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. Plain Language Summary Internal solitary‐like waves (ISWs) are ubiquitous in the ocean. They can propagate several hundreds of kilometers, and induce strong turbulence and mixing. Revealing generation mechanisms and propagation of ISWs is key to further investigating their influence on the distributions of energy dissipation, heat, nutrients, sediment, and pollutants, as well as the safety of submarine voyages and offshore drilling. In this paper, ISWs in the northern South China Sea (SCS), where the most energetic ISWs in the world's oceans are generated, are studied. It is widely accepted that these ISWs generally originate from the Luzon Strait (LS). They usually have regular occurrence, and are phase‐locked to tidal forcing in the LS. However, we present field measurements showing irregular occurrence of ISWs on the northern shelf of the SCS. This irregular occurrence is in striking contrast to the prominent predictability of ISWs originated from the LS. We reveal that the intermittent nature of the occurrence is due to the local generation of ISWs on the shelf; the coupling of different flow components seems to play a significant role in ISW generation on the shelf. The results reported here are expected to be applicable to other shelf regions of the world's oceans. Key Points Field measurements documented large numbers of internal solitary‐like waves (ISWs) with irregular occurrence on the northeastern shelf of the South China Sea More energetic and larger numbers of ISWs ar