Kuroshio Countercurrent Research Articles

Does the Kuroshio Current transport planktonic larvae of the hydrothermal-vent crabXenograpsusTakeda & Kurata, 1977 (Decapoda: Brachyura: Grapsoidea)?

AbstractThe grapsoid crab XenograpsusTakeda & Kurata, 1977 inhabiting shallow-water hydrothermal vents shows a limited distribution in the western Pacific, thus providing a good opportunity to observe how a species expands its distribution and establishes and maintains a metapopulation. We analyzed four Japanese populations of Xenograpsus (Kueishan-dao, Showa Iwo-jima, Shikine-jima, and Omuro-dashi) distributed along the Kuroshio Current to assess whether population connectivity is affected by the current. The reconstructed phylogenetic tree indicated that the specimens analyzed belong to a single clade, including X. testudinatusNg, Huang & Ho, 2000 in Kueishan-dao. We could not include genetic data of X. novaeinsularisTakeda & Kurata, 1977 and tentatively refer to the specimens analyzed as X. testudinatus. All four populations shared the two major haplotypes. Statistically significant population differentiation was nevertheless recognized between Showa Iwo-jima and Omuro-dashi, about 1,000 km apart. The number of individuals analyzed in Shikine-jima (N = 11) and Kueishan-dao (N = 5) might not be enough to detect differences. The relative age of population expansion seemed to correlate with the direction of the Kuroshio Countercurrent. Together with knowledge on the distribution of the planktonic larvae, larvae of X. testudinatus stay and mostly contribute to its source population. The Kuroshio Current system occasionally transports the larvae and juveniles, or even adult individuals on floating pumice and other materials, to eventually settle into newly formed habitats after volcanic eruptions, thus expanding the distributional range of the species.

Anatomy and dynamics of a mixed contourite sand sheet, Ryukyu Island Arc, northwestern Pacific Ocean

Contourites are well-known from many continental margins under the influence of bottom currents but have been little reported from the Pacific Ocean. This paper documents a new area of contourite-controlled sedimentation in the NW Pacific Ocean, which we call the Ryukyu Sand Sheet. This contourite sand sheet has an area of around 35,000 km2 and extends from the narrow island shelves to over 1500 m water depth. It comprises mainly moderate to well-sorted fine-grained sands, with current ripples and giant sediment waves and is also associated with small drifts. It is formed under the influence of three principal current systems – the Kuroshio Current, the Kuroshio Countercurrent and the Ryukyu Current. The interaction of these currents with each other and with a complex seafloor topography, spawns a series of meso-scale gyres, eddies and vortices that shape the seafloor and lead to deposition of an extensive sandy substrate, locally with gravels and exposed seafloor. Strong surface currents, as well as deep-water thermohaline circulation, both influence the depositional and erosional processes of deep-sea sediments. The role of the modern Kuroshio Current in this context supports earlier work that proposed an ancestral Kuroshio Current for the deposition of Miocene contourites onshore Japan. Sediment supply to the Ryukyu Sand Sheet is by a mixed process of seafloor polishing and sand spillover that involves combined oceanographic and gravitational processes.

Variations in sedimentary environments in the forearc and backarc regions of the Ryukyu Arc since 25 ka based on CNS analysis of sediment cores

In this study, we analyzed 14C dates, grain size, and carbon, nitrogen, and sulfur element contents using sediment cores collected at the eastern edge of the backarc and forearc of the Ryukyu Arc, and demonstrated different temporal variations in CaCO3 contents and ratios of total organic carbon and total nitrogen contents (C/N ratios) compared with previous studies of the Okinawa Trough cores. Increases in CaCO3 contents and decreases in C/N ratios, from the Last Glacial Maximum to the Holocene, in the Okinawa Trough backarc basin correspond with sea-level changes. These results demonstrate that the sedimentary environment in the Okinawa Trough is influenced by sediment supply from mainland China, and reveal temporal variations reflecting changes in the Yangtze River mouth that accompany sea-level changes. In contrast, CaCO3 contents of cores in this study are double those in the Okinawa Trough cores, and show an increasing trend at 7.5–6.0 cal ka BP. Additionally, in the forearc core, the profile of the CaCO3 content decreases at 11.5–8.1 cal ka BP. These variations in CaCO3 contents suggest that marine productivity changed with sea surface trophic conditions. In addition, the lateral transport of terrigenous sediment by the Kuroshio and Ryukyu currents might contribute to an increased CaCO3 content by reducing the influence of dilution. Variations in the C/N ratio after 7.8 cal ka BP suggest that the Kuroshio Countercurrent has varied in strength at 1–2 ka intervals since the early Holocene, influencing the deposition of terrigenous organic matter from the Okinawa Islands in the deeper parts of the east backarc.

