Intermittent Generation of Internal Solitary-Like Waves on the Northern Shelf of the South China Sea

Predicting the occurrence of internal solitary-like waves (ISWs) is important for parameterizing turbulent dissipation. ISWs in the northern South China Sea (SCS) are well understood to mainly originate from the Luzon Strait (LS). They are generally predictable due to their phase-locked relation with tidal forcing in the LS. However, irregular occurrence of ISWs is noted from our 9-day moored measurements on the SCS shelf. More energetic ISWs unexpectedly appeared during the neap tide than the spring tide. Their occurrence was related to subtidal shelf flows. The tidal and shelf flows had comparable magnitudes. We thus hypothesize that when the total flow meets critical conditions ISWs are generated locally over irregular seafloor. This hypothesis is supported by numerical experiments and validated by observations. Consequently, local generation of ISWs enhances both the intensity and complexity of internal wave field on the shelf, and thus the unpredictability of ISW occurrence.

Automated summary

Predicting the occurrence of internal solitary-like waves (ISWs) is crucial for understanding turbulent dissipation. In the northern South China Sea, ISWs mainly originate from the Luzon Strait and their occurrence is predictable due to their phase-locked relation with tidal forcing. However, our measurements on the SCS shelf show irregular occurrence of ISWs, with more energetic waves appearing during the neap tide instead of the spring tide. This unexpected behavior is related to subtidal shelf flows, and can be explained by the hypothesis that locally generated ISWs occur when the total flow meets critical conditions over irregular seafloor.