Mesoscale eddies are prevalent in the Philippine Sea and frequently impinge on the Kuroshio Current, affecting internal tides (ITs) originating from the Luzon Strait (LS). This process is investigated in this study using idealized experiments, which provide a controlled environment to quantify variations of the M2 ITs as a single cyclonic eddy (CE) or anticyclonic eddy (AE) approaches the LS from the east. Our results reveal an asymmetric influence: AEs induce more pronounced changes in the M2 tidal conversion than CEs. Variations in conversion rates are primarily controlled by changes in bottom pressure perturbations, which are linked to the product of subtidal buoyancy gradients and IT vertical velocity. Another noteworthy finding is that these mesoscale eddies influence the M2 ITs differently in two stages: during propagation across the Philippine Sea and upon impingement with the Kuroshio Current. In the first stage, mesoscale eddies modulate the propagation of the M2 ITs through direct interaction, inducing incoherent M2 ITs in the Philippine Sea. The percentage of incoherent ITs exceeds 50% along the eddy paths. Advection from eddy-induced currents contribute significantly more than background stratification and relative vorticity. During the second stage, as the eddies impinge on the Kuroshio Current and move northward, variations in the M2 IT energetics within the LS are evident. The IT incoherence increases not only in the Philippine Sea but also in the LS and South China Sea. These findings are important for understanding the IT variability and incoherence in the Philippine Sea and South China Sea.

The Japan Sea is a semi-enclosed marginal sea characterized by complex hydrodynamics and prominent seasonal variability in oceanic fronts and currents. While physical dynamics are known to influence marine ecosystems, the specific spatiotemporal dependence of chlorophyll a (Chl a) on frontal activities and other environmental factors in this region remains quantitatively under-explored. This study utilized satellite-derived sea surface temperature (SST) and Chl-a data to detect fronts using the gradient method and investigated their relationships using empirical orthogonal function (EOF) and correlation analyses. The results revealed distinct seasonal patterns, with the subpolar frontal zone exhibiting a phase reversal in frontal activity compared to the coastal shelf zone. A significant positive correlation between frontal probability (FP) and Chl a was identified in the central and southern basins (35°N–40°N), indicating that frontal dynamics are a primary driver of phytoplankton growth in this region, likely through nutrient supply via vertical circulation. In contrast, wind forcing and surface currents were identified as the dominant factors regulating Chl-a variability in the northern and coastal regions. Furthermore, these regional dynamics were found to be significantly modulated by the North Pacific index on interannual scales. These findings provide a comprehensive quantitative assessment of the physical-biological coupling in the Japan Sea, highlighting the spatially varying roles of fronts and atmospheric forcing in sustaining marine productivity.

Conventional point-based measurements are inadequate for capturing the spatial variability, seasonal evolution, and debated tidal ellipticity of currents in the hydrodynamically complex Yellow River estuary. This study employed two high-frequency surface wave radar systems to acquire high spatio-temporal resolution surfacecurrent data in the northern Yellow River estuary from April to September 2021. Through spectral and harmonic analysis, we examined the characteristics of tidal currents and their seasonal variations. The results indicate that the area is predominantly influenced by semidiurnal tides, with M2 constituent as the principal component. We identified two regions with enhanced M2 velocities exceeding 50 cm/s: one near Shenxiangou, which is adjacent to an M2 amphidromic point, and another near the Yellow River estuary. Notably, radar observations revealed a predominance of rotational flow near the estuary, a pattern that diverges from the bidirectional flow typically described by conventional models. Besides, during the observation period, both the M2 amplitude and the intensity of clockwise rotational flow increased in July. We further investigated these intensifications using river discharge records and CTD profiles, identifying water-column stratification as the primary driver. Near the estuary, enhanced summer stratification results primarily from the vertical salinity gradients induced by increased river discharge; near Shenxiangou, it is influenced by both temperature and salinity gradients, with salinity exerting a more substantial effect. Collectively, these results demonstrate that high-frequency radar observations provide critical insights into the dynamic processes in a rapidly evolving estuarine environment. It offers a novel mechanistic understanding of how river discharge and stratification regulate coastal tidal flows.

