J Oceanogr (2011) 67:173–182
DOI 10.1007/s10872-011-0016-5

ORIGINAL ARTICLE

Reciprocal sound transmission measurement of mean current
and temperature variations in the central part (Aki-nada)of the Seto Inland Sea, Japan
Yudi Adityawarman • Arata Kaneko •Koji Nakano • Naokazu Taniguchi •Katsuaki Komai • Xinyu Guo • Noriaki Gohda

Received: 3 August 2010 / Revised: 9 October 2010 / Accepted: 21 February 2011 / Published online: 12 April 2011Ó The Oceanographic Society of Japan and Springer 2011

Abstract A 30 km-range reciprocal sound transmission
experiment was carried out on the line connecting Honshu
and Shikoku (the first and fourth biggest main Japaneseislands, respectively) in the central part (Aki-nada) of the
Seto Inland Sea, Japan, during March–May 2010 to measure the mean current and temperature variations over thesea. The range-averaged current along the sound transmission line was estimated to have a mean and standard
deviation of (3.8–4.4) ± (1.7–1.8) cm/s after converting
the travel time difference data into currents, including afortnightly tidal variation in the range of ±30 cm/s. The
positive mean current implies slow water movement fromthe west to east through Aki-nada. The range-averaged
speed of sound was estimated by converting from the mean
travel time or one-way travel time into the speed of sound,and further converted into temperature for fixed values ofsalinity and depth, according to the standard speed of soundformula. Besides the precise measurement (to an accuracyof 0.01°C) of semidiurnal and diurnal tidal variations andseasonal warming, the temperature data showed periodic
variations with periods of 7.0 and 21.1 days that had neverbeen observed in Aki-nada before. This study suggests thatreciprocal sound transmission is a powerful technique forthe long-term accurate measurement of mean current andtemperature variations in coastal and inland seas.

Y. Adityawarman  A. Kaneko (&)  K. Nakano 
N. Taniguchi  K. Komai  N. Gohda
Graduate School of Engineering, Hiroshima University,
1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japane-mail: akaneko@hiroshima-u.ac.jp
X. GuoCenter for Marine Environmental Studies, Ehime University,
2-5 Bunkyo-cho, Matsuyama 790-8577, Japan

Keywords Aki-nada  Seto Inland Sea  Mean current 
Temperature variation  Long-term reciprocal soundtransmission experiment

1 Introduction
The Seto Inland Sea is the biggest inland sea in Japan, andis surrounded by three main Japanese islands (Honshu,
Kyushu and Shikoku). The east–west length, horizontal
area and average depth of the sea are 450 km, 23203 km2
and 38 m, respectively. Water pollution is present aroundthe urban and industrial regions surrounding the inland sea,
and this sometimes leads to disastrous events—such as red
tides that damage the fish and oyster aquaculture industries—in a number of places inside the inland sea. However, the environmental problems are confined to rather
specific regions around industrial areas; they do not extend
over the central regions of the inland sea.
The Seto Inland Sea is connected to the Pacific Ocean
through the Bungo Channel and the Kii Channel—locatedon the southwestern and the southeastern sides of the sea,
respectively—and is significantly affected by the Kuroshio,
which flowing off both of the channels. Although the
Kanmon Strait also represents an outlet to the Sea of Japan
at the western edge of the inland sea, its effect on the inland
sea may be negligible due to the narrowness of the strait.
The near-surface water near the northern front of the Kuroshio enters the Bungo Channel with a mean periodicity of
10 days (warming season) or 8 days (cooling season) as the
Kyucho (rapid tides) (Takeoka et al. 1993; Akiyama andSaitoh 1993). On the other hand, the northern part of theKuroshio surface water enters the Kii Channel as part of
the Furiwake-cho (Bifurcation Current) when the Kuroshio
is close to the Kii Peninsula (Takeuchi et al. 1998).

