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Abstract: Internal solitary waves (ISWs) in the northern South China Sea (SCS) are investigated using historical observational data, linear theory, global inverse tidal model, and a fully nonlinear nonhydrostatic numerical model. It was found on the basis of mooring data that the appearance of ISWs in the northern SCS is strongly correlated with the maximum of semidiurnal barotropic tidal forcing in the Luzon Strait (LS), but not with the largest peaks of diurnal tidal currents. The role of the latter lies in the modulation of the baroclinic tidal signal in such a way that the diurnal intermittency of ISW characteristics known as A-type waves (large-amplitude rank-ordered ISW packets) and B-type waves (single weak ISWs) is introduced into the internal wave fields. Both A- and B-waves were reproduced in a series of numerical experiments. It was found that due to the neap-spring variability of the tidal forcing, a permanent transition of A-type waves into B-type waves and vice versa takes place. This effect is treated in terms of a multi-harmonic approach. It is assumed that the internal wave field in the SCS is a sum of progressive semidiurnal and diurnal internal tidal waves radiated from the LS. Their superposition in space creates an alternation of large and small troughs. In the course of nonlinear steepening and ultimate disintegration the large and small troughs give rise to type A and type B internal wave packets, respectively. The neap-spring variability of tidal forcing alters the specific contribution of diurnal and semidiurnal constituents and leads to the wave transition from one type to another. [Copyright &y& Elsevier]

Abstract: A fully nonlinear, non-hydrostatic model, MITgcm, is used to investigate internal solitary waves (ISWs) from the Luzon Strait (LS). As the ISWs in the South China Sea (SCS) have drawn more and more attention in recent years, they are studied in various ways, i.e., via remote sensing images, in situ measurements, and numerical simulations. The inspiration of this paper derived from the potential flaws of different numerical models that were employed to examine ISWs. In this study, we performed three-dimensional (3D) experiments with realistic topography and stratification, as well as with fully non-hydrostatic terms in the model, which was rather important for investigating the ISWs. Modeling results showed that baroclinic tides in the LS were essentially three-dimensional (3D), and that wave structures around two ridges in the strait were complicated with interesting internal oceanic phenomena. Several zonal cross-sections were chosen to illustrate vertical structures of zonal velocity field, and to show their meridional variances together with surface horizontal velocity gradients in order to highlight the advantages of 3D modeling with fully nonlinear, non-hydrostatic terms. Following , analysis of two parameters (Froude number and slope parameter that is defined as the ratio of inclination of topography to slope of radiated rays) that govern generation regime indicated that internal waves produced in the LS were subject to a mixed lee wave regime rather than baroclinic tide regime or unsteady lee wave regime. The propagation of ISWs beyond the generation area showed that manifestation of 3D effects was not very obvious, which, through further analysis, was mainly attributed to homogeneity of topography, inaccuracy of barotropic forcing, and Kuroshio intrusion in the LS. To better understand the necessity of 3D modeling, we chose several zonal cross-sections and performed various sensitivity experiments to show discrepancies between 2D and 3D cases. [Copyright &y& Elsevier]