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. 2025 Aug 23;15:30986. doi: 10.1038/s41598-025-16226-8

Fluctuations in stratification and nutrient dynamics during the pre-bloom period in a Western margin of the East sea

Hae Kun Jung 1,#, Changsin Kim 1,#, Beom Sik Kim 2, Jeong Hee Shim 1, In Seong Han 1, Chung Il Lee 2, Dongyoung Kim 2,✉
PMCID: PMC12373884  PMID: 40847047

Abstract

Stratification is a fundamental process influencing nutrient availability and biological productivity in coastal marine ecosystems. In this study, we examine multi-decadal variability in winter stratification and nutrient distribution within Western margin of the East Sea (WES), using observational data collected between 1990 and 2023. Stratification characteristics were quantified using the Brunt-Väisälä frequency (N²), with particular focus on changes in intensity and the depth of maximum stratification. These variables were then analyzed in relation to environmental factors such as upper-layer temperature, surface wind speed, and East Korea Warm Current (EKWC) transport. Over the study period, stratification intensity increased at an average rate of 1.1% per year, while the maximum stratification depth shoaled by approximately 0.6% annual trends that became more pronounced after 2015. These shifts were closely linked to enhanced thermal stratification and increased EKWC volume transport. Enhanced stability of the water column was accompanied by reduced vertical mixing, which in turn intensified the nitrate gradient between surface and subsurface waters. The most marked nitrate accumulation was observed near 100 m, where stratification was strongest. This study highlights how physical oceanographic changes during winter can regulate nutrient supply prior to the spring bloom, potentially influencing seasonal productivity in marginal seas. The results emphasize the importance of long-term monitoring of vertical structure in assessing the ecological impacts of ocean variability.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-16226-8.

Keywords: Stratification, Nutrient dynamics, Northwest pacific marginal sea, Vertical mixing, Winter hydrography

Subject terms: Physical oceanography, Element cycles

Introduction

Stratification, shaped by the vertical distribution of temperature, salinity, and pressure, regulates key physical and biogeochemical processes in the ocean1. It influences vertical exchanges of heat, oxygen, and nutrients, and controls light and nutrient availability for primary productivity. Stratification also plays a critical role in modulating heat and carbon fluxes, with implications for global climate. In recent decades, upper-ocean stratification intensity has increased due to surface warming, driven by anthropogenic climate change2,3. Since the 1960 s, the global average stratification intensity has risen by approximately 6.1% compared to the climatological mean.

The East Sea, part of the Northwest Pacific, mirrors these global patterns while exhibiting distinct regional responses. Between 1968 and 2022, its annual mean sea surface temperature increased by 1.36 °C, about 2.6 times faster than the global average4. This rapid warming is linked to intensified Tsushima Warm Current (TWC)5 flow and weakened Siberian High and East Asian Winter Monsoon4,6. Marine heatwaves have also intensified, especially in winter and spring7. Within this context, Western margin of the East Sea (WES) shows heightened sensitivity to environmental variability, largely due to complex interactions between contrasting water masses like the North Korean Cold Current and the East Korea Warm Current (EKWC) (Fig. 1)4,8. This dynamic hydrography results in strong vertical and horizontal variability, making the WES a key region for detecting localized climate-driven changes9–11. Although the Kuroshio Current does not flow directly into the East Sea, it plays a critical upstream role in regulating the hydrographic properties of the TWC, the main conduit through which heat and salt enter the East Sea. The TWC originates from the Kuroshio off southeastern Kyushu, and observational studies indicate that more than 80% of the TWC volume during winter is derived from Kuroshio source waters12. The volume and pathway of the Kuroshio Current are modulated by basin-scale atmospheric forcing, particularly the Aleutian Low (AL) and the Pacific Decadal Oscillation (PDO). Under positive PDO conditions, an intensified AL enhances the zonal wind stress curl in the subtropical gyre, strengthening the Kuroshio and shifting its axis eastward6,13,14. This eastward shift reduces the interaction between the Kuroshio and the East China Sea shelf, leading to a decrease in the northward bifurcation that feeds the TWC12. Consequently, warm water inflow into the East Sea via the TWC and its northern extension, EKWC, weakens, while cold, low-salinity coastal waters expand offshore, modifying the vertical thermal structure and nutrient distribution4.

Fig. 1.

Fig. 1

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Map of the study area in the WES, generated using Ocean Data View (ODV, version 5.7.0; https://odv.awi.de). The enlarged panel on the right shows the locations of five observation transects (Lines 1–5) along the WES, with black and red dots representing in situ monitoring stations (red dots indicate stations where nutrient data were collected). The cyan line indicates the 500 m isobath. The lower inset displays the locations of sea level stations used to estimate volume transport through the Korea Strait, based on tide gauge data from Busan, South Korea, and Izuhara, Japan.

Furthermore, recent studies highlight the role of remote climatic forcing, such as El Niño events, in altering Kuroshio transport and thereby influencing TWC dynamics and biological productivity in the East Sea through remote teleconnection15. These basin- to regional-scale linkages demonstrate the importance of Kuroshio transport variability as a physical control on East Sea stratification, even if its influence is indirect and mediated through regional current branches.

PDO is a dominant mode of decadal-scale variability in sea surface temperature and atmospheric circulation over the North Pacific. Its associated index (PDOI) reflects large-scale shifts in SST anomalies, pressure gradients, and wind patterns. The PDO interacts closely with the East Asian Winter Monsoon (EAWM), as their indices tend to co-vary positively due to shared control by the Aleutian Low and Siberian High systems6. During the positive phase of the PDO, the Aleutian Low intensifies, strengthening the EAWM and enhancing zonal wind stress. This intensifies Kuroshio Current transport and shifts its axis eastward, weakening the connectivity between the Kuroshio and EKWC, the northern branch entering the East Sea12. As a result, warm water transport into the western East Sea decreases, while cold, low-salinity bottom waters intrude into the upper layer, altering thermal structure and nutrient distributions4.

