Observation of Mode-2 Internal Solitary Waves in the Northern South China Sea Based on Optical Remote Sensing

Due to the small scale and short lifetimes of mode-2 internal solitary waves (ISWs), they are seldom observed by satellite images. This study investigated the optical remote sensing imaging characteristics of mode-2 ISWs through physical simulation experiments, proposed a method for identifying mode-2 ISWs in optical remote sensing images, and conducted multisource optical remote sensing observation of mode-2 ISWs in the northern South China Sea (SCS). We have gathered over 10 000 images from seven optical remote sensing satellites, including Terra/Aqua, Gaofen-1/6, HJ-2A/B, and CBERS-04. A total of 342 mode-2 ISWs events were identified, exhibiting primary concentrations near Dongsha Atoll and the northern Heng–Chun Ridge in the northeastern SCS, as well as the Shenhu–Yitong Shoals in the northwestern SCS. Most mode-2 ISWs were observed within the 150–400 m water depth range along the continental shelf. The mode-2 ISWs primarily propagate in a northwestern direction, with propagation speeds approximately half that of the mode-1 ISWs. In the case of the northern Heng–Chun Ridge, mode-2 ISWs tended to propagate in a southwest direction. This study underscores the significant potential of high-resolution optical remote sensing satellites in observing mode-2 ISW and provides a new direction for studying mode-2 ISW.