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Using eddy-resolving numerical simulation data and historical hydrological observation data, this study investigates the sources, seasonal and interannual variability of two subsurface undercurrents under the Indonesian Throughflow—the Ombai Undercurrent located in the Ombai Strait and the Timor Undercurrent located in the Timor Channel. The results indicate that these two undercurrents exist at depths of approximately 200-800 m, which are a quasi-permanent undercurrent system. The formation of the Ombai Undercurrent is mainly related to the eastward extension of the South Java Undercurrent, while the water source of the Timor Undercurrent is more complex, mostly a mixture of the South Java Undercurrent and the Leeuwin Undercurrent. Both subsurface undercurrents exhibit significant seasonal and interannual variations, with a significant semiannual period at the seasonal scale, typically peaking during the Indian Ocean monsoon transition period (April, May, and October). Combining historical wind, satellite altimeters, and temperature and salinity observation data, it is found that the meridional pressure gradient in the subsurface layer related to local wind and their upwelling is the dominant factor leading to their seasonal changes. At the interannual scale, there is a period of 2-4 years for subsurface undercurrents, which is significantly correlated with the Indian Ocean dipole.

The occurrence of a “triple-dip” La Ni?a event is historically rare, yet it has exerted profound impacts on global weather and climate systems. To enhance the understanding of the causes of multiple La Ni?a events and improve the prediction capabilities for weather and climate, a comparative analysis of the ocean-atmosphere processes in the tropical Pacific during the 2020-2023 “triple-dip” La Ni?a period was conducted based on multiple sets of observational and reanalysis data, employing composite analysis and other methods. Results showed that: The peak of the 2020 La Ni?a event occurred in winter and lasted the longest among this “triple-dip” La Ni?a events; the peak of the 2021 La Ni?a event also occurred in winter, with the cold anomaly centered near the eastern Pacific, classified as an “Eastern Pacific” type; the peak of the 2022 La Ni?a event occurred in autumn, relatively weaker in intensity and the shortest in duration, with the cold anomaly centered in the central Pacific, classified as a “Central Pacific” type. Further research revealed a coupling relationship between zonal wind and sea surface temperature (SST) variations. However, during this “triple-dip” La Ni?a period, the intensity and location of the eastward wind anomalies showed little variation across different La Ni?a events. In contrast, subsurface SST changes align with changes in SST anomaly centers, it may be a crucial factor influencing the intensity and type differences among this “triple-dip” La Ni?a events. Although eastward-propagating Kelvin waves had a certain impact on the ocean system, but their propagation speeds and intensities exhibited minimal variations during this “triple-dip” La Ni?a events. Additionally, the study found that variations in the growth rate of warm water volume contributed to the differences in La Ni?a intensities, while the meridional convergence and divergence of warm water led to the seasonal phase-locking phenomenon of La Ni?a events.

Using the data of high resolution satellite sea surface temperature (SST) from January 1, 1990 to December 31, 2020, the spatial characteristics of marine heatwaves (MHWs) in the South China Sea were identified with a deep-first-search algorithm, and the characteristics of marine heatwaves at different spatial scales were further investigated. The results indicated that the small-scale marine heatwave events in the South China Sea (Type I MHWs, area<1.8×10⁴ km²) occurred the most frequently, accounting for 94.20% of the total marine heatwave occurrences. Large-scale marine heatwaves with areas exceeding 1.2×10⁵ km² (Type III MHWs) occurred only 74 times during the 31-year period, with the largest event recorded in 2015. Further analysis revealed significant differences in the spatial distribution of intensity, duration, and frequency of marine heatwaves for different spatial scales. Compared to Type I MHWs, Type II MHWs (1.8×10⁴~1.2×10⁵ km²) exhibited a noticeable increase in the average coverage area with an intensity exceeding 1.5 ℃. Statistical analysis showed that the intensity, duration, and cumulative intensity of South China Sea MHWs increased with the spatial scale of the MHWs. The intensity of Type III MHWs was 1.4 times that of Type I MHWs and 1.2 times that of Type II MHWs. In addition, the response of South China Sea MHWs areas to the El Ni?o-Southern Oscillation (ENSO) was also investigated. The results showed a significant increase in the areas of Type I to III MHWs during El Ni?o periods, with a lag of 6 to 7 months. The duration of Type III MHWs during El Ni?o was longer by 2 days compared to La Ni?a periods. This study explored the fundamental characteristics of South China Sea MHWs areas and further analyzed the commonalities and differences of MHWs at different spatial scales, providing new insights into the characteristics and mechanisms of the formation and dissipation of South China Sea MHWs.

