Sea level rise, strong typhoon and storm surge have increasingly serious impacts on coastal flood disasters in the southeastern China in the context of climate change. To address the climate change and estimate the hazard of extreme sea level in the future, based on the multiple data of oceanographic and atmospheric observation and methods of mathematical statistics, the impact of historical strong typhoon No.9914 (Dan) and No.1614 (Meranti) on the extreme sea level in coastal waters of Xiamen and estimates the changes and return of extreme sea level at scenarios of future sea level rise under RCP4.5 and RCP8.5 was analyzed in this paper. The results show that: (1) astronomical tide, storm surges and heavy precipitation induced the extreme sea level event (732 cm) during the period of strong typhoon No.9914; (2) the combined impacts of multiple hazards such as wind (ashore gale), rainfall (heavy precipitation), waves (giant waves), tides (high tide) and current (torrents) is an important cause of serious disasters in coastland of Xiamen; (3) under the medium and high greenhouse gas emission scenarios (RCP4.5 and RCP8.5), by 2050 and 2100, recent 1-in-100-year extreme sea level event will become 1-in-30-year (1-in-2-year) and 1-in-25-year (1-in-less than 1-year). It shows that the hazards of extreme sea level on coastland of Xiamen will increase significantly, and high adaptation measures in Xiamen should be taken to reduce the risk on coastal flood hazards in the future.

Salinity mixing and stratification in the river-dominated estuaries are important dynamic mechanisms for controlling transport and diffusion of suspended sediment. Based on the synchronous field investigation with three surveying vessels in the flood season in 2017, covering the spring and neap tidal cycles, the influence mechanism of salinity stratification on suspended sediment distribution in the Modaomen Estuary was analyzed in this study. Vertical distribution of salinity in the estuary also displayed spatial differences under the influence of interaction between riverine and tidal dynamics. The salinity at M1 Station (Guading Jiao), dominated by runoff, was mixed well vertically over the tidal cycles; salinity stratification occurred at the M2 Station (outlet location) and at M3 Station (outside the mouth), which were influenced by interaction between runoff and tide over the tidal cycles. Spatial distribution of suspended sediment was closely related to spatial distribution of salinity. In general, salinity mixing promoted the vertical mixing of suspended sediment, while salinity stratification constrained the suspended sediment to be concentrated in the bottom water layer, and high suspended sediment concentration (SSC) tended to appear at the layer where salinity stratified occur. The vertical distribution curve of SSC was L-shape or paracurve shape, while high SSC always concentrated in the front of the salinity wedge in the longitudinal direction, indicating a significant sediment trapping effect caused by salinity stratification. Comparing the stratification ratio and vertical diffusion coefficient at three gauging stations, there was a negative relationship between them, the larger stratification ratio is, the smaller vertical diffusion coefficient is, indicating the suppression effect of stratification on vertical diffusion. Furthermore, the higher stratification is, the larger suppression effect is. Such mechanism contributes to the sediment trapping caused by stratification. This study is helpful to reveal the mechanism of fine sediment movement and the mechanism of evolution of mouth bar in a complicated estuary, and provide scientific basis for regulation of mouth bar in the Modaomen Estuary.

Numerical simulation, as the major research method of tsunami, plays a key role in tsunami warning. The present paper develops a second-order accuracy numerical tsunami model in the spherical coordinate using the Godunov-type finite volume method and MUSCL-Hancock scheme. An HLLC approximate Riemann solver is employed to evaluate fluxes across cell interfaces. The well-balanced expression format of shallow water equations ensures the numerical stability, while the local topography reconstruction method is used to deal with the moving shoreline boundary. The model is used to investigate the propagation of the 16 September 2015 Chile tsunami. The capability of the model is verified by comparison with the observational data from 14 coastal tidal-gauge stations near Chile and 20 DART buoys covering the Pacific Ocean.

