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Based on the monthly seawater temperature and salinity from the World Ocean Atlas 2013 and the sea surface wind from the National Centers for Environmental Prediction reanalysis data, the spatial distribution of the North Pacific Tropical Water (NPTW) and Intermediate Water (NPIW) east of Taiwan are studied using a similarity coefficient method. We also discuss the mechanism underlying the seasonal variation in the NPTW and NPIW. The climatological results show that the NPTW east of Taiwan is mainly located at 25–300 m depth, with a volume of 6.76 × 1013 m3. It is larger in winter and smaller in autumn, which is mainly modulated by monsoon and the Kuroshio. The NPIW is generally located at 400–1100 m depth, with a volume of 1.39 × 1014 m3, which is more than twice that of the NPTW. It is larger in summer and smaller in winter, which is mainly affected by mesoscale eddies and the Kuroshio.

The Lombok Strait, as one of the outlet straits, is part of the ITF route, which is directly adjacent to the Indian Ocean. There is a sill in the Lombok Strait, which is a place for internal wave generation. Leg-1 data from the Japan Agency for Marine-Earth Science and Technology in collaboration with the Agency for the Assessment and Application of Technology which is part of the Tropical Ocean Climate Study Expedition including CTD Yoyo and ADCP taken using ship vehicles R/V Kaiyo. CTD Snapshot from PUSHIDROSAL using the KRI Spica 934 vehicle part of the Opssurta Baruna Jaya 2 Expedition . Determination of seawater mass stratification with the criteria for the thermocline layer is ≥ 0.05 °C.m -1 . Four types of water masses were identified, Java Sea, mixed seawater mass (Java Sea - ITF) which occurred diapycnal mixing, North Pacific Subtropical Water (NPSW) and North Pacific Intermediate Water (NPIW). The seawater mass stratification in the Lombok Strait based on temperature, salinity and density which are seen to follow the internal tidal pattern. The average values for energy dissipation and vertical diffusivity for each layer and replication were 5.73 x 10 -7 W.Kg -1 and 3.67 x 10 -2 m 2. s -1 for CTD Yoyo and 2.25 x 10 -6 W.Kg -1 and 7.38 x 10 -2 m 2 .s -1 for CTD Snapshot . The value obtained is greater than the open ocean and straits in other studies. The high shear value confirms this in the thermocline layer. The Richardson gradient value> 0.25 is relatively constant in the thermocline layer.

Vertical mixing in the ocean is an important physical process that brings deep nutrients to the upper layer. The Tokara Strait, where the Kuroshio flows out to the North Pacific from the East China Sea, is known for a mixing hotspot, but the associated vertical nutrient transport has not been well quantified yet, especially in the intermediate layer below 200 m depth. This study involved flow, turbulence, and nitrate observations to quantify the vertical turbulent nitrate flux in the Tokara Strait and discuss its impact on the Kuroshio nutrient transport. Temporal variation of current velocity, derived from a 3.5-day mooring observation, captured semi-diurnal internal waves propagating vertically at depths between 200 and 400 m. Vertical shear associated with the semi-diurnal flow was elevated at this depth range. Vertical shear due to the mean and the diurnal flow was also elevated at about 220 m and 400 m depths, respectively. Mean vertical eddy diffusivity was elevated to O(10–3–10–2 m2 s−1) at the density layers of 25.2–26.4 σθ (200–450 m depth). Associated vertical turbulent nitrate flux reached O(10 mmol m−2 day−1), especially at around 25.6 σθ (250 m depth) and 26.2–26.4 σθ (400–450 m depth). The enhanced vertical turbulent nitrate flux suggests a large amount of nitrate is injected vertically to the nutrient stream from the layer just above the North Pacific Intermediate Water and is transported throughout the core of the Kuroshio nutrient stream situated at 25.0–26.2 σθ.

