The Lifecycle of Nonlinear Internal Waves in the Northwestern South China Sea

Abstract

The life cycle of nonlinear internal waves (NIWs) to the southeast of Hainan Island in the northwestern South China Sea is investigated using synergistic satellite observations, in situ measurements, and numerical simulations. A three-dimensional, fully nonlinear and nonhydrostatic model with ultrafine resolution shows that a diurnal internal tide emanates from a sill in the Xisha Islands at approximately 215 km away from the local shelf break. The internal tide transits the deep basin toward the shelf break and reflects at the sea bottom and seasonal thermocline in the form of a wave beam. Arriving at the shelf break, the internal tide undergoes nonlinear transformation and produces an undular bore. Analyses of in situ measurements reveal that the undular bore appears as sharp depressions of the strong near-surface seasonal thermocline. The undular bore gradually evolves into an internal solitary wave train on the midshelf, which was detected by the spaceborne synthetic aperture radar. This finding has great implications for investigating NIWs in other coastal oceans where waves are controlled by remotely generated internal tides.

Generation of Shoreward Nonlinear Internal Waves South of the Hainan Island: Synthetic Aperture Radar Observations and Numerical Simulations

“…The internal tidal beam is formed at the Xisha Islands, which also has been found in Liang et al. (2019). The internal tidal beam scatters into an interfacial internal wave on the continental slope within the moderately stratified pycnocline.…”

“…Li et al (2008) and Z. Xu et al (2010), Su and Pohlmann (2009), Liang et al (2019), D. Li et al (2011 and Yan et al (2020), and J. Xu, Chen, et al (2016), respectively.…”

“…Liang et al. (2019) have recently investigated the lifecycle of NLIWs southeast of the Hainan Island, and their studies revealed that the NLIWs are generated by the internal tides formed at a sill in the Xisha Islands (also known as the Paracel Islands), approximately 215 km away from the continental shelf break. For the westward NLIWs observed around 16°N close to the Vietnam coast, D. Li et al.…”

“…Su and Pohlmann (2009) proposed that the NLIWs east of the Hainan Island are generated by surface winds and buoyancy forcing rather than barotropic tides. Liang et al (2019) have recently investigated the lifecycle of NLIWs southeast of the Hainan Island, and their studies revealed that the NLIWs are generated by the internal tides formed at a sill in the Xisha Islands (also known as the Paracel Islands), approximately 215 km away from the continental shelf break. For the westward NLIWs observed around 16°N close to the Vietnam coast, D. Li et al (2011) proposed that they could be generated by the mixed lee waves over sills around the Zhongsha Islands (also known as the Macclesfield Bank).…”

The inference of internal solitary waves in the northern South China Sea from data acquired by underwater gliders

“…The in situ observation of ISWs contributes to the development of these theories. Major observational methods include 1) remote sensing, which owns the characteristic of large-scale space (Zhao et al, 2004;Jackson, 2007), but with limited capability in sensing the sea surface; 2) moorings, which can obtain high-resolution thermohaline and current structures (Ramp et al, 2004;Alford et al, 2012), whereas conduct observation at a fixed position; 3) research vessels, which can conduct the well-designed and targeted survey in combination with field data (Farmer et al, 2011;Liang In the previous decades, the autonomous platforms (such as float and glider) played a significant role in the observation of some essential ocean variables. As an important member, gliders have been widely used in the oceanographic measurements (Whitt et al, 2020;Rudnick, 2016;Testor et al, 2019).…”