1154 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 35

The Tsushima Warm Current through Tsushima Straits Estimated from Ferryboat ADCP Data

TETSUTARO TAKIKAWA* Department of Earth System Science and Technology, Interdisciplinary Graduate School of Engineering Science, Kyushu University, Kasuga, ,

JONG-HWAN YOON Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka, Japan

KYU-DAE CHO Department of Oceanography, Pukyong National University, Nagu, Pusan,

(Manuscript received 20 October 2003, in final form 19 November 2004)

ABSTRACT Current structures across the Tsushima Straits are studied using results from long-term acoustic Doppler current profiler (ADCP) observations by a ferryboat between Hakata and Pusan conducted since February 1997. Two maxima of the northeastward current are observed in the central parts of the eastern and western channels, and the maximum velocity in the western channel is stronger than that of the eastern channel. Downstream of the Tsushima Islands, a southwestward countercurrent is observed associated with a pair of cyclonic and anticyclonic eddies. In the western channel, the deep countercurrent is observed pronouncedly on the bottom slope of the Korean side from summer to winter. The volume transport of the Tsushima Warm Current through the straits has strong seasonal variation with a minimum in January and two maxima from spring to autumn (double peaks). The spring peak of the volume transport through the eastern channel is more pronounced than the autumn peak, and the autumn peak of the western channel is more pro- nounced than the spring peak. The inflow volume transport into the Japan Sea through the western channel significantly increases in autumn because of an incrementation of the freshwater transport. The total volume transport averaged over the observation period (5.5 yr) is 2.64 Sv (Sv ϵ 106 m3 sϪ1). The average volume transports through the eastern and western channels are 1.10 and 1.54 Sv, respectively.

1. Introduction Tsushima Islands. According to Hase et al. (1999), the current through the eastern channel of Tsushima Straits The Tsushima Warm Current flows into the Japan feeds the first branch of the Tsushima Warm Current Sea from the East Sea through the Tsushima roughly following the 200-m isobath of the continental Straits with width, length, and mean water depth of shelf along the Japanese coast. The current through the about 180 km, 330 km, and 100 m, respectively. The western channel of Tsushima Straits feeds the second straits are divided by into the eastern branch, which flows along the continental shelf break and western channels with widths of about 140 and 40 and slope along the Japanese coast. Another branch km, respectively. Most of the water flows out to the along the Korean coast, which is called the East Korean Pacific Ocean and the through the Warm Current, flows as a western boundary current Tsugaru and Soya Straits. The Tsushima Warm Current is divided by the (Kawabe 1982; Yoon 1982a,b) and separates from the Korean coast at about 37°–39°N with synoptic mean- ders accompanied by warm and cold mesoscale eddies * Current affiliation: Department of Fishery Science and Tech- (Beardsley et al. 1992; Isoda and Saitoh 1993; Jacobs nology, National Fisheries University, , Japan. et al. 1999). A part of the East Korean Warm Current flows southward as a countercurrent and the rest of it flows northeastward toward , forming Corresponding author address: Tetsutaro Takikawa, Depart- ment of Fishery Science and Technology, National Fisheries Uni- the polar front at about 40°N with the northern cold versity, 2-7-1 Nagata-Honmachi, Shimonoseki 759-6595, Japan. water (Kim and Yoon 1996). Furthermore, a water E-mail: [email protected] mass with a vertical salinity minimum called the Japan

© 2005 American Meteorological Society

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FIG. 1. (a) The track (thick solid line) of the ferry Camellia along which current measure- ments by ADCP were carried out. The eastern and western channels are defined as the black (south of 34.75°N) and gray (north of 34.75°N) lines, respectively. Stations A and B are the deepest points on the observation line in the eastern and western channels, respectively. CTD observations were carried out at stations 1–5. Contour lines show the water depth in meters. (b) Cross-section view along the ferry track. Solid line indicates the water depth (m). Outflow volume transports associated with countercurrent downstream of the Tsushima Islands and deep countercurrent in the western channel flow through black and gray regions, respectively.

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FIG. 2. Monthly averaged current vectors at 18-m depth on the Camellia line from Feb 1997 to Aug 2002 (1 kt ϵ 51.4 cm sϪ1).

