Tsushima Warm Current Research Articles

Drastic hydrographic changes inferred from radiolarian assemblages in the central Japan Sea since the Last Glacial Maximum

Assessing modern radiolarian distribution in the marginal seas is essential for understanding its relationship to hydrographic and environmental variables, upon which subsequent interpretation of past radiolarian records can be made reliably. Here, we combine published radiolarian assemblages in 69 surface sediments from the Japan Sea with those obtained from 15 new surface sediments in this study to determine the modern distribution of radiolarians. The Tsushima Warm Current (TWC) taxa are only found in the south of the Subpolar Front. The results of redundancy analysis confirm that the temperature at 10 m depth (temperature10m) is the main factor controlling the spatial variability of TWC taxa. The water depth plays a key role in affecting the abundances of Japan Sea Proper Water (JSPW) Assemblage including Larcopyle buetschlii Dreyer (adult), Cycladophora davisiana Ehrenberg, and Actinomma group. The JSPW Assemblage can be used as a reliable indicator of ventilation changes in the Japan Sea. Based on the newly developed TWC taxa and JSPW Assemblage, we further reconstructed the water column structure in the central Japan Sea over the past 25 ka. There was weak ventilation in the central Japan Sea even during the Last Glacial Maximum (LGM), although a distinct decrease in the TWC inflow resulted in surface-water freshening and water column stratification. After the LGM, the TWC taxa together with JSPW Assemblage resumed in the central Japan Sea at the 19 ka meltwater pulse (MWP). The MWP-1A was clearly marked by an enhanced intrusion of the TWC. Synchronously, JSPW was ventilated deeply down after ~14.6 ka attaining to the similar level of the Holocene as a result of changing sea surface conditions. Along with the continuing sea-level rise, the matured TWC entered the Japan Sea at 7 ka and since then the modern hydrographic condition has been established. We suggest that the changes in the sea level and TWC strength predominantly determined the surface hydrography and deep ventilation in the central Japan Sea since 25 ka, and the TWC might have notably affected the Japan Sea since the Bølling-Allerød.

Study on the Behavior of the Water Temperature Inversion Layer in the Northern East China Sea

To investigate the behavioral characteristics of the water temperature inversion layer (TIL), we used data (KODC) from areas in the Northern East China Sea from 1995 to 2016. Water temperature and salinity surveys were conducted 8820 casts over 22 years. Of these, 1589 water temperature inversion layers were found, and the probability of occurrence was 18.0%. In the Gageo island, probability of TIL occurrence in winter was 25 times higher than in summer. On the other hand, in the south of Jeju Island, summer values were 3.7 times higher than winter values. A T–S diagram analysis shows the components of the water temperature inversion layers. Yellow Sea Cold Water was mainly found in the winter, while Jeju Warm Currents and Tsushima Warm Currents were found in summer. The correlation between the probability of the occurrence of a monthly water temperature inversion layer and the amount of seawater volume transported into the study area was analyzed. The correlation coefficient was higher than r = 0.8 in parts of southern Jeju Island. On the other hand, the correlation coefficient was r = −0.6 in the Gageo Island. The spatial correlation index for the seawater volume transport and the water temperature inversion layer is presented.

Microbial Diversity Responding to Changes in Depositional Conditions during the Last Glacial and Interglacial Period: NE Ulleung Basin, East Sea (Sea of Japan)

