Kuroshio South Research Articles

Estimating the sea surface dynamic topography from Geosat altimetry data

Optimal interpolation method is applied to Geosat altimetry data both to remove orbit error and to separate temporal mean sea surface dynamic topography (SSDT) from temporal fluctuations around the mean. Loss of long-wavelength oceanic signals at orbit error reduction procedure is smaller in this method than that in conventional collinear methods, but the areal average height over the study domain is still removed as the orbit error. The fluctuation SSDT is quantitatively evaluated by sea level data from tide gauge stations at Japanese islands. The correlation coefficient of the two sea-level variations is 0.83 when the loss of the areal average is compensated by the seasonal variation of the areal average height determined from the climatological monthly-mean SSDT. In addition, the improvement of the geoid model by combined use of Seasat altimetry data and hydrographic data is validated through the estimated temporal mean SSDT. In a local area where hydrographic data contemporary with the Seasat mission exist, the geoid model has been significantly improved so that the absolute SSDT can be determined from combination of the altimetry data and geoid model; the absolute SSDT describes the onset event of a quasi-stationary large meander of the Kuroshio south of Japan very well. Outside this local area, however, errors of several tens of centimeters still remain in the improved geoid model.

Dynamics of the Kuroshio/Oyashio current system using eddy‐resolving models of the North Pacific Ocean

A set of numerical simulations is used to investigate the Pacific Ocean circulation north of 20°S, with emphasis on the Kuroshio/Oyashio current system. The primitive equation models used for these simulations have a free surface and realistic geometry that includes the deep marginal seas, such as the Sea of Japan. Most of the simulations have 1/8° resolution for each variable but range from 1/2°, 1.5‐layer reduced gravity to 1/16°, six layer with realistic bottom topography. These are used to investigate the dynamics of the Kuroshio/Oyashio current system and to identify the processes that contribute most to the realism of the simulations. This is done by model‐data comparisons, by using the modularity of layered ocean models to include/exclude certain dynamical processes, by varying the model geometry and bottom topography, and by varying model parameters, such as horizontal grid resolution, layer structure, and eddy viscosity. In comparison with observational data, the simulations show that the barotropic mode, at least one internal mode, nonlinearity, high “horizontal” resolution (1/8° or finer), the regional bottom topography, and the wind forcing are critical for realistic simulations. The first four are important for baroclinic instability (eddy‐mean energetics actually show mixed barotropic‐baroclinic instability), the wind curl pattern for the formation and basic placement of the current system, and the bottom topography for the distribution of the instability and for influences on the pathways of the mean flow. Both the Hellerman and Rosenstein (1983) (HR) monthly wind stress climatology and 1000‐mbar winds from the European Centre for Medium‐Range Weather Forecasts (ECMWF) have been used to drive the model. East of about 150°E, they give a mean latitude for the Kuroshio Extension that differs by about 3°, approximately 34°N for HR, 37°N for ECMWF, and 35°N observed. The subarctic front is the northern boundary of the subtropical gyre. It is associated with the annual and April–September mean zero wind stress curl lines (which are similar), while the Kuroshio Extension is associated with wintertime zero wind stress curl. This means that part of the flow from the Kuroshio must pass north of the Kuroshio Extension and connect with the Oyashio and subarctic front. Realistic routes for this connection are flow through the Sea of Japan, a nonlinear route separated from the east coast of Japan, and bifurcation of the Kuroshio at the Shatsky Rise. In addition, the six‐layer simulations show a 3‐Sv meridional overturning cell with southward surface flow and northward return flow centered near 400 m depth. Baroclinic instability plays a critical role in coupling the shallow and abyssal layer circulations and in allowing the bottom topography to strongly influence the shallow circulation. By this means, the Izu Ridge and Trench and seamounts upstream and downstream of these have profound influence on (1) the mean path of the Kuroshio and its mean meanders south and east of Japan and (2) on separating the northward flow connecting the Kuroshio and the Oyashio/subarctic front from the east coast of Japan. Without the topographic influence, the models show an unrealistic northward current along the east coast of Japan. In essence, the topography regulates the location and strength of the baroclinic instability. The baroclinic instability gives eddy‐driven deep mean flows that follow the f/h contours (where f is the Coriolis parameter and h is the depth of the water column) of the bottom topography. These abyssal currents then strongly influence the pathway for subtropical gyre flow north of the Kuroshio Extension and steer the mean meanders in the Kuroshio south and east of Japan. This is corroborated by current meter data from the Kuroshio Extension Regional Experiment (World Ocean Circulation Experiment line PCM 7). The meander path south of Japan depends on the occurrence of baroclinic instability west of the Izu Ridge; otherwise, a straight path occurs. The pathway shows little sensitivity to the Tokara Strait transport over the range simulated (36–72 Sv in yearly means). However, interannual increases in wind forcing or Tokara Strait transport give rise to a predominant meander path, while decreases yield a predominant straight path. Resolution of 1/8° in an ocean model is comparable to the 2.5° resolution used in atmospheric forecast models in the early 1980s based on the first internal mode Rossby radius of deformation. Model comparisons at 1/8° and 1/16° resolution and comparisons with current meter data and Geosat altimeter data show that 1/16° resolution is needed for adequate eastward penetration of the high eddy kinetic energy associated with the Kuroshio Extension.

