The Current Structure of the Tsushima Warm Current along the Japanese Coast

Transcription

1 Journal of Oceanography, Vol. 55, pp. 217 to The Current Structure of the Tsushima Warm Current along the Japanese Coast HIDEAKI HASE 1, JONG-HWAN YOON 2 and WATARU KOTERAYAMA 2 1 Department of Earth System Science and Technology, Interdisciplinary Graduate School of Engineering Science, Kyushu University, 6-1 Kasuga Koen, Kasuga , Japan 2 Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga Koen, Kasuga , Japan (Received 25 September 1998; in revised form 18 November 1998; accepted 18 November 1998) The branching of the Tsushima Warm Current (TWC) along the Japanese coast is studied based upon intensive ADCP and CTD measurements conducted off the Wakasa Bay in every early summer of , the analysis of the temperature distribution at 100 m depth and the tracks of the surface drifters (Ishii and Michida, 1996; Lee et al., 1997). The first branch of TWC (FBTWC) exists throughout the year. It starts from the eastern channel of the Tsushima Straits, flows along the isobath shallower than 200 m along the Japanese coast and flows out through the Tsugaru Strait. The current flowing through the western channel of the Tsushima Straits feeds the second branch of TWC (SBTWC) which develops from spring to fall. The development of SBTWC propagates from the Tsushima Straits to Noto Peninsula at a speed of about 7 cm sec 1 following the continental shelf break with a strong baroclinicity. However, SBTWC cannot be always found around the shelf break because its path is influenced by the development of eddies. It is concluded that SBTWC is a topographically steered current; a current steered by the continental shelf break. Salient features at intermediate depth are the southwestward subsurface counter current (SWSCC) between 150 m and 300 m depths over the shelf region in with the velocity exceeding about 5 cm sec 1, although discrepancies of the velocity and its location are observed between the ADCP data and the geostrophic currents. Keywords: Tsushima Warm Current, double core structure, mean path, intensive direct measurements, analysis of 100 m depth temperature for 27 years. 1. Introduction After passing through the Tsushima Straits, the Tsushima Warm Current (TWC) along the Japanese coast is characterized by strong variabilities in connection with many meanders and eddies. Due to this strong variabilities, the mean path and characteristics of variations of TWC along the Japanese coast have not yet been well clarified. Many possible mean flow patterns (single meandering path, branching path and combination of these paths) of TWC have been proposed. For example, Ohwada and Tanioka (1971) and Moriyasu (1972) proposed, from an analysis of the dynamic topography referred to 300 db or more, that the mean path of TWC could be described as a single meandering current rather than a branching current. Naganuma (1973, 1977, 1985) suggested that TWC took the branching path and the single meandering path alternately. On the other hand, numerical and theoretical studies suggested that TWC along the Japanese coast was controlled topographically and divided into two branches. One branch (nearshore branch) was trapped along the shelf with a strong barotropy throughout the year, as a topographically steered current. The other branch (offshore branch) developed as the volume transport through the Tsushima Straits increased in early summer, also being trapped along the continental shelf break and slope between 200 m and 500 m depths with a strong baroclinic structure (e.g., Yoon, 1982, 1991). According to Kawabe (1982b, c), the continental shelf break and slope act as a wave guide for higher modes of topographic Rossby waves with the coast to the right in the northern hemisphere, being accompanied by baroclinic structures in a baroclinic ocean. However, due to the shallowness in the shelf region, the nearshore branch had been hardly detected by geostrophic calculations and the offshore branch had also not been detected due to the strong variability. Recent direct current measurements (Katoh, 1993, 1994; Katoh et al., 1996) using ADCP suggested that TWC comprises two branches along the Japanese coast west of 135 E in summer; the first branch (nearshore branch) basically flowing eastward along the isobath of 100 m, and the second one (offshore branch) flowing eastward along the thermal front at 100 m depth. Copyright The Oceanographic Society of Japan. 217

2 The existence of the first branch in summer was also suggested by the tracking of ARGOS buoys (Ishii and Michida, 1996; Lee et al., 1997). Most of the buoys passing through the eastern channel of the Tsushima Straits in summer reached the Tsugaru Strait in about a month, roughly following the m isobath along the Japanese coast (Fig. 1(a)), suggesting a coastal current (the first branch) trapped within a narrow belt along the coast. In winter, the Fig. 1. Trajectory of the surface drifter (ARGOS buoy) since 1991 (after Lee et al., 1997). (a) Selected drifter trajectories that pass the eastern channel of the Tsushima/Korea Straits. (b) Selected drifter trajectories that pass the western channel of the Tsushima/Korea Straits. The tracks are color coded with respect to season: summer (full lines) and other seasons (broken lines). The black arrow heads are marked at every ten days. 218 H. Hase et al.

