branching and joining of the tsushima current around the...

Journal of OceanographyVol. 52, pp. 747 to 761. 1996

Branching and Joining of the Tsushima Currentaround the Oki Islands

OSAMU KATOH1, KENJI MORINAGA1, KUNIAKI MIYAJI2* and KAZUYUKI TESHIMA1**

1Seikai National Fisheries Research Institute, 49 Kokubu-machi, Nagasaki 850, Japan2National Research Institute of Fisheries Science, 2-12-4 Fukuura, Kanazawa-ku, Yokohama 236, Japan

(Received 29 January 1996; in revised form 5 April 1996; accepted 13 April 1996)

We discussed the branching and joining of the Tsushima Current around the OkiIslands, based on ADCP and CTD measurements carried out in June 1990 by thequadrireciprocal method (Katoh, 1988). The volume transport of the northeastwardcurrent northwest of the Izumo Coast was about 2 Sv. The triple-branch structureof the Tsushima Current was obscure there. This northeastward current dividedinto the eastward and northward currents, with volume transports of 0.5 Sv and1.5 Sv, respectively, at the west entrance of the Oki Strait. Most of the first branchof the Tsushima Current seemed to be separated again from the other confluentbranches and to pass through the Oki Strait as this eastward current. Thenorthward current was composed of the second and the third branches of theTsushima Current. It detoured the Oki Islands, and almost all of it returned southto the Tajima Coast. In the vicinity of the Tajima Coast, the eastward current wasabruptly strengthened through the confluence of the southward one which wasoriginated from the northward current west of the Oki Islands. This showed thatthe first branch finally joined the compound of the second and the third branchesdetouring the Oki Islands. Between the Oki Strait and the Tajima Coast, the two-layer structure of currents was clearly found.

1. IntroductionThe sea around the Oki Islands (cf. Fig. 1) is very important as a major fishing ground of

crabs and prawns as well as pelagic fishes, e.g. the Japanese sardine and mackerel. Hence, it hasbeen desired to clarify the structure of the Tsushima Current and its spatial variation processaround the Oki Islands not only by oceanographers but also by fisheries scientists.

The Tsushima Current shows the triple-branch structure in many cases, although it is verychangeable spatially and temporally just after entering into the Japan Sea. In the sea northwestof the Izumo Coast, however, the first branch of the Tsushima Current flowing eastward alongthe 100 m isobath from the Eastern Channel of the Tsushima Strait, joins the second branchflowing eastward south of 37°N from the Western Channel. In the sea north of Hamada,moreover, the second branch often becomes confluent with the third branch flowing northwardalong the east coast of Korea. Consequently, the triple-branch structure of the Tsushima Currentcomes to be obscure near the Oki Islands (Katoh, 1994b). In the sea around the Oki Islands, thecontinental shelf extends northward, and thus, the Tsushima Current is much influenced by the

*Present Address: National Research Institute of Far Seas Fisheries, 5-7-1 Orido, Shimizu 424, Japan.**Present Address: Tohoku National Fisheries Research Institute, 3-27-5 Shinhama-cho, Shiogama 985,

Japan.

748 O. Katoh et al.

topographic feature.The main Tsushima Current is thought to detour the Oki Islands, because the Oki Strait,

which is between the Oki Islands and Honshu, is shallower than 100 m depth (Fukuoka, 1957;Moriyasu, 1972). However, a current with a velocity of 0.6 kt (31 cm s–1) was recorded in the OkiStrait (Minami et al., 1984), and some satellite-tracked drifters launched in the East China Seapassed through the Oki Strait, via the Eastern Channel of the Tsushima Strait (Beardsley et al.,1992; Michida and Ishii, 1993). Fukuoka (1957) also showed that the current velocity in the OkiStrait was 0.5–0.6 kt (26–31 cm s–1) and the volume transport through the strait was estimatedat 0.5 Sv. These indicate that the current passing through the Oki Strait cannot be ignored whenthe Tsushima Current east of the Oki Islands is discussed.

