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There are abundant mesoscale phenomena in the Bay of Bengal and near the mouth of the bay. In this study, a 2.0 version of the Chelton's data set was used, which can distinguish mesoscale eddies in low-latitude regions, and obtain the genesis distribution of mesoscale eddies by tracing. The northwest sea off Sumatra (the area with 5 °N,94 °E as the core) is an important origin of mesoscale eddies. The eddy tracking of Lagrangian method shows that there were 57 cyclonic and 40 anticyclonic mesoscale eddies in the origin (3°N—6°N, 92°E—95°E) during 19932017. Spectrum analysis indicates that there are two significant periods of 180-day and 360-day in sea surface height anomalies. The dynamic mechanism of mesoscale eddies in this origin is the combined effect of topography and wind field, the Rossby wave propagating westward along 5°N increases instability and triggers the mesoscale eddy generation under the action of the ridge topography. The equatorial wind field is an important source of energy in the origin, and the local wind field can induce the polarity of the mesoscale eddy. This study also reveals the reason why the previous literatures described the northwest sea area off Sumatra as a high eddy kinetic area, but failed to identify many mesoscale eddies.

The mid-ocean ridge is where the plates spread and the new oceanic crust forms. There are often well-developed central eruptive volcanic cones in addition to fissure eruptive lavas parallel to the ridge axis. These volcanic cones are of great significance for understanding the local and regional magmatism and tectonic activities. These volcanic cones can be identified using manual or machine interpretation methods based on seabed multi-beam bathymetric data. In this study the automatic extraction method of volcanic cones near the mid-ocean ridge was tried by means of unsupervised classification using the DEM data from Carlsberg Ridge obtained from the Chinese DY24 Cruise. The slope, surface roughness, positive and negative topography and other derivative parameters were calculated based on the original DEM data. The morphological characteristics and spatial features of volcanic cones were enhanced by feature transformation. The ISO clustering unsupervised classification method was selected to cluster and analyze the images after feature transformation. Then the method of landscape shape index in landscape ecology was introduced to complete the automatic extraction of volcanic cones. The accuracy of automatic recognition can reach around 0.8 with overlapping rate of ~0.7 comparing with manual extraction. Thus, the automatic extraction of submarine volcanic cones by means of unsupervised classification is robust and efficient, which can be of great help to large-scale data process and interpretation.

The passive continental margin ocean-continent transition zone of the southwest subbasin of the South China Sea is located in a strong extension area of the continental margin, which contains rich information about the critical extension and rupture of the lithosphere and the expansion of the ocean basin. In this research, the multi-channel seismic profile and gravity anomaly data were used to classify the passive continental margin structural units of the southwestern subbasin, the structural features of the southern and northern ocean-continent transition zones of the continental margin were studied, and the evolutionary process of continental margin extension was discussed. Multi-channel seismic profile data show that the northern ocean-continent transition zone develops rift-period rifts and tilting fault blocks that tilt toward the sea; in the southern, there are rift-period rifts controlled by low-angle normal faults, submarine volcanoes, and local uplifts. From land to ocean, the gravity anomaly changes significantly. According to above results, the passive continental margin of the southwest subbasin of the South China Sea is divided into three structural units: the proximal zone, the ocean-continent transition zone and the ocean basin, corresponding to the three stages of its extensional evolution process: pre-rift stage, continental margin rift stage and seafloor spreading stage.

Sea surface salinity is an important physical quantity for studying ocean changes and its climate effects. In this study, it was compared the monthly and daily average sea surface salinity products of the SMAP satellite in 2018 with the monthly average grid products and real-time scattered salinity data of Argo respectively, its accuracy was assessed and the global sea surface salinity distribution characteristics were analyzed. The results show that the average monthly product RMSE of SMAP satellite is 0.17, BIAS is 0.11, STD is 0.17, R is 0.98, and the t-test shows a significant correlation; the average daily product RMSE of SMAP satellite is 0.28, BIAS is 0.23, STD is 0.26, and R is 0.81. Compared with the monthly average product, the accuracy of average daily product is lower. The monthly average product deviation of SMAP satellites is small in mid-latitude seas and larger in high-latitude seas; the average daily product deviation of SMAP satellites is -0.6~0.6 in the Pacific Ocean and exceeds 1.0 in the Mediterranean Sea. The global sea surface salinity is between 25.0 and 40.0, among which the Atlantic sea surface salinity is generally higher than those of the Pacific and Indian Oceans, and the global sea surface salinity has an obvious zonal distribution pattern in the latitude direction.