The dimensional and temporal distribution of Antarctic Intermediate Water (AAIW) and North Pacific Intermediate Water (NPIW) in the Philippine Sea were explored using Argo profiles and gridded Argo data. As the salinity minimum of intermediate water from mid-high latitude of the southern and northern hemisphere of the Pacific Ocean, the properties of AAIW and NPIW merge at about 10°N with different properties in the Philippine Sea. The core of AAIW is located below 600 dbar with potential density of 27 ≤ σθ ≤ 27.3 kg/m3 and salinity of 34.5 ≤ S ≤ 34.55. The core of NPIW is located between 300-700 dbar with potential density of 26.2 ≤ σθ ≤ 27 kg/m3 and salinity of 34 ≤ S ≤ 34.4. The volume of AAIW and NPIW during January 2004 to December 2017 is negatively correlated. The time series of AAIW and NPIW is dominated by significant periods of 6 and 8 months, respectively. The variations of AAIW and NPIW are mainly affected by volume transport through a 130°E section by the North Equatorial Current (NEC) and North Equatorial Undercurrent (NEUC).

An investigation was carried out in the Y3 seamount area of the Western Pacific Ocean in December 2014, and the distribution of dissolved inorganic carbon (DIC) and its relationship with environmental factors in this area were explored. The results show that DIC concentration was higher in the adjacent waters of the Y3 seamount area, and the uplift of DIC isolines at the stations was close to the seamount. Meanwhile, interaction between the North Equatorial Current (NEC) and the Y3 seamount aff ected the DIC distribution, i.e., the upwelling in the same direction of the NEC was obvious, resulting in a decreasing trend of average concentration of DIC in the 200 m water column from the top to the two sides in this direction but in the cross direction. The DIC concentration increased with the water depth increase, and its distribution was aff ected by various environmental factors. In the surface water, high temperature was a decisive factor for the decrease of the DIC concentration, but the photosynthesis of phytoplankton showing only a weak influence. In the North Pacific Tropic Water (NPTW), DIC production rate from organic matter decomposition was higher than that of DIC consumption by phytoplankton photosynthesis, leading to a continual increase of DIC. In the North Pacific Intermediate Water (NPIW), organic matter decomposition played a leading role in the increase of DIC. In the deep water, decomposition of organic matter weakened, and the dissolution of CaCO3 controlled the carbonate system, and DIC had the smallest variation range.

The deep-ocean carbon cycle, especially carbon sequestration and outgassing, is one of the mechanisms to explain variations in atmospheric CO2 concentrations on millennial and orbital timescales. However, the potential role of subtropical North Pacific subsurface waters in modulating atmospheric CO2 levels on millennial timescales is poorly constrained. An increase in the respired CO2 concentration in the glacial deep-ocean due to biological pump generally corresponds to deoxygenation in the ocean interior. This link thus offers a chance to study oceanic ventilation and coeval export productivity based on redox-controlled sedimentary geochemical parameters. Here, we investigate a suite of geochemical proxies in a sediment core from the Okinawa Trough to understand sedimentary oxygenation variations in the subtropical North Pacific over the last 50 000 years (50 ka). Our results suggest that enhanced mid-depth western subtropical North Pacific (WSTNP) sedimentary oxygenation occurred during cold intervals and after 8.5 ka, while oxygenation decreased during the Bölling-Alleröd (B/A) and Preboreal. The enhanced oxygenation during cold spells is linked to the North Pacific Intermediate Water (NPIW), while interglacial increase after 8.5 ka is linked to an intensification of the Kuroshio Current due to strengthened northeast trade winds over the tropics. The enhanced formation of the NPIW during Heinrich Stadial 1 (HS1) was likely driven by the perturbation of sea ice formation and sea surface salinity oscillations in the high-latitude North Pacific. The diminished sedimentary oxygenation during the B/A due to a decreased NPIW formation and enhanced export production, indicates an expansion of the oxygen minimum zone in the North Pacific and enhanced CO2 sequestration at mid-depth waters, along with the termination of atmospheric CO2 concentration increase. We attribute the millennial-scale changes to an intensified NPIW and enhanced abyss flushing during deglacial cold and warm intervals, respectively, closely related to variations in North Atlantic Deep Water formation.

