Quantitative analysis on the properties of marine phytoplankton bloom events is helpful to understand the marine ecology, environment, and dynamic processes. In the South China Sea, remote sensing is vulnerable to clouds. Previous studies were mostly conducted with discontinuous observation or remote sensing data, which failed to comprehensively understand the characteristics and controlling factors of marine bloom events in the South China Sea. This study applied the statistical framework for defining marine heat waves, based on previously reconstructed daily full coverage remotely sensed chlorophyll a product from 2005 to 2019, to extract marine bloom events in two representative subregions, i.e., the northwestern Luzon in winter and southeastern Vietnam in summer. The duration, intensity, and corresponding long-term trends of marine bloom events in the two representative sea areas were analyzed. The results showed that the bloom frequency in the Luzon Strait has decreased, while the intensity has increased. These trends were significant in most areas for the winter Luzon Strait, while the trends for most of the summer Vietnam coasts were not significant. We further analyzed the influencing factors of marine bloom events and found that the winds (positive correlation) and sea surface temperature (negative correlation) had the greatest impacts on the bloom events. In both two representative sea areas, the marine bloom events were mainly dominated by upwelling, and the wind was the most important influencing factor. Analysis on the precursor conditions of marine bloom events found that the sub-mesoscale activity represented by the temperature front is also an important influencing factor. The study applied in South China Sea as a case study, as well can provide a new perspective to study marine ecosystem and environment.

Using mesoscale eddy trajectory product from 1993 to 2019 provided by the AVISO, this study analyzes the climatology characteristics and seasonal variations of mesoscale eddies in the Andaman Sea (AS). The results show that a total of 328 mesoscale eddies were generated in the AS during the past 27 years, of which anticyclonic eddies (AEs) (171) were more than cyclonic eddies (CEs) (157). The eddies are mainly distributed in the deep waters of the central and western basin of the AS. The average life span of total eddies is 46.4 days, with average eddy radius of 111.8 km, average amplitude of 4.7 cm, rotating and propagating speeds of 24.8 cm/s and 15.0 cm/s, respectively. The AEs have large radius, amplitude, and rotating speed than CEs, but smaller propagating speed. During the eddy life, the composite radius, amplitude, and rotating speed of eddies all increase in the generation stage and then decrease in dissipation stage, while the eddy propagation speed has opposite trend. For the seasonal variation, the comparison of AEs to CEs shows seesaw phenomena in winter and summer, CEs are stronger and larger than AEs in summer, but weaker and smaller in winter. The distribution of AEs and CEs also shows seasonal polarity reversal, a ‘CEs-AEs-CEs’ pattern from north to south in summer, but a reversed ‘AEs-CEs-AEs’ pattern in winter. Dynamic analysis showed that the vorticity of background current may affect the alternating distribution pattern of mesoscale eddies in the AS, in which positive (negative) vorticity favors CEs (AEs). The energy analysis shows that wind forcing is dominant in the eddy kinetic energy (EKE) variation, and the seasonal wind work is coherent to the EKE variation in the AS.

Based on the extreme value theory of non-stationary sequences, this study carried out the quantitative analysis of the effect of sea level rise on the exceedance probability of extreme water levels. A new method for the estimation of extreme water level with sea level rise was proposed by adopting the overall exceedance probability of extreme water level within the design lifetime of coastal facilities as a critical constraint. With the incorporation of sea level rise in the location parameter of Gumbel distribution, the new method allows the adjustment of the annual exceedance probability of extreme water levels along with sea level rise over time. The validity of the proposed method was examined using the long term sea level measurement data at 10 tide gauge stations globally. Using the five global mean sea level rise scenarios projected by IPCC, the extreme water levels for different design lifetime of coastal facilities with sea level rise were estimated, and the return periods of the extreme water levels were also evaluated.

Ocean fronts variations in strength are key to the terrestrial material transport and global material cycle. Ocean temperature fronts are formed between the branches of West Pacific Boundary Current and the coastal current during the winter and spring seasons in the eastern shelf of China. In order to investigate the multi-time scale variation and main controlling factors of temperature front over the Yellow Sea under the dual influence of winter storms and shelf circulation, we investigate the coupling of low-latitude driven circulation systems and high-latitude driven winter storms on frontal variability with the methods of signal decomposition and explainable deep learning on the decadal and weather scale. On the decadal scale, empirical orthogonal decomposition and ensemble empirical modal decomposition are used to relate temperature changes in the Yellow Sea to the strength of the Yellow Sea Warm Current. The results indicate that the spatial distribution of first sea surface temperature (SST) EOF mode has obvious characteristics of the Yellow Sea Warm Current-coastal current system; the time series of the first SST EOF mode correlates well with the Yellow Sea Warm Current intensity index and is modulated by the low frequency ENSO signal. On the weather scale, this paper trains CNN-LSTM models and uses interpretability metrics to conduct the research. The results show that, in windless or weak wind conditions, the strength of ocean front is maintained by the combination of pressure gradient forces resulted from sea surface height and Coriolis forces caused by flow field. However, in the storm conditions, influenced by Kelvin Wave propagation and shear front fragmentation, the flow field is responsible for the ocean front variation. The results of this study show that big data and machine learning methods are important means to establish connections between many ocean parameters and discover some unique physical ocean processes, which have broad application prospects.

