*Article* **Response of Spatial and Temporal Variations in the Kuroshio Current to Water Column Structure in the Western Part of the East Sea**

**Chung-Il Lee <sup>1</sup> , Yong-Woo Jung <sup>2</sup> and Hae-Kun Jung 3,\***


**Abstract:** Using geographic sea surface current data, long-term changes in spatial and temporal variations in the Kuroshio Current 1993–2021 were analyzed, and the relationship between the Kuroshio Current and oceanic conditions, such as water column structure and intensity of East Korea Warm Current (EKWC) in the western part of the East Sea (WES), was investigated. Longterm changes in the Kuroshio Current intensity were positively correlated with the Pacific Decadal Oscillation and East Asian Winter Monsoon indices. When the Kuroshio Current was strong, its main axis passing around the Ryukyu Islands moved eastward, and the intensity of EKWC separated from the Kuroshio Current and flowed into the WES, indicating weakened conditions. When the intensity of the EKWC was weakened, its main axis moved away from the inshore area of the WES. As a result, the vertical distribution range of the cold and low saline water mass located in the bottom layer extended to shallower depths in the inshore area of the WES with increasing chlorophyll-*a*.

**Keywords:** western part of the East Sea; Kuroshio Current; East Korea Warm Current; Pacific decadal oscillation

### **1. Introduction**

The East Sea is a continental sea in the Northwest Pacific that connects to open oceans such as the North Pacific and East China Seas (ECS). Hence, changes in oceanic conditions in the East Sea are influenced by both the direct and teleconnection effects of atmospheric and oceanic circulation in the North Pacific [1,2]. The Kuroshio Current, which originates in the equatorial current system, plays a key role in transmitting heat energy from the equatorial region into the East Sea, and its intensity affects oceanic and biological conditions in the East Sea [1,3–6]. The Tsushima Warm Current, which passes into the western channel of the Korea Strait separated from the Kuroshio Current, is divided into two branches, one of which is the East Korea Warm Current (EKWC), which flows into the western part of the East Sea (WES) [7,8]. Thus, the EKWC originating from the Kuroshio Current is a key factor for the variations in the upper layer and deep-water circulation in the East Sea [8–10]. However, the composition of the water mass comprising the TWS and EKWC indicated distinct seasonal patterns.

The Tsushima Warm Current that flows into the WES primarily comprises two types of water masses: one that originates in the Taiwan Current and another a branch of the Kuroshio Current [11–14]. In the summer, the composition of the warm water mass moving through the Korea Strait has a similar ratio between the Taiwan Current and Kuroshio Current. In the winter, the composition of the Kuroshio Current exceeds 80% of the total ratio [15]. Therefore, the Kuroshio Current accounts for a large proportion of the warm water mass flowing into the WES via the Korea Strait during winter, and changes in the

**Citation:** Lee, C.-I.; Jung, Y.-W.; Jung, H.-K. Response of Spatial and Temporal Variations in the Kuroshio Current to Water Column Structure in the Western Part of the East Sea. *J. Mar. Sci. Eng.* **2022**, *10*, 1703. https:// doi.org/10.3390/jmse10111703

Academic Editor: Rafael J. Bergillos

Received: 10 October 2022 Accepted: 8 November 2022 Published: 9 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

intensity of the Kuroshio Current may be critical for controlling the volume transport of warm water masses flowing into the WES [1,2,7,8,10].

Variations in EKWC volume transport are an essential factor for the changes in oceanic conditions in the WES, such as water temperature and salinity [1,16], and the intensity of the EKWC fluctuated similar to that of the Kuroshio Current [3]. The changes in oceanic conditions in the WES caused by the changes in the intensity of the Kuroshio Current were also closely associated with biological conditions, such as the spatial and temporal distribution of fishery resources in the WES [4,17]. During the strong EKWC period, the distribution range of warm water species, such as common squid, and their biomass expanded in the WES [3]. In addition, the EKWC and Tsushima Warm Current, separated from the Kuroshio Current, contain the nutrients transported by the Changjiang River entering the ECS. The intensity of the Kuroshio Current contributes to the variations in surface nutrients in the WES [18]. However, variations in oceanic conditions associated with the current system are simultaneously affected by current intensity and spatial changes [19,20]. The intensity and spatial changes in the Kuroshio Current influence the changes in the water column structure and chlorophyll-*a* (chl-*a*) in southern Japan and the Kuroshio Extension region in the eastern Pacific [21,22]. Previous studies have primarily focused on the intensity of the Kuroshio Current to explain oceanographic and biological conditions in the WES [1], and volume transport in the Korea Strait and Kuroshio Current has similar fluctuation patterns [3]. Therefore, the intensity of the Kuroshio Current influences the changes in oceanic conditions, such as the intensity of the Tsushima Warm Current and EKWC. However, spatial and temporal variations in the location of the main stream of currents can be one of the fundamental drivers of the changes in physical and biological environments [23,24]. Spatial changes in the main axis of the Kuroshio Current are also major factors influencing the changes in warm-water mass inflow in the WES, such as the Tsushima Warm Current and EKWC [1–3]. In addition, to understand the relationship between the Kuroshio Current and oceanic conditions in the WES, we must focus on both changes in the upper layer and water column structure in the WES.

