Seasonal Volume Transport Variation in the Tsushima ... - Springer Link

Journal of Oceanography, Vol. 66, pp. 539 to 551, 2010

Seasonal Volume Transport Variation in the Tsushima Warm Current through the Tsushima Straits from 10 Years of ADCP Observations K EN-ICHI FUKUDOME1*, JONG-HWAN Y OON1, A LEXANDER OSTROVSKII2, TETSUTARO TAKIKAWA3 and IN-SEONG HAN4 1

Research Institute for Applied Mechanics, Kyushu University, Kasuga-kouen, Kasuga, Fukuoka 816-8580, Japan 2 P.P. Shirshov Institute of Oceanology, Moscow 117218, Russia 3 National Fisheries University, Nagata-Honmachi, Shimonoseki 759-6595, Japan 4 National Fisheries Research and Development Institute, Sirang-Ri, Gijang-Eup, Gijang-Kun, Busan 619-902, Korea (Received 14 April 2010; in revised form 28 May 2010; accepted 28 May 2010)

The seasonal variation in the structure and volume transport of the Tsushima Warm Current through the Tsushima Straits is studied using the acoustic Doppler current profiler (ADCP) data obtained by the ferryboat Camellia between Hakata, Japan and Pusan, Korea from February 1997 to February 2007. A robust estimation method to eliminate the effects of aliasing and tidal signals more accurately leads to a significant increase in the volume transport in winter time compared to the previously reported one by Takikawa et al. (2005) who analyzed this ADCP dataset for the first 5.5 years. The 10 years average of volume transport through the Tsushima Straits is 2.65 Sv, and those through the channels east (CE) and west (CW) of the Tsushima Islands are 1.20 Sv and 1.45 Sv, respectively, which represent a 13% increase and an 8% decrease from those of Takikawa et al. (2005). The transport through the CE increases rapidly from winter to spring and then decreases gradually as winter approaches. On the other hand, the transport through the CW increases gradually from winter to autumn and then decreases rapidly as winter approaches. The transport through the CE is larger than that of through the CW from February to April. The contribution of the Ekman transport near the sea surface, which is not measured with the ADCP, to the seasonal volume transport variation across our ADCP section is not significant.

Keywords: ⋅ Tsushima Warm Current, ⋅ Tsushima Straits, ⋅ seasonal variation, ⋅ ADCP.

revealed that the TWC in the CW is stronger than in the CE (Kaneko et al., 1991; Egawa et al., 1993; Isobe et al., 1994) and becomes stronger in summer and autumn (Miita and Ogawa, 1984; Egawa et al., 1993). According to the studies by Miita and Ogawa (1984), Mizuno et al. (1989), and Egawa et al. (1993), the amplitude of seasonal variations of the TWC is larger in the CW than in the CE. It should be noted that a southwestward countercurrent was found in the lee of the Tsushima Islands (Miita and Ogawa, 1984; Katoh, 1993; Isobe et al., 1994) and was later observed throughout the year by Takikawa et al. (2005). Although many studies using a ship-mounted or towed acoustic Doppler current profiler (ADCP) were conducted on the volume transport and structure of the TWC in the Tsushima Straits (Kaneko et al., 1991; Kawano, 1993; Egawa et al., 1993; Katoh, 1993; Isobe et al., 1994), most of these studies were based on short-term

1. Introduction The Tsushima Straits have a width, length, and mean water depth of about 180 km, 330 km, and 100 m respectively, connecting the East China Sea and the Japan Sea. The Tsushima Islands are located in the middle of the Tsushima Straits and separate the strait into two channels, western and eastern (hereafter referred to as the CW and CE, respectively). Through this straits, the Tsushima Warm Current (hereafter referred to as the TWC) carries heat, salt, nutrients, organic matter, and so forth from the East China Sea to the Japan Sea (Isobe et al., 2002). Many studies on the structure and temporal variation of the TWC

* Corresponding author. E-mail: [email protected] Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer

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measurements. Recent measurements using bottommounted ADCPs (Teague et al., 2002) conducted for about 11 months gave 2.38 and 2.65 Sv as mean volume transports of the TWC through the northeast and southwest sections intersecting the Tsushima Straits, respectively. The value at the southwestern section seems to be more accurate because the northeast one was likely underestimated due to unresolved strong currents near the coast of Korea. Average transports through the western and eastern channels during the period of their observations were 1.46 and 1.19 Sv, respectively (Teague et al., 2005). The Research Institute for Applied Mechanics (RIAM) of Kyushu University has been conducting longterm current measurements since February 1997 using an ADCP mounted on a ferryboat between Hakata and Pusan along its cruise line (Fig. 1) with the cooperation of RIAM and Pukyong National University (February 1997 to February 2004) and the National Fisheries Research and Development Institute (NFRDI, July 2004 to the present). The total observation period is more than 13 years and observation is still going on now. Using the first 5.5 years’ ADCP data after tide removal, Takikawa et al. (2003, 2005) revealed that the annually averaged volume transport of the TWC is 2.64 Sv, with 1.54 Sv through the CW and 1.10 Sv through the CE. In July 2004, the ferryboat Camellia was replaced by the New Camellia which makes six or seven round trips a week (twice as many as the Camellia). The number of observations for the last 10 years exceeds 3400 sections (1300 sections for Takikawa et al., 2003). The accumulation of data is expected to improve the understanding of the temporal and spatial variation of the TWC. Additionally, we found an overestimation of the volume transport of TWC by about 5% from late autumn to winter, and an underestimation by 5~10% in summer in the total transport of Takikawa et al. (2005). This is due to programming errors in the treatment of missing data near the bottom, the mean velocities just above the bottom were defined incorrectly. In this study, we adopt a different approach from Takikawa et al. (2003) to obtain a more accurate data set, avoiding tidal aliasing as accurately as possible using the Camellia and the New Camellia ADCP data obtained for 10 years from February 1997 to February 2007. Results of the harmonic analyses are compared with those from surface velocities measured with high frequency ocean radars. Using this new data set, we again try to clarify the temporal and spatial characteristics of the TWC, primarily by examining volume transport through the Tsushima Straits. 2. Data and Methods 2.1 ADCP data processing The data used in this study were initially obtained