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.

Seasonal Variation in Volume Transport of the Kuroshio South of Japan

Abstract A two-layer numerical model driven by the observed wind stress compiled by K. Kutsuwada and T. Teramoto is used to study the seasonal variation in volume transport of the Kuroshio south of Japan. The authors focus on the observational evidence that the geostrophic volume transport of the Kuroshio is maximum in summer and minimum in winter; however, Sverdrup transport estimated by the observed wind stress indicates a maximum in winter and a minimum in summer. It is shown from a flat-bottom model that the western boundary current has a maximum transport of 90 Sv (Sv ≡ 106 m3 s−1) in late winter and a minimum transport of 40 Sv from summer to early autumn. The time lag of the calculated transport from Sverdrup transport is about one month, which shows that the barotropic response is dominant for the seasonal change in wind stress. However, the vertical velocity difference at the western boundary region reaches a maximum in summer and a minimum in autumn, of which the summer maximum coincides with th...

黒潮と日本人の遭遇史（第二部） 近世の和書と洋書にもとづく論考

After the national isolation policy was established in 1639, the building of large ships and ocean navigation were prohibited in Japan. Thus, it is difficult to find old books describing the Kuroshio. By searching and deciphering early books, however, I have found some historical descriptions and illustrations about the Kuroshio. By examining these records, coupled with records in the Western books, I have reached the following conclusion. The term originated from a local word Kurose River, used within inhabitants of the Izu Islands, which indicated a branch of the actual Kuroshio flowing over the Izu Ridge. About 1800 it became meaning the Kuroshio south of the Tokaido District, and became popular among Japanese people by the fashion of publishing maps, local geography, accounts of trips and novels. However, a view of fragmental currents at that period might interrupted the recognition of the Kuroshio as a long current. From the end of the 18th century to the mid 19th, the Western collected information about Japan's geography published by the Japanese and made surveys and analyses of the Kuroshio on the occasion of cruises to ask for establishment of commercial relationship with Japan. Before the Meiji Restoration (1868), the term had already turned to an international word, which meant the entire Kuroshio, by the international diffusion of information about the Kuroshio. However, the undersanding of the Kuroshio by the Japanese was not practical as shown in Kanrin Maru journals by them.

Temporal and spatial variability of volume transport of the Kuroshio in the East China Sea

The temporal and spatial variability of absolute geostrophic volume transport of the Kuroshio is examined using hydrographic and surface current data collected by the Japan Meteorological Agency on 39 transport sections made at six transects in the Fast China Sea and one transect just outside the East China Sea during the three-year period 1986–1988. The Kuroshio volume transport ( KVT) is estimated by integration over each transect of the downstream (positive) part of the geostrophic velocity referenced to the observed surface current. Our analysis indicates that, on average, only about 25% of the KVT inside the East China Sea is due to the baroclinic or vertically varying component. While KVT exhibits a small-scale temporal and spatial variability with downstream phase speeds of order 8–19 km day −1 and wavelengths of order 150–375 km, mean KVT is spatially uniform within the East China Sea. KVT has significant annual and interannual variations: KVT is large in summer, is positively correlated with local downstream wind stress, and is large during large meander events of the Kuroshio south of Japan. Our best estimate of the mean KVT in the East China Sea during 1986–1988 is 23.7 Sv ± 2.0 Sv. This represents about one-half of the mean interior Sverdrup transport at this latitude. The Kuroshio quickly absorbs most of the remaining interior Sverdrup transport after it exits the East China Sea through the Tokara Strait.

