Journal of Oceanography
Vol. 49, pp. 491 to 501. 1993
Tidal Currents Influenced by Topographic Eddies in Uchiumi Bay
HIDETAKA TAKEOKA and HAJIME MURAO
Department of Civil and Ocean Engineering, Faculty of Engineering, Ehime University,
Bunkyo 3, Matsuyama 790, Japan
(Received 27 November 1992; in revised form 9 March 1993; accepted 16 March 1993)
In the current record taken in Uchiumi Bay during summer in 1990, we find
anomalous features that the tidal currents during spring and neap tides are quite
different in both speed and direction, and the transition between them takes place
suddenly. Relation between the variation of tidal current and the observed internal
tide is examined, but they are poorly correlated. By examining the direction and
phase of the current, it is inferred that these anomalous features are caused by the
influence of topographic eddies formed periodically in Uchiumi Bay.
1. Introduction
Uchiumi Bay is one of the small bays on the eastern coast of the Bungo Channel, Japan (Fig.
1). Recently, a phenomenon called Kyucho, which is a periodic intrusion of oceanic warm water
into the Bungo Channel, was reported by Takeoka and Yoshimura (1988) and Takeoka et al. (1993).
To monitor this phenomenon, long term observations of water temperature and current speed
have been carried out at many stations along the coast including the station in Uchiumi Bay. In
the current record, taken in Uchiumi Bay, we found some anomalous features that the tidal
currents during spring tides are rather strong compared to those during neap tides with the ratio
much larger than the expected one in the surrounding sea areas. Moreover, transition in the
current speeds between spring and neap tidal phases takes place fairly suddenly. In the present
paper, we examine the cause of these anomalous features of the observed tidal currents in
Uchiumi Bay.
2. Observations
The observations were carried out at Stn. M in Fig. 1 during 13 July to 28 September 1990.
Water depth at Stn. M is about 70 m. A current meter (AANDERAA RCM4) and a thermistor
chain of 40 m long (AANDERAA TR-2) were moored to a cage of yellowtail culture at Stn. M.
The current meter was placed at the depth of 10 m below the sea surface. Intervals of data
sampling were 15 minutes for the current meter and 30 minutes for the thermistor chain.
Unfortunately, current data after 6 September were not obtained due to biofouling of the current
meter.
3. Results
Figure 2 shows the north and east components of the current velocity observed at Stn. M.
Tidal currents are found to be dominant in this figure, where semi-diurnal and spring-neap
changes of the current can be clearly seen in both components. The time variations are, however,
somewhat irregular, suggesting that there are some factors distorting the tidal currents. During
neap tides, northward current speeds are quite low, whereas they become rather high in spring
tides. Moreover, the transition between them takes place fairly abruptly. These features can be
Fig. 1. Map of the Bungo Channel (left) and Uchiumi Bay (right).
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Fig. 2. Raw data of northward and eastward current speeds at the depth of 10 m at Stn. M during 13 July
to 6 September 1990.
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Fig. 3. Predicted tidal currents at Hayasui Straits (positive northward) during 15 to 29 July 1990.
seen more or less also in the eastward component of the current velocity. To see whether or not
these are the common features in the surrounding sea areas, we examine the predicted tidal
currents in Hayasui Straits during 15 to 29 July shown in Fig. 3. We may regard that the pattern
of the variation in Fig. 3 represents that of the tidal currents in the Bungo Channel off Uchiumi
Bay. The difference in the current speeds between neap and spring tidal phases is much smaller
in Fig. 3 than in Fig. 2; in Fig. 3 the difference is about twice whereas it is three to five times in
Fig. 2. A sudden transition in current speeds between spring and neap tidal phases cannot be
found in Fig. 3. Hence, the features seen in the observed currents at Stn. M seems quite special.
Figure 4 shows the water temperature at the depths of 2, 10, 20, 30 and 40 m at Stn. M; after
10 August, only the data at 40 m depth are shown. Diurnal, semi-diurnal and long term variations
are recognized in these records. Presumably, the long term variations are caused by the Kyucho.
The diurnal and semi-diurnal variations are larger at the depths of 30 to 40 m, namely mid depths
(note that the water depth is 70 m at Stn. M). The range of the variation sometimes exceeds 5°C.
It can be hardly expected that there is such a large horizontal difference of water temperature
within the scale of the tidal excursion in this area. Therefore, the diurnal and semi-diurnal
variations of the water temperature are inferred to be caused by internal tides.
4. Analyses of the Tidal Currents
In this section, some analyses of the observed tidal currents are made to figure out their
features more clearly.
