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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]

The ice shelves in the Bellingshausen Sea are melting and thinning rapidly due to modified Circumpolar Deep Water (mCDW) intrusions carrying heat toward ice‐shelf cavities. Observations are, however, sparse in time and space, and extensive model‐data comparisons have never been possible. Here, using a circulation model of the region and ship‐based observations, we show that the simulated water mass distributions in several troughs traversing mCDW inflows are in good agreement with observations, implying that our model has the skills to simulate hydrographic structures as well as on‐shelf ocean circulations. It takes 7.9 and 11.7 months for mCDW to travel to the George VI Ice Shelf cavities through the Belgica and Marguerite troughs, respectively. Ice‐shelf melting is mainly caused by mCDW intrusions along the Belgica and Marguerite troughs, with the heat transport through the former being ∼2.8 times larger than that through the latter. The mCDW intrusions toward the George VI Ice Shelf show little seasonal variability, while those toward the Venable Ice Shelf show seasonal variability, with higher velocities in summer likely caused by coastal trapped waves. We also conduct particle experiments tracking glacial meltwater. After 2 years of model integration, ∼33% of the released particles are located in the Amundsen Sea, supporting a linkage between Bellingshausen Sea ice‐shelf meltwater and Amundsen Sea upper ocean hydrography. Plain Language Summary: The Bellingshausen Sea Ice Shelves have been affected by high ice‐shelf melt and ice‐loss rates in Antarctica. The reason for the ice loss is the presence of warm modified Circumpolar Deep Water (mCDW) over the continental shelf, which intrudes into ice‐shelf cavities. Recently, studies combining observations and models have significantly advanced our understanding of Ice‐Ocean interactions around Antarctica. For example, in the Amundsen Sea, the region neighboring the Bellingshausen Sea and contributing the most to Antarctic glacial ice loss, such an approach has revealed the drivers and pathways of ocean heat intrusions as well as the impact of glacial meltwater on the Antarctic coastal circulation. We follow this approach and evaluate our ocean model by comparing it with ship‐based and satellite‐based observations. Using this model simulation, we show that the simulated mCDW intrudes from the continental shelf break toward ice‐shelf cavities mainly through two deep cross‐shelf troughs. The glacial meltwater is transported by the westward Antarctic Coastal Current (AACC) flowing into the Amundsen Sea. The mCDW inflow and AACC in the coastal area show seasonal variabilities with higher velocities in the summer, which are likely caused by coastal trapped waves. Key Points: The simulated hydrographic structures over the Bellingshausen Sea continental shelf region are in good agreement with observationsThe warm water intrusions toward the Venable Ice Shelf show peaks of higher velocities in summer, likely caused by coastal trapped wavesGlacial meltwater from about 33% of the area underneath the BS ice shelves flows into the Amundsen Sea within 2 years [ABSTRACT FROM AUTHOR]

The Labrador Sea and contiguous Baffin Bay play an important role in the formation of the upper layer of the North Atlantic Deep Water, an essential component of the Atlantic Meridional Ocean Circulation. The hydrography of these two seas is strongly influenced by the melting of sea-ice and glacier-ice, which has likely been affected by long-term climate changes. In this study, we use historical data of ocean temperature and salinity from 1950 to 2022 to estimate the summer freshwater volume (SFV) in Baffin Bay and the Labrador Sea, establish climatologies, and assess the impact of multi-decadal climate change. The SFV climatology (1956 km3)and the summer freshwater budget (2286 km3)estimated from various components are in good agreement. Sea ice and glacial melt account for 37 % and 26 % of the freshwater budget, respectively. SFV climatologies before and after 1995 reveal an increase in Baffin Bay (+226 km3) because of enhanced glacier melting, and a decline (-112 km3) in the Labrador Sea because of recent sea ice volume decreases. The time series of Labrador Sea SFV and total freshwater content are uncorrelated at the multi-decadal scale possibly because the influx of freshwater from the Beaufort Sea dominates the long-term variability. [ABSTRACT FROM AUTHOR]

