**5. Concluding Remarks**

Nonseasonal (intraseasonal, interannual, and decadal) variations in NIW kinetic energy, *KENIW\_obs* at 400 m (VARNIW\_obs\_int) were presented from long time-series (from 2000 to 2020) moored observations in the southwestern East Sea. In total, nine periods of high (period high) and seven periods of low (period low) intraseasonal variance of *KENIW\_obs*, or VARNIW\_obs\_int at 400 m, were identified and analysed statistically, providing composite means for different mesoscale conditions, and suggesting a significant effect of mesoscale flow fields on VARNIW\_obs\_int. Although a high rate of wind work, sometimes associated with typhoon passage, may significantly enhance the near-inertial kinetic energy (*KENIW\_model* within or just below the surface mixed layer as simulated by the damped slab model), in our study, the intraseasonal variance of *KENIW\_model*, or VARNIW\_model\_int, hardly accounted for VARNIW\_obs\_int at 400 m depth, which was dissipated mostly in the upper layer. Instead, the condition parameters of relative vorticity (*ζ*) and the total strain (*S* 2 ), representing mesoscale flow fields, better explained VARNIW\_obs\_int at 400 m, yielding a significantly higher *KENIW\_obs* at 400 m when *ζ* < 0 and/or *S* <sup>2</sup> > *ζ* 2 , and vice versa (Figure 11). Our results, based on 21-year-long observations, statistically support previous (mostly theoretical) suggestions that NIW kinetic energy may have been enhanced through nonlinear interactions with the mesoscale flow field, when the strain exceeded the vorticity, and was trapped when the effective Coriolis frequency was lowered due to a negative relative vorticity. We believe that future process-oriented studies focusing on specific NIW events will provide a more comprehensive understanding of the interaction between the mesoscale flow fields and NIWs, testing the statistically derived proposition obtained in this study from rare, long-term, high-resolution, and continuous observations. *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 17 of 20

egory I (ζ > 0 and ܵ

in the right bottom of (**d**).

ζ ଶ <sup>ଶ</sup> > ζ ଶ

), and (**d**) Category IV (ζ < 0 and ܵ

**Figure 11.** Schematics of behaviour of near-inertial internal waves (NIW, red arrow) under

pycnal layers. Vertical scales of MLD and 400 m reference level are shown with thick grey arrows

**Author Contributions:** Conceptualisation, S.N. (Suyun Noh) and S.N. (SungHyun Nam); methodology, S.N. (Suyun Noh) and S.N. (SungHyun Nam); validation, S.N. (Suyun Noh) and S.N. (SungHyun Nam); formal analysis, S.N. (Suyun Noh); investigation, S.N. (Suyun Noh) and S.N. (SungHyun Nam); data curation, S.N. (Suyun Noh); writing—original draft preparation, S.N. (Suyun Noh); writing—review and editing, S.N. (Suyun Noh) and S.N. (SungHyun Nam); visualisation, S.N. (Suyun Noh); supervision, S.N. (SungHyun Nam); project administration, S.N. (SungHyun Nam). All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the Ministry of Oceans and Fisheries, Republic of Korea through the Deep Water Circulation and Material Cycling in the East Sea (20160040). This work was also partially funded by the Civil Military Technology Cooperation Program (18-SN-RB-01) from

**Data Availability Statement:** All EC1 data are available at SEANOE (https://www.seanoe.org/data/00677/78916/, accessed on 22 December 2021) and OceanS-ITES (http://www.oceansites.org, accessed on 22 December 2021). Satellite altimetry-derived daily sea-surface height of gridded level 4 data provided by the Copernicus Marine Environment Monitoring Service are available at https://marine.copernicus.eu/ (accessed on 22 December 2021). The sea surface wind data, ERA5, were provided by the European

<sup>ଶ</sup> < ζ ଶ ଶ ; (**a**) Cat-

<sup>ଶ</sup> >

), (**c**) Category III (ζ < 0 and ܵ

). Different blue shading colours represent different iso-

mesoscale flow fields (streamlines, grey arrows) yielding different conditions of ζ and ߙ

), (**b**) Category II (ζ > 0 and ܵ

<sup>ଶ</sup> < ζ ଶ

the Institute of Civil Military Technology Cooperation, Republic of Korea.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