123

174

Although the Kuroshio intrusions may provide a significant
influence on the overall environment of the Seto InlandSea, little is known about their effect on the central part ofthe inland sea, in contrast to the Bungo and Kii Channels.
The mean current—in other words the subtidal current—in the Seto Inland Sea has been studied as residual
currents in various regions of the inland sea. An overview
of the mean current was reported by Yanagi and Higuchi(1979) based on currentmeter data from limited periods ofa few days. However, these data were not sufficient togauge the long-term variations in the mean current in a
specified region. Most recent studies have been based on
numerical models (Guo et al. 2004; Arai 2004; Chang et al.
2009), and observational data have been acquired in only
limited areas due to the difficulty involved in acquiringsuch data when fisheries are active (Takasugi et al. 1994a,b). It has been proposed that the mean currents through theinland sea are generated by the pressure gradient between
the western and eastern parts of the inland sea (Kunii and
Fujiwara 2006) or between the Bungo and Kii Channels at
the outlets to the Pacific Ocean (Komai et al. 2008a). It has
also been reported that density differences produced in theinland sea by Kuroshio intrusion cause the movement ofinland sea water (Komai et al. 2008b). However, the meancurrents have never been validated by direct observations,as this is a rather difficult task.
The coastal acoustic tomography system (CATS), which
has been developed by Hiroshima University as an application of deep-sea acoustic tomography (Munk et al. 1995),
and was shown to operate effectively in the coastal seasaround Japan, is a large step towards solving the above
problem (Park and Kaneko 2000; Yamaguchi et al. 2005;
Lin et al. 2005; Kaneko et al. 2005; Nguyen et al. 2009). In
the present work, the CATS was applied to measure themean current and temperature variations in the central
region (Aki-nada) of the Seto Inland Sea, where considerable tidal currents occur. The mean current throughAki-nada between Honshu and Shikoku must be measured
over a long period to remove tidal components from thedata over the sea.

2 Experiment
A 30 km-range reciprocal sound transmission experiment
was carried out to measure the mean current and temperature variations in the sea between the stations T1 and T2(distance 30.17 km) at the central part (Aki-nada) of the
Seto Inland Sea, Japan, during March–May 2010 (Fig. 1).
The mean floor depth was 8 m at T1 and increased
monotonically to 33 m at 15 km away from T1. The floor
depth reached a maximum of 49 m around 20 km from T1,
and suddenly became shallower toward T2, where the123

Y. Adityawarman et al.

mean floor depth was 14 m. The floor depth was over 40 m
16–23 and 25–28 km away from T1, which remained themain traffic route of the inland sea. A CATS was placed at theedge of the breakwater that protects the fishery port, and a
4 kHz acoustic transducer was suspended by rope 2 m fromthe bottom in front of the offshore side of the breakwater andconnected to the CATS via a cable (Fig. 2). Power was
supplied to CATS by 12 and 24 V solar batteries.
The transducer (ITC 2002A) was of the broad-band typewith a central frequency of 4 kHz and a band width of
1.4 kHz. The transducer can be applied to not only soundtransmission but also sound reception. The source level ofthe broad-band transducer was 192 dB re 1 lPa at 1 m, andthe average power consumption was 480 W during soundtransmission. In this experiment, the transmitted signal wasphase-modulated by a twelfth-order M sequence toincrease the signal-to-noise ratio (SNR) of the received
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
signals by 20 log 212  1 = 36 dB. This gain in SNR was
attained by cross-correlating the received signals with the
M sequence used in transmission. Three cycles per digit
was selected to transmit the phase-modulated sound fromthe broad-band transducer. The transmission of 4 kHz
sound at 3 cycles per digit requires a frequency bandwidthof 4 kHz/3 = 1.3 kHz, which the present transducer
satisfied. The time resolution for multi-arrivals (one-digit
width of the M sequence) was 0.75 ms. Six repeatedperiods of sound (18.43 s), phase-modulated by the M
sequence, were released every 15 min upon synchronizedtiming from T1 and T2. Repeat transmission was also
pﬃﬃﬃﬃ
devised to increase the SNR by 20 log N dB (N: repeat
number) through an ensemble average. The fluctuation in
the data received during the six repeated periods of soundprovides the short-term accuracy of the system under fixedenvironmental conditions.
The travel times are identified at the maximum peak
point in the correlation waveform received for each of thesix repeated periods of sound. The mean travel times forthe periods of sound received are used to calculate therange-averaged current and speed of sound, and the standard deviation (STD) from their mean values serves as anindex of short-term variations, which is equivalent to anerror bar. The travel times for the reciprocal sound transmission between the stations T1 and T2 can be expressed
by
t1 ¼

