Search

Abstract Solid Earth tide represents the response of solid Earth to the lunar (solar) gravitational force. The yielding solid Earth due to the force has been thought to be a prolate ellipsoid since the time of Lord Kelvin, yet the ellipsoid’s geometry such as major semi-axis’s length, minor semi-axis’s length, and flattening remains unresolved. Additionally, the tidal displacement of reference point is conventionally resolved through a combination of expanded potential equations and given Earth model. Here we present a geometric model in which both the ellipsoid’s geometry and the tidal displacement of reference point can be resolved through a rotating ellipse with respect to the Moon (Sun). We test the geometric model using 23-year gravity data from 22 superconducting gravimeter (SG) stations and compare it with the current model recommended by the IERS (International Earth Rotation System) conventions (2010), the average Root Mean Square (RMS) deviation of the gravity change yielded by the geometric model against observation is 6.47 µGal (equivalent to 2.07 cm), while that yielded by the current model is 30.77 µGal (equivalent to 9.85 cm). The geometric model will greatly contribute to many application fields such as geodesy, geophysics, astronomy, and oceanography.

IntroductionBivalve aquaculture is an important pillar of China's fisheries, with over 1 million tonnes of scallop shells produced annually. However, most of these shells are directly discarded into the sea, leading to continuous pollution of the marine and coastal environments, especially the coast of Yantai in the Bohai Sea where a large number of discarded scallop shell have accumulated.MethodsTo trace the fate of scallop shells in the ocean, this study established a model for the transport of scallop shells, coupling a two-dimensional tidal current model using the adjoint method with a Lagrangian particle model. By simulating nested tidal models, the distribution of tidal residual current in the Yantai coastal region was obtained. Then, a Lagrangian particle model was used to track the transport pathways of pollutants in the sea.ResultsDriven by the residual current calculated from the tidal model with the actual situation, possible pollutant release areas were inferred. The results of Lagrangian particle tracking experiments indicate that pollutants were released from the upstream accumulation area, specifically the area near Penglai Hulushan, confirming previous speculation.DiscussionThe scallop shells transport model can accurately simulate the spatiotemporal profile of scallop shells, which is helpful for managing scallop shell resources and improving the level of shell reuse.

Near-inertial waves (NIWs), a special form of internal waves with a frequency close to the local Coriolis frequency, are ubiquitous in the ocean. NIWs play a crucial role in ocean mixing, influencing energy transport, climate change, and biogeochemistry. This manuscript briefly reviews the generation and propagation of NIWS in the oceans. NIWs are primarily generated at the surface by wind forcing or through the water column by nonlinear wave-wave interaction. Especially at critical latitudes where the tidal frequency is equal to twice the local inertial frequency, NIWs can be generated by a specific subclass of triadic resonance, parametric subharmonic instability (PSI). There are also other mechanisms, including lee wave and spontaneous generation. NIWs can propagate horizontally for hundreds of kilometers from their generating region and radiate energy far away from their origin. NIWs also penetrate deep into the ocean, affecting nutrient and oxygen redistribution through altering mixing. NIW propagation is influenced by factors such as mesoscale eddies, background flow, and topography. This review also discussed some recent observational evidence of interactions between NIWs from different origins, suggesting a complicated nonlinear interaction and energy cascading. Despite the long research history, there are still many areas of NIWs that are not well defined.

This study employs the two-dimensional Legendre polynomial fitting (2-D LPF) method to fit M2 tidal harmonic constants from satellite altimetry data within the region of 53°E–131°E, 34°S–6°N, extracting internal tide signals acting on the sea surface. The M2 tidal harmonic constants are derived from the sea surface height (SSH) data of the TOPEX/Poseidon (T/P), Jason-1, Jason-2, and Jason-3 satellites via t-tide analysis. We validate the 2-D LPF method against the 300 km moving average (300 km smooth) method and the one-dimensional Legendre polynomial fitting (1-D LPF) method. Through cross-validation across 42 orbits, the optimal polynomial orders are determined to be seven for 1-D LPF, and eight and seven for the longitudinal and latitudinal directions in 2-D LPF, respectively. The 2-D LPF method demonstrated superior spatial continuity and smoothness of internal tide signals. Further single-orbit correlation analysis confirmed generally higher correlation with topographic and density perturbations (correlation coefficients: 0.502, 0.620, 0.245; 0.420, 0.273, −0.101), underscoring its accuracy. Overall, the 2-D LPF method can use all regional data points, overcoming the limitations of single-orbit approaches and proving its effectiveness in extracting internal tide signals acting on the sea surface.

The Nordic Sea overflow, being a significant driver of the thermohaline circulation, exerts a substantial influence on environmental dynamics in the Arctic and globally. A better understanding about the trend of the storage volume of the Nordic Seas Overflows is of paramount importance to a realistic assessment of the North Atlantic circulation and variability. EN4.2.2 reanalysis data were utilized to acquire the monthly average time series of overflow water storage volume in the Nordic Sea from 1950 to 2022. The storage volume demonstrates seasonal variations, with fluctuations of approximately 10% around the average. Over the period from 1980 to 2022, the average volume per decade exhibited a consistent decrease. Linear fitting of the annual average data estimated the overall relative change trend in the last 43 years to be -7.2 ± 2.6%. However, the quality of the EN4.2.2 data brings the average error of about 12% in the calculation of the overflow storage volume, which implies that the downward trend requires further validation. From a spatial standpoint, the Norwegian Sea, particularly the Lofoten Basin, is the primary region where overflow water storage volume in the Nordic seas have decreased. This decrease corresponds to an increase in ocean temperatures within the upper layer (0-600m) of both the Lofoten Basin and the Norwegian Basin. The warming of these regions has directly impacted the overflow water storage volume, leading to its reduction. It is noteworthy that the ocean temperature rise in the upper layer is more influenced by the Atlantic inflow rather than air-sea flux, particularly in the Norwegian Basin. This is attributed to the substantial increase in SST in the North Atlantic, which aligns with the warm current regions in the Atlantic Ocean, and the lack of statistical significance in linear trend of the air-sea heat flux. Furthermore, there has been a more rapid reduction in the overflow storage volume in the Norwegian Sea from 2011 to 2022. Specifically, the annual average overflow volume from 2018 to 2022 dropped below the previous lowest value. Concurrently, the salinity of the upper Norwegian Sea and the Atlantic inflow decreased considerably, while there has been no significant change in ocean temperature. Therefore, this short-term fluctuation is predominantly attributed to the decrease in Atlantic inflow salinity. Interestingly, the decrease in overflow water volume in the Nordic Seas does not follow a linear pattern in relation to increasing ocean temperature; instead, it exhibits an accelerating trend. If the ocean temperature rises by 1°C, the overflow water volume in the Nordic Seas will decrease to 86% of the total volume. Overall, the overflow water storage in the Norwegian Sea undergoes complex interannual variations and is notably influenced by the influx from the Atlantic Ocean.