In the coastal region east of the Oki Strait, mean current velocities near the surface aregenerally 0.4–0.5 kt (21–26 cm s–1) (Matsuyama and Nazumi, 1986; Matsuyama 1990; Yoshi etal., 1995). On the other hand, Michida and Ishii (1993) showed that the velocity of the satellite-tracked drifter remarkably increased after passing the Oki Strait, reaching 1.5–2.5 kt (77–129cm s–1) between the Oki Strait and the Noto Peninsula. Matsuyama et al. (1988) also measuredlarge velocities of 1.0–2.0 kt (51–103 cm s–1) near the Tango Peninsula. Moreover, Matsuyama(1990) discovered a westward current with mean velocities of about 0.2 kt (10 cm s–1) near thesea bed off the Tajima Coast; he pointed out that the surface layer moved eastward while thedeeper layer moved westward. These mean that the structure of the Tsushima Current varieslargely between west and east of the Oki Strait. It is well known that a warm eddy often existseast of the Oki Islands (Matsuyama et al., 1988, 1990; Isoda and Nishihara, 1992); Matsuyamaet al. (1988, 1990) clearly showed that the warm eddy was confirmed even at a depth of 300 min 1987 and 1988. It is natural to think that the warm eddy has some influence on the variationof current structures east of the Oki Islands.

Fig. 1. Observation region (left) and transects and stations for ADCP and CTD measurements with theR/V Kumamoto-Maru in June 1990 (right). ADCP measurements with the quadrireciprocal methodwere taken only on transects from A to E. CTD casts were carried out at the stations of solid dots, andon transects A–E at an interval of about 3.3 nautical miles (6.1 km). Volume transports throughtransects A–E were calculated from ADCP data and the geostrophic calculation, while those throughtransects F–H were calculated from the geostrophic calculation only. The abbreviations “E.C.” and“W.C.” indicate the Eastern and Western Channels of the Tsushima Strait, respectively.

Branching and Joining of the Tsushima Current around the Oki Islands 749

In this paper, detailed current structures around the Oki Islands are described on the basisof ADCP and CTD data, and the branching and joining of the Tsushima Current is quantitativelydiscussed.

2. Data and MethodIn order to investigate current structures around the Oki Islands, extensive ADCP and CTD

measurements were carried out on 14–29 June 1990 using the R/V Kumamoto-Maru (380 ton,Kumamoto Prefectural High School of Fisheries).

The installed ADCP was JLN-612 (Japan Radio Company Ltd.) whose frequency was 125kHz. Flow direction and velocity were recorded in units of 0.1 degrees and 0.1 kt (5 cm s–1),respectively. This ADCP could measure velocities relative to the sea bed shallower than 300 m,and thus, the ADCP velocities were obtained at three depths only along transects from A to E (Fig.1). The shallowest sampling depth was fixed at 20 m, and the deepest and intermediate depthswere adjusted to within 70% of the sea bottom depth.

The authors took ADCP measurements with the “quadrireciprocal method” (Katoh, 1988)which requires four round-trip surveys along a transect during a diurnal tidal period of 24 hoursand 50 minutes. In this paper, a flow calculated by this method is called a “diurnally averagedflow”, in other words, a flow averaged in the diurnal tidal period. In cases of transects B and C,the transects were divided into three and two sections with length of about 30 nautical miles (56km), respectively. A run for the quadrireciprocal ADCP measurements, which took 24 hours and50 minutes, was carried out along each divided section with length of 30 nautical miles (56 km).Analysis of current data were made according to Katoh (1988).

The tidal flows east of the Oki Islands are generally thought to be weak, since the amplitudeof tidal fluctuation is very small on the coast. In fact, the measurements with mooring arraysshowed that the tidal flows were hardly seen near the Tajima Coast (Matsuyama et al., 1986). In

Fig. 2. Observed and tidal flows averaged for each round-trip survey at an interval of about six hours ontransect E in June 1990. Northerly direction and stick’s length for 1 kt (51 cm s–1) are shown, too.