Trans-Pacific dispersal of loggerhead turtle hatchlings inferred from numerical simulation modeling

We used Lagrangian numerical simulations to examine the trans-Pacific dispersal processes of loggerhead turtle hatchlings. Ten thousand simulated particles were released from each of the three nesting regions in Japan and tracked for 5 years. Results showed many particles moving eastward, drifting in the Kuroshio Current followed by the Kuroshio Extension Current. However, no particles reached Baja California, a known feeding area, through passive processes, indicating that trans-Pacific transportation requires active swimming by turtles. The duration of the trans-Pacific dispersal was estimated to be at least 1.6–3.4 years, with some turtles drifting in the Kuroshio Countercurrent and remaining in the western Pacific even after 5 years. This indicates that as revealed by previous genetic studies, not all loggerheads always disperse along a trans-Pacific route. The findings showed that survival and expected growth rates varied widely according to ambient temperatures during drifting, which in turn depended on nesting location.

Temporal variations of the net Kuroshio transport and its relation to atmospheric variations

Temporal variations of the net Kuroshio transport are investigated using long-term hydrographic data from repeat section of the 137°E meridian from the south of Japan (34°N) to New Guinea (1°S) conducted by the Japan Meteorological Agency. In this study, boundaries of the Kuroshio and the Kuroshio Counter Current (KCC) are defined based on the sea surface dynamic height distribution. Westward flows associated with the KCC and cold-core eddy north of the Kuroshio are removed from the eastward flow associated with the Kuroshio in order to estimate the net Kuroshio transport strictly. The net Kuroshio transport reveals low-frequency variations: significant signals on a decadal (about 10-year) timescale. The variations of net Kuroshio transport are predominantly caused by changes in the magnitude of oceanic current speed fields associated with a vertical movement of the main pycnocline depth around the southern boundary of the Kuroshio: deepening of the main pycnocline around the southern boundary of the Kuroshio forms a sharp northern upward-tilting slope of the isopycnal surfaces at the Kuroshio region, and eventually the net Kuroshio transport increases together with the Kuroshio eastward transport. By using a wind-driven hindcast model, it is found that the main pycnocline depth variation results from the first-mode baroclinic Rossby waves attributable to two types of Aleutian Low (AL) changes: a change in the magnitude of AL and meridional movement of AL.

Radiocarbon marine reservoir ages in the western Pacific estimated by pre-bomb molluscan shells

Apparent 14C ages of the pre-bomb molluscan shells show new estimations of ΔR values for the western Pacific including the East Sakhalin Current, Oyashio Current, Kuroshio Current, Kuroshio Countercurrent, and Pacific North Equatorial Current. Clear differences in ΔR between the subarctic and subtropical gyres water masses around the Japanese archipelago are observed.

The eastward subtropical countercurrent on isopycnal surface in the western North Pacific

The monthly circulations on isopycnal surface in the western North Pacific are calculated based on the Navy's Generalized Digital Environmental Model climatological tem- perature and salinity data on a 1/2° × 1/2° grid using a P-vector method. The eastward Sub- tropical Countercurrent (STCC) in the central subtropical gyre has been studied with emphasis on its spatial distribution, vertical structure, volume transport and its nearby eddy phenomenon. The results reproduce the observed feature of STCC, and reveal some new phenomena on isopycnal surface, such as: (1) At σθ = 24.6, there is strong seasonal variability in the distribution, origin and flow status of STCC. There is no STCC in January, February and March. (2) From May to October, a branch of the Kuroshio Countercurrent to the south of Japan merges into STCC, which increases the velocity of STCC and widens its width. (3) The velocity vector field shows that STCC and nearby eddies coexist. (4) The eastward volume transport of STCC across 137.5°E is the strongest in summer with the maximum of 6.9 Sv ( 1 Sv ≡ 10 6 m 3 s −1 ) in August, and weaker in spring and autumn with 1.7 Sv in April and 1.6 Sv in November. Most of the transports of STCC occur between isopycnal surfaces 24.0σθ and 25.0σθ. (5) The distributions of the zonal flow along 137.5°E show that the flow core of STCC is between 24.3σθ and 24.7σθ .