The estuary area of the Zhujiang River Delta is significantly influenced by its drainage and offshore environment. Since being under the interaction of sea and land, the depositional system within the estuary area could be of great potential to reveal the simultaneous succession of geological and geomorphic processes. This paper presents a magnetic investigation report of a sedimentary core drilled from the Lingdingyang Bay at the Zhujiang River Estuary. Results of this study show that, the concentration of magnetic minerals or the source area of the high-coercivity magnetic minerals within the core sediments is relatively constant during the two periods of relatively high sea-level (i.e., under marine transgression) since late Pleistocene, revealing a stable sedimentation environment or a relatively fixed sediment source, respectively; however, during the period of marine regression, when the weathering and denudation processes dominate, the magnetic mineral composition in sediments is characterized by significant large-scale fluctuations, possibly indicating that the source areas of detrital materials have undergone a dramatic climate evolution. Multiple environmental magnetic parameters, including low-frequency magnetic susceptibility, anhysterestic remanent magnetization, saturation isothermal remanent magnetization, and HIRM (the ‘hard’ isothermal remanent magnetization), reveal a synchronous pattern of magnetic variations through the core. In comparison, the S-ratio record of the core sediments is more sensitive to the sedimentation evolution history during the marine transgression and regression in the Lingdingyang Bay at the Zhujiang River Estuary.

The complex subaqueous channel system of shallow and transition facies is developed in the Pliocene-Pleistocene strata at the conjunction area of Malay Basin and West Natuna Basin, and can not be characterized in detail by conventional seismic data. By applying Hilbert-Huang transform to post-stack seismic data, the high frequency components of seismic data were extracted, the resolution of seismic data was improved, and the interaction characteristics of thin layer sand and mudstone and small sedimentary bodies were identified effectively. Through the instantaneous attribute extraction of the high frequency component, the plane distribution characteristics of the underwater branch channels in the target interval are defined. Compared with conventional post-stack seismic attributes, the instantaneous attributes extracted from post-stack seismic data after Hilbert-Huang transform show more detailed features of sedimentary system. And it provides high-resolution seismic data for the study of spatio-temporal evolution of underwater branch channels, such as internal structure, development period and cutting relationship.

Samples of International Ocean Discovery Program (IODP) 368 Site U1501 between core depth 264.0−331.1 m were collected and analyzed in this study. Through foraminifera oxygen isotopic stratigraphy and Sr isotopic dating, the age of this depth interval ranges from early Miocene 20.3 Ma to late Oligocene 32.0 Ma (with seismic reflector T60 bottom age at 28−30.5 Ma). After T60 event, 3 proxies including the content of organic carbon, stable δ¹³C of benthic foraminifera and the difference of δ¹³C between planktonic and benthic foraminifera (Δδ¹³C_P-B) showed the sea surface paleo-productivity weakened; meanwhile, the carbonate content and the organic carbon/nitrogen ratio revealed the terrigenous input decreased. Combined with the changes of relative abundance of planktonic foraminifera and the benthic foraminifera faunal combinations, these paleo-environmental proxies implies that during late Oligocene to early Miocene, the regional tectonic subsidence in the northern South China Sea leaded to the deepening of the paleo-water depth and increasing the offshore distance at Site U1501 after the T60 event. These conclusions provide micropaleontology evidence for better understanding of T60 tectonic event and subsequent sedimentary environment researches.

The seasonal variability of M₂ constituent exerts a major influence on the coastal ocean environment. The enhanced harmonic analysis (EHA) can synchronously extract the temporally varying amplitudes and phase lags of significant constituent and the constant values of the other constituents. In this study, the seasonal variability of the M₂ constituent in the Bohai Bay, China, is investigated by analyzing one-year sea level observations at two stations with EHA. In order to evaluate the accuracy of EHA, artificial "sea level" is designed in the ideal experiment. Both the estimated temporally varying amplitude and phase lag of the M₂ constituent and constant values of the S₂, K₁ and O₁ constituents using EHA were much closer to the prescribed values than those obtained using the other methods, indicating the capability and efficacy of EHA. When the real sea level observations were analyzed using EHA, the estimated M₂ constituent amplitude has significant seasonal variability, with large values in summer and small values in winter. The sensitivity experiments show that the trend of seasonal variation of M₂ amplitude in Bohai Bay is not affected by the experimental settings, and it is robust and can reflect the real seasonal variation of M₂ constituent. The seasonal variations in the M₂ amplitude is possibly induced by the seasonally alternating Asia monsoon, as the Asia monsoon dominates the seasonality of mean sea level, stratification and eddy viscosity, which are main influence factors summarized in the previous studies.