These coupled changes were particularly evident following the climate regime shift of 1988, when PDOI, EAWMI, and the North Pacific Index (NPI) simultaneously changed phase. According to Jung et al. (2017), this shift contributed to long-term hydrographic transitions in the marginal seas, including SST rise and enhanced stratification in the East Sea. Previous studies have also reported similar effects of PDO-driven changes in Kuroshio and TWC pathways on the marginal seas13,14underscoring the role of basin-scale climate variability in regional ocean dynamics. These findings highlight that the PDO–EAWM system is a critical large-scale driver of winter hydrography and stratification in the WES. These basin-scale changes provide a broader climatic framework within which more recent regional hydrographic trends in the WES—such as weakening monsoon winds and increased TWC transport—can be understood.

Recent studies have shown that this surface warming is closely associated with both atmospheric and oceanic changes. In particular, the weakening of EAWM has resulted in reduced winter wind stress and surface cooling, thereby suppressing convective mixing and promoting upper-ocean heat retention6,16. Concurrently, the volume transport of TWC, which carries warm water from the Korea Strait into the East Sea, has increased since the late 1980s4. This enhanced advective heat input, combined with weaker atmospheric forcing, has contributed to elevated sea surface temperatures and intensified thermal stratification during winter. Such physical changes have also led to a long-term decline in surface nitrate and phosphate concentrations from 1995 to 2021, likely due to the reduced vertical entrainment of nutrients from deeper layers17which may affect winter nutrient accumulation and the magnitude of the spring phytoplankton bloom.

Winter is characterized by enhanced vertical mixing of the water column compared to other seasons, facilitating the upward transport of nutrient-rich deep waters to the surface layer16. The seasonal enrichment of surface nutrients plays a pivotal role in initiating the spring phytoplankton bloom, which constitutes a fundamental pulse of primary production in temperate marine ecosystems. The magnitude of the spring bloom directly influences the productivity of lower trophic levels and propagates through the food web, ultimately affecting the annual productivity of higher trophic levels, including commercially important fish species18. Consequently, interannual variations in winter stratification strength, which modulate vertical nutrient fluxes, emerge as a critical determinant of the biological productivity and trophic dynamics of marine ecosystems19,20 in the WES, these winter driven vertical nutrient fluxes play a particularly important role in determining seasonal and interannual fluctuations in biological productivity.

Previous studies have focused on horizontal and latitudinal analyses to understand ocean variability, although vertical structure is crucial for a comprehensive assessment of physicochemical response in water columns. This study aims to investigate long-term changes in the vertical structure of the water column in the WES in response to climate change. In particular, we focused on stratification intensity during winter as a season critical for vertical mixing and nutrient replenishment identifies key environmental drivers and evaluates how changes in stratification modulate nutrient dynamics in this hydrographically dynamic coastal region.

Results

Long-Term changes in the intensity and depth of stratification

During winter, the Brunt-Väisälä frequency squared profiles (N2 in the WES exhibited a distinct depth-dependent pattern, where its intensity increased with depth, reaching a peak at approximately 100 m, before gradually decreasing below this depth. The standard deviation of N2 was highest between 50 and 100 m, indicating a greater variability in stratification intensity within this depth range (Fig. 2). The maximum N² was consistently observed at 100 m, with no significant difference in its occurrence depth between the northern and southern regions. However, despite this similarity in depth, stratification intensity at 100 m in the southern region (0.79 × 10-3) was notably higher than in the northern region (0.68 × 10-3). Furthermore, while N2 values in the northern region declined sharply below 100 m, the southern region exhibited a more gradual decrease, suggesting a more stable stratification structure at greater depths (Fig. 2).

Fig. 2.

Fig. 2

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Vertical distribution of stratification intensity (N2_max × 10-3) in the WES averaged over the period 1990–2023. Panels show (a) the Total WES, (b) the northern part of the WES (Lines 1–2), and (c) the southern part of the WES (Lines 3–5). Vertical bars represent mean values, and error bars indicate standard deviations across regions and years.

The annual variation in stratification intensity also displayed strong regional differences, particularly between different latitudinal zones. The magnitude of interannual variability was greatest at lines 1 and 2, both located in the northern part of the WES, where frequent and abrupt fluctuations were observed (Fig. S1). In contrast, at lower latitudes, annual fluctuations in stratification intensity were more subdued, with line 5 exhibiting the weakest interannual variability among the study regions (Fig. S1). A climatological analysis of maximum N2 over the period 1990–2023 revealed that its mean values at lines 1 to 5 were 1.16 × 10-3, 1.20 × 10-3, 1.19 × 10-3, 1.39 × 10-3, and 1.61 × 10-3, respectively, with line 5 recording the highest value. Although the maximum N2 generally formed near 100 m depth, its occurrence depth showed a clear latitudinal dependence (Fig. S1). In the northern part of the WES, maximum N2 frequently formed at shallower depths, while at lower latitudes, it tended to occur at progressively greater depths. The climatological mean depth of maximum N² from 1990 to 2023 was 68.2 m at line 1, 82.9 m at line 2, 81.8 m at line 3, 106.2 m at line 4, and 109.2 m at line 5, further confirming this depth dependent pattern (Fig. S1). In summary, distinct spatial patterns and regional variability in stratification characteristics were evident across the WES. Both stratification intensity and depth demonstrated significant regional differences, with the southern part of the WES exhibiting stronger stratification intensity than the northern part, while stratification depth was observed at greater depths in lower latitude regions. These findings indicate that latitudinal and regional factors exert a considerable influence on stratification intensity and its depth distribution, shaping the long-term stratification characteristics of the WES.

