The coast of Hainan Island is dominated by embayed beaches, which is an important coastal tourist destination in China and also has a high incidence of drowning accidents caused by rip currents. To investigate the distribution characteristics of rip currents on embayed beaches, the occurrence and distribution characteristics of rip currents are interpreted from the satellite images of 54 embayed beaches around Hainan Island over the past 20 years, and compared with the field survey results. The results show that, in terms of regional distribution, the occurrence probability of rip currents on the eastern and southern coasts is significantly higher than that on the western and northern coasts of Hainan Island. In terms of location distribution, the number of rip currents in the middle of the headland bays is higher than that on the two sides. In terms of geometric characteristics, the average rip length was positively correlated with the average rip width, and the average rip spacing. The distribution density of rip currents is negatively correlated with significant wave height, average wave period, incident wave power, and wave energy. The scale of the headland has a significant effect on the number of rips, which is positively correlated with the bay width, the maximum bay indentation, and length of the headland, but the curvature of the shoreline has no significant correlation with the average number of rips. The results of satellite image interpretation for the occurrence of rips are consistent with the results of the field survey and the evaluation of the Ω-RTR model. These conclusions can provide useful references for the beach safety management and rip current warning work on Hainan Island.

In order to understand the long−term variations in the nutritional environment and the key influencing factors of Bohai Bay, sediment cores from the mouth of the bay (BH15) and the outer area of the bay (BH47) were collected in April and June 2018 separately. The contents of total organic carbon (TOC), total nitrogen (TN), phosphorus (P), biogenic silica (BSi), and stable isotopes of carbon and nitrogen (δ¹³C, δ¹⁵N) were analyzed, and the causes of the changes of the above parameters were discussed in combination with the changes of climate and surrounding human activities. The results showed that the TN content in both sediment cores exhibited an increasing trend since the 1980s, which is consistent with the long-term variations in dissolved inorganic nitrogen (DIN) in the sea water. Sewage discharge, fertilizer application and mariculture are the main sources of nitrogen in the Bohai Bay, among which the input of agricultural fertilizer has decreased since 2007, while the discharge of mariculture and domestic sewage into the sea has shown a continuous upward trend. Inorganic Phosphorus (IP) was the main form of phosphorus in sediments. The IP in BH15 in the bay has shown a decreasing trend since the mid-1990s, while the IP in BH47 from the mouth of the bay has shown a decreasing trend after 1970s and a slow increasing trend since 1990s. All of them show obvious terrigenous input characteristics. The results of TOC/TN ratio、δ¹³C value and δ¹³C two-endmember mixing model indicated that the sediment organic matter in the mouth of Bohai Bay and adjacent sea area was influenced by both terrestrial input and marine autochthonous sources, with marine organic matter being the dominant source, the contribution of marine organic matter has decreased Since the 1990s. Compared with the BH47 core from the mouth of the bay, the BH15 core is closer to the coastal area, more significantly affected by terrestrial input, with higher TN content and contribution of terrestrial organic matter. This study shows that in order to effectively manage the water environment of Bohai Bay, it is necessary to strengthen the control of mariculture and domestic sewage discharge into the sea while controlling the use of agricultural fertilizers in the future.

