The marine hydrological process is still unclear due to scarce observations. Based on stable water isotopes in surface seawater along the 33rd Chinese National Antarctic Science Expedition from November 2016 to April 2017, this study explored the hydrological processes in the Pacific, Indian and Southern oceans. The results show that the Northwest Pacific (0°–26°N) is a region with strong evaporation (the δ¹⁸O-δD slope is 6.58), while the southern Indian Ocean is a region with strong precipitation (the δ¹⁸O-δD slope is 9.57). The influence of continental runoff and water mass mixing reduces the correlation between δ¹⁸O and salinity in the eastern Indian Ocean. The characteristics of the isotopes and hydrological parameters indicate that the Agulhas Front and sub-Tropical Convergence do not merge in the Antarctic–Indian Ocean region. The freezing of sea ice near the Antarctic continent decreases the δ¹⁸O and δD by 0.40‰ and 7.0‰, respectively, compared with those near 67°S. This study is helpful for understanding marine hydrological processes and promoting the understanding and research of the nature of ocean responses in the context of climate change.

Global carbon cycle has received extensive attention, among which the river-estuary system is one of the important links connecting the carbon cycle between land and ocean. In this paper, the distribution and control factors of particulate organic carbon (POC) were studied by using the data of organic carbon contents and its carbon isotopic composition (δ¹³C) in the mainstream and estuary of Passur River in the Sundarbans area, combined with the hydrological and biological data measured by CTD. The results show that POC content ranged from 0.263 mg/L to 9.292 mg/L, and the POC content in the river section (averaged 4.129 mg/L) was significantly higher than that in the estuary area (averaged 0.858 mg/L). Two distinct stages of POC transport from land to sea in the Sundarbans area were identified. The first stage occurred in the river section, where POC distribution was mainly controlled by the dynamic process of runoff and the organic carbon was mainly terrestrial source. The second stage occurred during estuarine mixing, where the POC distribution was mainly controlled by the mixing process of seawater and freshwater. The source of POC was predominantly marine and exhibiting vertical differences. The surface and middle layers were primarily influenced by marine sources, while the bottom layer was jointly controlled by terrestrial and marine sources of organic carbon. These findings are of great significance for understanding the carbon cycle in such a large mangrove ecosystem like the Sundarbans mangrove.

The element iron limitation is one of the crucial factors contributing to high nutrient low chlorophyll in the Southern Ocean (SO). Mixed layer dynamics regulate the availability of iron to phytoplankton. In this paper, we investigate the influence of surface iron supplementation triggered by the mixed layer depth (MLD) variation on chlorophyll-a (Chl-a) concentration in the SO on seasonal and interannual timescales. This analysis is based on the Biogeochemical Southern Ocean State Estimate for the period from 2013 to 2021. We provide a comprehensive and systematic mapping of the regions within the SO, where Chl-a is affected by iron input related to MLD deepening. The relationship between the MLD and the Chl-a varies with the latitude on the seasonal time scale. Both the MLD and sea ice melting affect the distribution of Chl-a. On the interannual scale, iron supply due to MLD deepening occurs primarily north of 60°S. Horizontal advection-induced entrainment enhances the surface iron input during the austral summer, which favors Chl-a increase. In addition to the MLD, the melting of sea ice and cooling of the sea surface can also alter iron input and subsequently affect Chl-a distribution in the austral summer. During the austral winter, entrainment can boost iron stocks, stimulating a subsequent spring increase of Chl-a in the SO. Furthermore, sea surface temperature declines during the austral winter, promoting an increased iron supply and creating favorable conditions for the subsequent spring Chl-a increase in the SO.

