This paper presents the development of a well-balanced gas-kinetic scheme (GKS) with space-time adaptive mesh refinement (STAMR) for the shallow water equations (SWE). While well-balanced GKS have been established on Cartesian and triangular meshes, the proposed STAMR framework utilizes arbitrary quadrilateral meshes with hanging nodes, introducing additional challenges for maintaining well-balanced properties. In addition to spatial adaptivity, temporal adaptivity is incorporated by assigning adaptive time steps to cells at different refinement levels, further enhancing computational efficiency. Furthermore, the numerical flux in the GKS adaptively transitions between equilibrium fluxes for smooth flows and non-equilibrium fluxes for discontinuities, providing the proposed GKS-based STAMR method with strong robustness, high accuracy, and high resolution. Standard benchmark tests and real-world case studies validate the effectiveness of the GKS-based STAMR and demonstrate its potential for interface capturing and the simulation of complex flows.

The slope current in the northwest South China Sea (SCS) plays a vital role in regulating mass and nutrient exchange between the coastal region and the open ocean. Over the past three decades, this current has exhibited a significant increasing trend, indicating a much stronger marginal sea-open ocean interaction. Using observational data, we show that the intensified anticyclonic warm eddy shedding from the Luzon Strait propagates along the continental slope, leading to a stronger slope current. During 1993-2022, a reduction in Subtropical Mode Water was accompanied by decreases in sea surface height (SSH) and Kuroshio transport in the Luzon Strait, as well as an enhanced looping pathway of the Kuroshio in the northern SCS. These conditions favor anticyclonic warm eddies (positive SSH anomaly) shedding from the northwestern Luzon Strait and propagating along the continental slope southwestward. At the same time, the accompanying cold eddies (negative SSH anomaly) propagate westward towards western SCS. The combined propagation of these dipole-like eddies further strengthens the SSH gradient between the shelf and inner ocean in the SCS, thereby intensifying the slope current. Importantly, it is the increased mean strength of the anticyclonic eddies, rather than their number, that drives this long-term current intensification. In contrast, neither local surface wind nor buoyancy forcing can account for the observed enhancement of the slope current, although wind forcing likely contributes to strengthening its southern segment. These findings highlight the crucial role of Kuroshio intrusion and eddy-current interaction in regulating the long-term variability of circulation in the SCS.

Atmospheric nitrogen (N) deposition critically affects ecosystem dynamics and carbon cycling, yet organic N (ON) remains under-monitored relative to inorganic N (IN), limiting constraints on global N budgets and impacts. We conducted year-long dry and wet deposition measurements of water-soluble ON (WSON) and IN at a coastal Hong Kong site using an innovative dual-surface collector, enabling simultaneous dry deposition to pure water and to a quartz filter. Dry deposition to water exceeded that to the filter by 26.6 & times; (NH4 +-N), 1.6 & times; (NO3 --N), and 4.3 & times; WSON, evidencing strong uptake of water-soluble gaseous N by the water surface. We provide the first quantification of gaseous versus particulate WSON in dry ON deposition, revealing a dominant gaseous contribution reaching up to similar to 80% to the total flux; the higher WSON fraction in wet deposition than in atmospheric particles further implicates gaseous WSON as a key rainwater ON source. Annual total N deposition (wet plus dry deposition-to-water) was 39.2 +/- 5.2 kg N ha-1 yr-1, comprising NH4 +-N (31.0 +/- 5.0%), NO3 --N (40.3 +/- 5.2%), and WSON (28.7 +/- 3.6%). These results demonstrate substantial ON inputs via wet and dry pathways-particularly gaseous-and underscore the need for coordinated measurements of gaseous and particulate ON and IN to better constrain N deposition budgets and ecological impacts.

