*4.1. Comparison to Known Characteristics of Near-Inertial Internal Waves (NIWs)*

We compared the results of nonseasonal variations of the observed and modelled NIW kinetic energy (VARNIW\_obs\_int) with previously known characteristics of NIWs. In general, period high often corresponded to the condition of *ζ* < 0 (Category III or IV), supporting that NIWs are trapped in a region of *ζ* < 0 because of the lowered *fe f f* due to mesoscale circulations, as suggested in previous studies [37,38]. The interannual variations in the region of *ζ* < 0 imposed by the meandering of the subpolar front and the activities of anticyclonic eddies off the east coast of Korea may be responsible for the interannual variations of NIW kinetic energy presented in [38] and the horizontal and vertical distributions of NIW kinetic energy shown in [37]. The NIWs presented in [39,59] under the condition of *ζ* > 0 corresponded to Categories I or II in our study. Notably, significant impacts of *S* <sup>2</sup> on the NIW kinetic energy in the region have not been reported previously, except for a study conducted by Noh and Nam [40], which suggested a strong case of NIWs extracting their energy from a mesoscale flow field under the favourable condition of *S* <sup>2</sup> > *ζ* 2 (strain exceeds vorticity, *α* <sup>2</sup> > 0) via the wave capture process [32,33]. The case found during events 2–4 in the study conducted by Noh and Nam [40], which supported the exponentially growing wavenumber, along with an increasing *S* <sup>2</sup> and a resulting small group velocity, corresponded to H2 in our study; this was generalised from statistically significant differences in the composite mean variance of NIW kinetic energy, for example, more period high events for *S* <sup>2</sup> > *ζ* 2 .

Wind-induced NIWs are known to propagate downward below the MLD [37,60,61]. However, in spite of large Π values, only VARNIW\_model\_int (and not VARNIW\_obs\_int) showed an occasional enhancement, such as that observed on 19 August 2004 and 3 September 2020 (green triangles in Figure 8d), when the typhoon passed the region. This means that the nonseasonal variations in VARNIW\_obs\_int could not be well-explained by the changes in the surface wind forcing alone, additionally requiring the consideration of mesoscale field conditions. Another circumstantial energy source for NIWs in the deep layer in the region suggested by Mori et al. [36] is topographic roughness, because barotropic currents flowing over the rough bottom could generate deep near-inertial oscillations. Indeed, in this case, the current-topography interaction will also be affected by the mesoscale circulation, because the mesoscale eddies in the region are quasi-barotropic, as discussed in [39], which accounts for the higher NIW kinetic energy observed inside (where *ζ* < 0) the anticyclonic circulation in the region.
