Hydrodynamics and Sediment Transport at Socheongcho Ocean Research Station, Korea, in the Yellow Sea

Abstract

A seasonal variability in flow and sediment flux at the Socheongcho Ocean Research Station (SORS) on the west coast of Korea in 2018 was investigated to elucidate the formation of a two-layered flow structure and changes in sediment transport during stratification. An analysis of SORS data revealed stable temperatures (5–10 °C) in deeper waters, while surface temperatures rose from 6 °C in April to a peak of 30 °C in late August, gradually declining and leading to full water column mixing by late November. This temperature variation induced stratification, influencing the development of a two-layered flow structure. In winter, a singular flow structure was observed, contrasting with the emergence of a two-layered structure as stratification progressed. In the surface layer, residual currents flowed northward in summer and southward in winter, consistent with previous studies. In deeper layers, a southward residual current persisted, irrespective of the season. Sediment flux consistently moved southward, regardless of the season or water depth, with notably higher cumulative sediment flux in the deeper layer (1300 kg·m⁻²s⁻¹) compared to the surface layer (300 kg·m⁻²s⁻¹). These findings diverge notably from previous studies, providing new insights into ocean currents and material transport in the Yellow Sea.

1. Introduction

The Yellow Sea, a marginal sea located between China and Korea, plays a crucial role in the circulation of matter and is of great geological, ecological, and social significance. Remarkably, the sediment discharged into the Yellow Sea accounts for approximately 10 percent of the global sediment discharge, despite the area of the Yellow Sea being merely 0.1 percent of the global ocean area [1]. This sediment discharge, transport, and deposition have profound implications for the marine ecosystem and fisheries in the region [2,3].

The Yellow Sea circulation system is largely influenced by the seasonal patterns of winds driven by the East Asian monsoon system [4,5,6]. During winter, strong southerly winds and cold air temperatures lead to a well-mixed water column, causing surface currents to flow southward, forming the Bohai Sea Coastal Current (BSCC) and the Yellow Sea Coastal Current (YSCC) along the western Chinese coast and the Korean Coastal Current (KCC) along the eastern Korean coast. Concurrently, the Yellow Sea Warm Current (YSWC) flows northward through the middle of the Yellow Sea, transporting warm saline water into the Bohai Sea (Figure 1). In summer, weaker southerly winds and warmer air temperatures result in surface warming and stratification, leading to the presence of the Yellow Sea Cold Water Mass (YSCWM) beneath the surface [5]. Along the Korean coast, the KCC flows northward, while the BSCC and YSCC are less observable in the Yellow Sea [4].

Although extensive research has been conducted on the detailed flow and sediment flux patterns along the Chinese coast and in the central Yellow Sea, there is a lack of understanding regarding the KCC in the eastern Yellow Sea. Previous studies have suggested a southward flow in the surface layer during summer, mainly based on the trajectories of satellite-tracked drifters and satellite observations [7,8,9,10]. However, the stratification in this region is known to be weak or broken due to active tidal mixing in the coastal areas, where tidal currents are strong [11]. Consequently, conflicting results have been reported, with some studies indicating northward flow along the Korean coast based on water-type distribution and current measurements [12,13]. Yet, due to the lack of flow measurements in this region, the flow structure, particularly the two-layered circulation under stratification during summer, remains poorly understood.

To address this knowledge gap, the Socheongcho Ocean Research Station (SORS) of Korea was established in 2014, about 30 km south of Socheong Island, to monitor the long-term oceanic variability in the Yellow Sea and facilitate interdisciplinary scientific investigations [14]. This research station has accumulated a comprehensive suite of oceanographic datasets, including temperature, salinity, winds, waves, and ocean currents, providing a valuable opportunity to enhance our understanding of the KCC and sediment transport. The objective of this study is to elucidate the flow structure and sediment flux patterns, as well as their seasonal variability, using the SORS monitoring data. Specifically, we examine the vertical, two-layered flow structure under unstratified and stratified conditions and estimate the vertical and temporal sediment fluxes to investigate the effects of stratification on sediment transport, with a particular focus on the Yellow Sea circulation along the KCC.