Microbial interaction with minerals are significantly linked with depositional conditions during glacial and interglacial periods, providing a unique redox condition in the sedimentary process. Abiotic geophysical and geochemical properties, including sedimentary facies, magnetic susceptibility, grain size, clay mineralogy, and distribution of elemental compositions in the sediments, have been widely used to understand paleo-depositional environments. In this study, microbial abundance and diversity in the core sediments (6.7 m long) from the northeastern slope of Dokdo Island were adapted to characterize the conventionally defined sedimentary depositional units and conditions in light of microbial habitats. The units of interglacial (Unit 1, <11.5 ka) and late glacial (Unit 2, 11.5–14.5 ka) periods in contrast to the glacial period (Unit 3, >14.5 ka) were distinctively identified in the core, showing a sharp boundary marked by the laminated Mn-carbonate (CaM) mud between bioturbated (Unit 1 and 2) and laminated mud (Unit 3). Based on the marker beds and the occurrence of sedimentary facies, core sediments were divided into three units, Unit 1 (<11.5 ka, interglacial), Unit 2 (11.5–14.5 ka, late glacial), and Unit 3 (>14.5 ka, glacial), in descending order. The sedimentation rate (0.073 cm/year), which was three times higher than the average value for the East Sea (Sea of Japan) was measured in the late glacial period (Unit 2), indicating the settlement of suspended sediments from volcanic clay in the East Sea (Sea of Japan), including Doldo Island. The Fe and Mg-rich smectite groups in Unit 2 can be transported from volcanic sediments, such as from the volcanic island in the East Sea or the east side of Korea, while the significant appearance of the Al-rich smectite group in Unit 1 was likely transported from East China by the Tsushima Warm Current (TWC). The appearance of CaM indicates a redox condition in the sedimentary process because the formation of CaM is associated with an oxidation of Mn2+ forming Mn-oxide in the ocean, and a subsequent reduction of Mn-oxide occurred, likely due to Mn-reducing bacteria resulting in the local supersaturation of Mn2+ and the precipitation of CaM. The low sea level (−120 m) in the glacial period (Unit 3) may restrict water circulation, causing anoxic conditions compared to the late glacial period (Unit 2), inducing favorable redox conditions for the formation of CaM in the boundary of the two units. Indeed, Planctomycetaceae, including anaerobic ammonium oxidation (ANAMMOX) bacteria capable of oxidizing ammonium coupled with Mn-reduction, was identified in the CaM layer by Next Generation Sequencing (NGS). Furthermore, the appearance of aerobic bacteria, such as Alphaproteobacteria, Gammaproteobacteria, and Methylophaga, tightly coupled with the abundance of phytoplankton was significantly identified in Unit 1, suggesting open marine condition in the interglacial period. Bacterial species for each unit displayed a unique grouping in the phylogenetic tree, indicating the different paleo-depositional environments favorable for the microbial habitats during the glacial and interglacial periods.

Seasonal and spatial variations of air-sea heat exchange in the seas around the Korean Peninsula: Based on the observations and reanalysis products from 2011 to 2016

It is important to better understand air–sea heat exchange for weather and climate systems prediction. In the seas around the Korean Peninsula (KP), the annual cycles of most oceanic and atmospheric variables have long been well recognized, whereas those of the air–sea heat exchange are still not completely clear primarily due to a lack of in-situ observations applied to estimate surface heat flux. This is particularly true over the region affected by the Tsushima Warm Current (TWC), where cold and dry air and a northwesterly wind over the warm sea surface enhance heat exchange during the winter monsoons (when the ocean loses heat), which may often be over or underestimated through atmospheric reanalysis model products. Herein, we present estimates of the mean annual cycles of air–sea heat fluxes from in-situ observations at two locations (I-ORS and ESROB) and from two atmospheric reanalysis products (CFSv2 and MERRA-2) during the period of 2011–2016. The results show that the net heat loss of the ocean (net heat flux (NHF) of −68 Wm-2, positive downward) at ESROB near the east coast of Korea is 146 Wm-2 larger than that (+78 Wm-2, net heat gain) at I-ORS located ~250 km southwest of KP, except in June when the incoming shortwave radiation flux decreases significantly (by 208 Wm-2) only at the I-ORS, a response associated with the meiyu-baiu rain band. Greater sensible and latent heat losses (SHF and LHF) of 32 and 47 Wm-2 during both winter and spring, respectively, and less LHF of 22 Wm-2 during fall, were observed at ESROB than at I-ORS. The CFSv2 yields −24, −84, −41, and −180 Wm-2 (−43, −110, −56, and −120 Wm-2) in terms of the average NHF, winter SHF, and spring and fall LHFs at I-ORS (ESROB), whereas MERRA-2 yields −28, −56, −26, and −161 Wm-2 (−108, −134, −71, and −155 Wm-2), which is indicative of significant biases in the air–sea temperature difference and the wind speed. Spatial distributions of SHF and LHF of both reanalysis products, which are consistent each other and with the observations despite their biases, indicate the significant impact of TWC and the associated currents through the sea surface temperature and saturation-specific humidity on the annual air–sea heat exchange cycles within the region.

