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The distinctive environmental attributes of the Southern Ocean underscore the indispensability of microorganisms in this region. We analyzed 208 samples obtained from four separate layers (Surface, Deep Chlorophyll Maximum, Middle, and Bottom) in the neighboring seas of the Antarctic Peninsula and the Cosmonaut Sea to explore variations in microbial composition, interactions and community assembly processes. The results demonstrated noteworthy distinctions in alpha and beta diversity across diverse communities, with the increase in water depth, a gradual rise in community diversity was observed. In particular, the co-occurrence network analysis exposed pronounced microbial interactions within the same water mass, which are notably stronger than those observed between different water masses. Co-occurrence network complexity was higher in the surface water mass than in the bottom water mass. Yet, the surface water mass exhibited greater network stability. Moreover, in the phylogenetic-based β-nearest taxon distance analyses, deterministic processes were identified as the primary factors influencing community assembly in Antarctic microorganisms. This study contributes to exploring diversity and assembly processes under the complex hydrological conditions of Antarctica., Competing Interests: Declaration of competing interest The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The South Scotia Ridge, in the Atlantic sector of the Southern Ocean, is a key region for water mass modification. It is the location of the Weddell‐Scotia Confluence, an area of reduced stratification which separates the Weddell Gyre to the south and the Antarctic Circumpolar Current to the north, and which receives input of shelf waters from the tip of the Antarctic Peninsula. To elucidate the transformations over the ridge, we focus on one of its largest seamounts, Discovery Bank, which has previously been observed as hosting a stratified Taylor column that retains water for months to years, during which time water masses are entrained from north and south of the Weddell Front and steadily mixed. Data from ship‐deployed sensors and autonomous platforms are analyzed to quantify and understand the diapycnal mixing, heat fluxes and water mass transformations over the bank. Ocean glider and free‐profiling drifting float data show that the mid‐depth temperature maximum of the Circumpolar Deep Water (CDW) is eroded between the northern and southern sides of the bank, while diapycnal diffusivity is enhanced by up to an order‐of‐magnitude over its steeply sloping portions. This is accompanied by heat fluxes from the CDW layer being increased by up to a factor of six, which may contribute to a reduction in mid‐depth stratification. Tidal model analysis shows that the southern side of the bank hosts strong barotropic to baroclinic energy conversion (>150 N m−2), emphasizing the role of internal tides in modulating water mass transformations in the Confluence. Plain Language Summary: The Weddell‐Scotia Confluence region, in the South Atlantic sector of the Southern Ocean, is a zone where ocean waters from the Antarctic Circumpolar Current to the north, the Weddell Sea to the south, and the Antarctic Peninsula to the west, come together and interact. In this paper, observations from a research ship, an autonomous underwater glider, and free‐drifting ocean floats are used to characterize the mixing between these different sources of water, and to understand what impact this mixing has on the downstream vertical structure of the water column. The results show that an anticlockwise circulation of waters occurs around a large underwater seamount in the region, causing the waters there to be trapped for months to years. In addition, interaction with the steeply sloping undersea topography generates particularly strong mixing between these different waters through the action of tides, which helps to sustain the characteristic low vertical density gradients present in the region. These processes have important implications for both the movement of mid‐depth waters south into the circulation of the Weddell Sea, and likely also for the supply of nutrients to the productive surface layer of the ocean. Key Points: New observations reveal enhanced mixing rates and mid‐depth heat fluxes over one of the largest seamounts of the South Scotia RidgeThis enhanced mixing may help to sustain the mid‐depth stratification minimum found in the Weddell Scotia ConfluenceResults from a tidal model show high rates of barotropic to baroclinic tidal energy conversion over steeply sloping parts of the bank [ABSTRACT FROM AUTHOR]

To understand the influence of the Fukushima accident on the Northwest Pacific, the distributions and transportations of 134Cs and 137Cs in the seawater in the Northwest Pacific in May and September 2015 were studied. The data showed that the Fukushima-derived 134Cs and 137Cs at some stations can still be distinguished from background level ~ 4 years later. On the whole, the activities of 137Cs and 134Cs in seawater were decreasing from May to Sep 2015. But the increased inventories and the surface activities of 137Cs imply that there has ever been an extra 137Cs from offshore water transported to this study area (from 31° N to 27° N, 145° E to 152.5° E) in May 2015. The average activities of 137Cs in subtropical gyre area in south of KE were the highest and the least were to the east of Luzon Strait in 2015. In vertical direction, 137Cs in subtropical gyre area were mainly distributed at 100 ~ 500 m layer and 137Cs only at 500 m layer in this area showed an increasing trend from May to Sep 2015 which reflects more 137Cs were still penetrating to deeper layer of 500 m from upper water. But they were almost not found below 1000 m layer. It was associated with the subsurface transport of radiocesiums by Northwest Pacific Mode Water (NPMW) and the diffusion of mesoscale eddy. Different distribution characteristics of 137Cs existed between north of KE and south of KE. The low-temperature-low-salinity water mass likely to be the first Oyashio Intrusion was the main factor that resulted in higher 137Cs appearing at the upper 100 m layers in north of KE. [ABSTRACT FROM AUTHOR]

The Indonesian seas, with their complex passages and vigorous mixing, constitute the only route and are critical in regulating Pacific–Indian Ocean interchange, air–sea interaction, and global climate events. Previous research employing remote sensing and numerical simulations strongly suggested that this mixing is tidally driven and localized in narrow channels and straits, with only a few direct observations to validate it. The current study offers the first comprehensive temporal microstructure observations in the south of Lombok Strait with a radius of 0.05° and centered on 115.54oE and 9.02oS. Fifteen days of tidal mixing observations measured potential temperature and density, salinity, and turbulent energy dissipation rate. The results revealed significant mixing and verified the remotely sensed technique. The south Lombok temporal and depth averaged of the turbulent kinetic energy dissipation rate, and the diapycnal diffusivity from 20 to 250 m are ε = 4.15 ± 15.9) × 10–6 W kg–1 and K ρ = (1.44 ± 10.7) × 10–2 m2s–1, respectively. This K ρ is up to 104 times larger than the Banda Sea [ K ρ = (9.2 ± 0.55) × 10–6 m2s–1] (Alford et al. Geophys Res Lett 26:2741–2744, 1999) or the "open ocean" K ρ = 0.03 × 10–4 m2s−1 within 2° of the equator to (0.4–0.5) × 10–4 m2s−1 at 50°–70° (Kunze et al. J Phys Oceanogr 36:1553–1576, 2006). Therefore, nonlinear interactions between internal tides, tidally induced mixing, and ITF plays a critical role regulating water mass transformation and have strong implications to longer-term variations and change of Pacific–Indian Ocean water circulation and climate. [ABSTRACT FROM AUTHOR]

The Atlantic inventory of anthropogenic carbon (Cant) and its changes between 1990 and 2020 are investigated by applying the transit time distribution (TTD) method to anthropogenic tracer data. In contrast to previous TTD applications, here we take into account the admixture of old waters free of anthropogenic tracers. The greatest difference from other methods based on direct carbon observations is the higher Cant storage in the deep ocean. Estimations of the decadal Cant increase based on direct carbon observations yield in general a smaller share of Cant storage in the North Atlantic and a larger share in the South Atlantic compared to our results. Changes in oceanic circulation and/or ventilation have significant impacts on the Cant inventory on the regional scale. The enhanced upwelling of older water in the Southern Ocean and the variability in the convection depth in the Labrador Sea lead to deviations in the inferred Cant increase between 1990 and 2020 from the rate equivalent to a steady-state ocean. For the total Atlantic Cant inventory, however, decadal ventilation variability of individual water masses partially compensates for each other. In addition, its impact on the Cant storage is small due to the much higher flushing time for the whole Atlantic of the order of hundreds of years. The total Cant inventory increases from 43.0 ± 7.3 Pg C in 1990 to 68.2 ± 10.8 Pg C in 2020, almost in unison with the rising CO2 in the atmosphere. So far, ventilation changes have impacted the Cant concentrations only on the regional scale, especially in the subpolar North Atlantic and the Southern Ocean. [ABSTRACT FROM AUTHOR]

The Amundsen Sea of West Antarctica is one of the regions of particularly interest to the oceanography community due to accelerated basal melting of its ice shelves. Quantitative estimates of the composition of water masses and their sources are essential for understanding the heat and salt budgets and the stability of ice shelves in this region. In this study, the water mass distributions on the Amundsen Sea continental shelf are estimated based on FOAM-GLOSEA5v13, a global oceanographic reanalysis product, using a modified statistical based optimal water mass analysis method. The reanalysis product effectively captures the signal of primary water masses, including the Winter Water (WW) and the Circumpolar Deep Water (CDW) below the Antarctic Summer Surface Water (SSW). The horizontal and vertical distributions and temporal evolution of three water mass endmembers (WW, CDW and DSW) and their mixing ratios are calculated, based on which an intrusion location detection method was established. Using the reanalysis product and intrusion detection method, all potential CDW intrusion sites along the shelf break are identified. Based on the reanalysis product, an intrusion site over the shelf break, away from major deep troughs was captured and confirmed by numerical particle tracking experiments. [ABSTRACT FROM AUTHOR]

The Ross Sea in the Southern Ocean is a key region for the formation of the Antarctic Bottom Water (AABW) that supplies the lower limb of the global overturning circulation, and contributes to 20–40 % of the total AABW production. AABW primarily originates from polynyas characterized by strong sea ice production and ocean convection that lead to the formation of Dense Shelf Water (DSW), the precursor of the AABW. The production and characteristics of DSW in the Ross Sea and AABW in the surrounding ocean are significantly affected by ice shelf meltwater transported from the nearby Amundsen Sea. The scarcity of long-term observations in the Ross Sea hinders the understanding of DSW and AABW variability, and numerical models are needed to explore the multi-scale variations of these water masses and the forcing mechanisms. In this work, a coupled high-resolution ocean-sea ice-ice shelf model is developed for the Ross Sea and Amundsen Sea, named RAISE (R oss- A mundsen Sea I ce- Se a Model). Detailed descriptions of the model configurations are provided. This study represents a first attempt to thoroughly evaluate the DSW properties and associated ocean-sea ice-ice shelf coupling processes among modelling studies in the Southern Ocean, using multiple datasets including satellite-based observations and hydrographic measurements from the World Ocean Database, Argo profilers and seal-tag sensors. In particular, the modelled temporal variations of DSW in polynyas and its key export passages are compared with long-term mooring observations, which are not seen in DSW studies before. RAISE demonstrates high skills in simulating the observed sea ice production rates in the Ross Sea polynyas, and the modelled spatial and temporal variability of DSW are significantly and strongly correlated with observations. RAISE can also effectively capture the observed long-term freshening trend of DSW prior to 2014 and the rebounding of DSW salinity after 2014. RAISE shows an overestimate of DSW density in the Ross Sea, which is associated with underestimate of ice shelf melting rates in the Amundsen Sea, missing ice shelf calving processes and subglacial discharge in the model. A sensitivity experiment simulating increased freshwater discharge from these processes can significantly improve the simulation of DSW properties. [ABSTRACT FROM AUTHOR]

