Tsushima Warm Current Research Articles

Revisiting the connection between variations in the Tsushima Warm Current and winter rainfall along the coast of Japan

AbstractEnhanced autumn transport via the Tsushima Warm Current (TWC) has been reported to increase winter rainfall along Japan’s northern coast. However, observations reveal a significantly weakened correlation after 2006. Although enhanced transport via the Eastern channel of the Tsushima Strait affects rainfall by enhancing latent heat flux along Japan’s northern coast, the role of oceanic flows in varying sea-surface temperatures has weakened since 2006. Stronger northwesterly winds co-occur with enhanced surface heat loss that overwhelms the impact of the TWC. Our findings imply that atmospheric conditions should be considered when using TWC transport as a precursor for winter rainfall.

Microplastic Distribution Characteristics Considering the Marine Environment Based on Surface Seawater Quality Parameters in Southern Sea of Korea, 2019

The present study determined the microplastic distributions in the surface water of the Southern Sea of Korea (SS01–SS09) in September 2019, depending on three groups (Groups A, B, and C) categorized via the principal component analysis and cluster analysis using the water quality parameters (water temperature, salinity, pH, dissolved oxygen, suspended particulate matter, and chlorophyll-a). The microplastic samples in the surface water were collected using a 300 μm-mesh neuston net. The microplastic abundance ranged from 0.10 to 5.08 (average 0.71 ± 1.64) particles/m3 in the entire sampling area. Median values in Groups A (SS01, SS02, and SS07), B (SS05, SS08, and SS09), and C (SS03, SS04, and SS06) were 0.14 ± 0.02, 0.12 ± 0.14, and 0.17 ± 2.85 particles/m3, respectively, and there were no statistically significant differences (p < 0.050). However, it was highlighted that the most considerably numerous microplastic abundance in SS04 (5.08 particles/m3) revealed relatively high water temperatures distinguished from other sampling areas. Polystyrene, consisting mainly of expanded polystyrene, was the predominant polymer type, accounting for 81.5% in Groups A, 84.4% in B, and 97.0% in C. The particle size in Group C (average 3.11 ± 1.08 mm) was statistically larger (p < 0.001) than those of Groups A (average 0.71 ± 1.06 mm) and B (average 0.98 ± 1.22 mm). Only fragment and sheet shapes were found in all the sampling points and the former, which consisted of the secondary microplastics regardless of the groups, was the dominant type. The fragment composition gradually increased from 77.3% in Group A, 96.7% in B, to 99.1% in C. However, the strategy studies should be investigated in the foreseeable future to supplement the current study limitations, such as verifying the effect of the Yangtze River and the Tsushima warm current, and seasonal fluctuation.

Seasonal Phytoplankton Characteristics Related with Region-Specific Coastal Environments in the Korean Peninsula

The seasonal dynamics of phytoplankton communities in Korean coastal waters (KCWs) are influenced by complex interactions between ocean currents and nearshore human activities. Despite these influences, the understanding of seasonal phytoplankton changes and their environmental relationships in KCWs remains limited. We investigate the influence of the distinct characteristics of the three seas surrounding the KCWs (the Yellow Sea, the South Sea, and the East Sea) on seasonal phytoplankton communities based on field surveys conducted at 23 stations between 2020 and 2021. The East Sea exhibited higher winter temperatures due to the Jeju and Tsushima warm currents, while summer temperatures were lower compared to the other regions, highlighting the role of currents and deeper oceanic waters. The Yellow Sea showed significant freshwater influence with low salinity levels from major rivers, contrasting with the higher salinity in the East Sea. These differences led to a disparity in the productivity of the two regions: the highest value of Chl. a was observed to be 6.05 µg L–1 in the Yellow Sea in summer. Diatoms dominated in nutrient-rich conditions, particularly in the Yellow Sea, where they comprised up to 80–100% of the phytoplankton community in summer, winter, and spring. PCA analysis revealed positive correlations between diatoms and Chl. a, while cryptophytes, which thrive in the absence of diatom proliferation, showed no such correlation, indicating their opportunistic growth in nutrient-limited conditions. This study highlights the significant impact of region-specific hydrographic factors on phytoplankton communities in KCWs, with diatoms dominating in summer and cryptophytes and dinoflagellates showing seasonal and regional variations. Understanding these dynamics is crucial for predicting phytoplankton bloom dynamics and their ecological implications in coastal ecosystems.

