Tracking the internal waves in the South China Sea withenvironmental satellite sun glint images

The propagation of internal solitary waves (ISWs) in the South China Sea (SCS) was tracked using National Aeronautics and Space Administration (NASA) Moderate Resolution Imaging Spectroradiometer (MODIS) and the Visible Infrared Imaging Radiometer Suite (VIIRS) sun glint image pairs. The acquisition times of the two images comprising the same pair is usually separated by about 2 hours. Four pairs of images in May and August 2013 were analysed. The ISW phase speeds were derived using the horizontal displacement of the ISW patterns and the time difference between the 2 satellite images. The phase speeds were in good agreement with the theoretical calculations using the Sturm–Louisville (S − L) equation with a non-linear term. Monthly ocean stratification data and bathymetry were used in the theoretical calculations. The ISW phase speeds are mainly affected by bottom depth, with seasonal variations. The ISWs propagated faster in August than in May. Our results also show that the ISW propagation in northern SCS can be mapped using environmental satellite sun glint images in short period of time with the advantage of large scale. 1. Introduction Waves occur at the interface of fluids with different density. As oceanic surface waves occur at the air–sea interface, oceanic internal waves occur in the ocean interior where the seasonal or permanent thermocline exists. Internal wave often behaves in solitary wave form in the deep ocean, which is referred to as internal solitary wave (ISW). ISW produces variability in temperature, salinity, currents, sound propagation, and primary production in the water column. In the stratified ocean, the water column is usually treated as a 2-layer system. Since the density difference between the upper mixed layer and the deeper water is about 3 orders of magnitude smaller between that of the air–sea interface, ISW tends to have much larger horizontal scale (up to 200 km), bigger vertical amplitude (up to 200 m), and lower frequency than surface waves. These large-wavelength waves are difficult to measure with in situ or shipboard instruments. Remote sensing has therefore been a vital tool for ISW observations in the ocean. Since the launch of the first meteorological satellite in the late 1960s, oceanic ISWs were among the first oceanic phenomena identified on these satellite visible images. Apel et al. (1975) first reported the coherent ISW pattern in New York Bight on the multi-spectral scanner image of Earth Resources Technology Satellite 1 (NASA, Washington, DC, USA). Later, using …