This study investigates the impacts of urbanization and anthropogenic heat (AH) on air temperature and precipitation in the Pearl River Delta subject to rapid socioeconomic development. Using the Weather Research and Forecasting (WRF) Model, coupled with a single-layer urban canopy model, three scenarios (nonurban, urban without AH, and urban with AH) were analyzed. Results reveal that urbanization significantly alters the regional climate, increasing air temperature, land-sea circulation, planetary boundary layer (PBL) height, and precipitation in the urban areas. Urbanization alters the surface heat balance, increasing sensible heat flux and reducing latent heat flux, while AH further amplifies the urban heat island (UHI) effect, leading to higher temperature and increased precipitation. Both urbanization and AH enhance vertical motion, redistributing moisture to higher atmospheric levels and increasing precipitation. The air temperature changes are driven primarily by the PBL process and temperature advection. Urbanization enhances the vertical divergence of turbulent heat flux within the PBL, and AH intensifies this effect, significantly contributing to rises in temperature. Moisture budget analysis highlights that the increased rainfall is driven primarily by intensified moisture flux convergence resulting from urbanization, with the dynamic effect (wind convergence) playing a dominant role and thermodynamic effect (moisture advection) playing a secondary role. AH further amplifies both the moisture advection and wind convergence, intensifying urban precipitation. These findings emphasize the significant impact of urbanization and AH on the regional climate, underscoring the need for better representation of urban processes in climate models to address the effects of rapid urban expansion. SIGNIFICANCE STATEMENT: This study explores how rapid urban growth and heat from human activities affect summer weather in China's Pearl River Delta. Using advanced weather modeling, we show that urbanization raises temperatures and increases rainfall in city areas. Added heat from buildings, vehicles, and people further intensifies these changes. These effects are linked to stronger air movement and shifts in moisture, making rainfall more likely. Our findings highlight the need to include urban features in climate models to better predict local weather.

This study proposes a well-balanced formulation of weakly compressible smoothed particle hydrodynamics (WCSPH) for free-surface flows, which preserves hydrostatic equilibrium exactly at the discrete level–a property essential for reliable long-term simulations. Although well-balanced schemes are well established for mesh-based methods, the property remains largely unaddressed in WCSPH, where the particle approximation of the pressure gradient fails to balance the gravitational force exactly. The imbalance stems from two difficulties: the nonlinearity of the pressure-gradient-over-density term, and the approximation error of gradients evaluated by particle summation. The first is resolved by introducing an auxiliary potential variable that recasts the nonlinear term as the gradient of a single scalar, which reduces to a linear function of position under hydrostatic conditions. The second is resolved by a Riemann-based gradient approximation with kernel correction, which is first-order consistent and recovers linear fields exactly. These two ingredients ensure that the discrete potential gradient balances gravitational force exactly. Widely used techniques, including δ-SPH, particle shifting and tensile instability control, are readily incorporated. The formulation is further extended to three dimensions and implemented on GPU with architecture-tailored optimizations. Hydrostatic tests with rectangular, triangular and Gaussian bottom topographies show that the proposed formulation attains the well-balanced property to machine precision, reducing the spurious velocity error of conventional SPH from 10^-3 to the order of 10^-13. More complex benchmarks confirm its robustness, accuracy and low pressure oscillation, with simulations of up to 17.53 million particles performed on a single consumer-grade GPU.

Western boundary undercurrents (WBUCs), present beneath almost all western boundary currents, are significant for transporting subsurface mass and energy and connecting regional circulations. Observations suggest that WBUCs can have strong ageostrophy and intra-seasonal variability, but many dynamic details remain unclear. Here, we analyze a representative model idealized from the Kuroshio Current and Luzon Undercurrent (KC and LUC); other instances like the Gulf Stream and the Deep Western Boundary Current are also feasible. A cross-shore mean flow section, extracted from realistic simulations, serves as the only input. We employ biglobal instability analysis (BIA) and high-resolution (up to 500m) regional simulations to reveal the nonlinear instability of the WBUC, its interaction with the upper-layer, and the induced mesoscale and submesoscale turbulence. First, we solidly verify BIA by showing that the predicted evolution of dominant eigenmodes closely agrees with the model results. Second, an upper-layer (depth < 500m) KC mode and a middle-layer (500 to 1500m) LUC mode are identified. The nonlinear instability of the KC mode leads to strong variability and periodic reversal of the LUC. The subthermocline-eddy-like LUC mode has stronger nonlinearity, but negligibly affects the upper layer. Qualitative and, in some cases, quantitative agreement with observations is obtained. The kinetic energy spectra for the subthermocline can exhibit the k scaling as the upper layer, jointly driven by the KC and LUC instability. Moderate centrifugal instability is identified for the LUC near the topography, leading to locally enhanced submesoscales and eddy fluxes. The present model has the potential to serve as a benchmark for global WBUCs, providing theoretical explanations to observational trends and helping improve the modelling for multi-layer circulations.