2. Study Area

The Yellow Sea, including the Bohai Sea, covers an area of 417,000 km² and is bordered by the Chinese mainland to the west and the Korean peninsula to the east. With an average depth of 44 m and a maximum depth of 130 m, the Yellow Sea is characterized by its shallow nature, leading to a macro-tidal range with a maximum amplitude of up to 10 m. Although tidal currents are strong, the residual tidal currents are relatively small, being around 2 cm/s in the central part of the Yellow Sea and up to about 8 cm/s in the southwestern part of Korea and around the Yangtze Bank [15]. The relatively strong residual currents are attributed to topographic and stratified tidal rectification [16].

The climate of the Yellow Sea is primarily influenced by the East Asian monsoon system. During the winter season from September to April, a high-pressure system develops over Siberia, and strong northerly winds blow from Siberia and the Mongolian Plateau, resulting in dry and cold air. In contrast, during the summer, a low-pressure system forms over eastern Siberia, and southerly winds dominate, bringing warm and moist air with relatively high precipitation. However, the wind force is significantly reduced during the summer, and the wind direction becomes more variable. The variation in air temperature is wide due to the monsoonal system. Mean air temperatures range from −5 °C to −2.5 °C in winter, depending on the latitude in the Yellow Sea, while the mean air temperature increases to about 22.5–25 °C in summer [5]. The sea surface temperature (SST) varies greatly seasonally, ranging from 4 °C in winter to 25 °C in summer [17]. The warming of the sea surface temperature and subsequent surface heating induces seasonal variability in the thermocline of the Yellow Sea, with formation in spring, maturation in summer, diminishing in autumn, and vanishing in winter [18,19].

The circulation in the Yellow Sea is mainly driven by the monsoonal influence of wind and water temperature. In winter, strong northerly winds drive surface currents southward, inducing a northward return flow at depth. This bottom return flow is referred to as the YSWC. Along the shallow coasts, coastal currents flow southward on both the Korean and Chinese coasts, known as the KCC and the YSCC, respectively (Figure 1). During summer, atmospheric heating warms surface waters, leading to a separation between the warmer surface water and the Yellow Sea Cold Water (YSCW) in the deeper Yellow Sea, while tidal mixing is dominant along the shallow coastal regions. As a result, a tidal mixing front forms between the deeper stratified and coastal mixed regions [5]. In the central Yellow Sea, currents are too weak to be observed [4]. Along the shallow coastal regions, the KCC flows northward along the Korean coast, while the YSCC flows southward along the Chinese coast.

Korea has established ocean-observing platforms in the western and southern coastal area of Korean peninsula, such as the Ieodo Ocean Research Station (ORS), Gageocho-ORS, and Socheongcho-ORS (SORS). These ocean observation stations were established to improve comprehensive oceanic and weather observations and provide core scientific information and data for regional and global environmental change studies [14]. The northernmost SORS is located 37 km south of Socheong Island (37.423056° N, 124.738056° E). It was constructed during 1995–2003 and completed in October 2014 at a depth of 50 m over a rocky bottom, with a 90 m tall structure (50 m underwater, 40 m above water). The SORS has continuously monitored 12 meteorological, 25 oceanic, and 11 environmental variables, including conductivity–temperature–depth (CTD), waves, and currents. Additional information about the SORS and its sensors can be found in [14,20].

3. Materials and Methods

3.1. Data Collection

3.1.1. Atmospheric and Oceanographic Environmental Data

This study primarily relied on data obtained from moored acoustic doppler current profiler (ADCP) and cast CTD measurements conducted at the SORS in the Yellow Sea. These datasets were instrumental in understanding the flow circulation and sediment flux in the region. To provide additional context for the collected ADCP and CTD data and support the analysis, multiannual atmospheric and oceanographic environmental data were compiled via the Korea Institute of Ocean Science and Technology’s (KIOST) online database [21].