3 ARGOS buoy tracking (see Fig. 1(a)) suggested the existence of the first branch west of the Oki Islands, but not east of Oki Islands because of the landing of ARGOS buoys due to the strong northwest monsoon. The existence of the first branch in winter east of the Noto Peninsula might be suggested by the movement of the spilled oil from the Russian tanker early January in 1997, which reached the coasts of Niigata Prefecture in about a month, flowing along the Japanese coast from the Wakasa Bay referred to Reports of drifting-landing state of spilled oil from Russian tanker Nakhodka by Eighth and Ninth Regional Maritime Safety Headquarters. The ARGOS buoys passing through the western channel of the Tsushima Straits flowed along the continental shelf break and slope with strong variabilities in the path west of the Noto Peninsula and flowed northward far off the Japanese coast after separating from the shelf and slope north of the Noto Peninsula, showing a strong correlation with the second branch of TWC (see Fig. 1(b)). To confirm above suggestions on the path of TWC along the Japanese coast in terms of observations and models, series of ADCP and CTD measurements were carried out off the Wakasa Bay in every early summer from 1995 to 1998, and the temperature data during the period from 1963 to 1989 were analyzed to detect the mean flow pattern of TWC and its variations along the Japanese coast. 2. Observations of TWC off the Wakasa Bay 2.1 Data and methods CTD and ADCP measurements off the Wakasa Bay (see Fig. 2) were conducted during May 27 June 3 in 1995, June in 1996, June in 1997 and June 5 7 in 1998 by the T/V Kakuyo-Maru of Nagasaki University. The CTD measurements were carried out along the line A in , the line B in 1995 and the line C in 1998 (Table 1). The CTD castings (Mark IIIB, Neil Brown) were performed from the sea surface to 1010 db depth, or to about 20 db above the bottom at depths shallower than 1000 db. Each data (pressure, conductivity and temperature) were taken every 1/32 seconds. Water samples were taken using Rosette water samplers for the calibration of salinity. After eliminating the abnormal data, such as the negative pressure and the salinity under 30.0 psu near the sea surface, the data beyond 3.0 times the standard deviation from the mean were eliminated for every 1 db range. We then repeated the same procedure with 2.5 times of the new standard deviation. Finally, these data were averaged in every 1 db interval. As Katoh et al. (1996) pointed out, the tidal currents are not negligible in the shelf region of the south-western Japan Sea. To remove the tidal current, ADCP surveys were repeated along the same line (e.g., Isobe, 1992; Katoh, 1993) four times at intervals of about six hours and twelve minutes Fig. 2. A map of the locations of the CTD and ADCP measurements by the T/V Kakuyo-Maru during May 27 June 3 in 1995, June in 1996, June in 1997 and June 5 7 in 1998 and the bottom topographic feature. The ADCP surveys were carried out along Stns. A4 A13 and B1 B10 in 1995, all stations along the line A in 1996 and 1997 and Stns. C2 C10 in The Current Structure of the Tsushima Warm Current along the Japanese Coast 219