The aim of this study was to elucidate the response mechanism of oceanic conditions in the WES to the variations in the intensity and spatial distribution of the Kuroshio Current. In particular, we addressed the following questions: when does the intensity of the Kuroshio Current strengthen, and does the volume transport of EKWC in the WES increase? In addition, we assessed the effect of change in the intensity of the Kuroshio Current on water column structure and chl-*a* in the WES, and its relationship with atmospheric and oceanic circulation in the North Pacific. To this end, we selected oceanographic data from eight fixed stations including the inshore and offshore areas near 37◦ N in the WES that passed the main axis of the EKWC, and then focused on the winter season with the highest proportion of the Kuroshio Current in the EKWC. We analyzed (1) long-term changes in the volume transport of warm water mass passing into the WES associated with the variations in the intensity and spatial distribution of the Kuroshio Current, (2) the effect of changes in the Kuroshio Current on changes in vertical structure and chl-*a* in the WES, and (3) the mechanisms underlying the response of oceanic conditions in the WES to the changes in the Kuroshio Current, focusing on the oceanic circulation system in the North Pacific.

### **2. Data and Methods**

### *2.1. Oceanic Conditions in the WES*

Oceanographic data were collected using conductivity, temperature, and depth probes provided by the National Institute of Fisheries Science in Korea. We used water temperature and salinity data in February between 1993–2021 from eight fixed stations near 37◦ N that passed the main axis of the EKWC (Figure 1). To analyze the long-term changes in oceanic conditions associated with the intensity and spatial variations in the EKWC, salinity in the upper 100 m layer was used, and the intensity of stratification (stratification index) was calculated using the water temperature difference between the upper 10 m and 100 m layers (Figure 1).

10 m and 100 m layers (Figure 1).

**Figure 1.** Study area for oceanic conditions such as water temperature and salinity in the western part of the East Sea and volume transport in the western channel of the Korea Strait. **Figure 1.** Study area for oceanic conditions such as water temperature and salinity in the western part of the East Sea and volume transport in the western channel of the Korea Strait.

Oceanographic data were collected using conductivity, temperature, and depth probes provided by the National Institute of Fisheries Science in Korea. We used water temperature and salinity data in February between 1993–2021 from eight fixed stations near 37° N that passed the main axis of the EKWC (Figure 1). To analyze the long-term changes in oceanic conditions associated with the intensity and spatial variations in the EKWC, salinity in the upper 100 m layer was used, and the intensity of stratification (stratification index) was calculated using the water temperature difference between the upper

#### *2.2. Kuroshio Current Intensity and Longitudinal Shift of the Main Axis 2.2. Kuroshio Current Intensity and Longitudinal Shift of the Main Axis*

The intensity of the Kuroshio Current and its longitudinal shift (west–east) were analyzed using geostrophic sea surface current data obtained from the Archiving, Validation, and Interpretation of Satellite Oceanographic database archived monthly 0.25° grids from 1993 to 2021. The Kuroshio Current intensity (KCI) was estimated using the average geostrophic sea surface current around 28° N and 125–129° E (Figure 2). Considering the time lag effect of change in the Kuroshio Current in the ECS on oceanic conditions in the WES [25–27], sea surface current data in January were used. The longitudinal shift in the main axis of the Kuroshio Current in January was calculated as the longitudinal position of the strongest current velocity around 125–129° E at the same latitude (28° N) (Figure 2). The intensity of the Kuroshio Current and its longitudinal shift (west–east) were analyzed using geostrophic sea surface current data obtained from the Archiving, Validation, and Interpretation of Satellite Oceanographic database archived monthly 0.25◦ grids from 1993 to 2021. The Kuroshio Current intensity (KCI) was estimated using the average geostrophic sea surface current around 28◦ N and 125–129◦ E (Figure 2). Considering the time lag effect of change in the Kuroshio Current in the ECS on oceanic conditions in the WES [25–27], sea surface current data in January were used. The longitudinal shift in the main axis of the Kuroshio Current in January was calculated as the longitudinal position of the strongest current velocity around 125–129◦ E at the same latitude (28◦ N) (Figure 2). *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 4 of 13

**Figure 2.** Mean geostrophic sea surface current velocity (m/s) from satellite in January during 1993– 2021. The white bar indicates the area where the intensity and longitudinal shift of Kuroshio Current was studied. **Figure 2.** Mean geostrophic sea surface current velocity (m/s) from satellite in January during 1993–2021. The white bar indicates the area where the intensity and longitudinal shift of Kuroshio Current was studied.