The data from SORS span from January 2016 to December 2020, comprising a five-year dataset. The atmospheric data include atmospheric pressure, air temperature, wind speed, and wind direction, while the oceanographic data encompass sea surface elevation, surface water temperature, surface salinity, wave height, wave direction, and wave period. The measurement instruments used are as follows: atmospheric pressure was measured using a digital barometer (Vaisala PTB210), air temperature was measured using a humidity and temperature probe (Vaisala HUMICAP HMP155), wind speed and direction were measured using a wind monitor (Campbell Scientific 5106), tide was measured using a MIROS Range Finder (SM-140) fixed to the deck of the research station, surface water temperature was observed nominally 5 m below the mean sea level using a temperature sensor (Aanderaa CT3919), and bottom water temperature was observed using a 300 kHz ADCP (Teledyne RDI; Section 3.1.3) [14,20]. The surface salinity was measured using a salinity sensor (Sea-Bird Scientific SBE 37-IM), while wave height, direction, and period were measured using a wave and current radar system (MIROS SM-050). The data collection intervals were generally every 10 min. It is important to note that the collection of surface water temperature and salinity data was not possible for the first year (2016), and the tide data show a difference in average sea level due to the relocation of the MIROS Range Finder in 2019.

3.1.2. Water Column Temperature and Salinity

To gain insights into the physical properties of seawater near SORS in the Yellow Sea and assess the stratification by season, CTD data were compiled from the KIOST online database [21]. The CTD data were available in two different datasets. The first dataset was collected by casting a CTD profiler (Sea-Bird Scientific 19plus V2 SeaCAT) from the deck of SORS, covering the period of 2015–2020, providing a six-year dataset. Vertical profiles of water temperature and salinity were sampled at 4 Hz, and a cast was performed for 2–3 days every month, excluding the winter season and typhoon events. The second dataset was collected using moored CTDs, distributed vertically and moored at constant heights above the bed (Figure 1B). In total, there were eight moored CTDs that measured at one-minute intervals during 2016–2020, constituting a five-year dataset.

3.1.3. Tide, Currents, and Acoustic Echo Intensity

For this study, vertical profiles of flow and acoustic backscatter intensity observed with a bottom-moored 300 kHz ADCP (Teledyne RDI) at SORS from January 2016 to November 2020 were acquired. Different settings were used during the observation period of the dataset. Data from 2016–2017, saved with a 5.12 m blanking distance and a profile of 35 bins with a 2 m bin size, were found to be unsuitable for analysis due to the ADCP being inclined and resulting in inaccurate data. However, from 2018 onward, the ADCP settings were adjusted to a 1.44 m blanking distance, 70 bins, a 1 m bin size, and a 2 Hz sampling rate, providing more suitable data for observing the flow and converting acoustic backscatter intensity to the suspended sediment concentration (SSC). However, there was a large gap in the ADCP dataset in 2019 due to Typhoon Lingling in September 2019. Consequently, this study focuses on the 2018 ADCP data, which are of the best quality and continuity.

3.1.4. Suspended Sediment Concentration

To calibrate the ADCP acoustic backscatter intensity to SSC, water samples were collected during two in situ surveys. The first survey took place from 2018-05-07 09:00 to 2018-05-08 10:00, conducted over a 25-h casting survey covering 2 tidal cycles. The second survey was from 2018-11-17 18:00 to 2018-11-20 02:00, during which 26 h of data were irregularly collected for about 2 tidal cycles. Hourly casts were performed from the lowest deck of SORS using an electric winch. During each survey, a CTD profiler (Sea-Bird Scientific 19plus V2 SeaCAT) with an auxiliary optical backscatter sensor (OBS; Seapoint Sensors, Inc.) was downcast to the bottom. On the upcast, it was stopped at the bottom, middle, and surface layers for nominally 2 min, and 1 L of water sample was collected at each of these layers using a Niskin bottle. The water samples were immediately filtered at SORS using glass fiber filters, and the filters were washed with freshwater to remove the salt. The filters were later dried and weighed in a laboratory to determine the SSC.

3.2. Data Processing and Analysis

The primary data analysis in this study involved evaluating the depth and strength of seasonal thermal stratification, calibrating the hADCP acoustic backscatter intensity to SSC and related data processing, computing sediment fluxes, and assessing the annual residual currents and fluxes. Detailed procedures for these analyses are outlined in the following sections.

3.2.1. Evaluation of Seasonal Thermal Stratification

The two CTD datasets were used to evaluate seawater properties and seasonal stratification at SORS. Before their use, the cast and moored CTD data (water temperature and salinity) were compared via a data quality check. While water temperature data obtained via casting and mooring exhibited similar values, the observed salinity in the moored CTD showed a large deviation from the average of the salinity obtained via casting. Therefore, both casting and moored CTD data were used to define water temperature, while only the casting CTD data were used to define salinity.

