Seasonal and interannual variations in material transport in the Korea Strait originating from the Taiwan Strait

The Tsushima Warm Current (TSWC), which flows through the Korea Strait (KS), is crucial in transporting nutrient-rich, warm, and saline water to the East/Japan Sea. Understanding the origins and variability of the TSWC is essential because of its significance in Northwestern marginal seas. Although previous studies have suggested that the Taiwan Strait (TS) partially originates from the TSWC, the seasonal and interannual variability in the material connectivity between the TS and KS remains poorly understood. In this study, we investigated this variability using a Lagrangian particle-tracking system and a three-dimensional numerical model. The model results showed that particles originating from TS passed through KS most frequently in August. Furthermore, particles traveling from the TS to KS exhibited distinct interannual variability. Composite analysis indicated that southerly winds increased sea surface height (SSH) in the southwestern East China Sea (ECS) shelf region via surface Ekman transport, weakening cross-shelf offshore currents and preventing particles from being transported offshore. Empirical Orthogonal Function (EOF) analysis indicated that the interannual variability of southerly winds over the ECS was associated with variations in SSH in the southwestern shelf region, thereby influencing material transport from the TS to the KS.

The sea surface height (SSH) variations observed by the TOPEX/POSEIDON satellite in the Yellow and East China Seas are examined over the 3 years from 1993 through 1995 assuming a linear response to wind stress across the region. The transfer function relating wind stress to SSH is modeled by a series of increasingly complex functions that gradually allow wind stress over successively broader regions and time periods to influence the SSH solution at a particular point. The SSH response to instantaneous wind stress at the same point in space implies an Ekman transport, but the response explains only a small fraction of the SSH variability. Relating SSH to wind stress averaged over two subregions and including both instantaneous and 12‐hour lagged wind stress indicates that a significant portion of SSH variability responds to winds that are remote in both space and time. An optimal estimation of the transfer function by minimizing the error variance leads to an extended empirical orthogonal function (EEOF) analysis of the wind stress field. The SSH response to each wind mode is determined. The analysis shows the principle variations in the wind stress to consist of northerly wind bursts during winter, which produce large SSH drops in the Bohai Bay and northern Yellow Sea region. The principal wind mode also is significantly related to variations along the shelf break near the Kuroshio Current. The third and fourth wind stress modes indicate the passage of typhoons.

This study investigates the sea surface height (SSH) and current responses to synoptic winter wind in the Bohai, Yellow, and East China Seas (BYES) using a numerical model. The basin‐scale responses are primarily interpreted as two leading coastal trapped waves (CTWs) identified through the complex empirical orthogonal function analysis of SSH. One CTW is a Kelvin wave (KW) explaining 76% of the total SSH variance; the other is a wind‐forced lowest shelf wave (SW1) explaining 17% of the total SSH variance. The KW dominates the current in the Bohai and Yellow Seas (BYS), and the SW1 dominates the current in the western East China Sea (ECS). The KW is a free relaxation response to large wind‐driven SSH variations and propagates cyclonically in the BYES around one SSH node. The SW1 propagates cyclonically in the semienclosed BYS around two nodes and southward along the western ECS with two nodes on the shelf. The wavelength and e‐folding scale of amplitude vary spatially, producing regionally distinct features and mechanisms. In the semienclosed BYS, the associated geostrophic current is along the cophase line of SSH, and its magnitude is proportional to both wave amplitude and wavenumber. In the western ECS, the associated geostrophic current is perpendicular to the cophase line of SSH, and its magnitude is proportional to the wave amplitude and inversely proportional to the e‐folding scale of amplitude. This study reveals not only the basin‐scale SSH and current variations in the BYES but also new features of CTWs in a semienclosed embayment.

Two types of models are used to examine the sea surface height (SSH) variability at the principle short time frequency bands in the Yellow and East China Seas. The models are a primitive equation numerical model and a statistical model based on TOPEX/POSEIDON altimeter data. In situ pressure gauge measurements at two mooring locations are first used to evaluate each model. In addition, a comparison of the numerical and statistical models is made through duplicate analysis of the response to the main extended empirical orthogonal function wind stress modes. In the northern Yellow Sea the wind stress, pressure gauge data, numerical model, and statistical model all indicate local spectral maxima in bands centered on the weekly and semiweekly periods. The coherence of the two models with the in situ data is significant down to 3‐day periods at the northern mooring and down to 5‐day periods at the southern mooring. The meridional wind stress variability in the semiweekly band is higher than the weekly band energy in the northern Yellow Sea and Bohai Bay and lower than the weekly band energy across the East China Sea. The SSH variability contained in the weekly and semiweekly bands is largest in the Bohai Bay and northern Yellow Sea with a region of high energy along the Chinese coast. The spatial structure of the weekly and semiweekly SSH variability is similar to the wind stress variability at these bands with semiweekly variability higher in the Bohai Bay and northern Yellow Sea and weekly variability dominant in the East China Sea.