The Okhotsk Sea, which connects the high latitude Asian continent and the North Pacific, plays an important role in modern and past climate changes due to seasonal sea ice coverage and as a precursor of the North Pacific Intermediate Water. The long-term glacial-interglacial changes of sea ice coverage and its impacts on terrigenous transport and surface primary productivity in the Okhotsk Sea remain, however, not well constrained. Base on the paleomagnetic, rock magnetic, micropaleontological (diatom), and geochemical studies of the marine sediment core MD01-2414 (53°11.77′N, 149°34.80′E, water depth: 1,123 m) taken in the central Okhotsk Sea, we reconstruct the terrigenous sediment transport and paleoceanographic variations during the past 1550 thousand years (kyr). Seventeen geomagnetic excursions are identified from the paleomagnetic directional record. Close to the bottom of the core, an excursion was observed, which is proposed to be the Gilsa event ∼1550 thousand years ago (ka). During glacial intervals, our records reveal a wide extension of sea ice coverage and low marine productivity. We observed ice-rafted debris from mountain icebergs composed of coarse and high magnetic terrigenous detritus which were derived from the Kamchatka Peninsula to the central Okhotsk basin. Still during glacial intervals, the initiation (i.e., at ∼900 ka) of the Mid-Pleistocene Transition marks the changes to even lower marine productivity, suggesting that sea-ice coverage became larger during the last 900 ka. During interglacial intervals, the central Okhotsk Sea was either devoid of sea-ice or the ice was at best seasonal; resulting in high marine productivity. The weaker formation of Okhotsk Sea Intermediate Water, lower ventilation, and microbial degradation of organic matter depleted the oxygen concentration in the bottom water and created a reduced environment condition in the sea basin. The freshwater supplied by snow or glacier melting from Siberia and Kamchatka delivered fine grain sediments to the Okhotsk Sea. During the stronger interglacial intervals after the Mid-Brunhes Transition (i.e., Marine Isotope Stages 1, 5e, 9, and 11), strong freshwater discharges from Amur River drainage area are in association with intensified East Asian Summer Monsoon. This process may have enhanced the input of fine-grained terrigenous sediments to the central Okhotsk Sea.

Due to processes that are poorly understood, the arctic/subarctic region is currently warming at approximately twice the rate of the rest of the globe, and these regional changes may in turn have important consequences on global climate. The Bering Sea is an ideal location to study the relationship between sea ice distribution, ocean circulation, and biological productivity, which are all thought to be components of important interactions that both respond to and influence climate changes on regional (North Pacific) to global scales. Geologic records are essential for understanding and predicting future oceanic and climatic changes, but most previous studies of Bering Sea and North Pacific climate and oceanic conditions were limited by short records that only spanned one glacial-interglacial climate cycle (~100 kyr). This dissertation presents new geochemical and sedimentological records of oceanic and climatic conditions over the past 1.2 Myr using sediment cores from Integrated Ocean Drilling Program Site U1342 (54.83N, 176.92E, 818 m water depth) in the Bering Sea, providing an unprecedented opportunity to examine trends over multiple glacial-interglacial cycles. Long benthic foraminiferal stable isotope records from Site U1342 provide the first evidence of an important recurring relationship in which extremely cold glacial conditions lead to enhanced Bering Sea ice extent and enhanced local formation of North Pacific Intermediate Water (NPIW), the water mass primarily associated with large-scale North Pacific circulation and heat transport. These results contradict model predictions for weaker NPIW formation during extreme glacial conditions and show that North Pacific and North Atlantic circulation acted in phase on glacial-interglacial timescales. Furthermore, new records of nitrate utilization based on nitrogen isotopes demonstrate that glacial climates were consistently associated with enhanced physical stratification in the Bering Sea, which resulted in increased utilization of nutrients by phytoplankton and a reduction in the carbon dioxide released from the ocean to the atmosphere. Finally, multiple new records that demonstrate high productivity during intervals of laminated sediments, which represent brief anoxic events, indicate that interglacial climates precondition the Bering Sea for rapid oceanic and biological change.

Processes controlling environmental change in the subarctic Pacific Ocean on millennial to orbital timescales are not well understood. Here we use a 230-kyr sedimentary record from the northwest Pacific Ocean to assess the response of late Pleistocene sediment dynamics to orbital forcing. Combining a source-to-sink perspective based on sedimentological records with climate model reanalysis, we reveal that fluctuations in sediment provenance were closely linked to obliquity-forced changes in atmospheric circulation modes. Specifically, the position of the Aleutian Low controlled sediment transport from the Bering Sea and Aleutian Arc sources. Furthermore, a distinct shift in North Pacific ocean circulation during the Last Glacial Maximum may have been related to a strengthened Siberian High. The coincidence of atmospheric mode switches with changes in sea-ice extent and North Pacific Intermediate Water formation in the marginal seas suggests that this coupled ocean-atmosphere system may have acted as a regional amplifier of global climate variability. (Less)

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 48(15), (2021): e2021GL094469, https://doi.org/10.1029/2021GL094469.