There exist unique northeastward counterwind currents (CWCs) over the China shelf seas, subject to the balance between the northeastward alongshore ageostrophic (effective) pressure gradient force (PGF eff , the residual PGF after balancing the Coriolis force) and southwestward surface wind stress forcing. The underlying physics for the formation of the alongshore PGF remains largely ambiguous. We used process-oriented modeling of the China shelf seas to investigate how the alongshore PGF and subsequent CWC form. Driven by a typical alongshore variable density field and wind forcing, our numerical model produced a realistic shelf current structure and CWC. Our results show that the alongshore sea level elevation gradient, induced by an alongshore variable wind, mainly contributed to the PGF, which is consistent with arrested topographic wave theory. However, the wind-induced elevation gradient alone is insufficient to overcome the frictional effects of wind forcing to form the CWC. The alongshore density gradient, due to heterogeneous heating, enhances the PGF because of the steric effect on sea level. The enhanced PGF produced by the density gradient and wind-induced elevation gradient critically forms the CWC. In addition, the seasonally variable density gradient and wind forcing determine the spatiotemporal variability of the CWC. We found that the dominant intrinsic dynamics of the wind and buoyancy forcing are enough to trigger the CWCs, and the shelf’s topographic features further shape the structure of the CWC. The study provides new insights into the formation of CWC, which has been widely observed over the shelves globally.

Eutrophication and hypoxia are intensifying in many estuarine systems globally, driven by complex interactions between physical and biogeochemical processes. In the Pearl River Estuary (PRE), the combined effects of large-scale river plume dispersion and wind-driven coastal upwelling remain poorly understood and rarely observed at high resolution. To address this gap, we conducted an intensive, high-resolution field campaign in the PRE in June 2021, capturing two contrasting hydrodynamic scenarios: high river discharge with plume-favorable northeasterly winds, and low river discharge with upwelling-favorable southwesterly winds. Using a well-validated three-endmember mixing model, we quantified surface nutrient consumption and bottom nutrient accumulation, and assessed the spatial extent of hypoxia under each scenario. In the high-discharge scenario, strong plume spreading enhanced water column stratification, promoting surface biological consumption of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). This resulted in elevated nutrient additions in bottom waters and a hypoxic area covering ∼1232 km2. In the low-discharge, upwelling-favorable scenario, the bottom-water hypoxic area contracted to ∼412 km2, while low-oxygen waters were brought to the surface and nutrient additions were moderated. Fitted slope values for DIN and DIP additions were 16.4 and 15.1 under the two scenarios, respectively, consistent with the Redfield ratio, indicating organic matter degradation dominated bottom-water nutrient addition and oxygen consumption. Our results reveal the dynamic interplay between river plume dispersion and coastal upwelling in shaping nutrient and dissolved oxygen distributions on intra-seasonal timescales, through coupled physical and biogeochemical processes, including surface primary production and bottom organic matter degradation. These findings provide critical new insights into biogeochemical modeling, coastal environmental forecasting and management.

Most studies on tropical cyclone (TC) rain rate focus on long-term variability, yet the short-term (days or shorter) variations across the TC lifecycle, with a particular focus on the period before landfall, are most critical because they strongly influence flood risk. Using satellite data, we show that, globally, the mean rain rate of TCs increases by over 20% from 60 hours before landfall to the time of landfall. This increase occurs across hemispheres, ocean basins, intensity categories, and latitudes, although the magnitude varies. As a TC approaches the coast, land-sea thermal contrasts raise low-level humidity over land, while frictional differences enhance convergence, upward motion, and instability on the offshore side of the circulation. These conditions collectively promote increased convection and precipitation of TCs as they near landfall. Our findings critically strengthen the current understanding of TC precipitation dynamics and support more effective flood management.