The seasonal variations of zooplankton communities in the nearby seas of the Island was investigated to know the relationship between the structure of zooplankton communities and environmental factors in the sea areas around Weizhou Island in Guangxi. Collecting samples of zooplankton communities were conducted in January (winter), April (spring), August (summer), and November (autumn) of 2022. The study examines the variations in zooplankton species composition, abundance, biomass, and dominant species across the four seasons, while also investigating their primary environmental influencing factors. A total of 224 species (classes) of zooplankton were identified throughout the four seasons of the year, including 25 species (classes) planktonic larvae, belonging to 18 groups. Copepods were the most abundant (78 species), followed by hydrozoan jellyfish (35 species). The number of zooplankton species shows a seasonal variation pattern of spring > autumn > winter > summer, and the composition of species shows a seasonal variation pattern of spring > winter > autumn > summer. The dominant species group exhibits a seasonal replacement phenomenon of copepods, red tide organisms, and glial organisms, as well as hairy jawbones and copepods. The seasonal variation characteristics of the zooplankton community structure are obvious. The annual average abundance and biomass of zooplankton are (246.50 ± 158.75) ind./m³ and (126.08 ± 192.27) mg/m³, respectively. The abundance and biomass in spring are the highest (712.80 ± 630.28) ind./m³ and (367.79 ± 264.33) mg/m³, while the abundance in winter is the lowest (62.09 ± 29.61) ind./m³ and the biomass in summer is the lowest (35.48 ± 19.88) mg/m³. The results of redundancy analysis indicate that water temperature, salinity, nutrients and chlorophyll a are the main environmental factors affecting the seasonal changes of zooplankton community structure in the sea area of Weizhou Island, Guangxi. The seasonal changes in water masses cause by the ocean monsoon drive the succession of dominant species of zooplankton. The seasonal variation characteristics of the abundance of copepods, an important group of zooplankton in the sea area, are consistent with the migration patterns of the Balaenoptera edeni in the waters of Weizhou Island, indicating that the basic feed organisms of the fishing industry affect the migration activities of the B. edeni. In addition, the ecological impact of the decreased dominant group of copepods caused by the proliferation and growth of protozoa and glial organisms in spring on the waters of Weizhou Island deserves attention and exploration.

Recognition of reef-building corals is important for protecting and monitoring coral reef ecosystems. Deep learning, as an advanced technology in image recognition, has been increasingly applied in coral recognition. However, its performance is still challenged by several issues, such as the imbalance of samples among different coral categories within a dataset and the limitation of data diversity. The former makes the deep learning model more likely to extract features from classes with a large number of samples and, therefore, decreases its ability to recognize small-sample-size corals, which often refer to endangered ones needing to be protected. The latter further reduces the performance of deep learning in recognizing corals with different appearances and are captured in variant environments. To solve these two problems, this study develops a reef-building coral recognition method by integrating a category-quantity adaptive deep data augmentation algorithm and transfer learning. To address the first problem, a category-quantity adaptive deep data augmentation algorithm named DeepSMOTE-F₁ is proposed. This algorithm improves the existing DeepSMOTE by introducing a sample-size determination stagey using an F1-score based evaluation metric. It can adaptively augment the number of samples of each category of corals according to its recognition performance so that the deep learning model can fully learn features from each class of corals. For the second problem, transfer learning is used to further enhance the model's ability to extract features. The experimental results on three widely used public coral recognition datasets, RSMAS, EILAT, and EILAT2 show that the recognition accuracy of the proposed DeepSMOTE-F₁ is improved by 2.88%, 0.39%, and 1.54%, respectively, compared with the traditional DeepSMOTE; and the accuracy of the integrated method is improved by 0.76%, 1.40% and 1.30% compared with the existing deep learning methods for coral recognition.

Benthic harpacticoid copepods are widely distributed in marine ecosystem, which is particularly abundant in the epilithic algal matrix(EAM) of coral reefs. Due to its unique habitat, there is currently limited research on its developmental process and cultivation methods. In this study, we collected epilithic algal matrix from the coral reef of Luhuitou, Sanya, Hainan in the summer of 2023, from which we isolated one species of Harpacticoida, which was belonging to the genus Robertgurneya based on morphological characteristics. The effects of mono-and mixed-algal cultures on the developmental dynamics of Robertgurneya sp. were observed and recorded. The results showed that the adult body length of the Robertgurneya sp. was 0.5−0.7 mm, with an average life cycle of about 61−68 days. There were no significant differences in egg number per female each time, brood number, reproductive cycle, and life cycle between different feed cultivation techniques. However, under mixed algae culture, the average egg diameter, larval survival rate, and maximum body length of nauplii stage VI in the embryonic development stage of the Robertgurneya sp. were significantly higher than those in the mono-algae culture group (p < 0.05). Moreover, the embryonic development time and cumulative copepodid development time were significantly shorter under mixed-algal culture (p < 0.05). The results indicate that Robertgurneya sp. in EAM have a short life cycle and strong reproductive ability. Considering the abundant benthic microalgae and organic debris in the mats, harpacticoida may be able to provide huge potential food resources for predators such as small fish in coral reefs. Therefore, it may play an important role in the material cycle and energy flow of coral reef ecosystems.

