The Kyucho in Sukumo Bay Induced by Kuroshio Warm Filament

Journal of Oceanography
Vol. 49, pp. 667 to 682. 1993
The Kyucho in Sukumo Bay Induced by
Kuroshio Warm Filament Intrusion
HIDEKI AKIYAMA1 and SEI-ICHI SAITOH2*
1Department
of Civil and Ocean Engineering, Ehime University, Bunkyo 3, Matsuyama 790, Japan
2Meteorological Information Center, Japan Weather Association,
4-5, Koji-machi, Chiyoda-ku, Tokyo 102, Japan
(Received 24 November 1992; in revised form 4 March 1993; accepted 7 June 1993)
Surface temperature data obtained in and out of the bay all year round from
March 1990 through February 1991, except from July through October 1990 were
analyzed to investigate seasonal variability of the Kyucho in Sukumo Bay,
southwest of Shikoku, Japan. The Kyucho periodically occurs in the bay during
both the warming period of March through June and the cooling period of
November through February. The onset period of the Kyucho is 8–15 days during
the warming period and 4–14 days during the cooling period, giving an average of
about 10 and 8 days, respectively. The position of the Kuroshio axis offshore in the
south of Cape Ashizuri-misaki is a significant factor with the Kyucho in the bay.
Thermal infrared images taken by the NOAA-11 in the sea off east of Kyushu were
also analyzed during the two observation periods. It is clearly found that a warm
filament derived from the Kuroshio (KWF) advects northeast to Cape Ashizurimisaki along the Kuroshio, then encounters the southwest coast of Shikoku,
followed by inducing the Kyucho in the bay by the warm water intrusion. The
alongshelf dimension of the KWFs is approximately 50–100 km, and the crossshelf distance from the western edge of the KWFs to that of the body of the
Kuroshio is about 30–50 km. The KWF sometimes closely approaches to the east
coast of Kyushu. An onshore meander of the Kuroshio front around Cape Toimisaki might grow into a KWF in the sea off east of Kyushu.
1. Introduction
The Kuroshio is flowing eastward along the southwest coast of Japan from the Tokara Strait,
south of Kyushu, to the sea off south of Shikoku (Fig. 1(a)). Its northern boundary is called the
Kuroshio front with a sharp temperature gradient between the Kuroshio and coastal waters,
where water exchange among them is intense. As take a bird’s-eye view of the Kuroshio front
from space by a satellite, its shape is usually rich in variability with disturbances of the synoptic
space-time scale.
The study area is the sea off east of Kyushu, where it is bounded to the southwest by Cape
Toi-misaki and to the north by both the Bungo Channel and Cape Ashizuri-misaki. The Kuroshio
is also the seaward boundary of the study area. The distance from Cape Toi-misaki to Cape
Ashizuri-misaki is approximately 200 km. Water depth at the shelf break, where lies relatively
close to the coast within 20–30 km wide, is about 200 m. The bottom topography falls abruptly
*Present address: Faculty of Fisheries, Hokkaido University, 3-1-1 Minato, Hakodate 041, Japan.
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H. Akiyama and S. Saitoh
Fig. 1. The sea areas off east of Kyushu (a) and around Sukumo Bay (b). HB and TG denote the Hyuga
Basin and the Tanegashima Island, respectively. Dotted lines are depth contours in meter.
into deep North Pacific Ocean to the offshore. Water depths in excess of 1,000 m are found within
20–30 km off the shelf break. Cape Ashizuri-misaki is a prominent shoal that projects from the
southwest coast of Shikoku out to the shelf break.
In the study area, some branch currents derived from the Kuroshio, such as a warm filament,
have been known to often appear so far (e.g., Yokota, 1953). Sakamoto (1971) investigated both
the sea area, where a warm filament derived from the Kuroshio (hereafter called the Kuroshio
warm filament, KWF) frequently appeared, and its appearance period using horizontal temperature
distributions obtained with a time interval of several days around the southwest coast of Japan.
According to his results, KWFs frequently appeared in both the center of Hyuga-nada and the
south of the Bungo Channel with periods of 6–13 days, giving its average of 9.5 days. Then the
KWFs usually migrated to the east along the Kuroshio. Yousho (1971) also found that the KWFs
migrated to the east at speeds of 9–15 cm s–1 using the same data in Sakamoto (1971).