4 Table 1. CTD observations off the Wakasa Bay. Date May 27, 1995 May 30, 1995 June 1, 1995 June 3, 1995 June 10, 1996 June 11, 1996 June 10, 1997 June 11, 1997 June 5, 1998 June 6, 1998 Stations A1 A7 A8 A13 B1 B7 B8 B19 A1 A3 A4 A13 A1 A5 A6 A13 C1 C2 C10 Table 2. ADCP plying surveys from 1995 to Period Transects Plying times May 30 31, 1995 A4 A13 4 June 1 2, 1995 B1 B10 4 June 11 12, 1996 A1 A7 4 June 12 13, 1996 A7 A June 11 12, 1997 A7 A June 12 13, 1997 A1 A7 3.5 June 6 7, 1998 C6 C June 7, 1998 C2 C6 0.5 Fig. 3. Comparisons of the ADCP-measured ship velocities with the calculated ship velocities using the GPS data (the ship speed (a) and direction (b)), where the subscripts g and b indicate the values obtained by the GPS and bottom-tracking ADCP, respectively. between Stns. A4 A13 and B1 B10 in 1995, four times between Stns. A1 A7, three and a half times between Stns. A7 A13 in 1996, three and a half times between Stns. A1 A7 and A7 A13 in 1997, and three and a half times between Stns. C6 C10 in 1998 (Table 2). On the assumption that tidal signals are sufficiently small off the shelf region, no ADCP plying surveys were conducted between Stns. C2 C6. Two types of ADCP were used for these surveys; one is a ship-mounted ADCP used in and the other is a towed ADCP used in The ship-mounted ADCP (CI-30, Furuno Electric Company Ltd.) measured the current velocities at three levels: 20 m, 70 m and 150 m along the line A and 30 m, 80 m and 150 m along the line B. The data (averaged over 2 minutes) were taken every 5 minutes in the unit of 0.1 knot and 0.1 degree for the current speed and direction, respectively. According to Joyce (1989), the ship-mounted ADCP data contain errors due to the misinstallation of the ADCP transducer. For the correction of the errors, we compare the ADCP-measured ship velocities with the calculated ones using the ship position data from the GPS, as shown in Fig. 3. Here (U g, U b ) and (θ g, θ b ) are the ship speed and direction, where the subscripts g and b indicate the values obtained by the GPS and bottom-tracking ADCP, respectively. The discrepancies between these values are regarded as the installation error. The estimated errors (Joyce, 1989) are 1 + β = and α = 1.154, where 1 + β is the error coefficient due to the tilt of the ADCP transducer from the horizontal plane and α is the anticlockwise error angle due to the misalignment between the gyrocompass and the transducer. After the correction of these errors, these data were averaged in every about 3.8 km interval along the observational lines to filter out small scale disturbances. The towed ADCP (RD-SC0150, RD Instruments) is equipped on the ocean observational vehicle Flying Fish, which measures CO 2, DO, temperature, salinity, depth, turbidity, chlorophyll, ph and multi-level current velocities 220 H. Hase et al.

5 Fig. 4. Observed currents (a), the tidal currents (b) and the residual currents (c) for each survey along Stns. A7 A13 in The residual currents indicate the observed currents minus the tidal currents. The velocity is the component perpendicular to the line A. The positive value indicates the north-eastward flow. Fig. 5. Vertical sections of the temperature along the line A (a) and B (b) in 1995, the line A in 1996 (c) and 1997 (d) and the line C (e) in Contour intervals of full and broken line are 1.0 C and 0.1 C, respectively. The Current Structure of the Tsushima Warm Current along the Japanese Coast 221

6 in real time (Koterayama and Yamaguchi, 1994). The data were taken every 11 seconds at every 8 m depth from 32 m to 368 m depth. In waters shallower than 368 m depth, the data within the range of 15% of total depth from the bottom were treated as missing. After the correction of the magnetic declination, we used the data with the percentage of good values (percent good) over 60, and took the mean in every vertical section with about 1.9 km width and 16 m depth. In waters deeper than 300 m depth (530 m depth), the ship-mounted ADCP (the towed ADCP) measured velocities relative to the ship because the sea bottom was not tracked. In order to obtain the velocities relative to the ground, the ship velocities calculated using the GPS data were subtracted from the ADCP data. The diurnal and semi-diurnal components of tidal cur- rent were calculated using the least square method (Isobe, 1992). Figure 4 shows the observed currents, the tidal currents and the residual currents (observed currents minus tidal currents) for each survey along Stns. A7 A13 in 1996 by the ship-mounted ADCP. Variations of the observed velocities (Fig. 4(a)) exceed 10 cm sec 1 at all of the stations and are larger than 20 cm sec 1 at Stns. A7 8 and A The amplitudes of the tidal currents over 4 12 cm sec 1 can be seen at most stations (Fig. 4(b)). Variations for the residual currents (Fig. 4(c)) become small enough, the standard deviations of which are reduced to 1 5 cm sec 1 compared with 4 8 cm sec 1 for the case of observed currents. In this study, the diurnally averaged currents were obtained by averaging these residual currents. Fig. 6. Vertical sections of the salinity along the line A (a) and B (b) in 1995, the line A in 1996 (c) and 1997 (d) and the line C (e) in Contour interval is 0.1 psu. Broken lines indicate the contour line of psu (or psu along the line B). 222 H. Hase et al.