Changes in the translation speed of landfalling tropical cyclones (TCs) pose great challenges in disaster preparedness. While some recent studies have discussed the increased chance of a reduction in the annual-mean translation speed of TCs after landfall, such changes before landfall have not been systematically investigated, especially for short-term variations (that is, hour-to-day timescales). Here we show, first based on observations, that globally, a TC about to make landfall tends to accelerate towards the coast, with an average acceleration of about 0.83 m s−1 per day, which means that the mean translation speed of a landfalling TC increases by 48

China Sea (CS) consists of South China Sea, East China Sea, Yellow Sea and Bohai Sea. CS connects with terrestrial input from landside and with Western Pacific Ocean (WPC) fluxes from seaside. A healthy, resilient and predictable CS is important to sustainable socioeconomic development in the region and is largely determined by the sustainability of the interlinked spheres that compose the regional earth system (RES), including the lithosphere (land), the hydrosphere (oceans and rivers), the atmosphere, and the biosphere (living things). Based on an unprecedented holistic study of the interactions among natural forcings, human activities, and climate change using an integrated system to create an RES framework (https://earthhk.hkust.edu.hk/), we integrate science, AI and develop a digital twin of the regional earth system that integrates streaming data from observations, an earth simulator of land-ocean-atmosphere and an immersive and interactive visual interface for diagnosis and prognosis in the CS and WPC.

Abstract The Kuroshio is well‐known for its variability south of Japan, where its complex dynamics lead to the formation of meandering jet and eddies along the stream. In this study, we investigated the relevant processes and physical mechanism that remain ambiguous, using a well‐validated high‐resolution China Sea Multiscale Ocean Modeling System and a process‐oriented numerical modeling. We found that this variability along Kuroshio is chiefly governed by the dynamics of the boundary jet separation. Our analysis shows that the variability follows a sequential dynamical pathway. First, the Kuroshio separation is induced by an inverse along‐stream pressure‐gradient force associated with the highly nonlinear jet, which decelerates the nearshore flow and induces a coastal countercurrent. In wind‐favorable events, Ekman transport can further reinforce this countercurrent, thereby promoting separation. Near protruding coastal capes, bottom pressure torque and topographic vorticity adjustment help anchor the separation point. An accumulation of shear vorticity on the shoreside/seaward side of the separated Kuroshio forms a cyclonic/anticyclonic eddy dipole. Through active dynamic adjustment along the jet, the increasing barotropic and baroclinic instabilities strengthen the meandering jet and eddy, thereby maintaining the separation. This study provides new insight into how boundary‐current separation, topographic constraint, wind‐favorable countercurrent formation, and jet instability jointly regulate western boundary current variability.

Abstract Chlorophyll‐a (Chl_a) concentration in the ocean is a critical indicator of primary production and plays a pivotal role in the global carbon cycle. Its accurate predictions are essential for reliable projections of future climate change. However, Chl_a is controlled by myriads of interactive physical and biogeochemical processes, posing a substantial challenge for its diagnosis and prognosis through numerical modeling. Machine learning (ML) approaches offer a promising venue to face the challenge, yet existing ML models often struggle in handling Chl_a's significant spatiotemporal variability. We propose a novel machine learning model, Spatiotemporal Dynamics Hunter (STD‐Hunter), to tame spatiotemporally heterogeneous dynamics of Chl_a and improve its prediction. STD‐Hunter effectively accommodates the heterogeneity in principal dynamics characterized by multiple basis ML models. By integrating these basis models with Chl_a's spatiotemporal characteristics, STD‐Hunter forms task‐specific predictive models that harvest the underlying dynamics. STD‐Hunter optimizes the trade‐off between effectively leveraging data and preserving idiosyncratic dynamics. Based on remotely sensing data, we demonstrate the superior performance of STD‐Hunter in predicting highly variable Chl_a in an active marginal sea. The extracted characteristics are also well aligned with Chl_a's intrinsic dynamics. By bridging between highly nonuniform observations and their underlying dynamics, STD‐Hunter offers an interpretable tool to obtain physically meaningful spatiotemporal characteristics from a data‐driven perspective.