Abstract Assessing the health of coastal ecosystems is crucial for maintaining ecological balance. One significant indicator of water eutrophication is the distribution of chlorophyll-a (Chl-a), which makes obtaining a complete three-dimensional spatial distribution of Chl-a essential for assessment. This study utilized the linear radial basis function (RBF-Linear) method to obtain a comprehensive and reasonable spatial distribution of Chl-a. The method was applied to obtain the three-dimensional spatial field of Chl-a concentration in the Bohai Sea in March, May, August, and October from 2016 to 2018. The distribution pattern of Chl-a concentration in the Bohai Sea displayed characteristic spatial and temporal variations. Spatially, high Chl-a concentration was most concentrated in coastal waters, particularly in estuaries and mariculture areas. Temporally, there were two peaks in March and August. The total Chl-a and areas with high Chl-a concentration in four sub-regions of the Bohai Sea were also calculated to enable a comprehensive assessment of the marine ecological environment. By analyzing the temporal and spatial variation of Chl-a in the Bohai Sea and evaluating the marine ecological environment, we confirmed the feasibility and rationality of RBF-Linear. Our findings have the potential to contribute to improve the accuracy of ecological models and assessment of the satellite products.

Route planning to evade potential pollution holds critical importance for aquaculture vessels. This study establishes a fish-feed pollutant drift model based on the Lagrangian particle tracking algorithm and designs four sets of sensitivity experiments in the East China Sea. The research investigates the impact of model input uncertainties on the drift trajectory, centroid position, and sweeping area of the fish-feed pollutants. Numerical results indicate that the uncertainty in the background flow field significantly affects the uncertainty in the centroid position and sweeping area in the numerical simulations. Specifically, when a 35% random error is added to the background flow field, the centroid shift distance reaches its maximum, and the sweeping area also attains its largest value. The uncertainty in the background wind field affects the centroid position of particles but to a much lesser extent compared to the background flow field. When considering only the uncertainty of the background wind field, the sweeping area does not significantly differ from the control experiment as the uncertainty of the background wind field increases. The initial release position has little effect on the drift direction of the fish-feed pollutants but does affect the drift distance; it has minimal impact on the trajectory but significantly affects the final position of the pollutant centroid. By analyzing the model uncertainties, this study reveals the key factors influencing the drift of fish-feed pollutants. This information is crucial for aquaculture vessels in planning routes, considering environmental factors, and reducing potential pollution risks.

Grid resolution and assimilation window size play significant roles in storm surge models. In the Bohai Sea, Yellow Sea, and East China Sea, the influence of grid resolution and assimilation window size on simulating storm surge levels was investigated during Typhoon 7203. In order to employ a more realistic wind stress drag coefficient that varies with time and space, we corrected the storm surge model using the spatial distribution of the wind stress drag coefficient, which was inverted using the data assimilation method based on the linear expression Cd = (a + b × U10) × 10−3. Initially, two grid resolutions of 5′ × 5′ and 10′ × 10′ were applied to the numerical storm surge model and adjoint assimilation model. It was found that the influence of different grid resolutions on the numerical model is almost negligible. But in the adjoint assimilation model, the root mean square (RMS) errors between the simulated and observed storm surge levels under 5′ × 5′ and 10′ × 10′ grid resolutions were 11.6 cm and 15.6 cm, and the average PCC and WSS values for 10 tidal stations changed from 89% and 92% in E3 to 93% and 96% in E4, respectively. The results indicate that the finer grid resolution can yield a closer consistency between the simulation and observations. Subsequently, the effects of assimilation window sizes of 6 h, 3 h, 2 h, and 1 h on simulated storm surge levels were evaluated in an adjoint assimilation model with a 5′ × 5′ grid resolution. The results show that the average RMS errors were 11.6 cm, 10.6 cm, 9.6 cm, and 9.3 cm under four assimilation window sizes. In particular, the RMS errors for the assimilation window sizes of 1 h and 6 h at RuShan station were 3.9 cm and 10.2 cm, a reduction of 61.76%. The PCC and WSS values from RuShan station in E4 and E7 separately showed significant increases, from 85% to 98% and from 92% to 99%. These results demonstrate that when the assimilation window size is smaller, the simulated storm surge level is closer to the observation. Further, the results show that the simulated storm surge levels are closer to the observation when using the wind stress drag coefficient with a finer grid resolution and smaller temporal resolution.

Exploring the spatial distribution of sea surface height involves two primary methodologies: utilizing gridded reanalysis data post-secondary processing or conducting direct fitting along-track data. While processing gridded reanalysis data may entail information loss, existing direct fitting methods have limitations. Therefore, there is a pressing need for novel direct fitting approaches to enhance efficiency and accuracy in sea surface height fitting. This study demonstrates the viability of Legendre polynomial surface fitting, benchmarked against bicubic quasi-uniform B-spline surface fitting, which has been proven to be a well-established direct fitting method. Despite slightly superior accuracy exhibited by bicubic quasi-uniform B-spline surface fitting under identical order combinations, Legendre polynomial surface fitting offers a simpler structure and enhanced controllability. However, it is pertinent to note that significant expansion of the spatial scope of fitting often results in decreased fitting efficacy. To address this, the current research achieves the precise fitting of sea surface height across expansive spatial ranges through a regional stitching methodology.

Abstract Based on the velocity and temperature data recorded by two acoustic Doppler current profilers (ADCPs) at a mooring system deployed in the northern South China Sea (SCS), this study investigates the characteristics of near-inertial waves (NIWs) induced by typhoons Bebinca, Barijat, Mangkhut and Yutu in 2018. For the dynamical response, besides the motion of near inertial frequency induced by typhoons, the motion of 2 f ([1.80–2.20] f, f is the local inertial frequency) and f D1 (a harmonic wave with a frequency equal to the sum of frequencies of NIWs and diurnal tides) frequency will also increase. For near-inertial motions, the maximum near-inertial kinetic energy (NIKE) is confined to depths above 150 m. For stronger (weaker) wind forcing, the longer (shorter) the response time of the ocean to the atmospheric forcing is, and the shorter (longer) the response time is required in relaxation stage. There are upward and downward propagating energies after the passage of typhoons, and the upward propagating energy mainly occur in the stage of the geostrophic balance adjustment. The current structure suggests that the NIWs in the vertical direction are two antisymmetric rotary vortices in a near-inertial period, which is similar to the structure of the Langmuir circulation. Besides, the horizontal near-inertial currents (NICs) are much stronger than the vertical NICs, and the stronger the NIWs are, the stronger the horizontal NICs relative to the vertical NICs are. For the temperature response, the temperature variation reflects a clear stratified vertical structure. In the forcing stage, the upper layer becomes colder, the lower layer becomes warmer, and the thickness and intensity of the thermocline decrease. In the relaxation stage, the upper layer warms and the lower layer cools, and the thickness and intensity of thermocline increase.

Abstract Based on the temperature data recorded by a mooring system deployed in the northern South China Sea (SCS), this study compares the traditional methods of identifying and extracting internal solitary waves (ISWs): the whole water column isotherm (WCI) method and the single isotherm (SI) method, and proposes a novel method: the temperature superposition (TS) method. Results indicate that the TS method identifies the most ISWs, followed by the WCI method, and the SI method identifies the fewest. When the ISW amplitudes are smaller than 60 m, the TS method can identify the most ISWs. When the ISW amplitudes are greater than 60 m, the number of ISWs identified by the three methods is the same. The TS method removes the internal tides, takes into account the temperature of all depth layers, and amplifies the signal of ISWs to a certain extent, so as to effectively identify ISWs.