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short-term direct measurements using current meters (Miita 1976). Miyazaki (1952) concluded that the vol- ume transport of the Tsushima Warm Current had a large seasonal variation with maximum of 2.5 Sv (Sv ϵ 106 m3 sϪ1) in summer and minimum of 0.2 Sv in winter, and Yi (1966) obtained almost the same result. Isoda and Yamaoka (1991) estimated the volume transport in summer to be about 3 Sv, where the ratio of the trans- port through the eastern channel to that of the western channel was about 1 to 3. Miita (1976) also showed that the volume transport attained a maximum in summer (3.3–3.7 Sv). Over the past decade, there have been many direct current observations using a ship-mounted or towed acoustic Doppler current profiler (ADCP) to elucidate the volume transport and spatial current structure of the Tsushima Warm Current in Tsushima Straits (Kaneko et al. 1991; Kawano 1993; Egawa et al. 1993; Katoh 1993; Isobe et al. 1994). These results were av- eraged by Isobe (1994), who obtained an annually av- eraged volume transport of 2.2 and 0.7 Sv through Tsushima Straits and the eastern channel, respectively. Teague et al. (2002) carried out current measure- ments at 12 stations along two lines (northeast and southwest of Tsushima Islands) across Tsushima Straits FIG. 3. Current vectors (kt) at 26-m depth from 16 to 18 Jun for about 11 months using ADCPs housed in trawl- 2000, using Camellia ADCP observations (west) and ADCP ob- servations from the T/V Kakuyo-Maru of University resistant bottom mounts (TRBM). Annually averaged (east). volume transports of the Tsushima Warm Current were estimated to be 2.3 Sv at the northeastern section and 2.7 Sv at the southwestern section. The disagree- Sea Intermediate Water is found below the Tsushima ment of the two transports between the northeastern Warm Current (Senjyu 1999; Yoon and Kawamura 2002). and southwestern sections is considered to be due to So far, many studies have been made to estimate the the large interpolation errors of the ADCP data near volume transport of the Tsushima Warm Current. How- the surface layer, especially near the Korean coast, ever, long-term current measurements in Tsushima where strong northeastward currents are observed (Ja- Straits are difficult because of heavy fishing and trawl- cobs et al. 2001). ing activities (Kawatate et al. 1988). Hence, most of the The Research Institute for Applied Mechanics estimates were given by geostrophic calculations (RIAM) of Kyushu University has been conducting (Miyazaki 1952; Yi 1966; Isoda and Yamaoka 1991) or long-term ADCP observations since 21 February 1997

Ϫ FIG. 4. Numerical simulation of the Tsushima Straits velocities (cm s 1) using the 1.5-layer reduced-gravity model of Maruyama et al. (2003). The horizontal grid spaceis2km.

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Ϫ FIG. 5. Monthly averaged velocities (cm s 1) on the vertical section along the ferry track from Feb 1997 to Aug 2002. The velocities are normal to the section, and positive velocities are toward the Japan Sea.

6 times per week using the ferryboat Camellia between multilevel ADCP (VMBBADCP, 300 kHz, RD Instru- Hakata and Pusan (Fig. 1). We use these data to esti- ments) mounted on the ferryboat Camellia. The Camel- mate the averaged volume transport and seasonal lia makes round trips between Hakata and Pusan 3 variations through Tsushima Straits. The characteristics times per week at a cruising speed of about 17 kt. The of the Tsushima Warm Current structure are discussed data sampling intervals are about 24 s and8mindepth first, and the volume transport is discussed second. from 18 to 258 m. However, the data within 15% of the total depth from the bottom are not reliable (missing 2. Data and method data). Surface and bottom velocities are obtained by The data used in this study were obtained for 5.5 yr extrapolating the values at 18-m depth and at the deep- from 21 February 1997 to 25 August 2002 from the est depth of reliable ADCP measurements to calculate

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Ϫ FIG. 6. Standard deviations (cm s 1) of the velocities in Fig. 5. the volume transport through the section. Since the current sections is roughly one day, the current data maximum water depth is about 240 m along the ferry may contain significant aliasing errors associated with track, the ship velocity relative to the bottom can be some of the tidal constituents. However, the sampling measured everywhere through the bottom tracking by intervals (the time between two successive cruises) are the ADCP, implying that the current velocity can be not constant and vary from point to point along the measured relative to the bottom. ferry track. The duration of the data is long enough to Since tidal currents are very strong and comparable decompose each tidal constituent. Ten tidal constitu- to the mean current due to the shallow depth and nar- ents (Q1, O1, P1, K1, N2, M2, S2, K2, MSf, and Mf) were row width of the straits, tides must be removed from the removed by harmonic analysis from the ADCP data ADCP data in order to study processes with time scales following the method by Takikawa et al. (2003), and the longer than a few days. Since the data interval between mean currents were accurately obtained without tidal

