Yellow Sea Research Articles

Extent and Sources of Heavy Metal Pollution from Discharging Rivers in the Bohai Region, China

Studies have investigated heavy metal (HM) contamination in the Bohai Sea, but primarily in seawater and associated sediments, or in single rivers. For the first time, 31 major rivers discharging into the Bohai Sea were analyzed, along with 27 uniformly distributed coastal seawater samples and selected invertebrates. The elements measured were As, Cd, Cr, Cu, Ni, Pb, V, and Zn. We calculated the ‘geo-accumulation index’, the ‘metal enrichment factor’, and the ‘contamination factor’, coupled with the ‘pollution load index’, and our findings suggested low-grade HM pollution, although two conspicuous associations of elements were found to stand out in particular: One is a combination of As, Cu, Cr, and V in seawater samples that may indicate pollution from intensive ship traffic. The other shows a significant pattern of Cr, Pb, and Zn in water samples from rivers discharging between Yantai and Weihai on the Shandong Peninsula at the south edge of the Bohai Sea. This is primarily a farming area, with a moderate share of industrial enterprises. Investigations including fertilizers and pesticides point to agricultural practices and textile printing/chrome tanneries as the causes of contamination. Overall, a significant decline was found in the HM load in the rivers, apart from those discharging into the Yellow Sea section.

Intraseasonal movements between staging sites by migrating great knots: Longer distances to alternatives decrease the probability of such moves

Distance is a key constraint for animals in moving between suitable habitats, but is this also the case in staging long-distance migrating shorebirds that habitually cover thousands of kilometers during migrations? We conducted multi-year field observations, benthic prey sampling and satellite tracking, to compare how endangered great knots Calidris tenuirostris respond to the food shortage at two similarly functioning staging sites (Gaizhou and Beijingzi) in the northern Yellow Sea, China. Food availability declined by >95 % at both sites across the study period, with the intake rates of great knots declining by 87 %. However, whereas the number of great knots declined by 91 % at Gaizhou, only a 29 % decrease was seen at Beiijngzi. Satellite tracking showed that during the time when food was poor in Gaizhou, tagged great knots crossed 20 km to suitable alternative high-quality sites where food was not scarce. From Beijingzi, tagged great knots flew at least 124 km to find a good alternative. We show that longer distances to alternative sites decreased the probability of a bird leaving. Thus, habitat degradation in staging sites induced great knots to move to alternative sites, but only if such alternatives were relatively close. As staging habitats become more isolated, the negative effects of habitat degradation will be more serious due to a distance constraint on exploratory movements. This emphasizes the importance of maintaining networks of nearby high-quality refueling sites for migratory birds to provide buffers in seasons when local food conditions are lean.

Distribution of Permian and Triassic strata in the Laoshan Uplift, Southern Yellow Sea, using seismic attributes

The Laoshan Uplift (LSU) in the South Yellow Sea Basin of the Lower Yangtze Plate is a potential site for hydrocarbon exploration; thus, determining the strata attributes and distribution is imperative. Tracking a wide range of seismic reflection horizons for lateral comparisons is challenging because the first well (CSDP-2) encountered in the Paleozoic strata of the LSU is located in a structurally complex deformation zone and has yielded poor seismic data quality. In this study, we present a novel approach to seismic horizon tracking in areas with few or no wells and adopt a cross-regional analogy and inverse analysis method to track the seismic horizon. Seismic attribute characteristics of the marine strata of the LSU were compared with the seismic horizons calibrated in the Wx1 well in the stable Qingdao Depression (QDD), and horizontal tracking was performed. An inverse analysis was then performed on the CSDP-2 well area to predict the distribution of marine strata in the LSU. The results revealed that the Permian and Triassic strata of the LSU and QDD have similar seismic attributes. Finally, the seismic horizon of the LSU was tracked horizontally based on a comparison of the seismic attributes of the LSU and QDD. Previous studies showed that seismic reflection horizons are difficult to calibrate, and the marine strata attributes in the LSU remain controversial. This study adopted a cross-regional comparison and inversion analysis method to perform a seismic multi-attribute comparison based on the calibrated results of the QDD for the Wx1 well. The remaining Permian and Triassic strata lie almost entirely within the LSU. Moreover, the maximum reported thicknesses of the Upper Permian and Lower-Middle Triassic strata in the LSU were approximately 500 and 2750 m, respectively. Our findings will aid in the exploration and evaluation of early hydrocarbon resources in the LSU.