The intensity of EKWC was defined as the volume transport of the western channel

), Δp is the pressure difference between each tidal station (hPa), and Δx is the dis-

To analyze the long-term variations in chl-*a* in the WES (35–40° N, 127–132° E) during February, the merged satellite data from the Copernicus Marine Environmental Monitoring Service (CMEMS) was used [29]. CMEMS GlobColour-merged chl-*a* product relies on the following sensors: SeaWiFS (1997–2010), MERIS (2002–2012), MODIS Aqua (2002–present), VIIRS-NPP (2012–present) and OLCI-S3A (2016–present) [29]. The monthly Level-4 product chl-*a* for the global ocean from 1998 to 2021 was obtained from the Copernicus website (https://marine.copernicus.eu/access-data (accessed on 31 August 2022)). Level 4 data were re-mapped to a standard projection with a spatial resolution of 4 km [29].

The Pacific Decadal Oscillation Index (PDOI) and East Asian Winter Monsoon Index (EAWMI) were used to elucidate the relationship between the intensity of the Kuroshio Current and the atmospheric and oceanic circulation systems in the North Pacific. The EAWMI is defined as the difference between the two regions (27.5°–37.5° N, 110°–170° E,

50°–60° N, 80°–140° E) in area-averaged zonal wind speed at 300 hectopascal [30].

V = 1/f/ρ\*Δp/Δx (1)

/s), f is the Coriolis force, ρ is the density of seawater

Izuhara, Japan, from the Japan Meteorological Agency (JMA) and Busan, South Korea

from the Korea Hydrographic and Oceanographic Agency (KHOA).

*2.3. Intensity of the East Korea Warm Current*

where V is the volume transport (hm<sup>3</sup>

tance between each tidal station (51.17 km) [1,28].

(kg/m<sup>3</sup>

*2.4. Chlorophyll-a*

*2.5. Climate Indices*

### *2.3. Intensity of the East Korea Warm Current*

The intensity of EKWC was defined as the volume transport of the western channel in the Korea Strait. The volume transport of the western channel in the Korea Strait during winter (January–March) from 1993 to 2019 was calculated using sea level data from Izuhara, Japan, from the Japan Meteorological Agency (JMA) and Busan, South Korea from the Korea Hydrographic and Oceanographic Agency (KHOA).

$$\mathbf{V} = \mathbf{1}/\mathbf{f}/\mathfrak{p}^\* \Delta \mathbf{p}/\Delta \mathbf{x} \tag{1}$$

where V is the volume transport (hm3/s), f is the Coriolis force, ρ is the density of seawater (kg/m<sup>3</sup> ), ∆p is the pressure difference between each tidal station (hPa), and ∆x is the distance between each tidal station (51.17 km) [1,28].

### *2.4. Chlorophyll-a*

To analyze the long-term variations in chl-*a* in the WES (35–40◦ N, 127–132◦ E) during February, the merged satellite data from the Copernicus Marine Environmental Monitoring Service (CMEMS) was used [29]. CMEMS GlobColour-merged chl-*a* product relies on the following sensors: SeaWiFS (1997–2010), MERIS (2002–2012), MODIS Aqua (2002–present), VIIRS-NPP (2012–present) and OLCI-S3A (2016–present) [29]. The monthly Level-4 product chl-*a* for the global ocean from 1998 to 2021 was obtained from the Copernicus website (https://marine.copernicus.eu/access-data (accessed on 31 August 2022)). Level 4 data were re-mapped to a standard projection with a spatial resolution of 4 km [29].

### *2.5. Climate Indices*

The Pacific Decadal Oscillation Index (PDOI) and East Asian Winter Monsoon Index (EAWMI) were used to elucidate the relationship between the intensity of the Kuroshio Current and the atmospheric and oceanic circulation systems in the North Pacific. The EAWMI is defined as the difference between the two regions (27.5–37.5◦ N, 110–170◦ E, 50–60◦ N, 80–140◦ E) in area-averaged zonal wind speed at 300 hectopascal [30].

$$\text{EAWMI} = \text{l} \text{l}\_{300} \text{ (27.5-37.5}^{\circ} \text{N, 110-170}^{\circ} \text{E}) - \text{l} \text{l}\_{300} \text{ (50-60}^{\circ} \text{N, 80-140}^{\circ} \text{E})$$

The PDOI is defined as the leading empirical orthogonal function of monthly sea surface temperature anomalies in the Pacific poleward of 20◦ N [31].