The variation in stratification intensity in the WES revealed both regional differences and a common long-term fluctuation pattern across different locations (Fig. 3a). Analysis of long-term changes in stratification intensity showed a gradual increasing trend after the late 1990 s, followed by a period of decline in the early 2010 s, and a subsequent increase beginning in 2015 (Fig. 3a). This trend suggests that, despite short-term fluctuations, the overall trajectory of stratification intensity has been increasing over the past few decades. However, while the long-term variation in stratification intensity followed a similar pattern in both the northern and southern part of the WES, distinct differences in variability were observed between the two regions. The northern part of the WES experienced greater interannual variability, characterized by frequent and abrupt shifts in stratification intensity, particularly over short timescales (Fig. 3b). In contrast, the southern part of the WES exhibited smaller fluctuations, indicating a more stable stratification structure over time, though it consistently maintained higher overall stratification intensity (Fig. 3c).

Fig. 3.

Fig. 3

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Time series of stratification intensity (N2max × 10-3; panels a–c) and stratification depth (panels d–f) in the WES from 1990 to 2023. Panels (a, d) show the entire WES, (b, e) the northern part (Lines 1–2), and (c, f) the southern part (Lines 3–5). Positive anomalies in stratification intensity and depth (pink shading) indicate stronger-than-average, while negative anomalies (purple shading) represent weaker-than-average.

Similarly, the stratification depth also exhibited regional differences while following a common long-term fluctuation pattern across different areas of the WES (Fig. 3d). During periods of strong stratification, the depth of stratification tended to be shallower, whereas during periods of weaker stratification, it tended to be deeper. Notably, in the late 1990 s and after 2010, when stratification was weaker than the climatological mean, the depth of stratification was deeper than usual. Conversely, after 2015, when stratification intensity increased significantly, the depth of stratification became shallower again (Fig. 3d). Just as stratification intensity exhibited greater short-term variability in the northern part of the WES, the depth of stratification also showed stronger interannual fluctuations in the northern region compared to the southern region (Fig. 3e-f). However, both regions exhibited a generally consistent long-term pattern, confirming the influence of broader-scale oceanic and climatic factors on stratification variability in the WES (Fig. 3e-f).

A statistically significant moderate negative correlation was found between intensity and depth of stratification in the WES (Pearson’s r = − 0.579, p < 0.01), indicating that enhanced stratification intensity is generally associated with shallower stratification depths (Table S1). A linear regression analysis shows that from 1990 to 2023 the stratification intensity of the WES increased by an average of approximately 1.1% per year (p = 0.103; 95% confidence interval = − 0.002 to 0.024). Decadal analysis revealed notable variability: relative to the 1990 s, stratification intensity rose by 40.7% in the 2000 s, fell by 24.4% in the 2010 s, and then climbed by 32.5% in the 2020s. Moreover, relative to the 1990 s, the mean stratification intensity in the post-2015 period was approximately 13.8% higher than the pre-2015 average. In contrast, the depth of stratification decreased at an average annual rate of approximately 0.6% across the full period (p = 0.344; 95% confidence interval [−0.019, 0.007]). On a decadal scale, the stratification depth decreased sharply (–28.4%) in the 2000 s, rebounded slightly (+ 11.4%) in the 2010 s, and declined again (–3.85%) in the 2020s. An additional decline of 8.42% was observed after 2015 compared to the earlier period (Fig. 3).

The relationship between stratification and oceanic/atmospheric conditions in the Northwest Pacific

The long-term variations in stratification intensity and depth in the WES were found to be strongly correlated with temperature. Stratification intensity exhibited a significant positive correlation with temperature at 10 m depth (r = + 0.77) and a negative correlation with temperature at 200 m depth (r = −0.50) (Table S1) (Fig. S2). The depth of stratification was negatively correlated with upper-layer temperature, though this relationship was not statistically significant, while it showed a significant positive correlation with temperature at 200–250 m depth (Fig. S2).

The intensity of the EKWC exhibited a gradual weakening trend after 2001, followed by a rapid intensification after 2011 (Fig. 4). These fluctuations in EKWC strength were significantly positively correlated with stratification intensity, whereas they showed a negative correlation with stratification depth, though the latter was not statistically significant (Table S1). In particular, during periods of strong stratification, such as the late 1990 s and after 2015, the EKWC was also stronger than usual (Fig. 4). Meanwhile, PDOI showed a significant negative correlation with stratification intensity but had no significant correlation with depth of stratification (Table S1). Notably, after 2015, when stratification intensity increased sharply, the PDOI transitioned into a negative phase. Both EKWC and the PDOI exhibited significant correlations with water temperature at 10 m, with EKWC transport displaying a positive correlation (r = + 0.55) and the PDOI showing a negative correlation (r = −0.44) (Table S1). However, neither factor showed significant correlations with temperature at 200 m depth, suggesting that their primary influence is exerted on the oceanic condition in the upper layers (Table S1).

Fig. 4.

Fig. 4

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Time series of (a) East Korea Warm Current intensity and (b) the PDOI. Bars represent annual anomalies relative to the long-term mean, with yellow and light green indicating positive and negative deviations, respectively.

The variations in stratification intensity and depth in the WES were significantly correlated with sea surface temperature across the broader Northwest Pacific, particularly around the Korean Peninsula (Fig. S3a and d). Specifically, stratification intensity exhibited a positive correlation with sea surface temperature in areas influenced by the Kuroshio Current and its branches, including the East China Sea (ECS), the South Sea of Korea, and the WES. Conversely, it showed a negative correlation with sea surface temperature in the northern East Sea, particularly near the Tatar Strait. In contrast, the depth of stratification exhibited a negative correlation with sea surface temperature in regions affected by the Kuroshio Current and its branches, while it was positively correlated with sea surface temperature near the Tatar Strait (Fig. S3a and d).