Dissolved oxygen (DO) plays a pivotal role in sustaining marine ecosystems. The Bohai Sea in China is a semi-enclosed sea, and oxygen-deficit events occur from time to time due to human activities. At present, there is a notable absence of any convenient and precise method for obtaining three-dimensional spatial data on DO, and the exploration of the physical mechanisms influencing oxygen deficit remains incomplete. This investigation uses the linear radial basis function (RBF-Linear) fitting method to conduct three-dimensional spatial interpolation for DO, which demonstrates minimal inaccuracy. Then, the RBF-Linear fitting method is utilized to collect a comprehensive three-dimensional spatial dataset encompassing temperature, salinity, and DO in the Bohai Sea in August from 2016 to 2018. The results indicate discernible interannual variations in the extent, area, and distribution of oxygen deficiency during summer in the Bohai Sea. Mechanism analysis reveals that intense precipitation episodes and an increase in wind stress curl exacerbate oxygen depletion. Additionally, the degree, location, and area of the two oxygen-deficit cores (off the Yellow River Estuary and off the Qinhuangdao) in the Bohai Sea are influenced by several factors, including current velocity, direction, local circulation position, and net horizontal transport rate. Furthermore, the study suggests that oxygen deficiency in the Bohai Sea region is currently in its early stages, with a limited degree of injury and a restricted range of influence. The use of a three-dimensional spatial interpolation method to create a complete DO field in three-dimensional space simplifies the research challenges associated with marine oxygen deficit. Moreover, this study holds particular significance for guiding the development of marine fisheries.

Near-surface PM2.5 estimates remain a global scientific research challenge due to their effect on human fitness and atmospheric environmental quality. However, practical near-surface PM2.5 estimates are impeded by the incomplete monitoring data. In this study, we propose the trigonometric polynomial fitting (TPF) method to estimate near-surface PM2.5 concentrations in south-central China during 2015. We employ 10-fold cross-validation (CV) to assess the reliability of TPF in estimating practical PM2.5 values. When compared to alternative methods such as the orthogonal polynomial fitting (OBF) method based on Chebyshev basis functions, Kriging interpolation, and radial basis function (RBF) interpolation, our results show that utilizing TPF31, with a maximum order of 3 in the x direction and a maximum order of 1 in the y direction, leads to superior efficiency through error minimization. TPF31 reduces MAE and RMSE by 1.93%, 24%, 6.96% and 3.6%, 23.07%, 10.43%, respectively, compared to the other three methods. In addition, the TPF31 method effectively reconstructs the spatial distribution of PM2.5 concentrations in the unevenly distributed observation stations of Inner Mongolia and the marginal regions of the study area. The reconstructed spatial distribution is remarkably smooth. Despite the non-uniform distribution of observation stations and the presence of missing data, the TPF31 method demonstrates exceptional effectiveness in accurately capturing the inherent physical attributes of spatial distribution. The theoretical and experimental results emphasize that the TPF method holds significant potential for accurately reconstructing the spatial distribution of PM2.5 in China.

Based on the method for identifying the boundary of movable water bodies (MWBB), the spatial distribution of reclamation projects in the Guangxi Beibu Gulf were identified over the past 40 years and the impact of these engineering facilities on hydrodynamics was also evaluated. The results showed that 163.8 km2 of natural sea areas in the Guangxi Beibu Gulf were occupied through reclamation in the last 40 years. The effects of land reclamation on tidal amplitude were more pronounced in the second period (2001–2018) than in the first period (1987–2001), particularly in the tidal channels of Qinzhou Bay and Fangcheng Bay, where the amplitude difference ranged from 8 to 15 cm, representing a 40–55% increase. The reduction in the sea area because of land reclamation has changed the hydrodynamics in the Guangxi Beibu Gulf, including reducing the tidal volume, altering the amplitude variations, and increasing the seaward residual currents, all of which could cause significant problems for the coastal environment.

Abstract Nonstationary internal tides (ITs) are formed from their interactions with background currents. Harmonic analysis (HA), which cannot be used to estimate the incoherent component, is almost exclusively used method to predict ITs from observations. This remains ITs prediction challenge. In this study, we establish a deep learning framework to predict semidiurnal ITs. The model is established and trained with observed semidiurnal internal tidal currents that are 172 days long, and then ITs over the next 42 days are forecasted. The prediction accuracy is greatly improved using the deep learning framework. The magnitudes of errors using the deep learning framework are approximately 35% of those obtained using HA. Most temporal and spatial variations in baroclinic currents can successfully be forecasted using deep learning. In addition, the kinetic energy and incoherent components of ITs can be accurately predicted. Moreover, the relatively high adoptability of the established deep learning model is shown.

Since 2000, coastal reclamation projects have been implemented in the Guangxi Beibu Gulf to alleviate the shortage of coastal land resources. As a result, the reclamation area has increased significantly. In this study, based on the Landsat program, we tracked coastal reclamation in the coastal waters of Guangxi in the 21st century. An indicator system was proposed to assess the driving forces of reclamation synthetically. Our results showed that 98.6 km2 of coastal waters of the Guangxi Beibu Gulf was reclaimed from 1997 to 2017, in which three regions underwent great changes: Fangcheng Bay, Tieshan Bay, and Qiznhou Bay. Furthermore, spatio-temporal changes of reclamation were affected by the combination of population growth, economic development, and marine industry development in coastal cities. These results provide an important historical reference for tracking the human development processes of the Guangxi Beibu Gulf, and also offer some suggestions for the rational allocation of reclamation resources.

IntroductionSuspended Particulate Matter (SPM) influences the primary production and the distributions of pollutants in the ocean. Besides, the regulation mechanisms of SPM in the Liaodong Bay were complicated.MethodTo analyze the distributions and influencing factors of SPM, based on the adjoint assimilation method, an interpolation method with dynamical constraint was established in the Liaodong Bay.ResultIn two ideal experiments, the cost function, Mean Absolute Error (MAE) and Normalized Mean Error (NME) all had reduced by more than 90%, which proved the accuracy of the interpolation method. Based on conventional observations of SPM, the distributions of dynamically constrained, Kriging and radial basis function (RBF) interpolations in March, May, August and October of 2015 were obtained.DiscussionThe cross-validation was carried out to compare the dynamically constrained interpolation and the unconstrained interpolations. Among seven unconstrained interpolation methods, the averaged MAE of RBF interpolation was the lowest, which was 10.976 mg/L. The averaged MAE of dynamically constrained interpolation was 7.703 mg/L, reduced by 29.8% compared with the RBF interpolation. It was indicated that RBF interpolation was the most accurate among the seven unconstrained interpolations and dynamically constrained interpolation was more accurate than unconstrained interpolations at the observation stations. The distributions of dynamically constrained and RBF interpolations were compared with Korean Geostationary Ocean Color Imager (GOCI) satellite-derived distributions of SPM concentrations in the Liaodong Bay. Fully considering the influences of the hydrodynamic processes, the dynamically constrained interpolation provided distributions more consistent with the satellite-derived distributions. However, due to the lack of observations in some areas and ignoring the influences of currents, some high values of SPM concentration were not captured by the distributions of RBF interpolation. Moreover, in accordance with the results of dynamically constrained interpolation, it was found that the SPM concentrations in the bay were affected by the SPM discharge from the Liao River Basin.

