Dynamic and Statistical Features of Internal Solitary Waves on the Continental Slope in the Northern South China Sea Derived From Mooring Observations

Abstract

For investigation of internal solitary waves (ISWs) in the South China Sea (SCS), most cruise observations are concentrated from Luzon Strait to Dongsha Atoll in the northeastern SCS but few on the continental slope far away from the west of Dongsha Atoll. In this study, we use 1‐year long mooring data to determine dynamic and statistical features of the ISWs on the shelf slope of the northwestern SCS. The analysis results of the mooring data reveal that the ocean internal waves on the shelf slope of the northwestern SCS have physical properties of highly nonlinear waves, which are well described by the solutions of the Korteweg‐de Vries equation. The mean nonlinear phase speeds of mode‐1 and mode‐2 ISWs are 1.38 ± 0.14 and 0.66 ± 0.12 m/s, respectively. The major direction of mode‐1 ISWs is northwestward 305° ± 21°. Strong ISW currents force the major direction of total current velocities to turn 67.5° in the upper layer and 135° in the lower layer. The monthly occurrence frequency distribution of ISWs shows a peak in July with a maximum frequency of 16.2% and a trough in March with a minimum frequency of 3.3%. Mode‐2 ISWs appear most in December, accounting for 50% of total mode‐2 ISWs. The largest mode‐2 ISW on record up today with the depressed amplitude as large as 91 m, and the elevated amplitude of 73 m was observed at mooring station. These new findings and new data are of significance to local internal wave prediction model development.

Observations of Internal Structure Changes in Shoaling Internal Solitary Waves Based on Seismic Oceanography Method

“…According to the study of the vertical structure of the internal solitary wave (Geng et al, 2019), the maximum amplitude of the internal solitary wave is 70 m, which is located at a water depth of 100 m. As the water depth increases, the amplitude gradually decreases. According to Chen et al (2019), the maximum amplitude of the mode-one internal solitary wave found near the Dongsha Atoll is 87 m, which is similar to the amplitude of the internal solitary wave. In addition, Ramp et al (2004) observed many internal solitary waves in the east of Dongsha islands and they showed that the amplitude of the internal solitary waves ranged from 29 to 140 m. We conclude that seismic reflection data are a useful and accurate tool to observe the internal solitary wave amplitudes.…”

A Directional Decomposition Method to Estimate the Reflection and Transmission of Nonlinear Internal Waves Over a Slope

“…In comparison with single soliton, NLIW packets are more common in field observations (e.g., Stanton & Ostrovsky, 1998) and/or satellite images (e.g., Brandt et al, 1996;Zheng et al, 2007). Apel (2003) and Chen et al (2019) suggested that a NLIW packet solution can be expressed by the dnoidal Jacobian elliptic functions. Therefore, the analytical solution of two NLIW packets, propagating in opposite directions (see Figure 1a), is written as…”

“…where 𝐴𝐴 𝐴𝐴𝑖𝑖 = 2 ∕𝐿𝐿 𝑖𝑖 is the wavenumber; 𝐴𝐴 𝐴𝐴𝑖𝑖 is the characteristic half-width of the leading wave in a NLIW packet; 𝐴𝐴 𝐴𝐴 2 is the nonlinear parameter, which is ranging from 0 to 1 (see Chen et al, 2019). 𝐴𝐴 𝐴𝐴𝑚𝑚 is a compensated quantity for amplitude.…”

Variation of Internal Solitary Wave Propagation Induced by the Typical Oceanic Circulation Patterns in the Northern South China Sea Deep Basin

“…The South China Sea (SCS), the largest semienclosed marginal sea in the western tropical Pacific Ocean, records the largest and fastest internal solitary waves (ISWs) in the global ocean (Huang et al., 2016; Klymak et al., 2006; Lien et al., 2014). A variety of in situ and satellite observations (Alford et al., 2010; Chen et al., 2018; 2019; Jackson, 2009; X. Li et al., 2013; Ramp et al., 2010; Wang et al., 2012) and theoretical and numerical models (Lai et al., 2019; Q. Li & Farmer, 2011; A. K. Liu et al., 2004; Xie et al., 2019; Zhao, 2014; 2020; Zhang et al., 2011; Zhao & Alford, 2006; Zheng et al., 2007) have been directed toward understanding and predicting the ISW's characteristics in the northern SCS over the last two decades (Alford et al., 2015; Guo & Chen, 2014). Although the background stratification and current have been found to play important roles in modulating these waves' characteristics and behavior (DeCarlo et al., 2015; Q. Li et al., 2016; Park & Farmer, 2013; Xie et al., 2015, 2016), how the background large‐scale mean motions, especially the typical wind‐driven oceanic circulations (Caruso et al., 2006; Su, 2004), affect them is still not well understood.…”