On the basis of the conductivity temperature depth (CTD) observation data off the coast of the Philippines (7.5˚–18˚N, 130˚E–the east coast of the Philippines) in the fall of 2005, the water mass distribution, geostrophic flow field, and heat budget are examined. Four water masses are present: the North Pacific Tropical Surface Water, the North Pacific Sub-surface Water, the North Pacific Intermediate Water, and the Antarctic Intermediate Water (AAIW). The previous three corresponded with the North Equatorial Current (NEC), the Kuroshio Current (KC), and the Mindanao Current (MC), respectively. AAIW is the source of the Mindanao Undercurrent. The mass transport of NEC, KC, and MC is 58.7, 15, and 27.95 Sv, respectively (relative to 1500 db). NEC can be balanced by the transport across the whole transect 18˚N (31.81 Sv) and 7.5˚N (26.11 Sv) but not simply by KC and MC. Direct calculation is used to study the heat flux. In sum, 1.45 PW heat is transported outwards the observed region, which is much more than that released from the ocean to the air at the surface (0.05 PW). The net heat lost decreased the water temperature by 0.75℃ each month on average, and the trend agreed well with the SST change. Vertically, the heat transported by the currents is mainly completed in the upper 500 m.

We investigated ocean variability at intermediate depths (300–750 m) in the western equatorial Pacific (WEP) using data from underwater sensors of the Triangle Trans-Ocean Buoy Network (TRITON) buoys. The data were specially processed because high accuracy is necessary for such an application, and we confirmed that sufficient accuracy was achieved. Variability of the temperature and salinity was large in the western boundary regions of the WEP. Seasonal variability is generally dominant at intermediate depths in the WEP; however, intraseasonal variability with period of 30–80 days is not negligible, particularly at 2°N, 138°E, where the New Guinea Coastal Undercurrent retroflects to the east as the Equatorial Undercurrent and/or Northern Subsurface Countercurrent. High correlation is seen between 12-month running means of the Nino 3.4 index anomaly and temperature at 300 and 750 dbar. In other words, the influence of El Nino/Southern Oscillation phenomena reaches to at least a depth of 750 dbar. We found a decreasing trend in salinity at intermediate depths in the WEP: − 0.0032 Practical Salinity Unit (PSU) year−1 at 300 dbar and − 0.00053 PSU year−1 at 750 dbar. This trend is likely associated with the freshening of the North Pacific Intermediate Water and Antarctic Intermediate Water.

Aluminum (Al), manganese (Mn), cobalt (Co), and lead (Pb) are key trace elements in seawater and thus significant in chemical oceanography research. However, although all of these elements are highly scavenged in the ocean, only a few studies focus on the intercomparison of their distributions. Here, we report the basin-scale and full-depth sectional distributions of these elements observed during three GEOTRACES Japan cruises in the North Pacific. We confirmed that a surface maximum of the dissolved (d) species is not a common feature for the four elements and that the d species have the lowest concentrations in the Pacific Deep Water (PDW) as compared to other oceans. The elements showed different speciations and distributions. The fraction of labile particulate (lp) species was calculated as the difference between the total dissolvable (td) species and d species. The lpM/tdM ratio, where M refers to an element, is highest for Al, at 0.66 ± 0.31 (average ± sd, n = 489), and lowest for Pb, at 0.02 ± 0.08 (n = 575). Further, the distribution of each element is uniquely related to ocean circulation. The tdAl concentration is high in the Equatorial Under Current (EUC), the North Equatorial Current (NEC), and the Lower Circumpolar Deep Water (LCDW). Manganese is supplied from reductive sources such as sediments on the continental shelves around the northern boundary. Cobalt is concentrated in the North Pacific Intermediate Water (NPIW) and in the Equatorial Pacific Intermediate Water (EqPIW) owing to the combined effects of supply from the continental shelves, biogeochemical cycling, and scavenging. Lead shows a subsurface maximum centered at ∼35°N and ∼200 m depth, implying an association with the formation of the Subtropical Mode Water (SMW) and the Central Mode Water (CMW). Although the subsurface Pb maximum in the Atlantic has diminished over the last three decades owing to the ban on leaded gasoline use, it has been sustained in the North Pacific through the growth of other anthropogenic sources in Asia and Russia. We propose that the enrichment factor of dM, defined as EF(dM) = (dM/dAl)seawater/(M/Al)upper crust, where (M/Al)upper crust is the molar ratio in upper crustal abundance, can be a good parameter for the sources. The median is 1.3 × 102 (n = 436) for EF(dMn), 3.2 × 102 (n = 430) for EF(dCo), and 1.2 × 103 (n = 413) for EF(dPb). The EF(dPb) found in this study is on the same order of magnitude as the EF values for aerosols found in the literature, suggesting that the deposition of aerosols is a major source for dPb. Because EF(dMn) and EF(dCo) are ten to hundred times higher than the EF for aerosols, sources other than the aerosol deposition are more significant contributors to the concentrations of Mn and Co.