Millennial-scale fluctuations in water volume transported by the Tsushima Warm Current in the Japan Sea during the Holocene

Abstract Cyclic changes in volume transport of the Tsushima Warm Current (TWC) have been argued from diatom records in the southern Japan Sea off the Japanese islands during the Holocene. Although this phenomenon has not yet been confirmed by various proxy data, determining whether or not these oceanographic changes occurred is crucial for clarifying the nature of oceanographic changes in the southern Japan Sea. Here, we conducted a coupled analysis of Mg/Ca ratios and oxygen isotopes (δ18O) in shallow-dwelling Neogloboquadrina incompta from 13 core-top sediments in southern Japan, and developed a new equation for Mg/Ca temperature calibration (Mg/Ca = 0.311 × exp (0.07 × T)) as a proxy for spring sea surface temperature (SST). Using the newly developed, species-specific Mg/Ca-paleothermometry, we reconstructed SST variability for the past 6800 years from core YK10-7-PC09 in the southern Japan Sea. The Mg/Ca-derived SST record clearly represented five warmer periods at 6200–6000, 4900–4500, 4200–3800, 2600–2100, and 900–400 cal. year BP, almost consistent with previously published diatom records. These warmer events also corresponded to the periods in which warm molluscan assemblages increased at the northern end of the TWC, suggesting that periods of higher SST can be seen as reflecting the increased volume transport of the TWC. We interpreted the results of a model study showing that higher solar irradiance provoked positive Arctic Oscillation (AO)-like spatial patterns and the negative phase of the Pacific Decadal Oscillation (PDO) to mean that increased (reduced) TWC volume transport on the multi-centennial to millennial time scales was caused by high (low) solar insolation via a potential link between AO and PDO. Given that larger and more frequent volcanic eruptions occurred in the mid Holocene than in the late Holocene, volcanic forcing on the TWC volume transport changes would have been more significant during the former, as seen in the highly variable SST from this period and distinct decreases in SST around ~5900 cal. year BP and ~6400 cal. year BP. The millennial-scale fluctuations seen in SSTs in the southern Japan Sea would have had a large impact on the evolution of vegetation and human adaptation in the northern Japanese islands, adjacent to the Japan Sea, over the last 6800 years.

Comparison of Particulate Organic Carbon to Chlorophyll-a Ratio Based on the Ocean Color Satellite Data at the Ieodo and Socheongcho Ocean Research Stations

Lee, D.; Jeong, J.-Y.; Jang, H.K.; Min, J.-O.; Kim, M.J.; Youn, S.H.; Lee, T., and Lee, S.H., 2019. Comparison of particulate organic carbon to chlorophyll-a ratio based on the ocean color satellite data at the Ieodo and Socheongcho ocean research stations. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Remote Sensing and Geoscience Information Systems of Coastal Environments. Journal of Coastal Research, Special Issue No. 90, pp. 267-271. Coconut Creek (Florida), ISSN 0749-0208.The Ieodo ocean research station (IORS) and the Socheongcho ocean research station (SORS) have been operated in South Korea to carry out a research across multiple disciplines. The IORS and the SORS are affected by various water masses such as Tsushima Warm Current (TWC), Yellow Sea Warm Current (YSWC) and Changjiang Diluted Water (CDW). Therefore, the IORS and the SORS are suitable for the research on the physio-ecological response of phytoplankton to environmental changes. The particulate organic carbon to chlorophyll-a ratio (POC:Chl-a) has been widely used for an indicator of the ecological and physiological conditions of the phytoplankton. The purpose of this study is to investigate the POC:Chl-a ratio and its controlling factors at the IORS and the SORS by using satellite dataset. At the IORS, POC:Chl-a ratio ranged from 166.16 to 431.20, and the average was 322.14 ± 46.35. On the other hand, at the SORS, POC:Chl-a ratio ranged from 131.92 to 703.98, and the average was 385.05 ± 123.95. The POC:Chl-a ratio was higher at the SORS from autumn to winter. However, POC:Chl-a ratio in the IORS showed similar ranges in every month. The difference in POC:Chl-a ratio between the IORS and the SORS appear to be caused by differences of environmental conditions such as light intensity, temperature, and nutrients. However, to reveal the detailed relationship between environmental conditions and POC:Chl-a ratio, a further study on seasonal variations of the phytoplankton community structure and several environmental parameters would be warrant.