L
C m þ Vm

ð1Þ

t2 ¼

L
;
C m  Vmð2Þ

where L denotes the station-to-station distance, and t1 andt2 denote the travel times from T1 to T2 and from T2 to T1,
respectively. By solving the coupled Eqs. 1 and 2, theReciprocal sound transmission measurement of mean current and temperature variations

Fig. 1 Map of the location of the experimental site and thesurrounding region with a bathymetric chart. The whole region of
the Seto Inland Sea is shown on a larger scale in the upper right of the175

figure. T1 and T2 are the acoustic stations, and C1–C5 are the CTD
stations. Vm indicates the positive current along the transmission line.
The tidal gauge station is indicated by an asterisk

Fig. 2 Schematic diagram ofthe long-range reciprocal soundtransmission experiment locatedat the breakwater at the fishery
port

123

176

Y. Adityawarman et al.

Fig. 3 Results of the range-independent ray simulation using CTD data at C4 (20 km from T1) on March 15 (upper panel) and May 12 (lower
panel). Vertical profiles of the speed of sound (C) are shown on the left of the figure

range-average velocity Vm and the speed of sound Cm forthe region between the stations T1 and T2 can related to thetravel time difference Dt and the mean travel time tm,
respectively, as shown in the following formulae:
Vm ¼

L
Dt
2t1 t2

ð3Þ

Cm ¼

L
tm :
2t1 t2

ð4Þ

CTD casts were performed at 5 km intervals on March
15 and May 12 along the sound transmission line. The
results were used not only as comparison data in soundtransmission measurements but also as data for a rangeindependent ray-tracing simulation. The results of thisrange-independent ray simulation for the March and May
data are shown in Fig. 3. Here, the speed of sound profile issmoothed using a spline function prior to the simulation.
Sound mainly traveled in the upper 20 m for the Marchdata and in the depth range from 10 to 25 m for the May
data, because the minimum speed of sound was found tooccur at a depth of 2 m in the March data and at a depth of123

10 m in the May data when constructing the corresponding
sound channel according to depth. A couple of rays
traveled at a depth of about 6 m in the May data. However,these rays were ignored as they diminish in the rangedependent ray simulation due to their sensitivity.

3 Sound transmission data and error evaluation
The sound transmission was sporadically interrupted byenvironmental factors such as the hulls of ships, surface
waves and coastal fronts. More data were missing from thedata set for T2 than from that for T1. This is because thebottom topography constructs gentle slopes around T1, in
contrast to the sudden deepening seen around T2. Thisasymmetric bottom topography causes traveling sounds toconvergence toward T1 but suddenly spread near T2.
The correlation waveforms obtained at T1 during
11:00–13:00 on March 28, 2010, are shown (as an example
of a typical case) with a stack diagram in Fig. 4a in such a
way that the transmission time is progressed upward.

Reciprocal sound transmission measurement of mean current and temperature variations

Fig. 4 Stack diagram of the correlation waveforms for the six
M-sequence periods of sound received, obtained at T1 during
11:00–13:00 on March 28, 2010 (a). The data for the third periodof sound received are magnified in b. The order that the repeatedsounds were received is indicated by the circled numbers above theplots

The correlation peak height varies considerably with not
only the repeat number (horizontal axis) but also thetransmission time (vertical axis). The peaks for the third
period of sound received tended to have the greatest heights.
Thus, the correlation waveforms for the third period ofsound received are magnified in Fig. 4b. The travel times
were determined for the maximum peak point in each correlation waveform. Only peak points with signal-to-noise
ratios (SNRs) [7 (equivalent to dB = 20 log (7) = 16.9)
are considered in the following travel time analyses.
The range-averaged current can be estimated only forperiods for which the reciprocal (travel time difference)
data were successfully acquired. The error bar for shortterm current measurement can be evaluated based on the
STD from the mean current, which was calculated fromthe data for the six repeated periods of sound received. The
STDs for the currents are shown in the time plots of Fig. 5.
Continuous data were successfully acquired during the
March 25–April 16 period (period 1) and the April 22–May
5 period (period 2). The STDs are scattered across theregion of \6 cm/s, and have mean values of 1.8 cm/s forperiod 1 and 1.7 cm/s for period 2. These values are significantly smaller than the average current values of 3.8
and 4.4 cm/s during the two periods, as shown in Fig. 6.
The error bar for short-term temperature measurements
can also be obtained from the STD from the mean temperature, as calculated from the data for the six repeated177