750 O. Katoh et al.

the offshore area, however, the tidal flows have not been studied in detail, and thus they arepreliminarily shown in Fig. 2, using the present ADCP data on transect E. The difference betweenthe observed flow and the diurnally averaged one is regarded as the tidal flow in this figure. Theobserved and tidal flows fluctuate largely at 20 m depth at bins E6–E10; the tidal flows, withvelocities larger than 0.5 kt (26 cm s–1), are found at the first and second round-trip surveys. Onthe other hand, the tidal flows are very weak at bins E1–E6 near the coast. The cause of such largetidal flows at bins E6–E10 has not been clarified, but at all events, Fig. 2 shows that the tidal flowscannot be ignored not only west but also east of the Oki Islands.

CTD (AST-1000, Alec Electronics Company Ltd.) measurements were made at the stationsof solid dots in Fig. 1, and on transects A–E at an interval of about 3.3 nautical miles (6.1 km).Temperature and salinity were recorded in units of 0.01°C and 0.01 psu, respectively. Salinitiesof the surface water sampled at all stations for CTD casts were examined using the salinometer(MODEL 601MkIII, YEO-KAL Environmental Electronics), in order to correct the CTD data.On transects F–H as well as A–E, geostrophic flows were calculated. The maximum of CTDmeasurement depth was 330 m in the offshore area deeper than 330 m, and thus, the referencedepth was set near the bottom in case water depth ≤330 m or at 330 m depth in case water depth>330 m.

On transects A–E, velocities at the surface and the reference depth were estimated, using theADCP data and the geostrophic calculations as follows. Let V1 and V3 be the diurnally averagedvelocity components perpendicular to the transect at the shallowest and deepest depths of theADCP measurements, respectively. Let G1 and G3 the velocities derived from the geostrophiccalculation at the shallowest and deepest depths of the ADCP measurements, respectively.Velocities at the surface and the reference depth, Vs and Vb, respectively, are expressed by thefollowing equations (Katoh, 1993),

Vs = Gs + (V1 – G1),

Vb = V3 – G3,

where Gs is the surface velocity obtained from the geostrophic calculation. The volume transportabove the reference depth through a bin with a width of about 3.3 nautical miles (6.1 km) wascalculated by the method of Katoh (1994b). On the other hand, the volume transports throughtransects F–H were calculated based on the geostrophic calculation only.

3. Results

3.1 Temperature distributionsHorizontal distributions of temperature at 100 m, 200 m and 300 m depths are shown in Fig.

3. At 100 m depth, very cold water (<6°C) appears west of the Oki Islands. A bunch of crowdedisotherms extends northward from the Izumo Coast, deflecting westward near 36°30′ N. Northof the Tajima Coast, too, cold water (<8°C) is found, and isotherms are crowded near the coast.At 200 m depth, a warm eddy is found northeast of the Oki Islands; the temperature at its centeris higher than 11°C. The warm eddy is clearly recognized at 300 m depth, too, and the temperatureat its center maintains 7°C. On the other hand, the very cold water with temperature lower than1°C is uniformly distributed west of 133°E.

The vertical sections of temperature, salinity and geostrophic velocity along transects G and


Fig

. 3.

Tem

pera

ture

(in

°C

) at

100

m, 2

00 m

and

300

m d

epth

s in

Jun

e 19

90.

752 O. Katoh et al.

Fig. 4. Vertical sections of temperature, salinity and geostrophic velocity along transects G and H in June1990. The reference depth is set near the bottom in case water depth ≤330 m or at 330 m depth in casewater depth >330 m. The hatched parts indicate westward and southward flows for transects G and H,respectively.


