South Coast Of Japan Research Articles

An initial attempt to date Pleistocene marine terraces in the south coast of Japan using in situ cosmogenic 10Be and 26Al

The southern coasts of Shikoku Island and the Kii Peninsula are located along the Nankai Trough where the Philippine Sea Plate is subducting under the Eurasian plate in Japan. Relatively well-preserved marine terraces along the coast, with elevations ranging between ca. 50 and 100 m, are suitable counterparts to reconstruct the uplift history of the southwestern coast of Japan due to the unique tectonic setting. In this study, we attempt to determine the age of the marine terraces by employing in-situ terrestrial cosmogenic radionuclides (TCN), 10Be and 26Al. TCN depth profiles provide information on the timing of terrace formation. General agreement between our TCN chronology and the existing tephrochronology show that the TCN method can be applicable to a mid-latitude coastal setting where erosion must also be considered. Thus, measuring TCN on uplifted terraces around Japan can be a promising technique for clarifying uplift rates and the seismic history of the region.

Unusually high sea level at the south coast of Japan in September 2011 induced by the Kuroshio

Using a coastal assimilation model, generation mechanism of unusually high sea level (UHSL) at the south coast of Japan in September 2011 is investigated. Both model results and tide gauge observations indicate that sea level rise associated with the UHSL event occurred twice in the middle and end of September. The first one, which is localized around the eastern part of the Seto Inland Sea, is caused by a cyclonic circulation in the Kii Channel formed as a result of northward migration of the Kuroshio axis toward Cape Shionomisaki. The second sea level rise, which is the main contributor to this UHSL event, is observed in wide areas not only at the south coast of Japan, but also at the coast of the Japan Sea. It is brought about by a coastal trapped wave (CTW) induced as a result of a fluctuation of the Kuroshio path to the south of the Boso Peninsula. The CTW with positive SSH anomalies propagates westward along the south coast of Japan, and then goes into the coast of the Japan Sea. Sensitivity experiments and a modal characteristic analysis indicate that the CTW is mainly characterized by the first mode baroclinic Kelvin wave. The phase speed for the first mode is calculated at 2.96 m s^{-1}, which compares well with that estimated by tide gauge observations.

South-coast cyclone in Japan during El Niño-caused warm winters

La Nina conditions during boreal winter sometimes brings excessive snowfall in Japan, especially on the East Sea/Sea of Japan coastal and mountain areas through intensified northwesterly cold winds caused by La-Nina related atmospheric teleconnection. Meanwhile, snowfall events also increase in the Pacific coast area of Japan during the El Nino state due to extratropical cyclones passing along the south coast of Japan (hereafter referred to as South-coast cyclone). In the present study, we investigated year-to-year snowfall/rainfall variations based on meteorological station data and cyclone tracks identified by using the Japanese 55-year Reanalysis. The result clearly indicates increase of the South-coast cyclone during El Nino-developing winters, which is consistent with excessive snow-fall in the northern part of the Pacific coast. Strong subtropical jet hampers cyclogenesis due to less vertical interaction through the trapping of upper-level eddies. During El Nino-developing winters, the subtropical jet is weakened over East Asia, indicating dynamic linkage to increased cyclone frequency. In addition to this, both the deepening of the upper-tropospheric trough over East Asia and anomalous low-tropospheric northwest anticyclones extending from the Philippines toward Japan are also consistent with the enhancement of cyclogenesis over the East China Sea as well as warm winter in Japan.

Simulating the Oceanic Migration of Silver Japanese Eels.