To evaluate the stratification by season, density was calculated based on water temperature, salinity, and pressure data, and the Brunt–Väisälä buoyancy frequency, i.e., N (Hz), was computed for quantitatively assessing the vertical density change as follows:

$$\mathrm{N}=\sqrt{-\frac{\mathrm{g}}{{\mathsf{\rho }}_{\mathrm{o}}}\frac{\partial \mathsf{\rho }}{\partial \mathrm{z}} }.$$

(1)

where g is the gravitational acceleration, ρ_o is the ambient water density, and ρ is the water density at depth z. The buoyancy frequency is a measure of the water column stability according to the vertical derivative of density. The larger the N, the more stabilized the water layer.

3.2.2. Backscatter Calibration to SSC and Computation of Residual Currents and Sediment Fluxes

The ADCP data analysis involved three main steps: calculating the average flow rate, calibrating the ADCP acoustic backscatter intensity to obtain SSC, and computing fluxes and annual residual currents and fluxes. The average flow rate was determined using 30-min burst averaging. To ensure data accuracy, measurements above the water surface and a few meters below the surface were excluded to avoid the impact of the acoustic side-lobe effect caused by reflections from the surface boundary. The quality-controlled and burst-averaged flow data were then transformed into a new coordinate system aligned with the tidal current ellipse. This transformation involved a 126° clockwise rotation from the original earth coordinates. The semi-major axis of the tidal ellipse, representing the dominant flow direction, was found to be directed northwest–southeast and defined as the “u” component of flow velocity. Conversely, the semi-minor axis, representing the secondary flow direction, was directed northeast–southwest and referred to as the “v” component of flow velocity. In this coordinate system, positive “u” corresponds to a northwestward current, while positive “v” corresponds to a northeastward current. Consequently, an overall northward flowing current will have both positive “u” and “v” components, while a southward flowing current will exhibit negative “u” and “v” components.

The conversion of the ADCP acoustic backscatter intensity to SSC was carried out following the methodology proposed in [22]. Before the conversion, however, two corrections were applied to address the influence of the diel vertical migration of zooplankton observed at the SORS in the Yellow Sea. The inspection of the raw acoustic backscatter intensity data revealed the presence of the diel vertical migration of zooplankton (Figure 2A,B), which has been observed in other studies conducted in the Yellow Sea [23,24]. To identify the diel vertical migration from the acoustic backscatter intensity data, spectral analysis was performed on the acoustic backscatter intensity dataset, which allowed the identification of two prominent peaks at frequencies corresponding to the vertical migration pattern (Figure 3A,B). To remove the backscatter intensity associated with the zooplankton diel migration, a bandstop filter was applied to the data at the frequencies of the identified peaks (12.19 h and 25.60 h). By filtering out the acoustic backscatter intensity at these frequencies, the influence of the zooplankton migration on the backscatter data was effectively eliminated, and the filtered data could then be used for the conversion to SSC (Figure 2C,D). The calibration results for OBS and ADCP backscatter conversion to SSC are shown in Figure 3B,C, respectively. The relatively weak correlation observed in the low SSC range for the OBS calibration (Figure 3C) can be attributed to the overall low concentrations of sediment at the study site. However, the mean relative error (MRE) of the ADCP backscatter calibration, reaching as low as 20%, appears to be good given that the MRE values for the ADCP-backscatter calibration fall within the range of 10–40% in various environments [22].

After obtaining the quality-controlled flow velocity data (u) and the estimated suspended sediment concentration (C), the sediment flux F (kg m⁻² s⁻¹) was calculated as F = u∙C. Then, the sediment flux data were integrated over time. This time, integration allowed the evaluation of the magnitude and direction of sediment transport during different seasons.

Additionally, this study aimed to investigate the residual currents in the eastern Yellow Sea, particularly during two distinct periods: the stratified summer and the well-mixed winter. To achieve this, a 72-h Lanczos low-pass filter was applied to the data. The Lanczos filter is a signal-processing technique used to extract low-frequency components from a time series while preserving the main features of the signal. By applying this filter to the data, we could isolate the low-frequency variations in the flow field, which are indicative of the residual currents.