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using a multilevel ADCP (VMBBADCP, 300 kHz, RD Instruments) mounted on the ferryboat Camellia from February 21, 1997 to February 6, 2004, which shuttled between Hakata and Pusan three times a week with a cruising speed of about 17 kt. The Camellia was replaced by a new ferryboat, New Camellia, on July 21, 2004, which shuttled six or seven times a week with a cruising speed of about 21 kt. The data sampling interval is about 24 sec and the measurements are performed in layers with a thickness of 8 m from a depth of 14 m (the top of the first layer) to the bottom in both cases. For each one-way cruise, the mean of the data is calculated in a horizontal segment within a latitude interval of 1/96 degree (about 1.6 km along the cruise line); the total number of horizontal segments along the cruise line is 130 in the first layer (14 to 22 m depth). Because the transducer beam angles are oriented 20° from vertical, the data within roughly the last 6% of the distance from the ADCP to the bottom could be contaminated by echo from the bottom. Therefore, the velocities within this range (hereafter called bottom layer velocities) are rejected and treated as missing data. As mentioned in the Introduction, due to programming errors in the treatment of missing data, the mean velocities near the bottom were defined incorrectly in Takikawa et al. (2003, 2005). If an observation is missing in the calculation of the mean velocity of a horizontal segment, the velocity previously observed in the same layer is added to the layer above the missing data, due to misnumbering of the data array and initializing errors in the program. Because distribution of missing data normally follows the shape of the bottom depth, values in deeper regions tend to spread to shallower regions along the bottom. Consequently, relatively strong northeastward currents in the central part of the CE and deep southwestward countercurrent in the CW spread above the bottom layer of each channel. Namely, in the CE, northeastward currents just above the bottom layer were overestimated. On the other hand, in the deep region of the CW (deeper than about 130 m), southwestward velocities just above the bottom layer were overestimated. Because the success rate of data acquisition near the bottom layer is high in spring, the effect of programming errors were small. While, in summer, this rate is low in the central part of the CE, but relatively high rate in the lee of the Tsushima Islands, leading to the spreading of relatively weak northeastward currents at the lees of the Tsushima Islands above the top of the bottom layer in the CE. 2.2 Tidal analysis Since tidal currents are very strong and comparable to mean currents due to the shallow depth and narrow width of the Tsushima Straits, tidal currents need to be removed from the ADCP data in order to study the proc-

Fig. 1. The observation line (thick solid line) is the track of the ferryboats Camellia and New Camellia along which current measurements using the vessel-mounted ADCP were performed. The channels east and west of the Tsushima Islands are defined as being south and north of 34.75°N (shown by the broken line), respectively.

esses associated with mean currents such as the mean volume transport. Takikawa et al. (2003) detided current data averaged in each horizontal segment with a length of 1/96 degree, using 4 years and 7 months of Camellia ADCP data, decomposing tidal currents into 10 major tidal constituents (Q1, O 1, P1, K1, N2, M2, S2, K2, MSf, and Mf). A further detailed harmonic analysis is performed for 52 tidal components using the current data detided by the method of Takikawa et al. (2003) after fixing the programming error as described above (hereafter called Takikawa’s detided current data) for the period from February 1997 to February 2004. Although Takikawa’s 10 major tidal constituents were effectively decomposed and removed, we found certain components (µ2, NO1, φ 1, and J1) that still contributed substantially to the transport variability. In particular, the µ2 component tide is significant and this component was previously used to calculate the tidal currents in the Tsushima Straits (Teague et al., 2001). Additionally, due to aliasing period of the NO1, φ 1, and J1 is large (29.8, 121.8, and 25.6 day, respectively), these tidal components need to be decomposed and removed to clarify the seasonal variation in the structure and volume transport of the TWC. Therefore, in this study, we modified the approaches of Takikawa et al. (2003) to permit more precise analysis. In addition to the 10 tidal constituents described by

