Northwestern Pacific Ocean Research Articles

Internal Lee Wave Generation from Geostrophic Flow in the Northwestern Pacific Ocean

Abstract Among the global mapping of lee wave generation, a missing piece exists in the northwestern Pacific Ocean (NPO), which features complex topographies and energetic circulations. This study applies Bell’s theory to estimate and map internal lee waves generated by geostrophic flows in the NPO using Mercator Ocean reanalysis data and the full topographic spectra obtained from the latest synthetic bathymetry product. Unlike the dominant contributions from abyssal hills in the Southern Ocean, multiple topographies, including ridges, rises, and continental margins, result in an inhomogeneous lee wave generation with multiple hotspots in the NPO. The generation rate is generally higher in the Philippine basin and lower in the central Pacific seamounts. Over ridges, the rough topography creates a high potential for triggering lee waves. Over rises and continental margins, the stronger currents at the shallow depths are favorable for lee wave generation. In the Kuroshio extension region, the rough topography and strong currents cause the strongest lee wave generation, with an energy flux reaching 100 mW m−2. By mean–eddy decomposition, it is found that the lee wave hotspots contributed by mean flow are concentrated in specific regions, while those by geostrophic eddies are widely distributed. Geostrophic eddies are the primary contributor to lee wave generation, which account for 74.6% of the total energy transferred from geostrophic flow to lee waves. This study also reveals that tides suppress the lee wave generation by 14%, and geostrophic flow can cause an asymmetric generation of internal tides.

Unraveling prokaryotic diversity distribution and functional pattern on nitrogen and methane cycling in the subtropical Western North Pacific Ocean

Prokaryotes play an important role in marine nitrogen and methane cycles. However, their community changes and metabolic modifications to the concurrent impact of ocean warming (OW), acidification (OA), deoxygenation (OD), and anthropogenic‑nitrogen-deposition (AND) from the surface to the deep ocean remains unknown. We examined here the amplicon sequencing approach across the surface (0–200 m; SL), intermediate (200–1000 m; IL), and deep layers (1000–2200 m; DL), and characterized the simultaneous impacts of OW, OA, OD, and AND on the Western North Pacific Ocean prokaryotic changes and their functional pattern in nitrogen and methane cycles. Results showed that SL possesses higher ammonium oxidation community/metabolic composition assumably the reason for excess nitrogen input from AND and modification of their kinetic properties to OW adaptation. Expanding OD at IL showed hypoxic conditions in the oxygen minimum layer, inducing higher microbial respiration that elevates the dimerization of nitrification genes for higher nitrous oxide production. The aerobic methane-oxidation composition was dominant in SL presumably the reason for adjustment in prokaryotic optimal temperature to OW, while anaerobic oxidation composition was dominant at IL due to the evolutionary changes coupling with higher nitrification. Our findings refocus on climate-change impacts on the open ocean ecosystem from the surface to the deep-environment integrating climate-drivers as key factors for higher nitrous-oxide and methane emissions.

Redescription of the hispidoberycid, Hispidoberyxambagiosus Kotlyar, 1981 from Taiwan, with comments on its morphology (Beryciformes, Stephanoberycoidei, Hispidoberycidae).

A rare spiny-scale pricklefish, Hispidoberyxambagiosus Kotlyar, 1981, is redescribed based on four specimens collected from Taiwan. Their sampling locality represents the northernmost record of the family, which extends the family's distribution from the eastern Indian Ocean and the South China Sea to northeastern Taiwan in the northwestern Pacific Ocean. A detailed description of these specimens and the first description of its sagittal otoliths are provided. In addition, the specimens are compared with other known specimens. Intraspecific variation of some morphological characters are discussed.

Observed interannual variability of the Kuroshio and Luzon Undercurrent associated with tropical and subtropical wind forcing

The interannual variability of the Kuroshio and Luzon Undercurrent (LUC) east of the Luzon Island is examined using observations from a moorings during December 2010–October 2012 and three moorings during January 2018–May 2020 deployed at 18°N. Measurements from acoustic Doppler current profiler (ADCP) equipped on the moorings indicate that the interannual variations of upper-layer Kuroshio and lower-layer LUC were in phase. The interannual variability is strong in the Kuroshio but weakens substantially in the LUC. A proxy of the Kuroshio and LUC based on sea level slope is developed and validated by comparing the ADCP and satellite altimeter measurements. The proxy during 1993–2020 shows that the interannual sea level slope across the Kuroshio is primarily associated with sea level fluctuations on its eastern flank. Diagnostic analysis and numerical sensitivity experiments using a simplified ocean model indicate that the wind forcing in the subtropical northwestern Pacific Ocean played an essential role in the interannual variability of Kuroshio and LUC’s strength and water mass properties during the observation periods in 2011 and 2018–2020, although the tropical winds dominated the interannual variability of the two boundary currents during most time of the past three decades (1993–2020). The mooring observations show that the subsurface water mass in the LUC at the depth of about 650–800 m experienced a considerable freshening event from July 2019 until at least January 2020 due to abnormally strong subtropical wind forcing.