Trophic niche of a neustonic copepod community sustained by local production in high- and low-salinity summer water masses in the northeastern East China Sea

We investigated the trophic dynamics between neustonic copepod communities and particulate organic matter (POM) sourced from contrasting water masses, i.e., high- and low-salinity areas, within the northeastern East China Sea. The sampling of neustonic copepod communities occurred three times in the summer (June, August, and September) of 2021, and considered the influx of the Yangtze River Diluted Water (YRDW) and the extension of the Tsushima Warm Current (TWC). To examine the relationship between the neustonic copepods and surface POM, we employed stable isotope techniques. Surprisingly, the neustonic copepod community exhibited a reluctance to feed on surface POM. The neustonic copepod community in the YRDW-dominated area in August showed higher δ13C values than those in the TWC-dominated area. The trophic groups within the neustonic copepod population appeared to consume selectively depending on the individual preferred prey sizes and composition. Using the POM from the water masses that affected the food sources consumed by neustonic copepods, we found a strong dependence on prey items originating from the TWC. This result suggests that the marine neustonic copepod community does not rely significantly on food sources derived from riverine freshwater.

Effects of water masses on the zooplankton community structure in the northern East China Sea during the East Asian Summer Monsoon in 2020

The prolonged rainy season in 2020 was classified as the second-largest rainfall event since 1961. In this study, we conducted a survey to assess fluctuations in the zooplankton community structure of the northern East China Sea (nECS) after the rainy season in August 2020. We examined 24 stations and observed several environmental factors, including water temperature, salinity, chlorophyll-a concentration, suspended particulate matter (SPM), and phytoplankton density. Zooplankton samples were collected from 12 stations using MOCNESS to target the surface mixed layer (SML), pycnocline layer (PNC), and lower layer (LL). In regions with low water temperatures, the Yellow Sea and northern East China Sea shelf mixed water community in each layer exhibited a decrease in the relative abundance of Calanus sinicus, whereas the incidence of Paracalanus parvus s. l. showed an increasing trend. Similarly, in areas with low salinity, the CDW and northern East China Sea shelf mixed water community in SML and PNC displayed comparable distributions, with a tendency for the relative abundance of Acartia pacifica and Paracalanus parvus s. l. to show a replacement pattern. Moreover, the Tsushima Warm Current (TWC) community in each layer, characterized by high water temperature and salinity, showed a high relative abundance of Oncaea spp. and a diverse species distribution, mainly in the eastern part of the study area. Our study identified multiple vertically distributed communities, providing detailed information on water mass structures through cluster analysis. This approach enhanced previous studies by offering novel insights into the vertical structure of water masses and their association with zooplankton communities in the nECS.

Palynological implications for the paleoclimate and paleoceanographic reconstruction of the East Sea since the early Pleistocene at IODP site U1430

Palynomorphs, including pollen and dinoflagellate cysts from marine sediments, can provide a history of paleoclimate and paleoceanography. We present new palynological profiles based on a core from the Integrated Ocean Drilling Program Site U1430, which was drilled in the Ulleung Plateau East Sea. This core covers the past 2 Ma and spans the Mid-Pleistocene Transition (MPT, 1.2–0.8 Ma) and Mid-Brunhes Event (MBE, ∼0.43 Ma). Terrestrial and marine palynomorph records indicate orbital-scale variations in paleoclimate and paleoceanography, reflecting changes in the interconnections between vegetation, climate, and the ocean. Before the MPT, the high pollen taxa ratio and abundant temperate pollen suggest a relatively warm temperature associated with astronomical forcing-induced insolation changes. At ca. 1.3 Ma, boreal coniferous pollen increased, suggesting intensification of the East Asian winter monsoon (EAWM). The long-term trend of boreal coniferous pollen concentration was similar to that of benthic δ18O. This orbital scale boreal conifer pollen assemblage variation indicates Quaternary EAWM evolution controlled by changes in global ice volume and climatic cooling. The marine palynological data reflect the ocean circulation history of the study area, where sea level fluctuations and tectonic activity played important roles around the marginal sea. The abundance of warm-water dinoflagellate species after ca. 1.9 Ma, including Tuberculodinium vancampoae, indicates Tsushima Warm Current intrusion related to the expansion of the Korea (Tsushima) Strait. Nematosphaeropsis labyrinthus and other cold-water species increased markedly during the MPT, suggesting a low sea surface temperature and weakened vertical mixing of the East Sea due to the cooled climate and sea ice during the MPT. The marked sea level fluctuation associated with the glacial–interglacial cycling after the MBE resulted in an assemblage composition shift from cold to warm water species. Tsuga and Betula thrived during mild winters, primarily influenced by winter insolation. These genera were particularly prosperous in the early Pleistocene, coinciding with a strong intrusion of the Tsushima Warm Current (TWC). Our findings should be considered in developing a comprehensive understanding of the paleoclimate and paleoceanographic history of northern East Asia across the major climate transitions of the Pleistocene.