The oceanic migration of silver Japanese eels starts from their continental growth habitats in East Asia and ends at the spawning area near the West Mariana Ridge seamount chain. However, the actual migration routes remain unknown. In this study, we examined the possible oceanic migration routes and strategies of silver Japanese eels using a particle tracking method in which virtual eels (v-eels) were programmed to move vertically and horizontally in an ocean circulation model (Japan Coastal Ocean Predictability Experiment 2, JCOPE2). Four horizontal swimming strategies were tested: random heading, true navigation (readjusted heading), orientation toward the spawning area (fixed heading), and swimming against the Kuroshio. We found that all strategies, except random swimming, allowed v-eels swimming at 0.65 m s−1 to reach the spawning area within eight months after their departure from the south coast of Japan (end of the spawning season). The estimated minimum swimming speed required to reach the area spawning within eight months was 0.1 m s−1 for true navigation, 0.12 m s−1 for constant compass heading, and 0.35 m s−1 for swimming against the Kuroshio. The lowest swimming speed estimated from tracked Japanese eels at sea was 0.03 m.s−1, which would not allow them to reach the spawning area within eight months, through any of the tested orientation strategies. Our numerical experiments also showed that ocean circulation significantly affected the migration of Japanese v-eels. A strong Kuroshio could advect v-eels further eastward. In addition, western Pacific ocean currents accelerated the migration of navigating v-eels. The migration duration was shortened in years with a stronger southward flow, contributed by a stronger recirculation south of Japan, an enhanced subtropical gyre, or a higher southward Kuroshio velocity.

Tilt of mean sea level along the Pacific coasts of North America and Japan

AbstractThe tilt of coastal mean sea level with respect to an equipotential surface is estimated using two fundamentally different approaches. The geodetic approach is based on tide gauge and GPS observations, and a model of the geoid. The ocean approach uses a high‐resolution, dynamically based ocean model to estimate mean dynamic topography. Along the Pacific coast of North America the two approaches give similar large‐scale profiles with a minimum at about 40°N and a maximum in the northern part of the Gulf of Alaska. Along the Pacific coast of Japan the geodetically determined coastal sea levels indicate an eastward drop of about 20 cm along the south coast and a further northward drop across Tsugaru Strait. Both of these features are reproduced by the ocean models. An analysis of the alongshore momentum balance suggests that alongshore wind stress acting over the inner shelf is the primary driver of the mean sea level profile along the coast of North America. Several large‐scale features are explained using arrested topographic wave theory. A similar momentum analysis, and an additional study of time variability of sea level and circulation, suggests that the Kuroshio is the main driver of the mean sea level tilt along the south coast of Japan. Discrepancies in the alongshore tilt of sea level estimated by the geodetic and ocean approaches along both coasts are discussed in terms of errors in the ocean and geoid models.

Development of a Four-Dimensional Variational Assimilation System for Coastal Data Assimilation around Japan

Abstract The authors have developed an assimilation system toward coastal data assimilation around Japan, which consists of a four-dimensional variational (4DVAR) assimilation scheme with an eddy-resolving model in the western North Pacific (MOVE-4DVAR-WNP) and a fine-resolution coastal model covering the western part of the Japanese coastal region around the Seto Inland Sea (MOVE-Seto). The 4DVAR scheme is developed as a natural extension of the 3DVAR scheme used in the Meteorological Research Institute Multivariate Ocean Variational Estimation (MOVE) system. An initialization scheme of incremental analysis update (IAU) is incorporated into MOVE-4DVAR-WNP to filter out high-frequency noises. During the backward integration of the adjoint model, it works as an incremental digital filtering. MOVE-Seto, which is nested within MOVE-4DVAR-WNP, also employs IAU to initialize the interior of the coastal model using MOVE-4DVAR-WNP analysis fields. The authors conducted an assimilation experiment using MOVE-4DVAR-WNP, and results were compared with an additional experiment using the 3DVAR scheme. The comparison reveals that MOVE-4DVAR-WNP improves mesoscale variability. In particular, short-term variability such as small-scale Kuroshio fluctuations is much enhanced. Using MOVE-Seto and MOVE-4DVAR-WNP, the authors also performed a case study focused on an unusual tide event that occurred at the south coast of Japan in September 2011. MOVE-Seto succeeds in reproducing a significant sea level rise associated with this event, indicating the effectiveness of the newly developed system for coastal sea level variability.