4. Results

4.1. Atmospheric and Oceanographic Conditions

Figure 4 displays time series data for pressure, air temperature, water temperature, surface salinity, tide, wind speed, wind direction, wave height, and wave period recorded at the SORS in 2018. The atmospheric and oceanographic conditions observed at the SORS were influenced by the Asian monsoon. During the winter months, air temperatures were near 0 °C, but they rose to approximately 35 °C in the summer. As summer temperatures increased, atmospheric pressure dropped to 1000 hPa, and surface water temperatures also rose to 30 °C. On the other hand, lower water temperatures remained relatively stable and were not significantly affected by seasonal temperature changes (Figure 4E). Surface salinity values were consistent at around 34 practical salinity units (psu) throughout the year, except in the summer, when they decreased due to increased precipitation.

In terms of wind patterns, the winter witnessed relatively strong northerly winds due to the seasonal wind pattern, while the summer brought milder southerly winds. This variation in wind direction led to wave heights exceeding 3 m in winter and dropping to less than 1 m in summer. Wave periods typically remained around 10 s throughout the year. Wave direction paralleled the wind, moving north in summer and south in winter. However, in August 2018, the area was affected by Typhoon Soulik (1819), causing significant damage in South Korea. During Typhoon Soulik (1819), there was a slight drop in air pressure and a decrease in surface water temperature. Notably, the strong winds, reaching speeds of 25 m per second, generated waves exceeding 4 m in height. These conditions were accompanied by heavy precipitation, leading to a reduction in surface salinity.

4.2. Seasonal Vertical Mixing and Residual Currents

Figure 5 illustrates the profiles of water temperature, salinity, and calculated density recorded by CTD throughout 2018. However, it is important to note that due to the fouling of the salinity sensors, data from moored sensors were only available for approximately two months, starting from March 2018. Furthermore, the moored CTD sensors malfunctioned during Typhoon Soulik (1819), resulting in casting data only being available after the typhoon’s occurrence (from 24 August 2018). An analysis of the water temperature and salinity data in the water column reveals the presence of stratification from May to November. During stratification (May to November), the average surface water temperature was approximately 19 °C, while the average temperature in the bottom layer was 11 °C. However, there was little variation in average surface salinity (31.5 psu) compared to that in the lower layers (32 psu). In contrast, during periods without stratification (December to April), the average water temperature remained similar between the surface (7.5 °C) and the bottom (7 °C). Salinity also exhibited minimal differences (surface layer: 31.5 psu, bottom layer: 32 psu) compared to the stratification period. This suggests that stratification near the SORS is predominantly influenced by water temperature rather than salinity.

To assess seasonal stratification, we classified the CTD profiles into four categories: well mixed, increasing stratification, fully stratified, and decreasing stratification for the year 2018, as shown in Figure 6. For consistent comparison across tidal cycles, the CTD profiles are presented based on data observed at low tide. Before the onset of stratification (well mixed/March 25), both water temperature and salinity remained constant throughout the water column, resulting in uniform density. The buoyancy frequency calculated from the density gradient within the water layer remained at 0.01 or less, indicating low stability in the water column and suggesting potential for active energy or material exchange. Stratification began to form in the surface layer in May (Figure 5), extending to a depth of approximately 20 m by June (Increasing stratification/June 22) (Figure 6). While the difference in water temperature between the surface and bottom layers reached 11 °C, the salinity difference was less than 1 psu, resulting in a density difference of about 3 kg/m³. Buoyancy frequency at depths of 10 to 20 m, where stratification was most pronounced, reached a maximum of 0.05, indicating a stabilized water column. The fully developed stratification occurred between August and September, reaching a depth of about 30 m in September (stratification/8 September). The difference in water temperature between the surface and bottom layers increased to 13 °C, with a more significant difference between the surface and lower layers compared to the initial stratification period (May to June). However, in the case of salinity, the difference remained similar to the initial stratification, suggesting a minimal impact on overall surface and stratification development. Stratification began to gradually weaken from October to November, resulting in increasing uniformity in water temperature and density between water columns. In November (decreasing stratification/17 November), as stratification diminished, water temperature changed by approximately 4 °C at a depth of about 5 m. However, salinity remained uniform, and the density showed a difference of less than 1 kg/m³ compared to its peak during the stratified period. Consequently, it exhibited low stability in the overall water column, except for a slight increase in stability in the 5-m section with a weak density difference.