Two new diatom species of the genus Gomphonemopsis (Bacillariophyceae) from the coast of China and two new combinations for the genus.

Two new diatom species belonging to the genus Gomphonemopsis are described, Gomphonemopsisnanasp. nov. and Gomphonemopsisgaoisp. nov. These two species were compared in detail with congeners. Gomphonemopsisnana is distinguished by its high stria density and small size. This species was found so far to be epiphytic only on the eelgrass collected from Qingdao Bay (Yellow Sea). Gomphonemopsisgaoi is characterized by its isopolar valves, simple proximal raphe endings and acutely rounded apices. This taxon was separated from the exoskeleton of marine copepods sampled from the Futian Mangrove Nature Reserve (South China Sea). In addition, two new combinations, Gomphonemopsisoahuensis (Hustedt) Lang Li, Yuhang Li & Changping Chen, comb. nov. and Gomphonemopsisplatypus (Østrup) Lang Li, Yuhang Li & Junxiang Lai, comb. nov. are proposed. This study increases the records and knowledge of Gomphonemopsis along the coast of China.

Projections of climate-driven biogeographical changes of benthic mollusks in the Yellow Sea and East China Sea

Climate change influences the distribution of many marine species. To project the biogeographical changes of benthic mollusks in response to climate change in the Yellow Sea and East China Sea, ensemble species distribution models (SDMs) were applied. Ensemble SDMs performed well for ten of the thirteen selected benthic mollusks with environmental variables including temperature, salinity, current velocity, and depth. Six cold water mollusks, including bivalves Acila mirabilis, Ennucula niponica, Ennucula tenuis, Nuculana yokoyamai, Pendaloma otohimeae and Megayoldia japonica, were projected to contract their habitats and move northward in 2050s and 2100s under all of the RCP2.6, 4.5, 6.0 and 8.5 climate scenarios, with temperature being the most important environmental variable. Two warm water mollusks (bivalves Nucula tokyoensis and Leptomya minuta) were projected to lose their suitable habitats under future climate scenarios (all RCP scenarios), while two (the gastropod Cylichna biplicata and the bivalve Moerella hilaris) were projected to expand their habitats to the deeper water area. The most important environmental variable varied among warm water species between temperature, salinity and depth. This study will contribute to better understanding the marine species biogeographical changes under climate change, and thus we can better protect their biodiversity.

Spatiotemporal dynamics of bacterioplankton communities in the estuaries of two differently contaminated coastal areas: Composition, driving factors and ecological process

Seasonal variations of environmental parameters usually lead to considerable changes in microbial communities. Nevertheless, the specific response patterns of these communities in coastal areas subjected to different levels of contamination remain unclear. Our results revealed notable fluctuations in the bacterioplankton community both seasonally and spatially, with seasonal variations being particularly significant. The diversity and composition of bacterioplankton communities in the estuaries varied significantly across seasons and between seas. Some bacterial phyla that were highly abundant in the dry season (e.g., Patescibacteria and Epsilonbacteraeota) were almost absent in the wet season. Furthermore, the network analysis revealed that the bacterioplankton networks were more complex during the wet season than in the dry season. In the wet season, the estuarine bacterioplankton network in the Yellow Sea region was more complex and stable, while the opposite was true in the dry season. According to the neutral community model, stochastic processes played a more significant role in the formation of bacterioplankton communities during the wet season than during the dry season. Estuarine bacterioplankton communities in the Yellow Sea region were more affected by stochastic processes compared to those in the Bohai Sea. In summary, in the estuaries of two differently contaminated coastal areas, the seasonal increase in nutrient levels enhanced the deterministic processes and network complexity of the bacterioplankton communities.

A climatological analysis of northward‐moving typhoon in environments of the Northeast China cold vortex

AbstractThis study investigates the influence of the Northeast China cold vortex (NCCV) on the northward‐moving typhoons (NTCs) over the western North Pacific (WNP). There is a significant inverse relationship between the NCCV during June–September and simultaneous NTCs during 1981–2021. Fewer (more) NTCs are observed during NCCV active (inactive) year a combination of less (more) NTC genesis, particularly over the central Pacific region of 10°–30° N and 130°–150° E, and fewer (more) NTCs moving northwestward and making landfall in coastal regions of the Yellow Sea and Bohai Sea. These regions are characterized by significantly decreased low‐level vorticity and mid‐level humidity, which impedes NTC genesis and notably enhances the deep‐layer subtropical straight westerly steering flow, thus blocking the northward movement of NTCs. These remarkable environmental changes during different NCCV years are clearly linked with the changes of an anomalous anticyclone in the subtropics (20°–30° N, 120°–160° E). In short, more (less) NCCV activity strengthens (weakens) the anomalous anticyclone, resulting in fewer (more) NTCs.

