**1. Introduction**

Inertia-gravity waves or internal waves (IWs) are ubiquitous in the stratified, rotating ocean. These waves are present at all frequencies between the local inertial frequency and buoyancy frequency, sometimes including diurnal and semidiurnal tidal frequencies. The two types of low-frequency IWs that originate from atmospheric disturbances (such as sea surface winds) and ocean tides (e.g., interactions between barotropic tides and bottom topography) are near-inertial waves (NIWs) [1] and internal (or baroclinic) tides (ITs) [2]. The energy of low-frequency IWs at larger scales transfers to smaller scales at higher frequencies. Continuum-frequency waves provide an illustration of this principle, where the characteristics of nonlinear internal waves (NLIWs) or internal solitary waves (ISWs) show that they will ultimately break and dissipate. IWs are known to play a key role in the redistribution of heat, momentum, and materials via turbulent mixing, such as diapycnal mixing [3]. It is clear to researchers that IWs may affect local and global climates [4], biogeochemistry and biological productivity [5], marine engineering and submarine navigation [6], and underwater acoustics [7].

Despite decades of study on IWs in other regions, the scientific understanding of IWs in the East Asian marginal seas is lacking detail. There exists a need for greater clarity on the mechanisms that underlie wave generation, propagation, evolution, and dissipation. In this editorial, we introduce eight publications of this Special Issue [8–15]. The wave types and regional waters covered in this review include ISWs/NLIWs in the Bali Sea; ITs, NIWs, ISWs/NLIWs in the South China Sea; ISWs/NLIWs in the East China Sea; and NIWs in the East Sea (also known as the Japan Sea). We discuss and collate the combined findings, with the aim of improving the understanding of the physical mechanisms of IWs (specifically the characteristics that define wave generation, propagation, and/or acoustic impacts).