Ｏｎ ｔｈｅ Ｐａｔｈ Ｄｙｎａｍｉｃｓ ｏｆ ｔｈｅ Ｋｕｒｏｓｈｉｏ Ｓｏｕｔｈ ｏｆ Ｊａｐａｎ．

Ｏｎ ｔｈｅ Ｐａｔｈ Ｄｙｎａｍｉｃｓ ｏｆ ｔｈｅ Ｋｕｒｏｓｈｉｏ Ｓｏｕｔｈ ｏｆ Ｊａｐａｎ．

Cross-stream velocity structures and their downstream variation of the Kuroshio around Japan

An acoustic Doppler current profiler (ADCP) mounted inside a towed fish was used to measure current velocity structure in the upper 400 m of the Kuroshio south of Honshu. Velocity sections were obtained along two different transects during spring 1988. The results illustrate the detailed cross-stream velocity structure of the Kuroshio, including the subsurface velocity core and the subsurface countercurrent along the shelf slope. These data, in addition to data previously collected further south near Okinawa, illustrate changes in the Kuroshio as it progresses downstream. The largest transport observed was 85 Sv, including estimated flow below the ADCP measurements down to 1000 m depth.

Outbreak of warm water from the Kuroshio South of Japan

During the concentrated observation (April–May 1988) conducted as a part of the Ocean Mixed Layer Experiment (OMLET) in the sea area south of Japan, a conspicuous outbreak of warm water occurred from the large-meander region of the Kuroshio toward the southwest in the direction of the former Ocean Weather Station “T”. A series of NOAA-AVHRR infrared images clearly showed the process of this event. A surface buoy-mooring system deployed in this experiment recorded the arrival of this outbreak of water, in terms of the rise of sea-surface temperature (SST) of 1.5°C and the flow of warm water of 1.5kt toward the northwest at “T”. We studied this phenomenon by combining time series of infrared SST images with the oceanographic data obtained by two research vessels. The warm water was about 100 m deep in the section at 137°E along the edge of the Off-Shikoku Warm Water. It was estimated that about twenty outbreaks of this kind in a year can compensate a large heat loss to the atmosphere above this ocean region.

Mixing in warm‐core rings of the Kuroshio

Water mass evolution and ventilation in warm‐core rings of the Kuroshio is discussed using hydrographic (T‐S) data, remote sensing, and tritium measurements. The Kuroshio warm‐core ring A3 detached from the Kuroshio in 1986, migrated northwest along the Japan and Kuril trenches, and reached the southern Kuril Islands in 1989. This ring is characterized by weak stratification to a depth of about 700 m. In its upper layer the ring displayed a pronounced thermostad and a high tritium concentration; in its lower portion, however, temperature and salinity were density compensated. On the basis of tritium data analyses, the thermostad water was shown to originate in intermediate layers of the Kuroshio south of the subarctic front. This implies that the warm‐core rings are a major factor influencing the tritium transport in this region and that they are an important link for water exchange between the subtropical and subpolar gyres in the North Pacific. The tritium data confirm the idea about the origin of the thermostad water of anticyclonic ring from intermediate layers of the Kuroshio. Therefore a mechanism must exist in the Kuroshio frontal zone, which selectively draws water from the Kuroshio to the rings and acts to ventilate the subtropical gyre. It is also noted that during the ring cooling period a surface layer of low tritium content was formed above the ring.