### *2.6. Statistical Analysis*

The sequential *t*-test analysis of regime shifts (STARS) developed by Rodionov (2006) was applied to determine the time scale of the regime and magnitudes of the intensity of Kuroshio Current and volume transport in the western channel of the Korea Strait. Sequential *t*-test analysis of regime shifts was designed for sequential data processing and can identify statistically significant departures from the mean.

### **3. Results**

### *3.1. Long-Term Changes in the Kuroshio Current and Their Relationship with Climate Effects*

The KCI showed a positive anomaly (strong velocity) before 2010 and a rapidly decreasing current velocity pattern after 2010 (Figure 3). The STAR analysis results also showed that step change in the KCI was detected in 2010 (Figure 3). These fluctuations in the KCI were related to the longitudinal shift in the main axis of the Kuroshio Current (r = 0.55, *p* < 0.01). The analysis of the correlation coefficient between the longitudinal shift in the main axis of the Kuroshio Current and surface current velocity around the ECS revealed that the surface current velocity around the Ryukyu Islands, which are located in the main stream of the Kuroshio Current, has a positive relationship with the longitudinal shift in the main axis of the Kuroshio Current (Figure 4). In contrast, the western part of the Kuroshio Current mainstream had a negative relationship with the longitudinal shift in the main axis of the Kuroshio Current (Figure 4), implying that when the Kuroshio velocity was high, the main axis of the Kuroshio Current migrated eastward. However, when the Kuroshio velocity was low, it moved westward. both climate indices had a positive relationship with surface current velocity around the Ryukyu Islands (Figure 5). In particular, a highly significant coefficient of variation was observed with a two-month lag phase than with a phase with no lag (Figure 5).

Long-term changes in KCI were related to atmospheric and oceanic conditions in the North Pacific. The EAWMI and PDOI are key climate indices for the intensity of the Northwest Pacific winter monsoon and fluctuations in sea surface temperature in the North Pacific associated with atmospheric and oceanic circulations. The correlation coefficients between the surface current velocity around the ECS and EAWMI and PDOI showed that

*J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 5 of 13

*2.6. Statistical Analysis*

**3. Results**

surface temperature anomalies in the Pacific poleward of 20° N [31].

can identify statistically significant departures from the mean.

ever, when the Kuroshio velocity was low, it moved westward.

EAWMI = *U*<sup>300</sup> (27.5°–37.5° N, 110°–170° E) − *U*<sup>300</sup> (50°–60° N, 80°–140° E)

The PDOI is defined as the leading empirical orthogonal function of monthly sea

The sequential *t*-test analysis of regime shifts (STARS) developed by Rodionov (2006) was applied to determine the time scale of the regime and magnitudes of the intensity of Kuroshio Current and volume transport in the western channel of the Korea Strait. Sequential *t*-test analysis of regime shifts was designed for sequential data processing and

*3.1. Long-Term Changes in the Kuroshio Current and Their Relationship with Climate Effects*

The KCI showed a positive anomaly (strong velocity) before 2010 and a rapidly decreasing current velocity pattern after 2010 (Figure 3). The STAR analysis results also showed that step change in the KCI was detected in 2010 (Figure 3). These fluctuations in the KCI were related to the longitudinal shift in the main axis of the Kuroshio Current (r = 0.55, *p* < 0.01). The analysis of the correlation coefficient between the longitudinal shift in the main axis of the Kuroshio Current and surface current velocity around the ECS revealed that the surface current velocity around the Ryukyu Islands, which are located in the main stream of the Kuroshio Current, has a positive relationship with the longitudinal shift in the main axis of the Kuroshio Current (Figure 4). In contrast, the western part of the Kuroshio Current mainstream had a negative relationship with the longitudinal shift in the main axis of the Kuroshio Current (Figure 4), implying that when the Kuroshio velocity was high, the main axis of the Kuroshio Current migrated eastward. How-

**Figure 3.** Long-term variations in the Kuroshio Current intensity in January. Black line represents step changes estimated using sequential *t*-test analysis of regime shifts analysis. **Figure 3.** Long-term variations in the Kuroshio Current intensity in January. Black line represents step changes estimated using sequential *t*-test analysis of regime shifts analysis. *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 6 of 13

**Figure 4.** Horizontal distribution of the correlation coefficient between the geostrophic surface current velocity in January and the longitudinal shift of the Kuroshio Current Index in January. Black lines represent significant levels: *p* < 0.01. **Figure 4.** Horizontal distribution of the correlation coefficient between the geostrophic surface current velocity in January and the longitudinal shift of the Kuroshio Current Index in January. Black lines represent significant levels: *p* < 0.01.