The deglacial history of CO2 release from the deep North Pacific remains unresolved. This is due to conflicting indications about subarctic Pacific ventilation changes based on various marine proxies, especially for Heinrich Stadial 1 (HS-1) when a rapid atmospheric CO2 rise occurs. Here, we use a complex Earth System Model to investigate the deglacial North Pacific overturning and its control on ocean stratification. Our results show an enhanced intermediate-to-deep ocean stratification coeval with intensified North Pacific Intermediate Water (NPIW) formation during HS-1, compared to the Last Glacial Maximum. The stronger NPIW formation causes lower salinities and higher temperatures at intermediate depths. By lowering NPIW densities, this enlarges vertical density gradient and thus enhances intermediate-to-deep ocean stratification during HS-1. Physically, this process prevents the North Pacific deep waters from a better communication with the upper oceans, thus prolongs the existing isolation of glacial Pacific abyssal carbons during HS-1., The role of the Pacific Ocean during the last deglacial is less well known. Here the authors used a complex Earth System Model and found that enhanced intermediate-to-deep ocean stratification, and an isolated carbon pool in the deep North Pacific during the last deglaciation when considered alongside proxy records.

On glacial-interglacial and millennial timescales, sea ice is an important player in the circulation and primary productivity of high latitude oceans, affecting regional and global biogeochemical cycling. In the modern North Pacific, brine rejection during sea-ice freezing in the Sea of Okhotsk drives the formation of North Pacific Intermediate Water (NPIW) that ventilates the North Pacific Ocean at 300 m–1000 m water depth. Glacial intervals of the late Quaternary, however, experienced a deepening of glacial NPIW to at least 2000 m, with the strongest ventilation observed during cold stadial conditions of the last deglaciation. However, the origin of the shifts in NPIW ventilation is poorly understood. Numerical simulations suggest an atmospheric teleconnection between the North Atlantic and the North Pacific, in response to a slowdown or shutdown of the Atlantic meridional overturning circulation. This leads to a build-up of salinity in the North Pacific surface ocean, triggering deep ventilation. Alternatively, increased sea-ice formation in the North Pacific and its marginal seas may have caused strengthened overturning in response to enhanced brine rejection. Here we use a multi-proxy approach to explore sea-ice dynamics, sedimentary redox chemistry, and benthic ecology at Integrated Ocean Drilling Program Site U1343 in the eastern Bering Sea across the last 40 ka. Our results suggest that brine rejection from enhanced sea-ice formation during early Heinrich Stadial 1 locally weakened the halocline, aiding in the initiation of deep overturning. Additionally, deglacial sea-ice retreat likely contributed to increased primary productivity and expansion of mid-depth hypoxia at Site U1343 during interstadial conditions, confirming a vital role of sea ice in the deglacial North Pacific carbon cycle.

Satellite data products and high-resolution in situ observations were combined to investigate the evolution and structure of the Kuroshio Extension Front in Spring 2019. The former reveals the variation of the front is influenced by the northward movement of the Kuroshio Extension through transporting warm and saline water to a cold and brackish water region. The latter indicates steep upward slopes of the isopycnals, tilting northward in the frontal zone, as well as several ~300 m thick blobs of North Pacific Intermediate Water between 26.25 and 26.75 kg/m3, where conspicuous thermohaline intrusions occur. Further analysis indicates these thermohaline intrusions prefer to alternate salt fingering and diffusive convection interfaces, and are affected by strong shears.

