Izu Ridge Research Articles

Variations of the Kuroshio in the Southern Region of Japan: Conditions for Large Meander of the Kuroshio

Conditions for the formation of large meander (LM) of the Kuroshio are inferred from observational data, mainly obtained in the 1990s. Propagation of the small meander of the Kuroshio from south of Kyushu to Cape Shiono-misaki is a prerequisite for LM formation, and three more conditions must be satisfied. (1) The cold eddy carried by small meander interacts with the cold eddy in Enshu-nada east of the cape. During and just after the propagation of small meander, (2) the Kuroshio axis in the Tokara Strait maintains the northern position and small curvature, and (3) current velocity of the Kuroshio is not quite small. If the first condition is not satisfied, the Kuroshio path changes little. If the first condition is satisfied, but the second or third one is not, the Kuroshio transforms to the offshore non-large-meander path, not the LM path. All three conditions must be satisfied to form the large meander. For continuance of the large meander, the Kuroshio must maintain the small curvature of current axis in the Tokara Strait and a medium or large range of velocity and transport. These conditions for formation and continuance may be necessary for the large meander to occur. Moreover, effects of bottom topography on position and structure of the Kuroshio are described. Due to topography, the Kuroshio changes horizontal curvature and vertical inclination of current axis in the Tokara Strait, and is confined into either of two passages over the Izu Ridge at mid-depth. The former contributes to the second condition for the LM formation.

Estimating the Kuroshio Axis South of Japan Using Combination of Satellite Altimetry and Drifting Buoys

By tracking the locally strongest part of the sea-surface velocity field, which was obtained by integrating data of satellite altimeters and surface drifting buoys, we extracted the Kuroshio axis south of Japan every 10 days from October 1992 to December 2000. The obtained axes clearly express the effect of the bottom topography; three modes were observed when the Kuroshio ran over the Izu Ridge. The axis was very stable to the south of ‘Tosa-bae,’ off the Kii Channel. Mean current speed at the Kuroshio axis gradually increased from 0.65 m/s south of Kyushu to 1.45 m/s off Enshu-nada.

Bedforms produced by the Kuroshio Current passing over the northern Izu Ridge

Current-generated bedforms were found on sandy seafloor at water depths of 200–400 m on the northern Izu Ridge, where the Kuroshio Current encounters and passes over the ridge. The observed bedforms include large dunes and sand ribbons and are interpreted to be products of present-day oceanographic conditions and to indicate intensive flow activity controlled by local topography. A comparison between the surface flow velocity estimated from empirical relationships for dune formation and the observed flow velocity suggests that the dunes are generated when the main axis of the meandering Kuroshio Current passes through this area, and that subsequent current velocities are sufficiently high to maintain the dunes up to the next event.

On the eddy‐Kuroshio interaction: Meander formation process

The Kuroshio flows along the southern coast of Japan during its non‐large‐meandering state, then separates from the coast near the Kii Peninsula and attaches at the north of the Izu Ridge. The amplitude of the offshore displacement of the Kuroshio changes as a result of mesoscale perturbations. Satellite SSH and SST observations (TOPEX/Poseidon: T/P and NOAA AVHRR) have suggested that the short‐term Kuroshio meander formation is triggered by anticyclonic eddies originating in the Kuroshio Extension. To simulate such an event, a numerical experiment using a high‐resolution regional GCM was conducted by initializing the eddy with the observed T/P sea level anomaly. An anticyclonic eddy was injected to the south of the Kuroshio (140°E, 30°N) by means of sequential data assimilation of the T/P data of October 1992 for 30 days. The volume transport of the model Kuroshio was kept constant at 25 Sv, a condition which would not cause large meander of the Kuroshio. The simulation successfully reproduced the four phases of the interaction: (1) westward propagation of the eddy; (2) advection of the eddy by the Kuroshio; (3) meander formation; and (4) detachment of the eddy from the Kuroshio and their repetition. The analyses revealed that the inshore high potential vorticity (PV) water is generated at the sharp coastal topography (the Kii peninsula). The cyclonic eddies shed will eventually coalesce with the existing inshore cyclonic circulation and the meander grows. During the growth of the meander, the necessary barotropic kinetic energy is produced through the shallowing of the thermocline of the anticyclonic eddy as it elongates and splits. The growth of the meander ceases when the split anticyclonic eddies merge, the thermocline deepens, and the eddy detaches itself from the Kuroshio as a result of its own westward thrust. Simultaneously, the accumulated high PV inshore is released to the Kuroshio Extension region as cyclonic eddies. This study provides evidence for the active role of the anticyclonic eddy in causing the variability of the Kuroshio path and suggests a mechanism of rapid discharge/recharge of the available potential energy of the eddy and production/release of high PV inshore Kuroshio to cause the short‐term Kuroshio meander.