While the reclamation project expands the land space to the coastal areas, it will also have a significant impact on the topographic changes of the project sea area. Based on the measured data and the results of the Delft3D mathematical modeling,  taking the eastern sea area of Ruian, Wenzhou as the research area in this study, the characteristics and dynamic mechanism of the topographic erosion and deposition changes caused by the Oufei Tidal Flat Reclamation Project were analyzed. Studies  show that the topography of the sea area has changed significantly after the implementation of the project, but the distribution characteristics of erosion and siltation in different areas are not consistent, and the dynamic mechanisms that cause changes are also different. From an overall point of view, the dominant feature of rising tides in the research sea area has been enhanced, and the coastal sea area maintains an overall siltation situation, and the siltation rate has increased slightly. From a local point of view, the vicinity of the dike bulge is affected by the effect of flip flow, which shows slow erosion, and at the concave corner of the dike affected by the blocking flow effect, it appears as rapid silting. The above results have important reference value for guiding the protection and utilization of Oufei Tidal Flat and other similar tidal flats.

In order to understand the geochemical characteristics in surface sediments from Sanya offshore area, the contents of 23 trace elements in surface sediments of 331 stations were determined. Multivariate statistical method was used for correlation and R-type factor analysis to explore the source, distribution characteristics and control factors of elements in the study area. The determination of trace elements show that the average values of Sr, Ba, Zr and Rb are higher than those of other elements; The contents of 15 elements such as Be, Cr, Co, Ni, Cu, Zn, Cd, Sb, Pb, U, Zr, Ba, Sc, V and Ga are close to those of stream sediments, surface soil and deep soil in Hainan Island. It is speculated that the sediments mainly come from Hainan Island. The spatial distribution of elements in the study area is complex. In the middle and east, the contents of most trace elements(15) represented by Li increase with the increase of offshore distances; The distribution characteristics of Cd, Sb, Zr, Ba, Rb, Sr, As and Hg are significantly different from other trace elements. The source and distribution of 15 elements such as Li are mainly controlled by the adsorption of terrigenous clay minerals, the source and distribution of Sr are mainly controlled by marine biogenic sedimentation, the source and distribution of Zr are mainly controlled by heavy minerals, the source and distribution of Ba and Rb are mainly controlled by the geological background of the study area, while the source and distribution of Cd and As are related to human activities.

Using the MODIS remote sensing data from Jan. 2003 to Dec. 2019, and combining with the data of sea surface temperature(SST) and wind speed(WS), the chlorophyll concentrations distribution in the mid-western South China Sea and their controlling factors were discussed. The results showed the seasonal variations of the spatio-temporal distribution of chlorophyll concentration in the study area. EOF decomposition indicated that EOF1 probably reflected the impact of extreme weather such as typhoons on the concentration of chlorophyll, while EOF2 and EOF3 reflected influence of the coastal upwelling. The correlation analysis showed that the concentration of chlorophyll had good positive correlation with the WS (r=0.87, p<0.01), and negative correlation with SST (r=-0.59, p<0.05). In summer, the upwelling current formed along the southeastern coast of Vietnam under the influence of southwest monsoon, and it resulted in flourishing of phytoplankton and increase of chlorophyll concentration. In winter, the mixing of upper ocean layer became strong and the nutrient supply increased under the influence of strong northeast monsoon. It promoted the growth of phytoplankton and resulted in higher chlorophyll concentration than that in other seasons.