Salinity shapes ocean circulation and marine biogeography, yet its long-term spatiotemporal variability and ecological impacts in marginal seas remain poorly constrained. We reconstruct a high-resolution sea surface salinity dataset (2000-2020) for the China Seas using a machine learning framework that integrates in situ cruises and buoys with satellite observations and diagnose drivers with an eigen microstates approach. The El Ni & ntilde;o/Southern Oscillation (ENSO) is the dominant control, modulating evaporation-precipitation, river discharge and Kuroshio intrusion. During El Ni & ntilde;o, sea surface salinity increases by up to 25% in ocean-dominated regions but decreases up to 21% in river-dominated zones, amplifying meridional salinity contrasts. Species-distribution models indicate a southward habitat shift up to 2.5 degrees latitude for 90% of key fish species. Under projected ENSO intensification, salinity inhomogeneity and associated ecological impacts are likely to strengthen. These results support an 'ENSO forcing-salinity-fishery' positive feedback framework and call for integrating salinity dynamics into adaptive, climate-informed fisheries management.

Non-point source (NPS) pollution has caused widespread degradation of riverine water quality, characterized by pronounced variability in nutrient species. However, the dominant controls on nutrient dynamics across heterogeneous landscapes remain poorly understood at the basin scale. Here, we combine observations with the Soil and Water Assessment Tool to investigate how land use and land surface processes regulate nutrient transport in the Pearl River Basin. Our results show that agricultural areas exhibit strong coupling among fertilizer inputs, surface flow, lateral flow, sediment transport, and interactions with organic soil pools, leading to the highest nutrient intensities (19.8 kg N ha(-1) yr(-1) and 0.37 kg P ha(-1) yr(-1)). In urban areas, compacted soils and impervious surfaces generate the highest nutrient transport efficiency (mean 18.6%), although total exports remain largely source-limited. In contrast, undisturbed regions selectively regulate nutrient species through soil processes. Forested areas exhibit substantial subsurface nitrate export (10.1 kg N ha(-1) yr(-1)) and elevated N:P ratios, driven by high soil nitrate storage and strong lateral flow. Pasture areas contribute relatively higher phosphate export (0.11 kg P ha(-1) yr(-1)), reflecting coupling between labile soil phosphorus pools and surface flow. Across the basin, nutrient intensity and transport efficiency are consistently lower for phosphorus than for nitrogen and lower for organic than for inorganic forms, promoting phosphorus limitation and the dominance of inorganic nutrient species in riverine systems. Overall, this study provides quantitative insights into the coupled hydrological and biogeochemical controls on NPS pollution across land use-specific processes.

Fertilizer application has been recognized as the major driver of nitrous oxide (N2O) emissions in rivers. However, the intrinsic risk of N2O emissions in responding to nitrogen discharge (i.e., ratios between N2O emissions and dissolved inorganic nitrogen or fertilizer) may vary across rivers but remains unclear. Here we uncover the hidden yet substantial control of riverine N2O emissions by bedrock geology through field observations and global data analyses. We discover lower denitrification but higher N2O production rates in rivers with silicate-dominated bedrocks compared to those with carbonate-dominated bedrocks. On one hand, the larger sediment grain size found in the silicate-dominated rivers shortens porewater residence time for reaction, which diminishes the likelihood of complete denitrification (NO3− → N2) but simultaneously increases the potential for N2O production through incomplete denitrification (NO3− → N2O). Concurrently, lower sediment total organic carbon and water pH abate N2O reductase and enhance N2O production in silicate-dominated rivers. More importantly, the dissimilarities in N2O production due to bedrock geology are reflected in N₂O emissions risks. By developing a geological factor, we interpret the heterogeneous N2O emission risks in rivers globally. Hence, we highlight the geologically uneven urgency of improving fertilizer management to mitigate N2O emissions. Silicate-dominated rivers exhibit lower denitrification but higher N2O production than carbonate-dominated rivers due to coarser sediments, lower organic carbon, and reduced pH, according to global statistical analyses and field sampling in the Pearl River Basin.