Takikawa et al. (2003), four additional tidal constituents (µ2, NO1, φ 1, and J1) were taken into consideration and removed from the original ADCP data. To deal with aliasing problems, sets of poorly separable tidal constituents and the corresponding unfavorable sampling intervals were identified in the ADCP data (Table 1). One in each pair of data with an unfavorable sampling interval was removed from the time series to avoid aliasing signals. In addition, seasonal variability in the M 2 tide in the Tsushima Straits (Kang et al., 1995; Okuno et al., 2005) was taken into consideration. The 10-year monthly mean values of the M 2 tide amplitude were computed; the annual cycles obtained were coherent with that of sea level near Hakata and Pusan. Finally, for fitting the tidal modes to the data, we used the robust least squares (RLS) method (Leffler and Jay, 2007), whereas Takikawa et al. (2003) adopted the ordinary least squares solution. Further details on our tidal analysis were given in another paper (Ostrovskii et al., 2009). Results of the harmonic analyses obtained above are compared with those from high frequency ocean radars (hereafter HF radar), which were installed at the Tsushima Islands and the Fukuoka prefecture in the Tsushima Straits (Yoshikawa et al., 2006). The tidal ellipses estimated from the hourly HF radar data along the ferryboat track were obtained by harmonic analysis using about 5 years of data

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Table 1. Sets of poorly decomposable tidal constituents and corresponding unfavorable sampling intervals. Sampling interval (h)

Sets of tidal constituents

22.25~22.40 23.05~23.15 23.75~24.05 24.65~24.95

K1 – P1 – Msf – Mf, O1 – S 2 – K2 M2 – Q1 , S 2 – K2 – Msf – Mf M2 – O2 – Msf – Mf, Q1 – N2 , K1 – P1 – S 2 – K2 – Sa – Ssa M2 – Sa – Ssa, O1 – N2 , Q1 – Msf – Mf – µ2

Table 2. Amplitude, phase, and directions of major tidal ellipses obtained by HF radar and in this study. Differences of major tidal ellipses between (a) HF radar and this study, (b) HF radar and ADCP obtained by the method of Takikawa et al. (2003), are also listed. HF radar

Q1 O1 P1 K1 N2 M2 S2 K2 MSf Mf

φ1 J1 µ2 NO1

ADCP

Difference (a)

Amp. (cm/s)

Phase (°)

Dir. (°)

Amp. (cm/s)

Phase (°)

Dir. (°)

Amp. (%)

Phase (%)

Dir. (%)

Amp. (%)

Phase (%)

Dir. (%)

2.5 11.7 4.9 13.7 4.1 19.6 8.5 2.4 1.5 1.5 1.0

285 330 195 210 313 322 16 357 346 117 337

32 35 42 40 48 48 49 47 35 38 −3

2.4 11.7 4.5 14.7 4.1 20.4 7.9 2.9 1.5 1.5 1.2

322 346 254 206 328 346 34 345 209 190 114

33 37 37 43 49 48 83 31 55 −10 −8

6.3 0.2 8.1 7.4 0.0 4.5 7.6 22.0 3.0 3.3 16.3

20.5 8.5 32.7 2.4 8.8 12.9 10.2 6.3 76.3 40.2 124.1

0.4 1.1 2.5 1.5 0.7 0.1 18.5 8.7 11.1 26.2 2.3

9.3 4.0 3.9 11.6 1.8 4.3 52.7 4.3 3.8 19.3

28.0 3.8 26.5 8.6 5.3 1.7 6.4 6.9 81.2 32.9

4.4 2.6 7.1 0.8 2.7 0.0 21.1 0.8 10.9 36.5

1.0 1.0 0.4

242 298 208

41 53 130

0.9 1.1 1.0

236 339 213

33 36 14

1.4 6.7 180.7

3.2 22.6 2.7

4.1 9.9 64.0

from February 2002 to February 2007. Because the HF radar measurement covers only the eastern half of the ferryboat track, the averages (32 stations along the ferryboat track) were used for comparison in this study. Ten major tidal constituents (Q1, O1, P1, K1, N2, M2, S2, K2, MSf, and Mf) and four additional tidal constituents (µ2, NO1, φ1, and J1) are compared (Table 2). Averaged amplitudes, phases and directions of 14 tidal ellipses obtained by HF radar and in this study and the differences of major tidal ellipses between (a) HF radar and this study, (b) HF radar and ADCP obtained by the method of Takikawa et al. (2003), are also listed. 2.3 Evaluation of the new data set A very good agreement between the HF radar measurement and ADCP measurements in this study is found in difference (a) of Table 2. The differences of amplitudes of major axis and directions of the eight major tidal constituents are less than 10%, except for the K2 amplitude (22.0%) and the direction of S2 (18.5%). The differences of phases are somewhat large compared with those 542

Difference (b)

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of amplitudes and directions especially for the Q1 and P1 (20.5 and 32.7%, respectively). The adoption of the new tidal estimation approach leads to significant improvements in amplitudes of the major axis especially for the S2, K1, and O1 (difference (a) and (b) in Table 2). Although location is somewhat displaced from our observation line, tidal amplitudes of major axis computed from vertically averaged currents for eight tidal constituents (O1, P1, K1, µ2, N2, M 2, S2, and K2) north-east of the Tsushima Islands (see table 4 of Teague et al., 2001) are in good agreement with the results of our calculations. The new tidal estimation approach made it possible to get rid of aliasing effects as false periodic signals at low frequencies with periods of 7.4, 25.6, and 121 days (Ostrovskii et al., 2009). The periods of removed false signals correspond to the aliasing period of µ2, J 1, and φ 1 (7.4, 25.6, and 122 days) with a one-day interval sampling. Although the annual variations in the M 2 tide amplitude is detected near the Hakata coast, Tsushima Islands, and the Pusan coast in the surface layer (Ostrovskii et al., 2009), contribute very little to the seasonal volume transport variation.