Understanding the dynamics of phytoplankton communities in coastal zones is crucial for the management and conservation of coastal ecosystems. Previous research indicated that the phytoplankton community structure and dominant taxa in the Bohai Sea (BHS) have exhibited significant shifts from the 1990s to the early 2010s in response to environmental changes, especially the change in nutrient structure. This study comprehensively investigated the variations in net-collected phytoplankton (>76 μm) community structure, diversity, and environmental factors in the BHS during the late summers of 2011−2020, aiming to understand the recent trend in phytoplankton community structure and to explore the interactions between the communities and the environment. During the study period, the nutrient status in the BHS was characterized by a decrease in dissolved inorganic nitrogen (DIN) concentration, an increase in dissolved inorganic phosphorus (DIP) concentration, and a return of the nitrogen-to-phosphorus (N/P) molar ratio (hereinafter referred to as N/P ratio) to the Redfield ratio since 2016. The eutrophication index (EI) in the BHS remained stable and was generally at a low level (<1). The Dia/Dino index fluctuated but did not show an obvious trend. Overall, the eutrophication, the imbalance in nutrient ratio, and the shift in phytoplankton community structure did not continue during the study period. The increased abundance of phytoplankton was strongly associated with elevated concentrations of DIN, as well as higher N/P and nitrogen-to-silicon (N/Si) ratios, whereas the greater diversity was strongly linked to higher concentrations of DIP. Diatoms and dinoflagellates showed significant differences in their interactions with the environment, and their relative dominance was related to water column depth and stratification intensity; their impacts on the phytoplankton community diversity were also significantly different. The variations of certain dominant species, i.e., Skeletonema costatum, Paralia sulcata, and Tripos longipes, exhibited strong links to the changes in nutrient structure in the BHS. The findings of this study contribute to understanding the regional environmental changes and provide insights into the adaptive strategies of coastal ecosystems in response to environmental shifts and fluctuations.

The three-dimensional displacements caused by ocean loading effects are significant enough to impact spatial geodetic measurements on sub-daily or longer timescales, particularly in the vertical direction. Currently, most tide models incorporate the distribution of vertical displacement loading tides; however, their accuracy has not been assessed for the equatorial and Indian Ocean regions. Global Positioning System (GPS) observations provide high-precision data on sea-level changes, enabling the assessment of the accuracy and reliability of vertical displacement tide models. However, because the tidal period of the K₂ constituent is almost identical to the orbital period of GPS constellations, the estimation of the K₂ tidal constituent from GPS observations is not satisfactory. In this study, the principle of smoothness is employed to correct the systematic error in K₂ estimates in GPS observations through quadratic fitting. Using the adjusted harmonic constants from 31 GPS stations for the equatorial and Indian Ocean, the accuracy of eight major constituents from five global vertical displacement tide models (FES2014, EOT11a, GOT4.10c, GOT4.8, and NAO.99b) is evaluated for the equatorial and Indian Ocean. The results indicate that the EOT11a and FES2014 models exhibit higher accuracy in the vertical displacement tide models for the equatorial and Indian Ocean, with root sum squares errors of 2.29 mm and 2.34 mm, respectively. Furthermore, a brief analysis of the vertical displacement tide distribution characteristics of the eight major constituents for the equatorial and Indian Ocean was conducted using the EOT11a model.

In the last 10 years (2012–2021), five hypoxic events have been observed in summer in the central Bohai Sea (CBS). Frequent and persistent hypoxia will have an impact on the ecosystem of the CBS. In this paper, historical sea temperature (ST), salinity (SAL), density (Den), and dissolved oxygen (DO) concentration data from three stations in the CBS are analyzed via the linear regression method, and the correlations between the stratification factors (ST, SAL, and Den) and DO concentration are determined. The thresholds of the stratification factors at the three stations in June in the year in which hypoxia occurred were determined and applied to survey data from 29 stations in late May to early June in 2022 in the CBS; this assessment found that the data from 19 stations indicated that hypoxia was about to occur. In August, the survey data showed that 14 out of the 29 stations indicated hypoxic conditions, of which 12 were from the predicted 19 stations, meaning that the estimation accuracy reached 63%. The same approach was applied to data from June 2023. The data for August from a bottom-type online monitoring system in the CBS verified the occurrence of hypoxic events around Sta. M2. The results show that the strength of the seawater stratification plays a leading role in hypoxic events in the summer in the CBS, and the thresholds of the stratification factors can be used to predict the occurrence of hypoxic events.

It is found that the winter (December–February) barrier layer (BL) in the Bay of Bengal (BoB) acts as a dynamical thermostat, modulating the subsequent summer BoB sea surface temperature (SST) variability and potentially affecting the Indian summer monsoon (ISM) onset and associated rainfall variability. In the years when the prior winter BL is anomalously thick, anomalous sea surface cooling caused by intensified latent heat flux loss appears in the BoB starting in October and persists into the following year by positive cloud-SST feedback. During January–March, the vertical entrainment of warmer subsurface water induced by the anomalously thick BL acts to damp excessive cooling of the sea surface caused by atmospheric forcing and favors the development of deep atmospheric convection over the BoB. During March–May, the thinner mixed layer linked to the anomalously thick BL allows more shortwave radiation to penetrate below the mixed layer. This tends to maintain existing cold SST anomalies, advancing the onset of ISM and enhancing June ISM precipitation through an increase in the land-sea tropospheric thermal contrast. We also find that most of the coupled model intercomparison project phase 5 (CMIP5) models fail to reproduce the observed relationship between June ISM rainfall and the prior winter BL thickness. This may be attributable to their difficulties in realistically simulating the winter BL in the BoB and ISM precipitation. The present results indicate that it is important to realistically capture the winter BL of the BoB in climate models for improving the simulation and prediction of ISM.