Deep water breaking wave is usually generated by the wave focusing method in laboratory. A wave with a significantly increased height can be generated by the superposition of components so that wave breaks when the steepness exceeds the limit value. However, using this method, the number of wave breaking is usually not unique leading to less obvious field characteristics after breaking and it’s hard to set research cases due to the difficulty in selecting wave-making parameters. As a result, the effect and efficiency of elaborate deep water wave breaking experiments is affected directly. Theoretical wave surface was calculated by the theory of wave focusing method and theoretical wave steepness was calculated using the wave height and wavelength defined by up-cross-zero method in this paper. On the other hand, wave breaking times and intensity were obtained from the physical experiments. The JONSWAP spectrum was used as the input spectrum and the effects of wave-making parameters, such as focusing amplitude, spectrum peak frequency, frequency bandwidth, on wave breaking were discussed so as to establish an approximate quantitative relationship between breaking times and input parameters. The research conclusions can provide a reference for selection of wave-making parameters so that the efficiency of the experiment can be improved.

The settling velocity is an important physical parameter of coral sand. Because of the rod coral sand is obviously different from other shapes of coral sand, it is not suitable to apply the settling velocity formula of the existing coral sand for calculation. The rod coral sand was selected to study the settling velocity and its influencing factors for single particle settlement experiment in stagnant water in this study. By analyzing the effects of different equivalent particle sizes and shape coefficients on the settling velocity of rod coral sand, it is found that the settling velocity of rod coral sand is strongly correlated with the diameter of the volume-equivalent sphere and Corey shape coefficient. Based on the experimental data, an empirical formula suitable for calculating the settling velocity of rod coral sand is deduced, which enriches the theory of coastal sediment.

Estuarine bay is a special geomorphological system, which is often charactered with the combination of shoals and channels. Its evolution and shifting balance of shoal-channel structure are not only the frontier hotspots of estuarine and coastal researches, but also the theoretical cornerstone of estuarine management and port waterway construction. Based on the nautical bathymetry data of different historical ages, using geomorphological information entropy and other analytical methods, this paper studies the phylogenetic evolution characteristics and shifting balance mechanism of the shoal-channel structure in Huangmaohai Estuary Bay (HEB), and the main findings include: (1) The geomorphological evolution of HEB has undergone a transition from siltation to erosion during 1940 to 2007. The average annual siltation rate was 1.4 cm/a. From then on, till 2015, the bay had shifted to erosion period and the average annual erosion rate was 1.2 cm/a. (2) The structure of shoal and channel in HEB was converted, and the structure of "three shoals and two channels" was transformed into the structure of "two shoals and one channel" between 2003 and 2007, and the elevation differentiation between shoals and channels tended to be enlarged due to the impact of reclamation in the shallow coasts and dredging projects in channels. As a result, the shallow shoals became shallower and the channels were deeper. (3) The entropy value of geomorphological information of HEB decreased first and then increased, indicating HEB it changed from the relatively stable mode of slow siltation to the abrupt changing mode, in which the uncertainty of the shoal-channel evolution was induced by human intervention; moreover, the steady states of the shoal and channel are different. The shoals showed higher uncertainty than the deep channel. (4) Human activities disturbed the original steady state of the shoal-channel structure, and the dredging project will enhance the vitality of the East Channel and promote the adaptive reconstruction in the deep channel to evolve in an orderly direction. The West Shoal and the East Shoal have undergone a disturbance balance of human activities such as reclamation, and the shoal instability has shown a disorderly evolution.