Figure 5 shows current ellipses of ten tidal constituents drawn on the basis of the harmonic
constants calculated by using the least square method. Location of the arrow attached to each
ellipse denotes the time at which the tide generating force of each constituent is at a maximum.
In Fig. 5, anomalous features can be seen as follows. The major axes of the current ellipses
are mostly directed parallel to the coast (north to south), and are not in the direction of the bay
axis (east to west). Moreover, the phase lag of the M2 constituent is quite different from that in
the Bungo Channel off Uchiumi Bay. The phase lag of the M2 constituent at Stn. M is 111°,
whereas that in the Bungo Channel is about 200° (Yanagi and Higuchi, 1981), namely, the phase
of M2 constituent at Stn. M leads that in the Bungo Channel by about a quarter tidal period.
To see the difference between the tidal currents during spring and neap tidal phases more
clearly, semi-diurnal component of the current on each day is obtained by decomposing each
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Fig. 4. Raw data of the water temperature at the shown depths obtained with the thermistor chain during
13 July to 28 September 1990 (only the data at 40 m depth are shown after 10 August).
24 h 50 m data into Fourier series. Figure 6 shows the current ellipse of the semi-diurnal
component on each day. Location of an arrow on each ellipse denotes the time at which the M2
tide generating force is at a maximum.
The ellipses in Fig. 6 show quite unusual features. The major axes of the current ellipses
Fig. 6. Current ellipses of the semi-diurnal component on each day.
Fig. 5. Current ellipses for ten tidal constituents obtained through the least square harmonic analysis.
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during spring tides are mostly directed north-northwest to south-southeast, whereas those in neap
tides are directed approximately east to west. Thus, the current structure in the bay seems to be
essentially different between spring and neap tidal phases. Transitions between these ellipses
occur suddenly, as can be seen in the current record in Fig. 2.
5. Discussion
5.1 Influence of internal tides on the tidal currents
The tidal currents accompanied by internal tides can become much stronger than that
accompanied by surface tides, because the amplitude of the elevation of the former can become
much larger than that of the latter. Therefore, it may be possible that the internal tides were
intensified in spring tides due to some unknown reasons, causing the anomalous features of the
observed tidal currents at Stn. M. To examine this possibility, variations of water temperature are
compared with those of the currents by taking dynamic spectra of both records. For the current,
the spectrum is calculated applying the FFT method to a set of 512 data in the first 5.3 days of
the observation. The spectra are obtained for the following series of the 5.3 days data each of
which is shifted by 2.7 days. For the temperature, the spectra are calculated for the series of 256
data. The results are shown in Fig. 7. The top panel shows the spectrum of the northward current
speed, whereas the bottom panel shows that of the water temperature at 40 m depth. Areas of
higher spectral density are hatched. In the current spectrum, higher values appear in the semi- and
quarter-diurnal bands, while they appear in the diurnal, semi-diurnal and quarter-diurnal bands
in the temperature spectrum. Coincidence of the peaks of the current and temperature spectra is
not generally good. The coincidence is quite bad, in particular, for the diurnal and quarter diurnal
bands. To check the coincidence in the semi-diurnal band, the days when the peaks appear in the
current spectrum in the spring tides are denoted by thick broken lines in Fig. 7. The coincidence
in the semi-diurnal band may be somewhat better than in the other bands, still it is fairly poor.
Accordingly, it is unlikely that the strong tidal currents during spring tides observed at Stn. M
are enhanced by the internal tides.
5.2 Influence of topographic eddies on the tidal currents
Figure 8 shows current vectors at 20 m depth in and around Uchiumi Bay observed with
ADCP during ebb on August 29, 1992, at spring tide. This observation was carried out by Ehime
Prefectural Fisheries Experimental Station on board the R/V Yoshu on the way of the survey of
the Kyucho in the Bungo Channel. From Fig. 8, the currents near Stn. M are directed northward
contrary to the southward ebb currents offshore, and it is suggested that an eddy is formed in the
inner part of Uchiumi Bay. The anomalous features of the tidal currents in Uchiumi Bay may be
attributed to such a topographic eddy.
Processes of formation, development and collapse of the topographic eddies in coastal
cavities facing to the strong tidal current are described by Takeoka and Higuchi (1982), who
examined the water exchange process in Matsuyama Harbor by carrying out the hydraulic model
experiments. Figure 9 schematically shows the transition of the topographic eddy during one
tidal period, drawn from the results of their hydraulic model experiments with a simplified
geometry. Just after the slack in the channel, a small eddy is formed in the upstream corner of the
bay mouth (Fig. 9(a)), and this eddy develops, occupying the bay (Figs. 9(b) and (c)). This eddy
further develops until the current in the channel comes to the following slack, when the closed
structure of the eddy is broken at the mouth and the current speed of the eddy in the inner part
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Fig. 7. Dynamic spectra of the northward current (top) and the water temperature at 40 m depth (bottom).