Long-term changes in KCI were related to atmospheric and oceanic conditions in the North Pacific. The EAWMI and PDOI are key climate indices for the intensity of the Northwest Pacific winter monsoon and fluctuations in sea surface temperature in the North Pacific associated with atmospheric and oceanic circulations. The correlation coefficients between the surface current velocity around the ECS and EAWMI and PDOI showed that both climate indices had a positive relationship with surface current velocity around the Ryukyu Islands (Figure 5). In particular, a highly significant coefficient of variation was observed with a two-month lag phase than with a phase with no lag (Figure 5).

**Figure 5.** Horizontal distribution of the correlation coefficient between the geostrophic surface current velocity in January: (**a**) East Asian Winter Monsoon Index (January–March), and (**b**) Pacific Decadal Oscillation Index (January–March). Black lines represent significant levels: *p* < 0.01).

*3.2. Long-Term Changes in the East Korea Warm Current*

**Figure 4.** Horizontal distribution of the correlation coefficient between the geostrophic surface current velocity in January and the longitudinal shift of the Kuroshio Current Index in January. Black

**Figure 5.** Horizontal distribution of the correlation coefficient between the geostrophic surface current velocity in January: (**a**) East Asian Winter Monsoon Index (January–March), and (**b**) Pacific Decadal Oscillation Index (January–March). Black lines represent significant levels: *p* < 0.01). **Figure 5.** Horizontal distribution of the correlation coefficient between the geostrophic surface current velocity in January: (**a**) East Asian Winter Monsoon Index (January–March), and (**b**) Pacific Decadal Oscillation Index (January–March). Black lines represent significant levels: *p* < 0.01). *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 7 of 13

#### *3.2. Long-Term Changes in the East Korea Warm Current 3.2. Long-Term Changes in the East Korea Warm Current*

lines represent significant levels: *p* < 0.01.

Long-term changes in the EKWC have a negative correlation with the KCI (Table 1). After 2010, the intensity of the EKWC exhibited a rapidly increasing pattern, whereas the KCI exhibited a decreasing pattern (Figures 3 and 6). The EKWC and KCI had a one-month lag phase (leading KCI) and had positive correlation coefficients (Table 1). The KCI in January had a more significant relationship with volume transport in February than it did with that in January and March (Table 1). Long-term changes in the EKWC have a negative correlation with the KCI (Table 1). After 2010, the intensity of the EKWC exhibited a rapidly increasing pattern, whereas the KCI exhibited a decreasing pattern (Figures 3 and 6). The EKWC and KCI had a onemonth lag phase (leading KCI) and had positive correlation coefficients (Table 1). The KCI in January had a more significant relationship with volume transport in February than it did with that in January and March (Table 1).

**Figure 6.** Long-term changes in East Korea Warm Current intensity in February. Black line represents step changes estimated using sequential *t*-test analysis of regime shifts analysis. **Figure 6.** Long-term changes in East Korea Warm Current intensity in February. Black line represents step changes estimated using sequential *t*-test analysis of regime shifts analysis.

**Table 1.** Correlation coefficient between volume transport in western channel of Korea Strait and

Kuroshio −0.26 <sup>N</sup> −0.47 \* −0.27 <sup>N</sup> −0.36 <sup>N</sup>

*3.3. Variations in Oceanic Conditions in the Western Part of the East Sea Associated with the*

ter column structure and EKWC intensity could not be analyzed in this study.

Long-term changes in salinity at a depth of 100 m in the WES near 37° E exhibited a distinct fluctuation pattern during weakened EKWC periods from the late 1990s to 2010; the salinity at the depth of 100 m around the inshore area ≤ 129.79°E was lower than 34.1 (Figure 7a). However, after 2010, the salinity at a depth of 100 m around the inshore area increased (Figure 7a), thereby indicating strong a EKWC (Figure 6). Furthermore, the stratification index was related to the intensity of the EKWC (Figure 7b). The stratification index increased after the late 1990s, which weakened EKWC. After the early 2010s, the stratification index was gradually reduced with a strong EKWC (Figure 7b). In summary, a low-saline water mass in the upper 100 m layer with strong stratification formed during the period of weakened EKWC. In contrast, during the strengthened EKWC period, highly saline water with weak stratification was observed (Figure 7). After 2019, the salinity and stratification index were slightly decreased (≤34.01) and increased (≥8), respectively; however, due to the absence of EKWC intensity data, the relationship between wa-

**January February March Mean**

**(January–March)**

Kuroshio Current intensity.