Nitrogen fluxes across the sediment-water interface and nitrogen removal from sediments are essential components of nitrogen cycle in semi-enclosed inland seas. However, the difficulty in observational sampling hinders continuous data availability that is necessary to understand their seasonal variations and underlying mechanisms. To address this issue, we developed a one-dimensional sediment nitrogen-cycle model and used sensitivity experiments to quantify the relative roles of environmental and biogeochemical drivers. Model results indicate that 48 % of particulate organic nitrogen (PON) settling into sediments is returned to the bottom water as dissolved inorganic nitrogen (DIN), while 6 % is removed via N-loss flux (dinitrogen gas and nitrous oxide). The seasonal variations of PON and DIN fluxes are controlled by fundamentally different mechanisms, with PON flux primarily regulated by bottom-water PON concentration and bottom stress, while DIN flux is mainly governed by temperature-dependent biogeochemical transformations and nitrogen availability. These contrasting responses reveal a decoupling between particulate and dissolved nitrogen fluxes, reflecting the buffering capacity of sediments. Denitrification controls nitrogen removal but its amount is not large because the oligotrophic conditions of the study site limits the nitrate availability.

In summer, the East China Sea (ECS) faces ecological challenges like harmful algal blooms, hypoxia, and jellyfish blooms, linked to nutrient distributions influenced by Changjiang Diluted Water (CDW), Kuroshio intrusion, and Zhejiang Coastal Upwelling (ZCU). The inconsistent interannual variations of these processes complicate nutrient dynamics. This study used the Empirical Orthogonal Function (EOF) analysis to reveal the interannual variations (1993-2022) of summer dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations in the ECS. We demonstrated that interannual variability in southerly wind strength was a key regulator controlling the relative influence of CDW and ZCU on surface nutrients. Weaker southerly wind combined with stronger Changjiang discharge (e.g., 1999, 2002, 2020) enhanced the nearshore influence of CDW and reduced the ZCU intensity. This suppressed DIP supply and biological consumption of DIN, leading to accumulated surface DIN but depleted surface DIP in the Zhejiang Coastal (ZC) region. Conversely, stronger southerly wind combined with stronger Changjiang discharge drove the CDW moving offshore that increased offshore DIN concentrations. Simultaneously, the stronger southerly wind also intensified ZCU that enhanced coastal DIP supply and biological DIN uptake, resulting in reduced coastal surface DIN. Critically, we identify the distinct drivers for bottom DIP. Weaker ZCU led to increasing (decreasing) of bottom DIP concentrations in an area deeper (shallower) than 20 m, while stronger Kuroshio intrusion substantially boosted bottom DIP, influencing productivity in both ZC region and the East Zhejiang coastal (EZC) region. The intricate interplay of these physically driven nutrient fluxes, characterized by differing N/P ratios, underpinned the ECS's complex and variable summer nutrient structures with profound implications for understanding its ecological responses.

The rivers around the Seto Inland Sea (SIS) have high perfluorooctanoic acid (PFOA) concentrations, and the SIS is expected to be a PFOA source for the Pacific Ocean. To investigate variability, transport, and fate of riverine PFOA in this sea, we use a hydrodynamic-ecosystem-PFOA model to simulate the behaviors of its dissolved and biogenic particle phases in the SIS. The sum of dissolved and particulate PFOA presents a mean concentration of 468.5 ng m-3 in the eastern part of SIS, which is much higher than that in the western part (11.5 ng m-3). The partitioning between particulate and dissolved phases is higher in nearshore than in offshore areas. The inputs from the Yodo and Yamato Rivers dominate the PFOA levels in the ESIS. After entering Osaka Bay, 25% of the PFOA is transported to Harima Nada, while 63% is transported to the Kii Channel. The latter is transported farther to the Pacific Ocean through the upper 20 m of the Kii Channel, which is the dominant fate of PFOA in the SIS. We use the proportions of PFOA outflow to the Pacific Ocean from the rivers discharging into Osaka Bay (88%) and Suo Nada (57%) as upper and lower limits, respectively, to estimate the amount of PFOA from the rivers along the coast south of Japan entering the Pacific Ocean. The annual amount is 328.0-348.9 kg, to which the SIS contributes the largest part.