A numerical experiment on the path dynamics of the Kuroshio with reference to the formation of the large meander path south of Japan

Path dynamics of the Kuroshio south of Japan are investigated with reference to the formation of the large meander of the Kuroshio south of Japan. The observed formation process is a composite of two subprocesses: generation of a small meander off eastern Kyushu (process I) and implification of the small meander south of central Japan (process II). Particularly, we have focused upon the observed evidence that the amplification of the small meander (process I) has a tendency to occur in a flat part of the Shikoku Basin, while a small meander staying on the continental slope decays in a short time without process II. On the basis of this evidence, a numerical experiment on the formation process of the large meander of the Kuroshio is carried out. topography south of Japan is modeled. In the numerical experiments, 10 runswith different in- and outflow transport, initial current path and/or bottom topography are performed. It is shown that if the inflow volume transport is less than 60 Sv, the current path is forced to flow along the coastal boundary due to the topography of the continental slope and the planetary β effect. Even if a small meander is given in the initial current path with inflow of 30 Sv, the small meander decays in a short time and the current path resumes a no-meander path. However, in the model with inflow greater than 60 Sv, the large meander path is formed. If a small meander path is given initially in the same model, a larger meander and a cyclonic eddy are formed with spin-up and spin-down processes; the results agree with the observational features of the cyclonic cold water mass south of Japan. The amplification of the large meander is carried out in a flat region. The spin-up of the large meander path and cold water mass is mainly due to the stretching of the water column when the meander shifts westward from the shallower west side slope of the Izu Ridge to the deeper region in the Shikoku Basin. If the topography of the Izu Ridge is excluded in the numerical model, the large meander is not formed. The bimodal path characteristics of the Kuroshio are discussed on the basis of the numerical results.

Major features and variability of the Kuroshio in the East China Sea

This is a brief introduction of the Kuroshio in the East China Sea (ECS). The main results of the study for this part of the Kuroshio system in recent years are reviewed and presented with emphases placed on the major features of the current structure, annual and inter-annual variations of the velocity and volume transport of the Kuroshio in ECS, and the relation between the variation of the Kuroshio in ECS and that of the Kuroshio south of Japan. And finally, an indirect relation between the variation of the Kuroshio in ECS and that of the North Equatorial Current system is suggested. It is shown that the fluctuation of the Kuroshio in ECS is also correlated with that of the North Equatorial Current and North Equatorial Counter-current. Ties of the above relation are the wind stress curl field over the tropical and subtropical belts.

Changing transport and curvature bias: A new type of meandering in the Florida Straits

Meanders in strong, inertial flows are generally asymmetric. Florida Current meanders with periods of 7–10 days and flows that tend to meander offshore when strong and onshore when weak, which we call type A, have been widely documented. We find a countertrend in the Florida Current data collected under the aegis of Subtropical Atlantic Climate Studies: a type B meandering where flows tend to meander offshore when weak and onshore when strong. On time scales of several months and depending on the strength of the flow, the tendency displays a largely one‐sided preference for either the onshore or the offshore component of the meander. In particular, the data reveal a negative curvature bias during the generally higher transports in spring and summer when most of the speed cores were inshore, and a positive curvature bias during lower transports in fall and winter when most speed cores were offshore. The data show that any use of cross‐channel sea level slope to estimate the transport would generally overestimate the weaker, but underestimate the stronger flows. Type B meander is interpreted as one of three types embodied in a meandering equation in which curvature changes are controlled primarily by the Coriolis effect of downstream acceleration and hence by that of changing transport. In this light the bimodal paths of the Kuroshio south of Japan is a type B meander.

Study of physical processes related to fisheries oceanography

For the audience or readers to deepen understanding of fisheries oceanography, especially physical fisheries oceanography, this lecture reviews my research work, including clues, failures and management problems encountered during the course of my studies. The contents of this lecture are: (1) introduction, (2) hydrographical feature in and around the Perturbed Area between the Kuroshio and Oyashio Fronts and its fluctuation, (3) conservative properties along a streamline or a trajectory and observation planning, (4) the Kuroshio south of Japan and warm and cold eddies pinched off from the Kuroshio meanders, (5) convergence-divergence of currents and accumulation-dispersal of marine organisms, (6) the Japan Sea (and a part of the East China Sea), and (7) conclusion: approaches to research in fisheries oceanography.