On the genesis of cold layer in the Kanto Plain accompanied by cyclones passing along south coast of Japan

On the genesis of cold layer in the Kanto Plain accompanied by cyclones passing along south coast of Japan

Rainfall Tendency in Winter Sugadairakogen Highlands, Nagano Prefecture

The temporal tendency of winter rain events in the Sugadairakogen Highlands was investigated using 33 years daily records of rainfall/snowfall discrimination at the Sugadaira Montane Research Center, University of Tsukuba, and the characteristics of atmospheric circulation in relation to the precipitation-phase changes were analyzed for 12 precipitation events by simple laser-type disdrometer observations with numerical weather simulations. The frequency of rainy days was 12 % from December to February, and 70 % of the rainy days were accompanied with traveling extratropical cyclones. A long-term increase of the snowfall frequency by extratropical cyclones was observed from November to April. The rainy days did not show a long-term trend. However, the large year-to-year variability of rainy days was significantly correlated with the index of the Pacific Decadal Oscillation. The year with an abrupt increase in the number of rainy days corresponded with the warm winter with less accumulated precipitation amount when the snow cover structure in high elevation areas is likely to be affected by the occurrence of rain. In the five rainfall-only cases and seven rainfall and snowfall cases identified in three winter seasons, rainfall dominant events were accompanied with broad warm-air mass expansion covering Honshu area with various courses of extratropical cyclones. On the other hand, rainfall with snowfall events were accompanied with a cyclone passing along the south coast of Japan or long-ranged cold front, and the precipitation phase changed mostly from rain to snow. In Sugadaira, southerly winds were suppressed before the phase change, and the air temperature stayed around 0 °C. A numerical simulation confirmed that weakening of the wind was due to the barrier effect behind the Echigo mountain range with prevailing east-west component winds by the traveling cyclone. A numerical model successfully simulated the rainfall-only events because the synoptic-scale circulation controlled the broad warm air migration. However, cases of a precipitation shift from rainfall to snowfall were not fully simulated. The causes of the discrepancies between the observation and simulation were discussed from the viewpoint of the low-level air-mass exchanges in the large valleys running in the south-north direction and the occurrence of Foehn in the lee-sides of mountain ranges accompanied with a traveling cyclone.

Ocean acidification off the south coast of Japan: A result from time series observations of CO2parameters from 1994 to 2008

[1] Ocean acidification resulting from increases in present and future atmospheric CO2 levels could seriously affect diverse coastal and oceanic ecosystems. In this work, we determine that a significant trend in ocean acidification is superposed on the large seasonal and interannual variabilities of acidity in surface waters off the south coast of Honshu, Japan, based on our repeated observations of partial pressure of CO2 (pCO2), total inorganic carbon (TCO2), and pH. Multiple regression analysis of TCO2 as a function of temperature, salinity, and timing of observations shows that TCO2 increased at a rate of +1.23 ± 0.40 μmol kg−1 yr−1 for the period 1994–2008, while no long-term change has been determined for total alkalinity calculated from TCO2 and pCO2 in seawater. These results indicate that pH and the aragonite saturation state (Ωarag) are decreasing at a rate of −0.020 ± 0.007 decade−1 and −0.12 ± 0.05 decade−1, respectively. If future atmospheric CO2 levels keep increasing as predicted by the Intergovernmental Panel on Climate Change emission scenario A1FI, which postulates intensive fossil fuel use associated with very rapid economic growth, a further reduction of −0.8 to −1.0 in Ωarag is likely in the next 50 years. Such a rapid reduction of Ωarag could have negative impacts on a variety of calcareous organisms.

Coastal Disturbance in Sea Level Propagating along the South Coast of Japan and Its Impact on the Kuroshio

