Numerical Simulations Of Internal Solitary Waves In The Northwestern South China Sea

Recently some internal solitary waves (ISWs) are observed from remotely sensedimages in the northwestern South China Sea (NWSCS). Scrutinizing the crest of theseISWs, it is supposed that they are generated locally as a result of tidal-topographyinteraction. To confirm this assumption, a fully nonlinear non-hydrostatic modelMITgcm is applied to investigate the ISWs in the NWSCS.A2-dimensional simulation with M2tidal forcing and a simplified topography isfirstly launched, and the generation process and mechanism of ISWs in the NWSCSare investigated. The model results confirm that the ISWs on the left side of the sillsare generated when the tidal flow on the sills top changes from ebb to flood tides,while those on the right side of the sills are associated with the beginning of ebb tidesat generation. Analysis of Froude number and slope parameter that govern generationregimes indicates that the generation of internal waves in this area is subject to mixedlee wave rather than baroclinic tide regime or unsteady lee wave regime. There arethree sills in the simplified topography, which are sills a, b, and c from west to eastrespectively. It is found in a series of sensitivity experiments that the ISWs on the leftside of these sills are generated from both sills b and c, while the sill a acts as the roleof intensifying nonlinearity of the internal waves and facilitating the generation ofhigher internal wave modes. However the ISWs on the right side of these sills areonly generated from sill c. Furthermore, scrutiny of the ISWs reveals that thesuperposition of different internal wave modes exists near the sills and thewell-developed first and second ISWs are pronounced further from the sills.Experiments simulated with different tidal forcing(s) reveal that the internal solitarywaves in this area are caused by semi-diurnal M2tide, and the nonlinear interactionbetween K1and M2at the sills’ crests showed no radical change on the generation mechanism of the ISWs, and the K1tidal harmonic alone can not generate ISWs withlarge amplitude. Furthermore, a series of numerical simulations exploring theinfluence of strength of tidal forcing, stratification, grid resolution andnon-hydrostatic approximation on ISWs generation are also carried out.The3-dimensional simulation is then carried out based on the2-dimensionalresults. The model results show that the edge of Zhongsha Island with large slopeparameter is the generation source of ISWs. The multi-mode internal waves propagatetowards northwestern and southeastern directions separately after they generated inthese areas. Wave amplitude, wavelength, propagating directions and the length ofcrests are changed during the propagating process of these internal waves and themulti-mode internal waves turn into first-mode ones gradually. In addition, several2-dimensional simulations with four profiles of the3-dimensional topography arelaunched. Comparison results show that the3-dimensional topography results inenergetics redistribution of the waves filed and affect the generation and evolution ofinternal waves.