Powerful observations in the same area of this study were deployed by the cooperation of
the Japan Fisheries Agency, the Ooita, Miyazaki, and Kagoshima Prefectural Fisheries Experimental Stations from 1974 through 1977 (Japan Fisheries Agency et al., 1978). They found
some fragmentary structures of KWFs. However, the detailed structure of a KWF and its spacetime modification are not led to clarify yet.
On the other hand, turning our attention to the coast of the study area, the phenomenon with
sudden changes in temperature and current, which is called “Hayashio” in Japanese, has been also
known to occur around the southwest coast of Shikoku (Uda, 1984). Akiyama and Yanagi (1984)
and Akiyama et al. (1987) pointed out that the same phenomena frequently occurred in Sukumo
Bay located at the northeastern part of the study area using the temperature and current data
obtained by the Ministry of International Trade and Industry, Japan. Then Akiyama and Yanagi
(1984) also found that these phenomena occurred in the bay during winter were induced by
intruding a warm water mass derived from the Kuroshio into the bay. Furthermore, they noted
The Kyucho Induced by Kuroshio Warm Filament Intrusion
669
that the current flowing into the bay rapidly increased its speed when these phenomena occurred.
Such swift current phenomena with a sudden change in temperature is generally called the
“Kyucho” in Japanese on the south coast of Japan. The same phenomena have been observed at
Sagami Bay (Kimura, 1942; Uda, 1953; Matsuyama and Iwata, 1977), Uragami Bay (Tanaka et
al., 1992) and Tanabe Bay (Kunishi et al., 1968) of the Kii Peninsula, and Uwajima Bay (Takeoka
and Yoshimura, 1988) in the Bungo Channel.
Akiyama (1991) reported seasonal characteristics of the onset of the Kyucho in Sukumo Bay
using the sea surface temperature (SST) data obtained in and out of the bay. Then he suggested
that the onset of the Kyucho in the bay was closely related to the appearance of a KWF in the sea
off east of Kyushu. However, the cause and the occurrence process of the Kyucho in the bay were
not led to clarify yet quantitatively.
Satellite oceanography has been advanced. In particular, consecutive satellite images of the
SST, contrasting the meanders of and the eddies derived from the Kuroshio with the colder
coastal waters, provides the means to monitor length scales, drift and growth rates of, and
interactions among the synoptic scale disturbances in the Kuroshio front (e.g., Qiu et al., 1990).
Moreover, the combination of both field and satellite observation data improves the spatial and
temporal coverages of those disturbances significantly (e.g., Toda et al., 1988).
Spatial structures of the Kuroshio front and its temporal variations have not been well known
in the study area. Then little is clarified conceptually or quantitatively for the physical dynamics
in the Kuroshio front of the study area up to the present. Therefore, the data of field and satellite
observations should be collected to examine for the understanding of the synoptic scale
disturbances.
This paper presents the results of field and satellite observations of both the Kyucho in Sukumo
Bay, southwest of Shikoku (Fig. 1(b)), and a KWF in the sea off east of Kyushu. The purpose of
this study is to clarify the relation between those two phenomena.
2. Data
Water temperatures at Stations 1, 2, and 6 in Sukumo Bay and that at Stations 3, 4, and 5 out
of the bay are measured twice a day by means of a thermometer attached on the side of a
commercial ferryboat made by the Sukumo City Agency since April 1985. AT-8000 typed selfmemorized thermometer, made by Ehime University, is set up about 4 m below the sea surface
at the port of the Okinoshima Island (OK, Fig. 1(b)). Water temperature data at OK are observed
every 30 minutes since late June 1990.
NOAA-11 Advanced Very High Resolution Radiometer (AVHRR) data were collected
from March 1990 through February 1991. We chose a thermal infrared (IR) channel, 10.5–11.5
µm, for observing the SST. For the AVHRR data, in-flight calibration was first carried out and
the calibrated data then processed to remove geometric distortions using a Mercator projection
method. We made no correction for atmospheric attenuation because it was possible to identify
different water masses separated by high thermal gradient in the images. The space resolution is
nearly 1.2 km in latitude by 1.2 km in longitude at nadir of the satellite.