Atmospheric conditions around Korean waters such as air temperature and wind speed were also found to have significant correlations with variations in stratification intensity and depth (Fig. S3b, c,e, f). Stratification intensity and depth were significantly correlated with air temperature in low-latitude regions, particularly the ECS, with stratification intensity exhibiting a positive correlation and stratification depth exhibiting a negative correlation (Fig. S3b, c,e, f). In contrast, wind speed around the East Sea showed a significant negative correlation with stratification intensity, whereas it exhibited a significant positive correlation with the depth of stratification.

The relationship between stratification intensity and nutrient supplement

Based on the principal component analysis (PCA) plot, we calculated the temporal variation of vertical NO3– gradient (Figs. 5 and 6a) and its relationship with N2 (Figs. 5 and 6b). The NO3– gradient was ranged from 0.004 mmol m[–4 in 2015 to 0.142 mmol m[–4 in 2021 with significant difference in the vertical gradient after 2015 (Mann-Whitney U test, p = 0.038). The vertical NO3– gradient was significantly increased in response to N2 elevation. With improved vertical data sets with 75 m, 200 m, and 500 m (for Line 3 and 4, 2018–2023), we obtained significantly different NO3– concentration in 100 m compared to 50 m and 75 m (one-way ANOVA, p < 0.001) while there was no difference between the concentrations in 50 m and 75 m. No relationship was detected between N2 and NO3– gradient in 200 m and 500 m from 2018 to 2023. Asymmetry Index based on latitudinal difference of NO3– at 100 m ranged from 0.020 in 2012 to 0.385 in 2017 (Fig. 7a). Asymmetry Index was significantly decreased with increase in volume transport of the EKWC (Fig. 7b).

Fig. 5.

Fig. 5

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Principal component analysis (PCA) plot showing the stratification intensity (N2 as a red arrow, and dissolved inorganic nutrients (NO3–, NO2–, PO43– and DSi) at upper four depths (0 m, 20 m, 50 m, and 100 m) as black arrows in the WES (2010–2023).

Fig. 6.

Fig. 6

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(a) Temporal variation in vertical gradient of nitrate (ΔNO3–/ΔZ) and (b) its relationship with stratification intensity (N2 in the WES (2007–2023).

Fig. 7.

Fig. 7

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(a) Temporal variation in Asymmetry Index and (b) its relationship with volume transport of the East Korea Warm Current (2010–2023).

Discussion

Our findings align with the globally observed intensification of upper-ocean stratification driven by anthropogenic climate change. Since the 1960 s, the global average stratification intensity has increased by approximately 6.1%, with more recent estimates (2006–2021) indicating a 7–8% 1,3 rise in annual mean maximum values. In the WES, stratification intensity increased at a mean annual rate of 1.1% from 1990 to 2023, with an additional 13.8% increase after 2015 relative to earlier decades. Simultaneously, the depth of maximum stratification shoaled at a rate of 0.6% per year, accompanied by a further 8.42% decline post-2015. These trends suggest that the WES is responding more rapidly and strongly to ocean warming than global trends. Notably, stratification intensified most markedly within the 20–100 m depth range, where N² values exceeded the Intergovernmental Panel on Climate Change (IPCC) AR5 and Special Report on the Ocean and Cryosphere in a Changing Climate projections by 28% and 87%, respectively2. These changes are consistent with broader North Pacific patterns, where wintertime surface warming, deepening of the mixed layer, and downward displacement of isopycnals have reshaped vertical density structures1. Additionally, shifts in basin-scale circulation, including the strengthening of subtropical highs and Ekman convergence, may further modulate stratification dynamics in the WES1,21,22.

In the North Pacific (NP), stratification intensity has increased by around 8–10%, the third-highest among studied regions, with winter showing an increase of 9.2 ± 1.8% over the past 58 years3. These changes are driven by localized surface warming and salinity variations, which enhance the density gradient between surface and deeper layers, significantly impacting nutrient transport and biological productivity, especially in higher latitudes where winter mixing is vital18,23,24. Seasonally, wintertime mixed layer warming leads to a strengthened temperature gradient near the thermocline. As a result, the depth at which the stratification intensity is strongest exhibits a shoaling trend, while the overall stratification intensity also strengthens1. According to Roch et al., (2023), the winter mixed layer depth (MLD) has deepened by approximately 30–50 m per decade, leading to the compression of underlying layers and a shoaling of the stratification maximum. Although MLD was not explicitly calculated in this study, the cited estimate serves as a useful reference framework for interpreting long-term changes in stratification structure. This process supports the hypothesis that deepening of the mixed layer does not necessarily result in a deeper stratification maximum but instead can lead to its shoaling due to increased vertical density gradients and water column compression. Additionally, large-scale atmospheric and oceanic circulation changes, such as Ekman-driven convergence and subtropical high variations, have been suggested as potential contributors to stratification trends21,22although their direct influence on the observed shoaling of the stratification maximum in the NP remains uncertain.