periods of sound received. The STDs for temperature areshown in the time plots of Fig. 7a, b for T1 and T2,
respectively. They are scattered across the region of\0.05°C and have mean values of 0.007 and 0.006°C forT1 and T2, respectively. These values are small enough toperform the accurate measurement of temperature.
The range-averaged speed of sound can be estimatedfrom not only the reciprocal data (mean travel times) but
also the one-way travel-time data. When the one-way
method is used, the effect of current becomes a source oferror in speed of sound estimates. The first error (i.e., theeffect of current) is diminished when the mean travel time
is calculated using the reciprocal travel-time data. Thiserror bar was found to be 0.05°C for the maximum diurnal
tidal current, as shown for the typical case of Fig. 8. The
temperature was estimated by converting from the speedof sound, using the standard speed of sound formula(Mackenzie 1981). Here, two more error sources (salinityand depth) arise in the conversion. The second error (i.e.,
the effect of salinity) is caused by its deviation from thereference value. During the experimental period, the rangeaveraged salinity varied from 33.0 in March to 33.2 inMay. This salinity deviation causes a temperature shift of
0.07°C for a fixed speed of sound (1493 m/s) and a fixed
depth (10 m) for March. The third source of error comesfrom the range of depths at which the sound travels.
According to the range-independent ray simulation resultsin Fig. 3, the depth ranges for the traveling rays are 0–20 m
for March and 10–25 m for May. The difference between
the mean depths for these months (7.5 m) causes a temperature shift of 0.04°C for a fixed speed of sound(C = 1493 m/s) and a fixed salinity (S = 33.0) for March.
However, these second and third errors can be diminishedby updating the salinity and depth values prior to theconversion from the speed of sound into temperature.

4 Temporal variations in current and temperature
The range-averaged current was calculated from the traveltime difference data obtained during the successful acquisition of reciprocal data (Fig. 6). Significant semidiurnaland fortnightly variations were visible, although there werelarge amounts of missing data during March 16–24 andApril 17–22. The amplitude of the fortnightly variation wasover 30 cm/s, with slightly larger positive values. The
along-line mean currents calculated for the two 13.66-day
periods during March 25–April 7 (period 1) and April
23–May 6 (period 2) were 3.8 and 4.4 cm/s, respectively,
referring to the direction from T1 to T2. The STDs (errorbars) for current variations were 1.8 cm/s for period 1 and1.7 cm/s period 2, and are indicated by thick vertical bars
in the figure.

123

178

Y. Adityawarman et al.

Fig. 5 STD versus time forcurrent variations, calculated
every 15 min from the data forthe six repeated periods ofsound received. The thickhorizontal lines show the mean
STDs for period 1 and period 2

Fig. 6 Time plot of the rangeaveraged current for the wholeobservation period, calculated
from travel-time differences.
The thick horizontal andvertical bars show the mean and
its STD calculated for periods 1
and 2, respectively

The range-averaged temperature was calculated fromthe mean travel-time data or one-way travel-time data.
Three kinds of temperatures (two one-way data sets andone reciprocal data set) were obtained during the successfulacquisition of the reciprocal data (Fig. 8). The tidal gauge
data at Matsuyama and the current data from the reciprocaltransmission for the same period are also shown in theupper and middle panels of the figure, respectively. The
effect of current to temperature variation is around 0.05°C.
A prominent semidiurnal variation in temperature is thestrongest feature of the figure. Upon comparing these123

figures, when there is no effect of current, the temperature(blue line) increases during the flood tide (eastward tidal
current), while this tendency reverses during the ebb tide(westward tidal current). During the flood tide with thealong-line current component directed from T1 to T2, thetemperature is underestimated for one-way data obtained at
T1 (green line) and overestimated for one-way dataobtained at T2 (red line). The effect of the tidal current on
the temperature reverses during the ebb tide when thealong-line current component is directed from T2 to T1. It
is worth noting that the eastward and westward tidal

Reciprocal sound transmission measurement of mean current and temperature variations

179

Fig. 7 Time plots of the STD in
the temperature variation at
a T1 and b T2, as calculated
every 15 min from the data forthe six repeated periods ofsound received. The thickhorizontal line shows the mean
STD for the whole observation
periodcurrents, which occurring in phase with the tides in
Aki-nada, correspond closely to the positive and negative
along-line currents in the reciprocal transmission experiment, respectively.
The length (Lt) of the tidal excursion for the semidiurnal tide during the spring tide can be expressed by
Lt ¼