H are shown in Fig. 4. On both transects, the change of salinity, generally between 34 and 34.5,is small, especially below 100 m depth. This means that the current structure in the offshore areaeast of the Oki Islands is almost determined by the temperature one. On transect G, configurationof isotherms in the upper 100 m layer is quite different from that in the lower layer; isothermsdescend to the south in the upper layer, while they ascend in the lower. Westward velocities aredistributed wholly north of 35°55′ N, while eastward ones are found south of the latitude. Thelayer of the maximum westward velocities deepens to the north in accordance with the depth ofnontilting isotherms. On transect H, too, the temperature structure similar to transect G is foundon the western section of 134°35′ E, but the two isotherms are contacted on the eastern section;the maximum of southward velocities is found at depth of 120–160 m in the western section,while near the sea surface in the eastern one. In the section north of 36°15′ N on transect G andthat west of 134°35′ E on transect H, horizontal gradient of temperature is very steep at a depthof 330 m, and this indicates that a considerably strong vertical shear of velocity exists even at thedepth. Hence, the velocities in these sections are actually larger than those shown in Fig. 4.

In order to grasp the path of the Tsushima Current, temperature distributions at 100 m and200 m depths in the southern Japan Sea during the two periods, 1–13 June and 5–18 July 1990,just before and after the present observations, respectively, are shown in Fig. 5, on the basis ofMaritime Safety Agency of Japan (1990). The bunch of isotherms at 9–14°C meanders largelyat 100 m depth; it detours around the Oki Islands and returns southward to the coast near thelongitude at 135°E in the later case. Comparing Fig. 5 with Fig. 3, we can say that the crowdedisotherms extending northward from the Izumo Coast in Fig. 3 are sequenced with those northof the Tajima Coast. The center position of the warm eddy, which can be clearly found at 200 mdepth, is almost steady through the two measurement periods.

Fig. 5. Temperature (in °C) at 100 m and 200 m depths in the southern Japan Sea just before and after thepresent observations, adapted from Maritime Safety Agency of Japan (1990).

754 O. Katoh et al.

Fig. 6. Temperature and salinity sections along transects A–E with diurnally averaged flow vectors atthree depths in June 1990. The sections across the Oki Strait, between 35°40′ N and 36°00′ N along133°10′ E, are shown, too.


3.2 Distributions of diurnally averaged flowsVertical distributions of diurnally averaged flows at three depths along transects A–E, with

temperature and salinity distributions, are shown in Fig. 6. In this figure, temperature and salinitysections across the Oki Strait, between 35°40′ N and 36°00′ N along 133°10′ E, are shown, too.On each of the transects, considerable eastward to northeastward flows are found in the upperlayer: mainly at stations A2, B5, C6, D3 and E4. On transect A, northeastward flows aresomewhat strengthened at station A7. In the Oki Strait, isotherms descend to the south, andrelatively high-temperature and low-salinity water is distributed near the coast of Honshu. Thisclearly indicates the existence of eastward flows through the Oki Strait. On transect C, relativelylow-salinity (<34.2) water is distributed at bins C1–C10, where the eastward flows are found. Thelow-salinity water widely appears in the Oki Strait, but it is found in a limited area at bins C10–C19. This means that the eastward flows at station C6 are sequenced from the Oki Strait, notdetouring the Oki Islands. On transects A, B and E, flow directions at 20 m and 100 m depths arevery similar, although the velocities at 100 m depth are much smaller than those at 20 m depth.On transects C and D, however, flow directions at the two depths are quite different from each

Fig. 7. Onshore sections of velocities perpendicular to transects in June 1990 (in kt).

756 O. Katoh et al.

other; mainly eastward on both transects at 20 m depth, while northwestward on transect C andsouthward on transect D at 100 m depth. The countercurrent at 100 m depth has velocities of 0.3–0.5 kt (15–26 cm s–1).