During August and September 2023, three giant icebergs, each bigger than Paris, successively grazed Clarence Island in the northeast of the Antarctic Peninsula, a home to a population of over 100 000 penguins. This incident may serve as a clarion call for the increasing iceberg calving due to global warming and its subsequent impact on the Antarctic ecosystem. Here we investigate this unexpected event and employ historical records and probabilistic analyses of iceberg grounding to assess the degree of impact on penguin colonies of Clarence Island. Among the eleven colonies, there is one with low impact, eight with medium impact, and two with high impact. The low-impact colony, Cape Lloyd, is located in the northern part of the island, while the high-impact colonies, False Ridge and Pink Pool, are in the southeast. The eight medium-impact colonies are distributed along both the eastern and western coasts of the island. This study provides essential support for evaluating the impact of iceberg activity on penguin colonies. We argue that penguin colonies located in areas prone to iceberg drift, such as Clarence Island, may become more vulnerable to the heightened risk of iceberg collisions or groundings in the warming future. Therefore, we hope the public will become more aware of the grave impacts of climate change on penguins and underscore the urgent need for effective conservation strategies.

The tide plays a pivotal role in the ocean, affecting the global ocean circulation and supplying the bulk of the energy for the global meridional overturning circulation. To further investigate internal tides and their impacts on circulation, it is imperative to incorporate tidal forcing into the eddy-resolving global ocean circulation model. In this study, we successfully incorporated explicit tides (eight major constituents) into a global eddy-resolving general ocean circulation model and evaluated its tidal simulation ability. We obtained harmonic constants by analyzing sea surface height through tidal harmonic analysis and compared them with the analysis data Topex Poseidon Cross-Overs v9 (TPXO9), the open ocean tide dataset from 102 open-ocean tide observations, and tide gauge stations from World Ocean Circulation Experiment. The results demonstrated that the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics (LASG/IAP) Climate System Ocean Model 3.0 (LICOM3.0) effectively simulated tides, with errors predominantly occurring in nearshore regions. The tidal amplitude simulated in LICOM3.0 was greater than that of TPXO9, and these high-amplitude areas exhibited greater errors. The amplitude error of the M₂ constituent was larger, while the phase error of the K₁ constituent was more significant. Furthermore, we further compared our results with those from other models.

Sediment-laden sea ice plays an important role in Arctic sediment transport and biogeochemical cycles, as well as the shortwave radiation budget and melt onset of ice surface. However, at present, there is a lack of efficient observation approach from both space and in situ for the coverage of Arctic sediment-laden sea ice. Thus, both spatial distribution and long-term changes in area fraction of such ice floes are still unclear. This study proposes a new classification method to extract Arctic sediment-laden sea ice on the basic of the difference in spectral characteristics between sediment-laden sea ice and clean sea ice in the visible band using the MOD09A1 data with the resolution of 500 m, and obtains its area fraction over the pan Arctic Ocean during 2000−2021. Compared with Landsat-8 true color verification images with a resolution of 30 m, the overall accuracy of our classification method is 92.3%, and the Kappa coefficient is 0.84. The impact of clouds on the results of recognition and spatiotemporal changes of sediment-laden sea ice is relatively small from June to July, compared to that in May or August. Spatially, sediment-laden sea ice mostly appears over the marginal seas of the Arctic Ocean, especially the continental shelf of Chukchi Sea and the Siberian seas. Associated with the retreat of Arctic sea ice extent, the total area of sediment-laden sea ice in June–July also shows a significant decreasing trend of 8.99 × 10⁴ km² per year. The occurrence of sediment-laden sea ice over the Arctic Ocean in June–July leads to the reduce of surface albedo over the ice-covered ocean by 14.1%. This study will help thoroughly understanding of the role of sediment-laden sea ice in the evolution of Arctic climate system and marine ecological environment, as well as the heat budget and mass balance of sea ice itself.