Variability of primary production among basins in the East/Japan Sea: Role of water column stability in modulating nutrient and light availability

The primary productivity of the East/Japan Sea (EJS) displays strong spatio-temporal variability, largely driven by the oceanographic forcings that vary across the different regions of the sea. Consequently, the ecosystem responses to climate variability could be region-dependent. A model-based analysis from this study synthesized the complex biological-physical interactions across the EJS, and suggested an intricate role of water column stability in driving the observed seasonal and basin-scale variability. Compared to the northern EJS (e.g., the Japan Basin), the southern EJS (e.g., the Ulleung and Yamato Basins) has a more favorable mixing regime for higher productivity in both winter and summer, balancing the need for phytoplankton growth in terms of nutrient and light availability. The wintertime water column in the southern EJS is relatively stable compared with the northern EJS due to the influx of warm Tsushima Warm Current (TWC) water and also weaker surface atmospheric cooling, leading to decreased light limitation for primary production in the south than in the north. During the stratified summer season, the surface stirring and mixing, due to the energy dissipation of strong TWC inflow, promote a relatively less stable surface mixed layer in the south than that in the north, and thus alleviate the nutrient limitation at the surface. In addition, the current- and wind-induced upwelling along the east coast of Korea further enhances the productivity in the southwestern EJS. The general south-north ‘see-saw’ pattern of mixing regime, combined with upwelling-downwelling switching, could be the key driver for the observed production patterns in the EJS.

Links between iron supply from Asian dust and marine productivity in the Japan Sea since four million years ago

AbstractAeolian dust input exerts significant influence on oceanic biogeochemical cycles and further potentially controls atmospheric CO2 concentrations. However, the possible link between long-term aeolian dust supply and primary productivity in the western North Pacific remains poorly understood. Here, we present a comprehensive study of major and trace elements and total organic carbon (TOC) concentrations of sediments from Integrated Ocean Drilling Program (IODP) Site U1430 in the southern Japan Sea, in order to reconstruct oceanic palaeoproductivity evolution and test its possible link to Asian dust input since 4 Ma. Palaeoproductivity proxies indicate remarkable increases in productivity at ∼3–2 Ma followed by high-frequency oscillations in productivity since 1.2 Ma. We suggest that higher dust-derived iron supply from Central Asia at 3–2 Ma, which was likely driven by the growth of the Northern Hemisphere ice sheets, could account for enhanced primary productivity and export production in the Japan Sea. Such increased oceanic palaeoproductivity could enhance organic carbon burial, which might contribute to the decrease in atmospheric CO2 concentrations, and provide a positive feedback to the global cooling. However, the Tsushima Warm Current (TSWC) intrusion via the southern Tsushima Strait, which was controlled by glacioeustatic sea level changes, has been the principal cause of the rapid changes in primary productivity and benthic redox condition since 1.2 Ma, regardless of continuously increased Asian dust input.

Convection of surface water in the northeastern Japan Sea: Implications from vertical profiles of 134Cs concentrations

This study examined the spatial profiles of 134Cs concentrations in the Japan Sea (particularly in the northeastern area) in July 2017 and July 2018. We detected 134Cs in water samples above a depth of ~200 m (~0.3–1 mBq/L decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant accident), which predominantly reflected the mixing of 134Cs-contaminated Tsushima Warm Current (TWC) water within 4 years after 134Cs re-entry from the Pacific Ocean into the Japan Sea in 2013. Furthermore, 134Cs was only detected at depths of 300 m and 500 m (~0.2 mBq/L) in the northern Japan Basin, which corresponded to ~30% of the annual mean concentration levels in TWC water (0.75 mBq/L). Observed spatial variations in 134Cs concentrations were explained by the surface TWC circulation and subsequent convection processes.