On the eddy‐Kuroshio interaction: Evolution of the mesoscale eddy

Presented are results from a numerical study of the interaction of a mesoscale eddy and the Kuroshio using a high‐resolution regional general circulation model (GCM). The distinct evolution of the mesoscale eddies is not new in the literature, but most of the previous studies were conducted with simpler model configurations. To our knowledge the present study is the first to use a high‐resolution GCM, proven to replicate a realistic ocean circulation. An anticyclonic eddy was injected to the south of Kuroshio (140°E, 30°N) by means of sequential data assimilation of TOPEX/Poseidon altimeter data of October 1992 for 30 days. The strength of the assimilation was varied to produce five eddies (Rossby number ϵ ∼ 0.012–0.014) comparable in scale to those observed in this region. The two results (ϵ = 0.0131, 0.0141) resemble the observed onset of short‐term meandering events in 1993 and 1998. The westward propagating anticyclonic eddy collides with the Kuroshio southeast of Kyushu, propagates downstream, and triggers the short‐term Kuroshio meander (occurring between the Kii Peninsula and Izu ridge with duration of half a year). The sequence resembles the scenario hypothesized on the basis of altimeter observations and other in situ measurements during Tokyo Ogasawara Line Experiment. The description of these cases is important, but we focus here on the differentiation between the strong eddy cases (ϵ = 0.0131, 0.0141) and the weak eddy cases (ϵ = 0.0118, 0.0124, 0.0125) classified on the basis of their subsequent evolutions: the strong geddy (meandering case) propagates west as it elongates zonally and barotropic tripolar vortices form, and the weak eddy (nonmeandering case) makes an abrupt southward migration while the eddy core splits as a result of advection by lower layer geostrophic motions. Such distinct evolutions are conjectured a result of a competition of the effects of nonlinearity, dispersion, and barotropicity of which their relative importance varied among the simulated eddies. Because the scale variation of the injected eddies are natural, we expect that even the weak‐eddy case exists in nature; in fact, such a case was observed in 1994.

Semidiurnal internal tide observed over Ohmuro-Dashi, the northern part of the Izu Ridge

Repeated measurements by acoustic Doppler current profiler and expendable bathythermograph were carried out across Ohmuro-Dashi, a rise at the northern part of the Izu Ridge, to investigate the generation and propagation of internal tides in October 1993. The barotropic current with semidiurnal period was perpendicular to the bottom contours on the ridge, indicating an effective generation of semidiurnal internal tide. The predominance of semidiurnal internal tide was confirmed from analysis of the temperature and current distributions. The vertical displacement of thermocline due to semidiurnal internal tide reached 35 m at the western side of the rise. The vertical shear of the horizontal current had a beam-like structure and was the strongest at the western side of the rise. From ray-trace of semidiurnal internal tide, the structure of vertical shear at the western side of Ohmuro-Dashi was found to be due to superposing of the rays emanating from two peaks of the ridge and the near critical slope. The distribution of gradient Richardson number calculated from the vertical shears for M 2 constituent and temperature inversions observed in the thermocline indicated that the vertical mixing was possibly occurring over Ohmuro-Dashi and was related to the internal tides.