In the open ocean, the development of the mixed layer and variations in the strength and depth of stratification are closely linked to large scale atmospheric and oceanic circulation. However, in coastal regions, stratification intensity and depth are primarily influenced by regional scale atmospheric and oceanic forcing. In coastal areas adjacent to major river systems, freshwater inflow can lead to the formation of a barrier layer, thereby enhancing upper ocean stratification16,25. For example, a thick barrier layer associated with low salinity surface waters is present near the Russian coast in the East Sea, creating a halocline within the isothermal layer. This structure is believed to result from freshwater input from the Amur River, which has one of the highest discharge rates among northern Eurasian rivers26,27. Similarly, reduced surface salinity near the Japanese coast in the East Sea may stem from Changjiang River discharge28 or substantial precipitation associated with monsoonal variability29. However, in the WES, there are no major freshwater sources with sufficient discharge flux to induce significant environmental changes. During winter, riverine input is further reduced to minimal levels30–33. In addition, the WES is a region where coastal upwelling is active34. In the WES, when southerly winds persist, surface waters in the coastal region are advected eastward toward the open ocean due to Ekman transport, leading to the upwelling of deeper waters near the coast, known as wind driven upwelling. This process causes the 10 °C isotherm to rise to shallower depths, weakening stratification strength in the WES. However, wind driven upwelling in the WES predominantly occurs between June and August, whereas during winter, prevailing northerly winds suppress upwelling34.

The major factor influencing winter stratification intensity changes in the WES is water temperature variation. In winter, the influence of low-salinity water inflow from the Changjiang River is minimal, making temperature induced density changes the dominant factor. These temperature variations in the WES are primarily driven by changes in the heat energy entering the region. Since 2015, ocean heat content (OHC) in Korean waters has increased rapidly, indicating a gradual warming trend. In recent years, ocean warming in this region has intensified significantly, largely due to the sharp rise in OHC in the East Sea4.

The results of this study support a coherent mechanistic framework linking basin-scale climate variability to regional hydrography and nutrient structure in the WES. During the positive phase of the PDO, both the AL and the Kuroshio Current intensify. This intensification enhances Ekman transport and shifts the Kuroshio’s main axis eastward in the East China Sea6,12. As a result, the Kuroshio becomes increasingly detached from the East China Sea shelf, reducing its interaction with the bifurcation region that supplies TWC and its northward extension, EKWC12,13,35. This shift decreases the volume transport of both currents into the WES, diminishing upper-ocean heat input and weakening winter stratification. Furthermore, the eastward-displaced Kuroshio, in combination with an intensified AL, reinforces EAWM, generating strong northwesterly winds over the Korean Peninsula. These winds not only enhance surface cooling but also act to block the northward intrusion of the EKWC through the Korea Strait into the WES12,15. As a result, EKWC transport is further suppressed, compounding the reduction in oceanic heat advection. At the same time, the strengthened EAWM enhances surface cooling and vertical mixing, both of which contribute to a sustained weakening of winter stratification in the WES. The positive correlation observed between EKWC transport and N² in the upper 100 m suggests that heat advection from the south, rather than surface buoyancy forcing, is the primary driver of stratification variability during winter. This is consistent with prior findings that EKWC variability, modulated by Kuroshio bifurcation and wind-driven transport, governs upper-ocean heat content and density structure in the WES4,12. In contrast, during periods of weak EKWC inflow, stratification was more susceptible to atmospheric cooling and wind-driven mixing, often leading to a deeper pycnocline and enhanced nutrient entrainment from below.

When EKWC accelerates, observations, moored-ADCP records, and HYCOM/MOM5 simulations all reveal a tightly coupled two‐layer system in WES: beneath the northward‐flowing EKWC, the southward NKCC simultaneously thickens, widens to ~ 35 km, and accelerates—its core speed rising from < 10 cm s-1 in winter to > 30 cm s-1 in late summer36,37. As the EKWC strengthens, it carries warm Tsushima Warm Water into the surface layer, raising the 5 °C and 10 °C isotherms by tens of metres and thinning the upper layer from ~ 140 m in February to < 60 m in August. This shoaling steepens the vertical density gradient (increasing N2) and uplifts the pycnocline, intensifying stratification and shifting its core closer to the surface36,38. With the pycnocline lifted, the expanded cold, fresh NKCC water beneath the EKWC occupies an ever‐greater fraction of the 100–300 m depth range. Long‐term cable and ADCP measurements in the Korea Strait record the consequence: near‐bottom temperatures reach their annual minimum in September—roughly five months after the surface maximum—marking the arrival of the thickened NKCC under an intensified EKWC36,39. Together, these observational and modeling studies demonstrate that when the EKWC strengthens, upper‐ocean stratification intensifies and shoals the pycnocline, while the sub‐pycnocline layer becomes increasingly dominated by the NKCC. The result is a warmer surface layer and a cooler subsurface layer throughout the WES36,39.

The WES is affected by two opposing currents: the southward-flowing NKCC and the northward-flowing EKWC. In the northern part of the WES, the NKCC core resides at relatively shallow depths. As the cold and dense NKCC flows southward, it encounters the warmer and less dense EKWC. Due to the density contrast between the two water masses, the NKCC is subducted and becomes subjacent to the EKWC, resulting in its core being located immediately beneath the EKWC layer in the southern WES. Our results revealed that stratification in the WES was typically most pronounced around the 100 m. Relative to this reference depth, the northern part of the WES is characterized by elevated N² values at shallower layers, whereas in the southern WES, higher N² values are predominantly found below the 100 m, reflecting regional differences in vertical density structure. The variability in stratification intensity might be closed linked with vertical structures in nutrient variability40. Our PCA results clearly separated the responses in vertical distribution in dissolved inorganic nutrients (e.g. NO3–, PO43–, DSi) in the subsurface (50–100 m) from that in the surface (0–20 m). These changes are mainly associated with rising SST, reduced upwelling, and decreased terrestrial nutrient inputs, leading to reduced primary productivity and an imbalance in the nitrogen-to-phosphorus (N: P) ratio in the WES8,19,41. Particularly, the N2 was closely related with NO3– at 100 m, indicating that the biogeochemical processes in subsurface layer can be considered as a proxy of stratification in the WES. Indeed, as a pathway of the EKWC, the 100 m depth in WES experiences the thermal and biogeochemical fluctuations42.