1.15

4.5 VT1 VT2 VT3

1.1 1.05 Transport ratio

Volume transport (Sv)

4

Total CE CW

(a)

3.5

3

1 0.95 0.9 0.85

2.5

0.8 0.75 Jan.

2

Mar.

May

Jul.

Sep.

Nov.

1.5 1.5

J A JOJ A JOJ A JOJ A JOJ A JOJ A JO 1997 1998 1999 2000 2001 2002

Total CE CW

(b)

We evaluate the contribution of the programming error correction and improved the tidal estimation mentioned above, to the volume transport of the TWC through the Tsushima Straits. Surface and bottom velocities are obtained by extrapolating the values in the 14 to 22 m layer and at the deepest ADCP measurements to calculate the volume transport. The monthly means of total volume transport through the Tsushima Straits from Takikawa et al. (2005) (VT1), after the correction of the programming error (VT2), and from the adoption of the improved tidal analysis method (VT3) from February 1997 to August 2002, are shown in Fig. 2. The ratio of the climatological monthly means of VT2 to VT1 and VT3 to VT2 through the entire Tsushima Straits (total), CE and CW averaged for 5.5 years are shown in Figs. 3(a) and (b), respectively. Comparing the VT1 and the VT2 in Fig. 3(a), the programming error leads to overestimation of transport by about 5% from late autumn to winter, and underestimation by 5~10% in summer in the total transport (cf. Takikawa et al., 2005). These differences are mainly contributed by the correction of the transport through the CE. After the adoption of the new tidal estimation approach, the total transport significantly increased in winter time (Fig. 2). This is because the tidal components having aliasing period longer than one month (especially K 1 and S 2 ) are estimated accurately. The climatological monthly means of the total transport shows increases of approximately 10~20% in winter time (Fig. 3(b)). This difference is also mainly contributed by the transport through the CE. The updated data set for the

Transport ratio

1.4

Fig. 2. Monthly average volume transport in Sverdrups (Sv) of the Tsushima Warm Current into the Japan Sea through the Camellia ADCP section from February 1997 to August 2002.

1.3 1.2 1.1 1 0.9 Jan.

Mar.

May

Jul.

Sep.

Nov.

Fig. 3. Ratio of climatological monthly means of volume transport of (a) VT2 to VT1, and (b) VT3 to VT2 averaged for 5.5 years.

Camellia and New Camellia ADCP survey extended to February 2007 is used in the following section. 3. Results 3.1 Structure of the Tsushima Warm Current Climatological bi-monthly variations of the TWC structure are shown in Fig. 4. Velocities normal to the observation section estimated from the Takikawa’s detided current data after fixing program errors are indicated in (a), and those from the new data set described in the previous section in (b). In both cases, the current structures change seasonally from a relatively weak baroclinic structure in winter and spring to a strong baroclinic structure toward summer and autumn, with strong northeastward TWC flow into the Japan Sea in the central parts of the CE and CW. Downstream of the Tsushima Islands, southwestward countercurrents are observed year-round and a


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Depth (m)

Depth (m)

Depth (m)

Depth (m)

Depth (m)

Depth (m)

(a)

(b)

0 50 100 150 200

Feb.

Feb.

0 50 100 150 200

Apr.

Apr.

0 50 100 150 200

Jun.

Jun.

0 50 100 150 200

Aug.

Aug.

0 50 100 150 200

Oct.

Oct.

0 50 100 150 200

Dec. 35.0 34.8 34.6 34.4 34.2 34.0 33.8 Latitude (oN)

Dec. 35.0 34.8 34.6 34.4 34.2 34.0 33.8 Latitude (oN)

Fig. 4. Vertical section of the climatological monthly mean velocities normal to the observation section estimated from (a) Takikawa’s detided current data and (b) the new data set. Positive velocities are toward the Japan Sea. White (shaded) regions indicate positive (negative) values. The contour interval is 10 cm s –1.

deep countercurrent on the slope of the Korean side is observed all year. Although the major features of the current structure are basically similar, there are some significant differences. The northeastward TWC in the central part of the CE in the new vertical section (Fig. 4(b)) became stronger through the year than that in the old vertical section (Fig. 4(a)). The northeastward TWC in the CW also became stronger through the year except in winter, and the deep southwestward countercurrent in the CW become stronger in spring and summer in the new vertical section. The countercurrents at the lees of the Tsushima Islands and Iki Island do not show big differences between the two vertical sections. The current maximum of the TWC in 544

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the central part of the CE varies from 17 cm s–1 in January to 41 cm s–1 in August, with an average of 29 cm s–1 in the new data set. That in the CW varies from 38 cm s–1 in February to 65 cm s–1 in August, with an average of 52 cm s–1. 3.2 Volume transport of the Tsushima Warm Current through the Tsushima Straits As mentioned in Subsection 2.1, the bottom layer velocities could be contaminated by echo from the bottom. Then, the bottom layer velocities are replaced by linear shear flow to estimate the volume transport through the Tsushima Straits. This is because the summer current is baroclinic, even near the bottom as shown in figure 8