To explore the source of platinum group elements (PGE) in cobalt-rich crusts, the samples from Caiwei Seamounts in western Pacific were chosen as the research object, for which XRD, ICP-OES and ICP-MS were used to analyze the mineral composition, major elements contents and PGE contents in cobalt-rich crusts. The results showed that, the main crystalline minerals were vernadites in cobalt-rich crusts, and the minor minerals included quartz, plagioclase, potassium feldspar and carbon fluoride apatite. Also many amorphous ferric minerals were contained in cobalt-rich crusts. In addition, Mn and Fe contents were the highest in cobalt-rich crusts, and PGE were enriched in cobalt-rich crusts. PGE contents were 142−1352 ng/g, and Pt contents were 114−1268 ng/g, in which Pt accounted for more than 80%. PGE contents in the old layers were higher than that in the new layers. And the phosphatization appeared in the old crust layers. Moreover, there was an obvious contrast in PGE of cobalt-rich crusts, Pd group (PPGE) contents were more than Ir group (IPGE). PGE diagrams showed the positive Pt anomalies and negative Pd anomalies. And Pt was enriched and Pd was poor in cobalt-rich crusts. Meanwhile, PGE in cobalt-rich crusts were probably derived from the reaction between seamount basalt and seawater during the oceanic shell erosion process. PGE had positive correlation with CaO, P₂O₅, Ba and Cu, so PGE were probably enriched in the phosphate phase. In addition, the growth profile of cobalt-rich crust was from top to bottom and from new to old, PGE contents increased gradually. Therefore, oxidative marine environment and high marine productivity had a positive impact on phosphatization, which further promoted the enrichment of PGE. This study provided a certain reference value for revealing the source of PGE in cobalt-rich crusts.

In order to study the carbon burial rates and sources of early to middle Holocene sediments in the Changjiang River palaeo-valley area, the determination of total organic carbon (TOC), total nitrogen (TN) and δ¹³C of sediments from ZK1 hole was carried out, and the spatial and temporal distribution characteristics and TOC sources of carbon burial in the Changjiang River palaeo-valley were analyzed by combining AMS¹⁴C (plant debris, shells) dating, foraminiferal and grain size data. Using a combination of historical geography and sedimentary geology combined with AMS¹⁴C data, the chronostratigraphic classification was carried out from bottom to top as tidal channel (U1), estuarine bay (U2), tidal sand ridge (U3), and pre-delta (U4). Sediments were influenced by the effects or factors of water depth, runoff, estuarine after-circulation, tidal currents, waves, storms and remineralization, and the mean value of TOC was 0.41%, which was lower than the surface layer of the Changjiang River Estuary. The ZK1 hole carbon burial flux (TOC_BF) ranged from 7.4 g/(m²·a) to 110.5 g/(m²·a), with large variations. The TOC_BF values were mainly controlled by the sedimentation rate. The results of δ¹³C and TOC/TN (C/N) projection points indicate that there are TOC sources with multi-source characteristics, and the overall performance is partial to terrestrial sources. The linear fit correlation between C/N and δ¹³C was high, which allowed the quantitative analysis of organic matter sources using C/N and δ¹³C. The organic carbon source analysis was carried out based on C/N and δ¹³C using a three-terminal source model. The U2 and U3 depositional periods were in the Holocene Great Warm Period, and the sea-derived and land-derived carbon was higher than that of the U1 and U2 depositional units, which was mainly related to the increase of marine and terrestrial primary productivity. The mean contribution of marine phytoplankton to total organic carbon was 31% in ZK1 hole. The mean contribution of estuarine phytoplankton to total organic carbon was 31%; the mean contribution of terrestrial organic carbon to total organic carbon was 38%. Overall, the contribution of terrestrial organic carbon was greater than that of estuarine phytoplankton and marine phytoplankton, which was basically consistent with the analysis results of the graphical method.

Marginal salt marsh patches play a crucial role in the morphological evolution of salt marsh-tidal flat systems by dissipating hydrodynamics and stabilizing sediment, and the tidal channel can also influence the growth, expansion and erosion of the salt marsh patches. However, the interactions between saltmarsh patch expansion and tidal channel formation are complex and poorly understood. In the study, we established a two-dimensional biomorphodynamic model and introduced a dynamic vegetation module to simulate the spatial-temporal distribution of saltmarsh patches and the geomorphic evolution of the tidal channel system. We explored the two-way feedback between the spatial patterns of the tidal trench system and salt marsh vegetation patches with different initial numbers. Model results showed that the tidal channel extended rapidly to both sides of the sea and land at first, and then developed a large number of creeks, and the salt marsh patches expanded to the periphery and gradually formed a large patch. Besides, the presence of marsh patches can increase the density of tidal channels and promote the development of tidal channels. Further, the orientation of tidal channels was affected by the spatial distribution of marsh patches, which can divert water flow and induce the concentration of tidal flow. Specially, in the early stage of saltmarsh evolution, more tidal channels were formed by the interactions between hydrodynamics and sediment motion with the increase of marsh patch numbers, and in the later stage, the influence of salt marsh clusters gradually changed from promotion to stabilization. However, the expansion and the spatial distribution pattern of salt marsh patches was later limited by the formation of tidal channels reciprocally. Our study extended current understanding of the mechanisms underlying the co-evolution of marsh patches and tidal channels, and can provide scientific basis for future works on coastal protection and restoration.