4.3. Seasonal Sediment Flux

The rotated current velocities are depicted in Figure 7, showing that current velocities are modulated with tidal motions. In the main axis, the current velocities reached up to approximately 1.5 m/s, which is stronger than the velocities in the minor axis, where they remained below 1 m/s. The sediment concentration, derived from ADCP acoustic intensity conversion, was relatively high in the lower layer, but the overall concentration averaged 0.01 kg/m³.

To assess the annual variations in current flow, the residual currents were calculated using a 72-h Lanczos low-pass filter (Figure 7). In winter and early spring (December–April), residual currents exhibited a consistent southeastward flow. This behavior indicates that residual currents flow southeasterly due to wind shear effects during the winter months when northerly or northwesterly winds prevail. This vertical flow pattern gradually shifted from surface layers to northwestward flow from May to November when stratification developed. This is associated with wind patterns shifting to the south during the summer monsoon, resulting in northwestward residual currents. In the lower layers, a two-layered circulation persisted, still moving in a southeast direction. With the disappearance of stratification from October to November, the flow returned to a vertically uniform southeastward direction. The residual current in the northeast–southwest direction displayed a similar pattern. Before stratification, there was vertically uniform southwestward flow, which transitioned to a northeastward flow with the development of stratification. This pattern reverted to a southwestward flow as stratification diminished.

Figure 8 illustrates the vertical profile of residual currents at different stratification stages. Overall, flow velocity in the northeast–southwest direction was smaller than that in the northwest–southeast direction, which is the main flow direction. In the fully mixed stage, the residual current in the northwest–southeast direction had a negative value, indicating southward flow caused by wind shear stress and resulting in a residual vertical shear. As stratification developed, the northwest–southeast residual currents shifted to a northward flow, and the residual vertical stress increased. In the fully stratified stage, a vertically oriented double-layer circulation occurred, likely influenced by wind direction changes. Finally, as stratification disappeared and winds blew from the north, the northwest–southeast flow velocity weakened, indicating a decrease in residual vertical stress. This observation suggests that the Korean coastal current exhibits a two-layered structure in the summer under the influence of stratification, with surface and lower layer flows decoupled by strong winds, causing a change in the flow direction.

In Figure 9, the cumulative water flux and sediment flux for both the surface (black) and bottom (blue) layers are presented in the main flow direction (northwest–southeast) and the minor direction. The cumulative water flux in the main flow direction exhibited distinct patterns in the surface and bottom layers. In the surface layer, the cumulative water flux was directed northwest in both the surface and bottom layers during the fully mixed phases of winter and spring. With the development of stratification, the surface layer shifted to a southwest direction, but it gradually returned to a southward flow after December. In the case of the bottom layer, the water flux remained southeastward, regardless of stratification development. The cumulative sediment flux was notably higher in the bottom layer, primarily due to the higher concentration of suspended sediment in this layer. Interestingly, the cumulative sediment flux in the main flow direction was directed southeastward in both the surface and bottom layers, demonstrating a different pattern compared to the water flux. The cumulative sediment flux in the auxiliary flow direction exhibited a similar trend to that observed in the main flow.

5. Discussion

The temporal and spatial variations in sediment transport in the Yellow Sea are influenced by various factors, including ocean currents, wind-driven flows, and seasonal variations. The strong YSWC flows northward through the middle of the Yellow Sea in winter, while the relatively weak YSCC and KCC flow southward [17,25]. During summer, the KCC flows northward, while the Southward Coastal Currents (BSCC and YSCC) flow southward. While the temporal and spatial variability in coastal currents along the Chinese coast has recently gained attention, similar investigations along the Korean coast remain limited.

