On the mechanism of A-type and B-type internal solitary wave generation in the northern South China Sea

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.

► Correlation between the ISWs in the northern SCS and the semidiurnal tidal harmonics in the LS.
► Different packets in a day (A- or B-waves) are the consequence of semidiurnal and diurnal harmonics in tide.
► Two progressive waves (semidiurnal and diurnal) produce large and small internal wave troughs.
► Wave troughs steepen and ultimately disintegrate into A- and B-wave packets, respectively.
► The number of waves in A and B packets varies with neap-spring periodicity of tide.