Current Status and Future Plans for Surface Current Observation by HF Radar in the Southern Jeju

The southern strait of Jeju is a divergence point of the Tsushima Warm Current (TWC), and it is the starting point of the thermohaline circulation in the waters of the Korean Peninsula, affecting the size and frequency of marine disasters such as typhoons and tsunamis, and has a very important oceanographic impact, such as becoming a source of harmful organisms and radioactively contaminated water. Therefore, for an immediate response to these maritime disasters, real-time ocean observation is required. However, compared to other straits, in the case of southern Jeju, such wide area marine observations are insufficient. Therefore, in this study, surface current field of the southern strait of Jeju was calculated using High-Frequency radar (HF radar). the large surface current field is calculated, and post-processing and data improvement are carried out through APM (Antenna Pattern Measurement) and FOL (First Order Line), and comparative analysis is conducted using actual data. As a result, the correlation shows improvement of 0.4~0.7 and RMSE of about 1~19 cm/s. These high-frequency radar observation results will help solve domestic issues such as response to typhoons, verification of numerical models, utilization of wide area wave data, and ocean search and rescue in the future through the establishment of an open data network.

Long-term variation in volume transport of the Tsushima warm current estimated from ADCP current measurement and sea level differences in the Korea/Tsushima Strait

ADCP current data and sea level data from tide gauges and satellite altimetry were used in order to analyze long-term variation of the Tsushima Warm Current (TWC) volume transport passing through the Korea/Tsushima Strait. The annual mean transport of the TWC estimated from ADCP current measurement for about 17 years (1997–2013) is 2.64 ± 0.41 Sv (Sv = 106 m3s−1) in the Korea/Tsushima Strait, 1.53 ± 0.36 Sv (about 58% of the total transport) in the western channel, and 1.11 ± 0.14 Sv (about 42%) in the eastern channel. The TWC shows the minimum transport (1.74 Sv) in January and the maximum transport (3.10 Sv) in October, with seasonal variation of up to 1.36 Sv (51.5% of the annual mean transport). The seasonal variation of the total TWC transport in the Korea/Tsushima Strait is mainly similar to that in the western channel. The annual mean transport of the TWC and its seasonal variation in the Korea/Tsushima Strait, the western and eastern channels are all similar in three datasets (ADCP, tide gauge, satellite altimetry). The annual mean volume transport of the TWC estimated from long-term tide gauge data was 2.57 ± 0.37 Sv (total volume transport) for 44 years (1975–2018) in the Korea/Tsushima Strait, 1.51 ± 0.32 Sv in the western channel and 1.06 ± 0.14 Sv in the eastern channel. Long-term variation of the TWC transport shows a decreasing trend for 1975 to 1988, and an increasing trend from 1989 to 2018. For the latter period (1989 to 2018), monthly transport tends to increase in all months throughout the years, showing a greater increase, especially in spring and summer months. The increase of the transport in the eastern channel was about 2.8 times larger than that in the western channel. It is suggested that the increasing trend of the TWC transport since 1989 is related to the Pacific Decadal Oscillation (PDO), which has been changed to negative direction since mid-1980s. During the period when the negative PDO index is strong, the negative wind stress curl weakens in the mid-latitudes of the North Pacific, and the Kuroshio in the East China Sea also weakens. As a result, the flow is strengthened toward the Korea/Tsushima Strait, increasing the TWC transport.