7 2.2 Results and discussion Vertical sections of temperature and salinity off the Wakasa Bay Figures 5 and 6 show the vertical sections of temperature and salinity along the lines A, B and C (see Fig. 2), respectively. In 1995 and 1996, the thermoclines (5 10 C) and haloclines ( psu) decline sharply toward the Japanese coast above the shelf break and slope, intersecting the bottom slope at about 200 m depth and the surface layer near the Stns. A3 and B10 (Figs. 5(a) (c) and 6(a) (c)). It is noted that the abrupt change of the temperature (about 2 C) between Stns. A7 A8 and B7 B8 in 1995 (Figs. 5(a) and (b)) was caused by the time gap of about three days between observations. Discernible above these thermocline (about m depths) are the water masses with relatively uniform high salinity (>34.5 psu) and temperature (>10.0 C). These waters are called the Tsushima Warm Water and can be found at least from March to August (Kawabe, 1982a), indicating the coastal trapping of TWC along the Japanese coast. Besides the tilting of the thermoclines as mentioned above, isotherms declining toward the coast are observed in the narrow regions adjacent to the Japanese coast (Stns. A12 A13 and B1 B2), suggesting the presence of a narrow boundary current along the Japanese coast. Compared with 1995 and 1996, the declinations of the thermocline and halocline toward the coast in 1997 (Figs. 5(d) and 6(d)) are much smaller, and a core of the high Fig. 7. Horizontal distributions of the diurnally averaged current at intermediate depth (70 m along the line A, 80 m along the line B and 68 m along the line C) obtained by the ship-mounted ADCP in 1995 (a), 1996 (b), 1997 (c) and by the towed ADCP in 1998 (d). Temperature distributions at 100 m depth in early June, referred to the Report of the Fisheries and Oceanographic conditions in the Japan Sea (Nihonkai Gyojyo-Kaikyo Sokuhou) by the Japan Sea National Fisheries Research Institute are superimposed. Contour interval is 1.0 C. The Current Structure of the Tsushima Warm Current along the Japanese Coast 223

8 salinity water larger than 34.6 psu cannot be found. On the other hand, the tilt of the isotherms adjacent to the Japanese coast (Stns. A11 A13) becomes larger, suggesting that the boundary current is stronger than in 1995 and In 1998, the thermocline declines offshore (Fig. 5(e)), and the halocline is not clear (Fig. 6(e)). A dome-like structure of the temperature (between Stns. C5 C8) and double cores of the salinity over 34.4 psu (about 50 m depth) are observed, suggesting the upwelling of the cold and low salinity water upwards on the shelf slope (Stns. C5 C8). The declination of the isotherms inshore (Stns. C7 C10) is also seen along the line C Diurnally averaged currents of TWC off the Wakasa Bay Figure 7 shows the horizontal distributions of the diurnally averaged current at intermediate depth (70 m along the line A, 80 m along the line B and 68 m along the line C) obtained by the ship-mounted ADCP in 1995, 1996 and 1997, and by the towed ADCP in Horizontal distributions of the temperature at 100 m depth in early June, (Japan Sea National Fisheries Research Institute, ) are superimposed on the figures. In 1995 and 1996 (Figs. 7(a) and (b)), the thermal fronts Fig. 8. Vertical distributions of the diurnally averaged current vectors by the ship-mounted ADCP along the line A (a) and B (b) in 1995, the compositions of the ship-mounted and towed ADCP in 1996 (c) and 1997 (d) and the towed ADCP only in 1998 (e). The potential density (σ θ ) distributions are superimposed. The hatched regions indicate the high salinity over 34.5 psu in and 34.4 psu in H. Hase et al.

9 (8 13 C) are located roughly above the continental shelf break and slope along the Japanese coast, and two current axes are observed off the Wakasa Bay; the nearshore one flows along the Japanese coast roughly following the isobath shallower than 200 m and the offshore one flows corresponding closely to the thermal fronts (8 13 C). Whereas, in 1997, only one northeastward current flows along the isobath shallower than 200 m depth (Fig. 7(c)), in 1998, the nearshore and offshore current axes are seen along the line C (Fig. 7(d)). The nearshore one is the same path as those flows through the line A and B, following the isobath shallower than 200 m depth. However, the offshore current changes its direction from southwest to northwest towards offshore. The temperature distribution (see Fig. 7(d)) as well as the vertical sections of temperature (see Fig. 5(e)) indicate that the southwestward offshore current is correlated with the declination of isotherms offshore (Stns. C3 C7), which is associated with the cold water tongue elongated southwestward from the cold eddy northwest of the Noto Peninsula. This offshore current axis does not seem to be the same current as those in 1995 and 1996 because it is not correlated to the thermal front and the maximum current speed is the only half that of 1995 and Fig. 9. Vertical sections of the geostrophic flows normal to the line A (a) and B (b) in 1995, the line A in 1996 (c) and 1997 (d) and the line C (e) in The hatched regions indicate the southwestward flow. The geostrophic calculations refers to the diurnally averaged currents at intermediate depth (70 m along the line A, 80 m along the line B and 68 m along the line C), or to the no motion at 1000 m depth where the ADCP measurements were not carried out. The Current Structure of the Tsushima Warm Current along the Japanese Coast 225