<sup>N</sup>: *p* ≥ 0.01.

*East Korea Warm Current*

\*: *p* < 0.01,


**Table 1.** Correlation coefficient between volume transport in western channel of Korea Strait and Kuroshio Current intensity.

\*: *<sup>p</sup>* < 0.01, <sup>N</sup>: *<sup>p</sup>* <sup>≥</sup> 0.01.

*3.3. Variations in Oceanic Conditions in the Western Part of the East Sea Associated with the East Korea Warm Current*

Long-term changes in salinity at a depth of 100 m in the WES near 37◦ E exhibited a distinct fluctuation pattern during weakened EKWC periods from the late 1990s to 2010; the salinity at the depth of 100 m around the inshore area ≤129.79◦ E was lower than 34.1 (Figure 7a). However, after 2010, the salinity at a depth of 100 m around the inshore area increased (Figure 7a), thereby indicating strong a EKWC (Figure 6). Furthermore, the stratification index was related to the intensity of the EKWC (Figure 7b). The stratification index increased after the late 1990s, which weakened EKWC. After the early 2010s, the stratification index was gradually reduced with a strong EKWC (Figure 7b). In summary, a low-saline water mass in the upper 100 m layer with strong stratification formed during the period of weakened EKWC. In contrast, during the strengthened EKWC period, highly saline water with weak stratification was observed (Figure 7). After 2019, the salinity and stratification index were slightly decreased (≤34.01) and increased (≥8), respectively; however, due to the absence of EKWC intensity data, the relationship between water column structure and EKWC intensity could not be analyzed in this study. *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 8 of 13

**Figure 7.** Long-term changes in (**a**) salinity at 100 m depth and (**b**) stratification index. **Figure 7.** Long-term changes in (**a**) salinity at 100 m depth and (**b**) stratification index.

#### *3.4. Long-Term Changes in Chlorophyll-a in the Inshore Area of the Western Part of the East Sea 3.4. Long-Term Changes in Chlorophyll-a in the Inshore Area of the Western Part of the East Sea*

Long-term fluctuations in the intensity of stratification in February were related to chl-*a* in February along the inshore area of the WES (Figure 8). The chl-*a* in inshore area of the WES had a more significant positive relationship than those in other regions, according to the analyzed correlation coefficients between the surface amount of Chl-*a* in February around the WES and the intensity of stratification in the inshore area (Figure 8). This suggests that the chl-*a* in the inshore area increased during winter owing to strengthened stratification (Figure 8). Long-term fluctuations in the intensity of stratification in February were related to chl-*a* in February along the inshore area of the WES (Figure 8). The chl-*a* in inshore area of the WES had a more significant positive relationship than those in other regions, according to the analyzed correlation coefficients between the surface amount of Chl-*a* in February around the WES and the intensity of stratification in the inshore area (Figure 8). This suggests that the chl-*a* in the inshore area increased during winter owing to strengthened stratification (Figure 8).

**Figure 8.** Horizontal correlation coefficient distribution between chlorophyll-*a* in February and

stratification index in February. Black lines represent significant levels: *p* < 0.01).

**Figure 7.** Long-term changes in (**a**) salinity at 100 m depth and (**b**) stratification index.

*3.4. Long-Term Changes in Chlorophyll-a in the Inshore Area of the Western Part of the East Sea*

Long-term fluctuations in the intensity of stratification in February were related to chl-*a* in February along the inshore area of the WES (Figure 8). The chl-*a* in inshore area of the WES had a more significant positive relationship than those in other regions, according to the analyzed correlation coefficients between the surface amount of Chl-*a* in February around the WES and the intensity of stratification in the inshore area (Figure 8). This suggests that the chl-*a* in the inshore area increased during winter owing to strengthened

**Figure 8.** Horizontal correlation coefficient distribution between chlorophyll-*a* in February and stratification index in February. Black lines represent significant levels: *p* < 0.01). **Figure 8.** Horizontal correlation coefficient distribution between chlorophyll-*a* in February and stratification index in February. Black lines represent significant levels: *p* < 0.01).

### **4. Discussion**

stratification (Figure 8).

