*2.1. Data*

Since 1996, long time-series data of zonal and meridional currents were collected using a subsurface mooring, named EC1 (37◦19.13 N, 131◦25.62 E), located between Ulleugdo and Dokdo, at a water depth of 2300 m (Figure 2). The EC1 was recovered and redeployed 24 times (as of December 2021) and equipped for most periods, with current meters at three nominal depths (400, 1400, and 2200 m). Rotary-type current meters (Aanderaa RCMs 7 and 8) and Doppler-type current meters (Aanderaa RCMs 9 and 11; Nortek Aquadopp) were attached to the mooring, and continuous time-series data were recorded with a sampling interval equal to or less than 1 h. An upward-looking acoustic Doppler current profiler (ADCP, 300 kHz) was mounted at 500 m with a depth interval (bin size) of 8 m, instead of a using single-type current meter at 400 m, from March 2011 to July 2012. All EC1 data collected from 1996 to 2020 were upgraded by quality control and quality assurance and were made available by SEANOE [41]. In this study, the time-series data of currents collected at 400 m of EC1 for almost 21 years, from January 2000 to November 2020, were used. Since 1996, long time-series data of zonal and meridional currents were collected using a subsurface mooring, named EC1 (37°19.13 N, 131°25.62 E), located between Ulleugdo and Dokdo, at a water depth of 2300 m (Figure 2). The EC1 was recovered and redeployed 24 times (as of December 2021) and equipped for most periods, with current meters at three nominal depths (400, 1400, and 2200 m). Rotary-type current meters (Aanderaa RCMs 7 and 8) and Doppler-type current meters (Aanderaa RCMs 9 and 11; Nortek Aquadopp) were attached to the mooring, and continuous time-series data were recorded with a sampling interval equal to or less than 1 h. An upward-looking acoustic Doppler current profiler (ADCP, 300 kHz) was mounted at 500 m with a depth interval (bin size) of 8 m, instead of a using single-type current meter at 400 m, from March 2011 to July 2012. All EC1 data collected from 1996 to 2020 were upgraded by quality control and quality assurance and were made available by SEANOE [41]. In this study, the time-series data of currents collected at 400 m of EC1 for almost 21 years, from January 2000 to November 2020, were used.

**Figure 2.** Location of subsurface mooring EC1 (red square) with bathymetry (colour) in the southwestern East Sea, off the east coast of Korea. Green dotted line indicates meridional line where sea surface wind data were extracted. Black dashed lines indicate trajectories of typhoon Maemi in 2003, Megi in 2004, and Maysak in 2020. The yellow circles indicate radii of 30, 100 and 200 km centred at EC1, respectively. **Figure 2.** Location of subsurface mooring EC1 (red square) with bathymetry (colour) in the southwestern East Sea, off the east coast of Korea. Green dotted line indicates meridional line where sea surface wind data were extracted. Black dashed lines indicate trajectories of typhoon Maemi in 2003, Megi in 2004, and Maysak in 2020. The yellow circles indicate radii of 30, 100 and 200 km centred at EC1, respectively.

To supplement the moored time-series data at a horizontally fixed position, the Met Office Hadley Center EN4.2.1. (hereafter referred to as EN4) data that passed the global quality-control processing were used. The EN4 data consist of temperature and salinity obtained from profiling instruments, and the main data source is the World Ocean Database 2009 [42]. The temporal and spatial resolutions of these data are monthly and 1 ◦ , respectively. The spatial resolution of EN4 is coarser than that of the first baroclinic