Mooring Measurements of Full-Depth Zonal Currents along the 143°E Meridian in the Northwestern Pacific Ocean

Abstract Full-depth ocean zonal currents in the tropical and extratropical northwestern Pacific (TNWP) are studied using current measurements from 17 deep-ocean moorings deployed along the 143°E meridian from the equator to 22°N during January 2016–February 2017. Mean transports of the North Equatorial Current and North Equatorial Countercurrent are estimated to be 42.7 ± 7.1 Sv (1 Sv ≡ 106 m3 s−1) and 10.5 ± 5.3 Sv, respectively, both of which exhibit prominent annual cycles with opposite phases in this year. The observations suggest much larger vertical extents of several of the major subsurface currents than previously reported, including the Lower Equatorial Intermediate Current, Northern Intermediate Countercurrent, North Equatorial Subsurface Current, and North Equatorial Undercurrent (NEUC) from south to north. The Northern Subsurface Countercurrent and NEUC are found to be less steady than the other currents. Seasonal variations of these currents are also revealed in the study. In the deep ocean, the currents below 2000 m are reported for the first time. The observations confirm the striation patterns of meridionally alternating zonal currents in the intermediate and deep layers. Further analyses suggest a superposition of at least the first four and two baroclinic modes to represent the mean equatorial and off-equatorial currents, respectively. Meanwhile, seasonal variations of the currents are generally dominated by the first baroclinic mode associated with the low-mode Rossby waves. Overall, the above observational results not only enhance the knowledge of full-depth current system in the TNWP but also provide a basis for future model validation and skill improvement.

Secondary invasion by an invasive shrub limits native tree seedling growth in a size‐dependent manner following the eradication of a different invasive species in an island ecosystem

Following the control of an invasive tree species, native forest ecosystem restoration can be prevented by a secondary invasion by another invasive species that inhibits the reestablishment of native trees. In this study, we evaluated the effects of the invasive shrub Leucaena leucocephala on the initial growth of planted seedlings of four native tree species (Pandanus boninensis, Rhaphiolepis indica, Schima mertensiana, and Terminalia catappa) after eradication of the invasive tree Casuarina equisetifolia in the Ogasawara Islands in the northwestern Pacific Ocean. In a 24‐month field experiment, we compared the sizes of planted seedlings (diameter at ground level, height, and leaf number) among four treatments in a 2 × 2 design of initial seedling size (large vs. small) × L. leucocephala invasion (sowing L. leucocephala seeds vs. control). More than 75% of the seedlings (excluding S. mertensiana) survived regardless of the treatments. For all four species, initially large seedlings tended to become larger plants at the end of the experiment than initially small seedlings. We detected significant effects of L. leucocephala invasion on one or more plant size parameters in all four species. Seedlings grown in L. leucocephala invasion quadrats were smaller than those in control quadrats at the end of the experiment. We detected significant interactions between initial seedling sizes and L. leucocephala invasion for three species. Together, these results indicate that secondary invasion by L. leucocephala after controlling C. equisetifolia inhibits the initial growth of native tree seedlings, depending on their size at the time of invasion.

Effects of a Subsurface Eddy on Acoustic Propagation in the Northwestern Pacific Ocean

Using a ray tracing model, acoustic propagation through a subsurface eddy is investigated. The eddy with a 460 km diameter lies at approximately 23° N, 126° E. The presence of the subsurface eddy is found to result in significant alterations to the received acoustic field. Affected by the subsurface eddy, the location of a given convergence zone can be different from usual with optimum propagation conditions. For a source deployed outside of the eddy, the subsurface eddy creates a surface duct by modifying the sonic layer depth. A maximum of 15 km difference in the range of a given convergence zone is found between the propagations with the presence and absence of the eddy when the source is placed around 100 m depth. However, for a source within the eddy, the intense shift in convergence zone patterns occurs only when the source is deployed around 300 m depth. These ocean-acoustic results can provide essential information for operating technologies such as communication devices, underwater sonar, and navigation systems.

