Rip current is a kind of high speed offshore current which occurs frequently in coastal. Based on Castelle’s rip current classification model, Google Earth images of 14 beaches in Guangdong Province were interpreted and classified, and the application of rip current classification model in remote sensing images was discussed. The results show that in high risk months, the composition of rip currents is relatively simple, the mixed rips are few, and the bathymetrically-controlled rip currents play a dominant role, and the number of rip currents remains at a high level. In the middle risk months, the proportion of hydrodynamically-controlled rip currents and mixed rips increases, while the proportion of bathymetrically-controlled rip currents still has a certain proportion. There is little or no rip currents in low-risk months. The structure of rip current is closely related to the state of beach. For the beach in the bay, the length of the bay affects the number of rip currents, and the concave degree of the bay affects the composition ratio of different types of rip currents. Considering the limitations of experimental conditions, this law needs to be further studied and verified in combination with the actual terrain and landform. The classification results obtained according to the interpretation criteria presented in this paper are in good agreement with the risk evaluation model proposed by previous authors, which further demonstrates the effectiveness of the interpretation criteria and provides some reference value for the future research and classification of rip currents.

Affected by fluvial sediment discharge, wind waves and tides, etc., suspended sediment in estuaries and adjacent waters has significant temporal and spatial changes. Based on the GOCI remote sensing images at hourly resolution, this paper uses the optimal remote sensing inversion algorithm, combined with spatial analysis and statistical methods to deeply study the temporal and spatial dynamic characteristics and driving mechanism of suspended sediment in the Yellow River Estuary and adjacent waters. The results show that the impact of fluvial sediment discharge on the concentration of suspended sediment is concentrated in the nearshore area of the estuary. The impact of high fluvial sediment discharge on the distribution of suspended sediment concentration can reach about 20 km offshore and spread to the Gudong nearshore area. Strong winds can cause intensive resuspension of sediment near the old river mouth of Qingshuigou, forming a high-concentration suspended sediment area. The flood and ebb tides have a significant impact on the hourly-scale suspended sediment concentration, and affect the dispersal of suspended sediment from north to south. The variation and dispersal range of the suspended sediment concentration in the spring tide are larger than that of the neap tide due to different tidal flow velocities. There is an obvious negative correlation between water depth and suspended sediment concentration. According to the difference of driving factors, the suspended sediment concentration present exponential, power function, and linear relationships with the increase of water depth.

Magnetic mineral diagenesis is an important early diagenetic process after the burial of sediments and its proper identification is the precondition of interpretations for the mineral magnetic properties in the sediments. This study carried out analyses of sedimentary facies, room temperature magnetic and thermomagnetic properties in a Holocene Core MZ collected in the Shunde Plain of the Zhujiang River Delta to identify the vertical changes in the assemblage of magnetic minerals, so as to explore the early diagenetic stages and possible linkage to the sedimentary facies. The results show that the Holocene sedimentary sequence of Core MZ includes tidal channel, embayment, and deltaic successions from bottom upward. The magnetic properties at room temperature lack correlation with sedimentary facies and demonstrate features of strong early diagenesis. In addition, the magnetic properties of the late Holocene sediments were strongly influenced by the human activities. The early diagenesis mainly includes the dissolution of magnetic minerals and the formation of authigenic pyrite. Greigite was also identified in the upper section of the delta-front succession and the bottom of embayment succession. The concentration of greigite increases with depth in the embayment succession. According to the magnetic mineral assemblages, we infer different formation mechanism of greigite in the two successions. We suggest that the greigite in the delta-front facies was formed in the sulfate reduction stage of early diagenesis, whilst it was formed in the anaerobic oxidation stage of methane in the embayment facies. These phenomena indicate that sedimentary environment has impacts on the early diagenetic stage of magnetic minerals by controlling the availability of organic matter and sulfate.