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FIG. 7. The time change of vertical profile of monthly mean velocity for 5.5 yr at stations (a) A and (b) B (see Fig. 1a). Contours indicate the isopleths of velocities normal to the section, and the positive velocities are toward the Japan Sea. The contour interval is 10 cm sϪ1. Gray shading indicates negative values, associated with the countercurrent from the Japan Sea. Length and direction of vectors indicate current velocity and direction. aliasing errors. In this paper, currents after removal of Downstream of the Tsushima Islands, a southwest- the 10 tidal current constituents are shown. ward countercurrent is almost always present. The countercurrent is relatively weak, but becomes stronger from summer to autumn. East of the observation line, 3. Results concurrent ADCP measurements were made during 16–18 June 2000 by the T/V Kakuyo-Maru of Nagasaki a. Structure of the Tsushima Warm Current University. Figure 3 shows the current vectors at 26-m Monthly averaged current vectors at 18-m depth on depth using both datasets, suggesting a pair of cyclonic the Camellia line from February 1997 to August 2002 and anticyclonic eddies east of the Tsushima Islands are shown in Fig. 2. The current maxima are located at with the southwestward countercurrent between the all times in the central parts of the eastern and western two eddies downstream of the islands. Takikawa et al. channels, flowing northeastward into the Japan Sea. (2003) calculated contributions of tidal currents to the The maximum velocity of the eastern channel current total kinetic energy and the eddy kinetic energy on the ranges from 15 cm sϪ1 in January to 35 cm sϪ1 in Au- section, suggesting that eddy activities are comparable gust with an average of 26 cm sϪ1. The maximum in the to the tidal currents and much stronger than the mean western channel ranges from 37 cm sϪ1 in January to 57 current downstream of the Tsushima Islands. Numeri- cm sϪ1 in November with an average of 49 cm sϪ1. The cal simulation of flow through Tsushima Straits using a current maxima tend to be weaker from winter to 1.5-layer reduced-gravity model (Maruyama et al. 2003) spring and stronger from summer to autumn, especially also showed a pair of eddies (Fig. 4) in the region. near the Korean coast. Strong eddy activities with a few days time scale, which

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FIG. 8. Monthly averaged volume transport in Sverdrups (open circle) of the Tsushima Warm Current into the Japan Sea through the Camellia line from Feb 1997 to Aug 2002, and transports of the eastern (open triangle) and western (open square) channels (see Fig. 1a). Straight lines indicate the mean value of the each volume transport. Error bar indicates standard error in each month. are much longer than tidal motions, were also observed barotropic model, suggesting generation of baroclinic by HF radar observation (Yamamoto et al. 2002). Since instability near the northern tip of the Tsushima Islands the countercurrent was also found in the result by in summer season. Teague et al. (2002) at the observation line located In the eastern channel, the velocity variations from about 50 km east of the Tsushima Islands, it is sug- the surface to the bottom in winter are very small (Fig. gested that the horizontal scale of the countercurrent is 5); that is, the current structure is barotropic. From almost 50 km. Such countercurrents were also found spring to early autumn, velocities increase near the sur- results of observations by Miita and Ogawa (1984), face, and the current structure becomes baroclinic. Egawa et al. (1993), Katoh (1993), and Isobe et al. From late autumn to winter, velocities decrease and the (1994). current structure becomes more barotropic. The large Downstream of , the current is also rela- and small standard deviations (Fig. 6) correspond to the tively weak and a southwestward countercurrent is baroclinic and barotropic current structures, respec- observed from August to February (Fig. 2). Near the tively. Kyushu coast, the northeastward current is weaker Figure 7 shows the time change of the vertical profile from autumn to winter and stronger from spring to of monthly mean velocity for the five and a half years at summer. stations A and B at the deepest parts of the two chan- The distributions of the monthly averaged current nels (Fig. 1a). The seasonal variation of the vertical velocities and standard deviations of these currents on current structure at the eastern channel mentioned the vertical section along the ferry track from February above is clearly visible (Fig. 7b). 1997 to August 2002 are shown in Figs. 5 and 6, respec- In the western channel, the current structure in win- tively. The velocity variability for all seasons in the ter becomes barotropic from the surface down to about western channel is larger than that in the eastern chan- 100 m, and the surface velocities gradually increase nel (Fig. 6). The standard deviation near the northern from spring (Fig. 5). From summer to autumn, the cur- tip of the Tsushima Islands (about 34.8°N) is large from rents become strongest with strong baroclinicity. The late spring to autumn, corresponding to the strong velocity increases are found not only at the core of the countercurrent downstream of the Tsushima Islands. northeastward current but also near the Korean coast According to Maruyama et al. (2003), because of baro- accompanied by the large standard deviation (Fig. 6). clinicity of current structure, although the countercur- In autumn, the current maximum shifts from the sur- rent is formed by the numerical simulation of the 1.5- face to the subsurface (about 40-m depth). layer reduced-gravity model (Fig. 4), it is not formed by The southwestward countercurrent is observed be-

