Nonlinear Internal Waves in the South China Sea
Ming-Kuang Hsu
Professor, Kuang Wu Institute of Technology, Taipei, Taiwan, R.O.C.
E-mail: Hsumk@kwit.edu.tw
Antony K. Liu
Senior Scientist, NASA/Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.
E-mail: Antony.A.Liu@nasa.gov
ABSTRACT
ENVISAT ASAR, RADARSAT-1 and ERS/2 SAR images have been used to study the internal
waves in the north part of South China Sea (SCS). Based on the internal wave distribution map
compiled from SAR images collected in the last 5 years, almost all internal waves in the SCS are
propagating westward and are generated from the shallow topography or sills in the Luzon Strait.
However, from ENVISAT ASAR images collected recently it shows no strong surface signature
of internal wave near the sources region. More study on radar imaging mechanism for internal
tides/waves near the generation area is needed. In the internal wave distribution map, the internal
wave crest in the SCS can be as long as 200 km and the westward propagating huge
well-developed internal solitons are often encountered and broken by the coral reefs on the shelf.
In some cases, the broken waves will merge after passing the island and interact with each other.
Also, the elevation internal wave and its induced re-suspension due to strong upwelling current
in the shallow water (around 100 m depth) were detected by using shipboard echo-sounder
EK500.
1. Introduction
The tidal flow over topographic features such as a sill or continental shelf in a stratified ocean
can produce nonlinear internal waves of tidal frequency and has been studied by many
investigators (Liu et al., 1985). Their observations provide insight into the internal wave
generation process and explain the role they play in the transfer of energy from tide to ocean
mixing. These nonlinear internal waves are apparently generated by internal turbulent mixing
or baroclinic shear instability due to tidal flow over bottom features. It has been demonstrated
that surface signatures of these nonlinear internal waves are observable in the SAR images from
the First and Second European Remote sensing Satellite ERS-1/2 (Liang et al., 1995; Liu et al.,
1998; Hsu, et al, 2000). The field experiments provide a unique opportunity to study the
evolution of nonlinear internal waves and to validate imaging mechanism for SAR-observed
internal wave signatures in the SCS. A validated and calibrated model can be very useful for
understanding of shelf processes and for the applications of the internal wave effect on oil
drilling platform, nutrient pump (for fishery) and sediment transport.