This study aimed to detect non-conservative processes that affect the distribution of fluorescent dissolved organic matter (FDOM) in the Baltic Sea. An extensive data set comprised of 408 FDOM data, optical and physical profiles, and the development of a water masses balance model allowed us to ascertain the sources of mixing anomalies. These were seen as second-order deviations in the FDOM distribution as a function of salinity in three layers: surface water, Baltic Sea Winter Water, and deep water. The difference between modeled and measured FDOM values at three different excitation/emission wavelengths allowed to show the strength of non-conservative processes, such as photochemical and microbial decomposition (negative residual values) or extracellular release of dissolved organic matter from phytoplankton, heterotrophic uptake and release from anoxic sediments (positive residual values). Humic-like FDOMfractions displayed positive residuals in all seasons for intermediate and deep layers and negative residuals in surface waters. Largest accumulation rates of humic-like fractions were reached in the Gulf of Gdańsk during summer in intermediate and deep layers, while the greatest removal in surface waters was observed during spring in the Bornholm and Gotland Basins and during summer in the Gulf of Gdańsk, probably due to photodegradation. Positive residuals of the protein-like fraction were observed at the surface in summer and autumn in the Gulf of Gdańsk, probably linked to the abundance of phytoplankton and also due to the low molecular weight by-products of photodegradation of humic-like components. Spatial transects revealed an increase in humic-like residuals with depth and a strong correlation with apparent oxygen utilization, increasing with higher fluorescence and exhibiting an asymptotic trend. A relationship was found between the protein-like fractions and phytoplankton biomass proxies. A generalized concept for FDOM cycling in the Baltic Sea was proposed, highlighting photobleaching as the dominant non-linear process determining the efficiency of humic-like FDOM removal. The protein-like component was found to be more efficiently taken up by aerobic prokaryotes at the surface. Microbial utilization and reworking of organic matter, release from sediments, and a decade-long stagnation of bottom water masses, all contribute to the observed accumulation of FDOM in mesohaline deep waters below the permanent pycnocline in the Baltic Sea. [ABSTRACT FROM AUTHOR]

The Sea of Okhotsk is a marginal sea that plays a major role in the ventilation of the North Pacific, being the key location where Dense Shelf Water (DSW) forms at the surface and sinks to the intermediate layer. The Okhotsk Sea Intermediate Water (OSIW) is a key water mass because it includes large amounts of DSW, outflows to the North Pacific, and supplies the ocean with the micronutrient iron. OSIW has been warming over the past few decades, which is attributed to a decreasing trend in DSW production. The acquisition of numerous hydrographic data after 2000 in the Kuril Basin, especially dissolved oxygen from profiling floats, offers an opportunity to better quantify the water mass composition of OSIW, and the changes in OSIW properties and DSW volume. Here, we used all available hydrographic records and a mapping technique specially adapted to polar and sub‐polar regions to revisit the Sea of Okhotsk water properties and document their long‐term changes. Our analysis revealed that the volume of heavier DSW (potential density above 26.9 kg.m−3) has decreased over the past three decades by 3,600 km3, or 15% of the volume present before 1990. This decline is nearly entirely compensated for by an increase in lighter DSW. This shift toward lighter DSW is possibly a sign of the weakening of the intermediate overturning circulation starting in the Okhotsk Sea. Additionally, we found that dense Soya Current Water only accounts for about 1% of OSIW, against the 5% previously estimated. Plain Language Summary: The water of the Sea of Okhotsk is a mix of several water masses which includes the Dense Shelf Water (DSW), a water mass generated in winter via sea ice formation. When sea ice forms, the surface water salinity and density increase, and the cold DSW is generated as the surface water sinks to the bottom of the continental shelf while micronutrients, especially iron, are also incorporated. DSW mixes with other water masses and becomes the Okhotsk Sea Intermediate Water (OSIW) before being exported to the North Pacific where it plays an important role for both the climate and the biological productivity. In this study, we used all the temperature, salinity, and dissolved oxygen data acquired since 1930 to evaluate how global warming may have affected the composition of OSIW. We found that, over the past century, the amount of denser DSW has strongly decreased whereas that of lighter DSW has increased, which is consistent with the decline in sea ice production in the Sea of Okhotsk. This also suggests that the ventilation of the Sea of Okhotsk and the circulation of the intermediate layer of the North Pacific is weakening, which could indirectly affect the ocean's biological productivity. Key Points: The composition of the Okhotsk Sea Intermediate Water (OSIW) is reevaluated using an updated data set and a new mapping techniqueOSIW properties have changed in the long‐term (≥50 years) scale, with a decrease (increase) in the amount of heavy (light) Dense Shelf WaterThis suggests that the North Pacific intermediate overturning is weakening, which could affect the ocean's biological productivity [ABSTRACT FROM AUTHOR]

This study conducted sensitive and precise analyses of dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) concentrations and trace concentrations of nitrate and nitrite (N+N) and soluble reactive phosphorus (SRP) in seawater. The methods were applied to investigate the distributions and controls of N+N, SRP, DON, and DOP in the oligotrophic Kuroshio Current (KC) area off eastern Taiwan during warm seasons. The water in the studied area was classified into four major types: typical Kuroshio water (KW), KW influenced by the East China Sea water, KC influenced by the South China Sea (SCS) water, and KW influenced by the SCS water and river plumes, which is confined to the coastal zone. Nutrient distributions in KC revealed very low N+N (0.002-0.40 mM) and SRP (0.015-0.125 mM) concentrations but high DON (<8 mM) and DOP (<0.3 mM) concentrations above the nutricline depth, which accounted for >80% of TDN and TDP, respectively; theses concentrations can primarily be attributed to strong, permanent surface stratification. Among the water types, KW had the lowest N+N, SRP, DON, and DOP concentrations but greatest chlorophyll maximum depth and nutricline depth, except for in locations influenced by island-induced upwelling. The concentrations of all nutrients increased by various degrees in the other water types, which was attributed to the exchange and mixing of different water masses and coastal uplift of subsurface waters. KW was not only highly oligotrophic but also N+N-limited reflected from very low [N+N]/[SRP] ratio (0.02-0.15) in the mixed layer (ML). Overall, the N+N limitation and high nitrate anomaly value (N*: 2.47 ± 0.16 mM) above the nutricline depth strongly indicate prevailing N2 fixation at the surface of KW. Very high DON/DOP ratio in KW (16.9-69.1) probably resulted from the release of N-rich organic nutrients from phytoplankton including N2 fixers at the surface and faster recycling of DOP than DON in deep waters. Persistent coastal uplift of subsurface water occurs everywhere over the shore-side region of the KC, resulting in increasing surface concentrations of nutrients and chlorophyll a. Overall, the aforementioned physical and biogeochemical processes determined the upper-ocean distributions of nutrient species in warm seasons. [ABSTRACT FROM AUTHOR]

Spatial variations of the nutrient concentration along the northwestward stream from the Bering Sea to the Chukchi Sea are considered for dissolved inorganic phosphorus (DIP) based on data from two surveys conducted in early September 2010 and 2020. The water mass from the Bering Sea has higher DIP than Chukchi Sea water, but in the upper layer its phosphorus is gradually lost due to both mixing with local water and consumption for photosynthesis. In the subsurface layer, the decrease in DIP is prevented by its recycling from degraded organic matter. The fluxes of phosphorus are estimated separately using the balance model with TS analysis, previously used in estuarial studies. Successive DIP utilization along the stream is traced, which forms in the Chukchi Sea two zones of high productivity divided by a wide low-productive zone. Nutrients from the upper layer are utilized within the southwestern Chukchi Sea, resulting in primary production 0.1–0.2 gC m–3 day–1, but the main stock of allochthonous nutrients from the subsurface layer is utilized further downstream (at Wrangel Island or in the northern Chukchi Sea) and results in production of 0.3–0.4 gC m–3 day–1. Localization of high-productive zones is determined by density stratification, which possibly depends on the stream strength: the stronger the advection through Bering Strait, the farther from the strait both zones are located. The recent tendency of strengthening of the stream tends to remove the zones of utilization the nutrients of Pacific origin from the southwestern Chukchi Sea. [ABSTRACT FROM AUTHOR]

The Ionian Sea is a crucial intersection for various water masses in the Mediterranean. Its hydrography and dynamics play a significant role in the seawater budgets and biogeochemistry of the neighboring sub-basins. Multiple theories have been formulated to gain a better understanding of the Ionian dynamics. These theories primarily attribute the variability of the near-surface Ionian circulation to internal processes. Here, we utilize horizontal currents and temperature–salinity profiles from the Copernicus reanalysis to examine the contribution of baroclinic modes to the variability of the basin horizontal circulation. Our findings demonstrate that, although the basin vertical structure is characterized by three layers, the primary patterns of the Ionian circulation can be attributed to the first baroclinic mode. This mode, along with the barotropic mode, accounts for over 85% of the overall variability in the Ionian circulation, suggesting that only one of the three interfaces separating the different water masses in the basin is dynamically active. We estimate the depth of this interface to be about 490 m. Additionally, our analysis shows that more than 90% of the kinetic energy over the water column is localized above this interface, indicating that the deep layer of the Ionian is dynamically nearly inert. [ABSTRACT FROM AUTHOR]