10 Although the offshore current is not always clearly seen due to eddy activities (Figs. 7(c) and (d)), these nearshore and offshore branches seem to be considered as the first and second branches of the Tsushima Warm Current (FBTWC and SBTWC) in previous studies (e.g., Yoon, 1991; Kawabe, 1982a, 1982c), respectively. Figure 8 shows the vertical distributions of the diurnally averaged current vectors by the ship-mounted ADCP in 1995, the compositions of ship-mounted and towed ADCP in and the towed ADCP only in The potential density (σ θ ) distributions are superimposed on the figures. The hatched regions indicate the high salinity exceeding 34.5 psu in and 34.4 psu in FBTWC with the maximum velocity of nearly 30 cm sec 1 is trapped over the steeply sloping bottom shallower than 200 m within the narrow band between Stns. A11 A13, B2 B4 and C7 C10 (Figs. 8(a) (e)), while SBTWC with the maximum velocity more than 50 cm sec 1 is located above the shelf break regions in 1995 and 1996 (Figs. 8(a) (c)). In 1997 and 1998 (Figs. 8(d) and (e)), SBTWC are not observed above the shelf break around Stns. A5 and C5 and the high salinity water core (>34.5 psu) over the shelf is ambiguous and the tilt of the pycnoclines (σ θ = ) is not larger than those in Although Kawabe (1982a) suggested strong correlations between FBTWC and the core of the high salinity water, the core of the high salinity water might be related with SBTWC rather than FBTWC since the appearance of high salinity core are found in 1995 and 1996 when SBTWC appears at the shelf break, but not clearly found in 1997 and 1998 when SBTWC is not observed. Salient features at intermediate depth are the southward current reversals exceeding 5 cm sec 1 between m depths on the shelf region at Stns. B3 B4 in Fig. 8(b), at Stns. A9 A10 in Fig. 8(c) and at Stns. C6 C7 in Fig. 8(e). These currents are considered as the southwestward subsurface counter current (SWSCC) suggested by the numerical model experiments (Yoon, 1991; Kim and Yoon, 1994; Seung and Yoon, 1995) in which the southwestward current reversal at intermediate depth over the shelf and shelf break region starts from spring and develops fully in summer along the shelf and shelf break. The southwestward flow from the surface to about 200 m depth at Stns. C4 C6 (see Fig. 8(e)) seems to be associated with the cold water tongue elongated southwestward from the cold eddy northwest of the Noto Peninsula (see Fig. 7(d)) Comparison with the geostrophic calculations The vertical sections of the northeastward geostrophic flows are shown in Fig. 9. The geostrophic calculations referred to the diurnally averaged currents at intermediate depth (70 m along the line A, 80 m along the line B and 68 m along the line C), or to the no motion at 1000 m depth where the ADCP measurements were not carried out. The geostrophic flows between Stns. A7 A8, and B7 B8 in 1995 (Figs. 9(a) and (b)) were regarded as missing due to the time gap between observations. Major features shown by the ADCP measurements are well confirmed along the line A, B and C, i.e., there exist the cores of FBTWC within a narrow region adjacent to the Japanese coast (in Figs. 9(a) (e)), SBTWC above the shelf break (in Figs. 9(a) (c)) and SWSCC exceeding 5 cm sec 1 between m depths over the shelf and shelf break region (in Figs. 9(a) (c)). Comparisons between the geostrophic current and the northeastward diurnally averaged current were made at Stns. A6 A7, A8 A9 and A12 A13 in 1996 (see Figs. 8(c) and 9(c)), as typical examples of SBTWC, SWSCC and Fig. 10. Comparisons of the geostrophic currents (full lines) with the northeastward diurnally averaged currents (dot marks) at Stns. A6 A7 (a), A8 A9 (b) and A12 A13 (c) in 1996, as typical examples of SBTWC, SWSCC and FBTWC, respectively. 226 H. Hase et al.