Paleoceanographic evolution of the Japan Sea over the last 460 kyr – A coccolithophore perspective

Abstract Changes in the intensity of the influx of the Tsushima Warm Current (TWC) in the south-central part of the Japan Sea (JS), or East Sea in Korean, were reconstructed for the last 460 kyr at IODP Site U1427 using the composition and abundance of the coccolithophore assemblage. In addition, the recent distribution of coccolithophore taxa in the JS and the East China Sea was assessed using electron microscopy. Coccolithophore assemblages, dominated by Emiliania huxleyi and Gephyrocapsa species, and coccolith abundance reveal a strong relationship between sea level and ocean variability over the last five glacial/interglacial cycles, in good agreement with planktic foraminiferal data. Three different circulation modes based on calcareous nannofossil and foraminifera TWC indicators were proposed for the JS. Coccolith production was low and TWC indicators (i.e., Gephyrocapsa oceanica, Calcidiscus leptoporus s.l., and Helicosphaera carteri) absent due to the isolation of the JS during glacials in response to global sea level falls (Mode 1). In contrast, coccolith abundance and TWC indicators reach a maximum due to the most intense TWC flow through the Tsushima Strait during interglacials (Mode 3). Intermediate conditions (Mode 2) are characterized by moderate/high coccolith numbers, presence of TWC coccolith indicators, and rare TWC planktic foraminifera indicators. This mode resulted in intermittent variations in the contribution of the TWC and East China Sea coastal water due to relatively low sea level stands (ca. −90 m to −20 m). Coccoliths dominated the carbonate sequence prior to MIS 8 and were major contributors to the total carbonate of the sediment at Site U1427, suggesting high coccolithophore productivity and a bloom-type environment during MIS 11 and 9. These changes in the carbonate chemistry caused by glacio-eustacy in the JS at the northwest margin of the Pacific Ocean should be considered in future paleoceanographic and paleoclimate models.

Seasonal changes in abiotic environmental conditions in the Busan coastal region (South Korea) due to the Nakdong River in 2013 and effect of these changes on phytoplankton communities

We studied phytoplankton community structure at 25 stations in the Busan coastal region (South Korea), including the Nakdong River estuary, during four seasons in 2013. Together with the observational study, we conducted an experimental study to assess the effects of nutrient enrichment on phytoplankton community structure over seasonal succession. Over the study period, abiotic environmental factors, such as water temperature, salinity, and nutrient concentrations, in the Busan coastal region were associated with the amount of discharge from the Nakdong River; salinity and nutrient levels varied depending on the amount of river discharge, and the water temperature in winter near the Nakdong River estuary was significantly lower due to the increase in river discharge compared to other regions. However, the water temperature in offshore stations was relatively high owing to the introduction of the Tsushima Warm Current (TWC). During winter and summer, diatoms were the predominant taxa; the primary dominant species were Stephanodiscus hantzschii (winter), Leptocylindrus danicus (spring), and small diatoms (summer). In summer, blooms of the dinoflagellate Cochlodinium polykrikoides occurred at several stations near the Nakdong River mouth and several offshore stations, and small diatoms were abundant at the sampling stations. In autumn, Cryptomonas spp. were dominant (>60%) at most sample stations. To investigate the response of phytoplankton communities to nutrient loading, bioassay experiments with phytoplankton samples collected from five sampling stations during four seasons were conducted through the addition of inorganic nutrients: nitrate (+N; 20 µM), phosphate (+P; 2 µM phosphate), and nitrate and phosphate (+NP; at the same concentrations). The +NP treatment exhibited the highest growth enhancement of phytoplankton in most samples. In addition, depending on the NP ratio and phytoplankton community structure, there was a distinct response of phytoplankton to nutrient loading. These results imply that phytoplankton can grow under conditions of pulsed nutrient loading, irrespective of season, but there may be seasonal difference in response of phytoplankton communities in the Busan coastal region.