Distribution patterns of larval myctophid fish assemblages in the subtropical-tropical waters of the western North Pacific

We examined larval myctophid fish assemblages and their distribution patterns, based on discrete depth sampling. Samples were collected at 19 stations along a transect that crossed the subtropical–tropical waters of the western North Pacific. In total, we collected 27 189 larvae of 40 myctophid species or types, belonging to 15 genera. Three assemblages were recognized, based on their species composition: Kuroshio Axis (KuA), Kuroshio Countercurrent (KCC), and Subtropical Countercurrent–North Equatorial Current (SCC–NEC) assemblages. The distributions of these assemblages were well defined by the positions of the KuA and the Subtropical Convergence. Each species had a specific distribution depth, and none showed diel vertical migration. Larvae of the subfamily Lampanyctinae were distributed in shallower waters (0–50 m) than larvae of the subfamily Myctophinae (50–150 m). Larvae of the same species were distributed at lower depths in the SCC–NEC area than in the KCC area. This corresponded to the abundance of chlorophyll a, which would reflect abundance of prey organisms such as copepod nauplii and copepodites.

Summertime Subtropical Countercurrent on isopycnals in the western North Pacific

Circulations on isopycnals (σθ) in the western North Pacific were investigated by using P-vector method; the data were taken from the U.S. Navy’s climatological temperature and salinity dataset (public domain) with 1/2° ×1/2° resolution. Results not only show the main circulation systems on isopycnals in the western North Pacific such as the North Equatorial Current (NEC), Kuroshio and Kuroshio Countercurrent, but also reveal the Subtropical Countercurrent (STCC) clearly. In this note we pay attention to discussing the distribution of STCC in summer (in June). The STCC flows eastward along a winding road; on shallow isopycnals, the STCC originates from the area east of Bashi Strait at about 122.5°E; with the isopycnals increasing, the origin and flow core of STCC move to north and east, but the main part of STCC is still between 18° and 23.5°N, i.e. near the Tropic of Cancer. There exists STCC on all isopycnals between the sea surface and 25.8 σθ.

Distribution of late-stage phyllosoma larvae of Panulirus japonicus in the Kuroshio Subgyre

The present study was undertaken in waters south of the Kuroshio Subgyre to examine and/or test Sekiguchi’s (1985, 1997) hypothesis about the larval recruitment processes of Panulirus japonicus by which the benthic populations of the species are maintained in Japanese waters. A total of 61 Panulirus phyllosoma larvae were collected in early summer; 56 belonged to P. japonicus, of which 30 were in the final stage, 24 in the subfinal stage, and 2 were too heavily damaged to permit identification of the stage. Most of the final-stage phyllosoma larvae were found in the northern part of the waters east of the Ryukyu Archipelago, whereas the larvae in the subfinal stage were found in the southern part. The present study supports Sekiguchi’s hypothesis that late-stage P. japonicus phyllosoma larvae are transported by the Kuroshio Countercurrent into waters east of the archipelago and then again enter the Kuroshio Current.

The Subtropical Mode Water Circulation in the North Pacific

Abstract Historical hydrographic data are analyzed to investigate the seasonal variation of the subsurace pycnostad in the northwestern part of the North Pacific and to relate it to the circulation of subtropical mode water (STMW). A gyre-scale scheme of STMW circulation is deduced based on its observed climatological formation area and climatological geostrophic flow fields. The deduced circulation is supported by climatological features of the subsurface hydrography presented separately for the non-large-meander period and the large-meander period of the Kuroshio: zonal and meridional sections and isopycnal maps of potential vorticity and apparent oxygen utilization. The Kuroshio Countercurrent composing the Kuroshio recirculation system advects STMW formed in the wintertime thick mixed layer immediately off the Kuroshio and the Kuroshio Extension. During the non-large-meander period, the recirculation system has a single anticyclonic gyre centered near 30°N, 137°E and advects STMW formed off the Kurosh...

Subtropical Mode Water south of Honshu, Japan in the spring of 1988 and 1989

The distribution and characteristics of Subtropical Mode Water (STMW) south of Honshu, the main island of Japan, were investigated using CTD, XBT, and dissolved oxygen data taken by the research vessels in the spring of 1988 and 1989. A comparatively low inventory of STMW was shown in spring 1988 during the large-meander period of the Kuroshio south of Honshu, while in spring 1989 during the non-large-meander period, the observation showed a considerable inventory of STMW which had outcropped east of 140°E in the preceding winter. These observations, together with published temperature maps, surface current charts, time series of vertical temperature profiles along 140°E, and wintertime Monsoon Index consistently support the climatology of the STMW circulation recently presented by the authors. That is, the change of the Kuroshio Countercurrent associated with the large meander of the Kuroshio most likely cuts off the westward/southwestward advection of STMW from its formation area east of 140°E.