4.5


Total Transport CE CW

(a)

4 3.5 3 2.5 2 1.5 1 0.5 J

A J O 1997

J

A

J O 1998

J

A

J O 1999

J

A

J O 2000

J

A

J O 2001

J

4 Total Transport CE CW

(b)


3.5 3 2.5 2 1.5 1 0.5 J

A J O J 2002

A J O J 2003

A

J O J 2004

A

J O J 2005

A

J O J 2006

Fig. 5. Monthly average volume transport of the Tsushima Warm Current into the Japan Sea through the Camellia ADCP section and transports of the eastern and western channels in Sv from (a) Feb. 1997 to Dec. 2001 and (b) Jan. 2002 to Feb. 2007. Straight dashed lines indicate the mean value of each volume transport. Error bars indicate the standard error in each month.

of Teague et al. (2002). Vertical gradients of the shear flow are taken to be equal to the gradients estimated from climatological monthly mean velocity profiles using the 10-year ADCP observations. In previous studies, Takikawa et al. (2003, 2005) replaced velocities in the bottom layer by the deepest velocity found by ADCP, assuming uniform bottom flow. Velocities at depth shallower than 14 m are assumed to be equal to the values in 14 to 22 m layer. The variation in the velocity structure in the surface layer due to wind forcing is discussed in Section 4. The monthly mean volume transports of the TWC through the Tsushima Straits, CE, and CW, estimated from the new data set, are shown in Fig. 5. As seen in Fig. 2(b), the modified tidal estimation method leads to a significant increase in the volume transport in winter time from February 1997 to August 2002, much of which is caused by an increase in volume transport through the CE. Consequently, the monthly averaged transport through the CE became comparable to or larger than that through the CW from winter to spring. The seasonal variation with double peaks in the total transport reported by Takikawa et al. (2005) appeared in 2002, 2003, 2005, and

also in 2006, which occurs after the period they analyzed. The total transport has two significant maxima from spring to early summer and in autumn and a minimum in winter in most years. The first maximum from spring to early summer tends to occur irregularly between March and July with an average value of about 2.9 Sv. The second maximum in autumn has an average value of about 3.2 Sv, and the minimum transport occurring mostly in January has an average value of about 2.0 Sv. The transport through the CE has a couple of maxima during a year, of which the maximum from spring to early summer is highest (except in 1999), and has a minimum mostly in January. The average values of the spring-early summer maximum and January minimum transport are about 1.5 and 1.0 Sv, respectively. An autumn maximum tends to occurs irregularly in time and does not show a clear peak in 2005. The transport through the CW shows relatively stable seasonal variation, in which spring and autumn maxima, and a winter minimum occur in June to July, October, and January to February, respectively, with an average of roughly 1.5, 2.0, and 1.0 Sv, respectively. The total volume transport averaged for the 10 years


545

is 2.65 Sv, which is almost same as that of Takikawa’s detided current data (2.64 Sv). The volume transports through the CE and CW averaged for the 10 years are 1.21 and 1.45 Sv, respectively, which represent a 13% increase and a 8% decrease from those of Takikawa’s detided current data (1.07 and 1.58 Sv, respectively). Figure 6 shows the climatological monthly means of the volume transport through the Tsushima Straits, CE, and CW estimated from the new data set. The climatological monthly mean transport through the CE starts to increase rapidly from a minimum (1.00 Sv) in

4 Total Transport Transport through the CE Transport through the CW


3.5 3 2.5 2 1.5 1 0.5

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Fig. 6. Climatological monthly mean total volume transport in Sv (open circles) of the Tsushima Warm Current into the Japan Sea through the Camellia ADCP section averaged for 10 years, and transports in the western (open triangles) and eastern (open squares) channels. Straight dashed lines indicate the mean values of each volume transport.

January and peaks (1.40 Sv) in March, and then decreases gradually as January approaches. On the other hand, that through the CW increases gradually from a minimum (1.01 Sv) in January and peaks (1.82 Sv) in October, and then decreases rapidly as January approaches. From winter to spring, the transport through the CE is comparable to that through the CW. Two local maxima in June and October in seasonal variation in the transports reported by Takikawa et al. (2005) are also seen in the monthly mean total transport. The annual ranges of climatological monthly mean volume transports through the CE and CW are estimated to be 0.40 and 0.81 Sv, respectively, and the total range is 1.09 Sv. The climatological monthly mean volume transports over the observation period are listed in Table 3. The monthly mean volume transports over the observation period are listed in Table 4. 4. Summary and Discussion In this study, we updated the method of analysis for the data set obtained during the Camellia and New Camellia ADCP surveys from February 1997 to February 2007 and studied features of the seasonal cycle of the TWC through the Tsushima Straits. Besides the 10 tidal constituents decomposed by Takikawa et al. (2003), four additional tidal constituents (µ2, NO1, φ1, and J1) were decomposed and removed from the original ADCP data. To deal with aliasing problems, sets of poorly decomposable tidal constituents and corresponding unfavorable sampling intervals were estimated, and data with unfavorable sampling intervals were removed from the tidal analysis. The aliased peaks in the power spectrum of the volume transport corresponding to aliasing periods of µ2, J1, and φ 1 with a one-day interval sampling are removed. In addition, seasonal variability of the M 2 tide in the Tsushima Straits was taken into consideration, but

Table 3. Climatological monthly mean volume transports of the Tsushima Warm Current through the Tsushima Straits (February 1997 to February 2007).