The data analysis is extended all year round from March 1990 through February 1991 to
investigate the seasonal variability of the Kyucho in the bay, except from July through October
1990 when a horizontal temperature gradient is not very sharp. Therefore, the period of the data
analysis is divided into two. One is the warming period (March through June) and the other is the
cooling period (November through February).
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3. Field Observations
3.1 Temperature variations at the mouth of Sukumo Bay
Figure 2 shows time variations in temperature observed at Station 4 during the warming
period and at OK during the cooling period, respectively. Each station located at the mouth of
the bay is marked as a closed circle in Fig. 1(b). Upward and downward arrows are explained later
in the text.
One can easily see that sudden increase in temperature with a range of 2–4°C by 1–2 days
frequently presents in each period. The temperatures past the peak gradually decrease for 3–4
days. The temperature at OK (Fig. 2(b)) for the duration from early January through early
February 1991 falls to a lower level than that for the other duration with no temperature jump at
the stations in the bay.
3.2 The Kyucho in Sukumo Bay
The sudden increase in temperature appears not only at the bay mouth but also at the other
Fig. 2. Time series of temperatures at the mouth of Sukumo Bay during the warming (a) and cooling (b)
periods. Downward arrows show the date with the Kyucho in the bay, while upward ones when a
satellite imagery is taken just before and/or when the Kyucho occurred in the bay. All the numbers
attached on each arrow are the date.
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671
stations in the bay with a little phase lag of half to one day. Temperature jump occurs as the water
column is abruptly warmed, which is indicative of advection of a warm mass past the stations.
Therefore, the temperature jumps in and out of the bay with a little phase lag are a good indication
of the onset of the Kyucho in the bay (Akiyama, 1991).
For instance, the temperature variations at OK, Stations 6 in the mid-bay and 1 in the bay
head from 20 to 28 February 1991 are shown in Fig. 3(b). The closed circles connected with a
broken line in Fig. 3(a) indicate a location of the stations. In the first place, the sudden increase
in temperature occurs at OK at the noon on 23 February, then occurs at Station 6 at the afternoon
on 24 February, followed by at Station 1 at the morning on 25 February.
According to Akiyama (1991), we were attached a downward arrow over the temperature
jumps with the date when the Kyucho occurred in the bay as shown in Fig. 2. One can find in Fig.
2 that the Kyucho frequently occurs in the bay during the two observation periods. The Kyucho
occurs 9 times during the warming period and 10 times during the cooling period. The onset
period is 8–15 days in the former and 4–14 days in the latter, giving an average of about 10 and
8 days, respectively. The dotted downward arrows mean that the Kyucho may have occurred in
the bay, though no temperature data existed.
As is usual with the onset of the Kyucho, the temperatures in and out of the bay at first
increase suddenly like a wall, followed by decreasing past the peak gradually. These are due to
a rapid intrusion of the Kuroshio warm water into the bay and to a mixture of the Kuroshio and
the bay waters, respectively (Akiyama and Yanagi, 1984). Then such a change pattern in
temperature appears similar form to a saw.
In cases with the Kyucho in the bay on 23 December 1990 and 9 February 1991 during the
cooling period, however, the temperature suddenly rises and falls for three times. The temperature jump in these cases appears in and out of the bay at the first stage as similar as that in
other cases with the Kyucho in the bay, while that only appears out of the bay at the second and
third stages. Therefore, it is not clear whether the Kyucho occurred in the bay or at the latter two
stages.
Fig. 3. Sudden increase in temperature at three stations (b) in and out of Sukumo Bay (a).
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4. Satellite Observations
4.1 Kuroshio warm filament in the sea off east of Kyushu
We examine here to a warm water derived from the Kuroshio, which intrudes as the Kyucho
into Sukumo Bay, using satellite IR imagery. The sea area detected with the satellite IR imagery
is within a solid square of 31° to 33°30′ N and 131° to 133°30′ E as shown in Fig. 4.