Mean sea surface heights of the South and East China Seas from ocean circulation model and geodetic leveling

The influence of climate regime shifts on the marine environment and ecosystems in the East Asian Marginal Seas and their mechanisms

Future global-mean warming and its intermodel spreads have shown to be greater in the coupled model intercomparison project phase 6 (CMIP6) models than in the earlier generation CMIP5 models, mainly due to increases in both forcing and climate sensitivity. However, regional future changes and their intermodel difference in CMIP6 models have been less known. In this study, we assessed biases in the sea surface temperature (SST) simulated from 30 CMIP6 models and then estimated future SST changes in the near-term (2021-2040) and the mid-term(2041-2060) periods under the scenarios of the low (SSP1-2.6) and high (SSP5-8.5) emissions, by using selected eight CMIP6 models with superior performance in the East Asian Marginal Seas (EAMS). The SST changes in the EAMS are projected to be more pronounced in the mid-term compared to the near-term under global warming. SST is expected to increase by approximately 1.8°C under SSP1-2.6 and by 4.5°C under SSP5-8.5, with the largest increases occurring in the Yellow Sea, East Sea, and East China Sea. Seasonal variations are significant, with summer (August) warming projected to be about 50% greater than winter (February) warming under SSP5-8.5, primarily due to the shallowing of the summer mixed layer. Uncertainty in future SST projections is higher under SSP1-2.6 than SSP5-8.5 because of the stronger signal in the SSP5-8.5 scenario. Spatially, uncertainty is lower in the coastal areas and the Yellow Sea, but higher in the East Sea and the mixed water region (interfrontal zone) between the Kuroshio and Oyashio currents, where ocean currents contribute to greater intermodel variation. In conclusion, the projected future surface warming in the East Asian Marginal Seas (EAMS) is estimated to be approximately twice the global mean, indicating that EAMS is a climate change hotspot with a high level of vulnerability to future global warming.

The East Asian marginal seas (EAMS) are one of the fastest‐warming ocean regions globally. This study presents the long‐term trends (1982–2020) of extreme ocean warming events called “marine heatwaves” over the EAMS and examines the relationships between marine heatwave trends and mean SST warming trends. We focus on five subregions with different influences from atmospheric perturbation and ocean currents: the northern East Sea (N‐ES), southern East Sea, Yellow Sea, Korea Strait (KS), and East China Sea (ECS). During the past four decades, marine heatwave duration and intensity in the EAMS have increased to approximately +4 days and +0.3°C per decade on average, respectively. In summer, the positive trend of marine heatwaves is the highest in the ECS, primarily due to the rapidly increasing mean sea surface temperature (SST). In winter, the N‐ES reveals remarkably rapid increases in marine heatwave properties in the last two decades, with increasing rates of approximately 6.2 (4.9) times longer total duration (stronger intensity) than the global average changes. Beyond the impact of the rapid increase in mean SST, the N‐ES marine heatwaves can be further extended due to the northward shift of the East Korea Warm Current. In general, mean SST changes are critical to the increasing trend in marine heatwave duration and intensity. This study further emphasizes that the changes in ocean circulation may expedite more rapid changes in extreme ocean events, which can produce more vulnerability in some places, such as the N‐ES, to marine heatwaves under continued global warming.

Modeling sea surface height (SSH) and tides is important but challenging in shelf seas. The Eastern China Seas (ECSs) is such a shelf sea with large inter-model deviations in prior studies. In order to assess and compare the possible uncertainty sources, numerical scenarios of varying the open boundary forcing, bottom roughness length scale, atmospheric forcing, grid resolution, and regional bathymetry were conducted in a hydrodynamic model of the ECSs. Results indicate that bathymetry data and open boundary forcing with inadequate accuracy generate uncertainties in SSH and tides locally and throughout the basin. An increase in bottom roughness enhances tidal dissipation and shifts amphidromes to the left relative to the incoming Kelvin waves, causing SSH variations in the ECSs. Refining the model resolution from 4 to 2 km mainly affects nearshore SSH and tides due to minor changes in depicted coastlines. Using different reanalysis meteorological data appears more important on the episodic than annual scale. It is highlighted that some uncertainty sources have opposing effects on SSH or tides and counteract their individual biases, making it difficult to achieve a realistic simulation. For example, increasing bottom roughness can not only compensate effects of overestimated tidal amplitude at open boundaries, but also balance out the overestimated M2 phase along the West Coast of Korea in a coarser-resolution model. Based on findings in this study, suggestions are provided for further reducing uncertainties in SSH and tide modeling in the ECSs and other shelf seas.