The Antarctic Slope Front and the associated Antarctic Slope Current dynamically regulate the exchanges of heat across the continental shelf break around Antarctica. Where the front is weak, relatively warm deep waters reach the ice shelf cavities, contributing to basal melting and ultimately affecting sea level rise. Here, we present new 2017–2021 records from two moorings deployed on the upper continental slope (530 and 738 m depth) just upstream of the Filchner Trough in the southeastern Weddell Sea. The structure and seasonal variability of the frontal system in this region, central to the inflow of warm water toward the large Filchner‐Ronne Ice Shelf, is previously undescribed. We use the records to describe the mean state and the seasonal variability of the regional hydrography and the southern part of the Antarctic Slope Current. We find that (a) the current is, contrary to previous assumptions, bottom‐enhanced, (b) the isotherms slope upwards toward the shelf break, and more so for warmer isotherms, and (c) the monthly mean thermocline depth is shallowest in February‐March and deepest in May‐June while (d) the current is strongest in April‐June. On monthly timescales, we show that (e) positive temperature anomalies of the de‐seasoned records are associated with weaker‐than‐average currents. We propose that the upward‐sloping isotherms are linked to the local topography and conservation of potential vorticity. Our results contribute to the understanding of how warm ocean waters propagate southward and potentially affect basal melt rates at the Filchner‐Ronne Ice Shelf. Plain Language Summary: Melting at the base of the Filchner‐Ronne Ice Shelf in the southeastern Weddell Sea is predicted to increase dramatically. The melt occurs within the ice shelf cavity filled with seawater, and the heat needed to melt the ice comes from warm water from the deep ocean. The warm water is carried toward the cavities over the relatively shallow continental shelves by ocean currents. The Antarctic Slope Current, a current that flows along the continental slope between cold and warm water masses, limits the amount of warm water that enters the continental shelf. Here, we use new four year‐long mooring records from the upper part of the continental slope north of the Filchner‐Ronne Ice Shelf and just east of the Filchner Trough to describe how the current changes with the season and how its vertical structure evolves throughout the year. We find that the warm water is lifted up toward the shelf break and suggest that this is linked to the presence of the Filchner Trough. An improved understanding of the dynamics of the Antarctic Slope Current in the region will allow us to better evaluate the potential for dramatic changes to the system in the future. Key Points: Four years of observations show that the Antarctic Slope Current north of the Filchner Trough is bottom‐enhanced and strongest during winterThe bottom enhancement is linked to isotherms that slope upward toward the shelf break, facilitating on‐shelf transport of warm waterOn monthly to seasonal time scales, positive temperature anomalies above the slope are associated with weaker‐than‐normal currents and vice versa [ABSTRACT FROM AUTHOR]

The slow water renewal endows the Baltic Sea a strong retention of pollutants/nutrients. Constraining water age is a practical way to depict the transport timescales for water masses and accompanying soluble substances. Although the water ages in the Baltic Sea have been resolved by 3D ocean models 20 year ago, the simulated results have not been verified. In this work, we exploited two anthropogenic radionuclides (129I and 236U) to constrain the ages of inflowing North Sea saline waters into the Baltic Sea by using the Tracer Age and Transit Time Distribution approaches. Our results indicate that the Baltic Sea has a highly stratified structure with a more diffusive bottom circulation and a more advective surface circulation. The circulation timescale for the saline water in the Baltic Sea was estimated to be >30 years from the bottom of Arkona Basin to the surface of central Baltic Sea. This work demonstrates the power of anthropogenic radiotracers in investigating circulation timescale and mixing processes in the Baltic Sea and provides the first observation‐based proof for the multi‐decadal retention of (radioactive) pollutants within the Baltic Sea. Plain Language Summary: This study focuses on using two man‐made radioisotopes, I‐129 and U‐236, to determine the ages of North Sea saline waters entering the Baltic Sea. Our results show that the Baltic Sea's circulation timescale is more than 30 years. This provides the first evidence, based on observations, of pollutants being retained within the Baltic Sea for several decades. The study highlights the usefulness of anthropogenic radioactive tracers in understanding hydrodynamic processes in the coastal areas of Northwestern Europe. Key Points: 129I and 236U released from nuclear reprocessing plants were used to determine the ages of saline water in the Baltic SeaThe transport timescales and mixing regimes are investigated by the tracer age and transit time distribution approachesThe saline water has a circulation timescale of >30 years within the interior Baltic Sea [ABSTRACT FROM AUTHOR]

Phthalic acid esters (PAEs) are widely used as plasticizers in industrial production. At present, this type of pollutant has been widely detected in the global environment. The distribution characteristics and ecological risk pollution of phthalate organic pollutants in seawater and sediments near Xiamen were evaluated, and gas chromatography-mass spectrometry was used to determine the concentration of PAEs in the samples. The results showed that a total of 11 PAEs compounds were mainly detected in the aqueous and sedimentary environmental media, and the total PAEs mass concentration in water ranged from 45.9-251.1 ng/L, with an average concentration of 109.2 ng/L. The concentration in sediment ranges from 8.12-50.29 μg/kg, with an average concentration of 27.7 μg/kg. The risk entropy method was used to assess the risk of PAEs in the near sea area, and the results showed that DBP has a moderate ecological risk level. It is necessary to further trace the pollution emissions of PAEs in these areas. [ABSTRACT FROM AUTHOR]

The spatial distribution of sediments on the seafloor reflects the various dynamic processes involved in the marine realm. To analyze sediment transport patterns in the North Aegean Sea, 323 surficial samples were obtained and studied. The granulometry data revealed a diverse range of grain sizes of surficial sediments, ranging from purely sandy to clay. The predominant size classes were silt and muddy sand, followed by sandy silt and mud. However, there were very few samples that fell within the clay classes. The sorting coefficient ranged from 0.21 to 5.48, while skewness ranged from −1.09 to 1.29. The sediment transport patterns were analyzed based on the grain-size parameters (mean, sorting, and skewness). The results showed the variability of flow parameters involved in sediment distribution. River influx and longshore drift near the shoreline are the most significant factors affecting sediment transport. At the open sea, sediment distribution is mainly controlled by general water circulation patterns, especially by the outflow of low-salinity waters from the Black Sea through the Dardanelles and the Marmara Sea. The heterogeneity of sediment textural parameters across the study area suggests that seafloor sediments are further reworked in areas where water masses are highly energetic. It can be concluded that open sea water circulation controls sediment distribution patterns at the open shelf, while close to the coast, river discharge plays a key role. [ABSTRACT FROM AUTHOR]

Changes in the masses of icebergs due to deterioration processes affect the drift of icebergs and should be taken into account when assessing iceberg risks in the areas of offshore development. In 2022 and 2023, eight laboratory experiments were carried out in the wave tank of the University Centre in Svalbard to study the melting of icebergs in sea water under calm and rough conditions. In the experiments, the water temperatures varied from 0   ℃ to 2.2   ℃ . Cylindrical iceberg models were made from columnar ice cores with a diameter of 24 cm. In one experiment, the iceberg model was protected on the sides with plastic fencing to investigate the iceberg's protection from melting when towed to deliver fresh water. The iceberg masses, water temperatures, and ice temperatures were measured in the experiments. The water velocity near the iceberg models was measured with an acoustic Doppler velocimeter. During the experiments, time-lapse cameras were used to describe the shapes and measure the vertical dimensions of the icebergs. Using experimental data, we calculated the horizontal dimensions of icebergs, latent heat fluxes, conductive heat fluxes inside the iceberg models, and turbulent heat fluxes in water as a function of time. We discovered the influence of surface waves and water mixing on the melt rates and found a significant reduction in the melt rates due to the lateral protection of the iceberg model using a plastic barrier. Based on the experimental data obtained, the ratio of the rates of lateral and bottom melting of the icebergs and lateral melting of the icebergs under wave conditions was parametrized depending on the wave frequency. [ABSTRACT FROM AUTHOR]

The East Australian Current (EAC), as the western boundary current of the South Pacific Gyre, governs the mesoscale circulation dynamics along the East Coast of Australia. It also exhibits meandering and instabilities that generate fine‐scale structures. Understanding the interplay between these fine‐scale processes and the larger‐scale EAC patterns is crucial in regards to chlorophyll‐a distribution, a key indicator of phytoplankton abundance and an essential component of marine ecosystems. However, the link between fine‐scale and large‐scale variability in the EAC's separation dynamics and its impact on chlorophyll‐a distribution remains poorly understood. In this study, we investigate this relationship using a clustering of high‐frequency radar data near the EAC's coherent jet and separation point, combined with satellite data on chlorophyll‐a concentration, sea surface temperature, and altimetry. Our findings reveal that the EAC acts as a frontal barrier for water‐mass and chlorophyll‐a concentration. The EAC separation enhances shelf‐offshore exchange and off‐shelf chlorophyll‐a concentration. In contrast, the extension of the current is characterized by fine‐scale structures with cyclonic vorticity and elevated strain, consequently contributing to an elevation of chlorophyll‐a levels in the region. In conclusion, the impact of separation dynamics on chlorophyll‐a distribution involves a complex interplay between large‐scale and fine‐scale processes, which may possibly compete under certain scenarios. Considering the implications of our findings in the context of future climate conditions, it becomes evident that purely dynamical changes in the separation latitude of the East Australian Current may hold consequences for chlorophyll‐a concentration distribution off the east coast of Australia. Plain Language Summary: The study focuses on the East Australian Current (EAC), a powerful ocean current flowing along Australia's East Coast. We investigate how small‐scale processes of the order of kilometers, and larger EAC patterns of the order of 100s kilometers affect chlorophyll‐a distribution, an indicator of phytoplankton abundance that forms the foundation of the marine food web. We used remote high‐frequency radar and satellite observations of surface currents, temperature and ocean color to investigate these interactions. We found that, when the current separates from the coast, it allows shelf and open ocean waters to interact, affecting chlorophyll‐a concentrations. Additionally, the EAC's twists and turns generate eddies (circular water movements) and fronts (boundary zones between different water masses), which have significant effects on chlorophyll‐a abundance in different areas. In conclusion, we highlight that the impact of the EAC dynamics involves a complex interplay between large‐scale and fine‐scale processes. Understanding these interactions is crucial for predicting how future climate changes may impact chlorophyll‐a distribution and marine life off the east coast of Australia. Key Points: The East Australian Current acts as a frontal barrier for water‐mass and chlorophyll‐a concentrationShelf‐offshore exchange of chlorophyll‐a concentration is enhanced when the East Australian Current separates from the coastThe strongest chlorophyll‐a anomalies are associated with cyclonic sub‐mesoscale strain inshore of the East Australian Current [ABSTRACT FROM AUTHOR]