87:6051 The short-period cold water mass accompanied by large meander of the Kuroshio south of Japan: Ishii, Haruo and Yoshihiko Sekine, 1986. Mer, Tokyo, 24(4):169–176

87:6051 The short-period cold water mass accompanied by large meander of the Kuroshio south of Japan: Ishii, Haruo and Yoshihiko Sekine, 1986. Mer, Tokyo, 24(4):169–176

87:6045 The Kuroshio (on the PN section) in the East China Sea during 1972–1983: Pu, Yongxiu and Xiaoyun Xu, 1987. Acta oceanol. sin., 6(1):37–45. Second Inst. of Oceanogr., State Oceanic Admin., Hangzhou, People's Republic of China

The current elements of the Kuroshio in the East China Sea are analysed and discussed using the temperature-salinity and GEK data of the PN section obtained during 1972-1983. The calculations have proved (i) that the 12-year's mean volume transport of the Kuroshio at PN in the 0-800 m layer is about 20×106m3/s; (ii) that the mean volume transport has seasonal variations, i. e. slightly stronger in spring and summer than in the other seasons; and (iii) that the period of high-volume transport thereof corresponds to the appearance of a meander of the Kuroshio south of Japan. It is also discovered that this Kuroshio meander usually appears after a sharp increase in such volume transport. Finally, the differences of the Kuroshio volume transport between the PN, E and Suao-Yonakuni-jima sections are discussed.

Transition processes between the three typical paths of the Kuroshio

Transitions between the three typical paths of the Kuroshio south of Japan (the nearshore and offshore non-large-meander paths and the large-meander path) are described using sea level data at Miyake-jima and HachijO-jima in the Izu Islands and temperature data at a depth of 200 m observed from 1964 to 1975 and in 1980.

Moored instrument observations in the Kuroshio south of Kyushu

In each of the (fiscal) years 1979–1982, multilayer current measurements by means of moored instruments were carried out in the Kuroshio off Tanegashima Island, south of Kyushu, with particular interest in the baroclinic layer (0–1,000 m) of the intense current. Most of the moored instruments were in position for more than 30 days, the maximum duration being 145 days. This paper describes some noticeable features of the mean and fluctuation fields in the surface Kuroshio as inferred from the the new data sets.

Spin-up and spin-down processes of the large cold water mass of the Kuroshio south of Japan

Time variation of the cold water mass of the Kuroshio south of Japan, which was formed in August 1975 and disappeared in August 1980, is studied. Its lifecycle includes several repetitions of spin-down and spin-up processes. The spin-down (or the spin-up) process is accompanied by warming (cooling) of the cold water mass and descending (ascending) motion of the inner water. Expansion of the cold water area is also associated with the spin-up period while shrinking occurs in the spin-down period. The rate of spin-down of the cold water mass is approximately equal to that of the Gulf Stream rings. The spin-up process is not observed in the Gulf Stream rings and the longer lifetime of the cold water mass off Japan, in comparison with the Gulf Stream rings, is due to the existence of the spin-up periods. The spin-up process tends to occur in late spring to summer, and it seems to be related to the seasonal variation in intensity of the Kuroshio.

A Numerical Study of the Kuroshio South of Japan

Abstract A numerical model is used to study the interaction of the Kuroshio with boundary constraints south of Japan. The model consists of two layers with the lower layer inert, and is both nonlinear and quasi-geostrophic. The boundary constraints are highly idealized. The Kyushu Peninsula and its adjacent continental shelf have the form of a V-shaped wedge on an otherwise rectilinear coastline; the Izu Ridge is represented as a square island. Solutions almost always form a meander downstream from Kyushu. The dynamics of meander formation involve the interaction of a Rossby lee wave generated by the Kyushu Peninsula and a westward disturbance forced by the Izu Ridge. Three different quasi-steady paths have been identified. The first path has a meander between Kyushu and the Izu Ridge, passes to the north of the ridge, and is associated with a current of low-volume transport. The second path has a large meander between Kyushu and the Izu Ridge, passes to the south of the Izu Ridge, and always occurs when ...