A simple tropical cyclone tracker based on wind stress characteristics was designed in this study, using the best track dataset from the United States Joint Typhoon Warning Center, wind speed data and sea level pressure field data from the European Centre for Medium-Range Weather Forecasts. The tracker was used to detect tropical cyclones in the Northwest Pacific from 1985 to 2014, and its performance metrics were evaluated. The results showed that the tracker was able to accurately reproduce the spatiotemporal structure of tropical cyclones in the Northwest Pacific. The peak period of activity was concentrated from August to October, and the latitudinal positions varied with the seasons, consistent with the observations. Additionally, the tracker used the minimum sea level pressure as a criterion for determining cyclone intensity, and the number of cyclones identified at different intensities closely matched the observations. The tracker performed well in terms of probability of detection and false alarm rate for tropical cyclones, comparable to previously used trackers. Regarding the tropical cyclones detected by the tracker, the research findings showed that approximately 90% of the center positions were within a deviation of 1 degree from the observed positions, and the lifetime deviation was within 2 days, indicating a good representation of the complete movement and evolution of tropical cyclones.

Antarctic krill (Euphausia superba) is a key species sustaining the biodiversity of the Southern Ocean and is a protected and restricted fishing target. In the context of significant impacts of climate change on the ecological environment of the Southern Ocean, it is urgent to understand the spatio-temporal distribution, change trends, and habitat suitability of Antarctic krill. In this study, based on Antarctic krill presence records and time series satellite and reanalysis data, a Maxent model for habitat suitability in the Cosmonauts Sea and the D’Urville Sea were constructed using timing parameters of phytoplankton phenology and sea-ice dynamics, along with related environmental parameters. It was found that timing parameters were more suitable for assessing habitat suitability for Antarctic krill compared to conventional environmental parameters. Using the Maxent model, the data over 20 years on the occurrence time and frequency of Antarctic krill in these two study areas were retrieved, and the mechanisms through the interannual trends of multiple environmental parameters were analyzed. Environmental parameters at the time of krill occurrence showed that the overall chlorophyll a mass concentration in the Cosmonauts Sea was lower than that in the D’Urville Sea, with a shorter ice-free period, lower temperatures, and later krill presence dates primarily composed of larval and young individuals along the coast. From 1997 to 2019, the presence time of krill in the coastal Cosmonauts Sea gradually advanced, and the number of presence days increased, mainly due to earlier onset of algal blooms, while increased chlorophyll a mass concentration provided more abundant overwintering food for krill larvae. In the D’Urville Sea, influenced by warming water, shortened ice-free period, and reduced chlorophyll a mass concentration, mature krill may migrate to a more suitable environment, leading to a decline in annual presence frequency. Based on the constructed habitat suitability model, this study showed the long-term distribution of Antarctic krill occurrence in the Cosmonauts Sea and the D’Urville Sea for the first time, which can help to understand the impact of climate change on the ecological environment in the Southern Ocean, and the planning of conservation areas and fishery management in the Southern Ocean.