The South China Sea throughflow (SCSTF) connects the South China Sea (SCS) with neighboring seas and oceans, transferring surface water of the global thermohaline circulation between the Pacific and Indian oceans. A configuration of the SYMPHONIE ocean model at high resolution (4 km) and including an explicit representation of tides is implemented over this region, and a simulation is analyzed over 2010–2018. Comparisons with in situ and satellite data and other available simulations at coarser resolution show the good performance of the model and the relevance of the high resolution for reproducing the spatial and temporal variability of the characteristics of surface dynamics and water masses over the SCS. The added value of an online computation of each term of the water, heat, and salt SCS budgets (surface, lateral oceanic and river fluxes, and internal variations) is also quantitatively demonstrated: important discards are obtained with offline computation, with relative biases of ∼40% for lateral oceanic inflows and outflows. The SCS water volume budget, including the SCSTF, is analyzed at climatological and seasonal scales. The SCS receives on average a 4.5 Sv yearly water volume input, mainly from the Luzon Strait. It laterally releases this water to neighboring seas, mainly to the Sulu Sea through Mindoro Strait (49 %), to the East China Sea via Taiwan Strait (28 %), and to the Java Sea through Karimata Strait (22 %). The seasonal variability of this water volume budget is driven by lateral interocean exchanges. Surface interocean exchanges, especially at Luzon Strait, are all driven by monsoon winds that favor winter southwestward flows and summer northeastward surface flows. Exchanges through Luzon Strait deep layers show a stable sandwiched structure with vertically alternating inflows and outflows. Last, differences in flux estimates induced by the use of a high-resolution model vs. a low-resolution model are quantified. [ABSTRACT FROM AUTHOR]

The Yellow Sea Cold Water Mass (YSCWM), a seasonal bottom‐layer water mass in the Yellow Sea (YS) most prominent in summer, has always been overlooked in previous studies investigating variations in the winter Sea Surface Temperature. Here, using observations and high‐resolution numerical modeling, we reveal for the first time the significant contribution of thermal stratification associated with the YSCWM to the sea surface cooling and the formation of the westward‐shifted Yellow Sea Warm Water Tongue (YSWT) in early winter. Heat budget analyses indicated that the impact of the YSCWM to surface cooling manifests in two aspects. First, the upward mixing of bottom cold water from the remnant YSCWM triggers cooling in the mixed layer. Second, the shallower local mixed layer above the YSCWM facilitates more pronounced cooling processes compared to the YSWT region under similar heat flux. The formation of the westward‐shifted YSWT is the result of intensified surface cooling in both the coastal shallow waters and the YSCWM region in the central YS. During the formation process of the YSWT, its main axis aligns with the western boundary of the YSCWM. As the YSCWM dissipates, the YSWT shows an eastward apparent displacement and eventually stabilizes near approximately 123°E. This study enriches our understanding of the transition from stratification to vertical homogeneity in the water temperature structure of the YS during early winter. Plain Language Summary: The Yellow Sea (YS), a shallow marginal sea in the northwestern Pacific, experiences significant seasonal variations in water temperature. During summer, the YS has layers of different temperatures with a colder water mass deep down. During winter, the water temperature becomes vertically uniform, and a tongue‐shaped warm water extends toward the northwest. Earlier studies of winter temperature changes in the YS often did not consider the effect of the cold water mass. Our research, using both observations and numerical models, shows that this cold water mass plays an important role in how the surface temperature changes in winter and proposes a novel mechanism for the formation of the winter warm water tongue. The thermal stratification associated with the bottom cold water mass contributes to stronger sea surface cooling through two processes: the upward mixing of cold deep‐water and the shallowing of the mixed layer. Additionally, the stronger cooling in both the deep waters and the coastal shallow waters result in the formation of the warm water tongue between these two regions. Understanding these changes is important for studying the YS's climate and its impact on marine life. Key Points: The Yellow Sea (YS) Cold Water Mass significantly contributes to the winter sea surface cooling in the YSThe effects of the YS Cold Water Mass are manifested in upward mixing of bottom cold water and shallowing of the mixed layerThe YS Warm Water Tongue forms under stronger cooling in the nearshore waters and the region of the YS Cold Water Mass [ABSTRACT FROM AUTHOR]

The interannual features of the salinity in the Chukchi Sea during the ice-free period of a year are investigated on the base of Soil Moisture Active Passive (SMAP) satellite measurements and GLORYS12v1 reanalysis data. Analysis of salinity measurements revealed two types of Bering Summer Waters (BSW) propagation: "western" and "eastern". The first is characterized by the penetration of Pacific waters into the northwest part of the sea, as well as the propagation of BSW to 180°W and 72.5°N. During the "eastern" type, salty waters are pressed to the eastern part of the shelf. Their area decreases and the northern boundary of the BSW area shifts to 174–176°W. Areas with low salinity, ~29 psu, are observed in the western part of the sea. Our study reveals that the formation of these types is affected not only by the inflow of Pacific waters through the Bering Strait but also by the East Siberian Current (ESC). Both factors are related and lead to correlated changes in the salinity of the Chukchi Sea waters. ESC carries Arctic freshwaters from west to east and leads to a decrease in salinity in the western part of the sea. At the same time, southward ESC caused the blockage of the northward currents in the Bering Strait and a decrease in the influx of saline Pacific waters in the southern part of the Chukchi Sea. The intensification of ESC occurred in 1994, 2002, 2012, and 2016, when the volume transport of ESC increased by approximately 0.2 Sv, while the influx through the Bering Strait decreased. As a result, in the years with intense ESC, the spatial structure of the salinity of the Chukchi Sea changed significantly and the shelf-averaged salinity decreased by 0.3–0.5 psu. [ABSTRACT FROM AUTHOR]

The Labrador Sea is a region of the North Atlantic known for its strong ocean currents and deep water formation, which contribute to the transport and mixing of water masses throughout the region. The absorbing and fluorescing properties of dissolved organic matter (DOM) were assessed to track the water masses and the in situ production in the Labrador Sea (>200 m). No significant differences in DOM composition were found in the mesopelagic waters (200–1,000 m). In the bathypelagic waters (1,000–4,000 m), the estimated dissolved organic carbon (DOC) and dissolved lignin concentrations, as well as humic‐like fluorescence intensities, allowed the discrimination of North East Atlantic Deep Water versus Denmark Strait overflow water. The humic‐like intensities were significantly different between upper Labrador Sea water (uLSW) and deep LSW (dLSW) suggesting their applications as tracers of deep winter mixing. The significant correlations with apparent oxygen utilization support the in situ production of humic‐like fluorescence and the net microbial consumption of DOC and lignin in the dark Labrador Sea. We also demonstrated that microbial activities play a role in the production of humic‐like compounds in the dLSW that experiences deep convection mixing. Plain Language Summary: The Labrador Sea is a region in the North Atlantic Ocean that is characterized by cold temperatures, and strong winds. It is known for its strong ocean currents and deep water formation, which contribute to the transport and mixing of water masses throughout the region. Dissolved organic matter (DOM) is a complex mixture of organic molecules and their concentration and composition can vary depending on several factors, including the depth of the water and the source of the organic material. In this study, we aim to better understand the sources, composition, and fate of the DOM pool in the main Labrador Sea water masses. Significant differences in the DOM composition were found in the deep waters (>1,000 m), which can be used to study water mass transformations and pathways in the deep Labrador Sea waters. Microbial respiration was shown to affect significantly the DOM composition in the dark Labrador Sea. Together these results help us understand the vertical DOM composition and its implication for the cycling of deep ocean DOM. Key Points: Dissolved organic carbon, lignin phenols, and humic‐like levels differentiate the bathypelagic water massesDifferences in humic‐like fluorescence intensities are proposed as winter deep convection mixing tracersNet consumption of dissolved organic carbon and lignin phenols by bacteria was found in the dark Labrador Sea [ABSTRACT FROM AUTHOR]

Dissolved inorganic carbon, total alkalinity, and dissolved N2O samples of upper 500 m were collected at the tip of the Antarctic Peninsula during the 32nd Chinese Antarctic National Research Expedition. The pH and anthropogenic carbon were calculated and the results show that the patterns of anthropogenic carbon uptake and acidification progresses are different in two adjacent regions of this study area. In the region of Weddell‐Scotia confluence, hydrographic processes such as convection prompt the transport of anthropogenic carbon into the subsurface layer, whereas in the region east of Powell Basin, where stratification existed, the downward transport of anthropogenic carbon to this depth is inhibited. However, the pH values indicate that the acidification status of the subsurface waters that are influenced by the above two hydrographic features are similar or even identical at a certain depth range. The progress of ocean acidification in the well‐ventilated region are dominated by anthropogenic carbon uptake, while in the adjacent well‐stratified region, anthropogenic carbon uptake and in situ remineralization of organic matter or horizontal advection of carbon rich water masses or both. In the later region, anthropogenic carbon uptake and in situ remineralization (or horizontal advection) contribute 40% and 60% to pH decline, respectively, suggesting that pH value in water mass of this region may significantly influenced by natural processes. Plain Language Summary: Ocean acidification is one of the most important marine environmental issues, which can result from absorbing CO2 from the atmosphere or by natural processes such as intrusion from CO2 rich deep water via upwelling. The Southern Ocean is an important region that absorbs large amounts of atmospheric CO2 and also a region with strong upwelling activity, so the above two processes may influence the status of acidification in this region. In this study, we investigated the region around the tip of the Antarctic Peninsula and compared areas of contrasting conditions, the result shows that anthropogenic CO2 absorption and natural process will both contribute to the change of pH value in the study region. Key Points: Vertical convection enhancing the intrusion of anthropogenic carbon into the subsurface layer at the tip of Antarctic PeninsulaBoth anthropogenic carbon and remineralization will contribute to ocean acidification at the tip of Antarctic Peninsula [ABSTRACT FROM AUTHOR]