The coastal sea level propagating westward along the south coast of Japan and the impact of the disturbance on the generation of the Kuroshio small meander have been examined. The propagation occurs in sea level variations for periods shorter than 10 days and is remarkable for periods of 4–6 days. Characteristics of the 4–6 day component have been studied using the extended empirical orthogonal function (EEOF). The first and second modes of EEOF are almost in-phase throughout the south coast of Japan. The higher four modes of EEOF are significantly excited when the Kuroshio takes the non-large-meander path, and propagate westward with phase speeds of 2.8 m s−1 (third and fourth modes) and 1.6 m s−1 (fifth and sixth modes) in the Kuroshio region west of Mera in the Boso Peninsula. The analysis shows that more than 70% of the small meanders generate in two months after a significant propagating disturbance reaches south of Kyushu when the velocity of the Kuroshio is high. This effect of coastal disturbance is examined by numerical experiments with a 2.5-layer model in which coastal disturbance is excited by vertical displacement of the upper interface. The result is that offshore displacement of the Kuroshio occurs southeast of Kyushu only in the case of significant upward displacement of the interface under the influence of a high Kuroshio velocity. The significant coastal disturbance, which is associated with upward displacement of the density interface, and a high Kuroshio velocity can therefore be important factors in generating small meanders.

Influence of the Kuroshio Fluctuations on Sea Level Variations along the South Coast of Japan

The correlation between the Kuroshio and coastal sea level south of Japan has been examined using the altimetry and tide gauge data during the period 1992–2000. The sea level varies uniformly in a region bounded by the coast and the mean Kuroshio axis, which stretches for several hundred kilometers along the coast. These variations are related with the Kuroshio velocity, as coastal sea level decreases (or increases) when the Kuroshio is faster (or slower). To the east of the Kii Peninsula, where sea level variations are different from these to the west, movement of the Kuroshio axis additionally affects coastal sea level variations.

Distribution of the oceanic insects Halobates (Hemiptera: Gerridae) off the south coast of Japan

AbstractSpecimens of ocean skaters Halobates were collected off the south coast of Japan in the East China Sea in 1995, and from the Kumano‐nada Sea to the East China Sea in 1998 and 1999. Three species were identified: H. micans, H. germanus and H. sericeus. We found two species co‐occurring in comparable densities in different years, a phenomenon not hitherto reported in other regions of the ocean. We discuss distributions of the three Halobates species with special reference to the influence of the Kuroshio Current, temporal variations of sea‐surface temperature, and monsoonal winds.

Monitoring of Generation and Propagation of the Kuroshio Small Meander Using Sea Level Data along the Southern Coast of Japan

The occurrence of the small meander of the Kuroshio, generated south of Kyushu and propagating eastward, was examined using sea level data collected during 1961–1995 along the south coast of Japan. Intra-annual variation of the sea level was expanded by the frequency domain empirical orthogonal function (FDEOF) modes, and it was found that the second and third modes are useful for monitoring the generation and propagation of the small meander. The third FDEOF for periods of 10–100 days has a phase reversal between Hosojima and Tosa-shimizu with significant amplitude west of Kushimoto, and the amplitude of its time coefficient is large during the non-large-meander (NLM) period and has a significant peak when the small meander exists southeast of Kyushu. The second FDEOF for periods of 20–80 days has a phase reversal between Kushimoto and Uragami, and the amplitude of its time coefficient is large when the small meander propagates to the south of Shikoku. The third FDEOF mode allowed us to conclude that the small meander occurred 42 times from July 1961 to May 1995, most of them (38) occurring during the NLM periods. The second FDEOF mode permits the conclusion that half of the 38 small meanders reached south of Shikoku. Of these, five small meanders influenced transitions of the Kuroshio path from the nearshore NLM path; one caused the offshore NLM path and four brought about the large meander. About one-tenth of the total number of small meanders are related to the formation of the large meander.

Analysis of an Intense Winter Extratropical Cyclone that Advanced along the South Coast of Japan.