Transport of reactive materials in the Sea of Japan: Implications from fine-resolution surface distribution of 228Th/228Ra ratio

This study aimed to explore the transport patterns of reactive materials under the Tsushima Warm Current (TWC) in the Sea of Japan. Fine-resolution seasonal and lateral variations in 228Th/228Ra activity ratio were examined along with 228Ra/226Ra ratio in surface seawater samples from the Sea of Japan, East China Sea (ECS), and Pacific Ocean in 2015–2020. Significant 228Th/228Ra ratio variations were observed in surface seawaters (<∼0.02–0.3), which partially correlated with the mixing ratios of the low-228Th/228Ra continental shelf side waters from the western ECS and high-228Th/228Ra Kuroshio Warm Current of the Pacific Ocean, in addition to the contributions of the subarctic current of the northern Sea of Japan. Conversely, in the Sea of Japan, local variation of 228Th/228Ra ratios were observed in samples with a similar fraction of the ECS continental shelf side waters. It is reasonable to speculate that these variations are due to local scavenging during the TWC circulation.

Long-term variation of nitrate in the East Sea, Korea.

Long-term variation of nitrate in the East Sea was monitored in order to investigate impact of Three Gorges Dam (TGD) in the Changjiang River's upstream, China and Nakdong River's estuary dam, Korea. Tracing source of nitrate was another objective in this study. For this study, nutrient data were collected for 20years from 1999 to 2018 in the East Sea, and divided into 4 sections, and evaluated whether a significant difference exists among the averages of nitrate concentrations. The concentrations of nitrate were affected by the major rivers (the Nakdong and the Taehwa Rivers) and Tsushima Warm Current (TWC) which diverged from the Kuroshio Current passing through East China Sea (ECS). Our results also indicated that long-term nitrate concentrations decreased and its reasons. First, the construction of TGD in the upstream of the Changjiang River may have resulted in the decrease of the nitrate supply in the river and ECS which is carried by TWC, toward the East Sea. Second, decrease in the nitrate flux of the Nakdong River's estuary due to the construction of the estuary dam and sewer treatment plant could also be a factor for the nitrate decrease in the East Sea. Therefore, anthropogenic activities from the Nakdong River and Changjiang River had a long-term effect on the East Sea's nitrate concentrations. The amount of nitrate runoff reduced by the anthropogenic activities influenced the nitrate levels over a long period by the flow of currents in the East Sea.

A Lacustrine Biomarker Record From Rebun Island Reveals a Warm Summer Climate in Northern Japan During the Early Middle Holocene Due to a Stronger North Pacific High

The summer climate of northern Japan since the last glacial period has likely been determined by atmospheric and oceanic dynamics, such as changes in the North Pacific High, the position of the westerlies, the Kuroshio Current, the Tsushima Warm Current (TWC), and the East Asian summer monsoon. However, it is unclear which factor has been most important. In this study, we analyzed leaf wax δ13C and δD and glycerol dialkyl glycerol tetraethers (GDGTs) in sediments from Lake Kushu, Rebun Island, northern Japan, and discuss changes in climate over the past 17,000 years. The GDGT-based temperature, the averaged chain length, δ13C and δD of long-chain n-fatty acids indicated that the climate was cold during the Oldest Dryas period ∼16 ka and warm in the early Middle Holocene from ∼9 to 6 ka. This climate change is consistent with the sea surface temperature in the Kuroshio–Oyashio transition, but inconsistent with changes in the TWC in the Sea of Japan. The results imply that the summer climate of northern Japan was controlled mainly by changes in the development of the North Pacific Highviachanges in the position of the westerly jet and East Asian summer monsoon rainfall, whereas the influence of the TWC was limited over a millennial timescale.