Outlier detection is the key to the quality control of marine survey data. For the detection of outliers in Conductivity-Temperature-Depth (CTD) data, previous methods, such as the Wild Edit method and the Median Filter Combined with Maximum Deviation method, mostly set a threshold based on statistics. Values greater than the threshold are treated as outliers, but there is no clear specification for the selection of threshold, thus multiple attempts are required. The process is time-consuming and inefficient, and the results have high false negative and positive rates. In response to this problem, we proposed an outlier detection method in CTD conductivity data, based on a physical constraint, the continuity of seawater. The method constructs a cubic spline fitting function based on the independent points scheme and the cubic spline interpolation to fit the conductivity data. The maximum fitting residual points will be flagged as outliers. The fitting stops when the optimal number of iterations is reached, which is automatically obtained by the minimum value of the sequence of maximum fitting residuals. Verification of the accuracy and stability of the method by means of examples proves that it has a lower false negative rate (17.88%) and false positive rate (0.24%) than other methods. Indeed, rates for the Wild Edit method are 56.96% and 2.19%, while for the Median Filter Combined with Maximum Deviation method rates are 23.28% and 0.31%. The Cubic Spline Fitting method is simple to operate, the result is clear and definite, better solved the problem of conductivity outliers detection.

In this study, we developed a transport model for typical pollutants in the Yellow Sea using the Lagrangian particle tracking method to analyze the trajectories of fish feed, a common pollutant in the Yellow Sea. The model incorporates the influence of ocean currents and surface winds on pollutant transport and utilizes a series of numerical experiments to simulate pollutant transport. Through statistical analysis of the numerical experiment results, we identified characteristic circles that represent the pollutant distribution patterns. Furthermore, based on the current and wind information within these characteristic circles, we derived an empirical formula to describe pollutant distribution. This formula enables us to predict the spatial distribution of pollutants using available current and wind data. Using this empirical formula, we designed an effective path to avoid pollutant contamination. This approach not only optimizes the utilization of computational resources within the study area but also contributes to the rational planning of navigation routes for aquaculture vessels. Overall, our study provides valuable insights into the transport behavior of fish feed pollutants in the Yellow Sea. The establishment of the empirical formula and the design of effective routes to avoid pollution contribute to the efficient management of pollution and facilitate the planning of marine activities in the region.

The optimal siting selection strategy for new tidal stations in the Bohai Sea is investigated using a two-dimensional tidal model with the adjoint method. Harmonic constants (HCs) of the M2 constituent at all computing grids are estimated in the Bohai Sea by assimilating observations from existing tidal stations and altimeter data processed by X-Track software. Several grid points on the coastline are selected as new tidal station sites, and different combinations of selected points are regarded as different siting strategies. The HCs at these new tidal stations are served as “simulated observations” (SOs) which are assimilated into the tidal model to re-estimate the HCs in the Bohai Sea. Through comparisons between the re-estimated HCs and SOs, we can evaluate the effects of different siting strategies. Divide the Bohai Sea into different subdomains, numerical experiments are constructed to investigate the effects of different strategies in different subdomains, while the effects of schemes including inversion variables and different tidal constituents on siting selection are experimented. By analyzing the root-mean-square (RMS) difference between re-estimated HCs and SOs on the coastline in different subdomains, the optimum strategy for siting of new tidal stations in the Bohai Sea is obtained.

Coastal regions are highly vulnerable to the impacts of human activities, land cover change, sea level rise (SLR), and climate extremes. In this study, we attempt to address this issue by simulating the spatial interactions among natural hazards, ever-increasing human activities, and social vulnerability in the south coast of the Bohai Sea, China. It is found that the activities occurring on land, especially within coastal environments, such as agricultural pond and ports, and those in the ocean (tide and residual current) both impose disaster risks on the marine environment. In 2020, 25.2% of the total study area is highly vulnerable, which is 21% larger than that in 1997. The socially vulnerable areas are mainly distributed in the southeast coast of the Laizhou Bay. These areas should be strengthened to reduce and prevent the storm surge and flood disasters. Ultimately, we emphasize the urgent needs to implement effective policy measures for reducing tomorrow’s risks from natural hazards.

The ecosystem parameters are critical for precisely determining the marine ecological process and improving the simulations of the marine ecological model. In this study, based on the NPZD (nutrient, phytoplankton, zooplankton and detritus) model, the surface chlorophyll-a observations obtained from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data were assimilated to estimate spatially ecosystem parameters in the Bohai, Yellow, and East China Seas using an adjoint assimilation method with characteristic finite difference scheme. The experiments of the moving Gaussian hump indicated that the characteristic finite difference method (CFDM) can get rid of the limit of stability and permit using large time steps, which reduces long computation durations and large memory requirements. The model performance was significantly improved after data assimilation with CFDM using a large time step of 6 hours. Moreover, the distributions of parameters of the NPZD model in winter in the Bohai Sea, the Yellow Sea, and the East China Sea were simulated by our method. Overall, the developed method can efficiently optimize the ecosystem parameters and the results can be beneficial for determining reasonable parameters of the marine ecological model.

Based on the modal decomposition and least square method, full-depth tidal currents and corresponding internal tidal modes can be obtained from limited observations. Through a series of experiments based on direct observations at the mooring MP1 and synthetic observations at the moorings MP2 and MP3 in the South China Sea, the essential observations for reconstructing full-depth tidal currents are determined. Both observations in the upper ocean (especially near the thermocline) and deep layer (below 500 m) are essential in the reconstruction. For the upper ocean, an up-looking acoustic Doppler current profiler near the thermocline is generally essential, which can also be replaced by at least 2-4 current meters (CMs). The number of CM depends on the complexity and range of buoyancy frequency variation. For the deep layer, more observations are needed as water depth increases. The observations with the CM interval/water depth ratios about 0.2 can effectively reconstruct full-depth tidal currents below 500 m based on one barotropic mode and first three baroclinic modes. Significantly, the CMs in the middle of the deep layer are more important than those at two ends of the deep layer, which may provide more cost-effective design at a small loss of accuracy. Furthermore, the CMs with the depth interval/water depth ratios less than 1/8 in the deep layer can effectively reconstruct full-depth tidal currents with high-mode baroclinic motions.

During the sinking process, overflow generates an entrainment phenomenon, accompanied by the occurrence, development, and dissipation of vortices. In this paper, particle image velocimetry (PIV) technology is used to measure the flow fields of overflow sinking, invasion, and mixing. In order to quantitatively analyze the mixing and flow processes during the sinking of the overflow, vorticity, turbulence dissipation, Froude number, local Richardson number, and entrainment coefficient are calculated. In a stratified environment, the overflow entrains the environment fluid of lower density to reach a terminal height where their density equals that of their surroundings and then spread out horizontally. The experimental results show the terminal depth is related to the density of the overflow. The lower the density, the smaller the terminal depth. The turbulent dissipation mainly occurs in the area along the slope and during the process of invading flow to the surrounding. The extreme of the turbulent dissipation is corresponding to the maximum velocity shear and vorticity. At the point where the overflow crosses the sill and at the front end of the overflow, there will be a more obvious phenomenon of entrainment. The entrainment parameter is positively correlated with the density of overflow. These preliminary results would require additional experimental validation and data observation in order to assess their relevance for realistic flow regimes.