Using temperature and salinity profiles from the Argo data repository, a detailed volumetric temperature‐salinity diagram is compiled for the upper 2,000 m layer of the Atlantic Ocean. It is generally accepted that, unlike the Pacific and Indian Oceans where the Equatorial Water is present, there is no Equatorial Water in the Atlantic Ocean and its place is occupied by the South Atlantic Central Water (SACW). However, the detailed volumetric T‐S diagram shows that the main thermocline in the latitude range of 10°S–10°N is characterized by its own tight T‐S relationship which is relatively close to but clearly distinguishable from the tight T–S relationship of SACW in the latitude range of 10°S–40°S. We argue that the Atlantic Equatorial Water can be considered as a separate water mass which is probably formed by isopycnal mixing of SACW and the North Atlantic Central Water (NACW) in proportion approx. 3.5:1. Plain Language Summary: One of the most astonishing properties of water in the ocean is the presence of a so‐called tight temperature‐salinity relationship, when the vertical profiles of temperature and salinity, sampled over a distance of hundreds and thousands of kilometers, express similar dependence of temperature versus salinity. The volume of oceanic water expressing similar temperature‐salinity relationship is considered a separate water mass. In the World Ocean, the water masses differ depending on their geographic location and origin. Oceanic water masses have been actively studied since the 1930s, and it would seem that everything is known about them. However, in 1998 the Argo program was launched, that collects information from inside the ocean using a fleet of robotic instruments that drift with the ocean currents and move up and down between the surface and a mid‐water level. Re‐examination of water masses using previously unavailable high‐quality large volume Argo data allowed us to distinguish a formerly unnoticed water mass in the main thermocline of the Equatorial Atlantic and thereby complete the phenomenological pattern of basic water masses of the World Ocean. Key Points: Main thermocline in the equatorial Atlantic in the depth range of 150–500 m is characterized by its own distinct tight T‐S relationshipThe low thermoclinicity water body in the equatorial Atlantic thermocline is separated from SACW and NACW by thermohaline frontsThe newly introduced Atlantic Equatorial Water (AEW) can be thought of as a mixture of SACW and NACW in proportion 3.5:1 [ABSTRACT FROM AUTHOR]

Neodymium (Nd) isotope ratios are potentially valuable for paleo‐ocean circulation reconstructions because they trace water‐mass transport and mixing in the present‐day oceans. Moreover, the Atlantic and Pacific global end‐member Nd‐isotope values can be constrained through time, and at in‐between sites their proportions vary with past climate changes. However, an important source of uncertainty about Nd‐isotopes' applicability for paleo‐circulation studies arises from the inability to constrain past Nd‐concentrations of the end‐members. Here we address this "paleo‐[Nd] problem" through a Bayesian analysis. Results show that even large variability in end‐member Nd‐concentrations over the Pleistocene is unlikely to significantly impact their concentration ratio, therefore end‐member concentration changes likely have only small impacts on Nd‐isotope ratios at in‐between locations. The results support their applicability to reconstruct past Atlantic paleo‐circulation. In addition, a Nd‐isotope mass balance for Antarctic Bottom Water shows that its present‐day Nd‐isotope values are consistent with the input from its sources. Plain Language Summary: Ocean circulation transfers heat around the Earth and characterizing its past variability is critical for understanding climate changes. Neodymium (Nd) isotopes are often used to trace past ocean circulation because they mimic deep ocean water mass mixing, and the North Atlantic and North Pacific global ocean mixing end‐member water mass compositions can be constrained through time. Therefore, temporal changes in Nd‐isotope ratios at intermediate locations like the Equatorial and South Atlantic are assumed to approximate changes in the proportions contributed by the global end‐members and thus reflect ocean circulation changes. However, Nd‐isotope ratios at intermediate locations are also sensitive to changes in Nd‐concentrations, which cannot be constrained in the global ocean end‐member water masses. This has caused the applicability of Nd‐isotope ratios for tracing paleo‐ocean circulation to be questioned. Our study examines the sensitivity of Nd‐isotope ratios at locations in‐between the global end‐members to changes in their Nd‐concentrations over recent interglacial‐glacial cycles. Results show that even substantial changes in their Nd‐concentrations likely have little impact on the Nd‐isotope ratios at intermediate locations, indicating that given the right intermediate location choices, Nd‐isotope ratios indeed reflect past deep ocean mixing, thus supporting their use to reconstruct past ocean circulation. Key Points: Neodymium (Nd) isotopes trace modern water mass mixing, but unconstrained end‐member Nd concentrations limit their paleocirculation useA Bayesian approach tests effects of Nd‐concentration changes in global water mass end‐members on Nd‐isotopes at intermediate locationsResults predict small impacts on Nd‐isotopes and constrain projected uncertainties in past ocean end‐member mixing reconstructions [ABSTRACT FROM AUTHOR]

The Yellow Sea Cold Water Mass (YSCWM) is an important component of the hydrodynamic system and it significantly impacts the primary production of the Yellow Sea. This study investigated the difference in the interannual variability and long-term trends between the northern YSCWM (NYSCWM) and southern YSCWM (SYSCWM), and explored the main physical environmental factors that led to their inconsistency using multiple wavelet coherence. On the interannual scale, the intensities of the NYSCWM and SYSCWM exhibited consistent variability, but the intensity of the SYSCWM had a larger standard deviation and longer periodic signal than that of the NYSCWM. The two-factor combination of surface air temperature (SAT)–Niño 3.4 in the NYSCWM and sea surface temperature (SST)–northward seawater velocity (Vgos) in the SYSCWM controlled the interannual variability, which meant the influencing intensity variability differed in the NYSCWM and SYSCWM. In the long-term trend, the intensities of the NYSCWM and SYSCWM both showed decreasing trends during the study period. However, the enhanced circulation provided more horizontal heat input into the SYSCWM, and the relatively higher increase in SST and decrease in the amplitude of variation in the thermocline depth promoted vertical heat exchange in the SYSCWM, thereby making the intensity of the SYSCWM decrease more quickly than that of the NYSCWM. These findings provide important references that facilitate a deeper understanding of the influence of hydrological processes on marine ecosystems in marginal seas. [ABSTRACT FROM AUTHOR]

The boundary of Yellow Sea Cold Water Mass (YSCWM) is a key ocean frontal structure influencing the regional ecosystem. Complex oceanic processes such as tidal currents, upwelling, and internal waves influence fine-scale hydrological structures, comprehensively resulting in a significantly highly productive area for plankton and fisheries. However, detailed research requires inaccessible high-resolution data. To investigate the fine-scale and high-frequency effects of oceanic processes on the local hydrological and ecological environment, we conducted comprehensive cruise acoustic observations and intensive station surveys of the hydrological environment around the YSCWM boundary in summer 2021 and 2022, and found that: (1) fine-scale hydrological structures across the YSCWM boundary were directly captured through this specific intensive station observation design; (2) clear zooplankton diel vertical migration (DVM) phenomena match well with the thermocline variation, showing that acoustics are effective indicators that reflect the water mass layering structure in summer in the YS; and (3) the shear excited by internal waves during propagation and flood tides enhances the upward and downward mixing of the water mass near the thermocline, thus thickening and weakening the layer, an effect that will be more pronounced when both are present at the same time, with ebb tide having the opposite effect. Topographically influenced tidal upwelling also causes significant vertical fluctuations in isotherms. This represents a new way of studying the fine-scale hydrodynamic–hydrologic–ecological aspects of key regions through acoustic remote sensing. [ABSTRACT FROM AUTHOR]

Mansor, K.N.A.A.K.; Roseli, N.H.; Ali, F.S.M., and Akhir, M.F.M., 2023. Physical properties of seawater in Malacca Strait (Southeast Asia) during monsoon seasons. Journal of Coastal Research, 39(5), 921–932. Charlotte (North Carolina), ISSN 0749-0208. Malacca Strait (MS) is a narrow passage in Southeast Asia mainly influenced by the Asian monsoon system. As a busy international maritime route, MS is highly exposed to seawater pollution, harmful algal blooms, and jellyfish blooms. To understand the physical properties of the water column in MS, scientific cruise data were used to examine mixed layer depth, stratification frequency, and water mass distribution during two monsoon seasons (March and August). Results showed that surface water in March is fresher and warmer than in August, whereas the bottom depth in March is more saline and cooler than in August. The mixed layer depth for both months did not exceed approximately 15 m for temperature and salinity, with thermocline and halocline layers observed below the mixed layer depth. The temperature and salinity diagram classified three water masses from the cruise dataset—surface warm water, mixed water, and subsurface water—with the potential density anomaly ranging between about 15.5 and 24 kg/m3. The highest density of water mass, subsurface water, was found only in March at a depth between 54 and 80 m. This cool, high-salinity water is the remaining NE monsoon water mass that settled near the bottom because March is the period of change from NE monsoon to SW monsoon. During this time, winds weaken and solar radiation increases, thus creating stable warm surface water. Strong stratification observed in March prevented mixing between warm surface water and cool bottom water. Meanwhile, August is characterized by a warm SW monsoon; thus, the whole MS is occupied by warmer water. This research presents the variation of physical properties in the water column and reveals the influences of monsoon season on shifts of stratification and water mass distribution. [ABSTRACT FROM AUTHOR]

Particulate Mn, given its high adsorptive capacity and oxidation potential, has profound impacts on the cycling of various trace elements and organic matter in the ocean. Moreover, particulate Mn acts as a sink (via oxidation and adsorption) or as a source (via remineralization and photoreduction) term of bioactive dissolved Mn(II). In the Canadian Arctic Ocean, particulate Mn distributions in the water column revealed the presence of distinctively high particulate Mn concentrations and an overwhelming dominance of the non‐lithogenic component to the bulk particulate Mn pool. This phenomenon is of particular importance in halocline waters in the Canada Basin, the Canadian Arctic Archipelago and Baffin Bay, and near‐bottom samples in Baffin Bay. Enhanced microbially‐mediated Mn oxidation in the water column is suggested as the main mechanism driving the non‐lithogenic dominance. Indeed, the microbial community composition data associated with high non‐lithogenic particulate Mn (i.e., Mn oxides) display a high relative abundance of taxa (e.g., f.Pirellulaceae, o.Phycisphaerales, f.Cryomorphaceae, g. Moritella) that have been identified in Mn oxide enriched environments. Furthermore, numerous taxa identified in the Canada Basin halocline water, where non‐lithogenic particulate Mn peaked, are phylogenetically related to known (cultured) Mn‐oxidizing bacteria (MnOB; e.g., Rhodobacteraceae, Oceanospirillaceae, Rhizobiaceae, and other Alphaproteobacteria). Putative MnOB appears to proliferate in certain water masses having a unique set of environmental conditions: low light intensity—alleviating photoinhibition—and high dissolved Mn concentrations, the main drivers known to influence MnOB dynamic, and hence, Mn oxidation. [ABSTRACT FROM AUTHOR]