VM2 TM2;pð5Þ

where VM2 is the amplitude of the M2 tidal current in theeast–west direction and TM2 is the period of the M2 tide.
Upon substituting VM2 = 0.3 m/s for the spring tide andTM2 = 12.42 h into Eq. 5, Lt drops to about 4.27 km. The
peak-to-peak range of temperature variation of 0.17°C
provides an east–west temperature gradient of 0.17°C/
4.27 km (0.40°C/10 km). The eastward decrease in the sea
surface temperature is usually visible in overviews fromsatellite SST images of the Seto Inland Sea, although theaccuracy of the satellite measurements is insufficient to beable to quantify the rate of eastward decrease intemperature.
The temperature data obtained at T1 and T2 at 15-minintervals are shown in the time plots of Fig. 9a, where
linear interpolation has been performed to account for

123

180

Y. Adityawarman et al.

Fig. 8 Time plots of the range-averaged temperature during March
28–30, when the reciprocal data were acquired. The temperature
variations in two one-way data sets (line with circles for T1 and linewith dots for T2) and one reciprocal data set (solid line) are shown inthe bottom figure. The tidal gauge data at Matsuyama and the current
data from the reciprocal transmission for the same period are also
shown in the upper and middle panels of the figure, respectively

missing data. The CTD temperatures acquired in Marchand May are plotted as solid circles in the figure, and theseshow good agreement with the sound transmission temperature. The difference between the T1 data (with lessmissing data) and the CTD data is 0.07°C for the Marchdata and 0.05°C for the May data. The seasonal warming at
a timescale of [30 days was subtracted from the 15-mininterval data, and the resulting high-pass-filtered data areshown in Fig. 9b. Here, a weak 10–20 day fluctuation in
the range of ±0.2°C can be seen together with the fluctuations due to semidiurnal and diurnal tides.
The specific timescales for the temperature variation
were then evaluated quantitatively by power spectral density analysis, and the results are shown in Fig. 10, together
with the 95% confidence interval. Besides the features
corresponding to semidiurnal and diurnal tides, significant
spectral peaks are visible at 7.0 and 21.1 days.

temperature variations in the central part of the Seto Inland
Sea (Aki-nada) were successfully measured during March–
May 2010 using a 30 km range reciprocal sound transmission experiment between Honshu and Shikoku. The
short-term accuracies of the current and temperature
measurements were examined by gauging the STD for thedata received for six repeated periods of sound lasting
18.43 s. The accuracy of the temperature measurements
were also validated by comparison with the CTD dataacquired during the sound transmission experiment.
The range-averaged mean currents calculated along thesound transmission line were 3.8 and 4.4 cm/s for the two
13.66-day periods (periods 1 and 2), respectively, and thesevalues were greater than the STDs (error bars) of
1.7–1.8 cm/s. It can thus be concluded that the positive
current (directed toward T2 from T1) reaches a significant
level. This positive along-line current points to an eastward
current rather than a westward current through Aki-nada.
On the other hand, the short-term accuracy of temperature measurements was estimated to be less than 0.01°C,
using the reciprocal transmission data. The effects ofsalinity and depth as other error sources were greatly
reduced when the salinity and depth were updated using
newly acquired CTD data at appropriate intervals during

5 Summary and discussion
Current and temperature measurements achieved using
long-range reciprocal sound transmission were obtained forthe first time for the Seto Inland Sea. The mean current and123

Reciprocal sound transmission measurement of mean current and temperature variations

181

Fig. 10 Power spectral density diagram for the interpolated temperature data, processed through a 30-day high-pass filter. The periods
for the significant spectral peaks are indicated with downward arrows
above the corresponding peaks. The dashed lines above and below the
spectral line shows 95% confidence intervals

the Bungo Channel and Aki-nada (Akiyama and Saitoh1993).
This study suggests that reciprocal sound transmission isa powerful technique for the long-term and accurate measurement of mean current and temperature variations incoastal and inland seas.

