*2.1. Satellite Images*

On account of the changes of sea surface roughness induced by convergence and divergence in the wave front and rear, ISWs are often manifested as bright and dark stripes in optical remote sensing images [23]. Accordingly, remote sensing images are regarded as a useful tool to investigate ISWs in the oceans. Zhao et al. analyzed the polarity transition of ISWs over the continental shelf of the northern South China Sea using SPOT-3 satellite optical image [24], and Jackson utilized Moderate Resolution Imaging Spectroradiometer (MODIS) images for global internal wave detection [25]. Furthermore, based on MODIS images, Huang and Zhao extracted the characteristic parameters of a typical ISW in the deep water of northern South China Sea [26] and Ning et al. further established a model that is able to derive the amplitude of ISWs [27].

The optical remote sensing images employed in this paper were acquired by the MODIS sensor equipped on the National Aeronautics and Space Administration's (NASA's) Terra/Aqua satellite and the Visible infrared Imaging Radiometer (VIIRS) sensor equipped on the NOAA/Suomi NPP satellite. The MODIS data are obtained in 36 visible and infrared bands with a spatial resolution between 250 m and 1 km, which depend on acquisition wavelength, and the VIIRS data are obtained from 22 channels at two resolutions, 375 m and 750 m.

A total of 10 satellite images with distinguishable ISWs over the BS were collected (Figures 1 and 2) and they were taken from two durations, either from 9:30 to 10:00 or from 12:00 to 13:00 local time. The 10 images involve six days of the period from 12 to 21 April, and there were two images on 12, 14 and 19 April photographed with an interval of 175 min. Unfortunately, no satellite images were available on 20 April.