Palynological study of paleoclimate and paleoceanographic changes in the Eastern South Korea Plateau, East Sea, during the Plio-Pleistocene climate transition

A palynological study was performed using the core obtained from the Integrated Ocean Drilling Program (IODP) Expedition 346 Site U1430 in the Eastern South Korea Plateau in the East Sea, to reveal variability of paleoclimate and paleoceanography during the late Pliocene to early Pleistocene climate transition period (56.0–68.0 meters below sea floor [mbsf]). Two pollen and dinocyst zones are established with a boundary at ca. 3.0 Ma. Prior to ca. 3.0 Ma (first pollen and dinocyst zone), long-term oscillation of the East-Asian winter monsoon (EAWM) is indicated by fluctuations in the abundance of pollen taxa, representing cold–dry and cool–wet climates. After ca. 3.0 Ma (second pollen and dinocyst zone), an increase in cold–dry conifer pollen indicates an enhanced and stable EAWM. In addition, the prevalence of the xerophytic herb, Artemisia, increases markedly after 3.0 Ma, suggesting open grassland on the exposed South Yellow Sea shelf during the sea level lowstand under cold–dry climate conditions. Artemisia pollen are interpreted as having been delivered to the coast through fluvial channels in this grassland environment, and subsequently transported to the East Sea by inflow of a weak Tsushima Warm Current (TWC) through the narrow Korea Strait. The TWC intrusion signal is represented by an influx of Tuberculodinium vancampoae, a warm water dinoflagellate species, in the western Pacific region. The high abundances of cold–dry conifer pollen and cold water dinocyst elements after ca. 2.8 Ma suggest an intensified EAWM. Meanwhile, a short-term increase in moisture-loving Tsuga pollen after ca. 2.5 Ma was likely due to soil moisture derived from snowfall caused by evaporation of heated seawater from the weak TWC in the East Sea under cold climate conditions. The palynological studies reported here help verify the linkages between the EAWM, TWC, precipitation and vegetation responses in/around the East Sea region.

Biological modulation in the seasonal variation of dissolved oxygen concentration in the upper Japan Sea

Dissolved oxygen (DO) can be influenced by both physical and biogeochemical conditions of the ocean and is hence an important index for predicting changes in ocean environments. However, the relationship between biogeochemical activities and the DO concentration has not been quantified clearly in the Japan Sea (JS). In this study, we developed a coupled physical-biogeochemical model and clarified the contributions of biological processes to the DO content in the upper JS. We conducted two numerical experiments: with and without the biogeochemical compartments. Taking the biogeochemical factors into account gave better estimations for the spring-to-autumn seasonal migration of the subsurface DO maximum. In the euphotic zone, high photosynthetic production increases the DO concentration from spring to autumn. The strongest positive effect of biological processes on DO concentrations appears in summer at a depth of 30 m and is more than 0.5 ml l−1. As summer progresses to autumn, the vertical pattern of DO concentration in the Tsushima Warm Current (TWC) region shows minima at depths between 50 and 100 m, which is caused by the intrusion of low DO water from the East China Sea. The strongest negative effect on DO concentrations mainly due to the decomposition of detritus occurs in the TWC region below 150 m depth because the water is poorly ventilated by the relatively weak winter convection.

Turbulent Mixing induced by Cold Water Intrusion over the Southwestern East Sea

ABSTRACT Seo, S.; Park, Y.-G.; Jeon, C.; Min, H. S.; Kim, D. G., and Park, J.-H., 2018. Turbulent mixing induced by cold water intrusion over the southwestern East Sea. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 476–480. Coconut Creek (Florida), ISSN 0749-0208. Along the southeastern coast of Korea the warm and salty Tsushima Warm Currents flows northeastward. During summer a southwestward cold water intrusion from the East Sea occurs along the bottom below the warm current. Thus, a strong shear and consequently strong turbulent mixing are expected along the boundary between the warm water and cold water. To quantify the mixing hydrographic surveys using a free falling microstructure profiler (VMP-500) were conducted. Enhanced dissipation of energy was indeed observed along the interface located between 15°C to 20°C layers. Along the bottom there was anot...