Effect of a current trapped by a continental slope on the pathway of a coastal current crossing Toyama Trough, Japan

To clarify the Tsushima Warm Current (TWC) system around a downstream region of Noto Peninsula (NP) where Toyama Trough (TT) causes a discontinuity of the along shelf-current, numerical experiments were performed using two-layered ocean model with simplified bottom topography. When a current trapped by the continental slope representing an offshore branch of TWC (OB) encounters the NP and TT, the OB was trapped by the coast facing the TT. A clockwise lee-eddy developed between a current axis and a coastline over the TT, which results in a current path transition offshore (CPT). The OB finally adjusted to a current path which crosses TT by shifting from the discontinuity point of the continental slope. If a coastal current representing a coastal branch of the TWC (CB) developed under this situation, the CB also formed lee-eddy within the identical region of the OB’s eddy, resulting in CPT. Period for the CPT of the CB was shorter with increase of the OB volume transport. These results indicate that the OB acted to accelerate CPT of the CB within the TT. To support these results, observational data comprising mooring current, tidal, and CTD measurements obtained in and around the TT, along with the output from high-resolution data-assimilated ocean model DR_C were analyzed. Although the lee-eddy of the CBs was generated in every summer, the CPT events did not fully develop when the OB was absent over the continental slope because of the slow growth rate of the lee-eddy.

Anatomical study of Tsushima Warm Current system: Determination of Principal Pathways and its Variation

The Tsushima Warm Current (TWC) forms a chaotic picture of surface currents in the Sea of Japan. This study, based on satellite-based sea surface topography, was conducted to clarify the complicated figures by identifying the main pathways of the TWC system. We specifically examine four main branches based on the initial position at the entrance region and their downstream trajectories. In regions west of 133°E, the four currents mutually flowed independently, whereas the streamlines became scrambled at eastern longitudes. Our analyses revealed that the three branches of TWC, excluding the Subpolar Front Current, showed a confluence in the regions around the Oki Islands and off the Noto Peninsula. The seasonality of those current pathways is also addressed by this study. In the Yamato Basin, the TWC pathways, particularly offshore branches, were affected strongly by the existence and the location of developed mesoscale eddies, where basin-wide anticyclone during winter and cyclone during summer are frequently observed. In the eastern area of the Sea of Japan, the TWC branches showed notable features of meandering during winter, but it was straightforward during summer, mostly exiting towards the Pacific Ocean through the Tsugaru Strait.

Holocene sedimentation in the Hupo Trough of the southwestern East Sea (Japan Sea) and development of the East Korea Warm Current

Various sediment properties, such as mean grain size, total organic carbon, total nitrogen, C/N ratio, CaCO3, and biogenic opal content, were analyzed for a box core (BC02; 45 cm long) and a gravity core (GC02; 628 cm long), which were collected from the western margin of the Hupo Trough located off the eastern coast of Korea. The study area has been affected by the East Korea Warm Current (EKWC), a branch of the Tsushima Warm Current (TWC). The analytical results obtained for BC02 and the upper part of GC02 were in agreement, affirming the core-top preservation of GC02. Based on the corrected calibrated AMS 14C dates, the sedimentation rate of GC02 changed abruptly at ~8.2 ka from ~4.0–10.2 cm/kyr in the lower part to ~56.6–91.0 cm/kyr in the middle to upper part. This corresponds to the lithologic change from sandy mud to mud sediments showing the mean grain size change from 6.9 to 46.0 μm. Diverse paleoceanographic proxies representing the surface water condition exhibited varying degree of change at ~8.2 ka, after which all the properties remain almost unchanged, implying stable and continuous depositional conditions following the complete development of the EKWC. Furthermore, it indicated that the sediment depositional conditions in the Hupo Trough in response to the EKWC might have stabilized at ~8.2 ka since the opening of the Korea Strait during the Holocene sea level rise. Moreover, microfossil data from previous studies on the establishment of the TWC in the East Sea (Japan Sea) support our interpretation that the sediment properties revealed the Holocene development of the EKWC in the Hupo Trough.