11 FBTWC, respectively. The full lines and the dots represent the vertical profiles of the geostrophic and the diurnally averaged flows, respectively. Focusing on SBTWC and SWSCC (Figs. 10(a) and (b)), the discrepancies between the geostrophic and the diurnally averaged currents exceed 25 cm sec 1 at a depth of 250 m, being caused by the strong discrepancies of the vertical shears between these currents at around a depth of 150 m (Hase et al., 1997). The southwestward current reversals are confirmed for both the diurnally averaged current (Stns. A8 A9) and the geostrophic currents (Stns. A6 A7) although these have different localities, the current velocities of which show large discrepancies (about 5 cm sec 1 and 15 cm sec 1 for the diurnally averaged and geostrophic currents, respectively). On the other hand, the geostrophic current of FBTWC closely coincides with the diurnally averaged current (Fig. 10(c)). These discrepancies between the geostrophic and diurnally averaged currents might be caused by both the inaccuracy of the geostrophic calculations and ADCP measurements. For example, as the incorporation of the centrifugal force (e.g., Liu and Rossby, 1993) for the meandering path (see Fig. 7(b)) in the calculations improved about 50% of these discrepancies, the inclusions of nonlinearlity or the time gap between observations or small scale internal wave motions might improve the accuracy of the geostrophic calculations. On the other hand, the strong thermocline with the strong density change might also give an influence on the accuracy of the ADCP measurements in terms of the sound reflection and transmission. More studies are required to conclude which is more accurate, ADCP measurements or the geostrophic calculations. 3. Variation of SBTWC Derived from the 100 m Depth Temperature Data during The temperature data at 100 m depth were analyzed to study the variation of SBTWC because SBTWC is strongly correlated with the thermal front at 100 m depth, as shown in Fig. 7. The period of the analysis of the temperature data provided by the National Research Institute of Fisheries Science is 27 years from 1963 to The data coverage, except in winter (December, January and February), seems to be sufficient to cover TWC region excluding the polar front (the third branch of TWC) along about 40 N, as shown in Figs. 11 and 12. For the quality control, the data beyond twice the standard deviation from the mean were eliminated in 1 squares and each month after removal of the negative values. The monthly data were interpolated into the grid points with 1/6 intervals in both zonal and meridional directions using the Gaussian filter with an e-folding scale of 30 km, and then the monthly mean values were obtained Fig. 11. Number of the temperature data at 100 m depth in each 1 square during the period from 1963 to Fig. 12. Seasonal variation of number of the data for 27 years. by averaging the interpolated monthly data for 27 years. Figure 13 shows the monthly mean temperature and its variance at 100 m depth for March, June and September. Two strong fronts are always seen and have strong localities; one of them develops along the continental shelf break and slope at the southern (35 36 N) and eastern rims ( E) of the Tsushima basin; the other develops far off the continental shelf break and slope along the Japanese coast ( E, N). An additional front, which is relatively weak, is seen along the continental shelf break and The Current Structure of the Tsushima Warm Current along the Japanese Coast 227

12 Fig. 13. Spatial distributions of the monthly mean of the temperature (in C) (upper) and its variance (in ( C) 2 ) (lower) at 100 m depth for March (left), June (middle) and September (right). slope between the Oki Islands and the Noto Peninsula (with the temperature between C in June and C in September). These fronts seem to correspond to SBTWC, which flows into the Japan Sea through the western channel of the Tsushima Straits, flows along the continental shelf break and slope west of Noto Peninsula and separates from the shelf break and slope east of the Noto Peninsula (see Fig. 1(b)). Although the fronts have strong localities, the temperature variance is very large along the fronts as shown in March and June. It becomes larger as the fronts become stronger, exceeding 6 ( C) 2 in most of the region in September. This fact suggests that SBTWC is very unstable and its mean path is masked by eddy activities. To detect the mean path of SBTWC more clearly, temperature gradients were calculated as the strong temperature gradients are well correlated with strong currents (see Fig. 7). The temperature differences were calculated between each two neighboring grid points along fourteen lines, as shown in Fig. 14. And then, the monthly mean 228 H. Hase et al.