Prediction of Convective Available Potential Energy and Equivalent Potential Temperature using a Coupled WRF and Deep Learning for Typhoon Identification

To predict typhoons in the western North Pacific Ocean, it is required to predict the determinants of typhoon activities. The formation of the typhoon can be controlled by Convective Available Potential Energy (CAPE) and Equivalent Potential Temperature (theta-e). To predict the variables, a mesoscale numerical model of Weather Research and Forecasting (WRF) can be used. However, the output of WRF needs to improve to obtain a more accurate CAPE and theta-e prediction. This study uses a coupled WRF model and Deep Learning (DL) Multilayer Perceptron Regressor approach to increase CAPE and theta-e prediction skills. Simulation with dataset scenarios with WRF outputs as predictors and sounding data as predictors are developed and tested to obtain the most appropriate package of deep learning simulation. The study found that coupled models provide increased mean accuracy of theta-e and CAPE, namely 16.6% and 32.0% higher than using original WRF, respectively. This study also shows the difference of skill scores in the spatial distribution of CAPE and theta-e of WRF result and its coupled model.

Downward Propagation and Trapping of Near-Inertial Waves by a Westward-Moving Anticyclonic Eddy in the Subtropical Northwestern Pacific Ocean

Abstract Mesoscale eddies can alter the propagation of wind-generated near-inertial waves (NIWs). Different from previous studies, the subsurface mooring observed NIWs are generated outside an anticyclonic eddy (ACE) and then interact with the arriving ACE. It is found that with the arrival of the ACE, the NIWs accelerate to propagate downward and the maximum vertical wavelength and group velocity of NIWs reach ∼500 m and ∼35 m day−1, respectively. When entering the core of the ACE, the near-inertial energy is trapped and finally stalls at a critical depth, which basically corresponds to the base of the ACE located at around 750-m depth. Through a ray-tracing model and dynamic analyses, this critical depth is much deeper than that of NIWs generated directly inside an ACE. By using depth–time varying stratification and relative vorticity, ray-tracing experiments further demonstrate that NIWs generated outside and passed over by an ACE can propagate to deep depths. Furthermore, energy budget analyses indicate that the net energy transfer from the ACE to NIWs plays an important role in the enhancement of downward-propagating near-inertial energy and its long-term persistence (∼45 days) in the critical layer. Within the critical layer, the enhancement of shear instability and nonlinear interactions among internal waves account for the loss of the trapped near-inertial energy and provide energy for furnishing deep ocean mixing. Significance Statement The interactions between near-inertial waves and a westward-moving anticyclonic eddy are investigated in this study. Knowledge about the propagation of near-inertial waves continues to be a topic of interest because near-inertial waves transfer energy from the mixed layer to the interior ocean, which is an important source of turbulent mixing. While much is known about how near-inertial energy propagates inside an anticyclonic eddy, few studies have examined how near-inertial energy propagates when it is generated outside an anticyclonic eddy and then enters the arriving anticyclonic eddy. In this study, the deep propagation and trapping of near-inertial energy by a westward-moving anticyclonic eddy is observed, which contributes greatly to the energy budget and the deep-ocean mixing.

Genetic structure and historical demography of the Pacific rudderfish Psenopsis anomala in the Northwestern Pacific Ocean

Genetic structure and historical demography of the Pacific rudderfish Psenopsis anomala in the Northwestern Pacific Ocean

Dating pelagic sediments from the northwestern Pacific Ocean by integration of Multi-geochronologic approaches

As the largest promising rare earth elements and yttrium (termed as REY) resource in the world, pelagic sediments enriched in REY (REY-rich sediments) have gained extensive attention. However, the mechanism responsible for the enrichment of REY in pelagic sediments remains unknown, primarily due to the challenge of obtaining robust geochronology. Despite previous attempts, integration of multi-geochronologic approaches seems better suited to determine robust geochronology for pelagic sediments. Here, we present a continuous chronostratigraphic framework for a pelagic sediment core collected from the northwestern Pacific Ocean, integrating authigenic 10Be/9Be dating, 230Thex dating, and magnetostratigraphy. Our results indicate that the REY-rich sediments deposited prior to ∼ 2.5 Ma, and a highly REY-rich sediment layer deposited at ∼ 11.5–9.5 Ma in the study area. We suggest that a low sedimentation accumulation rate (SAR) was necessary for REY enrichment in pelagic sediment, while the deposition of highly REY-rich sediments should be attributed to the additional contribution from active bottom currents. Our study proposes that the enrichment of bioapatite fossils and micronodules related to enhanced bottom currents under a low SAR sedimentary environment could promote the widespread highly REY-rich sediments in the study region.

