Variation in the arrival time of internal solitary waves in the northern South China Sea

Mooring observations conducted at a station in the northern South China Sea (SCS) in 2019 and 2020 revealed two daily packets of internal solitary waves (ISWs) during spring tides, a finding consistent with previous reports. On the basis of arrival time, ISWs in the northern SCS are usually classified into two types: type-a waves that appear at 24-h intervals and type-b waves that appear at approximately 25-h intervals. However, the two daily ISW packets observed in this study exhibited no notable differences in arrival time intervals. Instead, their arrival times mutually transitioned; that is, the arrival time of the two ISW packets interchanged during two consecutive neap-spring cycles, demonstrating that ISW arrival time is modulated by the tide itself rather than by a tidal constituent of specific frequency. In addition to the transition phenomenon, daily deviations were found between the arrival time interval of the ISWs and the background tidal peak intervals. The daily deviations were modulated by the tidal inequality of the neap-spring cycle, whereby stronger tides generate larger ISWs with higher phase speeds. The difference in phase speed resulted in the different arrival time intervals recorded at the observing station. Furthermore, daily deviation magnitudes increased with the distance between the observing station and the ISW generation site. These findings highlight the challenges in classifying field-observed ISWs into type-a or type-b categories.