Fluctuations in the abundance of chub mackerel in relation to climatic/oceanic regime shifts in the northwest Pacific Ocean since the 1970s

Abstract Chub mackerel (Scomber japonicus) in the northwest Pacific Ocean is generally divided into the Tsushima Warm Current (TWC) and Pacific stocks, based on habitat and migratory patterns. Since the 1970s, the abundance of the Pacific stock has undergone more extreme fluctuations than the TWC stock, which decreased sharply in the late 1970s and remained at low levels until it began to recover after the 2000s. In this study we focused on the Pacific stock, and analysed the variability in its abundance based on time series data for catch, biomass and recruitment. Sea surface temperature is considered as a major environmental factor that affects the wintering and spawning of chub mackerel. Consequently, we used wintering and spawning ground indices, including the average sea surface temperature (SST), the meridional positional deviance of the 15 °C (MPD15) and 18 °C (MPD18) isotherms and the SST-suitability weighted size of the potential spawning ground (WSSG) in Kuroshio Current waters to analyse the impacts of oceanic environmental conditions in the wintering and spawning grounds on variations in chub mackerel abundance. The wintering and spawning ground indices showed clear decadal variability, with abrupt changes in the late 1970s, the late 1980s and the 2010s. These were correlated with major fluctuations in the abundance of chub mackerel. A combination of correlation and gradient forest analyses showed that the thermal environment in the wintering ground were relatively important predictors of chub mackerel abundance; these predictors (with a 1- or 2-year time lag) may better explain the variations in abundance after the 1970s. Variations in the SST and MPD18 during the spawning period correlated well with the recovery of chub mackerel abundance after the 2000s. Both the Asian Monsoon Index and Southern Oscillation Index correlated with the wintering and spawning ground temperature conditions, indicating that large scale climatic factors have a major influence on variability in temperature in the wintering and spawning grounds of chub mackerel. We conclude that wintering and spawning ground indices for the Kuroshio Current region may be useful for explaining the variations in chub mackerel abundance.

Paleoceanographic insights on meridional ventilation variations in the Japan Sea since the Last Glacial Maximum: A radiolarian assemblage perspective

The Japan Sea is ideal for investigating deep water formation due to its unique topography and hydrography. However, because of the scarcity of reliable indicator and high-resolution ventilation records, the driving mechanisms behind ventilation changes in the entire Japan Sea since the Last Glacial Maximum (LGM) remain elusive. In this study, we analyze the radiolarian assemblage in three sediment cores from the southwestern, central, and northwestern Japan Sea to reveal ventilation variations over the last 25 ka. Our results suggest that weak ventilation was a widespread phenomenon in the Japan Sea during the LGM. This is inferred by the low relative abundances of species belonging into the Japan Sea Proper Water (JSPW) Assemblage, which are indicative of deep water ventilation in the Japan Sea. The JSPW Assemblage have gradually become the predominant species in studied cores from the last deglaciation to the early Holocene, suggesting an abrupt transition from oxygen poorer to oxygen richer conditions. All investigated regions show re-ventilation during the deglaciation, albeit at different times. The JSPW ventilation in the central Japan Sea started during the 19 ka Melt-water Pulse (MWP) and remained in an interglacial mode after MWP-1A (~14.7 ka), presumably due to the opening of the Tsushima Strait and intrusion of the Tsushima Warm Current (TWC). An abrupt increase in productive shallow water export during the Bølling-Allerød (B/A) was likely the key factor causing poorly oxygenated deep-water conditions in the southwestern Japan Sea. In contrast, the weakening ventilation in the northwestern Japan Sea during the B/A and Younger Dryas was probably caused by the blocking effect of the sea ice.

