Monthly variation on the propagation and evolution of internal solitary waves in the northern South China Sea

In this paper, internal solitary waves (ISWs) in the northern South China Sea (NSCS) are investigated with respect to their propagation and evolution processes in different months. To achieve that, environmental parameters associated with ISW are first calculated using climatological datasets derived from the Simple Ocean Data Assimilation 3.3.1 (SODA3.3.1) reanalysis. The environmental parameters are significantly distinct from month to month, especially the quadratic nonlinearity coefficient alpha. It is negative in the deep basin and turns positive on the shallow continental shelf. A clear dividing line that alpha equals to zero can be found along the shelf from northeast to southwest from January to February and from October to December. The dividing line is interrupted starting from March and is mostly constrained on the northeast shelf, and from May to September only limited areas can positive alpha be found. Further simulations based on the variable extended Korteweg-de Vires (veKdV) model indicate that the polarity conversion process of ISWs therefore exhibits significant seasonality. It can be concluded from the simulation results that near the 200-m isobath on the continental shelf the elevation waves are more likely to appear from November to March owing to the positive values of alpha. The large-scale background current exhibits little effect on the deformation and polarity conversion of ISW in the NSCS although it prominently affects the environmental parameters around the shelf currents from Luzon strait.