This study examines the influence of the Kuroshio intrusion into the South China Sea (SCS) on the sea surface temperature cooling (SSTC) induced by tropical cyclones (TCs) through numerical simulations. Two types of the Kuroshio intrusion are investigated, i.e. the looping path characterized by the formation of an anticyclonic eddy in the SCS, and the leaping path characterized by the minimal intrusion into the SCS. For comparison, a no-Kuroshio case with initial horizontally homogeneous stratification is also simulated. Results indicate that both the looping and leaping Kuroshio modify TC-induced SSTC compared to the no-Kuroshio case, through distinct mechanisms. The looping Kuroshio suppresses TC-induced SSTC to both sides of the Luzon Strait (LS). In the anticyclonic eddy induced by the looping Kuroshio to the west side of the LS, the suppression is related to the warmer subsurface water, smaller vertical temperature gradient and larger mixed layer depth, which lead to weaker vertical mixing. Whereas to the east side of the LS, the cause is horizontal advection. The leaping Kuroshio also suppresses TC-induced SSTC to the east side of the LS through strong horizontal advection, but it enhances the SSTC to the west side of the LS through enhanced vertical mixing. Moreover, it is found that TC-induced SSTC spreads downstream along the major axis of the leaping Kuroshio, which is caused by vertical mixing rather than horizontal advection.

Under global warming, the frequency and intensity of extreme events have significantly increased, exerting dramatic impacts on the natural ecosystems and human society. We report that typhoons, as extreme weather events, can trigger subsurface marine heatwaves (MHWs), which are extreme ocean events. Based on 27-year ocean reanalysis data, a total of 9 typhoon-induced subsurface MHW events are identified in the South China Sea, with most of them occurring in the nearshore shallow waters. The enhanced mixing induced by the typhoon, relatively high subsurface temperature percentile prior to the typhoon, and sufficiently long duration of subsurface warming are three key factors for typhoon-induced subsurface MHWs. Although the occurring probability of typhoon-induced subsurface MHW events is relatively low, the co-occurrence of such extreme events poses a serious threat to the marine environment and requires special attention.

Previous studies have demonstrated that the internal tides (ITs) near the Luzon Strait (LS) are significantly modulated by the Kuroshio Current (KC). However, the modulation, particularly the contributions of the KC's velocity and stratification to the modulation, remains incompletely understood. Based on the interaction theory between subtidal motions and ITs, and 9-month numerical simulation results, this study investigates the KC's modulation on the M2 IT energetics at the LS, with a focus on quantifying the contributions of the KC's velocity and stratification to the modulation. The KC directly modulates the IT energy budget through buoyancy production, shear production, and advection. Among them, the buoyancy production, associated with the KC's stratification, dominates over the shear production and advection, which are related to the KC's velocity. Furthermore, the KC influences the tidal conversion and energy flux of ITs by changing the pressure perturbation as well as barotropic and baroclinic tidal currents through nonlinear interaction with ITs. Sensitivity analysis indicates that the bottom pressure perturbation plays a more important role in the tidal conversion, whereas the baroclinic tidal currents are more crucial in the energy flux. Further analysis shows that the contributions of KC's velocity and stratification to the buoyancy and pressure perturbation are generally comparable. Moreover, the KC's velocity plays a more important role than the stratification in determining the phase speed of ITs, which leads to different energy flux patterns under different KC paths.