Numerical study on blue mackerel larval transport in East China Sea

Abstract An Individual-based model (IBM) for studying the blue mackerel larval transport in East China Sea (ECS), for the first time, is developed based on Ichthyop model. Coastal and Regional Ocean Community model (CROCO), a typical hydrodynamic model, is configured as well to provide Ichthyop with the physical variables (e.g. temperature, salinity, current). Effects of physical environments (current and temperature), spawning ground/depth/time and diel vertical migration (DVM) on aspects of the early life stage of blue mackerel, including connectivity between spawning ground and nursery area, temperature-dependent mortality, transportation and dispersal distance, are discussed in detail for the first time based on IBM. Two transport scenarios are considered in our simulations: (1) passive Lagrangian transport and (2) DVM. The experimental results show that the transport of the blue mackerel in ECS is principally controlled by the Kuroshio Current (KC) and Tsushima Warm Current (TSWC). Most of the larvae that affected by TSWC are mainly remained in ECS stock, while those controlled by KC are generally confined in the Pacific stock. In passive Lagrangian transport, the larvae from the largest spawning ground do not only remain in ECS stock but also drift to the Pacific stock in February. In travelling to the Pacific stock, the larvae will experience a long-distance trip and may spread away from the suitable growth area, which is not conducive to their survival. Whereas in DVM scenario, cases with different vertical migration mechanisms all have shorter dispersal distances comparing to the first scenario, the long-distance movement is significantly reduced during the simulating period, particularly in February. DVM tends to prevent the larvae entering the Pacific stock from the largest spawning ground in ECS, thus could increase the recruitment and survival for ECS stock.

New insights towards an integrated understanding of NE Asian monsoon during mid to late Holocene

Speleothem records from NE Asia are essential to understand the spatial patterns of the Asian Summer Monsoon (ASM) variability. In this paper, we present a new high-resolution and chronologically well-resolved speleothem record from Liu-li (LL) cave, NE China, through δ18O, δ13C and Sr/Ca ratio characterizing a dynamic ASM history over the past 6680 years. The LL δ18O record shows a long-term trend to higher values, following the North Hemisphere summer insolation (NHSI), with the exception of the last 2 kyr characterized by a negative δ18O excursion. The ‘2-kyr shift’ is evident in NE Asia over the past 2 kyr, originally inferred from the speleothem δ18O negative excursion in South China, as well as in many other proxy records from East China, but it merely manifests a flat trend in most of the western East Asian Summer Monsoon (EASM) and Indian Summer Monsoon (ISM) regions, rather than a negative excursion. On multi-centennial to millennial timescales, the intensification of the Tsushima Warm Current (TWC) can amplify rainfall on both sides of the Japan Sea. A close comparison between the monsoon reconstruction and the cultural changes in NE Asia suggests contemporary climatic and cultural regime-shifts, implying an important role of the ASM weakening in the collapse of Neolithic and Early Bronze cultures in this region.

Increased occurrence of red-tides of fish-killing dinoflagellate Karenia mikimotoi and related environmental conditions in Imari Bay, Japan

Blooms of the dinoflagellate Karenia mikimotoi cause mass mortality of aquaculture fishes and have become a worldwide environmental problem in coastal waters, especially in Asia. One area is Imari Bay, Japan, where blooms have recurrently formed in recent years. This study examines the meteorological and oceanographical conditions causing the recent increase in occurrence. The recent frequency of occurrence (2012–2018) is nearly four-fold previous years (1999–2011). In recent years, total nitrogen increased in the eastern area where the bloom initially develops, and The Tsushima warm current (TWC) have intensified during spring. Spring total nitrogen has increased as a result of climate change involving increasing rainfall and weakening monsoon rather than eutrophication because of the asynchrony between increasing and total nitrogen in rivers. Intensity of TWC changed in synchrony with M2 amplitude with lunar nodal cycle. Potential methodology for the medium term prediction of blooms is suggested in Imari Bay using environmental parameters.

Behavior of Water Mass Beneath the Tsushima Warm Current in the Japan Sea

To better understand the behavior of water mass beneath the Tsushima Warm Current (TWC), we use the vertical cross-sections of potential temperature, salinity and dissolved oxygen in the Japan Sea obtained by the T/V Oshoro Maru of the Hokkaido University during 8–29 June in 2011 to analyze its origins and variations. The results show that the potential temperature and salinity beneath the TWC varies little, but the dissolved oxygen varies largely with the geographical location. There are two deep water masses with different dissolved oxygen content below the TWC. One is on the coastal side with the low dissolved oxygen, and the other is on the offshore side with the high dissolved oxygen. It is inferred that the former one is relatively old water and the latter is the new water. By using the phosphate (PO4) and the apparent oxygen utilization (AOU) relationship, we calculate the PO40 (preformed PO4) as a water mass tracer. These results suggest that the water masses beneath the TWC with high and low dissolved oxygen originate from the same surface water mass in the central Japan Sea.