Nonlinear Internal Waves in the South China Sea
2. Nonlinear internal waves in the South China Sea
The Kuroshio moving north from Philippine Basin branches out near the south tip of Taiwan
and part of the Kuroshio intrudes into the South China Sea through the Bashi Channel and the
Luzon Strait. Surface signature of huge internal wave packets has been observed in the
ERS-1/2 SAR images, RADARSAT-1 wide swath SAR and ENVISAT ASAR images.
Recently, the internal wave distribution map in the SCS have been compiled from hundreds of
ERS-1/2, RADARSAT and Space Shuttle SAR images from 1993 to 1999 by Hsu et al. (2000)
as shown in Figure 1 for reference.
Figure 1. The bathymetry and internal wave distribution in the South China Sea.
Based on internal wave distribution map, most of internal waves in the northeast part of SCS are
propagating westward. The crest of soliton is more than 200 km long and each packet contains
more than ten rank-ordered solitons with a packet width of 25 km. Within a wave packet, the
wavelengths appear to be monotonically decreasing, front to rear, from 5 km to 500 m. In the
SCS, internal wave amplitude larger than 100 m had been observed in the sea expeditions and
mooring stations. These huge wave packets propagate and evolve into the SCS and finally
reach the continental shelf of southern China.
Bole (Bole et al., 1994) suggested that the solitons are generated in a 4 km wide channel
between Batan and Sabtang islands in Luzon Strait. The proposed generation mechanism is
similar to the lee wave formation from a shallow topography in the Sulu Sea (Apel et al., 1985).
The generation is caused by the semi-diurnal tide and the Kuroshio intrusion across the Luzon
Strait. But, from ENVISAT ASAR (Fig. 2) images collected in 2003 show no strong surface
signature of internal waves near the sources in these images as observed in Sulu sea (Hsu, et. al,
2003). Various generation mechanisms and SAR imaging algorithms of internal tides/waves
are under investigation.
Figure 2. ENVISAT ASAR image in Luzon Strait, up-left is the south tip of Taiwan and lower part is Luzon island.
Both elevation internal waves and depression waves under various ocean conditions have been
studied. The effects of water depth on the evolution of solitons and wave packets have been
modeled by Kortweg-deVries (KdV)-type equation and linked to satellite image observations.
The numerical results indicate that the depression internal waves evolve into rank-ordered
solitons on the shelf with a shallow mixed layer. However, when the mixed layer is thicker
than the bottom layer, only elevation waves can be evolved. Based on the SAR images and
hydrographic data, internal waves of elevation type had been identified in shallow water due to a
thicker mixed layer as compared with the bottom layer on the continental shelf (Fig.3).
Figure 3. Schematic diagram of depression and elevation internal waves, surfaces current field,
wind waves, and the resultant SAR image intensity variations. (After Liu, et al.,
1998).