Eccentricity signal in the nannofossil time-series across the Mid-Pleistocene Transition in the northwestern Pacific Ocean (ODP Site 1209)

The Mid-Pleistocene Transition (MPT; 1.25–0.6 million years ago, Ma) is one of the most important and still debated climate reorganizations during which the glacial/interglacial cycles switched from a 41-thousand years (kyr) cycle (i.e. obliquity) to a quasi-periodic 100-kyr cycle (associated with orbital eccentricity). Variations in the orbital geometry can affect the abundance and distribution of certain marine biota such as the coccolithophores, a group of unicellular calcifying phytoplankton, whose skeletal remains – called nannofossils – represent a valid tool within the geological archives to infer change in surface water conditions and/or coccolithophore productivity and how orbital variations may have impacted them. Here, we apply for the first time various time series analytical techniques to the nannofossil dataset from mid-latitudinal Ocean Drilling Program (ODP) Site 1209 in the northwest Pacific Ocean for the interval spanning the last 1.6 Myr. To better interpret the orbital signal recorded by different nannofossil species we used time series analyses (i.e. wavelet, autocorrelation and cross correlation) to identify the main periodicities by single nannofossil species during the MPT, and to investigate further their response timings to those orbital drivers. In addition, we investigated how the recorded periodicities can improve understanding of the paleoecological preferences of particular species. The combination of multiple time series analyses allowed identification of the 100-kyr periodicity as the main cyclicity recorded in most analyzed species at Site 1209, documenting the predominance of the eccentricity-related signal at mid-latitudes and a reduced or absent influence of the obliquity response. Thus, our data highlight how orbital influence varies by latitude impacting the nannofossil species. The lag between eccentricity and species abundance fluctuations was also investigated, identifying a fast response ranging between 20 and 40 kyr for the taxa Calcidiscus leptoporus subspecies leptoporus, Gephyrocapsa caribbeanica small, and Reticulofenestra spp. (>5 μm). This study corroborates the potential of nannofossils to deepen understanding of the dynamics and effects of variations in orbital geometry through time. It also underlines the need to extend the study of the responses of specific species through the use of different time series analysis techniques in order to return complementary information and detect clearer orbital signals.

Acoustic Target Strength According to Different Growth Stages of Japanese Anchovy (Engraulis japonicus): A Comparison of Juvenile and Adult Fish

Japanese anchovy (Engraulis japonicus) is an economically important species found in the coastal waters of the northwestern Pacific Ocean. To conduct effective acoustic surveys on the spawning or fishing grounds of this species, the target strength (TS, dB re 1 m2) during its growth stages must be understood. However, knowledge regarding the TS of juvenile Japanese anchovies is limited. To address this knowledge gap, we measured the TS and proposed equation models for juvenile Japanese anchovies while describing the TS differences between young and adult anchovies. We evaluated 21 live anchovies using 38, 120, and 200 kHz transducers. TS models were fitted for the juvenile stage of anchovies, considering different frequencies. The mean TS values of young and adult anchovies were similar to those reported in previous TS studies, whereas the TS values of juvenile anchovies were significantly lower. Thus, applying the same TS equation based on the acoustic characteristics of anchovies at different growth stages, especially for juvenile anchovies, may not be appropriate. These findings underscore the importance of adopting a suitable TS equation specific to the growth stages of Japanese anchovies for accurate data analysis in acoustic surveys conducted on spawning and fishing grounds.

The Diversity and Metabolism of Culturable Nitrate-Reducing Bacteria from the Photic Zone of the Western North Pacific Ocean