Water color, often quantified using the Forel-Ule Index (FUI), is a crucial parameter for assessing the water quality and ecological health of coastal waters. However, there is limited research on the spatiotemporal variations of FUI and the associated influencing factors in the Bohai and Yellow Seas. In this study, we utilized multi-sensor satellite datasets to retrieve monthly FUI products for the Bohai and Yellow Seas spanning the period from September 1997 to December 2019. Subsequently, we examined significant spatial disparities and variations across multiple timescales in the remotely sensed FUI time series. The climatological annual mean FUI map reveals a decreasing trend from nearshore to offshore regions, with similar spatial patterns observed in terms of overall and interannual FUI variability. The annual variations in wind field, sea surface temperature (SST), and ocean stratification play a key role in the seasonal dynamics of FUI by modulating the sediment resuspension process, resulting in low FUI values in summer and high FUI values in winter. Linear regression analysis of FUI anomaly indicates a long-term decreasing trend in FUI for the three bays of the Bohai Sea, while upward trends in FUI predominantly prevail in the central Yellow Sea. Factors related to interannual FUI variations, such as surface winds, SST, river outflow, rainfall, and anthropogenic activities, are qualitatively discussed. The findings of this study provide the first comprehensive evaluation of water color variations and their underlying mechanisms in the Bohai and Yellow Seas.

Numerical simulations of the eight major tidal constituents (M2, S2, K1, O1, N2, K2, P1, and Q1) in the Bohai, Yellow and East China Seas (BYES) were conducted using the Regional Ocean Modeling System (ROMS) based on altimeter products from X-TRACK. Tidal harmonic constants and two-dimensional tidal current data with a spatial resolution of 1/12° were obtained. To validate the simulation results (SRs), harmonic constants from altimeters and tide gauges, two sea level anomaly time series, and velocity observations from 12 current meters were utilized. Additionally, data from five tidal models were used for comparison. The validation and comparison results demonstrated the accuracy of SR, especially when compared with coastal tide gauge data where SR performs exceptionally well. The cotidal charts and tidal current ellipses obtained through SR exhibited good continuity and consistency with the previous studies, effectively reflecting the tidal characteristics of the BYES. The SR can serve as a valuable reference and support for tidal-related fields in the BYES, including the supplement and verification of ocean measurements and the calculation of reference planes for ocean engineering.

The vertical eddy viscosity coefficient (VEVC) is an important parameter used in ocean dynamics studies to describe the intensity of mixing in the vertical direction and the process of momentum transport. In this paper, an adjoint assimilation method was used to invert the VEVC, based on the Ekman model, with the measured wind field and current data. The main purpose was to study the effect of changes in the background wind field on VEVC, and thus investigate the role of wind stress in the inversion process. The results indicated that the inverse vertical eddy viscosity coefficient increased slightly at the surface layer, reaching its maximum at around 10–12 m, and then decreased monotonically with depth. The maximum VEVC value corresponds to different depths for different wind speed ranges. Additionally, wind steering could affect the VEVC inversion curve, causing it to deviate from the general trend. The kinetic energy ratio increased with depth, peaked at 18–20 m, and then rapidly decreased to nearly zero beyond 24 m. The impact of wind field strength and steering on VEVC was observed in the kinetic energy ratio curve, which confirms the speculative wind stress effect. This study revealed the characteristics and mechanisms of VEVC in coastal waters under different wind conditions, which could provide a reference for further research on physical oceanography.

Ice melting plays a crucial role in ocean circulation and global climate. Laboratory experiments were used to study the dynamic mechanisms of the influence of waves and currents on ice melting. The results showed that under near stable air temperature and water temperature conditions, the ice melting rate was significantly greater with waves than that without waves, as well as the higher the wave height, the greater the melting rate. This is related to the increase in the contact area between ice and water by waves. Further research was carried out to observe the flow field at different locations on the ice bottom, ice sides, and behind the ice by particle image velocimetry (PIV) and dyeing experiments. At different flow velocities, the changes in the side melting rate and bottom melting rate were not the same. Meltwater is attached to the bottom in the form of plume at low background flow velocity, which leads to the slowness of the heat exchange between the ice with a higher ambient temperature. Therefore, the melting of the ice bottom and the ice side were slower at low flow velocity. At high background flow velocity, there is strong shear instability and vortex at the ice bottom and behind the ice. The dissipation and mixing effects caused by vortices accelerate the melting of the ice bottom and the ice back. The thermodynamic factors, such as air temperature and water temperature, had significant impacts in the experiments. Further research needs to improve the accuracy of temperature control of experimental equipment. Computational fluid dynamics and sensitive tests of numerical simulation may also be carried out on ice melting.

The harmonic constants of long-period tidal constituents are critical for simulating the tide process, obtaining theoretical datum sounding reduction, and conducting further ocean research. In this paper, the equidistant node orthogonal polynomial fitting (ENOPF) method is employed to fit the harmonic constants of the T/P satellite altimeter data and the gridded altimeter data from the Data Unification and Altimeter Combination System (DUACS) to obtain the full-field harmonic constants of the long-period tidal constituents (Sa and Ssa) in the Bohai Sea, Yellow Sea, and East China Sea (BYECS). To verify the validity of the ENOPF method, the long-period tidal harmonic constants of the ENOPF method, the Finite Element Solutions 2014 (FES2014) model, the Empirical Ocean Tide 20 (EOT20) model, and the DUACS data were compared with the observations of the X-TRACK products and the tide gauges. In addition, the root-mean-square errors (RMSEs), amplitude differences (ΔH), and phase-lag differences (ΔG) of the comparison results were calculated. According to the error analysis and the cotidal charts, the ENOPF method is better than the other three methods or models for comparing tide gauges (6.19 cm, 5.85 cm, 25.44°; 1.10 cm, 0.72 cm, 59.09°, respectively. The cotidal charts obtained by the ENOPF method are smoother and have better consistency with the actual track data. The results indicate that, due to the polynomial fitting method adopted in ENOPF method, which has the characteristics of improving the resolution infinitely, it is easier to obtain the harmonic constant of full field than with other models or methods. This study proves that the ENOPF method is a reasonable and simple tool in extracting the harmonic constants of the BYECS.

