Nonlinear internal wave interaction in the China Seas

NONLINEAR INTERNAL WAVE INTERACTION IN THE CHINA SEAS

Antony K. Liu Oceans and Ice Branch NASA/Goddard Space Flight Center Greenbelt, Maryland 20771 USA

Ming-K. Hsu Department of Oceanography National Taiwan Ocean University Keelung, Taiwan

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.

WAVE-WAVE INTERACTION

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.

ACKNOWLEDGMENT

This research was supported by the National Science Council of Taiwan, National Aeronautics and Space Administration and Office of Naval Research.

REFERENCES

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