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.