Antony K. Liu

Evolution of nonlinear internal waves in the East and South China Seas

Synthetic Aperture Radar (SAR) images from ERS-I have been used to study the characteristics of internal waves northeast and south of Taiwan in the East China Sea, and east of Hainan Island in the South China Sea. Rank-ordered packets of internal solitons propagating shoreward from the edge of the continental shelf were observed in the SAR images. On the basis of the assumption of a semidiurnal tidal origin, the wave speed can be estimated and is consistent with the internal wave theory. By using the SAR images and hydrographic data, internal waves of elevation have been identified in shallow water by a thicker mixed layer as compared with the bottom layer on the continental shelf. The generation mechanism includes the influences of the tide and the Kuroshio intrusion across the continental shelf for the formations of elevation internal waves. The effects of water depth on the evolution of solitons and wave packets are modeled by the nonlinear Kortweg-deVries (KdV) type equation and linked to satellite image observations. The numerical calculations of internal wave evolution on the continental shelf have been performed and compared with the SAR 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 “turning point” of approximately equal layer depths that has been observed in the SAR image and simulated by the numerical model. The importance of the dissipation effect in the coastal area is also discussed and demonstrated.

The Sulu Sea Internal Soliton Experiment

Abstract We present the results of a comprehensive study of large-amplitude internal solitons generated in the Sulu Sea in the Philippines by intense tidal flow over a sharp bathymetric feature. Surface signatures of these waves have been observed in images from the DMSP, Landsat, and Nimbus-7 satellites and the space shuttle SIR-A synthetic aperture radar. A two-week experiment was conducted in the Sulu Sea in May 1980 from R.V. Oceanograher, during which a three-mooring phased array of current meters and thermistors was implanted near the wave source and at 82 km and 200 km from the source. Seventeen soliton packets were observed whose properties underwent significant change as they propagated across the sea. Vertical profiles of density, temperature, and acoustic backscatter, as well as radar and photographic surface signatures, were obtained from the ship. Data from satellites, moorings, and the ship give unprecedented information on the spatial and temporal characteristics of solitary waves, which ha...

A study of internal waves in the China Seas and Yellow Sea using SAR

Abstract Synthetic aperture radar (SAR) images from ERS-1 and ERS-2 have been used to study the characteristics of internal waves in the East China Sea. Rank-ordered packets of nonlinear internal waves in the East China Sea are often observed in the SAR images, especially in the northeast of Taiwan. In this region, the internal wave field is very complicated, and its generation mechanisms include the influence of the tide and the upwelling, which is induced by the intrusion of the Kuroshio across the continental shelf. The internal wave distributions in the East and South China Seas have been compiled based on the SAR observations from satellites. 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 merging of two wave packets from nonlinear wave–wave interaction in the Yellow Sea has been observed in the SAR image and is demonstrated by numerical results.

Nonlinear Internal Wave Evolution in the Sulu Sea

Abstract This paper presents an analytical investigation of nonlinear internal wave evolution in the Sulu Sea. A solitary wave theory that describes tie evolution of internal solitons has been developed and expanded to include effects of vertical shear, variable bottom topography, radial spreading, and dissipation. A numerical parametric study has been performed in order to understand the relative importance of radial spreading and dissipation effects on soliton decay across the Sulu Sea. Using Sulu Sea environmental and initial data, the numerical simulations reproduce reasonably well the measurements at the downstream mooring from the Sulu Sea Internal Soliton Experiment. Comparison of simulations with data on the evolution of lead wave amplitude across the Sulu Sea indicates that the variable depth effect is appreciable, and the radial spreading and dissipation effects are very significant The effects of the Earths rotation, vertical shear, variable bottom topography, radial spreading and dissipation ...

Wave propagation in a solid ice pack

Abstract The analysis presented in this paper was inspired by the report that the R/V Polarstern has encountered surface waves of large amplitude hundreds of kilometers inside the ice pack in the Weddell Sea. This paper presents analysis of processes that affect waves in an ice pack, namely the refraction of waves at the pack edge, the effects of pack compression on wave propagation, wave train stability and buckling stability in the ice pack. Sources of pack compression and interaction between wave momentum and pack compression are discussed. Viscous damping of propagating waves are also studied. Significant results include the conditions for total reflection of waves at the pack edge, the strong effect of pack compressive stress on wave group speed, with the concomitant possibility of extreme local concentration of wave energy. The result that compressive stress in the pack leads to very rapid development of wave packets, through changes in the parameters for weakly nonlinear modulational instability of...