Based on a survey in Hangzhou Bay in March 2022, the distribution and sea-air exchange flux of N₂O in Hangzhou Bay and its adjacent waters in spring were investigated, and the influencing factors were analyzed. The dissolved N₂O concentration and saturation of surface water were 12.5-21.3 nmol·L^-1 and 115%-183%, respectively. The average N₂O concentration in the upper, middle and lower waters were 17.2±2.9, 14.1±0.8 and 13.2±0.7 nmol·L^-1, respectively, and the average saturation were 151%±17%, 125%±6% and 123%±6%, respectively. Dissolved N₂O in all sampling sites were in a supersaturated state. The spatial distribution of N₂O concentration and saturation in the surface water exhibited significant variations, with high values concentrated in the upstream area and gradually decreasing from west to east, while these showed a decreasing trend from north to south in middle and lower areas. The distribution of N₂O in the Hangzhou Bay and its adjacent waters was influenced by multiple factors such as temperature, estuarine mixing, river input and in situ bioproduction. The sea-air N₂O exchange flux ranged from 11.4 to 71.2 μmol·m^-2·d^-1, with an average of 29.5±16.0 μmol·m^-2·d^-1. Comparing with other domestic estuaries and bays, this is relatively high in Hangzhou Bay, indicating a significant potential for N₂O release. Combining the sea-air exchange flux and sea area, this study preliminarily estimated that the N₂O emission in Hangzhou Bay and its adjacent waters in spring was 3.5×10⁵ mol·d^-1, indicating its important role in atmospheric N₂O emissions.

The topographic and geomorphologic features of mid-ocean ridges are directly controlled by tectonic movements and magmatic activities, and study of them can help us to understand the tectonic evolution history and magmatic processes of mid-ocean ridges, and is also of great significance to the exploration of deep-sea mineral resources. In this paper, the shipboard multibeam sonar data collected during the China Ocean 24 Cruise were utilize to study the topographic and geomorphologic features of the Carlsberg Ridge in the Northwest Indian Ocean by applying the quantitative analysis method with the 61°24'E-61°48'E segment as the research target, the magma intrusion ratio and the fault correlation index were calculated, and the magma-tectonic significance of the study area was discussed. The study area can be divided into four secondary mid-ocean ridge segments (A, B, C, and D). The active intervals of magma-tectonic periods for segments A, B, C, and D are 0.15, 0.50, 0.70 and 0.21 Ma, respectively. The mid-ocean ridge segments A and B are asymmetric dilatation sections with poor magma and mainly tectonic action, belonging to the period of tectonic activity; the mid-ocean ridge segment C is a symmetric dilatation section with sufficient magma and mainly magmatic action, belonging to the period of axial volcanic ridge construction; the mid-ocean ridge segment D is a symmetric dilatation section with poor magma and mainly tectonic action, belonging to the period of tectonic activity. Faults area with high kernel density on the two flanks of the mid-ocean ridge section has a possibility of forming an area of hydrothermal activity, which is a target area of further exploration.

This study was based on the actual gravity, magnetic and multichannel seismic data obtained during the international cooperative survey in 2021. Through joint inversion of gravity and seismic along the CMD01 profile traversing the southwestern margin of Madagascar, combined with interpretation and analysis of seismic profiles, the crustal structure, tectonic units, and evolution of the southwestern margin of Madagascar were studied. A crustal density model of the southwestern margin of Madagascar was established, revealing thinned continental crust and the presence of magmatic underplating in certain segments. Structural units such as thinned continental crust, continental-ocean transition zone and oceanic crust domain can be delineated from land to sea, among which, due to the influence of Davie Fracture Zone strike-slip tectonics on the margin, the continental-ocean transition zone exhibited typical characteristics of a transform margin. By combining plate reconstruction and crustal density model, the developmental evolution of the southwestern margin of Madagascar was reconstructed. The research results have important theoretical significance and practical application value for deepening the understanding of the breakup and separation of the Gondwana, the developmental evolution of the East African passive margin, and maritime delineation.