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

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Total transport (Sv)

Western channel (Sv)

Eastern channel (Sv)

Number of ADCP sections

2.01 2.28 2.52 2.60 2.66 2.70 2.68 3.05 2.93 3.10 2.84 2.36

1.01 1.02 1.12 1.30 1.39 1.42 1.48 1.72 1.78 1.92 1.71 1.32

1.00 1.26 1.40 1.30 1.28 1.28 1.20 1.33 1.15 1.18 1.13 1.04

310 259 240 265 269 260 333 288 313 316 310 284

Table 4. Monthly mean volume transports of the Tsushima Warm Current through the Tsushima Straits (February 1997 to February 2007). Total transport (Sv)




1997 Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

2.34 2.51 2.44 2.58 2.70 3.01 2.91 2.65 3.11 2.49 2.46

1.26 1.29 1.24 1.41 1.46 1.75 1.68 1.76 1.95 1.63 1.54

1.08 1.22 1.20 1.17 1.24 1.26 1.23 0.89 1.16 0.86 0.92

7 21 26 22 26 29 25 24 22 20 26

1998 Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

1.76 2.03 2.30 2.79 2.95 2.93 2.91 2.69 2.72 2.98 2.91 2.48

1.01 0.97 1.02 1.45 1.55 1.44 1.53 1.43 1.64 1.74 1.77 1.43

0.75 1.06 1.28 1.34 1.40 1.49 1.38 1.26 1.08 1.25 1.14 1.05

27 23 14 27 26 28 27 26 23 27 26 22


2.05 2.59 2.56 2.69 2.79 3.08 2.87 3.56 3.43 3.96 3.53 2.65

1.10 1.21 1.24 1.23 1.42 1.70 1.66 1.95 2.03 2.29 2.06 1.39

0.94 1.38 1.31 1.46 1.37 1.38 1.21 1.60 1.41 1.68 1.47 1.26

25 12 25 22 17 25 26 20 23 26 27 23

its contribution to the seasonal variation of volume transport was negligible. The adoption of this modified tidal estimation approach led to a significant increase in the total volume transport in winter time, much of which was contributed by increased transport through the CE. This is due to the tidal components having an aliasing period of longer than one month (especially K1 being S2) are estimated accurately. Consequently, the monthly averaged transport through the CE became comparable to that through the CW from winter to spring. The monthly mean

total volume transport showed an increase of approximately 10~20% in winter time (Fig. 3). Characteristics of the TWC through the Tsushima Straits have been studied using the new data set. The TWC in the central part of the CE was found to be stronger throughout the year than that of Takikawa’s detided current data. The TWC in the central part of the CW was also found to be stronger throughout the year except in winter. The northeastward current maximum in the CE varies from 17 cm s–1 in winter to 41 cm s–1 in summer,


547

Table 4. (continued). Total transport (Sv)





1.83 2.22 2.60 2.42 2.64 2.67 2.90 3.06 2.85 2.78 2.54 2.47

1.01 1.02 1.07 1.22 1.56 1.50 1.54 1.66 1.57 1.85 1.81 1.48

0.82 1.20 1.53 1.20 1.08 1.17 1.36 1.40 1.29 0.93 0.73 0.99

24 23 17 27 30 26 28 24 24 26 19 15


2.11 2.67 2.89 2.91 2.70 2.68 2.70 3.00 3.12 3.01 2.76 1.93

1.15 1.38 1.48 1.57 1.44 1.47 1.64 1.75 2.10 1.85 1.69 1.24

0.96 1.29 1.41 1.34 1.25 1.21 1.06 1.25 1.02 1.16 1.07 0.69

27 23 16 29 30 25 27 26 24 28 25 25


1.82 2.31 2.90 2.93 3.22 2.73 2.81 2.92 3.22 2.96 2.53 2.11

0.87 1.02 1.40 1.50 1.55 1.58 1.68 1.74 1.96 1.91 1.53 1.20

0.95 1.29 1.50 1.43 1.67 1.16 1.13 1.18 1.26 1.05 1.00 0.91

17 6 26 26 17 25 28 27 28 18 27 26

with an average of 29 cm s–1. In the CW, the current maximum varies from 38 cm s –1 in winter to 65 cm s–1 in autumn, with an average of 52 cm s –1. The southwestward deep countercurrent was found to be stronger all yearround in the CW. The total volume transport of the TWC averaged for the 10 years is 2.65 Sv, which is almost the same as that of Takikawa’s detided current data (2.64 Sv). The volume transports of the TWC through the CE and CW averaged for the 10 years are 1.21 and 1.45 Sv, respectively, 548

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which represent an 13% increase and 8% decrease from those of Takikawa’s detided current data (1.07 and 1.58 Sv, respectively). There is a remarkable difference in the climatological seasonal volume transport between the CE and CW. The transport through the CE increases rapidly from winter to spring, and then decreases gradually as winter approaches. On the other hand, the transport through the CW increases gradually from winter to autumn, and then decreases rapidly as winter approaches. It should be noted that the volume transport through the CE