Hydroxymethylfurfural (HMF) derivatives such as 2,5-bis(hydroxymethyl)furan (BHMF) and furandicarboxylic acid (FDCA) are promising alternative of fossil-based diols and dicarboxylic acids for synthesis of polyesters such as polyethylene terephthalate (PET). However, high cost for preparing HMF from biomass discourages the commercialization of HMF-derived polyesters. Since producing furfural (FUR) from five-carbon sugars (e.g., xylose) via dehydration is an inexpensive and commercialized process, we herein reported a method to synthesize BHMF derivatives (5-(ethoxymethyl)furan-2-methanol (EMFM), 2,5-bis(hydroxymethyl)furan monoacetate (BHMFM) and 2,5-bis(hydroxymethyl)furan diacetate (BHMFD) from furfural derivatives, i.e., (2-(ethoxymethyl)furan (EMF) and furfuryl acetate (FA)). To avoid strong acid-induced side reactions (e.g., furan ring opening, condensation and carbonization), two reaction systems, i.e., a low-concentration HCl aqueous solution combined with formaldehyde and anhydrous acetic acid combined with paraformaldehyde, were found to be suitable for such a hydroxymethylation reaction and could lead to decent product yields. In order to improve the carbon utilization, condensed furanic byproducts were further converted into hydrocarbon fuels via a reported two-step hydrodeoxygenation (HDO) process. This study not only validates the possibility of synthesizing functional HMF derivatives (EMFM, BHMFM, and BHMFD) from commercially-available FUR derivatives (EMF and FA), but also provide a new way to transform condensed furanics to value-added hydrocarbon fuels.

The coastal reclamation, as one of the most extreme transformations of the ocean space by humans, still lacks scientific quantitative evaluating methods to a large extent, compared with the evolution of land use patterns. A cumulative ecological impacts of reclamation (RCEI) was established in our study based on ecological influence characteristics of different reclamation types, and the attenuation effect of reclamation on adjacent areas. It was characterized by spatio-temporal features in decades. Here, we estimated that the cumulative reclamation area in the Bohai Sea from 1985 to 2018 was 5839.5 km2. Under the influence of human activity, proportions of the industrial and urban boundary, marine construction boundaries (e.g., ports, wharves, and bridges), and protective dams were increased significantly, which led to a sharp increase of the RCEI. In addition, spatio-temporal changes of reclamation were affected by the combination of population growth, economic development, urbanization, industrialization, and marine industry development in coastal cities. These results provided an important historical reference for tracking future development of the Bohai Sea by humans and provided basic data support for the development and protection of the ocean.

The Southern Ocean plays a vital role in the global climate system and the life cycle of high-latitude marine life. Phytoplankton is an important source of primary productivity in this ecosystem. Its future changes could affect Southern Ocean geochemistry, carbon export, and higher trophic organisms. To better protect Antarctica, three different marine protected areas (MPA) have been established in the Ross Sea region. Because time-continuous and regionally complete data are difficult to obtain in this region, we obtained data from ocean model outputs to understand the spatiotemporal variability of phytoplankton biomass in this region. This study explored the correlation between phytoplankton biomass and key environmental factors. Phytoplankton biomass peaks in February as temperatures rise and sea ice melts. Correlations also vary between different protected areas. The correlation between biomass, nitrate, and salinity in the Krill Research Zone (KRZ) area was significantly different from other protected areas. In addition, in the context of global warming, Antarctica lacks temperature perception. The model results show a downward trend in temperature and an increase in sea ice coverage in the western Ross Sea that other studies have also pointed to. How phytoplankton biomass will change in protected areas in the future is a question worth considering. Finally, the study simply simulates future regional trends by comparing the biomass distribution in hot years to average years. This will increase our knowledge of the polar system.

Modulations of internal tides (ITs) including the baroclinic tidal energy budget, the incoherency, and the nonlinear interactions among different tidal components by turbulent mixing in the South China Sea (SCS) are investigated through numerical simulations. The baroclinic tidal energy budget can hardly be affected by the structure of mixing. Meanwhile, change in the mixing intensity in a reasonable range also cannot obviously modulate the baroclinic tidal energy budget in the SCS. Compared to the baroclinic energy budget, the distributions of conversion and dissipation are more sensitive to the change of mixing. Turbulent mixing also modulates the incoherency of ITs by changing the horizontal density in the ocean. The horizontal variation of density adds incoherence to ITs largely by affecting the internal tidal amplitudes. Furthermore, nonlinear interactions among different components of ITs are generally modulated by the mixing intensity, whereas the variation of the mixing structure can hardly influence the nonlinear interactions. Therefore, the diapycnal diffusivity can set to be horizontally and vertically homogeneous in most of the internal tidal simulations, except for those in which the incoherency of ITs needs to be simulated. However, excessive strong mixing will destroy the stratification. Thus, the optimum range for IT simulations in the SCS is from O (10–5) to O (10–3) m2s–1.

Chemical pretreatment followed by enzymatic hydrolysis has been regarded as a viable way to produce fermentable sugars. Phenylsulfonic acid (PSA) pretreatment could efficiently fractionate the non-cellulosic components (hemicelluloses and lignin) from bamboo and result in increased cellulose accessibility that was 10 times that of untreated bamboo. However, deposited lignin could trigger non-productive adsorption to enzymes, which therefore significantly decreased the enzymatic hydrolysis efficiency of PSA-pretreated bamboo substrates. Herein, poly(N-vinylcaprolactam) (PNVCL), a non-ionic surfactant, was developed as a novel additive for overcoming the non-productive adsorption of lignin during enzymatic hydrolysis. PNVCL was found to be not only more effective than those of commonly used lignosulfonate and polyvinyl alcohol for overcoming the negative effect of lignin, but also comparable to the robust Tween 20 and bovine serum albumin additives. A PNVCL loading at 1.2 g/L during enzymatic hydrolysis of PSA pretreated bamboo substrate could achieve an 80% cellulosic enzymatic conversion and meanwhile reduce the cellulase loading by three times as compared to that without additive. Mechanistic investigations indicated that PNVCL could block lignin residues through hydrophobic interactions and the resultant PNVCL coating resisted the adsorption of cellulase via electrostatic repulsion and/or hydration. This practical method can improve the lignocellulosic enzymatic hydrolysis efficiency and thereby increase the productivity and profitability of biorefinery.

Tidal open boundary conditions (OBCs) of the M2 tidal constituent in the Bohai and Yellow Seas (BYS) are estimated via the assimilation of multi-satellite altimeter data to optimize regional tidal numerical simulation. A two-dimensional adjoint assimilation model is used for tidal numerical simulation and, as an improvement, trigonometric polynomials fitting (TPF) is applied in the inversion of OBCs. It is assumed that the linearized amplitudes/phases in the open boundary are spatially varying and can be represented by nonlinear functions. Based on the discrete Fourier series, taking the trigonometric function as the basis function, the spatially varying OBCs are constructed by selecting the maximum truncation period. The independent points scheme used in previous studies was also compared in the experiments. Twin experiments show that the errors of simulations with TPF are the smallest in different schemes, and their results show the highest correlation with observations while maintaining the best performance in terms of observation errors. The mean absolute errors (MAEs) in amplitude/phase between the simulated results using estimated OBCs and the satellite altimeter records are 2.82 cm and 2.26°, respectively.