Long-term changes in the oceanic conditions of the WES are affected by the fluctuations in both the thermal energy of the atmosphere and the current system in North Pacific [1,2,4]. In particular, the EAWMI and PDOI, which show atmospheric and oceanic circulations in the NP, are important factors influencing the changes in oceanic conditions in the WES [1,2,4]. In this study, the EAWMI and PDOI exhibited positive relationship with the KCI (Figure 9). In previous studies, the relationship between the Kuroshio Current and Pacific Decadal Oscillation has been reported. During the positive mode of the Pacific Decadal Oscillation, the intensity of the Kuroshio Current was increased by strong Aleutian low-pressure [31,32]. With strengthened Aleutian low-pressure, the pressure gradient between the high- and middle-latitude areas of the North Pacific as well as the westerlies around the middle-latitude area increased [31,32]. Consequently, the sea surface temperature in the central North Pacific exhibited a decreasing trend owing to strong winds, heat loss, and vertical mixing; however, the intensity of subtropical gyres such as the Kuroshio Current was enhanced by strong Ekman transport [33–35]. Additionally, variation in Aleutian low-pressure intensity is one of the primary factors controlling the EAWM by interacting with the Siberian High pressure [36]. The pressure gradient between the enhanced Aleutian low-pressure and Siberian High pressure, as well as the intensity of the winter monsoon over the WES, increased [1,2]. Consequently, during periods of strong Aleutian low-pressure, northwesterly winter winds, and cold, dry air masses from Siberia were intensified near the WES [1,2].

Fluctuations in wind stress curl associated with Aleutian low-pressure and Pacific Decadal Oscillation intensity variations have a positive relationship with the barotropic transport of the Kuroshio Current [37]; however, in this study, the response of geostrophic velocity in the ECS indicates regional differences. The surface current velocity around the ECS, which are located in the main stream of Kuroshio Current, had a positive relationship with the longitudinal shift in the main axis of the Kuroshio Current (Figure 9). This implies that when the KCI was enhanced, the main axis of the Kuroshio Current migrated eastward. The geostrophic velocity in the mainstream region of the Kuroshio Current and the western

section of the Ryukyu Islands indicates different responses to the changes in atmospheric and oceanic circulations in the North Pacific [38,39]. Previous studies have explained that in the period with strong Kuroshio Current and enhanced Aleutian low-pressure, the effect of Ekman transport on the flow of the Kuroshio Current moving through the ECS was amplified, and the main axis of the Kuroshio Current migrated eastward with strong velocity [1,16]. Consequently, the warm water mass separated from the Kuroshio Current, such as the Tsushima Warm Current, as well as the volume transport in the Korea Strait was weakened [1,16]. Furthermore, atmospheric conditions near the Korea Strait have a critical role in controlling volume transport in the Korea Strait. During winter, volume transport in the Korea Strait was reduced with increased northwesterly winds related to a strong EAWM [16,40]. In this study, similar results were obtained. When the intensity of the EKWC was enhanced, the KCI showed a decreasing pattern with strong EAWMI. Consequently, the response of oceanic conditions to the KCI revealed regional differences in the Northwest Pacific [1,2,41]. In previous studies, oceanic conditions, such as sea level and water temperature in the WES, driven by branch currents of the Kuroshio Current including the Tsushima Warm Current and EKWC indicated different fluctuation patterns with the Kuroshio Current region, such as the ECS and Kuroshio Extension in North Pacific [1,2,41]. *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 10 of 13

**Figure 9.** Schematic of the relationship between intensity of Kuroshio Current and East Korea Warm Current. Abbreviations: ECS, East China Seas; EAWM, East Asian Winter Monsoon; PDO, Pacific Decadal Oscillation. **Figure 9.** Schematic of the relationship between intensity of Kuroshio Current and East Korea Warm Current. Abbreviations: ECS, East China Seas; EAWM, East Asian Winter Monsoon; PDO, Pacific Decadal Oscillation.