Onshore sections of velocities perpendicular to transects A–E are shown in Fig. 7. Ontransect A, the existence of two velocity cores is barely confirmed near stations A2 and A7. Ontransects B–D, the area where velocities are larger than 0.2 kt (10 cm s–1) is limited above 80 m,which is nearly equivalent to the sill depth of the Oki Strait, but on transect E it reaches 150 mdepth.

Fig. 9. Topography (in meter) of the 7°C isothermal surface in June 1990. Transects C–H are shown, too.

Fig. 8. Diurnally averaged flow vectors at 20 m depth (left) and 100 m depth (right), and temperature (in°C) at 75 m depth (left) and 150 m depth (right), in June 1990.


Fig. 10. T-S diagrams in the area of the eastward to northeastward flows on transects A–E, together withthat at station M in the Oki Strait.

Horizontal distributions of diurnally averaged flows at 20 m and 100 m depths in the onshorearea shallower than 300 m are shown in Fig. 8. Judging from Fig. 6, flow distributions at 20 mand 100 m depths along transect C are well correspondent to the temperature structures in thelayers upper and lower than 100 m depth, respectively. Hence, isotherms at 75 m and 150 mdepths are contoured in the left and right panels of Fig. 8, respectively, in order to detect thesequence of currents in the onshore area. On the other hand, the flow distribution in the offshorearea deeper than 300 m coincides basically with the configuration of the lower thermocline, asshown in Fig. 4. Hence, the depth of the 7°C isothermal surface, the middle of the lowerthermocline, is contoured in Fig. 9, in order to investigate the sequence of currents in the offshorearea.

Figure 8 shows that the offshore part of the northeastward flows around transects A and Boff the Izumo Coast detours the Oki Islands. On the other hand, the onshore part of them passesthrough the Oki Strait, as shown by the extension of isotherms at 17°C and 18°C in the left panel.After passing through the Oki Strait, the current at 20 m depth has velocities of 0.7–0.9 kt (36–46 cm s–1) on transects C and D, but it is abruptly strengthened on transect E; the velocity reaches1.5 kt (77 cm s–1). This abrupt strengthening of the current reflects the confluence of thesouthward current near the Tajima Coast, as shown in Fig. 5. Distributions of the flows at 100m depth and the temperature at 150 m depth in the right panel indicate that the southward currentdivides into an eastward current and a westward current near transect D, and that the westwardcurrent bends clockwise along the isobath at 100 m. Figure 9 clearly shows that the strongsouthward current flows through transect H. The major part of this current circulates clockwise

758 O. Katoh et al.

north of transect D as the warm eddy, and its remainder which does not circulate, flows eastwardthrough transects E and F.

3.3 T-S diagramsT-S diagrams in the area of the eastward to northeastward flows on transects A–E are shown

in Fig. 10. Stations for T-S diagrams, A2, A7, B5, C6, D3 and E4, are located in the parts wherethe velocity cores are found in Fig. 7. The T-S diagram for the station at 35°50′ N and 133°10′E in the Oki Strait, named as station “M”, is shown in this figure, too. The T-S diagrams at stationsA2, B5, M, C6 and D3 are similar to each other in the range between 17°C and 21°C, which isthe representative range of the water temperature of the eastward to northeastward flows. On theother hand, the T-S diagram at station A7 is largely different from that at station A2. This indicatesthat the water passing through the Oki Strait is different from that detouring the Oki Islands. TheT-S diagram at station E4 shows an intermediate feature between the T-S diagram at station A2and that at station A7.

3.4 Volume transports through transectsVolume transports through the sections of transects A–H are shown in Table 1. The values

through transects B and C are calculated between stations B1 and B10, and stations C1 and C10,respectively. The values through transects F–H are based on the geostrophic calculations only.The temperature distribution at 300 m depth in Fig. 3 shows that considerable velocities exist ontransects G and H even at 330 m depth which is regarded as the reference depth in this paper.Hence, the real volume transports through transects G and H are probably larger than those shownin Table 1. In order to detect the sequence of currents from the Oki Strait whose sill depth is about80 m, the volume transports are divided into the value for the upper layer (0–80 m depth) and thatfor the lower layer (80 m–reference depth).