In the storm surge model, the wind drag coefficient Cd is a critical parameter that has a great influence on the forecast of the storm surge level. In the present study, the effect of various wind drag coefficient parameterizations on the storm surge level is investigated in the Bohai Sea, Yellow Sea and East China Sea for Typhoons 7203 and 7303. Firstly, the impacts of initial values of a and b in the linear expression Cd = (a + b × U10) × 10−3 on the pure storm surge model are evaluated based on the data assimilation method. Results indicate that when a and b (i.e., the wind drag coefficients given by Smith, Wu, Geernaert et al. and Mel et al.) are non-zeros, the performance of the model has little difference, and the result from Wu is slightly better. However, they are superior to the performance of the model adopting zero initial values. Then, we discuss the influences of diverse ways of calculating wind drag coefficients, which are inverted by data assimilation method (including both linear and constant Cd) and given in the form of linear formulas, on simulating pure storm surge level. They show that the data assimilation-based coefficients greatly exceed those of the ordinary coefficient formulas. Moreover, the wind drag coefficient in the linear form is a little better than that in constant form when the data assimilation method is used. Finally, the assessment of the impact of astronomical tides on the storm surge level is conducted, and the simulation demonstrates that the storm surge model, which has the combination of four constituents (M2, S2, K1, O1) and wind drag coefficient inverted by the data assimilation method with the linear Cd, exhibits the best performance.

Over the past 40 years, increasing coastal reclamation and natural sedimentation has changed coastline positions and resulted in variation in the hydrodynamic environment in the Bohai Sea (BHS), China. Based on the Landsat series images, an interpretative identifier for identifying the coastline was proposed to assess the hydrodynamic changes caused by the coastline change and was applied to a typical case of the Bohai Sea (BHS), China. We combined a grid-based coastline position with an adjoint data assimilation method to seamlessly map the distribution of the amplitude, phase lag, and tidal current of the M2 tidal constituent along the BHS’s coast from 1985 to 2018. Our findings reveal that the coastline change at long time scales dominated reclamation, and around 72.9% of the coastline of the BHS mapped in 2018 had seaward movement compared with its position in 1985. From 1985 to 2018, the BHS volume decreased by 0.17%, the sea surface area decreased by 4.54%, and the kinetic energy increased by 2.53%. The change in the coastline increased the amplitude of the M2 tidal constituent in the Bohai Bay by 6–14 cm and increased the residual current in the eastern coast of the Liaodong Bay by up to 0.07 (0.01) m/s.

Gridded reanalysis data has been widely used in oceanic mesoscale eddy identification. However, there is little research on identifying oceanic mesoscale eddies based on along-track data. In this paper, the B-spline surface fitting model is established for oceanic mesoscale eddy identification. This innovative model can embody along-track data’s advantages in accuracy, universality, and instantaneity, and directly identify oceanic mesoscale eddies solely based on along-track data. We start with ideal experiments using simulated data to explore the error sources of B-spline surface fitting for one oceanic mesoscale eddy, followed by practical experiments with measured data to analyze the effect of the B-spline surface fitting and the fitting errors further. Compared with eddies obtained from gridded reanalysis data, results show that the B-spline surface fitting model based on along-track data has an ideal effect in identifying oceanic mesoscale eddies; little difference exists between the fitting result and measured data. The model proposes a new technological means to deal with along-track data, which contributes to oceanic mesoscale eddy identification and marine element data analysis.

In the original publication [...]

High-precision tidal harmonic constants are necessary for studies involving tides. This study proposes a new method combined with the adjoint assimilation model and the Chebyshev polynomial fitting (CPF) method to obtain the tidal harmonic constants in the shallow-water region of the Bohai and Yellow Sea (BYS). Based on the CPF method, the full-field harmonic constants and reliable cotidal charts of the eight major constituents (M2, S2, K1, O1, N2, K2, P1 and Q1) were fitted from the X-TRACK products briefly and this method was effectively for coastal conditions. Compared with the observations of the X-TRACK products and tidal gauges, for the M2 constituent, the TPXO9, Finite Element Solutions 2014 (FES2014), National Astronomical Observatory 99b (NAO.99b) and Empirical Ocean Tide 20 (EOT20) models yield the root-mean-square errors (RMSEs) of 18.50, 7.31, 18.73 and 13.32 cm, respectively, while the CPF method yields an RMSE of 10.74 cm. These results indicate that the CPF method could maintain high resolution and obtain accurate cotidal charts consistent with the simulations of the four models in shallow-water regions.

Sea ice concentration is an important observation element and a necessary input parameter for the numerical prediction of sea ice. In this study, a new method is developed for the inversion of sea ice concentration (SIC) in Liaodong Bay from MODIS data based on the Normalized Difference Water Index (NDWI). By comparing the inverted results of the NDWI method and the WU’s methods, as well as observations from six shore-based observation points on the east coast of Liaodong Bay in the winter of 2016/2017 and Landsat 8 OLI data, the method based on NDWI is shown to be a useful tool to estimate the SIC. The SIC obtained by the NDWI method is in good agreement with the survey data. The mean absolute error between the SIC obtained by the NDWI method and visual observation is about 10%. In the comparison data, 92% of the absolute error values of SIC are less than 20%. The average value of the error of the SIC obtained by the NDWI method is 1%, and that of the WU’s method is about −10%, which means the accuracy of SIC obtained by the NDWI method is better. The mean deviation of the error of the SIC obtained by the NDWI method and the WU’s method is about 9% and 13%, which shows a certain improvement in precision. The validation results with the Landsat 8 OLI data show the precision of the SIC obtained by the NDWI method is better than the WU’s method, which is consistent with the validation result of the survey data.

In this study, a triangular polynomial interpolation (TPI) scheme was developed to estimate the vertical eddy viscosity coefficient (VEVC) on the basis of the Ekman model with adjoint assimilation. In the twin experiments, the advantages and disadvantages of estimating the VEVC using the TPI scheme under different factors are discussed. The results indicated that (1) the TPI scheme proves to be better than the cubic spline interpolation (CSI) and Cressman interpolation (CI) schemes; (2) the inversion results are more sensitive to observations from upper ocean layers than those from lower layers, and the TPI scheme is less likely to be influenced by missing data; (3) for various boundary layer depths, the inversion results of the TPI scheme remain consistent with the given distributions; (4) the inversion results can be influenced considerably by observational errors, and the TPI scheme is more resistant to noise than the CSI and CI schemes; and (5) the inversion accuracy of the TPI scheme can be improved by selecting the temporal wind stress drag coefficients. In practical experiments, the adjoint method with the TPI scheme was developed to estimate the Ekman currents by assimilating the observations from a buoy stationed in the Yellow Sea. The results showed the successful estimation of the VEVC and demonstrated that more precise current velocities can be obtained with this estimation scheme. In summary, this study provides a useful approach for the effective estimation of the VEVC.