Ocean circulation plays a key role in structuring marine ecosystems in the Southern Ocean. However, the seasonal dynamics of ocean circulation are poorly understood in the ice‐covered continental shelves due to difficulties in conducting observations. We, therefore, investigated spatial and temporal variations in oceanographic conditions and their biological effects on the continental shelf off East Antarctica (35°E–50°E) by deploying conductivity–temperature–depth (CTD) tags on Weddell seals. The seals moved up to 633 km east from the tagging location. We successfully obtained 1254 CTD casts from seven seals. Winter Water (WW) was most prevalent (77.4% of the total data), followed by Supercooled Water (14.2%), Antarctic Surface Water (AASW: 7.4%), Modified Circumpolar Deep Water (mCDW; 0.9%), and Modified Shelf Water (0.1%). During our study period, landfast ice broke up extensively, and the easterly wind was most prevalent during autumn. AASW was observed in the subsurface layer over the shelf in autumn, suggesting that AASW intruded from the surface of off‐shelf areas through Ekman transport. Particular water masses (mCDW, AASW, and WW below the AASW) had positive effects on the seals' foraging behavior. These results highlight the importance of easterly wind‐driven shoreward intrusion of oceanic surface waters onto the shelf in autumn. This physical process may enhance transport and accumulation of additional prey and increase local prey availability during winter. Such a process may play important roles in the Antarctic coastal marine ecosystems that are influenced by landfast ice. [ABSTRACT FROM AUTHOR]

The Kuroshio is important to climate, weather prediction, and fishery management along the northeast coast of Asia because it transports tremendous heat, salt, and energy from east of the Philippines to waters southeast of Japan. In the middle of its journey northward, the Kuroshio's velocity mean and its variability east of Taiwan crucially affect its downstream variability. To improve understanding of the Kuroshio there, multiple platforms were used to collect intensive observations off Taiwan during the three-year Observations of the Kuroshio Transports and their Variability (OKTV) program (2012-2015). Mean Kuroshio velocity transects show two velocity maxima southeast of Taiwan, with the primary velocity core on the onshore side of the Kuroshio exhibiting a mean maximum velocity of ~1.2 m s-1. The two cores then merge and move at a single velocity maximum of ~1 m s-1 east of Taiwan. Standard deviations of both the directly measured poleward (v) and zonal (u) velocities are ~0.4 m s-1 in the Kuroshio main stream. Water mass exchange in the Kuroshio east of Taiwan was found to be complicated, as it includes water of Kuroshio origin, South China Sea Water, and West Philippine Sea Water, and it vitally affects heat, salt, and nutrient inputs to the East China Sea. Impinging eddies and typhoons are two of the principal causes of variability in the Kuroshio. This study's models are more consistent with the observed Kuroshio than with high-frequency radar measurements. [ABSTRACT FROM AUTHOR]

Current structure, transport, and water mass properties of the northward-flowing Kuroshio and the southward-flowing Luzon Undercurrent (LU) were observed for nearly one year, June 8, 2012--June 4, 2013, across the Kuroshio path at 18.75°N. Observations were made from four platforms: an array of six subsurface ADCP moorings, two Seagliders, fivepressure inverted echo sounders (PIES), and five horizontal electric field (HEF) sensors, providing the most detailed time series of the Kuroshio and Luzon Undercurrent water properties to date. Ocean state estimates of the western boundary current system were performed using the MIT general circulation model--four-dimensional variational assimilation (MITgcm-4D-Var) system. Prominent Kuroshio features from observations are simulated well by the numerical model. Annual mean Kuroshio transport, averaged over all platforms, is ~16 Sv with a standard deviation ~4 Sv. Kuroshio and LU transports and water mass pathways east of Luzon are revealed by Seaglider measurements. In a layer above the salinity maximum associated with North Pacific Tropical Water (NPTW), Kuroshio transport is ~7 Sv and contains North Equatorial Current (NEC) and Western Philippine Sea (WPS) waters, with an insignificant amount of South China Sea water on the shallow western flank. In an intermediate layer containing the core of the NPTW, Kuroshio transport is ~10 Sv, consisting mostly of NEC water. In the lower layer of the Kuroshio, transport is ~1.5 Sv of mostly North Pacific Intermediate Water (NPIW) as a part of WPS waters. Annual mean Luzon Undercurrent southward transport integrated to 1,000 m depth is ~2.7 Sv with a standard deviation ~2 Sv, carrying solely WPS waters below the salinity minimum of the NPIW. The transport of the western boundary current integrated over the full ocean depth east of Luzon Island is ~14 ± 4.5 Sv. Sources of the water masses in the Kuroshio and Luzon Undercurrent are confirmed qualitatively by the numerical model. [ABSTRACT FROM AUTHOR]