Recent mooring observations have suggested that geostrophic currents flowing over large-scale topography with wavenumbers outside of the radiating internal lee wave range can generate strong, upward-radiating Near-inertial Internal Waves (NIWs), but the generation dynamics are unclear. In this study, a two-dimensional nonhydrostatic numerical model based on MITgcm is conducted to investigate the interaction between geostrophic currents and large-scale deep seamounts, discerning the generation mechanism of NIWs and clarifying the influence of seamount height and summit width on the wave generation. The results suggest that weak depth-independent inertial oscillations are first excited when the flows encounter seamounts. Radiating NIWs are subsequently generated at the summit edge of seamounts. Near the summit edge on the lee side of the seamount, the accelerated mean flow can further develop and enhance NIWs via the nonlinear interaction between the mean flow and NIWs. As the seamount height increases, larger mean flows and stronger nonlinearity occur at the summit edge of the seamounts, generating stronger NIWs. Compared to flat-topped seamounts (i.e., the more width summit), the sharp-topped feature (i.e., the conical seamount) can induce stronger mean flows and nonlinear interaction near the summit edge on the lee side of seamounts via the hydraulic control, resulting in the generation of stronger NIWs.

Abstract. We present a summer 2024 comparative study of aerosol nitrogen speciation across the Guangdong–Hong Kong–Macau Greater Bay Area (GBA), contrasting marine air over the coastal ocean with a coastal urban site in Hong Kong. Inorganic nitrogen (IN) and organic nitrogen (ON) were quantified, and a high-resolution time-of-flight aerosol mass spectrometer was operated offline to characterize water-soluble nitrogen-containing organics. Total nitrogen and IN showed a west-to-east increase along the coastal ocean, indicating stronger anthropogenic influence in the more populated eastern GBA. ON showed a contrasting pattern: while its concentration decreased offshore, its fraction in total nitrogen peaked in the western marine region (34.6 ± 12.4 %), highlighting the relative importance of ON under lower PM2.5 loadings. Urban aerosols were enriched in ammonium and exhibited more oxidized ON signatures, including higher NO+/NO2+ ratios (7.9 ± 2.6), consistent with NOx–VOC photochemistry. Marine aerosols showed lower NO+/NO2+ ratios (5.3 ± 1.3) and molecular signatures consistent with reduced, amine-related ON, reflecting marine biogenic inputs in the marine boundary layer. Using an inferential approach with deposition velocity (Vd) assumptions, PM2.5-bound nitrogen deposition over the ocean was estimated to be comparable to that at the urban site (0.14 vs. 0.15 kg N ha−1 yr−1), indicating non-negligible fine-particle nitrogen input to adjacent coastal waters. These results demonstrate a notable coastal transition in nitrogen chemical form and suggest that ON speciation should be considered when assessing nitrogen deposition to coastal waters and potential ecosystem responses in the South China Sea.

The three-dimensional circulation in the Japan Sea (JS) plays an important role in its water mass and biogeochemical substances exchange with neighboring oceans. However, characterizing the spatiotemporal circulation pattern in the JS, and diagnosing its complex forcing mechanism between intrinsic flow-topography interaction and extrinsic flux through the straits connected with adjacent seas remain challenge. Combined observations with numerical modeling and a novel Stokes-based layer-integrated vorticity equation (LIVE) dynamics, we discovered a three-layer circulation with alternating cyclonic, anti-cyclonic, and cyclonic circulation in the upper (0-150 m), middle (150-250 m), and bottom (>250 m) layers in the JS, respectively. The strong cyclonic and weak anti-cyclonic circulations in the upper and middle layers show similar seasonal phase: the domainintegrated vorticity anomaly is positive during winter and negative from summer to early autumn. In contrast, cyclonic circulation in the bottom layer remains relatively stable throughout the year. We diagnosed that besides vorticity input from wind stress curl in the upper layer, the lateral planetary vorticity fluxes from inflow/outflow through the straits surrounding the JS lead to vortex stretching in all layers and extrinsically control the structure of the layered circulation. The joint effects of baroclinicity and relief (JEBAR) arising from flow-topography interaction is an intrinsic dynamic response to the extrinsic forcing and dynamically shapes the layered circulation. Based on Stokes circulation theorem, this study characterizes the layered circulation pattern, and based on LIVE dynamics, effectively identifies intrinsic and extrinsic forcing mechanisms for the layered circulation in the JS and other marginal seas.