Submarine earthquake is one of the most major factors causing deep-water international submarine cables damage. Understanding the process of submarine cables damage and the mechanism of submarine cables damage caused by turbidity currents after earthquake are of great significance to the security maintenance of international submarine communications. Combined with the lastest research result of global seabed topography and using professional international submarine cables engineering software Makaiplan, the process of plenty of submarine cables damage after Grand Banks Earthquake and Hengchun Earthquake were studied, then the relationship between the pattern of submarine cable damage and the developing process of turbidity currents after earthquake was found, and the mechanism of submarine cables damage caused by turbidity currents after earthquake was summarized. Study result shows that submarine cables break points are located intentively in submarine canyons and trenches. The movement speed of turbidity currents in submarine canyon and submarine trench, which caused submarine cable damage, can reach several ten kilometers to several hundred kilometres per hour. Terrestrial rivers and continental shelf undersea river channels provide materials transportation for the development of turbidity currents. Submarine canyons and trenchs are the pathes of turbidity currents movement then damage plenty of submarine cables. The turbidity currents that developed from upper continental slope in passive continental margin after earthquake can damage submarine cables laid on continental slope, continental rise and abyssal plain. This kind of turbidity currents achieves maximum speed on continental slope, then self-accelerate on abyssal plain. Multiple turbidity currents can develop at different positions of continental slope at the same time in active continental margin, then strike submarine cables which laid on canyons and trenches for multiple times. This kind of turbidity currents achieves maximum speed and self-accelerates in submarine trenches. There are several earthquake-resistance measures: submarine cable routes trying to avoid crossing submarine canyons and trenches which connected to terrestrial rivers or continental shelf channels; using shallow water type submarine cable which has outer armor protection when crossing inevitably; laying submarine cables suspended slightly on the bottom of canyons or trenches with Uraduct protection on them; changing the cross-section shape of submarine cable.

Island group is a typical form of existence for offshore islands in China. Investigating the evolution of landscape patterns in an island group and among individual islands can provide theoretical foundations and technical support for island resource management. Based on the high-resolution remote sensing images of six inhabited islands (Dongtou Island, Banping Island, Dasanpan Island, Huagang Island, Niyu Island and Zhuangyuanao Island) in Dongtou District in 2008, 2014, 2018 and 2022, the landscape pattern indices of island groups and individual islands were calculated. The results indicate the following: (1) During the study period, the proportions of forest land, water areas, and cultivated land in the landscape of island group have been continuously declining, while the proportions of building land, industrial and mining land, and road areas have been steadily increasing. (2) The landscape pattern changes on Dongtou Island, Dasanpan Island, Niyu Island, and Zhuangyuanao Island are significant, with the former two primarily characterized by an increase in building land and the latter two by an increase in industrial and mining land; the landscape patterns of Banping Island and Huagang Island are relatively stable. (3) Development activities among the islands are interrelated, with the landscape evolving in an orderly manner. The transformations of forest land into mining areas on Niyu Island and Dasanpan Island have supported the reclamation and subsequent development of water areas on Dongtou Island and Zhuangyuanao Island; as the larger islands develop, the development of surrounding smaller islands has also begun.

Accurate monitoring and assessment of the temperature rise intensity and spatio-temporal distribution of thermal discharge from power plants are of great significance for ensuring safe operations of the power plants and protecting the ecological environment of surrounding marine areas. Based on thermal infrared remote sensing observation technology, information such as the spatial distribution, temperature rise intensity, and temporal variation of thermal discharge from coastal power plants can be obtained, making it an important means for monitoring thermal discharge from these plants. The current research status and progress in monitoring thermal discharge from coastal power plants using thermal infrared remote sensing technology are systematically reviewed, including remote sensing data sources, water temperature retrieval, thermal discharge background temperature information extraction, spatio-temporal statistics of thermal discharge, and analysis of influencing factors. Furthermore, the paper proposes future directions for remote sensing research on thermal discharge from coastal power plants.