Velocity Measurement of Internal Waves in the South China Sea by Multi-Temporal and Multi-Sensor Satellite Images

Abstract

Oceanic internal waves are generated at the boundary of stratified water layers of different densities associated with salinity and/or temperature. They are often generated by the interaction between the bottom topography and tidal currents. As internal waves propagate the water particles are pushed upward and pulled downward by the stratified boundary, yielding varying surface currents, and these currents move in phase with the internal waves. The varying surface currents produce rough and smooth zones of sea surface, and these differences in surface roughness can be detected by optical imagers and synthetic aperture radars (SARs). In this study, MODIS/Terra and ENVISAT/ASAR were used to obtain sequential images of internal waves in the South China Sea for estimating the propagation properties of internal waves. Most of internal waves in the South China Sea were propagating westward due to strong currents of the Kuroshio. The phase velocities of these internal waves were measured by two methods using 1) multi-temporal and multi-sensor images acquired in a very short time interval, 2) several packets of internal waves associated with semidiurnal tidal period in a single image. The internal wave velocities estimated by the method 1 ranged from 0.84 m/s to 1.91 m/s depending on the water depth, while those by the method 2 were from 0.37 m/s to 0.59 m/s.. As internal waves propagate the water particles are pushed upward and pulled downward by the stratified boundary, yielding varying surface currents, and these currents move in phase with the internal waves. The varying surface currents produce rough and smooth zones of sea surface, and these differences in surface roughness can be detected by optical imagers and synthetic aperture radars (SARs). In this study, MODIS/Terra and ENVISAT/ASAR were used to obtain sequential images of internal waves in the South China Sea for estimating the propagation properties of internal waves. Most of internal waves in the South China Sea were propagating westward due to strong currents of the Kuroshio. The phase velocities of these internal waves were measured by two methods using 1) multi-temporal and multi-sensor images acquired in a very short time interval, 2) several packets of internal waves associated with semidiurnal tidal period in a single image. The internal wave velocities estimated by the method 1 ranged from 0.84 m/s to 1.91 m/s depending on the water depth, while those by the method 2 were from 0.37 m/s to 0.59 m/s.