Rossby radius of deformation of ~O (10 km), representing the horizontal scales at which the stratification can significantly vary within the EN4 grid [43]. Since the surface mixed layer depth (MLD) estimated from EN4 might not accurately represent the MLD for given spatial resolution, it was verified against the MLD estimated by Lim et al. [44], which were based on the World Ocean Database 2005 and multisource hydrographic data. Three vertical profiles of temperature observed using the profiling floats located within 80 km from EC1 were used to estimate the buoyancy frequency (Figure 3c,d). Satellite altimetry-derived daily sea-surface height (SSH; absolute dynamic topography above geoid) of gridded level four data provided by the Copernicus Marine Environment Monitoring Service were used to calculate the surface geostrophic currents, at a spatial resolution of 0.25◦ . The absolute dynamic topography was obtained by adding the mean dynamic topography to the sea level anomaly field and processed to provide the multimission merged altimeter data [45] used in this study. The mean dynamic topography is an estimate of the mean over 1993–2012 of the SSH above the geoid [45]. Hourly sea-surface wind data from January 2000 to November 2020 along a meridional line at 131◦ E (green dotted line in Figure 1), with a horizontal resolution of 30 km, were used to calculate the local surface wind stress, <sup>→</sup> *τ* = *τx*, *τ<sup>y</sup>* , retrieved from the European Centre for Medium-Range Weather Forecasts reanalysis version 5 (ECMWF, ERA5). respectively. The spatial resolution of EN4 is coarser than that of the first baroclinic Rossby radius of deformation of ~O (10 km), representing the horizontal scales at which the stratification can significantly vary within the EN4 grid [43]. Since the surface mixed layer depth (MLD) estimated from EN4 might not accurately represent the MLD for given spatial resolution, it was verified against the MLD estimated by Lim et al. [44], which were based on the World Ocean Database 2005 and multisource hydrographic data. Three vertical profiles of temperature observed using the profiling floats located within 80 km from EC1 were used to estimate the buoyancy frequency (Figure 3c,d). Satellite altimetry-derived daily sea-surface height (SSH; absolute dynamic topography above geoid) of gridded level four data provided by the Copernicus Marine Environment Monitoring Service were used to calculate the surface geostrophic currents, at a spatial resolution of 0.25°. The absolute dynamic topography was obtained by adding the mean dynamic topography to the sea level anomaly field and processed to provide the multimission merged altimeter data [45] used in this study. The mean dynamic topography is an estimate of the mean over 1993–2012 of the SSH above the geoid [45]. Hourly sea-surface wind data from January 2000 to November 2020 along a meridional line at 131° E (green dotted line in Figure 1), with a horizontal resolution of 30 km, were used to calculate the local surface wind stress, ߬⃗ = (߬௫, ߬௬), retrieved from the European Centre for Medium-Range Weather Forecasts reanalysis version 5 (ECMWF, ERA5).

To supplement the moored time-series data at a horizontally fixed position, the Met Office Hadley Center EN4.2.1. (hereafter referred to as EN4) data that passed the global quality-control processing were used. The EN4 data consist of temperature and salinity obtained from profiling instruments, and the main data source is the World Ocean Database 2009 [42]. The temporal and spatial resolutions of these data are monthly and 1°,

*J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 4 of 20

**Figure 3.** (**a**) Monthly mixed layer depth (MLD) estimated from EN4 (black line) data compared to that from Lim et al. [44] (yellow dot), (**b**) vertical structure of mean buoyancy frequencies, estimated from the EN4 data, at the upper 400 m nearby the EC1 mooring averaged over summer (June–July– August, red line) and winter (December–January–February, blue line) where the *Nmax* and MLD are remarked with circles and dashed horizontal lines with corresponding colours. Vertical dashed **Figure 3.** (**a**) Monthly mixed layer depth (MLD) estimated from EN4 (black line) data compared to that from Lim et al. [44] (yellow dot), (**b**) vertical structure of mean buoyancy frequencies, estimated from the EN4 data, at the upper 400 m nearby the EC1 mooring averaged over summer (June–July– August, red line) and winter (December–January–February, blue line) where the *Nmax* and MLD are remarked with circles and dashed horizontal lines with corresponding colours. Vertical dashed line in (**b**) indicates the depth-averaged reference *<sup>N</sup>*<sup>0</sup> (4.6 <sup>×</sup> <sup>10</sup>−<sup>3</sup> rad/s). Vertical profiles of (**c**) temperature, (**d**) buoyancy frequency and (**e**) WKB scaling factor observed using Argo floats located within 80 km from the EC1 (red, blue and orange lines were observed on 20 August 2014, 25 June 2015, and 23 December 2016, respectively.