To understand the internal physical mechanism of wave breaking, it is important to study the distribution characteristics of the particle velocity field under breaking wave. In addition, a comparative study of the evolution characteristics of the gas-liquid mixing zone caused by different types of breaking is beneficial to the improvement of the whitecap coverage model. In the laboratory wave flume, a critical wave, a single spilling wave, and a single plunging wave are generated in deep water based on the linear phase focusing theory. The velocity fields below the wave surface and the velocity fields in the gas-liquid mixing zone are measured using particle image velocimetry (PIV) and bubble image velocimetry (BIV), respectively. The distribution characteristics of the velocity field at the extreme state of different breaking types are compared and discussed. The results show that the horizontal velocity u and vertical velocity v of the spilling wave are extremely asymmetrical in the front part and back part of the wave crest. In addition, the maximum horizontal velocity u_max is not at the top of the wave peak, but at the pre-peak position about 0.7$\eta_{\max} $ front part and back part of the wave crest. In addition, the maximum horizontal velocity of the peak. For plunging wave, the maximum horizontal velocity u_max appears at the top and very front of the wave peak with a very small region, and the velocity gradient between this area and the surrounding area is very large. The development characteristics of the gas-liquid mixing zone produced by different wave breaking also have differences. For spilling wave, the gas-liquid mixing zone generated by breaking has high horizontal movement speed, long influencing area, short influence time at each location, and small thickness. For plunging wave, the gas-liquid mixing zone has relatively slow horizontal movement speed and larger vertical input, shorter affected area, longer affected time at each location, and greater thickness.small thickness. However, for plunging wave, these characteristic parameters are in contrast with those of the spilling wave.

The research of tidal changes is vital to ocean engineering, the protection of flooding as well as the utilization of ocean resources in coastal areas. Previous studies are mainly based on long-term hourly tide gauges whose number and location are highly limited. Previous study indicated that the long-term trends of tidal amplitudes are abnormally large in the central deep basin of the South China Sea (SCS) based on 25-year satellite altimeter data, which is not realistic and the resultant of the interference of ocean mesoscale variability on tidal harmonic analysis. In this paper, we use 27-year T/P-Jason satellite altimeter data processed by X-TRACK software to study the long-term tidal trends in the SCS. The satellite altimeter data processed by X-TRACK obviously improves the accuracy and completeness of satellite data in the SCS. Meanwhile, the weighted least square method is used to eliminate the effect of tidal aliasing caused by long period sampling. We find that in most areas of the SCS, the amplitudes of four major tidal constituents have significant long-term trends. The significant positive and negative long-term trends of amplitudes and phases are mainly distributed in the coastal areas such as the western Luzon Strait, the Strait of Malacca and Taiwan Strait, where the water depth and shoreline change dramatically. The largest positive long-term trend of amplitudes can reach 2.75 mm/a and the largest negative long-term trend of amplitudes can reach −2.16 mm/a. The long-term trends of tidal amplitudes in the SCS should be related to river flow and human interference.

When oil spill occurs and the large scale covering on the sea surface has not been formed, it is hard to find oil film by existing oil spill detection technology. To solve this problem, a novel method for discriminating of oil spill by monitoring the area of oil film is presented based on thermal infrared video image, which combined with the diffusion characteristic of oil spill. Firstly, foreground regions (real oil film region and look-alikes interference region) on the sea surface are extracted and the actual physical area of each region is calculated based on single-frame thermal infrared image processing (i.e., the pixel area calculation method from the previous research). According to the video image processing, the change of the actual physical area of each region is tracked in real-time. The area change rate threshold is set to discriminate whether oil film on the foreground regions, then whether oil spill happened can be determined. The experimental results show that the proposed method can effectively discriminate oil film formed by different viscosity of oil and maintain good identification accuracy under sea surface with waves and floating objects. This strategy is suitable for specific scenes such as wharves and ships, and also can provide technical support for pollution control of oil spill.

The marine ecological disasters occurred frequently in recent years. A large number of floating algae gather in the sea surface and inshore water, which brings serious harm to the economic activities and ecological health of coastal cities. In this study, HY-1C, GF-1 and HJ-1A/1B satellite remote sensing images were used to extract information and divide the growth stages of Sargassum in the South Yellow Sea from April to June in 2016 to 2020, combined with the MODIS sea surface temperature data, photosynthetically active radiation data and sea surface wind data to explore the effects of environmental factors on the temporal and spatial distribution of Sargassum. The results show that: (1) In terms of time, the Sargassum mainly appeared from April to June every year. Sargassum had the largest range of influence in 2017 and smaller in other years. In terms of space, the Sargassum was first detected in the far sea northeast of the Changjiang River Estuary and disappeared in the sea near 35°−36°N. (2) In terms of growth rate, the growth phase of Sargassum could be divided into three stages: “development-outbreak-extinction”. (3) At different growth stages, sea surface temperature and photosynthetically active radiation had different degrees of influence on Sargassum. The larger area of Sargassum in 2017 was mainly influenced by higher sea surface temperature and photosynthetically active radiation. The southeast monsoon promoted the drift of Sargassum from the southeast to the northwest and north. This shows that the spatiotemporal characteristics of Sargassum are affected by a variety of environmental factors.