Marine ranching is a stock enhancement project that has been an important part of aquaculture in China. Due to the lack of scientific management, disasters have occurred, resulting in millions of economic losses. Based on the observation system of marine ranches, a three-dimensional water quality model of marine ranches was developed to study the spatio-temporal variation of key ecological indicators, including the concentrations of chlorophyll-a, zooplankton, detritus, nutrients, and dissolved oxygen (DO). The model is coupled offline with the residual current, temperature, and salinity simulated by a regional oceanic modeling system (ROMS). The conservative characteristic finite difference (C-CFD) scheme is introduced to solve the equations, which guarantees model stability and mass conservation and allows for a larger time step compared to traditional difference schemes. In state-of-the-art water quality models, the biogeochemical processes are parameterized. Due to the complexity of the water quality model, a combination of global sensitivity analysis (GSA) and the adjoint method is introduced as the methodology to optimize the model parameters. Morris’ sampling method is implemented as the GSA method to find out the key factors of the water quality model. The optimization of sensitive parameters with the adjoint method significantly improves the model precision, while the other parameters can be set as empirical values. The results indicate that the combination of GSA and the adjoint method is efficient in the parameter optimization of the water quality model. The model is applicable in marine ranches.

Understanding nodal tidal characteristics is essential for accurate long-term tidal prediction. Observational nodal evolution of tides is mainly based on tide gauge records in coastal areas which are limited in time and space, thus impeding coherent determinations of basin-wide patterns of tidal variability. In this paper, we indicate the potential of satellite altimeter data to investigate 18.61-year nodal modulations of main constituents in the Gulf of Tonkin. Three tide gauges and multi-source satellite altimeter observations (TOPEX/Poseidon, Jason1, Jason2, and Jason3) revealed that 18.61-year nodal cycles in tidal amplitudes have noticeable deviations from the equilibrium tidal theory in the Gulf of Tonkin. In general, M2 and N2 nodal modulations are anomalously larger than theoretical values while K2, K1, and O1 nodal modulations are noticeably smaller than theoretical values. Compared to point-based tide gauges, satellite altimeter records can provide basin-wide features of nodal modulations of main constituents. Although overlapping geographical blocks are applied to eliminate the effect of tidal alias originated from long-period sampling intervals, the estimation of nodal cycles of minor constituents are still questionable. Nevertheless, the methods described here provide a strong foundation for future research on time-varying tidal dynamics using the combination of tide gauges and satellite altimeter data.

The water mass in the East China Sea (ECS) shelf has a complicated three-dimensional (3D) hydrologic structure. However, previous studies mostly concentrated on the sea surface based on the sparse in situ and incomplete satellite-derived observations. Therefore, the 3D interpolation technology was introduced for the reconstruction of hydrologic structure in the ECS shelf using in situ temperature and salinity observations in the summer and autumn of 2010 to 2011. Considering the high accuracy and good fitness of the radial basis function (RBF) methods, we applied the RBF methods to the in situ observations to completely reconstruct the 3D hydrologic fields. Other 3D interpolation methods and 2D methods were also tested for a comparison. The cubic and thin plate spline RBFs were recommended because their mean absolute error (MAE) in the 10-fold cross-validation experiments maintained the order of ~10−2. The 3D RBF reconstructions showed a reasonable 3D hydrologic structure and extra details of the water masses in the ECS shelf. It also helps evaluate regional satellite-derived sea surface temperature (SST). Comparisons between the interpolated and satellite-derived SST indicates that the large bias of satellite-derived SST in the daytime corresponds to weak mixing during low-speed wind and shows seasonal variation.

Ocean colour data are crucial for monitoring and assessing marine ecosystems. In this study, the Data Interpolating Empirical Orthogonal Functions (DINEOF) approach was applied to the Ocean Colour Climate Change Initiative (OC-CCI), chlorophyll-a (Chl-a) and Secchi disk depth (Zsd) to completely reconstruct the missing pixels in the Bohai Sea during 1997–2019. The results of cross-validation demonstrate that the DINEOF reconstructed data have a good agreement with the satellite-measured data. Based on monthly cloud-free satellite data reconstructions, the Zsd series showed high negative correlation with log10 (Chl-a). The Zsd as a function of log10 (Chl-a) can be well fitted by the cubic polynomial in the offshore waters. The Chl-a in the entire Bohai Sea showed a significant decreasing trend (−0.013 mg/m3/year), while the Zsd exhibited a significant increasing trend (0.0065 m/year), and both had regional-seasonal variations. In addition, the ensemble empirical mode decomposition (EEMD) results reveal highly nonlinear trends of Chl-a and Zsd. The linear and nonlinear trends of Chl-a and Zsd suggest the deterioration of water quality in the Bohai Sea was not continued over the past two decades. This study presents the first simultaneous investigation of Chl-a and Zsd using the 23 years of cloud-free reconstructions in the Bohai Sea.

In this study, the time-varying wind stress drag coefficient in the Ekman model was inverted by the cubic spline interpolation scheme based on the adjoint method. Twin experiments were carried out to investigate the influences of several factors on inversion results, and the conclusions were (1) the inverted distributions with the cubic spline interpolation scheme were in good agreement with the prescribed distributions of the wind stress drag coefficients, and the cubic spline interpolation scheme was superior to direct inversion by the model scheme and Cressman interpolation scheme; (2) the cubic spline interpolation scheme was more advantageous than the Cressman interpolation scheme even if there is moderate noise in the observations. The cubic spline interpolation scheme was further validated in practical experiments where Ekman currents and wind speed derived from mooring data of ocean station Papa were assimilated. The results demonstrated that the variation of the time-varying wind stress drag coefficient with time was similar to that of wind speed with time, and a more accurate inversion result could be obtained by the cubic spline interpolation scheme employing appropriate independent points. Overall, this study provides a potential way for efficient estimation of time-varying wind stress drag coefficient.

The independent point scheme (IPS) is applied to inverting initial condition with the adjoint method for the ocean pollutant transport model in this work. As an improvement, the linear Cressman interpolation is removed and the surface spline interpolation is implemented in the IPS. A series of numerical experiments are carried out to test and compare the improved IPS. And experiment results show that through applying the improved IPS, what is further reduced is mean absolute errors between simulation results and observations. Moreover, the inverted distributions are more smooth, accurate and reasonable. In addition, the application of improved IPS also reduces the variables that need to be inverted and promotes the computational efficiency. By these numerical experiment results, it is demonstrated that the combination of improved IPS and adjoint method can be used for the inversion of initial conditions and parameters estimation more effectively and reliably.

The wind stress drag coefficient plays an important role in storm surge models. This study reveals the influences of wind stress drag coefficients, which are given in form of formulas and inverted by the data assimilation method, on the storm surge levels in the Bohai Sea, Yellow Sea, and East China Sea during Typhoon 7008. In the process of data assimilation, the drag coefficient is based on the linear expression Cd = (a + b × U10) × 10−3 (generally speaking, a and b are empirical parameters determined by observed data). The results showed that the performance of the data assimilation method was far superior to those of drag coefficient formulas. Additionally, the simulated storm surge levels obviously changed in the neighborhood of typhoon eye. Furthermore, the effect of initial values of a and b in the Cd expression on the storm surge levels was also investigated when employing the data assimilation method. The results indicated that the simulation of storm surge level was the closest to the observation when a and b were simultaneously equal to zero, whereas the simulations had slight differences when the initial values of a and b were separately equal to the drag coefficients from the work of Smith, Wu, and Geernaert et al.. Therefore, we should choose appropriate initial values for a and b by using the data assimilation method. As a whole, the data assimilation method is much better than drag coefficient parameterization formulas in the simulation of storm surges.