Analysis of kinematic parameters of Internal Solitary Waves in the Northern South China Sea

Abstract

The monthly climatology of observed temperature and salinity from the U.S. Navy Generalized Digital Environment Model (GDEM-Version 3.0) is used to derive the geographical and seasonal distribution of kinematic parameters of nonlinear internal waves in the Northern South China Sea (NSCS). Coefficients of the Generalized Extended Korteweg-de Vries Equation (GEKdV) with a background current are investigated (phase speed, dispersion, quadratic and cubic nonlinearity parameters, normalizing factor). These parameters are used to evaluate the possible polarities, shapes of internal solitary waves, their limiting amplitudes and propagation speed. We show that the long wave phase speed and dispersion parameters mainly depend on topography characteristics and have no obvious seasonal variation. The nonlinear parameters and normalizing factor are sensitive to variations in the density stratification and topography. Background current also exerts the distinct effects on the kinematic parameters; especially the nonlinear parameter can change by an order of magnitude. The nonlinear parameters take on larger values in the summer (July), and linear internal waves are prone to become steeper and develop into large-amplitude internal solitary waves under such circumstances. This explains why nonlinear internal solitary waves occur more frequently in summer. From the kinematic viewpoint, the dispersion parameter takes on larger values in the Pacific Ocean (PO) due to deeper water depth when compared with that in the NSCS. The stronger dispersion effect in the PO hinders the formation of large amplitude internal solitary waves, explaining why nonlinear internal solitary waves are rarely found to the east of the Luzon Strait. Large near-bottom velocities dominate the shallow area and tend to increase in the warm season. The largest values are induced by internal solitary waves, indicating that internal waves are the major drivers of sediment re-suspension and erosion processes.