The southern part of the WES is consistently influenced by the EKWC, while the northern part of the WES exhibits stronger spatial-temporal fluctuations in response to fluctuations in the EKWC’s strength and pathway. When the EKWC intensifies, its core closely follows the coastline, extending its influence into the northern WES11,43,44. Conversely, during periods of weakened flow, the EKWC shifts offshore and exhibits enhanced meandering, thereby reducing its coastal influence11,43–45. Under these weakened conditions, the NKCC becomes more dominant in the northern part of the WES. These circulation shifts substantially affect the latitudinal distribution of nutrients in the WES. During periods of strong EKWC transport, warm and oligotrophic waters expand across the region, resulting in a reduced latitudinal gradient in nutrient concentrations. In contrast, when EKWC transport weakens, the northward intrusion of cold, nutrient-rich NKCC waters enhances latitudinal variability, reflecting the hydrographic contrasts between the northern and southern WES. The spatial asymmetry of hydrographic condition in WES was reflected with the negative relationship between the Asymmetry Index and the EKWC transport.

Upper ocean stratification plays a key role in regulating winter MLD, which directly influences nutrient availability on surface layer in NP46. Indeed, nutrient supply driven by depth changes in the WES, although there is seasonality in concentration, is closely linked to phytoplankton uptake9. In case that vertical resolution was improved by additional data of NO3– concentration in 75 m from 2018 to 2023, we obtained the significant difference of the values in 100 m from that in 50 m and 75 m, supporting 100 m might be the vertically sensitive zone in the WES. Furthermore, no relationship between N2 and vertical gradient in deeper layer (e.g. 200 and 500 m) suggested that stratification in the winter results in physicochemical response in upper 100 m layer although the mixing layer is formed in 200 m47. In the North Pacific, the wintertime dipole pattern near 200 m provides a possible bifurcation in nutrient supply process between the subsurface and deep layers48. Indeed, as a pathway of the EKWC, WES experiences the thermal and biogeochemical fluctuations in the upper 100 m layer42. The influence of horizontal nutrient supply via the TWC is likely limited under the physically well-mixed conditions of the WES, although several studies have suggested its possible contribution to surface nitrogen availability with interannual variability49. In addition, the contribution of atmospheric nitrogen deposition to the regional nitrogen budget is relatively minor with little evidence for substantial surface enrichment50. Our study focused on a period characterized by strong atmospheric forcing, where vertical convection is likely dominant driver of nutrient supply16. When upper-layer warming intensifies stratification, the shoaling pycnocline allows nutrient-rich NKCC waters to intrude into shallower layers; however, the stronger stratification also suppresses vertical mixing, limiting the upward flux of nutrients and steepening the vertical nutrient gradient.

The WES has emerged as a particularly sensitive and productive area, serving as a key site for commercial fisheries and climate-related oceanographic research17,51,52. It is generally agreed that nutrient supply is the major factor driving the biological responses in lower-trophic-level organisms. Our PCA analysis clearly demonstrated that the stratification in the WES amplified the vertical gradient in nutrient represented by increased concentrations at 50 m and 100 m contrasting to diminishment at 0 m and 20 m. In particular, in response to stability intensification, nitrate in 100 m was significantly increased thereby strengthening the vertical gradient weaken flux on upper layer. The amplification of vertical gradient might be explained by phytoplankton uptake, as indicated by negative relationship between phytoplankton biomass (e.g. chlorophyll a concentration) and dissolved inorganic nutrients (NO3–, PO43–, and DSi) under frontogenesis condition in the WES53. The nitrate utilization of phytoplankton in subsurface layer (< 50 m) can be enhanced in the East Sea of Korea where warm water overlies cold and dense water masses, resulting in shoaling pycnocline and intensified stratification9. Furthermore, in the opened system of the WES, phytoplankton on surface layer can be affected by nutrient-depleted condition which is resulted from weaken vertical mixing54. Weaken vertical mixing can induce the light limitation to phytoplankton through temporal reduction of photosynthetic activity, simultaneously accompanied with the weaken regeneration of nutrient. In terms of the phytoplankton community structure, dominance of diatoms characterized with high sinking rates is readily affected by both light and nutrient limitation resulting from weaken stratification. Thus, considering that diatoms are the dominant phytoplankton in the subsurface chlorophyll maximum in the WES during the spring bloom, concurrent responses of NO3– and DSi under intensified stratification suggest that hydrographic variability in winter may trigger potential changes in phytoplankton bloom dynamics in the WES53,55. Further studies can explore how winter hydrographic shifts in the WES influence the magnitude and community structure of phytoplankton during the pre-bloom and bloom period.