North Pacific Intermediate water (NPIW) is a dominant water mass controlling ~400-1200m depth North Pacific Ocean, characterized by its low salinities and relatively lower temperatures. In the modern climate, the interplay between NPIW-related physical and biogeochemical processes among seasons determines annual-mean budget and efficiency of carbon sink into the North Pacific Ocean. Thus, to understand the NPIW physics is key to project roles of the North Pacific Ocean in changing Earth climate and carbon systems in the future. In this study, we provide a modelling view of the NPIW history since Yr 1850 (historical experiment) and its projection to near future (IPCC-defined RCP 4.2 and 8.5 experiments until Yr 2100), using new-generation Alfred Wegener Institute Earth System Model (AWI-ESM). Our results suggest an important role of regional hydroclimate feedback over the NW Pacific and Sea of Okhotsk in determining the NPIW from recent past to near future.

The deep ocean carbon cycle, especially carbon sequestration and outgassing, is one of the mechanisms to explain variations in atmospheric CO2 concentrations on millennial and orbital timescales. However, the potential role of subtropical North Pacific subsurface waters in modulating atmospheric CO2 levels on millennial timescales is poorly constrained. Here, we investigate a suite of geochemical proxies in a sediment core from the northern and middle Okinawa Trough to understand variations in intermediate-water ventilation of the subtropical North Pacific over the last 50,000 years (50 ka). Our results suggest that enhanced mid-depth western subtropical North Pacific (WSTNP) sedimentary oxygenation occurred during cold intervals during the last deglaciation and last glaciation, while oxygenation decreased during the Bölling-Alleröd (B/A) and warm interstadials. The enhanced oxygenation during cold spells is linked to the intensified North Pacific Intermediate Water (NPIW), while interglacial increase after 8.5 ka is linked to an intensification of the Kuroshio Current due to strengthened northeast trade winds over the tropics. The enhanced formation of NPIW during Heinrich Stadials was likely driven by the perturbation of sea ice formation and sea surface salinity oscillations in high-latitude North Pacific. The diminished sedimentary oxygenation during the B/A and interstadials due to decreased NPIW formation and enhanced export production, indicates an expansion of oxygen minimum zone in the North Pacific and enhanced CO2 sequestration at mid-depth waters. We attribute the millennial-scale changes to intensified NPIW and enhanced abyss flushing during deglacial cold and warm intervals, respectively, closely related to variations in North Atlantic Deep Water formation. Out study extends the millennial-scale links between ventilation in the subtropical North Pacific Ocean and the Atlantic Climate into the last glaciations, highlighting the key roles of Atlantic Meridional Overturning Circulation in regulating the North Pacific environment at millennial timescales. Note: Financial support was provided by the National Program on Global Change and Air-Sea Interaction (GASI-GEOGE-04) and by the National Natural Science Foundation of China (Grant Nos.: 41876065, 41476056, and U1606401).

The Kuroshio is the most significant current in the western North Pacific Ocean and affects a wide area. This work shows that the intrusion of the oligotrophic upper-layer West Philippine Sea seawater into the South China Sea (SCS) as the branch of Kuroshio reduced the productivity and hence the fluxes of sinking particles in the SCS between 2013 and 2017. Conversely, the productivity in the SCS increased during a large scale Kuroshio intrusion in 1998–2006, indicating that other factors also affected the productivity. Further, the western North Pacific Ocean is acidifying, with the surface seawaters to the west having lower acidification rates. This phenomenon is likely a consequence of enhanced productivity owing to more anthropogenic nutrient inputs from the continent in the west, but needs further investigation. In the East China Sea, the Kuroshio Intermediate Water has increased nutrient concentrations, but decreased in both dissolved oxygen (DO) concentration and pH, most likely owing to reduced ventilation in the North Pacific Intermediate Water. Further warming of the surface oceans would strengthen the stratification of the surface ocean, weakening ventilation. Consequently, DO and pH would continue to decline while nutrients level increases.