A winter extratropical cyclone underwent rapid development along the south coast and east of Japan on 12 February 1994. The evolution and structure of this cyclone were investigated from its formation through the rapid developing stage using subjective surface re-analyses, objective analyses for upper levels, satellite images and other data. The cyclone formed over eastern China within a broad low-level baroclinic zone. A cold air mass covered the East China Sea and around Japan behind a preceding cyclone, while strong southerly winds brought warm, moist air above and near the incipient cyclone. The surface cyclone progressed eastward over the East China Sea with a moderate deepening rate. It then began developing rapidly south of Japan, over the warm Kuroshio Current. A deep mid-tropospheric trough approached the surface low from northwest and the cyclone extended horizontally as well as vertically. A subtropical jet streak and an associated upper-tropospheric PV anomaly advanced from the rear of the surface low. This cyclone had a cloud system of about 2000 km scale. Active cloud areas were found mostly to the north and east sides of the surface low. The incipient cyclone lacked well-defined surface fronts. As it advanced over the East China Sea, frontogenesis proceeded rapidly to form a warm front with appreciable temperature gradient, which extended zonally from the cyclone center. A low-level cold air flow, corresponding to a cold conveyer belt, occupied the northern side of the cyclone. Cold frontgenesis in terms of thermal fields was inactive to the rear of the cyclone center. A strong wind shear line extended in a north-south direction from the low center, between the southerly winds to the east and the westerly winds to the west. The shear line was associated with weak temperature gradient. As the cyclone developed the warm front extended westward relative to the cyclone center to form a bent-back front. The bent-back front was accompanied by significant wind shear between the northeasterlies to the north and westerlies to the south. The frontal system of the developing cyclone thus exhibited a characteristic configuration that differs from the classical conceptual cyclone model.

ADCP Repeat Tracking in Irago Suido, South Coast of Japan

ADCP Repeat Tracking in Irago Suido, South Coast of Japan

黒潮と日本人の遭遇史 （第一部） 中世のころまでの和書と近世までの漢籍にもとづく論考

Since ancient times the Japanese had encountered the Kuroshio frequently. For instance some records about long-distance driftings along the south coast of Japan or across the East China Sea are found in Nihon Shoki (Official Record of Japan before 697), Shoku Nihongi (Official Record of Japan from 697 to 806) and Todai Wajo Tosei Den (Biography of Ganjin, an outstanding Chinese Buddhist). In the seventh and eights centuries, some ships of the Japanese envoy to China in the Tang Dynasty crossed the Kuroshio in the East China Sea, despite frequent incidents of ship wrecks. In the Momoyama and early Edo periods (1592-1635) the Shuinsen, Japanese ships with the Shogunate license for foreign trade, crossed the Kuroshio east of Taiwan bound for Luzon. Nevertheless, no mention of the Kuroshio was found in these periods. Since records of no mention of the Kuroshio in maritime literatures in old days will be helpful in this kind of study in the future, efforts are made to list a detailed bibliography. Finally, studies on terms "Rakusei" and "Biryo", the Chinese equivalents of "Kuroshio", are made on the basis of Chinese classics. The term "Rakusei" was cited in Japanese books in the Edo period, but the term "Biryo" was too legendary to be used in Japan as indicating the Kuroshio, although Needham et al. (1971) made extensively a historical study on "Biryo".

Kuroshio path variation south of Japan: 1. Barotropic inflow‐outflow model

We examined the dynamics of the variation in the path of the Kuroshio current along the south coast of Japan by use of a barotropic inflow‐outflow ocean model. The model basin has an inclined coastline and includes a part of the East China Sea so that we could investigate the effect of Kyushu on the Kuroshio. We obtained three regimes of the Kuroshio's path variation according to the magnitude of its inlet velocity Vmax, similar to the previous study. The Kuroshio takes a straight path along the south coast of Japan in regime I with a low Vmax, a meandering path in regime II with a high Vmax, and both of these paths (multiple‐equilibrium state) in regime III with an intermediate Vmax. The transition from the straight state to the meandering state occurs with an increase in Vmax from regime I (III) to II, preceded by an eastward progression of a small meander which is produced as a separated vortex by the nonlinear effect of the flow and the effect of Kyushu. It takes 120 days, consistent with observations. The nonlinearity of the flow has two effects in the transition. One causes a downstream shift of the small meander, and the other enlarges it by pulling out positive vorticity from the no‐slip coast. With an increase in the meander scale due to the latter effect, the former effect becomes small to be balanced with the beta effect, causing an upstream shift of the meander. Thus a large‐meander path is formed. A small meander does not progress eastward to develop into a large meander until Vmax exceeds a critical value. This behavior causes a multiple‐equilibrium state in Regime III and only the meandering path in Regime II.