Using temperature-salinity profiles and current measurements, satellite data and reanalysis data in April 2018, this study analyses the acoustic field characteristics and effects of an abnormal anticyclonic eddy (AAE) on acoustic propagation on the continental slope area in the northwestern South China Sea (SCS). The results show that the AAE has a lens-shaped structure with a surface cold core, a shallower mixed layer, and subsurface intensified velocities. Unlike the concave sound-speed contours in the normal anticyclonic eddy (NAE), the sound-speed distribution in the abnormal anticyclonic eddy (AAE) exhibits a lens-shaped structure with an upward convexity and downward concavity. The surface sound speed within the eddy is lower than that outside, showing a negative anomaly (<−2 m/s). Conversely, the sound speed in the subsurface layer of the eddy is higher than that outside, showing a positive anomaly (>11 m/s). This results in the thickness of the original double thermocline extending up and down by a total of 47 meters in the presence of the eddy. As the sound propagates from the eddy outside on the shelf to the deep sea, the surface sound channel disappears as the propagation distances decreasing in the AAE, contrast to the increased distance in the NAE. As the sound propagates from the eddy outside in the deep ocean to the shelf, the location of the sound energy convergence zone moves backward and downward in the AAE, with the maximum distance exceeding 24 km and 0.3 km, respectively. This is similar to the situation in the NAE. As the sound propagates from the eddy core to outside in the deep sea, the turning depth of the sound deepens and the distance between the sound energy convergence zones doubles in the AAE, while no changes in the NAE.

Based on the Acoustic Doppler Current Profiler carried by deep-sea moorings, we investigated the diel vertical migration of the acoustic scattering layer and the impact of internal solitary waves on it in the northern South China Sea. The observational results reveal that the acoustic scattering layer, influenced by zooplankton, ascends to shallower depth within approximately an hour after sunset, remains there throughout the night, then migrates to the deeper depth within about an hour before sunrise. The average migration velocities are 4.7 cm/s (upward) and 5.8 cm/s (downward). Additionally, internal solitary waves observed in the region induce a pair of downward and upward currents, with maximum vertical velocities exceeding 50 cm/s, leading to fluctuations of tens to hundreds of meters in the acoustic scattering layer. The acoustic backscattering strength of the ocean’s upper layer reaches its maximum value at the troughs of internal solitary waves. Further research indicates that the daytime internal solitary waves exhibit a stronger correlation between the vertical velocity and the depth-averaged backscattering strength variation compared to the nighttime internal solitary waves. When the vertical velocities induced by both types of waves are equal, the depth-averaged backscattering strength variations during the day are typically greater than those at night.

It prevails southwesterly monsoon in summer in the northern South China Sea. Consequently, the seasonal coastal upwellings are frequently driven up in summer off the southeastern coast of the Hainan Island, the eastern coast of the Leizhou Peninsula and the eastern coast of Guangdong Province, and these upwellings have attracted common research attention. The present paper revisits the historic hydrologic survey data in the northern South China Sea. It is found that, besides the coastal upwellings frequently reaching the sea surface in the above-mentioned zones, there are more subsurface upwelling patterns in the northern South China Sea: namely, (1) the subsurface upwelling throughout the eastern coastal zones of Hainan Province and all the western and eastern coastal zones of Guangdong Province under strong overlying southwesterly monsoon; (2) the subsurface one over the wide continental shelf off the western coast of Guangdong Province; (3) the subsurface one driven by the diluted Pearl River runoff off the river mouth; and (4) the bottom continental water invasion into the bays of eastern Guangdong Province and the cyclonic eddies in the bays. These four upwelling patterns have not been reported in the existing research papers, or little attention has been paid to them. Besides the Ekman’s transport due to strong summer southwesterly monsoon and Ekman’s pumping due to overlying wind stress curl, the bottom Ekman’s effect due to the basin-scale South China Sea circulation is possibly the non-negligible driving mechanism for the subsurface upwelling throughout all the coastal zones. In the light of the simultaneous atmospheric circulation over the northern South China Sea, Ekman’s pumping due to positive wind stress curl seems to be the major driving force for the subsurface upwelling over the continental shelf off western Guangdong coast. The positive wind stress curl as well as the diluted Pearl River runoff drives a cyclonic circulation off western Guangdong coast. The seawater divengence in the cyclonic circulation is favorable to the formation and maintenance of the subsurface upwelling. The vertical circulations driven by southwesterly monsoon and horizontal cyclonic ones in the coastal bays of eastern Guangdong Province are the major driving forces for the bottom continental water intruding into the bays. In the Daya Bay, the thermal plume due to the unclear power plants is favorable to strengthen the horizontal cyclonic circulations. The western and eastern capes of the Honghai Bay are favorable to the formation of horizontal cyclonic circulations in the bay. So we need attach more importance to the other driving mechanisms of the upwelling but to surface Ekman’s effect in the northern South China Sea.