This study utilized a high-resolution, three-dimensional hydrodynamic model with improved model evaluation to investigate seasonal variations in key hydrographic conditions, including sea level, water temperature, salinity, current speed, and circulation in the Gulf of Thailand (GoT), as well as its interaction with the South China Sea (SCS). The analysis focuses on a climatological year calculated from a 15-year average for 2006-2020, which is categorized into four seasons: northeast monsoon, the first inter-monsoon, southwest monsoon, and the second inter-monsoon. Evaluation of model performance, based on observational data with temporal resolutions ranging from 30 min to monthly average with a duration from 10 months to 5 years, demonstrated good accuracy through high coefficients of determination and low root mean square errors. Results clearly depicted seasonal variability in hydrographic properties, characterized by alternating patterns of high and low sea level, high and low water temperatures, saline and fresh water, along with a persistent anticyclonic gyre in the central area of GoT and a smaller anticyclonic gyre in the southern area. Seasonal exchange flows between the SCS and the GoT were also evident, with the strongest outflow in northeast monsoon and the weakest in the second inter-monsoon.

Seasonal oxygen depletion has been increasingly reported on the East China Sea (ECS) shelf since the early 2000s; however, its long-term evolution and underlying drivers remain poorly understood. Using historical observations from the World Ocean Database (1951–2006), we reconstruct basin-scale variability of dissolved oxygen (DO) across the ECS shelf and analyze both seasonal patterns and multi-decadal trends. Our results show that oxygen depletion (DO ≤ 4 mg L⁻1) on the mid–outer ECS shelf has occurred regularly since the 1950s, substantially earlier than previously reported observations, which mainly documented its occurrence after the early 2000s. Oxygen depletion recurs from summer to autumn, with oxygen stress shifting from the inner shelf in late summer toward the mid–outer shelf along the 50–100 m isobaths in autumn. A significant decline in bottom DO is detected over the mid–outer shelf during 1960–1999 and is accompanied by increasing apparent oxygen utilization (AOU). By combining AOU diagnostics with a three-endmember mixing model, oxygen deficits are partitioned into locally generated consumption and remotely advected components. The results indicate that local remineralization dominates oxygen consumption, while the intrusion of oxygen-poor Kuroshio Subsurface Water reduces the background oxygen inventory and modulates the intensity of seasonal depletion. These findings reveal that oxygen depletion on the ECS shelf has a longer history and broader spatial extent than previously recognized.

This research examines the phenomenon of saltwater intrusion in the Yellow River estuary (YRE) through utilization of shipboard surveys and mooring investigations. Hydrographic data were gathered in May and October 2023 to elucidate the relationship between saltwater intrusion in YRE and various factors, including tides, winds, and river discharge. The fortnightly cycle of the diurnal tide within the estuary is controlled by the declination of the moon above and below the equator, rather than by the lunar phase, as is the case in a semidiurnal system. The spring tides occur two days after the maximum lunar declination and neap tides occur two days following the zero value of lunar declination. Surface salinity within estuary demonstrates semidiurnal fluctuations, with an initial peak occurring prior to high tide and a subsequent peak during early ebb phase. The earlier peak is attributed to interplay between upstream tidal currents and downstream fluvial flow, while the later peak is likely influenced by saltwater which has intruded into river channel. Conversely, bottom salinity exhibits diurnal variations, as seawater is driven into and out of river channel by diurnal tidal currents. The peak salinity levels in the estuary correspond to fortnightly variations in water elevation, indicating stronger intrusion during spring tides than in neap tides. Additionally, strong northeasterly winds can enhance saltwater intrusion; however, intrusion is hindered once the winds subside in following day. This phenomenon of wind-induced hindrance has not been previously documented and may be attributed to significant accumulation of freshwater within the river due to northeasterly winds. Overall, saltwater intrusion is more intense during winter and spring than in summer and autumn.