Fig. 9 a Time plot of the range-averaged temperatures for T1 (solidline) and T2 (dashed line), where linear interpolation has been
performed to account for missing data. The dots show the CTD dataobtained on March 15 and May 12. High-pass-filtered data obtainedby subtracting the 30-day running mean data from the original dataobtained at 15-min intervals are shown in b

the sound transmission experiment. This high accuracy of
the temperature measurements was also supported by thefinding that the temperature differences between the soundtransmission and the CTD data were as small as 0.07°C for
the March data and 0.05°C for the May data.
Temperature variations with 7.0- and 21.1-day periods
were found in the power spectral density diagram with a
95% confidence interval. These peaks in temperature havenever been reported previously for Aki-nada, which may bepartly due to their weak energies. It should be noted that
relatively weak temperature variations in the range of±0.2°C can only be correctly measured using the longrange sound transmission technique. The typical periods ofthe temperature variations mentioned above are close to theperiods of the Kyucho, implying a connectivity between

Acknowledgments Dr. Yoshio Takasugi is greatly acknowledged
for his kind support in relation to the instrumentation. This study was
supported by grants from the Japan Society for the Promotionof Science and the Dean of the Graduate School of Engineering,
Hiroshima University.

References
Akiyama H, Saitoh S (1993) The Kyucho in Sukumo Bay induced by
Kuroshio warm filament intrusion. J Oceanogr 49:667–682
Arai M (2004) Development of a semi-spectral coastal ocean modeland its application to the Neko Seto Sea in the Seto Inland Sea.
J Oceanogr 60:597–611
Chang P, Guo X, Takeoka H (2009) A numerical study of the seasonal
circulation in the Seto Inland Sea, Japan. J Oceanogr 65:721–736
Guo X, Futamura A, Takeoka H (2004) Residual currents in a semienclosed bay of the Seto Inland Sea, Japan. J Geophys Res
109:C12008. doi:10.1029/2003JC002203
Kaneko A, Yamaguchi K, Yamamoto T et al (2005) A coastal
acoustic tomography experiment in Tokyo Bay. Acta Oceanologica Sinica 24(1):86–94
Komai K, Hibino T, Ohkama T (2008a) Influence of the Kuroshio
Meander/straight on flow in the Seto Inland Sea. Proc Jpn Soc
Civ Eng 63(3):165–179
Komai K, Kim K, Ikehara K et al (2008b) Annual variation ofthroughflow in the Seto Inland Sea. Coast Eng Jpn 55:406–410(in Japanese with English abstract)

123

182
Kunii M, Fujiwara T (2006) Through flow of the Seto Inland Sea
driven by the sea level difference between western and easternboundaries. Umi-to-Sora 81(2):13–21 (in Japanese with English
abstract)
Lin J, Kaneko A, Gohda N et al (2005) Accurate imaging andprediction of Kanmon Strait tidal current structures by thecoastal acoustic tomography data. Geophys Res Letts
32:L14607. doi:10.1029/2005GL022914
Mackenzie KV (1981) Nine-term equation for sound speed in theocean. J Acoust Soc Am 70:807–812
Munk W, Worcestor PF, Wunsch C (1995) Ocean acoustic tomography. Cambridge University Press, Cambridge
Nguyen H, Kaneko A, Lin J et al (2009) Acoustic measurement ofmulti sub-tidal internal modes generated in Hiroshima Bay,
Japan. IEEE J Ocean Eng 34(2):103–112
Park JH, Kaneko A (2000) Assimilation of coastal acoustic
tomography data into a barotropic ocean model. Geophys Res
Letts 27(20):3373–3376

123

Y. Adityawarman et al.
Takasugi Y, Hoshika A, Noguchi H et al (1994a) The role of tidal
vortices in the material transport around straits. J Oceanogr
50:65–80
Takasugi Y, Fujiwara T, Sugimoto T (1994b) Formation of sandbanks due to tidal vortices around straits. J Oceanogr 50:81–98
Takeoka H, Akiyama H, Kikuchi T (1993) The Kyucho in the Bungo
Channel, Japan–Periodic intrusion of oceanic warm water.
J Oceanogr 49:369–382
Takeuchi J, Honda N, Morikawa Y et al (1998) Bifurcation Current
along the southwest coast of the Kii Peninsula. J Oceanogr
54:45–52
Yamaguchi K, Lin J, Kaneko A et al (2005) A continuous mapping of
tidal current structures in the Kanmon Strait. J Oceanogr
61:283–294
Yanagi T, Higuchi H (1979) Constant flows in the Seto Inland Sea.
Bull Coast Oceanogr 16:123–127