The total volume transport through transect A is 2.0 Sv (1 Sv = 106 m3s–1). Volume transportsthrough the upper layers of transects B–D are about 0.5 Sv, which are equivalent to the volumetransport through the Oki Strait, and thus, the total volume transport of the northward currentdetouring the Oki Islands is about 1.5 Sv. The large increase of the volume transport through

*The reference depth is set near the bottom in case water depth ≤330 m or at 330 m depth in case waterdepth >330 m.

Transect A B(B1–B10)

C(C1–C10)

D E F G H

Volume transporttotal (0 m–ref. depth*) 1.96 0.53 0.48 0.21 1.30 0.45 –3.03 –4.85upper layer (0–80 m) 1.37 0.51 0.53 0.42 0.99 0.27 –0.72 –1.95lower layer (80 m–ref. depth) 0.59 0.02 –0.05 –0.21 0.31 0.18 –2.31 –2.90

Table 1. Volume transports (in Sv) through transects A–H. The values through transects B and C arecalculated between stations B1 and B10, and between stations C1 and C10, respectively. The valuesthrough transects F–H are based on the geostrophic calculation only. The plus and minus for transectsA–G indicate the eastward and westward volume transports, respectively. The minus for transect Hindicates the southward volume transport.


transect E verifies the confluence of the southward current, which is originated from the currentdetouring the Oki Islands.

The sum of the total volume transports through transects E and F is 1.75 Sv, which is about88% of the total volume transport of 2.0 Sv through transect A. This means that almost all of thenorthward current detouring the Oki Islands returns near the Tajima Coast. The total volumetransport through transect G is 3.03 Sv, which circulates to compose the warm eddy. With regardto the balance of total volume transports in the area enclosed by transects E–H and the TajimaCoast line (cf. Fig. 9), the inflow of 4.85 Sv through transect H is almost equal to the outflow of4.78 Sv which is the sum of volume transports through transects E–G.

4. DiscussionsThe volume transport through transect A northwest of the Izumo Coast is about 2.0 Sv.

Kawano (1993) and Isobe et al. (1994) estimated the volume transport through the TsushimaStrait as about 2.7 Sv and 1.3 Sv, respectively, early in July 1990, just after the presentmeasurements, their mean is 2.0 Sv. This indicates that almost all of the volume transport throughthe Tsushima Strait is converged again northwest of the Izumo Coast in the observation period,and it confirms the result of Katoh (1994b) that the triple-branch structure of the TsushimaCurrent comes to be obscure through the confluence of the first, second and third branches nearthe Oki Islands.

The volume transport through the Oki Strait, about 0.5 Sv, is equal to the value reported byFukuoka (1957). Katoh (1994a) showed that the volume transport of the first branch of theTsushima Current northwest of Yamaguchi Prefecture, the most western prefecture in Honshu,ranged from 0.43 Sv to 0.86 Sv, with the average of 0.61 Sv in June and July 1989. Judging fromthe extension of isotherms at 17°C and 18°C (Fig. 8), the difference of T-S diagrams betweenstations A2 and A7 (Fig. 10) and the average of the volume transport of the first branch, most ofthe first branch seems to be again separated from the other confluent branches and to pass throughthe Oki Strait. The trajectories of satellite-tracked drifters passing through the Oki Strait via theEastern Channel of the Tsushima Strait (Beardsley et al., 1992; Michida and Ishii, 1993) stronglysupport this assumption. In order to confirm the assumption, it is necessary to investigate howthe volume transport through the Oki Strait fluctuates in accordance with the fluctuation of thefirst branch in the west of the Oki Strait. On the other hand, the volume transport of the northwardcurrent detouring the Oki Islands is about 1.5 Sv. It is concluded that this northward current witha volume transport of 1.5 Sv is composed of the second and the third branches of the TsushimaCurrent.

