaken the barotropic-to-baroclinic conversion rate and the divergenceof baroclinic energy flux as well as the baroclinic energy dissipation. What is differentabout their roles is that, even less baroclinic energy was generated in LS, Hagubitdramatically reinforced the westward baroclinic energy flux, while Kalmaegi actedmore like a baroclinic energy obstructer. The measurements from a mooring site nearLS during the typhoon Nanmadol are analysed. Large-amplitude ISWs clearly due toNanmadol are captured.