Vertical structure of internal solitary waves and its force exerted on a horizontal cylinder in the northern South China sea

Abstract

To date, although longitudinal evolution and latitudinal structure of internal solitary waves (ISWs) in Northern South China Sea (NSCS) has been sufficiently investigated, vertical structure of ISWs is rarely discussed. Here we analyse the modulation of the nonhydrostatic and nonlinear effects on ISW vertical structure in NSCS quantitatively. The results show that the depth of maximum vertical displacement of ISWs gets shallower by the nonhydrostatic or nonlinear effect, especially in the continental shelf and slope. Further, the observations show that vertical structures of wave amplitude and ISW-induced vertical velocity could be well predicted by the theoretical nonlinear vertical structures, with their maximums locating at a depth of 200–400 m at the continental slope. The nonlinear effect could be twice stronger than the nonhydrostatic effect in modulating the depth of maximum vertical structure. In spite of varying depth of maximum vertical structure of a westward propagating ISW due to bathymetry, downward vertical force exerted on a horizontal transverse cylinder, calculated under the continuously stratified model, reaches its most fixedly at a depth near the pycnocline (∼100 m) instead of the corresponding depth of maximum ISW vertical structure. Because of positive correlation with wave nonlinearity, the estimated vertical force in the continental shelf and slope is roughly 2–3 times larger than that in the deep basin. The vertical force in eKdV theory could be up to 1.5 times larger than that in KdV theory.