Two field observations were conducted around the Lembeh Strait in September 2015 and 2016, respectively. Evidences indicate that seawater around the Lembeh Strait is consisted of North Pacific Tropical Water (NPTW), North Pacific Intermediate Water (NPIW), North Pacific Tropical Intermediate Water (NPTIW) and Antarctic Intermediate Water (AAIW). Around the Lembeh Strait, there exist some north-south differences in terms of water mass properties. NPTIW is only found in the southern Lembeh Strait. Water mass with the salinity of 34.6 is only detected at 200–240 m between NPTW and NPTIW in the southern Lembeh Strait, and results from the process of mixing between the saltier water transported from the South Pacific Ocean and the lighter water from the North Pacific Ocean and Sulawesi Sea. According to the analysis on mixing layer depth, it is indicated that there exists an onshore surface current in the northern Lembeh Strait and the surface current in the Lembeh Strait is southward. These dramatic differences of water masses demonstrate that the less water exchange has been occurred between the north and south of Lembeh Strait. In 2015, the positive wind stress curl covering the northern Lembeh Strait induces the shoaling of thermocline and deepening of NPIW, which show that the north-south difference of airsea system is possible of inducing north-south differences of seawater properties.

The first direct measurements of vertical turbulent nitrate flux, FluxNO3, were conducted in the Kuroshio across the Izu Ridge and revealed large FluxNO3 near the sill crest at the 1% light depth. This large FluxNO3 possibly enhances downstream phytoplankton growth. Extremely large FluxNO3 was also observed at 26.0–26.6 σθ between the core of the Kuroshio nutrient stream and the North Pacific Intermediate Water. We hypothesize that strong vertical mixing in the Izu Ridge draws sufficient nitrate upward from the North Pacific Intermediate Water to the nutrient stream, which eventually impacts the downstream biogeochemistry in the western North Pacific.

Recent studies suggest that transport and water mass mixing may play a dominant role in controlling the distribution of dissolved rare earth element concentrations ([REE]) at least in parts of the North and South Atlantic and the Pacific Southern Ocean. Here we report vertically and spatially high-resolution profiles of dissolved REE concentrations ([REE]) along a NW–SE transect in the West Pacific and examine the processes affecting the [REE] distributions in this area. Surface water REE patterns reveal sources of trace element (TE) input near South Korea and in the tropical equatorial West Pacific. Positive europium anomalies and middle REE enrichments in surface and subsurface waters are indicative of TE input from volcanic islands and fingerprint in detail small-scale equatorial zonal eastward transport of TEs to the iron-limited tropical East Pacific. The low [REE] of North and South Pacific Tropical Waters and Antarctic Intermediate Water are a long-range (i.e., preformed) laterally advected signal, whereas increasing [REE] with depth within North Pacific Intermediate Water result from release from particles. Optimum multiparameter analysis of deep to bottom waters indicates a dominant control of lateral transport and mixing on [REE] at the depth of Lower Circumpolar Deep Water (≥3000 m water depth; ∼75–100% explained by water mass mixing), allowing the northward tracing of LCDW to ∼28°N in the Northwest Pacific. In contrast, scavenging in the hydrothermal plumes of the Lau Basin and Tonga-Fiji area at 1500–2000 m water depth leads to [REE] deficits (∼40–60% removal) and marked REE fractionation in the tropical West Pacific. Overall, our data provide evidence for active trace element input both near South Korea and Papua New Guinea, and for a strong lateral transport component in the distribution of dissolved REEs in large parts of the West Pacific.

Radiocarbon (14C) in dissolved inorganic carbon (DIC) was measured for water samples collected from six deep stations in the Kuroshio Extension (KE) region in the northwestern North Pacific in April–May 2015. Vertical profiles of Δ14C-DIC indicate that bomb-produced 14C was present from the surface to ~1500 m water depth. Large variations in Δ14C-DIC values (300‰) were observed at 500 m water depth among the stations and the differences were likely controlled by transport and mixing dynamics of different water masses in the region. The major Pacific western boundary currents, such as Kuroshio and Oyashio and regional mesoscale eddies, could play important roles affecting the observed Δ14C-DIC variability. The depth profiles of both Δ14C-DIC and DIC concentrations can be predicted by the solution mixing model and can be used as conservative tracers of water mass movement and water parcel homogenization in the ocean.