Characteristics, distribution patterns and sources of atmospheric microplastics in the Bohai and Yellow Seas, China

Although the prevalence of microplastics in the atmosphere has recently received considerable attention, there is little information available regarding the distribution of atmospheric microplastics over oceanic regions. In this study, during the summer and autumn months of 2022, we investigated atmospheric microplastics in four marine regions off the eastern coast of mainland China, namely, the southern, middle, and northern regions of the Yellow Sea, and the Bohai Sea. The abundance of atmospheric microplastics in these regions ranged from 1.65 to 16.80 items/100 m3 during summer and from 0.38 to 14.58 items/100 m3 during autumn, although we detected no significant differences in abundance among these regions. Polyamide, chlorinated polyethylene, and polyethylene terephthalate were identified as the main types of plastic polymer. On the basis of meteorological data and backward trajectory model analyses, we established that the atmospheric microplastics detected during summer were mainly derived from the adjacent marine atmosphere and that over the continental landmass in the vicinity of the sampling area, whereas microplastics detected during autumn appear to have originated mainly from the northeast of China. By influencing the settlement and migration of microplastics, meteorological factors, such as relative humidity and wind speed, were identified as potential factors determining the distribution and characteristics of the detected microplastics. Our findings in this study, revealing the origin and fate of marine atmospheric microplastics, make an important contribution to our current understanding of the distribution and transmission of microplastics within the surveyed region and potentially worldwide.

Assessing the predictability of the marine heatwave in the Yellow Sea during the summer of 2018 based on a deterministic forecast model

Understanding the predictability of marine heatwaves (MHWs) and identifying the sources of their forecast errors are essential for enhancing their forecast accuracy. In the summer of 2018, a powerful MHW struck the Yellow Sea, resulting in significant economic losses for the sea cucumber culture industry in China's coastal areas. However, the ability to predict the evolution of this MHW remains uncertain. In this study, several forecast experiments were conducted based on a deterministic ocean forecast model to address this issue. The results demonstrate that this MHW can be effectively predicted with a lead time of less than 3 days. Specifically, the mean MHW forecast accuracy is 0.66 and the mean absence/presence accuracy is 0.79 at a 3-day lead time. Beyond a 3-day lead time, the MHW forecast accuracy steadily decreases, which is primarily due to the overpredicted “False Alarms” during its growth and decay phases. The overpredicted “False Alarms” are largely attributed to uncertainties in predicting wind and air temperature related to two typhoons passing through the Yellow Sea. Additionally, anomalous ocean circulation induced by atmospheric forcing uncertainties may also trigger MHW forecast errors through advection. Future efforts involving parameter optimization, air-sea coupling, ensemble forecasts and integration with artificial intelligence-based weather forecasts are suggested to improve the prediction of MHWs. Our findings may provide implications for stakeholders in preparation for any future occurrences of MHWs in the Yellow Sea.

Near-Future Projection of Sea Surface Winds in Northwest Pacific Ocean Based on a CMIP6 Multi-Model Ensemble

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.

Significant CO2 emission in the shallow inshore waters of the southeastern Yellow Sea in 2020

This study investigated the carbonate system and air-sea CO2 exchange in the inshore waters along South Korea's western coastline in 2020. Overlooking these waters might introduce significant errors in estimating air-sea CO2 fluxes of the southeastern Yellow Sea, given their interaction with land, offshore regions, and sediments. During periods other than summer, seasonal variations in seawater CO2 partial pressure (pCO2) could be generally explained by thermal effects. Tidal mixing and shallow depths resulted in weaker stratification-induced carbon export compared to offshore regions. However, during summer, inshore waters exhibited high spatial variability in pCO2, ranging from approximately 185 to 1000 μatm. In contrast to offshore waters that modestly absorbed CO2, inshore waters shallower than 20 m emitted ∼100 Gg C yr−1 to the atmosphere. However, considering the high heterogeneity of the study area, additional observations with high spatial and temporal resolution are required to refine estimates of air-sea CO2 exchange.