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FIG. 9. The inflow volume transport (Sv; open circle) into the Japan Sea through the western channel (see Fig. 1a), the salinity (psu; open diamond) averaged vertically and horizontally at stations 1–5 in Fig. 1a, and the outflow volume transport (open triangle) from the Japan Sea in deep layer in the western channel (see Fig. 1b). The salinity data were provided by the Korea Oceanographic Data Center. low about 130 m in the western channel from summer the deep countercurrent is clearly visible. This counter- to winter but is absent in spring (Figs. 5 and 7b). Since current exists from April to January with two maxima this countercurrent is on the slope of the Korean side in September (16 cm sϪ1) and December (12 cm sϪ1)at (Fig. 5), it might be associated with the topographic ␤ station B. effect. According to Fig. 7b, the seasonal variation of According to Lim and Chang (1969), the deep coun-

FIG. 10. The inflow volume transport (Sv; open circle) into the Japan Sea through the eastern channel (see Fig. 1a) and the outflow volume transport (open triangle) from the Japan Sea downstream of the Tsushima Islands (see Fig. 1b). The open diamonds are as in Fig. 9.

Unauthenticated | Downloaded 09/29/21 03:19 PM UTC JUNE 2005 NOTES AND CORRESPONDENCE 1163 tercurrent is an intrusion of cold water into Tsushima Straits from the Japan Sea. According to Cho and Kim (1998), the bottom cold-water advection has a seasonal cycle with maximum in summer and minimum in winter associated with variation of the salinity minimum layer in the Japan Sea. On the other hand, Johnson and Teague (2002) did not find an annual cycle in the bot- tom cold water using temperature records between May 1999 and March 2000 (Teague et al. 2002). They hypothesized that the bottom cold-water intrusions are the product of relatively weak advection augmented by horizontal diffusion. However, the deep countercurrent was weaker during their observation period than usual (transport associated with the deep countercurrent is given by open triangles in Fig. 9, described below). Al- though the maximum of the deep countercurrent in summer (Fig. 7b) supports the result of Cho and Kim (1998), that of winter suggests that the bottom cold water intrudes by horizontal diffusion when transport to the Japan Sea is reduced (Johnson and Teague 2002). b. Volume transport of the Tsushima Warm Current through Tsushima Straits Monthly averaged volume transports of the Tsushi- ma Warm Current into the Japan Sea across the Ca- mellia line from February 1997 to August 2002 are shown in Fig. 8. The total volume transport has a sea- sonal variation with a minimum in January and two maxima in spring and autumn. The seasonal variation with double peaks is found to be more pronounced in FIG. 11. (a) The inflow volume transports (Sv) into the Japan 1997 (July and October), 1998 (May and November), Sea through the eastern (open triangle) and western (open and 2001 (March and October) than in 1999 (July and square) channels averaged monthly for 5.5 yr, and (b) the outflow volume transports of the countercurrent east of the Tsushima October) and 2000 (May and September). Over the Islands (open triangle) and the deep countercurrent in the west- observation period, the amplitude of seasonal variation ern channel (open square) averaged monthly for the observation of volume transport is relatively stable except in 1999 period. when the total volume transports exceeds 3.5 Sv in Oc- tober and November. The total volume transport through the observation line averaged for five and a Although the seasonal variation of volume transport half years is 2.64 Sv. The average volume transports through the western channel has double peaks, the through the eastern (south of 34.75°N) and western spring peak is relatively smaller than that of autumn (north of 34.75°N) channels are 1.10 and 1.54 Sv, re- (Fig. 8). The inflow transport in the western channel spectively. (Fig. 9) varies seasonally with a single peak (minimum Isobe (1994) showed that the average volume trans- in winter and maximum in autumn), except in 2001. port through Tsushima Straits from 1987 to 1991 is 2.2 Salinity data in the western channel were provided by Sv, without any pronounced double peaks of volume the Korea Oceanographic Data Center (KODC). The transport. The differences of volume transport between salinity is inversely correlated with the inflow volume this study and Isobe (1994) might originate from the transport, except in spring of 2001 (Fig. 9). The low difference of observation period. According to Taki- salinity from August to October in the western channel kawa and Yoon (2005), who estimated the volume is likely caused by the seasonal freshwater discharge transport using sea level data from 1965 to 2001, the from the Changjiang River into the , transport over the same period of Isobe (1994) is 2.4 Sv. with a maximum in July and minimum in January Time series of the volume transport from May 1999 to (Chen et al. 1994; Yanagi 1994; Zhu et al. 2001). The March 2000 are shown by Teague et al. (2002), showing freshwater transport through Tsushima Straits in- good agreement between that of the southern section creases pronouncedly in summer and autumn with two described by Teague et al. (2002) and this study in the or three months delay to the river discharge (Isobe et time variations and the time-averaged mean (2.7 Sv). al. 2002). Although the variation of the outflow volume