Sea ice leads in the Arctic, accounting for only 1%−10% area of the whole ice area, play a crucial role in the exchange of energy and moisture between the ocean and the atmosphere. Currently, the analysis of the numerical simulation of the leads mainly focuses on the spatial distribution of the occurrence frequency and the spatio-temporal variations of the lead area proportion within the cell, while few analysis concerns the simulated lead morphology (length, width and orientation). This article is based on the high-resolution (2 km) ice sea coupling model using visual plastic rheologies to simulate sea ice thickness to extract leads, and the lead morphology is compared to three MODIS lead products respectly. The results show that the spatial distribution of simulated leads occurrence frequency in Beaufort Sea is basically consistent with WH2015 and H2019 products. The number density and total length of leads with a width greater than 6 km follow the power law distribution as presented in remote sensing products, while that of the narrow (2−4 km) leads are underestimated due to limited model’s resolution. The correlation between the total length of simulated leads and remote sensed products is high in January and March, but the model fails to reproduce the trends in February and April shown in the remote sensing products. The overall orientation of the simulated leads aligns with the remote sensing products, both show that leads along the north of the Canadian archipelago and the southeast of Beaufort sea are almost parallel to the coastline and the ice drift direction, while the orientation of the simulated leads is more restricted by the continent than that of remote sensing products, and the location of the lead and ice speed turning is not consistent in the middle of the Beaufort Sea. This study highlights the capability of the state-of-the-art high-resolution sea ice-ocean coupled models in simulating various morphological characteristics of sea ice lead, and provides insights for further model improvements.

Antarctic sea ice is a crucial component of the polar climate system, with profound implications for global climate. Sea ice thickness, as one of the key properties of sea ice, holds significant importance for understanding and predicting the influences of climate change by revealing its spatial and temporal distribution patterns and variation trends. However, current monitoring of Antarctic sea ice thickness is constrained by limited ground observations with restricted spatial and temporal coverage or short-term satellite observations, long-term sea ice thickness data remains elusive. To address this issue, this study utilized the continuous satellite radar altimetry data from Envisat and CryoSat-2 and constructed a consistent dataset of radar freeboard of Antarctic sea ice. Then, the penetration depth of radar signals through the snow covers over Antarctic sea ice was quantitatively estimated, and a method applicable to various sea ice-snow scenarios for estimating Antarctic sea ice thickness was developed. The estimated sea ice thickness shows an average absolute bias of approximately 0.28 m compared to in situ measurements from the Australian Antarctic Data Centre, and an average absolute bias of approximately 0.65 m compared to ICESat laser altimeter with a high correlation coefficient of 0.71. Analysis of the spatiotemporal variations of Antarctic sea ice thickness from 2002 to 2023 reveals that thick ice is predominantly concentrated in the western Weddell Sea and Bellingshuan/Amundsen Seas, while ice in other sea sectors is relatively thin. Antarctic sea ice thickness exhibited a slight decreasing trend before 2011, followed by an accelerated decline after 2011 (−0.03 m/a). The distribution and trends of Antarctic sea ice thickness exhibit distinct seasonal and regional characteristics.