Sea level changes south of Japan associated with the non-large-meander path of the Kuroshio

Sea levels south of Japan from 1964 to 1975 are examined in terms of the nearshore and offshore non-large-meander (NLM) paths of the Kuroshio and the transitions between them. The sea-level anomalies from the annual variations on the south coast of Japan are much larger during the transition from the nearshore to offshore NLM paths than during the reverse transition by 9 cm on average. This characteristic can be seen only in the coastal region of the Kuroshio-flowing area, so that the sea-level difference of Naze minus Nishinoomote (indicator of Kuroshio velocity) during the offshore to nearshore transition is larger by 15 cm than during the reverse transition. The transition from the offshore to nearshore NLM paths occurs when the velocity of the Kuroshio is large or increasing, while the nearshore to offshore transition occurs when it is small or decreasing. The former transition occurs whenever the velocity increases greatly, whereas the latter one does not always occur even though the velocity decreases. The sea-level difference between Kushimoto and Uragami is highly coherent with the alternate appearance of the nearshore and offshore NLM paths. Offshore NLM paths longer than 2.5 months appear during large falls of the sea-level difference of Kushimoto minus Uragami, while large rises of the sea-level difference correspond to long-lasting nearshore NLM paths. The mean sea-level difference during the nearshore NLM path is larger by 4 cm than that during the offshore NLM path.

Interannual correlations between sea level difference at the south coast of Japan and wind stress over the north pacific

Relationships of the sea level differences between Naze and Nishinoomote and between Kushimoto and Uragami with wind stress over the North Pacific are examined for interannual variability. These sea level differences are considered to be indications of Kuroshio transport in Tokara Strait and Kuroshio path south of Enshu-nada, respectively. In the sea level difference between Kushimoto and Uragami, dominant variations are found to have periods of about seven years and 3–4 years. The variation of about 7-year period, which corresponds to that in the Kuroshio path between the large meander and non-large meander, is coherent with the variation of the wind stress curl in a region about 2,400 km east of the Kii Peninsula, where negative stress curl weakens about two years before the sea level difference drops (i.e. the large meander path in the Kuroshio generates). The variation of the 3–4 year period is coherent with that of the wind stress in a large area covering the eastern equatorial Pacific, which suggests that it links with global-scale atmospheric variations. Interannual variation in sea level difference between Naze and Nishinoomote is not coherent with that between Kushimoto and Uragami, which suggests that it is not related to the variation of the Kuroshio path south of Enshu-nada, but is coherent with that of the zonally-integrated Sverdrup transport in the latitudinal zone along 30°N. It is suggested that the interannual variation of the Kuroshio transport in Tokara Strait can be explained by the barotropic response to the wind stress.

Spectral properties of sea level and time scales of Kuroshio path variations

Spectral properties of sea levels at Naze, Nishinoomote, Kushimoto, Uragami, Miyake-jima and HachijO-jima are examined for the non-large-meander (February 1964 – May 1975) and large-meander (October 1975 – December 1979) periods, and the periodicity of variation of the Kuroshio path is clarified. The large meander of the Kuroshio occurs with a primary period of about 20 years and secondary period of 7 to 8. 5 years. During the non-large-meander period, the Kuroshio alternately takes the nearshore and offshore non-large-meander paths with a primary period of 1. 6–1. 8 years. This variation is moreover composed of 110-day, around 195-day and annual periods. The 110-day variation of the Kuroshio path appears to have influence on the coastal sea levels between the Kii Peninsula and the Izu Ridge;i. e., the coastal sea levels rise and fall with one-month time lag after the Kuroshio has begun to approach and leave the Japanese coast. During the large-meander period, the 70 and 110-day variations are remarkable in sea levels south of Japan except Miyake-jima and HachijO-jima. The 70-day variation is highly coherent throughout the south coast of Japan; the coherent area of the 110-day variation seems to be smaller. The sea-level variations at Naze and Nishinoomote are not significantly coherent for any of the periods except for annual and semiannual cycles during both the non-large-meander and large-meander periods. That is, the sea-level variations are incoherent between the onshore and offshore sides of the Kuroshio, except for seasonal variation.