A numerical study on the formation of the Kuroshio Counter Current and the Kuroshio Branch Current in the East China Sea

The Kuroshio Counter Current (KCC) and the Kuroshio Branch Current (KBC) are two unique features of the Kuroshio in the East China Sea. The mechanisms that generate the KCC and the KBC are studied using a barotropic inflow-outflow model with a simplified basin configuration of the East China Sea. The present study shows that the KCC can only exist on a β-plane frame of reference and its flow pattern is independent of the offshore Ryukyu Islands. In the East China Sea, the continental slope plays the role of a western boundary for the Kuroshio. Whether the Kuroshio protrudes onto the slope region, however, is found to have little influence on the KCC. Concerning the formation of the KBC, we found that the planetary β-effect and the existence of Taiwan Island are two indispensible conditions: the planetary β-effect drives part of the Kuroshio inflow to branch southwestward and Taiwan Island blocks this branched current causing it to protrude onto the continental shelf. Based on the numerical calculation, we further found that the branch current is reinforced by topographic Rossby waves induced by the repeated crossing of the Kuroshio over the continental slope.

Interannual variability of the Kuroshio Extension and its relation to the Southern Oscillation/El Niño

The interannual variability of the temperature structure of the Kuroshio Extension is studied by establishing time series for the period 1950 to 1970 and then comparing it with the time series of sea level differences across the North Equatorial Current obtained by Wyrtki (1975). First, the present analysis shows a significant correlation between the interannual fluctuation of the Kuroshio Extension and the eddy activity south of the Kuroshio axis, suggesting the importance of the eddy-driven mechanism. Secondly, spectral analysis shows close connections between the Kuroshio Extension and the North Equatorial Current with a reasonable time lag of about 1.5 years. This time lag of the mid-latitude variability is also supported by other independent data. In particular, the present preliminary study strongly suggests that the bimodal behavior of the Kuroshio path south of Japan and the intensity of the Kuroshio Countercurrent are closely connected with the Southern Oscillation/El Nino.

Large‐scale seasonal and secular variability of the subtropical front in the western North Pacific from 1954 to 1974

Examination of the Subtropical Front in the western North Pacific along 137°E, using routine hydrographic cruise data from 1967 to 1974, reveals it to be narrow, located between 17.5° and 25°N, and associated with a sharp northward rise of the upper portion of the main thermocline which detaches itself from the deeper main thermocline near 17.5°N. The climatological front depicted in various oceanographic atlases (e.g., USSR Navy Atlas, 1976) is thus revealed as a smoothed version of the more narrow synoptic frontal feature that moves latitudinally and varies in strength on both a seasonal and a secular basis. The narrow Subtropical Front is superimposed upon the weaker north/south gradient of thermocline temperature that exists between the North Equatorial Current and the Kuroshio Countercurrent. The present study extends our knowledge on the space/time variability of the Subtropical Front by analyzing all available mechanical bathythermograph, expendable bathythermograph, and hydrographic data taken in the western North Pacific from 1954 to 1974. These data are interpolated onto a regular space grid (2.5° latitude by 5° longitude) on both a monthly climatological basis and an individual yearly basis from 1954 to 1974. Because the data distribution lacked the data density required, the method of interpolating could not reproduce the narrow character of the Subtropical Front but rather smoothed it in space and over a year's time. However, a maximum in the meridional gradient in temperature over the upper 200 m consistently appeared each year from 1954 to 1974 in the approximate vicinity of the synoptic front, although it showed much less strength. Changes in both the latitude location and the strength of this smoothed version of the Subtropical Front show interesting behavior on both the seasonal and the secular time scale. On the seasonal scale (i.e., 3 months) the Subtropical Front is twice as strong in spring as it is in fall, with some tendency for it to be located farther north when it is stronger and farther south when it is weaker. This seasonal variability has a maximum north/south displacement of ±4° of latitude in the west at 135°E, decreasing to the east, associated with a maximum range in strength of ±35% about the annual mean. On the secular scale (i.e., 1–1.5 yr) the Subtropical Front from 1954 to 1974 had a maximum range in strength of ±15% about the annual mean, and it had some tendency to be more intense when it was displaced farther north, showing a maximum north/south displacement of ±2° latitude. From 1954 to 1974 the years 1957–1958, 1963–1965, and 1970–1971 find the Subtropical Front to have been stronger than normal, in phase with the secular variability of the two major current systems both north and south of it (i.e., the Kuroshio and the North Equatorial Current). These periods of years (i.e., 1957–1958, 1963–1965, and 1970–1971) are the same as those characterizing the El Niño phenomenon in the equatorial and tropical Pacific, an indication of the more broad‐scale nature of this secular variability.