In this study, the vertical and seasonal variations in water and sediment fluxes within the KCC were examined using CTD and ADCP data collected in 2018 at the SORS of Korea. Our results revealed stratification in the water column from May to November, with surface temperatures averaging 19 °C and bottom layer temperatures averaging 11 °C (Figure 4 and Figure 5). Salinity exhibited little variation in the surface layer (31.5 psu) compared to the lower layers (32 psu). Thus, stratification at SORS was primarily influenced by water temperature rather than salinity. In contrast, during non-stratified periods (December to April), water temperature and salinity remained relatively uniform between the surface and bottom layers (Figure 5). The inter-annual variation in stratification has been observed in the other part central part of the Yellow Sea and can be explained by the trapping of the surface heat fluxes in the upper layer in summer and the surface cooling and mixing in winter [26,27,28].

The current velocities revealed modulation with tidal motions, with stronger currents reaching up to approximately 1.5 m/s in the main axis compared to the minor axis, which remained below 1 m/s (Figure 5). Residual currents exhibited a fully-mixed, southeastward flow in winter and early spring, transitioning to a northwestward flow during stratification from May to November (Figure 7 and Figure 8). A two-layered circulation developed due to the stratification and persisted in the lower layers during summer, with a return to vertically uniform southeastward flow in the absence of stratification (Figure 7). The northeast–southwest residual currents displayed similar patterns, shifting from southwestward to northeastward flow with stratification and reverting with its disappearance (Figure 7B). The residual currents in the surface layer were directed northwest due to the dominant wind shear, while the baroclinic pressure gradient force due to the strong tidal-induced temperature front drove the bottom residual current in a southeasterly direction along the Korean coast [29]. Therefore, the cumulative water flux is directed northwestward at the surface layers, while it is southeastward at the bottom layers (Figure 9A,B). By inspecting the output of the Lanczos low-pass filter and comparing the behavior of the residual currents during the stratified summer and well-mixed winter periods, we could gain valuable insights into the seasonal variability in the flow patterns. Understanding how residual currents change with the seasons is crucial for comprehending the overall circulation patterns in the eastern Yellow Sea and how sediment transport is influenced by the changing oceanographic conditions.

The sediment flux exhibited distinct patterns, both in the surface and bottom layers, flowing southward, regardless of seasons (Figure 9C,D). Also, it was notably higher in the bottom layer because the sediment concentration was relatively higher, averaging 10 mg/L, while it was low about 4 mg/L in the surface layer (Figure 2D). The depth- and time-integrated sediment flux was 0.04 Mt/yr to the south. Although this value is about 5–10 times smaller than the sediment flux (0.17~0.4 Mt/yr) at the Bohai Strait [30], it is still significant as no large rivers contribute sediment along the Korean coast. The sediment flux is mainly driven by the southward currents in winter along the Korean coast and contributes to the sediment transport in the Yellow Sea [31]. Since the sediment transport in the Yellow Sea shows seasonal characteristics and spatial variations, a further study is required with respect to the regional circulation, including the Northern Shandong Coastal Current, the Yellow Sea Coastal Current, and the Yellow Sea Warm Current.

6. Conclusions

While recent attention has been paid to the temporal and spatial variability in coastal currents along the Chinese coast, investigations along the Korean coast remain relatively limited. This study specifically delved into examining the vertical and seasonal shifts in water and sediment fluxes within the KCC. Atmospheric and oceanographic data sets collected at the SORS of Korea in 2018 provided insights. From May to November, rising water temperatures in the upper layer prompted water column stratification at SORS, giving rise to a distinctive two-layered flow structure. The residual current of the surface layer exhibited the seasonal pattern, flowing north in summer and south in winter, consistent with prior observations. However, it consistently flowed southward in the deeper layer, irrespective of the season. Similarly, sediment fluxes consistently headed south, regardless of the depth and season. The temporal and seasonal variability in sediment transport in the Yellow Sea represents strong dynamic processes influenced by various seasonal factors, such as winter storms, river discharge, wind forcing, and intra-tidal variations. These observations notably diverge from those of previous studies, shedding new light on ocean currents and material circulation within the Yellow Sea. Deeper comprehension of these variations is essential for a comprehensive understanding of sediment transport dynamics and their impact on the geomorphology and environmental conditions of the Yellow Sea. We recommend further research to collect a new set of flow and SSC data, as well as investigate the mechanisms governing flow circulation and sediment transport along the KCC and YSWC.