After the atmosphere, the ocean is the most well-mixed and homogeneous global geochemical reservoir. Both physical and biological processes generate elemental and isotope variations in seawater. Contrasting geochemical behaviors cause elements to be susceptible to different fractionation mechanisms, with their isotopes providing unique insights into the composition and evolution of the ocean over the course of geological history. Supplementing the traditional stable isotopes (H, C, O, N, S) that provide information about ocean processes and past environmental conditions, radiogenic isotope (Sr, Nd, Os, Pb, U) systems can be used as time markers, indicators of terrestrial weathering, and ocean water mass mixing. Recent instrumentation advances have made possible the measurement of natural stable isotope variations produced by both mass-dependent and mass-independent fractionation for an ever-increasing number of metal elements (e.g. Li, B, Mg, Si, Ca, V, Cr, Fe, Ni, Cu, Zn, Se, Mo, Cd, Tl, U). The major emphasis in this review is on the isotopic composition of the light elements based on a comparatively large literature. Unlike O, H and S, the stable isotopes of C, N and Si do not have a constant isotopic composition in the modern ocean. The major cations Ca, Mg, and Sr fixed in carbonate shells provide the best proxies for reconstruction of the composition of the ocean in the past. Exhibiting large isotope enrichments in ocean water, B and Li are suitable for the investigation of water/rock interactions and can act as monitors of former oceanic pH. The bioessential elements Zn, Cd, and Ni are indicators of paleoproductivity in the ocean. Characteristic isotope enrichments or depletions of the multivalent elements V, Cr, Fe, Se, Mo, and U record the past redox state of the ocean/atmosphere system. Case studies describe how isotopes have been used to define the seawater composition in the geological past. [ABSTRACT FROM AUTHOR]

A new species, Fragilaria shirshovii sp. nov., is described on the basis of molecular and morphological investigations. Cells were isolated from the surface levels of desalinated water masses in the zone of mixing river and marine waters in the Gulf of Ob. The morphology of this species is analyzed with light microscopy (LM) and scanning electron microscopy (SEM). The valves of F. shirshovii sp. nov. are fusiform to almost linear. The frustules are linked together in the central part, forming double comb-shaped colonies. The axial area is narrow and linear, with a distinct rounded or rectangular fascia in the center. Ghost striae are present on the central area. Areolae are not discernible in LM. Apical pore fields are composed of small pores and closed papillary outgrowths. The lips of the rimoportula are well pronounced and the striae are uniseriate. Phylogenetic analysis based on partials 18S rRNA and rbcL genes infers the species F. shirshovii sp. nov. as a member of Fragilariaceae. The species most resembling F. shirshovii sp. nov. are F. crotonensis, F. pararumpens, F. bidens and F. perminuta. The differences between F. shirshovii sp. nov. and close taxa are discussed. This work is a pioneer investigation of Fragilariaceae taxa from the freshwater ecosystems of the Siberian Arctic using a combination of morphological and molecular tools. [ABSTRACT FROM AUTHOR]

We present a model for the thermal evolution of Magma Ocean (MO) in interaction with a degassing atmosphere of H2O and CO2. The interior model is based on parameterized convection and is coupled to the atmospheric Model of Marcq et al. (2017, https://doi.org/10.1002/2016JE005224) through heat and volatiles. A new equation for the mass balance of volatiles is implemented, correcting Salvador et al. (2017, https://doi.org/10.1002/2017je005286). We found that the domain for water condensation is extended: for instance, depending on the cloud cover and resulting albedo, 0.13 Earth's ocean mass might be sufficient to form a water ocean on early Venus (instead of 0.3 MEO in Salvador et al. (2017, https://doi.org/10.1002/2017je005286)). Comparing our results with other recent models, we discuss the relative influence of the model hypotheses, such as mantle melting curves (which depend on mantle composition), the treatment of the atmosphere (e.g., gray or convective‐radiative) and the treatment of the last stages of the MO solidification (e.g., episodic resurfacing, stagnant lid...). We also apply our results to exoplanets. They suggest that liquid water might be present at the surface of Trappist‐1e and 1f, provided that those planets' volatile primitive contents were dominated by H2O and CO2. Plain Language Summary: We present a numerical model of a cooling magma ocean (MO) and the atmosphere degassing from it. The solidification of the MO leads to the enrichment of the silicate melt in volatiles, thus favoring degassing. Both reservoirs interact via heat and volatile exchange, where the volatiles are H2O and CO2. The aim of this model is to explore the influence of the atmosphere on the surface conditions after the MO stage, and especially the conditions required for the condensation of a water ocean to occur. For example, for an early Earth at 1 AU initially containing 1 Earth's water ocean mass, a water ocean could form for initial CO2 content as large as 1,000 bars. Moreover, a tenth of the actual Earth's water ocean mass would be sufficient to generate a water ocean on early Venus. Liquid water could also be present on the surface of the two exoplanets Trappist‐1e and 1f. Comparing our results with other recent models, we discuss the relative influence of the model hypotheses, such as mantle composition, the treatment of the heat transfer in the atmosphere, and the treatment of the last stages of the MO solidification. Key Points: An initial CO2 content up to 1,000 bars would not prevent condensation of a water ocean on the early Earth0.13 Earth's ocean mass would be sufficient to form a water ocean on early VenusLiquid water may be present on the surface of Trappist‐1e and 1f [ABSTRACT FROM AUTHOR]

The Atlantic inventory of anthropogenic carbon (Cant) and its changes between 1990 and 2010 are investigated by applying the transit time distribution (TTD) method to anthropogenic tracer data. In contrast to previous TTD applications, here we take into account the admixture of old waters free of anthropogenic tracers. The greatest difference to other methods based on direct carbon observations is the higher Cant storage in the deep ocean. The results from the TTD method better reflects the observed distribution of other transient tracers such as chlorofluorocarbons (CFCs). Changes in oceanic circulation/ventilation are important on the regional scale. The enhanced upwelling of older water in the Southern Ocean and the decline in the convection depth in the Labrador Sea lead to deviations of the inferred Cant increase between 1990 and 2010 from the rate equivalent to a steady state ocean. For the total Atlantic Cant inventory, however, decadal ventilation variability of individual water masses is partially compensating each other, and the effect is small due to the much higher flushing time for the total Atlantic of the order of hundreds of years. The total Cant inventory increases from 39.7 ± 7.7 Pg C in 1990 to 54.6 ± 9.5 Pg C in 2010, almost in unison with the rising CO2 in the atmosphere. Only a reduction of the Atlantic ventilation over several decades would severely change this relationship. [ABSTRACT FROM AUTHOR]

Manganese (Mn) is an essential element involved in photosynthesis, yet its concentrations in Southern Ocean open waters are very low, arising from biological uptake and limited external inputs. At southern latitudes, waters overlying the Antarctic shelf are expected to have much higher Mn concentrations due to their proximity to external sources. In this study, we investigated the potential export of Mn‐rich Antarctic shelf waters toward depleted open Southern Ocean waters. Our results showed that while high Mn concentrations were observed over the shelf, biological uptake decreased dissolved Mn concentrations in surface waters north of the South Antarctic Circumpolar Current Front (<0.1 nmol kg−1), limiting its export. Conversely, mixing between Mn‐rich Antarctic Bottom Waters and Mn‐depleted Lower Circumpolar Deep Waters combined with oxidative and scavenging processes led to a decrease in dissolved Mn concentrations within bottom waters, with distance from the coast. Particulate Mn concentrations also showed a decreasing trend with distance from the coast. A comparison with other Antarctic coastal regions suggests this bottom water Mn removal may be widespread and that East Antarctica may be characterized by lower Mn concentrations compared to other regions. Still, subsurface dissolved Mn maxima (0.3–0.6 nmol kg−1) represented a potential reservoir for surface waters. We hypothesize that these high subsurface values result from external sources near the shelf. Overall, these results suggest that the moderate lateral export of trace metal‐enriched waters contributes to the extremely low and potentially limiting Mn concentrations previously reported further north in this Southern Ocean region. Key Points: High shelf manganese export is limited by biological uptake in surface watersIn bottom waters, dissolved and particulate manganese concentrations can be explained by water mass mixing and removal processesEast Antarctica is characterized by lower dissolved manganese concentrations compared to other coastal Antarctic regions [ABSTRACT FROM AUTHOR]

We explore historical variability in the volume of Labrador Sea Water (LSW) using ECCO, an ocean state estimate configuration of the Massachusetts Institute of Technology general circulation model (MITgcm). The model's adjoint, a linearization of the MITgcm, is set up to output the lagged sensitivity of the watermass volume to surface boundary conditions. This allows us to reconstruct the evolution of LSW volume over recent decades using historical surface wind stress, heat, and freshwater fluxes. Each of these boundary conditions contributes significantly to the LSW variability that we recover, but these impacts are associated with different geographical fingerprints and arise over a range of time lags. We show that the volume of LSW accumulated in the Labrador Sea exhibits a delayed response to surface wind stress and buoyancy forcing outside the convective interior of the Labrador Sea, at key locations in the North Atlantic Ocean. In particular, winds and surface density anomalies affect the North Atlantic Current's (NAC) transport of warm and saline subtropical water masses that are precursors for the formation of LSW. This propensity for a delayed response of LSW to remote forcing allows us to predict a substantial fraction of LSW variability at least a year into the future. Our analysis also enables us to attribute LSW variability to different boundary conditions and to gain insight into the major mechanisms that drive volume anomalies in this deep watermass. We point out the important role of buoyancy loss and preconditioning along the NAC pathway, in the Iceland Basin, the Irminger Sea, and the Nordic Seas, processes which facilitate the formation of LSW both in the Irminger and in the Labrador Sea. [ABSTRACT FROM AUTHOR]

Coastal municipalities and industries often discharge poorly treated wastewater into proximate marine and estuarine environments. The urban and/or effluent input can lead to eutrophication and lower water quality, as it holds high concentrations of nutrients and pollutants. One widely applied tool to increase effluent dispersion and direct it away from coastal areas, thus causing fewer impacts on human activities, is sea outfall. In Aveiro, Portugal, the São Jacinto sea outfall construction was completed in 1998; however, limited literature regarding the sea outfall's monitoring using satellite data is available. The methodology in this study involved collecting four years' worth (2016–2019) of satellite data to investigate visible traces of the interaction between the S. Jacinto sewage water mass and the Ria de Aveiro lagoon ecosystem using ocean color and spectral analysis, and producing new qualitative data regarding sea outfall plume dispersion monitoring through high-resolution Sentinel-2 imagery. The results showed a clear spectral signature of the sewage water mass, and a seasonal pattern was observed in which the plume was more evident in winter and autumn. Additionally, the coastal longshore current and the Aveiro lagoon's runoff were able to restrict the marine outfall's dispersion superficially. Ocean color data were revealed to be a factual and cost-effective tool to monitor the plume water. Finally, an exchange between the marine outfall water mass and Ria de Aveiro lagoon could happen in high tide under northern wind conditions. Therefore, it is important to monitor the water quality to ensure the coastal ecosystem's good environmental state. [ABSTRACT FROM AUTHOR]