Impacts of Northward Typhoons on Autumn Haze Pollution Over North China Plain

AbstractAlthough air quality in China has improved substantially over recent years, haze pollution events still occur frequently, especially over the North China Plain (NCP). Previous studies showed that typhoons are conducive to regional pollution events in eastern China; however, the underlying mechanism and quantitative understanding of the typhoons' impact on haze pollution remain unclear there. Here, based on ground‐based and satellite observations, reanalysis data, and model simulations, we show that northward typhoons approaching China are essential for autumn haze pollution over NCP. Elevated relative humidity levels and enhanced pollution accumulation, caused by northward typhoons and the corresponding high‐pressure systems, are responsible for the pollution enhancements over NCP. Compared with episodes without typhoon influence, cities near Taihang and Yan Mountain suffer from heavier haze pollution when typhoons approach, with PM2.5 concentrations increasing from 87.1 to 106.4 μg m−3. More water vapor from the Yellow and Bohai Seas and pollutants from eastern China are transported to these cities by typhoon‐induced southeasterly wind anomalies, facilitating the chemical formation of aerosols there. In addition, by the block of mountains, these southeasterly wind anomalies also lead to stronger local accumulation over cities and an elevation of pollutants along the mountains. What is more, with the implementation of emission reduction, the relative changes of PM2.5 concentrations between typhoon‐induced episodes and no‐typhoon episodes increase. This work highlights the importance of understanding the impact of synoptical weather on PM2.5 transport, accumulation, and formation processes in haze pollution mitigation in eastern China.

Ocean acidification and desalination increase the growth and photosynthesis of the diatom Skeletonema costatum isolated from the coastal water of the Yellow Sea

Global climate changes induce substantial alterations in the marine system, including ocean acidification (OA), desalination and warming of surface seawater. Here, we examined the combined effects of OA and reduced salinity under different temperatures on the growth and photosynthesis of the diatom Skeletonema costatum. After having been acclimated to 2 CO2 concentrations (400 μatm, 1000 μatm) and 2 salinity levels (20 psu, 30 psu) at temperature levels of 10 °C and 20 °C, the diatom showed enhanced growth rate at the lowered salinity and elevated pCO2 irrespective of the temperature. The OA treatment increased the net photosynthetic rate and biogenic silica (Bsi) contents. Increasing the temperature from 10 to 20 °C raised the net photosynthetic rate by over twofold. The elevated pCO2 increased the net and gross photosynthetic rates by 20%–40% and by 16%–32%, respectively, with the higher enhancement observed at the higher levels of salinity and temperature. Our results imply that OA and desalination along with warming to the levels tested can enhance S. costatum's competitiveness in coastal phytoplankton communities under influence of future climate changes.

Effect of hydrodynamic and ecosystem conditions on persistent organic pollutant temporal-spatial variations in the Yellow Sea

Coastal seas are important pools of persistent organic pollutants (POPs) discharged from land. Considering the complex conditions in coastal seas and various biochemical features of POPs, special temporal-spatial variations in POPs have been reported. To understand these variations, we developed a three-dimensional hydrodynamic–ecosystem–POP coupled model and applied it to the Yellow Sea. We selected two POP species (polychlorinated biphenyl congener 153 (PCB-153) and decabromodiphenyl ether (BDE-209)), which have different biochemical properties, as target materials. The dissolved PCB-153 simulated concentration was high in late spring and low in autumn, whereas that of BDE-209 was high in summer and low in winter. Both PCB-153 and BDE-209 showed high particle-bound concentrations in early spring. In summer, dissolved PCB-153 accumulated at the sea bottom, whereas dissolved BDE-209 accumulated at the sea surface. Seasonal and spatial variation differences in the two POPs are likely caused by greater Henry’s Law Constant (H′) and bioconcentration factor (BCF) of PCB-153 than that of BDE-209, which leads to higher volatilization and stronger absorption by the particles for PCB-153 than BDE-209. As a component of such differences, the “biological pump” of PCB-153 in the central Yellow Sea is more apparent than that of BDE-209.