Abstract Internal tides (ITs) usually exhibit incoherent characteristics during their propagation. However, estimations of incoherent ITs from different platform (mooring and satellite) observations exhibit discrepancies, partly due to their different sampling intervals and observation durations. Based on 10‐year numerical simulation of ITs near the Luzon Strait, we examine influences of sampling interval and observation duration on the coherent and incoherent characteristics of semidiurnal ITs, that is, the M2 IT steric height and incoherence of semidiurnal ITs in steric height. Results indicate that for 1‐hr interval (typical sampling interval of mooring observations), both the coherent and incoherent characteristics of semidiurnal ITs exhibit convergence with increasing duration. However, for 238‐hr interval (approximate repeating cycle of TOPEX/Poseidon/Jason‐1/Jason‐2 satellites), the coherent characteristic of semidiurnal ITs has a larger bias than that for 1‐hr interval, and this bias does not monotonously decrease with duration; the semidiurnal IT incoherence has a larger uncertainty than that for 1‐hr interval.

Kuroshio frontal eddies in the East China Sea (ECS) have been observed and investigated for a long time; however, the energetics of frontal eddy-mean flow interactions remain unclear. In this study, we revisit the Kuroshio frontal eddies in the ECS from the perspective of the eddy energy budget using an eddy-resolving ocean general circulation model (OGCM) named OFES2. By decomposing the variables into frontal eddy (5 < T < 20 days) and background components (T > 20 days), we diagnose each term in the eddy energy budget equation to examine their contributions and spatiotemporal variations. The results demonstrate that the frontal eddy energy increases downstream along the Kuroshio in the ECS, with high values in the upper 500 m. Baroclinic instability is the primary source of frontal eddy energy in the entire ECS Kuroshio region, while barotropic and shear instabilities also contribute to one-third of the total energy conversion. In contrast, the work done by wind to eddy energy is insignificant. Further instability analysis suggests that these eddies can be generated either locally within the ECS or come from the upstream region east of Taiwan Island, with a faster growth rate in the ECS. The temporal variation of the frontal eddy energy is largely controlled by baroclinic instability, which is further modulated by the Kuroshio volume transport. The results of this study provide new insights into the Kuroshio frontal eddies, which may be extended to other western boundary current regions where the frontal eddies exist.

Large-scale marine heatwaves in the mid-latitude northeast Pacific have garnered significant attention due to their vast spatial extent, intensity, prolonged duration, and detrimental impacts on marine ecosystems and fisheries. Contrary to the conventional understanding that surface heat fluxes drive their formation, here we show that the two severe and impactful multi-season marine heatwave events in 2013-15 and 2019-20 were caused by the large-scale northward displacement of warm subtropical waters into the typically colder subarctic region. These oceanic changes are mainly explained by wind-driven circulation changes and Sverdrup balance adjustment. The marine heatwave decay phase corresponds with anomalous northwesterly winds which transport cold, dry air, enhancing latent heat loss and leading to ocean surface cooling. The physical driver is linked to the Tropical/Northern Hemisphere (TNH) teleconnection pattern, which aligns with previous studies. Collectively, the characteristic interannual timescale of the oceanic dynamic response and TNH explains the multi-season persistence of these extreme events.

The Kuroshio carries a large amount of nutrients from the east of Luzon Island to the south of Japan. However, only nutrients transported into the euphotic layer can be utilized by phytoplankton. We use the results of an eddy-resolving coupled physical-biological model to investigate (1) the horizontal and vertical transport of nitrate into the euphotic layer (0–100 m) in the Kuroshio and Kuroshio Extension and (2) the contribution of different sources of nitrate (coastal, deep layer and open ocean) to the spatial variation of downstream transport within the euphotic layer along the Kuroshio and Kuroshio Extension. As a mean state, the downstream transport of nitrate in the euphotic layer varies as 2.8 kmol s−1 east of Luzon Island, 7.9 kmol s−1 east of Taiwan, 8.9 kmol s−1 near the Tokara Strait, 21.5 kmol s−1 near the Izu-Ogasawara Ridge and 19.6 kmol s−1 around 160°E. Vertical transport from the bottom of the euphotic layer around the Luzon Strait due to the uplifting of the potential density layer is an important contribution to the increase in downstream transport of nitrate by 3.8 kmol s−1. Horizontal transport from Japan coast area contributes 14.3 kmol s−1 of nitrate to the Kuroshio mainstream. An interesting staggered upward and downward distribution of vertical velocity, which can be explained by conservation of the potential vorticity, was found in the Kuroshio Extension and contributed a total nitrate transport of 5.2 kmol s−1 to the euphotic layer. We also calculated the vertical flux of nitrate resulting from vertical mixing based on observations and demonstrated that vertical mixing can provide a nitrate flux into the euphotic layer in the Kuroshio region comparable to that supplied by vertical velocity.