The distribution of water masses in August 2019 in the Chukchi Sea and their hydrochemical features were investigated using shipboard data from the transarctic expedition on board the R/V Professor Levanidov. The work was carried out in the southern shallow-water part of the sea and in the northern part beyond the 70-m isobath. Six main water masses were identified: Alaskan and Anadyr Coastal Water (ACW), Siberian Coastal Water (SCW), Melt Water (MW), Summer Bering Sea Water (BSW), Remnant Pacific Winter Water (RWW) and Atlantic Water (AW). Their features are described and analyzed. It was revealed that the water masses of the Chukchi Sea are characterized by a high spatial variability of hydrochemical parameters, in particular, the BSW, RWW and AW. In the southern Chukchi Sea, very high values of ammonium nitrogen in the bottom layer were noted (up to 12.6 µM). In August 2019, transformed AW, typically residing below 150 m, was observed at a depth of 117–137 m. This transformed AW had salinity values from 34.2 to 35.7 which are typical for AW, but anomalously high content of silicate (93–97 µM), low apparent oxygen utilization values (240–270 µM), and low N* (a measure of fixed dissolved inorganic nitrogen excess relative to dissolved inorganic phosphorus) values (down to −24.0 µM). At the same time, the AW observed at depths greater than 140 m did not have such anomalous hydrochemical features. [ABSTRACT FROM AUTHOR]

Coherent ocean vortices, or eddies, are usually tracked on the surface of the ocean. However, tracking subsurface eddies is important for a complete understanding of deep ocean circulation. In this study, we develop an algorithm designed for the detection of subsurface eddies in the Arabian Sea using Nucleus for European Modelling of the Ocean (NEMO) model simulations. We optimize each parameter of our algorithm to achieve favorable results when compared with an algorithm using sea surface height (SSH). When compared to similar methods, we find that using the rescaled isopycnal potential vorticity (PV) is best for subsurface eddy detection. We proceed to demonstrate that our new algorithm can detect eddies successfully between specific isopycnals, such as those that define the Red Sea Water (RSW). In doing so, we showcase how our method can be used to describe the properties of eddies within the RSW and even identify specific long-lived subsurface eddies. We conduct one such case study by discerning the structure of a completely subsurface RSW eddy near the Chagos Archipelago using Lagrangian particle tracking and PV diagnostics. We conclude that our rescaled PV method is an efficient tool for investigating eddy dynamics within the ocean's interior, and publicly provide our optimization methodology as a way for other researchers to develop their own subsurface detection algorithms with optimized parameters for any spatiotemporal model domain. Significance Statement: Eddies are a key part of ocean circulation both at the surface and in the subsurface. The purpose of our study was to design the first detection method comprehensively optimized for subsurface eddy detection from numerical simulations. We demonstrate that potential vorticity (PV) is the best field to use when algorithmically tracking eddies in subsurface water masses, using our new method to identify and track eddies in the Red Sea Water (RSW). Additionally, our method allows us to efficiently evaluate the dynamics of eddies through potential vorticity diagnostics, exemplified with a previously undescribed eddy near the Chagos Archipelago. Our methodology can be used by future researchers to study the eddy dynamics hidden within subsurface water masses around the world. [ABSTRACT FROM AUTHOR]

Despite intensive study of the early Cambrian Nanhua Basin (ECNB) in South China, its degree of connectedness to the open ocean remains uncertain, impeding analysis of contemporaneous oceanic redox evolution and its role in the Cambrian Explosion. We analyzed bulk-shale B/Ga, a newly proposed proxy for watermass salinity in ancient depositional systems, for three sections (Daotuo, Yuanjia, and Zhalagou) representing a range of water depths in the ECNB and spanning early Cambrian Stages 2 and 3. The B/Ga proxy records a salinity gradient in the basin, with increasing salinity from shallow- to deep-water areas, particularly during the deposition of black shales of the lower Jiumenchong/Niutitang/Zhalagou Formation. The gradient was probably due to the mixing of freshwater from adjacent landmasses with marine waters within a moderately restricted basin. All three study sections exhibit a monotonic rise in salinity over the ~9-Myr study interval, reflecting increased watermass exchange with the open ocean due to regional tectonic and/or global sea-level changes. The improved watermass exchange may have weakened water-column stratification and reduced deep-water anoxia in the ECNB, contributing the enhanced oceanic oxygenation along with global environmental controls and further modulating the expression of the Cambrian Explosion recorded in Lower Cambrian sections of South China. Our study thus highlights the need for comprehensive analyses of hydrographic dynamics in marginal-marine settings that are used to explore co-evolutionary relationships between early animals and environmental oxygen levels. [ABSTRACT FROM AUTHOR]

The Heuningnes Catchment in the Republic of South Africa was used as a case study in this research to describe the application of saltwater fraction/quantification and hydrogeochemistry methods to evaluate the extent of saline intrusion in the coastal aquifers. The argument of the research is that the presence of seawater incursion may be conclusively determined by combining the examination of the major ions, seawater fraction, stable isotopes of water, bromide, and geochemical modeling. Using stable isotopes of oxygen (18O) and deuterium (2H), major ions chemistry, seawater composition, and geochemical modeling, the genesis of salinity and mixing of different water masses were examined. Twenty-nine (29) samples of groundwater were examined. All samples showed water facies of the Na-Cl type, indicating a seawater-related origin. The significance of mixing in coastal aquifers under natural conditions was shown by the hydrogeochemical characteristics of key ions derived from ionic ratios, which demonstrated substantial adherence to mixing lines among endmembers for freshwater as well as saltwater (seawater). The quantification of seawater contribution in groundwater percentages varied from 0.01 to 43%, with three samples having concentrations of seawater above 50%. It was clear from the hydrogeochemical analysis and determination of the proportion of saltwater that the seawater intrusion impacted the coastal fresh groundwater. In addition, the chloride concentration in the groundwater ranged from 81.5 to 26,557.5 mg/L, with the corresponding δ18O values ranging from −5.5‰ to −0.9‰, which suggested that freshwater and saltwater were mixing. The Br−/Cl− ratios showed that evaporation had played a part in elevating groundwater salinity as well. Since saturation indices were below zero, the mineral dissolution could also contribute to the salinization of groundwater. Further proof of seawater incursion in the investigated catchment was supplied by geochemical modeling and bromide. Even though such tools were not verified in multiple coastal aquifers for widespread generalization, the study offered a scientifically significant understanding of the application of such tools on seawater intrusion in coastal aquifers and has useful recommendations for the aquifer setting of similar environments. [ABSTRACT FROM AUTHOR]

Antarctic Bottom Water (AABW) is a major component of the global overturning circulation, originating around the Antarctic continental margin. In recent decades AABW has both warmed and freshened, but there is also evidence of large interannual variability. The causes of this underlying variability are not yet fully understood, in part due to a lack of ocean and air‐sea‐ice flux measurements in the region. Here, we simulate the formation and export of AABW from 1958 to 2018 using a global, eddying ocean–sea‐ice model in which the four AABW formation regions and transports agree reasonably well with observations. The simulated formation and export of AABW exhibits strong interannual variability which is not correlated between the different formation regions. Reservoirs of very dense waters at depth in the Weddell and Ross Seas following 1–2 years of strong surface water mass transformation can lead to higher AABW export for up to a decade. In Prydz Bay and at the Adélie Coast in contrast, dense water reservoirs do not persist beyond 1 year which we attribute to the narrower shelf extent in the East Antarctic AABW formation regions. The main factor controlling years of high AABW formation are weaker easterly winds, which reduce sea ice import into the AABW formation region, leaving increased areas of open water primed for air‐sea buoyancy loss and convective overturning. Our study highlights the variability of simulated AABW formation in all four formation regions, with potential implications for interpreting trends in observational data using only limited duration and coverage. Plain Language Summary: Antarctic Bottom Water (AABW) originates over the Antarctic shelf when sea ice formation and mixing leave behind very cold, salty and dense waters. One third of the total ocean volume is filled by AABW, the densest water mass in the global ocean. AABW is important for climate as it regulates the global ocean overturning of heat, freshwater, oxygen, carbon and nutrients. Although we know that AABW has been warming in recent decades, there are only very limited observations of the year‐to‐year variability in AABW formation rates. Here, we use a numerical simulation of the global ocean to quantify how much AABW is formed and exported to depth over the period 1958 to 2018. We found that the AABW formation and export changes strongly from year to year. We attribute this variability to changes in wind and sea ice around Antarctica. In general, more AABW is exported during years when the polar easterly winds are weaker, as this influences the sea ice distribution and sea ice formation in the regions of AABW formation. Our findings help to interpret trends and variations in observational data of AABW where only limited duration and coverage is available. Key Points: Strong interannual variability of modeled Antarctic Bottom Water (AABW) formation and export is identified at the four AABW formation sitesReservoirs of dense waters in the Weddell/Ross Sea due to strong water mass transformation can lead to higher AABW export for up to a decadeIncreased AABW formation and export is linked to weaker easterlies, less sea ice import, and more areas of open water and sea ice formation [ABSTRACT FROM AUTHOR]

Air-sea fluxes of dimethyl sulphide (DMS) and methanethiol (MeSH) from surface seawater in the remote Southern Pacific Ocean were measured in three Air-Sea Interface Tank (ASIT) experiments during the Sea2Cloud voyage in March 2020. The measured fluxes of 0.78 ± 0.44 ng m-2 s-1 and 0.05 ± 0.03 ng m-2 s-1 for DMS and MeSH, respectively, varied between experiments reflecting the different water mass types investigated, with lowest fluxes with subtropical water and highest with biologically-active water with sub-Tropical water and highest from the sub-Tropical Front. Measured DMS fluxes were consistent with calculated fluxes from a two-layer model using DMS concentration in the ASIT seawater. The experiments also determined the influence of elevated ozone, with one ASIT headspace amended with 10 ppbv ozone while the other provided an unamended control. Elevated ozone resulted in a decrease in DMS flux, corresponding to decreased conversion of dimethylsulfoniopropionate (DMSP) to DMS in the seawater. The MeSH:DMS flux range was 11–18 % across experiments, in line with previous observations, indicating that MeSH represents a significant contribution to the atmospheric sulfur budget. Using the ASIT results in combination with ambient seawater concentrations during Sea2Cloud, significant linear correlations were identified for both DMS and MeSH fluxes with nanophytoplankton cell abundance (rDMS= 0.73 and rMeSH= 0.86), indicating an important role for this phytoplankton size class, and also its potential as a proxy for estimating DMS and MeSH emissions in chemistry-climate models. [ABSTRACT FROM AUTHOR]