To better understand bacterial communities and metabolism under nitrogen deficiency, 154 seawater samples were obtained from 5 to 200 m at 22 stations in the photic zone of the Western North Pacific Ocean. Total 634 nitrate-utilizing bacteria were isolated using selective media and culture-dependent methods, and 295 of them were positive for nitrate reduction. These nitrate-reducing bacteria belonged to 19 genera and 29 species and among them, Qipengyuania flava, Roseibium aggregatum, Erythrobacter aureus, Vibrio campbellii, and Stappia indica were identified from all tested seawater layers of the photic zone and at almost all stations. Twenty-nine nitrate-reducing strains representing different species were selected for further the study of nitrogen, sulfur, and carbon metabolism. All 29 nitrate-reducing isolates contained genes encoding dissimilatory nitrate reduction or assimilatory nitrate reduction. Six nitrate-reducing isolates can oxidize thiosulfate based on genomic analysis and activity testing, indicating that nitrate-reducing thiosulfate-oxidizing bacteria exist in the photic zone. Five nitrate-reducing isolates obtained near the chlorophyll a-maximum layer contained a dimethylsulfoniopropionate synthesis gene and three of them contained both dimethylsulfoniopropionate synthesis and cleavage genes. This suggests that nitrate-reducing isolates may participate in dimethylsulfoniopropionate synthesis and catabolism in photic seawater. The presence of multiple genes for chitin degradation and extracellular peptidases may indicate that almost all nitrate-reducing isolates (28/29) can use chitin and proteinaceous compounds as important sources of carbon and nitrogen. Collectively, these results reveal culturable nitrate-reducing bacterial diversity and have implications for understanding the role of such strains in the ecology and biogeochemical cycles of nitrogen, sulfur, and carbon in the oligotrophic marine photic zone.

A census and properties of mesoscale Kuril eddies in the altimetry era

The ocean-side area of the Kuril Islands is a region with high bioproductivity and rich fishing and a convergence of transformed subtropical and subarctic waters with significant impact on weather and climate. Waters of different origin mix in the region including Kuroshio Extention, Japan Sea, Oyashio, Okhotsk Sea and Alaskan Stream waters. Despite the important role of Kuril eddies (KE) in transporting heat, salt, nutrients and their biological activity, there are still major gaps in our knowledge of the properties and dynamics of these energetic and mesoscale features in the northwestern Pacific Ocean. Using altimetry data during 1993–2021, we conducted the first systematic census of KE with the help of the automatic eddy tracking algorithm AMEDA. We separated KE into four groups according to their places of formation in relation to the Kuril-Kamchatka Trench and found a strong inhomogeneous spatial distribution of the Kuril cyclones (KC) and anticyclones (KAC). Different kinematic properties of KE have been studied and analyzed. The Trench has been found to play a role of an eddy guide for long-lived eddies with a comparatively small number of KE being able to cross the Trench. The highest occurrence frequency of KAC was observed along the Trench, especially in the area east off the Bussol Strait, whereas KC occurred more frequently along the ocean side of the Trench. Among the observed long-lived KE, there were more than 90% of eddies with the nonlinearity parameter exceeding 1 implying that the majority of KE are coherent vortices transporting water with its properties. The relative concentrations of waters of different origin inside KAC and KC cores were computed for each group applying the new Lagrangian algorithm. It allowed us to detect transportation of subtropical water in the KE cores to north and subarctic water to south over the distance of the order of 1000 km and more.

Large-Scale Distribution and Variations of Active Salt-Finger Double-Diffusion in the Western North Pacific

Abstract Large-scale distribution and variations in active salt fingers (SF) in the western North Pacific were examined by detecting the active SF with a vertical density ratio Rρ = 1–2 at depths of 10–300 m using a monthly gridded hydrographic dataset from 2001 to 2016. The active SF is distributed most frequently in March along 40°N around the Subarctic Boundary (SAB), where the mixed layer deepens northward and corresponds to the Central Mode Water formation site with a density from +0.02σθ to +0.2σθ of surface density and mainly in 26.1–26.4σθ. This active SF along 40°N underwent seasonal variation and decayed rapidly from March to August from the shallower and less dense parts of the active SF with increasing mean density. The features of the active SF in March are consistent with the hypothesis that surface water with a horizontal density ratio RL = 1–2 is subducted and vertically superposed, resulting in an active SF. The mean density of the active SF in March is well correlated with the surface density with RL = 1–2, and both mean densities showed a decreasing trend from 2001 to 2016, following the surface warming trend (∼0.057°C yr−1) in the surface water with RL = 1–2. Large year-to-year variations in the active SF in March are explained by both horizontal and vertical extensions, and can be reproduced by four conditions: 1) from 1°N to 3°S of SAB, 2) RL = 1–2, and 3) northward deepening of the mixed layer depth, and 4) the part with a density from +0.02σθ to +0.2σθ of surface density. Significance Statement It has been recognized that salt-finger (SF) double-diffusive convection is not active in the western North Pacific Ocean. This study demonstrated the distribution and seasonal/interannual variations of active SF in the western North Pacific for the first time: the formation of active SF along 40°N in 140°E–180° around the Subarctic Boundary in March and rapid decay until August, and large year-to-year variations of vertical and horizontal extensions with density decreasing trend. This study also proposed a formation mechanism that is relevant to the active SF density decrease and warming trend in the western North Pacific.