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FIG. 12. The total volume transport (Sv; open circle) of the Tsushima Warm Current through Tsushima Straits averaged monthly for 5.5 yr, and those of the eastern (open triangle) and western (open square) channels. Each solid line is fitted by functions with annual and semiannual cycles. transport from the Japan Sea in deep layers below 100 The inflow volume transport averaged monthly for m (north of 34.85°N) is small (Fig. 9), the outflow trans- 5.5 yr through the eastern channel (Fig. 11a) has a mini- port associated with the intrusion of the cold water mum in January (0.90 Sv) and two maxima in March (Lim and Chang 1969) in Figs. 5 and 7b from summer to (1.38 Sv) and October (1.45 Sv). Although the variation winter increases with the increase of the inflow trans- of the inflow transport is very small from spring to au- port, except in 1999. The cold water is not observed tumn, its transport has a minimum in July (1.23 Sv) west of the Tsushima Islands (Johnson and Teague between two maxima. The average inflow transport 2002), suggesting that the outflow water is recirculated through the eastern channel is 1.28 Sv. Double peaks back into the Japan Sea. are not found in the inflow volume transport through In autumn 1999, a large volume transport, exceeding the western channel with a minimum (1.16 Sv) in Janu- 4 Sv, is observed (Fig. 8). Since the salinity decreases ary, a maximum (2.12 Sv) in October, and an average of abruptly in the western channel (Fig. 9) in the same 1.67 Sv. period, it is suggested that a large amount of freshwater The monthly averaged transport downstream of the is transported from the rivers around the Tsushima Islands (Fig. 11b) has a minimum (0.10 Sv) in and the East China Sea. In fact, according to the Asian February and a maximum (0.26 Sv) in September with Disaster Reduction Center (ADRC), the precipitation an average of 0.17 Sv. The monthly averaged transport over increased and river floods occurred fre- in deep layer through the western channel has two quently in July during that year. minima in February (0.04 Sv) and November (0.08 Sv) Double peaks in the seasonal variation of the volume and two maxima in January (0.26 Sv) and August (0.17 transport through the eastern channel (Figs. 8 and 10) Sv) with an average of 0.10 Sv. are more pronounced than that in the western channel Total volume transport and the eastern and western (Fig. 8). The countercurrent downstream of the channel transports are averaged monthly (Fig. 12). Tsushima Islands is associated with a dipole of eddies Each of the three is fit by functions with annual and (Fig. 3), suggesting that the outflow water returns to the semiannual cycles, Japan Sea by eddy recirculation. The countercurrent transport (Fig. 10) shows relatively clear seasonal varia- TABLE 1. Coefficients of the volume transport, Eq. (1). tion with a maximum in summer and a minimum in winter. The outflow transport peaks when the inflow T0 a1 a2 b1 b2 transport is a minimum in summer. The autumn peak of Eastern channel 1.098 Ϫ0.107 0.014 Ϫ0.081 Ϫ0.123 the inflow volume transport is weakly correlated with Western channel 1.536 Ϫ0.077 Ϫ0.287 Ϫ0.055 Ϫ0.159 Ϫ Ϫ Ϫ Ϫ low salinity (Fig. 10). Total 2.635 0.184 0.273 0.136 0.282