The intrusion of the warm and saline Atlantic Water (AW) has significant implications and feedback on the dynamics and thermodynamics of the Arctic Ocean. The AW enters the Arctic Ocean through two gateways: Fram Strait (FS) and the Barents Sea Opening (BSO). The AW is the dominant source of oceanic heat to the Arctic Ocean. In conjunction with the measurements in key hydrographic sections, numerical ocean modeling provides us with a useful tool to characterize and corroborate the temporal and spatial variability of the AW branches. Simulations are conducted using the regional configuration Arctic and North Hemispheric Atlantic (ANHA) of the ocean/sea‐ice model Nucleus for Modeling of the Ocean running at 1/4° and 1/12° resolution. Online passive tracers from the model configuration are used to trace the pathways of the AW inflow in the Arctic Ocean. While studying the variability of the AW through FS and the BSO, a cold mode of AW (Cold Atlantic Water [CAW]) with a temperature below 0℃ is detected along the rim of the eastern Eurasian Basin. It is formed primarily due to the intense sea surface cooling of the AW in the Barents and Kara Seas. The CAW exhibits the cascading process as it propagates downstream through the St. Anna Trough, leading to a considerable mixing with the ambient water. The CAW pulses eventually result in a significant heat content reduction in the eastern Arctic. This study provides insight into the changing AW property and its potential impact on the Arctic Basin. Plain Language Summary: Atlantic Water (AW) is a warm and salty water mass that distinguishes itself from the cold and fresh Arctic Water within the Arctic Ocean. The AW flows into the Arctic Ocean in two branches, Fram Strait Branch Water and Barents Sea Branch Water (BSBW). We define the AW using temperature and salinity as constraints in an ocean model. We first evaluate the interannual and seasonal variations of the AW properties at these two gateways, then quantify the AW volume and heat transport on the interannual and seasonal timescales. We also compare long‐term transport means with the available observations. We discover two strong Cold Atlantic Water (CAW) events along the rim of the eastern Eurasian Basin in 2013 and 2014. The dominant contributor to the CAW formation is the strong sea surface cooling in the Barents and Kara Seas over the turn of 2013 and 2014. Releasing artificial particles at Fram Strait and the Barents Sea Opening using particle tracking software, we find that the source of the CAW is primarily from the BSBW. The CAW signals progress along the typical AW poleward pathway and reduce the heat that is contained in the AW layer of the eastern Arctic Basin. Key Points: A cold mode of Atlantic Water with a temperature below 0℃ that reduced the heat content of the eastern Arctic is identified and trackedCold Atlantic Water (CAW) originates primarily from the Barents Sea Branch Water, with secondary contributions from the Fram Strait Branch WaterA Barents Sea surface cooling in 2013–2014 proved critical preconditioning for the transformation into CAW [ABSTRACT FROM AUTHOR]

At 26.5°N in the North Atlantic, a continuous transbasin observational array has been established since 2004 to detect the strength of the Atlantic meridional overturning circulation. The observational record shows that the subtropical Atlantic meridional overturning circulation has weakened by 2.5 ± 1.5 Sv (as mean ± 95% interval; 1 Sv ≡ 106 m3 s−1) since 2008 compared to the initial 4-yr average. Strengthening of the upper southward geostrophic transport (with a 2.6 ± 1.6 Sv southward increase) derived from thermal wind dominates this Atlantic meridional overturning circulation decline. We decompose the geostrophic transport into its temperature and salinity components to compare their contributions to the transport variability. The contributions of temperature and salinity components to the southward geostrophic transport strengthening are 1.0 ± 2.5 and 1.6 ± 1.3 Sv, respectively. The variation of salinity component is significant at the 95% confidence level, while the temperature component’s variation is not. This result highlights the vital role that salinity plays in the subtropical Atlantic meridional overturning circulation variability, which has been overlooked in previous studies. We further analyze the geostrophic transport variations and their temperature and salinity components arising from different water masses, which shows that a warming signal in Labrador Sea Water and a freshening signal in Nordic Sea Water are two prominent sources of the geostrophic transport increase. Comparison of the temperature and salinity records of the 26.5°N array with the upstream records from repeated hydrographic sections across the Labrador Sea suggests that these thermohaline signals may be exported from the subpolar Atlantic via the deep western boundary current. [ABSTRACT FROM AUTHOR]

A new three-dimensional method is proposed for calculating the annual mean subduction and obduction rate in the ocean and applied to the North Pacific Ocean. Due to the beta spiral, the subducted/obducted water at a given station can spread over/come from a wide range with different densities in the subsurface ocean. This new method can provide the three-dimensional feature of subduction/obduction and more accurate distribution of the annual subduction/obduction rate in density space. The spatial patterns of annual subduction/obduction rate obtained from both the classical and new methods are similar, although at individual stations the rate can be different; however, the new 3D method can greatly improve the density structure of subducted/obducted water mass. In spite of the assumption of idealized fluid in most previous studies, our analysis showed that subducted water masses can change their density due to diapycnal mixing, especially for water masses subducted at relatively shallow depths. In the North Pacific, the subduction process mainly takes place for about 1–2 months in most of the subtropical basin, while the time window for obduction is ∼100 days in the major obduction regions. Based on the SODA monthly mean climatology, the subducted/obducted water in the North Pacific is primarily distributed at depths of 80–120 m. Significance Statement: The annual mean subduction/obduction rate is a term quantifying the large-scale irreversible downward/upward water transport between the mixed layer and the permanent pycnocline; these processes are crucially important for climate and the biogeochemical cycle in the oceans. However, the widely used classical Lagrangian method for calculating the annual subduction/obduction rate does not take the three-dimensional structure of ocean currents into consideration, which may induce errors in the destinations/sources of subducted/obducted water masses and the associated water properties. This study is focused on refining the three-dimensional features of subduction/obduction and providing a more accurate distribution of the annual subduction/obduction rate in the density space. In addition, the time window for subduction/obduction and the distribution of subducted/obducted water in the ocean interior are explored based on the SODA monthly mean climatology. [ABSTRACT FROM AUTHOR]

Influenced by local mixing and coastal runoff, water masses in the South China Sea degenerate significantly. The K-means algorithm is used to classify the water masses based on WOD13 temperature and salinity observations from 1966 to 2013 because its principle is consistent with the definition of a shallow water mass. The numbers and initial centers of the water masses are determined using functions of in-cluster distance and density values. The result shows that there are ten water masses in the South China Sea. In combination with the T-S scatter diagram, the properties of the South China Sea water masses were analyzed, including their distribution, the seasonal variability, and the degeneration processes. The temperatures of water masses were higher in summer and lower in winter, with the amplitudes of variation gradually reduced from the surface to the bottom. The seasonal variation in salinity of the surface water masses was high in winter and low in summer, which mainly depends on the amount of river discharge and precipitation. The subsurface water masses were strongly affected by water from the Pacific Ocean; thus, the seasonal variability of these water masses is weak, especially for the intermediate water mass that characterized by prominent low salinity. The water mass formed by the Kuroshio water invading the South China Sea has insignificant seasonal variations in temperature and salinity. The properties and seasonal variabilities of the water masses derived from the K-means algorithm are in agreement with the existing conclusions, suggesting that the improved K-means algorithm is efficient and accurate in the shallow water mass division. [ABSTRACT FROM AUTHOR]

The location of the Subtropical Front (STF), the boundary between Subtropical and Subantarctic Water in the Southern Ocean is proposed to be influenced by the strength and location of the Southern Hemisphere westerly winds. We use a hydrodynamic hindcast model and recent observations to test if changes in the westerly winds can cause meridional shifts in the STF over interannual to decadal timescales by modulating local Ekman transport. We find that increased, or northward, shifted westerly winds lead to an enhanced northward Ekman transport over large parts of the Southern Ocean, resulting in a northward shift in the STF. Conversely for weaker or southward shifted westerly winds where the STF migrates south. However, this relationship breaks down in regions with strong eddy variability and western boundary current systems. In these regions an increase in westerly winds lead to a southward shift in the STF. Observation data from 2004 to 2019 display a southward shift of STF associated with the positive Southern Annular Mode. However, the shift is smaller than the latitudinal shifts in the location of the zero wind stress curl and maximum westerly winds (−0.4° latitude/decade). This discrepancy is due to positive Ekman trends resulting from the intensification of the westerly winds, which oppose the southward migration. Changes in the Ekman transport and the overall southward shift of the STF have also resulted in an observed positive trend in chlorophyll‐a concentrations south of the STF, which could have ramifications for marine ecosystems and the biological pump in the Southern Ocean. Plain Language Summary: The Subtropical Front (STF) is an important water mass barrier, between warm, salty, and nutrient‐depleted Subtropical Waters of the subtropical gyre to its north, and cold, fresh, but nutrient‐rich Subantarctic Waters of the Southern Ocean to its south. The position of the STF is thought to be influenced by the westerly winds. In this study we investigate if the STF shifts in relation to changes in these westerly winds. Our model experiments show that over large parts of the Southern Ocean, the changes in the location of the STF follow changes in the westerly winds, except in regions of strong oceanic currents. The observations show that between 2004 and 2019 a small southward trend of the STF has been detected over most parts of the Southern Ocean as a consequence of southward shift of the westerly winds due to positive Southern Annular Mode (SAM). However, the shift in the STF has not been as large as the shift in the winds, as it has been opposed by the strengthening of the westerly winds. The recent southward shift in the STF has led to an increase in plankton growth south of the STF due to increased mixing of the Subtropical and Subantarctic Waters. Key Points: Stronger westerly winds shift the Subtropical Front (STF), due to enhanced Ekman transports, northward, except in regions of strong currentsThe shift of the STF (2004–2019) is smaller than the shift in the westerly winds, possibly due to increased Ekman transportsSouthward shifts of the STF triggers a positive chlorophyll‐a response south of the front, and a negative response north of the front [ABSTRACT FROM AUTHOR]