Solitons northeast of Tung-Sha Island during the ASIAEX pilot studies

In a recent study, satellite images have shown that internal solitons are active in the northern South China Sea (SCS). During the Asian Seas International Acoustic Experiment (ASIAEX) pilot studies, current profiler and thermistor chain moorings were deployed in the spring of 1999 and 2000 to investigate internal solitons northeast of Tung-Sha Island on the continental slope of the northern SCS. Most of the observed internal solitons were first baroclinic mode depression waves. The largest horizontal current velocity, vertical displacement, and temperature variation induced by the internal solitons were around 240 cm/s, 106 m, and 11/spl deg/C, respectively, while the estimated nonlinear phase speed was primarily westward at 152 /spl plusmn/ 4 cm/s. The observed internal solitons could be categorized as four types. The first type is the incoming wave from deep water and can be described reasonably well with the KdV equation. The second and third types are in the transition zone before and close to the turning point (where the upper and lower layer depths are equal), respectively. These two types of solitons were generally near the wave-breaking stage. The fourth type of soliton is a second baroclinic mode and probably was locally generated. The time evolutions are asymmetric, especially at the middle depths. A temperature kink following the main pulse of the soliton is often seen. Higher order nonlinear and shallow topographic effects could be the primary cause for these features. The appearance/disappearance of internal solitons coincides mostly with spring/neap tide. The internal soliton is irregularly seen during the neap tide period and its amplitude is generally small. The time interval between two leading solitons is generally around 12 h. The first baroclinic mode of the semidiurnal tide has a larger amplitude than the diurnal tide and could redistribute its energy into the soliton.

A case study of internal solitary wave propagation during ASIAEX 2001

During the recent Asian Seas International Acoustics Experiment (ASIAEX), extensive current meter moorings were deployed around the continental shelf-break area in the northeastern South China Sea. Thirteen RADARSAT SAR images were collected during the field test to integrate with the in situ measurements from the moorings, ship-board sensors, and conductivity/temperatire/depth (CTD) casts. Besides providing a synoptic view of the entire region, satellite imagery is very useful for tracking the internal waves, locating surface fronts, and identifying mesoscale features. During ASIAEX in May 2001, many large internal waves were observed at the test area and were the major oceanic features studied for acoustic volume interaction. Based on the internal wave distribution maps compiled from satellite data, the wave crests can be as long as 200 km with an amplitude of 100 m. Environmental parameters have been calculated based on extensive CTD casts data near the ASIAEX area. Nonlinear internal wave models have been applied to integrate and assimilate both synthetic aperture radar (SAR) and mooring data. Using SAR data in deep water as an initial condition, numerical simulations produced the wave evolution on the continental shelf and compared reasonably well with the mooring measurements at the downstream station. The shoaling, turning, and dissipation of large internal waves at the shelf break have been studied and are very important issues for acoustic propagation.

Wavelet analysis of satellite images for coastal watch

The two-dimensional wavelet transform is a very efficient bandpass filter, which can be used to separate various scales of processes and show their relative phase/location. In this paper, algorithms and techniques for automated detection and tracking of mesoscale features from satellite imagery employing wavelet analysis are developed. The wavelet transform has been applied to satellite images, such as those from synthetic aperture radar (SAR), advanced very-high-resolution radiometer (AVHRR), and coastal zone color scanner (CZCS) for feature extraction. The evolution of mesoscale features such as oil slicks, fronts, eddies, and ship wakes can be tracked by the wavelet analysis using satellite data from repeating paths. Several examples of the wavelet analysis applied to various satellite images demonstrate the feasibility of this technique for coastal monitoring.

Towards an automated ocean feature detection, extraction and classification scheme for SAR imagery

Spaceborne synthetic aperture radar (SAR) observation is an important tool for monitoring and studying changes in various geophysical elements in and above world oceans. Because of SARs ideal imaging capability and high resolution, the collection of SAR data will likely extend well into the 21st century. As the data become increasingly abundant and computers faster and more affordable, it naturally leads to an increasing need for an automated procedure to replace the labour-intensive manual screening process. In this paper, an integrated scheme for detection, extraction and classification of linear ocean features in SAR imagery is attempted for the purpose of automated screening. The methodology consists of feature detection based on greyscale histogram screening, feature extraction based on two-dimensional wavelet analysis and feature classification based on texture analysis. Using these algorithms on SAR data, several case studies of linear ocean features, including fronts, ice edges and a polar low, are presented herein. Though not fully automated at this stage, the integration of these algorithms seems to lay a promising foundation for the future development of a more automated ocean feature detection, extraction and classification scheme.

Validation of sea ice motion from QuikSCAT with those from SSM/I and buoy

Arctic sea ice motion for the period from October 1999 to March 2000 derived from QuikSCAT and ocean buoy observations.Special Sensor Microwave/Imager (SSM/I) data using the wavelet analysis method agrees well with ocean buoy observations. Results from QuikSCAT and SSM/I are compatible when compared with buoy observations and complement each other. Sea ice drift merged from daily results from QuikSCAT, SSM/I, and buoy data gives more complete coverage of sea ice motion. Based on observations of six months of sea ice motion maps, the sea ice motion maps in the Arctic derived from QuikSCAT data appear to have smoother (less noisy) patterns than those from NSCAT, especially in boundary areas, possibly due to constant radar scanning incidence angle. For late summer, QuikSCAT data can provide good sea ice motion information in the Arctic as early as the beginning of September. For early summer, QuikSCAT can provide at least partial sea ice motion information until mid-June. In the Antarctic, a case study shows that sea ice motion derived from QuikSCAT data is consistent with pressure field contours.