Conclusions

This study provides a comprehensive assessment of long-term changes in the vertical water column structure in the WES during winter from 1990 to 2023, with particular emphasis on variations in stratification intensity and its vertical position. The findings reveal a statistically significant intensification of winter stratification across the WES, with an average annual increase of 1.83%, accompanied by a shoaling of the stratification maximum depth at a mean rate of 2.08% per year. These trends have become more pronounced since 2015, reflecting the region’s heightened sensitivity to ocean warming. The observed changes in intensity of stratification were closely linked to both regional and basin-scale climatic drivers. Stratification intensity exhibited strong correlations with upper ocean temperature and the volume transport of the EKWC, while the PDOI was significantly associated with interdecadal variability. These relationships underscore the influence of large-scale atmospheric-oceanic interactions on the hydrographic conditions of the WES. The 100 m depth in the WES represents a climatically sensitive horizon where physical stratification and nutrient gradients converge. Variations in hydrographic and chemical conditions at this depth offer a robust indicator of climate-driven changes in vertical structure, biogeochemical cycling, and ecosystem productivity. Accordingly, changes in the depth and intensity of stratification at this horizon have important implications for nutrient accessibility and primary productivity, particularly during the pre-bloom period. Furthermore, enhanced stratification was associated with a marked steepening of the vertical nitrate gradient, primarily driven by the suppression of vertical mixing and the resultant accumulation of nutrients below the pycnocline. Principal component analysis further supported a robust linkage between the intensity of stratification and subsurface nitrate concentrations. These findings suggest that intensified stratification alters nutrient supply pathways and may modulate biological productivity, particularly during the pre-bloom period (Fig. 8; Fig. S4). In summary, the WES is undergoing rapid hydrographic transformations under ongoing climate change, with significant implications for nutrient dynamics and ecosystem functioning. This study highlights the necessity of incorporating vertical structural analyses into regional climate impact assessments and underscores the importance of long-term, high-resolution monitoring to evaluate the cascading effects of stratification on biogeochemical processes and marine ecological responses in marginal seas.

Fig. 8.

Fig. 8

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Conceptual illustration of the effects of winter stratification intensity on vertical nitrate distribution and atmospheric–oceanic interactions in the WES. (Left) During periods of strong stratification, weakened wind stress and warm air temperature enhance water column stability, suppressing vertical mixing and resulting in a steep vertical nitrate gradient. (Right) During periods of weak stratification, stronger wind forcing and cold air temperature promote vertical mixing, facilitating upward nutrient transport and reducing the vertical nitrate gradient.

Methods

Oceanographic data

We used historical in situ observed temperature and salinity by standard depth (0, 10, 20, 30, 50, 75, 100, 125, 150, 200, 250, 300, 400, 500 m) in the WES archived from Korea Oceanographic Data Center from National Institute of Fisheries Science (KODC; https://www.nifs.go.kr/kodc/eng/index.kodc). A total of 24 fixed stations were observed in February during 1990–2023. However, data from some stations in 1995 were missing, and therefore, the 1995 data were excluded from the analysis of long-term variability trends (Fig. 1).

To investigate the long-term changes in the intensity and depth of stratification in the WES and their relationship with the EKWC, volume transport data from the western channel of the Korea Strait during winter (January, February, March) from 1991 to 2023 were used (Fig. 1). The strength of the EKWC was determined based on this volume transport. It was estimated for the winter season (January–March) from 1991 to 2023 using sea level data from tidal stations in Izuhara, Japan (provided by the Japan Meteorological Agency, JMA), and Busan, South Korea (provided by the Korea Hydrographic and Oceanographic Agency, KHOA). The volume transport in the western channel of the Korea Strait were calculated using:

graphic file with name d33e1132.gif 1

where V is the volume transport (hm3/s), f is the Coriolis force, ρ is the density of seawater (kg/m3), Δp is the pressure difference between each tidal station (hPa), and Δx is the distance between each tidal station (51.17 km)6,56.

To investigate the long-term changes in the intensity and depth of stratification in the WES and their relationship with SST in the broader region around the Korean Peninsula, Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) data from 1990 to 2023 during winter (February) were used (Fig. 1). OSTIA provides daily global-scale sea surface temperature and sea ice data in a 0.05° lattice, which are reanalyzed for public consumption based on field observations of the International Comprehensive Ocean-Atmosphere Data Set (ICOADS)57 and data from the Advanced Very High-Resolution Radiometer (AVHRR; http://www.nodc.noaa.gov/SatelliteData/) and the Along-track Scanning Radiometer (ATSR) (http://neodc.nerc.ac.uk/).

Long term changes in intensity and depth of stratification in the WES

The Brunt-Väisälä frequency, a measure of the maximum vertical density gradient, is often used to identify stratification maxima, providing insights into ocean stability and the extent of vertical mixing58,59. N2 were computed using:

graphic file with name d33e1185.gif 2

Where g is gravitational constant, 9.8 m/2, is the density, z is depth in meter. The vertical stratification maximum and its corresponding depth were identified from each Brunt-Väisälä frequency profile as the maximum value of N2 (intensity of stratification) and the depth (location of stratification) at which it occurs.

The intensity and depth of stratification were defined based on this maximum value and its associated depth. The stratification intensity and depth in the WES during winter (February) were analyzed for the period from 1990 to 2023. Each line consists of 4 to 5 in-situ monitoring stations, and the annual N2 value for each line was determined by averaging the N2 values of the stations along that line. For regional analysis, the WES was divided into the northern part (lines 1 and 2) and the southern part (lines 3 to 5) to investigate latitudinal changes in stratification strength and distribution (Fig. 1). In this study, the N2 was calculated using standard depth temperature and salinity data to define long term trends in intensity and depth of stratification. To verify the appropriateness of using standard depth data, N2 values were calculated separately from both 1-meter interval temperature and salinity data and standard depth data for the period from 2010 to 2023, and the results were compared. Prior to statistical analysis using IBM SPSS software (v. 22.0, IBM Corp., Armonk, NY, USA), the normality of both datasets, obtained from different depth intervals, was assessed using the Kolmogorov-Smirnov test. The results indicated that neither dataset followed a normal distribution (p > 0.05). Although differences were observed in the paired comparison results (Wilcoxon Matched-Pairs Signed-Ranks Test, p < 0.001), a significant positive correlation was identified between the two datasets (p < 0.001, r2 = 0.758).