ζ ଶ

**Figure 11.** Schematics of behaviour of near-inertial internal waves (NIW, red arrow) under mesoscale flow fields (streamlines, grey arrows) yielding different conditions of ζ and ߙ ଶ ; (**a**) Category I (ζ > 0 and ܵ <sup>ଶ</sup> > ζ ଶ ), (**b**) Category II (ζ > 0 and ܵ <sup>ଶ</sup> < ζ ଶ ), (**c**) Category III (ζ < 0 and ܵ <sup>ଶ</sup> > ), and (**d**) Category IV (ζ < 0 and ܵ <sup>ଶ</sup> < ζ ଶ ). Different blue shading colours represent different isopycnal layers. Vertical scales of MLD and 400 m reference level are shown with thick grey arrows in the right bottom of (**d**). **Figure 11.** Schematics of behaviour of near-inertial internal waves (NIW, red arrow) under mesoscale flow fields (streamlines, grey arrows) yielding different conditions of *ζ* and *α* 2 ; (**a**) Category I (*ζ* > 0 and *S* <sup>2</sup> > *ζ* 2 ), (**b**) Category II (*ζ* > 0 and *S* <sup>2</sup> < *ζ* 2 ), (**c**) Category III (*ζ* < 0 and *S* <sup>2</sup> > *ζ* 2 ), and (**d**) Category IV (*ζ* < 0 and *S* <sup>2</sup> < *ζ* 2 ). Different blue shading colours represent different isopycnal layers. Vertical scales of MLD and 400 m reference level are shown with thick grey arrows in the right bottom of (**d**).

**Author Contributions:** Conceptualisation, S.N. (Suyun Noh) and S.N. (SungHyun Nam); method-

ology, S.N. (Suyun Noh) and S.N. (SungHyun Nam); validation, S.N. (Suyun Noh) and S.N. (SungHyun Nam); formal analysis, S.N. (Suyun Noh); investigation, S.N. (Suyun Noh) and S.N. (SungHyun Nam); data curation, S.N. (Suyun Noh); writing—original draft preparation, S.N. (Suyun Noh); writing—review and editing, S.N. (Suyun Noh) and S.N. (SungHyun Nam); visualisation, S.N. (Suyun Noh); supervision, S.N. (SungHyun Nam); project administration, S.N. (SungHyun Nam). All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the Ministry of Oceans and Fisheries, Republic of Korea **Author Contributions:** Conceptualisation, S.N. (Suyun Noh) and S.N. (SungHyun Nam); methodology, S.N. (Suyun Noh) and S.N. (SungHyun Nam); validation, S.N. (Suyun Noh) and S.N. (SungHyun Nam); formal analysis, S.N. (Suyun Noh); investigation, S.N. (Suyun Noh) and S.N. (SungHyun Nam); data curation, S.N. (Suyun Noh); writing—original draft preparation, S.N. (Suyun Noh); writing review and editing, S.N. (Suyun Noh) and S.N. (SungHyun Nam); visualisation, S.N. (Suyun Noh); supervision, S.N. (SungHyun Nam); project administration, S.N. (SungHyun Nam). All authors have read and agreed to the published version of the manuscript.

through the Deep Water Circulation and Material Cycling in the East Sea (20160040). This work was also partially funded by the Civil Military Technology Cooperation Program (18-SN-RB-01) from the Institute of Civil Military Technology Cooperation, Republic of Korea. **Institutional Review Board Statement:** Not applicable. **Funding:** This research was funded by the Ministry of Oceans and Fisheries, Republic of Korea through the Deep Water Circulation and Material Cycling in the East Sea (20160040). This work was also partially funded by the Civil Military Technology Cooperation Program (18-SN-RB-01) from the Institute of Civil Military Technology Cooperation, Republic of Korea.

**Informed Consent Statement:** Not applicable. **Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** All EC1 data are available at SEANOE **Informed Consent Statement:** Not applicable.

(https://www.seanoe.org/data/00677/78916/, accessed on 22 December 2021) and OceanS-ITES (http://www.oceansites.org, accessed on 22 December 2021). Satellite altimetry-derived daily sea-surface height of gridded level 4 data provided by the Copernicus Marine Environment Monitoring Service are available at https://marine.copernicus.eu/ (accessed on 22 December 2021). The sea surface wind data, ERA5, were provided by the European **Data Availability Statement:** All EC1 data are available at SEANOE (https://www.seanoe.org/ data/00677/78916/, accessed on 12 November 2021) and OceanSITES (http://www.oceansites.org, accessed on 12 November 2021). Satellite altimetry-derived daily sea-surface height of gridded level 4 data provided by the Copernicus Marine Environment Monitoring Service are available at https://marine.copernicus.eu/ (accessed on 12 November 2021). The sea surface wind data, ERA5, were provided by the European Center for Medium-Range Weather Forecasts (https://www.ecmwf. int/en/forecasts/datasets/reanalysis-datasets/era5, accessed on 12 November 2021). The EN4 dataset was collected and provided by the Met Office Hadley Centre (https://www.metoffice.gov. uk/hadobs/en4/, accessed on 12 November 2021).

**Acknowledgments:** We would like to thank two reviewers for constructive comments, and Jae-Hun Park and Sehan Lim who generously provided the hydrographic data required for our study, along with their thoughtful insights on the preliminary results.

**Conflicts of Interest:** The authors declare no conflict of interest.
