NONLINEAR INTERNAL WAVE INTERACTION IN THE CHINA
Antony K. Liu
Oceans and Ice Branch
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771 USA
Department of Oceanography
National Taiwan Ocean University
EAST CHINA SEA
Synthetic Aperture Radar (SAR) images have been used to study the characteristics
of internal waves in the East and South China Seas (Liang et al., 1995;
Liu et al., 1998). Rank-ordered packets of nonlinear internal waves in
the East and South China Seas are often observed in the SAR images. Recently,
the internal wave distribution maps have been compiled from hundreds of
ERS-1/2, RADARSAT and Space Shuttle SAR images in the East and South China
Seas from 1993 to 1998. In the northeast of Taiwan, the internal wave field
is very complicate, and waves are propagating in all directions. Its generation
mechanisms include the influence of the tide and the upwelling, which is
induced by the intrusion of Kuroshio across the continental shelf (Hsueh
et al., 1993). The Kortweg-deVries (KdV) type equation has been used to
study the evolution of internal wave packets generated in the upwelling
area. Depending on the mixed layer depth, both elevation and depression
waves can be generated based on numerical simulations as observed in the
SAR images. The merge of two wave packets from nonlinear wave-wave interaction
in the East China Sea has been observed in the SAR image and demonstrated
by numerical results.
SOUTH CHINA SEA
While most of internal waves in the north part of South China Sea are propagating
westward. Some of these internal waves are generated from the shallow topography
or sills in the Luzon Strait. The suggested mechanism is similar to the
lee wave formation (Liu et al., 1985) due to strong current from the Kuroshio
branching out into the South China Sea. The wave crest can be as long as
200 km with amplitude of 100 m. Some small internal waves observed on the
continental shelf may be generated from the shelf break in the South China
Sea. At the shelf break, a depression area may be induced by mixing or
shear flow instability in the pycnocline. The disturbance of mixed area
is then driven by the semi-diurnal tide onto the shelf and evolves into
a rank-ordered wave packet (Liu 1988). In the summer, where the mixed layer
is thinner than the bottom layer, depression wave train can be generated.
During the spring/winter, as observed in the SAR image, elevation solitons
can be evolved, because the mixed layer deepens caused by strong winds
and its thickness is thicker than the bottom layer.
Based on the RADARSAT ScanSAR images collected on April, 26 and May
4, 1998, huge internal solitons were observed near Dong-Sar Island with
crest more than 200 km long and wave speed of 1.9 m/s. Most interesting
process is the detection of elevation internal waves in shallow water (220
m) and depression waves on the shelf break (500 m depth) in the same SAR
image (5/4/98). The effects of water depth on the evolution of solitons
and wave packets have been modeled by KdV-type equation and linked to satellite
image observations. For a case of depression waves in deep water, the solitons
first disintegrate into dispersive wave trains and then evolve to a packet
of elevation waves in the shallow water area after they pass through a
"critical depth" of approximately equal layer thickness as demonstrated
by numerical model (Liu et al., 1998). Based on the numerical simulations,
the evolution time for conversion is about 20 hours, and the wave propagation
distance can be as far as 200 km. Also, in the ScanSAR image near Dong-Sar
Island, the westward propagating huge 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.
The wave-wave interaction has been observed in many SAR images of the East
China Sea. The internal solitons are nonlinear, thus their interaction
are much more complicated than the regular linear waves. One of the well-known
phenomena is the phase shift when two solitons are collided. In the northeast
of Taiwan, the interaction patterns are very complicated. In the southeast
of Yellow Sea near South Korea coast, the internal waves are generated
from several islands, so the wave interaction pattern is much more organized.
Especially, during the summer time, a shallow mixed layer of 15 m persists
in the water of 100 m depth. The internal wave packets with more than 15
solitons of equal amplitudes were observed and measured by the thermistor
chain from a research ship in the Yellow Sea during the field test in August
1996. These many solitons in a wave packet may be caused by the internal
wave-wave interaction in the Yellow Sea, which results in the merge of
solitons to a single large internal wave.
From the SAR images obtained on July 23, 1997, several internal wave
packets were generated from the islands near the southwest tip of Korea
Peninsula by the collision of Korea coastal current and semi-diurnal tide.
There are at least two generation sources (islands), one from the east
and the other from northeast. The phase/front of internal wave packets
are shifted and distorted in the interaction areas due to the nonlinear
wave-wave interaction. The direction of shifted wave train is in between
two incident wave packets without interaction. Not only the direction shifted
after the wave-wave interaction, but the number of waves in the wave packet,
wavelength, and amplitude of the waves are also changed. In order to demonstrate
the wave-wave interaction, a numerical calculation with two wave packets
moving in the same direction is performed. Although the wave-wave interaction
in the Yellow Sea is definitely a two-dimensional process, however, the
one-dimensional results may shed some lights on the merge of wave packets.
This research was supported by the National Science Council of Taiwan,
National Aeronautics and Space Administration and Office of Naval Research.
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