Splitting of the subtropical gyre in the western North Pacific

Examined here is a hypothetical idea of the splitting of the subtropical gyre in the western North Pacific on the basis of two independent sources of data,i.e., the long-term mean geopotential-anomaly data compiled by the Japanese Oceanographic Data Center and the synoptic hydrographic (STD) data taken by the Hakuho Maru in the source region of the Kuroshio and the Subtropical Countercurrent in the period February and March 1974. Both of the synoptic and the long-term mean dynamic-topographic maps reveal three major ridges, which indicate that the western subtropical gyre is split into three subgyres. Each subgyre is made up of the pair of currents, the Kuroshio and the Kuroshio Countercurrent, the Subtropical Countercurrent and a westward flow lying just south of the Countercurrent (18°N–21°N), and the northern part of the North Fquatorial Current and an eastward flow at around 18°N. The subgyres are more or less composed of a train of anticyclonic eddies with meridional scales of between 300 and 600 km, so that the volume transport of the subgyres varies by a factor of two or more from section to section. The upper-water characteristics also support the splitting of the subtropical gyre; the water characteristics are fairly uniform within each subgyre, but markedly different between them. The northern rim of each subgyre appears as a sharp density front accompanied by an eastward flow. The bifurcations of the sharp density fronts across the western boundary current indicate that the major part of the surface waters in the North Equatorial Countercurrent is not brought into the Kuroshio. The western boundary current appears as a continuous feature of high speed, but the waters transported change discontinuously at some places.

豆南海域におけるサンマ<i>Cololabis saira</i> (BREVOORT) 稚仔の分布-II

During 1955 and 1956 the authors conducted 87 survey cruises for distribution of larval forms of the saury pink, Cololabis saira (Brevoort) in Zu'nan Region extending from Sagami Bay to Torishima (Fig. 1). On the basis of the materials and data made available from the surveys, analyses have been made, in succession to a previous work, in regard to the distribution of the larvae and the length composition of the catch, which may vary depending on locality and year and be related with changes in oceanographical conditions in the region. The number of individuals of the larvae collected at each station is listed in Tables 1 and 2 together with the data pertinent to total length, gear used and so forth. Noticeable features those tables reveal are as follows. 1) Stations in the vicinity of Torishima showed the distribution in a greater density than any other offshore station. 2) In the inner niche of Sagami Bay, where a water boundary exists, about 150 individuals accrued from a single haul of a larval net. 3) Generally speaking, 856 larvae collected with a dip net between Aogashima and Torishima were greater in total length than those obtained at most of larval netting stations. By sorting the samples into several groups according to the locality, time and gear of collection, the total length frequency has been compared among the different groups (Fig. 2). As is evident in Fig. 2, the catches both by dip net and by larval net indicate that the length class where the mode takes place is greater in 1956 than in 1955. Considerable numbers of the larvae measuring about 10 centimeters or less in total length occurred in Sagami Bay and in the neighbourhood of Torishima. Furthermore, 18 adults which occurred off Torishima in early March of 1956 and measured 28.83±1.46cm in body length apparently belonged to a spawning group. These facts suggest that the saury pink spawn substantial quantity of the young in these general localities. According to oceanographical surveys carried out by the R. V. To'nan Maru along a longitudinal line connecting the Izu Islands during the period from 1954 to 1956, the locality of the water boundary between the Kuroshio Current and the littoral waters is moving further offshore one year after another (Fig. 3). Apparently the movement of the water boundary has been dependent on the status of a cold water mass off Enshu Nada. On the other hand, the larval distribution has been nearly twice greater in the Kuroshio Countercurrent area than in the Kuroshio Current area flowing eastwards (Tables 3 and 4). On the basis of these findings it is inferable that a close relation exists between translocation of the major basin of the Kuroshio Current and changes in the spawning conditions of the saury pike and particularly in the length composition of the young whose mode has been found greater one year after another. Frequency of surface temperature at the collecting stations, showed that nearly 75 percent of the stations had the temperature ranging from 18° to 21°C (Table 5).