Intraspecific bloom succession in the harmful dinoflagellate Cochlodinium polykrikoides (Dinophyceae) extended the blooming period in Korean coastal waters in 2009

Although there have been extensive studies on dinoflagellate blooms in recent decades, the mechanism that allows the maintenance of blooms over long periods remains uncertain, and studies on genetically differentiated subpopulations may provide insights into this mechanism. In this study, the influence of two genetically distinct subpopulations of the dinoflagellate Cochlodinium polykrikoides, referred to as Group I and IV, on bloom duration in Korean coastal waters (KCW) was examined using a quantitative PCR (qPCR) assay. In this study, a C. polykrikoides bloom occurred over a longer period in 2009 (49 days), whereas the bloom period was shorter in 2010 (35 days). The qPCR results indicate that intraspecific bloom succession between Groups I and IV occurred in 2009, whereas only a single subpopulation (Group I) was responsible for the bloom in 2010. Based on the statistical analysis, the Group I and Group IV blooms occurred under significantly different environmental conditions (p ≤ 0.05) in terms of water temperature, pH, and phosphate concentration, and these subpopulations exhibited significantly different relationships with environmental factors, particularly water temperature (p < 0.01). This variability may allow blooms to continue through intraspecific bloom succession even after environmental conditions change. Southern KCW are affected by outer regions via the Tsushima Warm Current (TWC) every summer. Group IV (≤1108 ± 69 cells L−1) was primarily observed along the route of the TWC in summer 2009, when the bloom of this subpopulation occurred in southern KCW. These results suggest that Group IV transported via the TWC may have influenced the bloom dynamics of this subpopulation in summer 2009.

Computing Ocean Surface Currents From GOCI Ocean Color Satellite Imagery

One of the significant challenges in physical oceanography is getting an adequate space/time description of the ocean surface currents. One possible solution is the maximum cross-correlation (MCC) method that we apply to hourly ocean color images from the Geostationary Ocean Color Imager (GOCI) over five years. Since GOCI provided a large number of image pairs, we introduce a new MCC search strategy to improve the computational efficiency of the MCC method saving 95% of the processing time. We also use an MCC current merging method to increase the total spatial coverage of the currents, proving a 25% increase. Five-year mean and seasonal time-average flows are computed to capture the major currents in the area of interest. The mean flows investigate the Kuroshio path, support the triple-branch pattern of the Tsushima Warm Current (TC), and reveal the origin of the TC. The evolution of a warm core ring shed by the Kuroshio near the northeast coast of Honshu, Japan, is clearly depicted by a sequence of three monthly MCC composites. We capture the evolution of the Kuroshio meander over seasonal, monthly, and weekly time scales. Three successive weekly MCC composite maps demonstrate how a large anticyclonic eddy, to the south of the Kuroshio meander, influences its formation and evolution in time and space. The unique ability to view short space/time scale changes in these strong current systems is a major benefit of the application of the MCC method to the high spatial resolution and rapid refresh GOCI data.

Pleistocene deep-sea ostracods from the Oki Ridge, Sea of Japan (IODP Site U1426) and condition of the intermediate water

AbstractThe Sea of Japan (also termed the East Sea) has a circulation system isolated from the Pacific Ocean and East China Sea. The East Asian winter monsoon drives the circulation system and cools the Tsushima Warm Current (TWC) to form the Japan Sea Intermediate–Proper Water (JSIPW). The intermediate water conveys oxygen to deep-sea floors, which is available for benthic animals. During the Pliocene (3.5–2.8 Ma), Temperate Intermediate Water (TIW) was formed under the weak winter monsoon, and extinct ostracod TIW taxa were found. Little is known about early Pleistocene intermediate water and the extinction mode of benthic ostracods. We studied radiolarians and ostracods from deep-sea sediments between 2.0 and 1.3 Ma (Marine Oxygen Isotope Stage [MIS] 77 to MIS 41) at Integrated Ocean Drilling Program Site U1426, Sea of Japan. The ostracod faunas contained TIW and JSIPW taxa. The radiolarian subtropical-water taxa and the JSIPW ostracods indicate a small influx of the TWC and the JSIPW. The TIW occasionally expanded to the middle bathyal zone. By analogy with the relationship between the modern JSIPW and winter monsoon, weak winter monsoon possibly caused gentle temperature gradients in the water column and the expansion of the TIW. The JSIPW taxa expanded their ranges into the deep sea during interglacial periods.