Figure 5 shows satellite IR images around the study area just before and/or when the Kyucho
occurred in the bay. The date when each imagery was taken was shown in Fig. 2 with an upward
arrow. Predominant feature seen in the Kuroshio front of these images is characterized by the
presence of a filament with a tongue-like structure. The disturbances with such a structure, which
are called the Kuroshio warm filament (KWF) in this study, grow and break backward onto the
shelf as the onshore portion of a small meander in the Kuroshio front folds back. The KWFs wrap
around the cold core of shelf water. The KWF sometimes closely approaches to the east coast of
Kyushu as seen in the satellite imagery on 21 December 1990.
Length scale of the KWFs is determined by direct measurements from the satellite images.
The alongshelf dimension of the KWFs is approximately 50–100 km, and the cross-shelf distance
from the western edge of the KWFs to that of the body of the Kuroshio is about 30–50 km. The
along- and cross-shelf scales of a KWF are roughly twice as long during the warming period as
during the cooling one.
Fig. 4. The satellite observation area in a square. Sea surface temperature data are observed by the NOAA11 using the channel 4 of the AVHRR.
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673
Fig. 5. Sea surface temperature (SST) patterns when Kuroshio warm filaments appeared in the sea off east
of Kyushu during the warming (a) and cooling (b) periods. Black and shading areas show the land and
a cloud, respectively. The SST patterns are depicted by the temperature contour with an interval of
0.2°C.
4.2 The cases with the Kyucho in Sukumo Bay
All the satellite IR images obtained during the two observation periods have indicated the
presence of a KWF in the study area as mentioned in Subsection 4.1. These images were taken
just before and/or when the Kyucho occurred in the bay (see Fig. 2). This suggests that a KWF
appeared in the study area is closely linked to the Kyucho in the bay.
Figure 6 shows the satellite IR images when the Kyucho events occurred in the bay during
the two observation periods. One can see in each imagery that a KWF encounters the southwest
coast of Shikoku and warm water of the KWF may intrude into the bay.
It is concluded here that a KWF enlarged in the sea off east of Kyushu migrates northeast
to Cape Ashizuri-misaki along the Kuroshio, then encounters the southwest coast of Shikoku,
followed by inducing the Kyucho in the bay by the warm water intrusion.
4.3 The case without the Kyucho in Sukumo Bay
The Kyucho has usually occurred in the bay during the two observation periods, while that
has not occurred for the duration from early January through early February 1991 (see Fig. 2(b))
based on no temperature jump at the stations in the bay. We examine here why the Kyucho did
not occur for that duration.
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Fig. 6. As in Fig. 5 but when the KWFs encountered the southwest coast of Shikoku. The SST pattern on
8 February 1991 is complicated by the coupling with two KWFs.
Figure 7(a) shows the satellite IR imagery on 18 January 1991 in the case without the Kyucho
in the bay. The distance (dotted shading area) offshore in the south of Cape Ashizuri-misaki to
the Kuroshio axis, which was obtained from the prompt oceanographic observation report by the
Japan Maritime Safety Agency, is also shown in Fig. 7(b).
The Kuroshio axis generally lies at distances of 30–50 km in the offing of south of Cape
Ashizuri-misaki. That is moving onshore and offshore around its mean location with amplitudes
of 10–20 km. However, there is no big seasonal variation of its location.
On the contrary, the Kuroshio axis temporarily moves offshore far from Cape Ashizurimisaki at distances of 120–170 km in January 1991. The satellite imagery in Fig. 7(a) presents
that the KWF (broken line) is located off Cape Ashizuri-misaki, whereas it advects to the east
along the Kuroshio without encountering the southwest coast of Shikoku.
This concludes that the Kyucho in the bay does not occur when the Kuroshio axis is far away
from Cape Ashizuri-misaki. Therefore, the position of the Kuroshio axis offshore in the south of
Cape Ashizuri-misaki is a significant factor whether the Kyucho occurs in the bay or not.
4.4 Appearance and development of a Kuroshio warm filament
Time series of satellite IR images have permitted studies of appearance and development of
a KWF as shown in Fig. 8. Each imagery in Fig. 8(a) presents that the Kuroshio front closely
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675
Fig. 7. Sea surface temperature pattern in the case without the Kyucho in Sukumo Bay (a). The distance
offshore in the south of Cape Ashizuri-misaki to the Kuroshio axis (b, dotted shading).
approaches to the east coast of Kyushu and meanders onshore around Cape Toi-misaki, where
is indicated by an arrow. The Kyucho events occur in Sukumo Bay several days after these images
were taken (see Fig. 2). This may suggest that an onshore meander of the Kuroshio front around
Cape Toi-misaki grows into a KWF.