Outflow of the Intermediate Oyashio Water from Sagami Bay to the Shikoku Basin

Since the Intermediate Oyashio Water (IOW) gradually accumulates in Sagami Bay, it can reasonably be supposed that the IOW also flows out from Sagami Bay, even though it may be altered by mixing with other waters. We have occasionally observed a water less than 34.2 psu with a potential density of 26.8 at the southeastern area off Izu Peninsula in July 1993 by the training vessel Seisui-maru of Mie University. Observational data supplied by the Japan Meteorological Agency and the Kanagawa Prefectural Fisheries Experimental Station show that the IOW of less than 34.1 psu was observed at northern stations of the line PT (KJ) off the Boso Peninsula and to the east of Oshima in the late spring 1993. Based upon these observations, it is concluded that the IOW flows out from Sagami Bay into the Shikoku Basin along southeastern area off the Izu Peninsula. The less saline water (<34.2 psu) was also observed to the west of Miyake-jima during the same cruise, and the westward intrusion of IOW from south of the Boso Peninsula to the Shikoku Basin through the gate area of the Kuroshio path over the Izu Ridge was detected. This event indicated that the IOW branched south of the Boso Peninsula and flowed into Sagami Bay and/or into the gate area over the Izu Ridge. The southward intrusion of IOW into the south of the Boso Peninsula is discussed in relation to the latitudinal location of the main axes of the Kuroshio and the Oyashio.

Index and Composites of the Kuroshio Meander South of Japan

Using the merged NOAA National Oceanographic Data Center (NODC)/Japan Oceanographic Data Center (JODC)/Marine Information Research Center (MIRC) historical hydrographic dataset, a new Kuroshio large meander (LM) index is introduced. This index helps to distinguish between the LM events and other types of Kuroshio Current (KC) variability south of Japan. Observations, re-systematized according to the index, provide composite patterns of typical formation and decay of the LM. The patterns reveal a remarkable similarity between individual LMs and support the deterministic rather than the stochastic model of LM evolution on a time-scale of one year. A “trigger” meander (TM) occurs on composite maps six months prior to the LM formation as a 1° latitude southward shift of the KC axis south of Kyushu. When propagating eastward along the coast of Japan, TM gradually increases in area. In principal the emergence of LM takes only one month. East of TM and LM a remarkable onshore shift of the KC is noticed, supplying the coastal region with warm water. Other warm anomalies are found on the warm side of KC next to the propagating TM and in the larger warm eddy area southeast of Kyushu. Different LMs survive for different times and decay in some months after KC “jumps” across the Izu Ridge. Changes of water properties on isopycnals in the interior of LM can be roughly described by two-layer kinematics with an interface at σθ = 27 which suggests a strong inflow of deep Kuroshio waters into the LM core during the formation of the latter.

Influence of Kuroshio Flow on the Horizontal Distribution of North Pacific Intermediate Water in the Shikoku Basin

The influence of the Kuroshio flow on the horizontal distribution of North Pacific Intermediate Water (NPIW) in the Shikoku Basin is examined based upon observational data collected by the training vessel “Seisui-maru” of Mie University together with oceanographic data compiled by the Japan Oceanographic Data Center (JODC). Although it has been stated that the NPIW with salinity less than 34.2 psu had been confined to the south of the Kuroshio main axis along the PT (KJ) Line on the eastern side of the Izu Ridge, a similar tendency can be detected on the western side of the Izu Ridge. Namely, the NPIW on the southern side of the Kuroshio main axis in the Shihoku Basin does not indicate a tendency to go northward across the Kuroshio main axis without an increase in salinity of more than 34.2 psu. However, the JODC data show that less saline water (<34.2 psu) was present on the northern side of the Kuroshio main axis south of the Kii Peninsula in May 1992. Satellite observed sea surface temperature (SST) data suggested that the Kuroshio approaches the Kii Peninsula after forming a small meander off Kyushu and some intrusions of the NPIW into the northern coastal side of the Kuroshio main axis occurred in this period. It is concluded that intrusion of the NPIW with salinity less than 34.2 psu to the northern coastal side through the Kuroshio main axis occurred during the decay period of the small meander path in May 1992. Based on these observational results, the source of the salinity minimum water on the northern coastal side of the Kuroshio main axis is discussed.