13 Fig. 14. Locations of the fourteen lines that the temperature gradients are calculated along. (TB: Tsushima Basin, NP: Noto Peninsula, WTS: western channel of the Tsushima Straits). temperature gradients in horizontal (unit in C km 1 ) were calculated by the simple average for the period of 27 years (Fig. 15). Figure 16 shows the time-space diagram of the maximum values of the monthly mean temperature gradients at the continental shelf break between 200 m and 500 m depths. The positive values in the temperature gradients indicate that the temperature increases shoreward along the line. Focusing on the temperature gradients at the continental shelf break and slope region west of the Noto Peninsula (see Fig. 15), we recognize that the strongest temperature gradient starts to develop from March at the continental shelf break ( m depths) cross the line 1 3 and the development propagates eastward. Whereas, east of Noto Peninsula, the strong gradients develops far off the Japanese coast (in waters deeper than 1000 m north of 38 N). The propagation speed of the strongest temperature gradients west of Noto Peninsula is estimated to be about 7 cm sec 1 along the continental shelf break between 200 m and 500 m depths until summer, as shown by the bold straight line in Fig. 16. From fall, the temperature gradients along the shelf break ( m depths) become weaker and weaker toward winter. The strongest temperature gradients east of the Noto Peninsula are not well correlated with the bottom topography because they are located far off the continental shelf and slope. The long-term means of temperature gradients and their standard deviations along fourteen lines clearly show strong correlations between the temperature gradients and the shelf break west of the Noto Peninsula and no correlations east of the Noto Peninsula (Fig. 17). East of the Noto Peninsula, the shelf region is immature and is narrower than west of the Noto Peninsula, suggesting that the wave guide for topographic Rossby waves is interrupted. The current axis estimated from the largest long-term mean temperature gradients is shown in Fig. 18. Considering the above results, together with the ARGOS buoy trackings (Lee et al., 1997) (see Fig. 1(b)), it is concluded that the strongest temperature gradients at 100 m depth corresponds to SBTWC. SBTWC flows roughly along the continental shelf break (between m depths) west of the Noto Peninsula after flowing into the Japan Sea through the western channel of the Tsushima Straits and separates from the continental shelf break north of the Noto Peninsula, flowing northeastward toward the Tsugaru Strait in the deep ocean. SBTWC is considered to be a current steered by the bottom topography (Yoon, 1991; Kawabe, 1982b, c), which follows the continental shelf break and slope. As Kawabe (1982b, c) pointed out, the current (SBTWC) with a baroclinic structure imposed at the western channel of the Tsushima Straits seems to be steered to flow along the continental shelf break through a quasi-geostrophic adjustment process conducted by topographic Rossby waves which propagate along the shelf break, leaving behind the geostrophic current flowing parallel to the contour of the ambient potential vorticity ( f/h, f; Coriolis parameter, H; water depth). The strong temperature variances which grow in time along the thermal fronts (Fig. 13) seem to be generated by the baroclinic instability of SBTWC. The inclination of the bottom topography perpendicular to the direction along the Japanese coast makes SBTWC baroclinically unstable, while the inclination of the bottom topography south of the Japan Islands makes the Kuroshio baroclinically stable (Ikeda and Griffiths, 1996). 4. Summary and Conclusions The branching of the Tsushima Warm Current (TWC) along the Japanese coast has been studied based upon the intensive ADCP and CTD measurements conducted off the Wakasa Bay in every early summer of , the analysis of the temperature distribution at 100 m depth and the tracks of the surface drifters (Ishii and Michida, 1996; Lee et al., 1997). Considering the tracks of the surface drifters (Ishii and Michida, 1996; Lee et al., 1997) as well as the results of the current measurements off the Wakasa Bay, it is concluded that FBTWC starts from the eastern channel of the Tsushima Straits, follows the isobath shallower than 200 m along the Japanese coast as a topographically steered current, and flows out through the Tsugaru Strait in summer. The same conclusion might be suggested for winter from the tracks of the surface drifter and the movement of the oil spilled from a Russian tanker in the winter of The Current Structure of the Tsushima Warm Current along the Japanese Coast 229

14 230 H. Hase et al.

15 The Current Structure of the Tsushima Warm Current along the Japanese Coast 231 Fig. 15. Spatial distributions of the monthly mean temperature gradients (unit in C km 1 ) at 100 m depth from March to November. The positive values of the gradients indicate that the temperature increase shoreward along the line. The magnitude of the gradient is shown at the top of each figure. The hatched regions indicate the bottom topography between 200 m and 500 m depths.

16 Fig. 16. Time-space diagram of the maximum values of the monthly mean temperature gradients at the continental shelf break between 200 m and 500 m depths (unit in C km 1 ). Contour interval is 0.01 C km 1. The analysis of the temperature gradient and the tracks of surface drifters (Lee et al., 1997) show that the current flowing through the western channel of the Tsushima Straits feeds SBTWC which develops from spring to fall, and the development of SBTWC propagates from the Tsushima Straits to Noto Peninsula at a speed of about 7 cm sec 1 following the continental shelf break with a strong baroclinicity. Topographic Rossby waves with baroclinicity are responsible for the eastward propagation of the development of SBTWC along the shelf break. SBTWC is concluded to be a topographically steered current; a current steered by the continental shelf break west of Noto Peninsula. East of Noto Peninsula, SBTWC flows northeastward toward the Tsugaru Strait being relieved from the topographic trapping. Fig. 17. Long-term mean (closed dots and full lines) of the temperature gradients at 100 m depth and its standard deviation (open dots and broken lines) for the period of 27 years along the line 1 14 and the bottom topography (hatch regions). 232 H. Hase et al.