Effect of climate change on changes in water column structure in the WES

To analyze changes in oceanic conditions, such as the intensity and depth of stratification in response to climate change, the PDOI was used for the period from 1990 to 2023. The PDOI represents the ocean-atmosphere state of the NP and is defined as the leading empirical orthogonal function of monthly sea surface temperature anomalies in the Pacific, poleward of 20°N60. In addition, 10 m wind speed and air temperature data were used to analyze the relationship between changes in the intensity and depth of stratification and atmospheric conditions around the Korean Peninsula. Inter-annual variations in atmospheric conditions, including wind speed and air temperature, over Korean waters during winter (January to March) were examined using ERA5 reanalysis data. ERA5 is the latest climate reanalysis dataset developed by the European Centre for Medium-Range Weather Forecasts (ECMWF)61. The analysis utilized monthly data from 1990 to 2023 with a spatial resolution of 0.25° × 0.25°.

Long-term changes in nutrient enrichment in the WES

Historical in situ nutrient data at four depths (0, 20, 50, and 100 m) in the WES were obtained from the KODC. Observations were conducted at a total of ten fixed stations each February from 2010 to 2023 (Fig. 1). Two stations per transect line were selected for analysis. To investigate long-term changes in nutrient concentrations by water depth across the WES, nutrient data from the ten stations were averaged at each depth. Temporal variability in the vertical nitrate gradient (ΔNO3-/ΔZ, mmol m-4) from 100 m to the surface layer was estimated for the period 2010–2023.

To quantitatively compare the latitudinal difference between the northern and southern regions, nitrate concentrations at 100 m depth were first standardized using Z-scores. Subsequently, the Asymmetry Index was calculated to assess the degree of NO3- imbalance between the two regions. The Asymmetry Index is defined as the absolute difference between the two values divided by the larger of the two, serving as a normalized measure of relative disparity. A higher Asymmetry Index indicates a greater difference in NO3- concentration between the northern and southern WES, reflecting a higher degree of spatial heterogeneity in nutrient conditions.

Statistical analysis

Prior to statistical analysis using IBM SPSS software (v. 22.0, IBM Corp., Armonk, NY, USA), the normality of both datasets, obtained from different depth intervals, was assessed using the Kolmogorov-Smirnov test. The results indicated that neither dataset followed a normal distribution (p > 0.05). Although differences were observed in the paired comparison results (Wilcoxon Matched-Pairs Signed-Ranks Test, p < 0.001), a significant positive correlation was identified between the two datasets (p < 0.001, r2 = 0.758). Temporal difference in the vertical nitrate gradient from 100 m to the surface layer between the period of 2010–2015 and 2016–2023 using Mann-Whitney U test.

Supplementary Information

Below is the link to the electronic supplementary material.

Supplementary Material 1 (4.2MB, docx)

Acknowledgements

This study used publicly available data from Korea Oceanographic Data Center, KODC, which can be accessed at https://www.nifs.go.kr/kodc/eng/index.kodc.

Author contributions

H.K.J. Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing the original draft; C.K. Data curation, Formal analysis, Investigation, Writing the original draft; B.S.K. Visualization, Writing the original draft; J.H.S. Investigation, Visualization; I.S.H. Data curation, Funding acquisition, Methodology, Project administration; C.I.L. Writing the original draft; D.K. Formal analysis, Conceptualization, Data curation, Methodology, Project administration, Writing the original draft. All authors reviewed the manuscript.

Funding

This research was supported by the National Institute of Fisheries Science, Ministry of Oceans and Fisheries, Korea (R2025014).

Data availability

The in situ oceanographic (temperature, salinity, and nutrients) data used in this study are publicly available from the Korea Oceanographic Data Center (KODC) operated by the National Institute of Fisheries Science ([https://www.nifs.go.kr/kodc/eng/index.kodc](https:/www.nifs.go.kr/kodc/eng/index.kodc)). Sea level data from Izuhara, Japan, were obtained from the Japan Meteorological Agency (JMA; https://www.jma.go.jp), and data from Busan, South Korea, were provided by the Korea Hydrographic and Oceanographic Agency (KHOA; https://www.khoa.go.kr). Sea surface temperature data were derived from the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA), available at the UK Met Office (https://www.metoffice.gov.uk/hadobs/ostia/). The Pacific Decadal Oscillation Index (PDOI) was accessed via the University of Washington Joint Institute for the Study of the Atmosphere and Ocean (JISAO; https://research.jisao.washington.edu/pdo/). ERA5 atmospheric reanalysis data (10 m wind and air temperature) were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF; https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5). All datasets are available upon reasonable request from the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Hae Kun Jung and Changsin Kim contributed equally to this work.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary Material 1 (4.2MB, docx)

Data Availability Statement

The in situ oceanographic (temperature, salinity, and nutrients) data used in this study are publicly available from the Korea Oceanographic Data Center (KODC) operated by the National Institute of Fisheries Science ([https://www.nifs.go.kr/kodc/eng/index.kodc](https:/www.nifs.go.kr/kodc/eng/index.kodc)). Sea level data from Izuhara, Japan, were obtained from the Japan Meteorological Agency (JMA; https://www.jma.go.jp), and data from Busan, South Korea, were provided by the Korea Hydrographic and Oceanographic Agency (KHOA; https://www.khoa.go.kr). Sea surface temperature data were derived from the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA), available at the UK Met Office (https://www.metoffice.gov.uk/hadobs/ostia/). The Pacific Decadal Oscillation Index (PDOI) was accessed via the University of Washington Joint Institute for the Study of the Atmosphere and Ocean (JISAO; https://research.jisao.washington.edu/pdo/). ERA5 atmospheric reanalysis data (10 m wind and air temperature) were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF; https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5). All datasets are available upon reasonable request from the corresponding author.

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