Seasonal axis migration of the upstream Kuroshio Extension associated with standing oscillations

The TOPEX/Poseidon altimeter data from October 1992 to October 1998 are analyzed to investigate the seasonal cycle of the sea surface height (SSH) in the upstream region of the Kuroshio Extension. After removing steric height changes caused by surface buoyancy fluxes, two maxima of the seasonal SSH amplitude are found around the crest and trough of the quasi‐stationary meanders as standing waves, accompanied with nodes of amplitude minima. Seasonal variations are identified in the subsurface temperature at 400 m depth similar to those of SSH in amplitude and phase. Associated with the standing waves, the current follows a straight path in fall and then changes to a meandering path in spring. From a composite map of the seasonal Kuroshio Extension axes, nodes of minimum displacement are found. One node is located over the Izu Ridge, while the others are located in the areas where meridional acceleration is large. The seasonal axis migration of the upstream Kuroshio Extension is regulated by these nodes.

Anticyclonic eddies and Kuroshio Meander Formation

A meander‐formation mechanism of the Kuroshio that involves anticyclonic eddies propagating from the Kuroshio Extension, is presented using the analysis of observations of the TOPEX/POSEIDON (T/P) altimeter data and in‐situ data. Support of the mechanism comes also from a numerical simulation. In October 1997, a current meter mooring over the Izu Ridge at 500‐m‐depth captured large velocity fluctuations of about 50 cm s−1 that were associated with a strong anticyclonic eddy. The observations from T/P show that the anticyclonic eddy propagated westward and collided with the Kuroshio off Kyushu. At collision, the anticyclonic eddy coupled with a cyclonic circulation accompanied by a small meander and formed a vortex pair. They were then advected downstream and finally formed a large‐meander‐like path off the south coast of Honshu. The meander lasted for a few months. Subsequently, the anticyclonic eddy was detached, and the meander grew smaller in amplitude. Numerical experiments successfully simulate this interaction between the Kuroshio and anticyclonic eddies.

Circulation of Intermediate and Deep Waters in the Philippine Sea

The circulation of intermediate and deep waters in the Philippine Sea west of the Izu-Ogasawara-Mariana-Yap Ridge is estimated with use of an inverse model applied to the World Ocean Circulation Experiment (WOCE) Hydrographic Program data set. Above 1500 m depth, the subtropical gyre is dominant, but the circulation is split in small cells below the thermocline, causing multiple zonal inflows of intermediate waters toward the western boundary. The inflows along 20°N and 26°N carry the North Pacific Intermediate Water (NPIW) of 11 × 109 kg s−1 in total, at the density range of 26.5σθ–36.7σ2 (approximately 500–1500 m depths), 8 × 109 kg s−1 of the NPIW circulate within the subtropical gyre, whereas the rest is conveyed to the tropics and the South China Sea. The inflow south of 15°N carries the Tropical Salinity Minimum water of 35 × 109 kg s−1, nearly half of which return to the east through a narrow undercurrent at 15–17°N, and the rest is transported into the lower part of the North Equatorial Countercurrent. Below 1500 m depth, the deep circulation regime is anti-cyclonic. At the density range of 36.7σ2, – 45.845σ4 (approximately 1500–3500 m depths), deep waters of 17 × 109 kg s−1 flow northward, and three quarters of them return to the east at 16–24°N. The remainder flows further north of 24°N, then turns eastward out of the Philippine Sea, together with a small amount of subarctic-origin North Pacific Deep Water (NPDW) which enters the Philippine Sea through the gap between the Izu Ridge and Ogasawara Ridge. The full-depth structure and transportation of the Kuroshio in total and net are also examined. It is suggested that low potential vorticity of the Subtropical Mode Water is useful for distinguishing the net Kuroshio flow from recirculation flows.