Enhanced Fenton-like catalysis through in-situ bismuth defection in Bi/CuBi2O4: Vacancy mechanisms

Alleviating the concentration of hazardous bacteria by photocatalysts is an efficient way for treating ballast water. In this work, we present the synthesis of in-situ Bi vacancies on Bi/CuBi2O4, which is assisted by H2O2 for Fenton-like photocatalytic potential in deactivating marine microorganisms in natural seawater (Yellow Sea, China), the optimal Bi vacancy content (3BCBO) is determined and exhibits superior photocatalytic inactivation performance, additional H2O2 activation enhances the production of •OH for comprehensive oxidation, with •OH confirmed as the dominant reactive species. Density functional theory (DFT) calculations highlight the synergistic of vacancies and metallic Bi play a pivotal role in decreasing the energy barrier of H2O2 adsorption for •OH improving production, promotes efficient separation of charge carriers and utilization of visible light, the co-effect of metallic Bi nanoparticles and bismuth defection mechanisms effectively inhibit the madness thriving of marine microorganisms, this work offers an ideal and promising perspective for ballast water treatment on ships.

Identification and simulation the response of storm-induced coastal erosion in the China Yellow sea

Storm events have a significant impact on coastal topography, which can subsequently lead to coastal erosion and ecological deterioration. Traditional field measurements are affected by environmental factors and consume substantial resources, making it difficult to conduct large-scale terrain, hydrodynamic and other monitoring tasks. Significant challenges still remain to identify the response of coastal processes to storm events due to the lack of field measured data. In this study, the response of coastal morphodynamical variations to extreme storm events was simulated by the coupled XBeach and FVCOM models and the storm-induced coastal evolution was evaluated in the China Yellow Sea. The costal morphodynamical variations under different wind conditions were also quantified. The results reveal that erosion dominated the supratidal zone while accretion mainly occurred in the subtidal zone. Comparing the coastal morphology before and after the storm, we found the alongshore currents were the primary factors influencing sediment transport. The degree of coastal erosion and accretion shift was positively correlated with the wind speed and direction. The present nested model may serve as a valuable tool for predicting future coastal erosion and accretion, contributing to more scientifically informed coastal management decisions. The study also provides a scientific basis for understanding the response mechanism of coastal morphodynamical process to extreme storm events and coastline evolution under global change.

An evaluation of microplastic contamination in the marine waters and species in the coastal region of the South Yellow Sea, China

Microplastics (MPs) contamination of marine environments poses a significant ecological risk, although impacts on species’ realized niche spaces remain unclear. The current study investigates MPs distribution across pelagic habitats, benthic sediments, and key biota in the South Yellow Sea, China. Samples were collected via trawling across estuarine transects, and tissues were digested to extract MPs. Density gradient separations and vacuum-filtrations prepared particle extracts for ATR-FTIR and Micro-Raman spectroscopic characterization. Sampling along industrialized river transects reveals ubiquitous plastic particle presence, with concentrations ranging from 0 to 51.68 item/L seawater. Contamination levels reach their peak at station estuaries before dispersing offshore, indicating significant waste stream inputs. Importantly, MPs detected in demersal and pelagic fish species, as well as in bivalves, confirm exposure across trophic niches. Gastrointestinal tract and gill concentrations reached 0.6 items/g fresh tissue, reflecting significant biological uptake and in vivo retention. The greatest population of organisms occurred adjacent to polluted areas. Overall, distribution of MPs from polluted rivers to coastal food webs was evident, suggesting potential negative impacts on key ecological functions in this system. These findings underscore the need to develop upstream mitigation efforts so as to minimize MPs contamination in areas where nearshore and offshore niches intersect.

Factors influencing the intensity of cross-front transport: An example from the offshore transport around the Shandong Peninsula, China

Cross-front transport is a vital process in the offshore transport of terrestrial materials. The factors influencing cross-front transport are multifaceted and exhibit regional variability. As important factors regulating material transport patterns in the shelf seas of eastern China, the East Asian Winter Monsoon (EAWM) and Kuroshio current (including its branches) vary on multiple time scales, yet their role in controlling offshore transport is not fully understood. In this paper, cross-front offshore transport in the North Yellow Sea is chosen to investigate the aforementioned issue using numerical simulation. By excluding other potential processes for offshore transport, frontal instability is identified as the primary mechanism for offshore transport, which is triggered by winter storms (bursts of the EAWM); hence, the intensity of offshore transport is highly associated with the strength of winter storms on synoptic scales. However, on the inter-annual scale, the intensity of offshore transport is more likely regulated by the strength of the Kuroshio current, and a warming Kuroshio facilitates offshore transport. Although the role of each factor in controlling the intensity of offshore transport is delineated, the interactive effects of the EAWM and Kuroshio current on offshore transport are quite intricate, and how to quantitatively estimate their roles on multiple time scales remains a challenge using modelling. The results obtained from this study can be applied to analyse cross-front transport throughout the eastern China seas and bear significant implications for future studies on pollutant diffusion, nutrient distribution, and sediment transport in the coastal area.

