Fission of Shoaling Internal Waves on the Northeastern Shelf of the South China Sea. Issue 7 (4th July 2019)

• Record Type: Journal Article
• Authors: Bai, Xiaolin; Liu, Zhiyu; Zheng, Quanan; Hu, Jianyu; Lamb, Kevin G.; Cai, Shuqun
• Is Part Of: Journal of geophysical research. Volume 124:Issue 7(2019)
• Page Start: 4529
• Page End: 4545
• Publication Date: 2019-07-04
• Subjects: internal solitary wave -- fission -- South China Sea; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2018JC014437
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Using high‐resolution full‐water column velocity data at a site of 60‐m depth on the northeastern shelf of the South China Sea, we observed high‐frequency internal waves (HIWs) occurring continuously on the continental shelf during the 15‐hr observation on 13‐14 July 2011. This phenomenon is in contrast to the regular occurrence of internal solitary waves (ISWs) that are generally phase‐locked to the tides. We hypothesize that the continuously occurring HIWs are generated from shoaling ISWs via fission. Analysis of the Moderate Resolution Imaging Spectroradiometer (MODIS) true‐color images supports this hypothesis. The mechanism of ISW fission is then investigated using an analytical model as well as a two‐layer weakly nonlinear numerical model based on the regularized long‐wave (RLW) equation. The results are also compared with those from a fully nonlinear continuously stratified numerical model. Simulation results are generally consistent with the field measurements, explaining the observations of the continuous occurrence of HIWs. Sensitivity experiments are conducted to investigate the role and effects of the initial wave amplitude, pycnocline depth and thickness, bathymetry, and viscosity on ISW fission. Although all these factors impact the properties of the shoaling ISWs, both the two‐layer and continuously stratified models suggest that fission mainly occurs near the critical point region, highlighting the importance of pycnocline depth. However, simulations using the two‐layer model suggest that fission occurs after ISWs pass through critical points, whereas the fission location predicted by the continuously stratified model is generally offshore of critical points. This points to the deficiency of the two‐layer model in simulating small‐amplitude short trailing waves, large‐amplitude ISWs, wave breaking, and higher‐mode waves. Key Points: High‐frequency internal waves were continuously recorded from moored observations in July 2011 in the northeastern South China Sea Satellite images indicate that the observed high‐frequency internal wave trains were generated from fission of internal solitary waves The ISW fission characteristics are investigated using an analytical model, a two‐layer weakly nonlinear numerical model, and a continuously stratified fully nonlinear numerical model