Interannual Variability of the Kuroshio Extension System and Its Impact on the Wintertime SST Field

Altimetry data from the first 7-yr TOPEX/Poseidon (T/P) mission (October 1992–September 1999) are analyzed to investigate the interannual variability in regions of the Kuroshio Extension and its southern recirculation gyre. Large-scale, interannual changes of the Kuroshio Extension system are characterized by the oscillation between an elongated state and a contracted state. In the elongated state, the Kuroshio Extension has a larger eastward surface transport, a greater zonal penetration, and a more northerly zonal-mean path. All these characteristics are closely connected to the presence of an intense, zonally elongated southern recirculation gyre. In its contracted state, the Kuroshio Extension has a smaller eastward surface transport, a more southerly mean path, and is accompanied by a weaker southern recirculation gyre. During the T/P period, the Kuroshio Extension changed from the elongated state in 1992–93 to the contracted state in 1996–97, and back to the elongated state since late 1997. In addition to these large-scale changes, the mesoscale eddy field also exhibited prominent interannual changes in the Kuroshio Extension region. In the upstream region between the Izu Ridge and the Shatsky Rise, the eddy kinetic energy level was generally low while the Kuroshio Extension was in its elongated state, and high while in the contracted state. Downstream of the Shatsky Rise, on the other hand, the EKE level was high (low) when the Kuroshio Extension was in the elongated (contracted) state. The large-scale, interannual changes in the Kuroshio Extension system have a significant impact on the regional wintertime SST anomaly field: the warm (cold) wintertime SST anomalies tend to persist in years when the Kuroshio Extension is in its elongated (contracted) state. A diagnostic analysis of the surface ocean heat balance indicates that the nonseasonal geostrophic advection by the ocean circulation works to reduce (increase) the wintertime SST anomalies when the Kuroshio Extension changes from an elongated (contracted) state to a contracted (elongated) state. The SST anomalies associated with the large-scale changes of the Kuroshio Extension have an area-averaged, peak-to-peak amplitude of >1°C and appear independent of the interannual SST changes in the tropical Pacific Ocean.

Two processes by which short-period fluctuations in the meander of the Kuroshio affect its countercurrent

Direct current velocity measurements in the countercurrent of the Kuroshio, south of Japan, were carried out to investigate the influence of short-period fluctuations in the small-scale meander of the Kuroshio on its countercurrent. When the Kuroshio took a path having a meander west of the Izu Ridge and approaching the Izu Peninsula, the countercurrent freely intruded into coastal seas with a period of 17 d and a phase velocity almost equal to that of the Kuroshio itself. However, when the Kuroshio did not significantly bend and deflect off the Izu Peninsula, even when taking the same path, the velocity of the countercurrent was considerably reduced and the periodic fluctuations propagated into the coastal seas as a continental shelf wave. The results indicate that a small change in the Kuroshio's path can cause a different process of propagation of the small-scale meandering; this difference probably explains why there are two kinds of phase velocity in coastal temperature fluctuations.

Topographic Effect of Izu Ridge on the Distribution of the North Pacific Intermediate Water South of Japan

The topographic effect of the Izu Ridge on the horizontal distribution of the North Pacific Intermediate Water (NPIW) south of Japan has been studied using observational data obtained by the Seisui-Maru of Mie University (Mie Univ. data) and those compiled by Japan Oceanographic Data Center (JODC data). Both data sets show that water of salinity less than 34.1 psu on potential density (σΘ) surface of 26.8 is confined to the eastern side of the Izu Ridge, while water of salinity less than 34.2 psu is confined to the southern area over the Izu Ridge at a depth greater than 2000 m and to the southeastern area in the Shikoku Basin. It is also shown by T-S analysis of Mie Univ. data over the Izu Ridge that water of salinity less than 34.2 psu dominates south of 30°N, where the depth of the Izu Ridge is deeper than 2000 m and NPIW can intrude westward over the Izu Ridge. JODC data reveal that relatively large standard deviations of the salinity on σΘ surface of 26.7, 26.8 and 26.9 are detected along the mean current path of the Kuroshio and the Kuroshio Extension. Almost all of the standard deviations are less than 0.05 psu in other area with the NPIW, which shows that the time variation in the salinity can be neglected. This observational evidence shows that the topographic effect of the Izu Ridge on the horizontal distribution of the NPIW, which is formed east of 145°E by the mixing of the Kuroshio water and the Oyashio water, is prominent north of 30°N with a depth shallower than 2000 m.