Table 4. (continued). Total transport (Sv)





1.78 2.11 2.22 2.42 2.74 2.87 2.42 2.87 2.79 3.24 2.86 2.43

0.73 0.97 1.02 1.27 1.41 1.42 1.50 1.79 1.85 1.97 1.75 1.38

1.05 1.14 1.19 1.15 1.34 1.46 0.92 1.08 0.94 1.28 1.11 1.05

26 23 17 26 26 26 27 26 26 27 27 24

2004 Jan. Feb. Jul. Aug. Sep. Oct. Nov. Dec.

1.79 1.83 2.80 3.55 3.02 3.01 3.00 2.32

0.89 0.72 1.40 1.87 1.70 1.86 1.77 1.26

0.90 1.11 1.40 1.68 1.32 1.15 1.23 1.06

21 6 39 38 48 43 43 40


2.34 1.91 2.19 2.58 2.43 2.44 2.21 2.67 2.57 2.90 2.61 1.90

1.12 0.67 0.79 1.18 1.25 1.21 1.07 1.42 1.49 1.76 1.54 0.88

1.22 1.25 1.40 1.40 1.19 1.23 1.14 1.25 1.08 1.14 1.07 1.01

45 40 51 33 53 51 54 24 43 47 45 31


1.76 2.38 2.65 2.38 2.51 2.49 2.61 3.09 2.99 3.14 2.96 2.71

0.88 0.98 1.13 1.14 1.23 1.25 1.43 1.77 1.88 2.05 1.68 1.44

0.88 1.40 1.52 1.24 1.29 1.24 1.18 1.32 1.10 1.10 1.28 1.27

43 45 53 49 48 28 48 52 50 52 51 52

2007 Jan. Feb.

2.32 2.47

1.16 1.14

1.16 1.33

55 52


549

0.1 Surface to bottom Surface to 14m (EV1) Surface to 14m (EV2)


0.05

0

-0.05

-0.1

-0.15 Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Fig. 7. Climatological monthly mean Ekman transport across the Camellia observation line (surface to bottom) and surface Ekman transports (surface to 14 m) with different eddy viscosities (February 1997 to February 2007).

is larger than that through the CW from February to March. The annual ranges of climatological monthly mean volume transport through the CE and CW are estimated to be 0.40 and 0.81 Sv, respectively, and the range of the total transport is 1.09 Sv. Finally, we investigate the effect of Ekman transport on the volume transport through the Tsushima Straits. Because our ship-mounted ADCP measurements are performed in layers with a thickness of 8 m from a depth of 14 m (the top of the first layer) to the bottom, the velocity structure from the surface to 14 m remains unknown, although the velocity structure in the surface layer can be strongly influenced by wind stress. To obtain more accurate estimates of the volume transport through the Tsushima Straits, the velocity profile in the Ekman layer needs to be assessed. Recently, Yoshikawa et al. (2007) moored ADCPs in the CE for two weeks in summer and successfully identified a current profile corresponding basically to the Ekman spiral. Two ADCPs of high and low acoustic frequencies were used simultaneously for high-resolution velocity measurements in both the surface boundary layer and the interior. They estimated the velocity relative to an interior flow in the surface boundary layer, subtracting the reference velocity (estimated from velocities at greater depths) from the velocity in the surface layer, and performed complex principal component analysis (PCA) of the lagged wind stress and the relative velocity. The result indicates that a wind-driven flow is balanced with wind stress after 11~13 h in the Tsushima Straits. To estimate the Ekman transport across the Camellia ADCP section, reanalyzed surface wind data 550

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published by the Japan Meteorological Agency (JMA) every 6 h with 1.25 (longitude) × 1.25 (latitude) resolution (GPV-GSM data) are used in this study. The surface wind stress is assumed to be spatially uniform in the Tsushima Straits and was estimated from the surface wind data using the drag coefficient formula of Yelland and Taylor (1996). Ekman transports across every ADCP section are computed using the surface wind stress of 12 h earlier, with uniform eddy viscosities of 5.0 × 10–3 (hereafter referred to as EV1) and 5.0 × 10–2 m2 s –1 (hereafter referred to as EV2). The EV1 is considered appropriate in the Tsushima Straits in summer (Yoshikawa et al., 2007) and the EV2 is chosen under the assumption of weak stratification. The corresponding Ekman depths are 11 and 35 m, respectively. The climatological monthly mean Ekman transports and surface Ekman transports (surface to 14 m) across the ADCP section with the two eddy viscosities are shown in Fig. 7. The vertically integrated total Ekman transport shows clear seasonal variation with a minimum (southwestward) in January (–0.11 Sv) and a maximum (northeastward) in July (0.03 Sv) and an annual average of –0.03 Sv. With a small eddy viscosity (EV1), the Ekman transport is concentrated in the surface layer, but with a large eddy viscosity (EV2), almost half the Ekman transport occurs in depth deeper than 14 m, the shallowest depth of our ADCP measurement. Although these estimates are relatively coarse, the Ekman transport across our ADCP section is not significant, and a certain amount of it is already included in our ADCP measurements under the weak stratification. The causes of the different seasonal variations in the CE and CW and the mechanism driving the TWC will be addressed in future studies. A better definition of the seasonal cycle leads to a more precise description of intraseasonal variations. We will also examine the intraseasonal variation in the Tsushima Straits in the future. Acknowledgements The authors deeply express our special thanks to captain and crew of the ferryboats Camellia, New Camellia and the staff of the Camellia Line Ltd. Naoki Hirose and Yutaka Yoshikawa of RIAM, Kyushu University are gratefully acknowledged for their useful discussion and suggestions. Thanks are also due to Kaoru Ichikawa and the anonymous reviewers for helpful comments. They also wish to express their thanks to the organizations which kindly provided observational data, especially the Japan Meteorological Agency. References Egawa, T., Y. Nagata and S. Sato (1993): Seasonal variability in the Tsushima Strait deduced from ADCP data of ship-ofopportunity. J. Oceanogr., 49, 39–50.