Chinese Ocean Color Temperature Scanner (COCTS) of Haiyang 1C/D (HY-1C/D) satellite is mainly used to detect ocean water color, water temperature and other elements. These elements can only be achieved by processing satellite data, and geometric positioning is the core of preprocessing, which directly affects quality of these elements. COCTS has the characteristics of 114° field of view and quaternary whisk broom point by point. A set of complete geometric positioning method has been developed based on COCTS characteristics. The satellite position and velocity corresponding to the sampling time are obtained by using the interpolation method in the satellite ephemeris extracted from 0 level data, and then the transformation matrix from orbital coordinate system (ORB) to earth-centered rotating coordinate system (ECR) will be achieved. Based on the quaternary whisk broom point by point, the ORB viewing vector of every sampling point in a sweep can be calculated by rotating center viewing vector around X and Y axis in corresponding angles. The relationship model of viewing vector and the earth intersection point can be established to carry out geolocation of remote sensing images obtained from band data. This article uses interpolation to replace the traditional complex method that requires 6 orbital elements to calculate the satellite position, and directly calculates ORB to ECR transformation matrix rather than the traditional two-step transformation method. After multiple sets of data calculation and qualitative and quantitative verification, the HY-1C/D COCTS geometric positioning results are consistent. As a result of the sampling pixel scale effect, the error increases gradually from Nadir to the edge of two sides and from the equator to two poles, all within the error of 2 pixels. This method meets certain positioning accuracy requirements and can be employed for geometric positioning of COCTS.

A support vector machine (SVM) sea ice classification method of Haiyang-1C (HY-1C) satellite coastal zone imager (CZI) images based on the optimal feature set is proposed in this paper. The spectral features and the texture features of CZI images are extracted, and then distance separability criterion is used for feature selection to obtain the optimal feature set. The sea ice classification experiment and analysis of the three CZI images of Liaodong Bay are carried out based on SVM classification method with the optimal feature set as the input of the classifier. The results show that the sea ice classification accuracy obtained by the proposed method is better than that of only using the spectral features or the texture features. The sea ice classification accuracy of December 19, 2020, January 10, 2021 and January 16, 2021 are 93.67%, 91.75% and 84.89%, respectively, all above 80%. The sea ice area of Liaodong Bay is estimated according to the sea ice classification map. It is found that the sea ice area of Liaodong Bay in the three images increased successively, and the maximum area is about 11 998.98 km².

The dramatic change of sea ice in the Beaufort Sea has an important impact on the regional ecosystem and economic activities. Based on the sea ice concentration released by the National Ice and Snow Center of the United States, the mechanism of extremely low summer sea ice in the Beaufort Sea in 2019 is discussed. The sea ice area in the melt season (May–September) of 2019 is 1.38×10⁵ km², far lower than the average of 2.28×10⁵ km² from 1998 to 2020. According to the statistics of the sea ice area in the preceding autumn (October–December 2018) and winter (January–April 2019) of 2019, there is no significant difference between the 2019 and the average results of 1998–2019, so it is not the main reason for the extremely low ice event. Combining the data of sea ice drift field, sea ice thickness, 10 m wind field, and net sea surface heat flux, it is found that the sea ice in May of 2019 decreased by 2.33×10⁵ km², which is the largest of sea ice loss in May since 1998, accounting for 62% of the loss of sea ice area in the melt season. Different from the mechanism of extremely low summer sea ice area in 1998, 2008, 2012, and 2016, decreasing sea ice thickness and abnormally strong wind field in May 2019 contribute to the rapid sea ice export, resulting in the formation of open water in the south of Beaufort Sea on May 16, 2019. The abnormally high sea surface net heat flux makes the sea ice melt more, resulting in the abnormal phenomenon of sea ice in the summer of 2019. With the continuous thinning of sea ice thickness, the response of sea ice to wind field is stronger and stronger, and the time of sea ice retreat is advanced, the phenomenon of extremely low summer ice condition in the Beaufort Sea may appear more frequently.