Numerical study of the generation and propagation of trigger meanders of the Kuroshio South of Japan

We examine the processes underlying the generation and propagation of the small meander of the Kuroshio south of Japan which occurs prior to the transition from the non-large meander path to the large meander path. The study proceeds numerically by using a two-layer, flat-bottom, quasi-geostrophic inflow-outflow model which takes account of the coastal geometries of Kyushu, Nansei Islands, part of the East China Sea, and the Izu Ridge. The model successfully reproduces the observed generation and propagation features of what is called "trigger meander" until it passes by Cape Shiono-misaki; presumably because of the absence of the bottom topography, the applicability of the present numerical model becomes questionable after the trigger meander passes by Cape Shiono-misaki. The generation of the trigger meander off the south-eastern coast of Kyushu is shown to be associated with the increase in the supply of cyclonic vorticity by the enhanced current velocity in the upper layer along the southern coast of Kyushu where the no-slip boundary condition is employed. Thereafter, the trigger meander propagates eastward while inducing an anticyclone-cyclone-cyclone pair in the lower layer. The lower-layer cyclone induced in this way, in particular, plays a crucial role in intensifying the trigger meander trough via cross-stream advection in the upper layer; the intensified trigger meander trough then further amplifies the lower-layer cyclone. This joint evolution of the upper-layer meander trough and the lower-layer cyclone indicates that baroclinic instability is the dominant mechanism underlying the rapid amplification of the eastward propagating trigger meander.

Coastal-trapped waves with several-day period caused by wind along the southeast coast of Honshu, Japan

Generation and propagation of several-day period fluctuations along the southeast coast of Honshu, Japan, were investigated by analyzing sea level data and by using a numerical model. The sea level data obtained at twelve stations from Choshi to Omaezaki in fall in 1991, showed energy peaks at the 3–6 day period at the eastern stations in this coast. Time lags of the 3–6 day period fluctuations between station and station indicate westward propagation along the coast. However, the energy level of the 3–6 day period fluctuations suddenly decreased south of the Izu Peninsula. Numerical experiments using a two-layer model were performed to clarify the generation and propagation mechanism of the several-day period fluctuations by periodical wind in fall. The amplitude distributions of observed sea level were qualitatively explained by a coastal-trapped wave (CTW) in the numerical experiment. From the discussions on propagation of a free wave, CTW with the characteristics of a shelf wave generated by the wind along the northeast of the Boso Peninsula was separated into two types of wave at the southeast of the peninsula. One is an internal Kelvin wave with large interface displacement and the other is the shelf wave propagating over the northern part of the Izu Ridge. The sudden decrease in the surface displacement with the 3–6 day period observed at the western stations is considered to be due to the local effect of the wind and phase relation between the internal Kelvin wave and shelf wave.