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TABLE 2. The monthly mean volume transport through the Tsushima Straits and its standard variation for the years from (a) 1997 through (f) 2002. Total (Sv) Eastern (Sv) Western (Sv) No. of observations Transport Std dev Transport Std dev Transport Std dev (a) for 1997 Feb 7 2.33 1.61 1.04 0.86 1.29 0.77 Mar 21 2.41 0.91 1.07 0.51 1.34 0.44 Apr 26 2.40 0.84 1.08 0.45 1.32 0.44 May 22 2.47 0.65 0.99 0.43 1.48 0.28 Jun 26 2.70 0.55 1.12 0.33 1.58 0.31 Jul 29 2.73 0.56 1.00 0.47 1.73 0.27 Aug 25 2.54 0.86 0.82 0.56 1.72 0.43 Sep 24 2.62 0.95 0.84 0.70 1.78 0.34 Oct 22 3.02 1.23 1.02 0.70 1.99 0.76 Nov 20 2.34 1.00 0.75 0.66 1.59 0.41 Dec 26 2.28 0.73 0.81 0.48 1.46 0.33

(b) for 1998 Jan 27 1.67 0.67 0.68 0.35 0.99 0.43 Feb 23 2.13 0.83 1.00 0.44 1.12 0.47 Mar 14 2.15 0.53 1.08 0.38 1.07 0.25 Apr 27 2.70 0.47 1.21 0.33 1.50 0.27 May 26 2.99 0.70 1.32 0.45 1.67 0.29 Jun 28 2.99 0.59 1.41 0.33 1.58 0.34 Jul 27 2.82 0.67 1.17 0.44 1.65 0.33 Aug 26 2.42 0.49 0.96 0.42 1.46 0.35 Sep 23 2.33 1.23 0.77 0.87 1.56 0.50 Oct 27 2.97 0.88 1.21 0.45 1.77 0.65 Nov 26 3.00 1.00 1.17 0.53 1.82 0.51 Dec 22 2.22 0.60 0.91 0.41 1.32 0.27

(c) for 1999 Jan 25 1.75 1.01 0.70 0.55 1.05 0.53 Feb 12 2.52 0.49 1.26 0.32 1.26 0.28 Mar 25 2.63 0.72 1.32 0.51 1.31 0.34 Apr 22 2.67 1.06 1.40 0.54 1.28 0.58 May 17 2.65 0.71 1.20 0.40 1.45 0.46 Jun 25 3.00 0.75 1.31 0.45 1.70 0.37 Jul 26 2.72 0.81 1.02 0.55 1.70 0.43 Aug 20 3.23 0.45 1.23 0.38 2.00 0.30 Sep 25 3.26 0.80 1.33 0.72 1.93 0.51 Oct 26 4.20 0.86 1.80 0.50 2.40 0.43 Nov 27 3.67 0.75 1.61 0.40 2.06 0.41 Dec 23 2.52 0.85 1.16 0.44 1.36 0.50

(d) for 2000 Jan 24 1.73 0.71 0.70 0.33 1.03 0.45 Feb 23 2.09 0.82 1.07 0.42 1.02 0.44 Mar 17 2.55 0.85 1.44 0.32 1.11 0.61 Apr 27 2.51 0.60 1.18 0.36 1.33 0.35 May 30 2.70 0.42 1.08 0.23 1.62 0.27 Jun 26 2.67 0.43 1.11 0.31 1.55 0.32 Jul 28 2.65 0.67 1.06 0.42 1.59 0.36 Aug 24 2.93 0.45 1.25 0.45 1.68 0.33 Sep 24 3.05 0.86 1.38 0.55 1.67 0.57 Oct 26 2.77 0.80 1.01 0.49 1.76 0.38 Nov 19 2.33 0.64 0.80 0.40 1.53 0.30 Dec 15 2.16 0.88 0.91 0.40 1.25 0.55