Isobe, A., S. Tawara, A. Kaneko and M. Kawano (1994): Seasonal variability in the Tsushima Warm Current, TsushimaKorea Strait. Cont. Shelf Res., 14, 23–35. Isobe, A., M. Ando, T. Watanabe, T. Senjyu, S. Sugihara and A. Manda (2002): Freshwater and temperature transports through the Tsushima-Korea Straits. J. Geophys. Res., 107(C7), 3065, doi:10.1029/2000JC000702. Kaneko, A., S. K. Nyun, S. D. Chang and M. Takahashi (1991): An observation of sectional velocity structure and transport of the Tsushima Current across the Korea Strait. p. 179– 195. In Oceanography of Asian Marginal Seas, ed. by K. Takano, Elsevier, Amsterdam. Kang, S. K., J. Y. Chung, S. R. Lee and K. D. Yum (1995): Seasonal variability of the M2 tide in the seas adjacent to Korea. Cont. Shelf Res., 15(9), 1087–1113. Katoh, O. (1993): Detailed current structure in the Eastern Channel of the Tsushima Strait in summer. J. Oceanogr., 49, 17– 30. Kawano, M. (1993): Monthly changes of the velocity and volume transport of the Tsushima Warm Current in the Tsushima Strait. Bull. Japan Soc. Fish. Oceanogr., 57, 219– 230 (in Japanese). Leffler, K. E. and D. A. Jay (2007): Enhancing tidal harmonic analysis: Robust (hybrid L1/L2) solutions. Cont. Shelf Res., 29, 78–88. Miita, T. and Y. Ogawa (1984): Tsushima current measured with current meters and drifters. p. 67–76. In Ocean Hydrodynamics of the Japan and East China Seas, ed. by T. Ichiye, Elsevier, Amsterdam. Mizuno, S., K. Kawatate, T. Nagahama and T. Miita (1989): Measurements of east Tsushima Current in winter and estimation of its seasonal variability. J. Oceanogr. Soc. Japan, 45, 375–384. Okuno, A., Y. Yoshikawa, A. Masuda, K. Marubayashi and M.

Ishibashi (2005): Tidal currents in the Tsushima Straits observed by HF radars. Eng. Sci. Rep., Kyushu Univ., 27(1), 9–18 (in Japanese with English abstract). Ostrovskii, A., K. Fukudome, J.-H. Yoon and T. Takikawa (2009): Variability of the volume transport through the Korea/Tsushima Strait as inferred from the shipborne acoustic Doppler current profiler observations in 1997–2007. Oceanology, 49, 338–349. Takikawa, T., J.-H. Yoon and K.-D. Cho (2003): Tidal current in the Tsushima Straits estimated from ADCP data by ferryboat. J. Oceanogr., 59, 37–47. Takikawa, T., J.-H. Yoon and K.-D. Cho (2005): The Tsushima Warm Current through Tsushima Straits estimated from Ferryboat ADCP data. J. Phys. Oceanogr., 35, 1154–1168. Teague, W. J., H. T. Perkins, G. A. Jacobs and J. W. Book (2001): Tide observations in the Korea-Tsushima Strait. Cont. Shelf Res., 21, 545–561. Teague, W. J., G. A. Jacobs, H. T. Perkins and J. W. Book (2002): Low-frequency current observations in the Korea-Tsushima Strait. J. Phys. Oceanogr., 32, 1621–1641. Teague, W. J., P. A. Hwang, G. A. Jacobs and J. W. Book (2005): Transport variability across the Korea/Tsushima Strait and the Tsushima Island Wake. Deep-Sea Res. II, 52, 1784–1801. Yelland, M. and P. K. Taylor (1996): Wind stress measurement from the open ocean. J. Phys. Oceanogr., 26, 541–558. Yoshikawa, Y., A. Masuda, K. Marubayashi, M. Ishibashi and A. Okuno (2006): On the Accuracy of HF Radar Measurement in the Tsushima Strait. J. Geophys. Res., 111, C04009, doi:10.1029/2005JC003232. Yoshikawa, Y., T. Matsuno, K. Marubayashi and K. Fukudome (2007): A surface velocity spiral observed with ADCP and HF radar in the Tsushima Strait. J. Geophys. Res., 112, C06022, doi:10.1029/2006JC003625.


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