<b>Reappeared period shifts of internal solitary waves </b><b>in the northern South China Sea</b>

The internal solitary waves occur in the ocean interior. Their huge shock momentum may lead to catastrophic threats to underwater navigation and ocean engineering facilities. The South China Sea is characterized by frequent occurrence of super intensive internal solitary waves with amplitudes as high as 200 m. However, daily reappeared times of waves manifest uncertainty. To accurately forecast the wave occurrence, dynamic mechanisms should be clarified. This study aims to solve this problem using theory of modulation of mean shear flow to the internal wave propagation. Approximate solutions are validated by the mooring array measurements in 2019 and 2020. The mechanisms of reappeared period shift and amplitude variation of internal solitary waves are attributed to the modulation of low-frequency flow of about 7d period to internal tide waves. The mean flow vorticity and the Doppler effect determine the signs of reappeared period shifts. Mooring observed strong reappeared period shift events occurred by the full moon days, 15 October 2019 and 4 September 2020, further revealing that the frequency doubling component of tidal constituent Msf is a dominant modulator with a high possibility. The results may help the development of local internal solitary wave forecast models.