Microplastics (MPs) pollution is a prevalent environmental problem that affects ecosystems globally. Despite the growing research on the environmental effects of MPs, a significant research gap remains in understanding the differences of environmental behavior and distribution patterns between biodegradable MPs and traditional MPs. Using a three-dimensional hydrodynamic model and treating MPs as tracers with vertical velocity, this study simulated the transport of positively, neutrally, and negatively buoyant biodegradable MPs from rivers. The results show that positively buoyant MPs have significant seasonal variations and are mainly distributed in the surface layer. Neutrally buoyant MPs are distributed in all water depths, with a high (low) concentration in the eastern (western) Seto Inland Sea (SIS), characterized by winter mixing and summer stratification. Negatively buoyant MPs accumulate in the sediments and exhibit lower concentrations in seawater. Positively and neutrally buoyant MPs mainly outflow from the SIS into the Pacific Ocean, whereas negatively buoyant MPs hardly leave the SIS and are primarily deposited and degraded near river mouths. A settling velocity of -10-6 to -5×10-5 m s-1 (downward) greatly affects the concentration of MPs in seawater. However, large upward and downward velocities outside this range do not result in pronounced changes. Compared with traditional MPs, biodegradable MPs are less environmentally persistent by not accumulating in sediments and keeping a low concentration there, which contributes to the reduction of transport flux of MPs to the Pacific Ocean.

The intertidal zone is one of the natural systems most vulnerable to threats from polycyclic aromatic hydrocarbons (PAHs). However, the natural attenuation rate of PAHs within intertidal zones is low, posing challenges for the short-term recovery of contaminated environments. This study developed a contaminated intertidal zone simulation system and used a composite bacterial consortium containing Cellulosimicrobium sp. RS and Brucella sp. BZ for bioaugmented remediation. The degradation rate of PAHs (initial concentration: 5000 μg/kg) in the sediments reached 85.37 % after 120 days of restoration, which was significantly higher than the 29.93 % observed in the control group. High-throughput sequencing was used to analyze the structure and function of sediment microbial communities. The exogenous bacteria Cellulosimicrobium became dominant after remediation, whereas Brucella did not dominate but contributed to synergistic degradation. Network analysis and PICRUSt predictions confirmed that the microbial community evolved toward stronger PAHs degradation capabilities and degraded PAHs through ring cleavage, side-chain metabolism, and central metabolism in bioaugmented sediments. This study provides theoretical guidance and data support for bioaugmented remediation of intertidal zone pollution.

The maritime migration to the South Ryukyu Islands of southwestern Japan, which occurred approximately 30,000 years ago, was one of the most difficult sea crossings accomplished by the Late Pleistocene Homo sapiens. This study performs numerical simulations to investigate the conditions that were needed to cross between Taiwan and Yonaguni Island, where one of the world's strongest ocean currents, the Kuroshio, remains active. We combined simulations based on three ocean models with data from an actual experimental voyage conducted in 2019. The results showed that travel across this sea would have been possible on both the modern and Late Pleistocene oceans if a dugout canoe was used with a suitable departure place and paddling strategy. Recognizing the Kuroshio, paddling to counteract this current, and using high-level navigation were crucial to success. This suggests that the Paleolithic maritime expansion in the Western Pacific involved both advanced technologies and strategic challenges.