Kim C.-H.; Jang C. J., and Kim M.-W., 2018. A Numerical Simulation of Long-Term Sea Level Change in the East Asian Marginal Seas. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 546–550. Coconut Creek (Florida), ISSN 0749-0208. To investigate the regional sea level rise (SLR) and its possible mechanisms in the East Asian Marginal Seas (Yellow Sea (YS), East China Sea (ECS) and Japan/East Sea (JES)) sea level variation was simulated using the global ocean-sea ice coupled model (GFDL MOM5-SIS) forced with European Centre for Medium-range Weather Forecasts reanalysis data for 58 years (1958–2015). Seasonal variation in the simulated sea level shows comparable amplitude and phase to those of the satellite altimetry for 1993–2014 in each sea region. Rates of the simulated SLR are compared with those of the reconstructed sea level (Hamlington et al., 2011) ...

A halo‐thermal, reduced‐gravity model with four active layers is used to investigate how interannual rainfall anomalies affect sea surface height (SSH) variability in the Indian Ocean. The model is forced by monthly varying winds observed over the period 1980–2000 in two experiments that differ by their rainfall forcing, Run FSU and Run Arkin, forced by climatological and interannually varying rainfall, respectively. Compared to the large impact of wind on SSH (about 30 cm), the impact of rain is much smaller. Its maximum (found in the southeastern Indian Ocean during the rainfall deficits of 1994 and 1997) is only 2 cm. Because rainfall significantly affects model salinity and temperature, the deficits make the layers of Run Arkin colder and saltier than in Run FSU, causing a −5 cm change in sea level. Baroclinic adjustments also occur such that the top (bottom) two layers are thicker (thinner), increasing sea level by 3 cm and hence significantly reducing the SSH change due to steric effects alone. SSH variability in either Run Arkin or Run FSU compares very well with TOPEX data. Although the impact of rainfall on SSH is negligible, salinity variations significantly affect dynamic‐height calculations of SSH. In the model, the neglect of salinity variations leads to an error of 5 to 10 cm along the eastern boundary, in the Bay of Bengal, and in the interior ocean south of 8°S. This error is validated by the difference between TOPEX data and SSH derived from observed temperature profiles.

On the observed annual gravity variation and the effect of sea surface height variations

As a ubiquitous movement in the ocean, tides are vital for marine life and numerous marine activities such as fishing and ocean engineering. Tidal dynamics are complicated in the East Asian marginal seas (EAMS) due to changing complex topography and coastlines related to human activities (e.g., land reclamation and channel deepening) and natural variability (e.g., seasonal variations of ocean stratification and river flow). As an important tool, numerical models are widely used because they can provide basin-scale patterns of tidal dynamics compared to point-based tide gauges. This paper aims to overview the development history of the numerical simulation of tides in the EAMS, including the Bohai Sea, the Yellow Sea, the East China Sea, the East/Japan Sea, and the South China Sea, provide comprehensive understanding of tidal dynamics, and address contemporary research challenges. The basic features of major tidal constituents obtained by tidal models are reviewed, and the progress in the inversion of spatially and temporally changing model parameters via the adjoint method are presented. We review numerical research on how a changing ocean environment induces tidal evolution and how tides and tidal mixing influence ocean environment in turn. The generation, propagation, and dissipation of internal tides in the EAMS are also reviewed. Although remarkable progresses in tidal dynamics have been made, nonstationary tidal variations are not fully explained yet, and further efforts are needed. In addition, tidal influences on ocean environment still receive limited attention, which deserves special attention.

Interannual and interdecadal variations of sea surface temperature in the East Asian Marginal Seas

Information about wind variations and future wind conditions is essential for a monsoon domain such as the Northwest Pacific (NWP) region. This study utilizes 10 Generalized Circulation Models (GCM) from CMIP6 to evaluate near-future wind changes in the NWP under various climate warming scenarios. Evaluation against the ERA5 reanalysis dataset for the historical period 1985–2014 reveals a relatively small error with an average of no more than 1 m/s, particularly in the East Asian Marginal Seas (EAMS). Future projections (2026–2050) indicate intensified winds, with a 5–8% increase in the summer season in the EAMS, such as the Yellow Sea, East Sea, and East China Sea, while slight decreases are observed in the winter period. Climate mode influences show that winter El Niño tends to decrease wind speeds in the southern study domain, while intensifying winds are observed in the northern part, particularly under SSP5-8.5. Conversely, summer El Niño induces higher positive anomalous wind speeds in the EAMS, observed in SSP2-4.5. These conditions are likely linked to El Niño-induced SST anomalies. For the application of CMIP6 surface winds, the findings are essential for further investigations focusing on the oceanic consequences of anticipated wind changes such as the ocean wave climate, which can be studied through model simulations.