**2. Recent Findings in the East Asian Seas**

1

The ISWs/NLIWs are a great threat to submarine navigation as discussed in [8]. In the early morning of 21 April 2021 (local time), the Indonesian Navy Submarine (KRI Nanggala-402) crashed to the seafloor. The authors of [8] analyzed the ISWs/NLIWs in and around the submarine wreck site in the Bali Sea. They surveyed satellite remote sensing data collected from 12 April to 21 April and found that the ISWs/NLIWs had travelled across the deep basin of the Bali Sea [8]. The pathway of the waves passed through the submarine wreck site and then shoaled onto the continental shelf (originating from the Lombok Strait).

Several papers have investigated the three types of IWs found in the South China Sea [9–13]. The study completed by the authors of [9] applied a general ocean circulation model to the ITs in the Luzon Strait. Their work showed the impact of fortnightly stratification variability (as induced by tide–topography interactions) on the generation of ITs. Their contribution has led to a better understanding of the energy transfer between barotropic

**Citation:** Nam, S.; Chen, X. Oceanic Internal Waves and Internal Tides in the East Asian Marginal Seas. *J. Mar. Sci. Eng.* **2022**, *10*, 573. https://doi.org/ 10.3390/jmse10050573

Received: 11 April 2022 Accepted: 21 April 2022 Published: 23 April 2022

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and baroclinic tides and shown a lead–lag relationship between barotropic tidal forcing and maximum baroclinic response within the fortnightly tidal cycle.

Using hybrid coordinate ocean model re-analysis, the authors of [10] reproduced the NIWs induced by Typhoon Megi in 2010 in the South China Sea. Their results revealed that typhoon-induced NIWs could propagate to a depth of 1000 m. They found that the damping and modal content of typhoon-induced NIWs were site-dependent. The results of their modelling showed that the first three baroclinic modes dominated and damped quickly in the region near the typhoon track, while the e-folding time of typhoon-induced NIWs could be longer than 20 days and higher modes were enhanced several days after the typhoon passage.

The three studies published by the authors of [11–13] addressed high-frequency IWs, particularly ISWs/NLIWs at frequencies close to the buoyancy frequency in the South China Sea. The study published by the authors of [11] investigated the distribution and source sites of ISWs/NLIWs northeast of Hainan Island, using satellite remote sensing data and a wavefront propagation model. Their work identified two types of ISWs/NLIWs originating from the Luzon Strait, spaced at both semidiurnal (northern region) and diurnal (southern region) tidal periods. On 22 May 2011, northeast of Dong-Sha Atoll, the authors of [12] observed strong ISWs/NLIWs with unprecedently large velocities (a peak westward velocity of 2.94 m/s and a peak downward velocity of 0.63 m/s), as measured by shipboard velocity observations. They inferred the wave's amplitude (~97 m) from backscatter observations and propagation speed (1.76 m/s estimated theoretically and 1.59 m/s inferred from the satellite remote sensing data). In further work conducted in the South China Sea [13], the authors reported on the substantial influence of ISWs/NLIWs on underwater sound propagation and ambient noise. Their paper described a passive acoustic monitoring and warning method for the strong velocity induced by ISWs/NLIWs, given that the power spectra of noise generated by the waves at frequencies below 100 Hz was almost 20 dB higher than ambient noise.

The authors of [14] developed a method to estimate the propagation speed and direction of ISWs/NLIWs using shipboard underway and moored observations. Their work applied two methods to estimate propagation speed and direction: apparent observations from a moving ship using the Doppler shift method (measuring change in frequency relative to the distance of the waves from the ship), and the time lag method (observing the distance between two locations of the wave at different times). The authors developed an optimal approach that then was applied to two cases of ISWs/NLIWs as observed in the northern regions of the East China Sea in May 2015 and August 2018.

The work published by the authors of [15] utilized a 21-year-long dataset of moored observations in the southwestern region of the East Sea (the Japan Sea), and focused on the non-seasonal (intra-seasonal, interannual, and decadal) variability of NIW kinetic energy far below the surface mixed layer. Their results identified nine periods of relatively high (*period high*), and seven periods of relatively low (*period low*) NIW kinetic energy. The work statistically revealed that the NIW kinetic energies in specific years and decades were significantly higher than those in other years and decades, in association with mesoscale circulation—NIW kinetic energy was enhanced/favored under conditions of negative relative vorticity and strong total strain.