Tidal Currents Influenced by Topographic Eddies in Uchiumi Bay
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Fig. 8. Current vectors at 20 m depth in and around Uchiumi Bay observed with ADCP of R/V Yoshu of
Ehime Prefectural Fisheries Experimental Station during 9 h 40 m to 13 h 14 m on August 28, 1992.
reaches nearly maximum (Fig. 9(d)). Thereafter, follows the structure in reverse to Figs. 9(a) to
(d). In the middle of the inner half of the bay in Fig. 9, the current direction is almost parallel to
that in the channel, and the phase leads that in the channel by about a quarter tidal period.
Therefore, if we assume that the topographic eddies similar to those shown in Fig. 9 are formed
in Uchiumi Bay, the direction and the phase of the M2 component in Fig. 5 can be explained well.
Two possible explanations can be made for the difference between the tidal currents during
spring and neap tidal phases. The eddy is initially formed at the bay mouth by flow separation
as shown in Fig. 9(a). Therefore, during the neap tides when the current speed is low, the flow
separation may not occur and the eddy may not be formed. Since the flow separation occurs at
the current speed higher than a critical value, the abrupt change between the currents during neap
and spring tides can be explained by this assumption. Another explanation is that, even if eddy
is formed during neap tides, it may not develop well to affect the tidal currents at the observation
station. In both explanations, the tidal currents at the observation station during neap tides are free
from the eddies. In the absence of such eddies, the currents in the bay are rather stagnant (Takeoka
and Higuchi, 1982). Hence the difference in the observed current speeds between neap and spring
tides can be explained by considering the influence of the topographic eddies.
Thus, the anomalous features of the observed tidal currents are explained well by assuming
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Fig. 9. Schematic diagram showing the formation, development and collapse of the topographic eddies
formed in a coastal cavity in one tidal period (redrawn from Takeoka and Higuchi (1982)).
that the topographic eddies are formed periodically in Uchiumi Bay, and hence we conclude that
such eddies are formed in Uchiumi Bay affecting the tidal currents.
It should be noted that the eddies in Uchiumi Bay can be formed under relatively low tidal
currents. In the case of Matsuyama Harbor studied by Takeoka and Higuchi (1982), tidal
excursion of M2 constituent off the bay is more than ten times the width of the bay mouth. In the
case of Uchiumi Bay, in contrast, the tidal excursion off the bay is at most nearly the same as the
width of the bay mouth. Under such circumstances, it seems difficult for the eddy to develop to
occupy the bay. However, eddies in the coral reef lagoon reported by Wolanski and King (1990)
are formed under the circumstance similar to that in Uchiumi Bay. This supports the formation
of topographic eddy in Uchiumi Bay, as well as the current structure shown in Fig. 8 does. The
fact that the eddies are formed under such relatively weak tidal currents is quite important. It
suggests that similar topographic eddies can be also formed in the bays along the coasts of the
Bungo Channel. The eddies cause vertical circulation, converging in the lower layer and
diverging in the upper layer, and the vertical circulation may effectively transport nutrients from
the bottom to the upper layer raising the biological productivity. Such an important effect of the
eddies may one of the causes of the high productivity in this region. Investigation of the eddies
seems to be important also from such a viewpoint.
Acknowledgements
The authors wish to thank Dr. T. Yanagi of Ehime University for helpful discussions. They
also thank the members of Minami-Uchiumi Fisherman’s Union for their kind cooperation in the
field observation, and Ehime Prefectural Fisheries Experimental Station for providing the ADCP
data.
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501
References
Takeoka, H. and H. Higuchi (1982): Water exchange due to local eddies in harbors and embayments. Bull. Coast.
Oceanogr., 19, 175–182 (in Japanese).
Takeoka, H. and T. Yoshimura (1988): The Kyucho in Uwajima Bay. J. Oceanogr. Soc. Japan, 44, 6–16.
Takeoka, H., H. Akiyama and T. Kikuchi (1993): The Kyucho in the Bungo Channel, Japan—Periodic intrusion of
oceanic warm water. J. Oceanogr., 49, 369–382.
Wolanski, E. and B. King (1990): Flushing of Bowden Reef Lagoon, Great Barrier Reef. Estuarine Coast. Shelf Sci.,
31, 789–804.
Yanagi, T. and H. Higuchi (1981): Tide and tidal current in the Seto Inland Sea. Proc. 28th Conf. Coast. Eng., Japan
Society of Civil Engineers, 555–558 (in Japanese).