In view of the problems of small available temperature difference and low utilization efficiency of ocean thermal energy conversion (OTEC). A new thermodynamic cycle of OTEC using non-azeotropic mixed working fluid is proposed. Based on the laws of thermodynamics, the thermodynamic analysis of the proposed thermodynamic cycle is carried out. Evaporation pressure, mass fraction of working fluid, evaporation temperature and condensation temperature are selected as influencing variables to study the proposed thermodynamic cycle. The results show that when the working fluid mass fraction is selected as a variable, with the evaporation pressure increases the cycle thermal efficiency and the net output of system increase first and then decrease. The system thermal efficiency achieves a maximum value of 5.28% when the working fluid mass fraction is 0.91. The maximum value of 3.83 kW is obtained when the mass fraction of the working fluid is 0.96. When the evaporation pressure is selected as the variable, the cycle thermal efficiency and the net output of the system increase first and then decrease with the mass fraction of the working medium increases. The cycle thermal efficiency achieves a maximum value of 5.26% when the evaporating pressure is 0.595 MPa, and the net output work obtains a maximum value of 3.57 kW when the evaporating pressure is 0.58 MPa. Compared with Uehara cycle and Yoon cycle under the same operating conditions, the proposed cycle system has the highest thermal efficiency. The analysis results of the proposed thermal cycle system can provide a theoretical basis and reference for improving the utilization efficiency of OTEC.

Cohesive sediments are widely distributed in the subaqueous delta of the Huanghe River and can easily cause problems such as sedimentation, erosion and sea-bed fluidization under the action of external load, which poses a great threat to the engineering facilities such as ports, waterways and submarine pipelines. Samples of cohesive sediment with different consolidation times and different water contents were prepared by using the undisturbed cohesive sediment taken from the fluid mud development area of the Huanghe River subaqueous delta. The rheological tests were carried out under the mode of full shear rate with R/S rheometer to analyze the rheological characteristics of cohesive sediment in the Huanghe River subaqueous delta and the effects of water content and consolidation time on the rheological characteristics. The results show that the cohesive sediment flows and becomes unstable under the shear load, and the phase changes. The yield stress increased by 35% after being consolidated for 120 min. The shear thickening behavior of the cohesive sediment with 50% water content at a high shear rate was more obvious with the increase of water content. Power model is applicable to the rheological behavior of the cohesive sediment with a water content greater than 50% at high shear rate. This study can provide a reference for the numerical simulation of subaqueous cohesive sediment and the prediction of subaqueous gravity flow.

Multidecadal variability of rapid intensification of tropical cyclones passing through the South China Sea (SCS) is investigated using the best track data set of tropical cyclones archived at the China Meteorological Administration, the National Center for Atmospheric Research/National Center for Environmental Prediction atmospheric reanalysis data set, and the China Ocean Reanalysis oceanic reanalysis data set from the National Marine Data and Information Service. During 1951−2017, tropical cyclones passing through the SCS are occurred concentratedly from June through December, among which the tropical cyclones with rapid intensification (RI) are concentrated from July through December. RI events have shown multidecadal variability, which is significantly correlated with the Pacific Decadal Oscillation (PDO). In positive PDO years, RI events, with relatively low frequency, mainly distribute in Eastern Philippines and north of the SCS, while in negative PDO years, RI events, with relatively high frequency, mainly distribute in a large area of eastern Philippines. The multidecadal variation of RI of tropical cyclones passing through the SCS is related to large-scale oceanic and atmospheric variables modulated by PDO. Regression analysis shows that tropical cyclone heat potential has the greatest effect on the multidecadal variation of RI, while relative humidity has a relatively small effect and the effect of vertical wind shear is smaller.