Length−Length and Length−Weight Relationships of Pacific Saury, Cololabis saira (Scomberesocidae), in the Northwestern Pacific Ocean

Length−Length and Length−Weight Relationships of Pacific Saury, Cololabis saira (Scomberesocidae), in the Northwestern Pacific Ocean

Impact of the Atlantic multidecadal variability on East Asian summer climate in idealized simulations

The impact of the Atlantic multidecadal variability (AMV) on the East Asian summer climate is analyzed using a set of idealized simulations performed by CESM1. The results show that the AMV exerts a remote impact on the East Asian summer climate via two pathways: a midlatitude Rossby wave train and equatorial Kelvin waves. Specifically, the positive phase of the AMV can enhance the land–sea pressure contrast between the Asian continent and western North Pacific Ocean through the midlatitude Rossby wave train, and induce an anomalous anticyclone in the Northwest Pacific through exciting an equatorial Kelvin wave. These anomalies in atmospheric circulation in turn enhance the East Asian summer monsoon, leading to a northern-wet–southern-dry and warm summer climate in East Asia.

High‐Dimensional Chemostratigraphy of Pelagic Clay in the Western North Pacific Ocean Revealed via an Unsupervised Clustering Approach

AbstractPelagic clay constitutes massive and apparently uniform lithology that limits the stratigraphic correlation between neighboring sediment core samples. Recent studies on the pelagic clay in the western North Pacific Ocean demonstrated that the bulk chemical composition of sediments constitutes multielemental chemostratigraphy, deducing the correlation between visibly featureless pelagic clay layers across several cores. However, this heuristic approach utilized only a few elements. Therefore, this study employed multivariate statistical techniques, includingk‐means cluster analysis, to analyze the chemical composition data set of 1,646 samples × 41 elements of the western North Pacific pelagic clay. The pelagic clay was classified into 10 clusters systematically aligned from the seafloor to the depth in a specific order, constituting stratigraphic units that reflected the high‐dimensional geochemical features of these 41 elements. This finding strongly supports the statistical robustness of the latent chemostratigraphy in the western North Pacific pelagic clay. Additionally, we performed Sr–Nd–Pb isotope analyses of the detrital silicate fractions of the centroid samples representing each cluster. The multi‐isotopic features of the detrital fraction varied from a mixture of North American and Asian dusts to a predominance of Asian dust superimposed by volcanic inputs. This secular variation in the matrix components is attributable to the northwestward motion of the Pacific Plate and the time‐varying influence of arc volcanism throughout sedimentary history. The proposed integrated approach of multivariate statistical and isotopic analyses effectively extracted the essential information hidden in the pelagic clay, which may postulate a new protocol for paleoceanographic reconstructions targeting the pelagic realm.

Microplastics in the tropical Northwestern Pacific Ocean and the Indonesian seas

The abundance of microplastics fragments with sizes larger than 0.30–0.35 mm is measured during two research cruises in the tropical Northwestern Pacific Ocean and inside the Indonesian seas. The fragments are confirmed to be primarily nylon, polyester, and polyethylene etc. microplastics in the western Pacific by Fourier transform infrared spectroscopy (FT-IR), whereas dominated by polyethylene in the Indonesian seas based on Raman spectrum analyses. The abundance of the microplastics is found to be very low (<0.018 particles m−3) in the surface waters of the North Equatorial Current and the Kuroshio, suggesting no westward extension of the Great Pacific Garbage Belt and lack of microplastics accumulation due to mass convergence in the western boundary current. The abundances of the microplastics in the Makassar Strait of the Indonesian seas are found to be higher by an order of magnitude than those in the Northwestern Pacific, believed to be due to river run-offs carrying microplastics from nearby lands. The largest microplastics abundance is found in the northeastern Maluku Sea, where concurrent current measurements suggest a branch of the Indonesian Throughflow bringing accumulated microplastics from the eastern Indonesian seas into the convergent area of multiple strong currents. The study underlines the importance of ocean circulation in the microplastics distributions and transportation, and suggests the importance of high-resolution spatial mapping in the budget analysis of global microplastics distribution.