17 Fig. 18. Location of the current axis (bold line) estimated from the long-term mean temperature gradients. A bold line links the largest peaks of the long-term mean temperature gradients of each line. Fig. 17. (continued). Although SBTWC is strongly correlated with the continental shelf break with a strong baroclinicity, SBTWC cannot be always found around the shelf break because its path is influenced by the development of eddies which seem to be generated by the baroclinic instability. Salient features at intermediate depth are SWSCC between m depths over the shelf region in with the velocity exceeding about 5 cm sec 1, although the discrepancies between the diurnally averaged and geostrophic currents are observed. The existence of the subsurface counter currents has been suggested by many numerical models (Yoon, 1991; Kim and Yoon, 1994; Seung and Yoon, 1995) in which the southwestward current reversal at intermediate depth over the shelf and shelf break region starts from spring, and develops fully in summer and fall along the shelf and shelf break. The generating mechanism of this SWSCC seems to be related to the development and propagation of SBTWC, for which topographic Rossby waves with baroclinicity are responsible (e.g., Yoon, 1991). A schematic view of the TWC system in the southern part of the Japan Sea is presented in Fig. 19 based upon the above results. The TWC system in the southern part of the Japan Sea comprises two branches along the Japanese coast; the stable first branch trapped on the steep shallow bottom, and the second branch following the shelf break with a strong baroclinicity and variability. The Current Structure of the Tsushima Warm Current along the Japanese Coast 233

18 Fig. 19. Schematic view of Tsushima Warm Current system and the bottom topographic feature. The nearshore and offshore lines indicate the paths of FBTWC and SBTWC, respectively. Acknowledgements The authors express their sincere thanks to the Captain, the officers and crew of the T/V Kakuyo-Maru of Nagasaki University. Special thanks are extended to Assistant Prof. A. Isobe of ESST, Kyushu University and Dr. T. Senjyu of National Fisheries University for their kind help in the field observations. Thanks are also extended to Prof. D.-K. Lee of Pusan National University for providing their figures. National Research Institute of Fisheries Science is acknowledged for providing the temperature data sets during Japan Sea National Fisheries Research Institute is also acknowledged for providing the Reports of the Fisheries and Oceanographic conditions in the Japan Sea (Nihonkai Gyojyo-Kaikyo Sokuhou). Reports of drifting-landing state of spilled oil from Russian tanker Nakhodka ( N -go Kainan Ryushutsu-yu no jyokyo) were provided by Eighth and Ninth Regional Maritime Safety Headquarters. References Hase, H., J.-H. Yoon, M. Takematsu, W. Koterayama and S. Yamaguchi (1997): The structure of the Tsushima Warm Current off the Wakasa Bay during Engineer. Sci. Rep. Kyushu Univ., 19, Ikeda, M. and C. Griffiths (1996): Baroclinic instability in the presence of topography: Comparison of the numerical models. Abstract volume, Fall meeting of Japan Oceanogr. Soc., (in Japanese). Ishii, H. and Y. Michida (1996): Tracking of the first branch of the Tsushima Warm Current with surface drifter. Rep. Hydrogr. Res., 32, (in Japanese with English abstract). Isobe, A. (1992): Studies on the removing tidal currents from ADCP data. Rep. J. Shimonoseki Univ. Fisheries, 40, (in Japanese with English abstract). Japan Sea National Fisheries Research Institute ( ): Report of the Fisheries and Oceanographic Conditions in the Japan Sea (Nihonkai Gyojyo-Kaikyo Sokuhou)., No Joyce, T. M. (1989): On in situ calibration of shipboard ADCPs. J. Atmos. Oceanic Tech., 6, Katoh, O. (1993): Detailed current structures over the continental shelf off the San in Coast in summer. J. Oceanogr., 49, Katoh, O. (1994): Structure of the Tsushima Current in the southwestern Japan Sea. J. Oceanogr., 50, Katoh, O., K. Morinaga, K. Miyaji and K. Teshima (1996): Branching and joining of the Tsushima Current around the Oki Islands. J. Oceanogr., 52, Kawabe, M. (1982a): Branching of the Tsushima Current in the Japan Sea, Part I. Data analysis. J. Oceanogr. Soc. Japan, 38, Kawabe, M. (1982b): Coastal trapped waves in a two-layer ocean: Wave properties when the density interface intersects a sloping bottom. J. Oceanogr. Soc. Japan, 38, Kawabe, M. (1982c): Branching of the Tsushima Current in the Japan Sea, Part II. Numerical Experiment. J. Oceanogr. Soc. Japan, 38, Kim, C.-H. and J.-H. Yoon (1994): Circulation of the Japan Sea as seen in the prognostic numerical model. Kaiyo Monthly, 26, (in Japanese). 234 H. Hase et al.

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