A brief history of recognition of the Kuroshio

Since ancient times the Japanese and Asians have encountered the Kuroshio. For instance, records of long-distance driftings that must have been greatly influenced by the Kuroshio are found in official chronicles of Japan from before 833 and a biography of Ganjin, the Chinese Buddhist who arrived in Japan on a religious mission in 754 across the Kuroshio. In the seventh and eighth centuries, some of the ships of Japanese envoys to China during 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) Japanese ships with the shogunate license for foreign trade crossed the Kuroshio east of Taiwan bound for Luzon. During those periods, however, no mention of the Kuroshio has been found, except for that of S. Vizcaı́no, the Spanish navigator who reported a segment of strong current east of the Boso Peninsula in 1614. After the National Isolation Policy was enforced in 1635, the building of large ships and navigation in waters beyond where the coast was visible were prohibited in Japan, and hence it seems unlikely that many early books from this period will mention the Kuroshio. However, by searching and deciphering early books, the author has found many important notes, descriptions and illustrations referring to the Kuroshio in maps and books from the Edo period (1603–1867). Terms “Luo-Ji” and “Wei-Lü”, Chinese equivalents of “Kuroshio”, were searched for in Chinese classics and were examined. The term “Luo-Ji” (Rakusai in Japanese) was cited by some Japanese books written in the Edo period referring to the Kuroshio in the East China Sea. Meanwhile, the term “Wei-Lü” (Biryo in Japanese) was considered to be too mythical to be used in Japan as indicating the Kuroshio, although Needham (1971) inferred that the “Wei-Lü” had been used since ancient times as the Chinese equivalent of “Kuroshio”. To clarify the evolution of types of image of the current recognized as the Kuroshio, early books were sought in which there was recognition by navigators from Western countries of the existence of the Kuroshio, although some of them were unavailable for reading. Examining and comparing synoptically these Japanese, Chinese and Western records, the author reached the following conclusions. The term “Kuroshio” (blackish stream or tide) originated as a local synonym for “Kurose Gawa” (a river over blackish shallows) among inhabitants of the Izu Islands. However, it really indicated only a branch of the actual Kuroshio that flows over shallows along the Izu Ridge. This type of image of the Kuroshio is called the “short concentrated stream”. Towards the end of the 18th century it came to refer to a somewhat longer (about 400 km) path of the Kuroshio, which had become better known among Japanese people, through the publishing of maps, local geographies, accounts of trips and novels. This type of image of the Kuroshio is called the “middle-distance concentrated stream”. However, the “conception of fragmented currents” prevalent at that period probably delayed the recognition that the Kuroshio is a long continuous stream. From the end of the 18th century to the mid-19th century, Westerners collected information about Japan's geography and carried out research and made surveys of the Kuroshio, when they sent naval vessels to Japan in order to establish commercial relations. Before the Meiji Restoration (1868), Westerners viewed the “Kuroshio” as a broadly dispersing stream along the Japanese Islands and Taiwan. This type of image of the Kuroshio is called the “long dispersed stream”, and it was predominant until the modern picture of the “long concentrated stream” was first recognized by the Japanese in about 1930. Ironically, written use of the term “Kuroshio” is found in documents that appeared after the Japanese had ceased oceanic activities under the National Isolation Policy. During these two hundred-plus years, the types of image of the Kuroshio underwent major changes, reflecting the turbulent national and international conditions encountered in Japan.

Description of the oceanographic condition off Sanriku, northwestern Pacific, and its relation to spring bloom detected by the Ocean Color and Temperature Scanner (OCTS) images

We describe the oceanographic condition as observed by hydrographic data and phytoplankton spring bloom detected by OCTS images off Sanriku, northwestern Pacific, during the spring bloom period in 1997. The relationship between the two is discussed. OCTS images detected the bloom in early April in the coastal area around the Izu ridge north of the Kuroshio and the eastern coastal area of Hokkaido to the Oyashio front. The bloom areas were seen along the offshore Kuroshio Extension from the end of April, in the upstream region of the Oyashio south of the Kurile Islands, except for a part of coastal area from the end of May, and in the Kuroshio warm-core ring 93A (KWCR 93A) from early June. The temperature difference between the surface and subsurface layer is used as a stratification index. This was large in the upstream region of the Oyashio south of the Kurile Islands and KWCR 93A from early June. Previous research has pointed out that the spring bloom usually corresponds to the development of stratification in the water column due to seasonal warming. In addition to that, we suggest that the transportation of water containing a high chlorophylla concentration by advection due to strong currents, like the Kuroshio and the Oyashio, is important for the formation of an area of high chlorophylla concentration. These results indicate that the OCTS images are useful for a knowledge of the distribution and the change of chlorophylla concentration in the northwestern Pacific region.

Monitoring of the transport of the ocean current by an acoustic transceiver array

Current velocity can be determined by measuring the difference of the sound wave traveling time between two points. The multipaths inverted echo sounder (MIES) [Takeuchi et al., J. Acoust. Soc. Jpn. 49, 543–550 (1993)] was developed in order to apply this method to the measurement of the volume transport of the Kuroshio. Measurement of mean current velocity can be made with a difference of reciprocal travel times of the sound wave along two sides of a triangle in a vertical plane, which is constructed by the acoustic paths with a base side of the mooring distance when two multipath inverted echosounders were deployed 10 km apart. The measurement of volume transport was attempted by applying the above method, where three multipath inverted echosounders were deployed so as to construct a regular triangle on the sea bottom. In this paper, the result of the measurement of the volume transport of the Kuroshio over Izu Ridge from 20 March to 25 April 1995 is presented.