The mean velocities near the surface are generally 0.4–0.5 kt (21–26 cm s–1) in coastal waterseast of the Oki Strait (Matsuyama and Nazumi, 1986; Matsuyama 1990; Yoshi et al., 1995). Inthe present observations, however, the eastward current passing through the Oki Strait is abruptlystrengthened near the Tajima Coast through the confluence of the southward current, and itsvelocity reaches 1.5 kt (77 cm s–1). The confluence of the eastward and the southward currentsmeans that the first branch, passing through the Oki Strait, finally joins the compound of thesecond and the third branches detouring the Oki Islands. This indicates that the first branchsometimes varies largely east of the Oki Strait and it can be hardly identified through theconfluence.

Current velocities larger than 1 kt (51 cm s–1) were measured east of the Oki Strait byMatsuyama et al. (1988) and Michida and Ishii (1993), too. In case of Matsuyama et al. (1988),the temperature distribution was very similar to the present case when the warm eddy exists.

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Hence, the strong current shown by Matsuyama et al. (1988) probably resulted from the confluencefrom the southward current related with the warm eddy. In case of Michida and Ishii (1993),however, the warm eddy implying the existence of the southward current did not exist in July1991, judging from temperature distributions issued by Maritime Safety Agency of Japan (1991).Matsuyama and Nazumi (1986) showed that the current near the Tajima Coast fluctuated largelyaccording to the wind fluctuation. These facts suggest that there are some factors for strength-ening the first branch near the Tajima Coast, for example the wind fluctuation, other than theconfluence of the southward current related with the warm eddy.

On the basis of these discussions, the schematic representations for current paths in the upperand lower layers in June 1990 are shown in Fig. 11.

5. ConclusionsThe main conclusions obtained from the present study are as follows:1) The volume transport of the northeastward current northwest of the Izumo Coast was

about 2.0 Sv, which was nearly equivalent to the volume transport through the Tsushima Strait.The triple-branch structure of the Tsushima Current was obscure there.

2) This northeastward current divided into the eastward and northward currents at the westentrance of the Oki Strait. The eastward current, passing through the Oki Strait, had a volumetransport of about 0.5 Sv. Most of the first branch of the Tsushima Current seemed to be againseparated from the other confluent branches and to pass through the Oki Strait as this eastwardcurrent. The northward current with a volume transport of about 1.5 Sv was composed of thesecond and the third branches of the Tsushima Current. It detoured the Oki Islands, and almostall of it returned south to the Tajima Coast.

3) In the vicinity of Tajima Coast, the eastward current was suddenly strengthened throughthe confluence of the southward one which was originated from the northward current west ofthe Oki Islands; its velocity reached 1.5 kt (77 cm s–1). This showed that the first branch finallyjoined the compound of the second and the third branches detouring the Oki Islands, and that itcould be hardly identified.

Fig. 11. Schematic representations for current paths in the upper (0–80 m depth) and lower (80 m–reference depth) layers around the Oki Islands in June 1990.


4) Between the Oki Strait and the Tajima Coast, the two-layer structure of currents wasclearly found; the countercurrent with velocities of 0.3–0.5 kt (15–26 cm s–1) was observed at100 m depth.

AcknowledgementsThe authors express their sincere gratitude to Dr. H. Kawai, Professor Emeritus of Kyoto

University, for his helpful guidance and critical reading of the manuscript. The authors are verygrateful to the crew of the R/V Kumamoto-Maru (Former Captain, N. Takaoka) for their valuableassistance, and to the two anonymous reviewers and the editor for their critical reading and usefulcomments.

This study is done as a part of the observation supported by the Fisheries Agency of Japanfor the improvement of fishery stock assessment system. This paper is contribution number 525from the Seikai National Fisheries Research Institute.

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branching and joining of the tsushima current around the...

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