Mapping the potential spatial distribution for offshore finfish aquaculture in China under climate change

Offshore aquaculture plays an important role in alleviating the pressure of coastal aquaculture and promoting the sustainable development of fisheries. And changes in ocean conditions caused by climate change will have a huge impact on mariculture. Considering the conditions of climate change, this study took the main mariculture fishes in China as the research object, used a species distribution model (SDM) to predict the potential mariculture areas of each fish under the current and different future (2040–2050) climate scenarios, divided the mariculture areas into three potential levels by natural breaks classification method, and calculated the area of different levels potential areas of each fish through the grid number and grid unit area. Additionally, this paper compares the potential area variation of different fishes under different climate scenarios. The results indicate that: (1)In the current climate, the potential area of Yellowtail kingfish(Seriola lalandi) is the largest among seven fishes, estimated at 1,288,500 km2. And the number of varieties suitable for aquaculture in the East China Sea and the South China Sea is more than that in the Bohai and Yellow Seas. (2)Compared with the high potential area of current and future climate scenarios, Large yellow croaker(Larimichthys crocea) has the largest increase, Yellowtail kingfish(Seriola lalandi) decreases under all four future climate scenarios, and both Cobia(Rachycentron canadum) and Allied Kingfish(Seriola dumerili) has small changes. (3)In the future climate scenarios, the number of species potentially suitable for culturing increases significantly in the Bohai and Yellow Sea area, and in the South China Sea, the suitable areas for cultured Large yellow croaker(Larimichthys crocea), gold pompanos(Trachinotus ovatus), Cobia(Rachycentron canadum) and Allied Kingfish(Seriola dumerili) decrease significantly. Overall, the potential areas of each fish show a northward trend.

Long-term trends and extreme events of marine heatwaves in the Eastern China Marginal Seas during summer

Marine heatwaves (MHWs) are a type of widespread, persistent, and extreme marine warming event that can cause serious harm to the global marine ecology and economy. This study provides a systematic analysis of the long-term trends of MHWs in the Eastern China Marginal Seas (ECMS) during summer spanning from 1982 to 2022, and occurrence mechanisms of extreme MHW events. The findings show that in the context of global warming, the frequency of summer MHWs in the ECMS has increased across most regions, with a higher rate along the coast of China. Areas exhibiting a rapid surge in duration predominantly reside in the southern Yellow Sea (SYS) and southern East China Sea (ECS, south of 28°N). In contrast, the long-term trends of mean and maximum intensities exhibit both increases and decreases: Rising trends primarily occur in the Bohai Sea (BS) and Yellow Sea (YS), whereas descending trends are detected in the northern ECS (north of 28°N). Influenced jointly by duration and mean intensity, cumulative intensity (CumInt) exhibits a notable positive growth off the Yangtze River Estuary, in the SYS and southern ECS. By employing the empirical orthogonal function, the spatio-temporal features of the first two modes of CumInt and their correlation with summer mean sea surface temperature (SST) and SST variance are further examined. The first mode of CumInt displays a positive anomalous pattern throughout the ECMS, with notable upward trend in the corresponding time series, and the rising trend is primarily influenced by summer mean SST warming. Moreover, both of the first two modes show notable interannual variability. Extreme MHW events in the SYS in 2016 and 2018 are examined using the mixed layer temperature equation. The results suggest that these extreme MHW events originate primarily from anomalous atmospheric forcing and oceanic vertical mixing. These processes involve an anomalous high-pressure system over the SYS splitting from the western Pacific subtropical high, augmented atmospheric stability, diminished wind speeds, intensified solar radiation, and reduced oceanic mixing, thereby leading to the accumulation of more heat near the sea surface and forming extreme MHW events.