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TABLE 2. (Continued) Total (Sv) Eastern (Sv) Western (Sv) No. of observations Transport Std dev Transport Std dev Transport Std dev (e) for 2001 Jan 27 1.98 0.95 0.88 0.48 1.10 0.58 Feb 23 2.83 0.73 1.25 0.42 1.59 0.47 Mar 16 3.12 0.75 1.39 0.32 1.74 0.49 Apr 29 2.88 0.82 1.32 0.48 1.56 0.41 May 30 2.71 0.79 1.22 0.53 1.48 0.43 Jun 25 2.72 0.61 1.17 0.46 1.55 0.30 Jul 27 2.44 0.56 0.78 0.37 1.66 0.38 Aug 26 2.78 0.65 1.13 0.47 1.65 0.34 Sep 24 2.89 1.05 0.97 0.64 1.92 0.50 Oct 28 3.04 0.76 1.22 0.48 1.82 0.40 Nov 25 2.97 0.55 1.22 0.39 1.74 0.31 Dec 25 1.79 0.63 0.62 0.33 1.17 0.41

(f) for 2002 Jan 18 1.60 1.09 0.85 0.70 0.75 0.57 Feb 8 2.28 0.63 1.13 0.32 1.15 0.39 Mar 26 2.77 0.76 1.29 0.43 1.48 0.45 Apr 26 2.70 0.67 1.25 0.43 1.45 0.37 May 17 2.99 0.58 1.43 0.34 1.56 0.32 Jun 25 2.72 0.76 1.10 0.43 1.62 0.39 Jul 28 2.80 0.65 1.00 0.46 1.80 0.32 Aug 23 2.65 0.62 0.90 0.42 1.75 0.39

␲ ␲ ␲ the Korean slope from summer to winter, with the av- T͑t͒ ϭ T ϩ a cos t ϩ b sin t ϩ a cos t 0 1 6 1 6 2 3 erage transport of 0.10 Sv. The total volume transport of the Tsushima Warm ␲ Current through Tsushima Straits varies seasonally ϩ b sin t, ͑1͒ 2 3 with a minimum in January and two maxima from spring to autumn (double peaks). Although the eastern where the unit of time t is month and coefficients (a1, a2, and western channel transports each have relatively b1, and b2) are listed in Table 1. According to Fig. 12, weak double peaks, they reinforce each other, yielding the eastern and western channel transports each have more pronounced double peaks in the total volume relatively weak double peaks. However, they reinforce transport. The monthly and yearly mean volume trans- each other, yielding more pronounced double peaks in ports over the observation period are listed in Tables the total volume transport. The annual ranges of vol- 2a–f and 3. The average volume transport through the ume transport through the eastern and western chan- observation line is 2.64 Sv. The average volume trans- nels are estimated at 0.5 and 0.8 Sv, respectively, and ports through the eastern and western channels are 1.10 the total range is 1.2 Sv. and 1.54 Sv, respectively. The inflow volume transport into the Japan Sea through the western channel varies seasonally (mini- 4. Summary and conclusions mum in winter and maximum in autumn), and is in- versely correlated with the salinity. The low salinity Characteristics of the Tsushima Warm Current have from August to October in the western channel is likely been described analyzing the ADCP data obtained by caused by the freshwater discharge from the rivers the ferry Camellia for 5.5 years. The maximum north- around the Yellow Sea and the East China Sea. eastward currents are observed in the central parts of Although the correlation between the transport the eastern and western channels, and the western core through the Tsushima Straits and the freshwater dis- is stronger than the eastern core. The current structure charge is mentioned in this paper, the detail of what is changes seasonally from a barotropic structure in win- driving the Tsushima Warm Current will be addressed ter and spring to a baroclinic structure in summer and as future issues and problems from the standpoint of autumn. Downstream of the Tsushima Islands, south- comparing with the variability of transport through the westward countercurrents are observed, corresponding Taiwan Strait or the Kuroshio variability in the East to a pair of the cyclonic and anticyclonic eddies. In the China Sea. According to You (2005), most of the vol- western channel, a deep countercurrent is observed on ume transport estimated using a general circulation

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TABLE 3. The yearly mean volume transport through the Tsushima Straits and its standard variation. Total (Sv) Eastern (Sv) Western (Sv) Year No. of observations Transport Std dev Transport Std dev Transport Std dev 1997 248 2.55 0.89 0.96 0.56 1.59 0.47 1998 296 2.56 0.87 1.08 0.51 1.48 0.49 1999 273 2.94 1.02 1.29 0.57 1.65 0.59 2000 283 2.54 0.77 1.09 0.44 1.45 0.48 2001 305 2.66 0.85 1.09 0.51 1.57 0.48 2002 171 2.62 0.82 1.12 0.49 1.50 0.50

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