The changes in the intensity of the EKWC flowing through the Korea Strait into the WES influence oceanic and biological conditions of the WES [42,43]. For the enhanced EKWC period, the main axis of the EKWC was adjacent to the inshore area but separated from the inshore area during the periods of reduced EKWC [44]. This spatial distribution along the main axis of the EKWC was related to the changes in the vertical structure of the water column in the inshore area [2,9]. During enhanced EKWC, the vertical volume of the warm water mass increased, and the vertical structure in the inshore area indicated mixed conditions. However, as the EKWC weakened and the main axis moved away from the inshore area, the North Korea Cold Current extended to the inshore area. Consequently, the vertical distribution range of the North Korea Cold Current, which was cold with less saline (33.9–34.1‰), and nutrient rich water mass with high oxygen content [9,45–46], extended to shallower depths in the inshore area of the WES [42]. Furthermore, vertical mixing became more active owing to the strong winter monsoon in the winter [2,43], and a replete condition of nutrient replenishment below the upper layer was supplied to the upper layer [43]. In February, chl-*a* in the inshore area of the WES increased. The changes in the intensity of the EKWC flowing through the Korea Strait into the WES influence oceanic and biological conditions of the WES [42,43]. For the enhanced EKWC period, the main axis of the EKWC was adjacent to the inshore area but separated from the inshore area during the periods of reduced EKWC [44]. This spatial distribution along the main axis of the EKWC was related to the changes in the vertical structure of the water column in the inshore area [2,9]. During enhanced EKWC, the vertical volume of the warm water mass increased, and the vertical structure in the inshore area indicated mixed conditions. However, as the EKWC weakened and the main axis moved away from the inshore area, the North Korea Cold Current extended to the inshore area. Consequently, the vertical distribution range of the North Korea Cold Current, which was cold with less saline (33.9–34.1‰), and nutrient rich water mass with high oxygen content [9,45,46], extended to shallower depths in the inshore area of the WES [42]. Furthermore, vertical mixing became more active owing to the strong winter monsoon in the winter [2,43], and a replete condition of nutrient replenishment below the upper layer was supplied to the upper layer [43]. In February, chl-*a* in the inshore area of the WES increased.

The results of this study provide novel insights into the possible mechanisms underlying the effect of climate change on oceanic conditions in the WES. Especially, these results provide major information that can be used to better understand the effects of the intensity changes in the Kuroshio current related to climate change on the change of The results of this study provide novel insights into the possible mechanisms underlying the effect of climate change on oceanic conditions in the WES. Especially, these results provide major information that can be used to better understand the effects of the intensity changes in the Kuroshio current related to climate change on the change of EKWC and

EKWC and water column structure in the WES. Furthermore, by explaining the mechanism underlying the changes in oceanic conditions in habitats ground, this study expands

tion, understanding the nutrient circulation cycle, supplement, and demand is necessary. In this study, we did not explain the process involving nutrient source. Therefore, in future studies, we will assess the possible mechanisms underlying the response of climate change to physical, chemical, and biological conditions in the WES, including nutrient,

Our study elucidated the impact of the intensity of the Kuroshio Current on the variations in the vertical water column structure in the inshore area of the WES. Long-term fluctuations in WES oceanic conditions are affected by both atmospheric and oceanic circulation. The intensity of the Kuroshio Current was related to atmospheric conditions, such as the intensity of the winter monsoon, as well as oceanic conditions, such as the volume transport of warm water mass into the WES. The interaction between atmospheric

primary production, and fisheries resources.

**5. Conclusions**

water column structure in the WES. Furthermore, by explaining the mechanism underlying the changes in oceanic conditions in habitats ground, this study expands our understanding of the changes in distribution and biomass of fisheries resources. However, to accurately investigate the changes in low trophic level, such as primary production, understanding the nutrient circulation cycle, supplement, and demand is necessary. In this study, we did not explain the process involving nutrient source. Therefore, in future studies, we will assess the possible mechanisms underlying the response of climate change to physical, chemical, and biological conditions in the WES, including nutrient, primary production, and fisheries resources.

### **5. Conclusions**

Our study elucidated the impact of the intensity of the Kuroshio Current on the variations in the vertical water column structure in the inshore area of the WES. Longterm fluctuations in WES oceanic conditions are affected by both atmospheric and oceanic circulation. The intensity of the Kuroshio Current was related to atmospheric conditions, such as the intensity of the winter monsoon, as well as oceanic conditions, such as the volume transport of warm water mass into the WES. The interaction between atmospheric and oceanic conditions can play a major role in controlling the changes in the water column structure and primary production. During periods of increased KCI, the severity of the winter monsoon increased; however, the warm water mass passing into the WES separated from the Kuroshio Current weakened. Consequently, the North Korea Cold Current range expanded, and cold-water masses were distributed under the upper layer in the inshore area of the WES. The water column structure is a possible primary cause of the high chl-*a* observed in the area.

**Author Contributions:** Conceptualization, Methodology, Validation, Formal analysis, and Writing— Original Draft, H.-K.J.; Supervision, Project Administration and Funding Acquisition, and Writing— Review and Editing, C.-I.L.; Data Curation and Visualization, Y.-W.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the National Institute of Fisheries Science, Ministry of Oceans and Fisheries, Korea (R2022035) and Korea Institute of Marine Science & Technology Promotion (KIMST) funded by the Ministry of Oceans and Fisheries (20220558).

**Data Availability Statement:** All data supporting the results of this study are provided in this manuscript.

**Acknowledgments:** We would like to thank the members of the Fisheries Resources and Environment Research Division in National Institute of Fisheries Science for their assistance in field observations and data analyses.

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

### **References**