Fossil diatom assemblages as paleoecological indicators of paleo-water environmental change in the Ulleung Basin, East Sea, Republic of Korea

The fossil diatom assemblage record from two sediment cores obtained from the Ulleung Basin, East Sea, Republic of Korea, revealed changes in the diatom assemblage zones in PG1 and PD3 core samples. The two sediment cores were δC14 dated and approximately represented the late Pleistocene–Holocene. The analysis of age zones in the PG1 core and PD3 core was assessed based on the frequency of variations, and occurrences of biostratigraphical fossil diatom species. During the Last Glacial Maximum (LGM), the sea level was lower than that at present and the Ulleung Basin became isolated from the Pacific Ocean. As a result, there would have been a limited Tsushima Warm Current (TWC) influence, and salinity would have decreased resulting in increased freshwater and coastal diatoms. The distribution pattern of diatoms presented in the cores was associated with changes in water temperature and salinity and the adding of terrigenous material brought about by the input of freshwater. Changes in the abundance of a tychopelagic diatom, Paralia sulcata, reflected the effect of the water currents. Diatom temperature (Td) values and the ratio of centric/pennate diatoms provided evidence of limited influences of the TWC and freshwater inflow. It is thought that all assemblage zones were influenced by the TWC, which had an important effect on the distribution and composition of fossil diatoms.

Genetic diversity and population structure of Hyporhamphus sajori (Beloniformes: Hemiramphidae) inferred from mtDNA control region and msDNA markers.

This paper presents preliminary data on the genetic diversity and population structure of Hyporhamphus sajori by analysing a 510 bp sequence in the mitochondrial DNA (mtDNA) control region and eight polymorphic microsatellite DNA loci. The H. sajori individuals from different locations were indistinguishable from one another based on mtDNA variation, as demonstrated with a neighbour-joining tree and minimum spanning network analysis. Low level of genetic diversity and the absence of population structure in H. sajori from the north-west Pacific Ocean, combined with negative indices for neutral evolution in these populations, suggest that H. sajori underwent a population expansion after a recent bottleneck. The Structure analysis, discriminant analysis of principal components (DAPC) and the pair-wise ΦST values after Bonferroni correction using eight microsatellite loci provided no clear inference on the genetic differentiation and thus no evidence of population structure of H. sajori. The genetic connectivity among locations might be due to fairly high gene flow via transport of eggs and larvae by the Kuroshio and Tsushima warm current. This study revealed low levels of genetic diversity and suggested high level of contemporary gene flow among populations of H. sajori in the East (Japan) Sea and the Pacific Ocean.

Evaluation of air-sea sensible and latent heat fluxes over the Japan Sea obtained from satellite, atmospheric reanalysis, and objective analysis products

In order to evaluate the air-sea sensible and latent heat fluxes (SHF and LHF, respectively) over the Japan Sea obtained from satellite-derived, atmospheric reanalysis, and objective analysis products, we conducted an intercomparison of climatological monthly mean fields and a validation of the daily means with in situ observations. As a result, we found significant differences in the climatological mean fields of SHF in products during the winter. The SHF values from the Japanese Ocean Flux Data Sets with Use of Remote Sensing Observations version 2 (J-OFURO2) showed finer-scale spatial structures, large values near the coastal regions, and small values over the center of the Japan Sea. From a comparison with in situ observations, we concluded that J-OFURO2 SHF has better performance compared with the other global products. The differences in LHF in the products are not so large compared with the results for SHF. However, in a comparison between the satellite-derived products and the others, the representations of the frontal features associated with the sub-polar front and the Tsushima warm current were different. A comparison in the near-coastal region showed that application of the creeping-sea-fill technique used in J-OFURO2 provided appropriate data in the region and succeeded in reducing the adverse effects from land contamination encountered when using coarse atmospheric reanalysis data.