On the other hand, the mean axis of the Kuroshio broadly follows the shelf break from the
Tokara Strait to Cape Toi-misaki, where it veers offshore owing to the influence of the Hyuga
Basin shaped with a cusp, where located between Kyushu and Shikoku (HB, Fig. 1(a)). It is
conceivable that the Kuroshio front over the basin becomes unstable and leads to the entrainment
of shelf water into the Kuroshio water. The similar feature has been taken in the Gulf Stream front
off Cape Canaveral, Florida on the southeastern U.S. continental shelf by Zantopp et al. (1987).
Most of the KWFs formed in the Kuroshio front advect northeast to Cape Ashizuri-misaki as
shown in Fig. 8(b).
As mentioned above, it is to be assumed that rapid enlargement of a KWF is brought about
by coupling of a large horizontal shear across the Kuroshio front with bottom topography,
suggesting an existence of the instabilities in the front region (e.g., Orlanski, 1969; Luther and
Bane, 1985).
By the way, with regard to the formation process of a KWF, recently Awaji et al. (1991)
investigated numerically the barotropic response of the shelf and coastal circulations south of
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H. Akiyama and S. Saitoh
Fig. 8. Appearance (a) and development (b) of a KWF. Arrow in (a) shows an onshore meander of the
Kuroshio front. Broken line in (b) indicates the deformation of western boundary of a KWF
accompanied with its development process.
Japan to short-term variations in the Kuroshio with an inflow-outflow model. They found that
an eddy appeared near the south of the Tanegashima Island (TG, Fig. 1(a)) developed into a large
eddy-like circulation. Then they examined the movement of the Kuroshio water accompanied by
this circulation with tracking numerous labeled particles numerically. The particle motion
clearly showed that significant intrusion of the Kuroshio water onto the shelf/slope region was
driven by this circulation. It is interesting that this visual pattern of the Kuroshio water intrusion
is similar to that of a KWF. More informations are needed to verify their model validation for the
generation mechanism of a KWF in the study area.
Furthermore, Toda (1993) found that a small meander of the surface front was induced by
the Kuroshio frontal eddy at the downstream side of Cape Ashizuri-misaki when the Kuroshio
flowed very close to the coast. He suggested that the topography of the cape triggers formation
of the frontal eddy. The small meander of the surface front, which looks like a KWF, makes a pair
with the frontal eddy. Such a prominent shoal as a cape that projects out to the shelf break may
play an important role in the generation process of the eddy/filament pair.
5. Vertical Structure of a Kuroshio Warm Filament
The ship surveys in the study area have been frequently equipped with both mechanical BT
by the Maritime Self-Defense Force (MSDF), Japan Defense Agency, and conventional CTD by
the Ooita Prefectural Fisheries Experimental Station (OPFES), respectively. They were fortu-
The Kyucho Induced by Kuroshio Warm Filament Intrusion
677
nate to observe some fragmentary structures of a KWF. The survey lines made by these
organizations are drawn as a solid line attached on the station number in each satellite IR imagery
of Figs. 9(a) and 10(a).
Figure 9 shows the satellite IR images (a) and the temperature cross-sections observed by
the MSDF (b). An onshore meander of the Kuroshio front around Cape Toi-misaki appears in the
imagery on 21 June 1990. Warm core of the Kuroshio migrates onshore, approaching to the east
coast of Kyushu, where is on the coastal side of the broken line. Isotherms in the Kuroshio frontal
zone slope upward and offshore by 50–100 m deep for the distance among the stations of about
20 km.
On the other hand, fully enlarged KWF (broken line) appears in the northern part of the study
area as seen in the imagery on 25 February 1991, even though clouds cover the wide (shading)
area. The KWF has already encountered the southwest coast of Shikoku. Cold shelf water
distributes between the KWF and the body of the Kuroshio on the southeast of Station 45. The
KWF is about 50 m deep around Station 45. A dome of uplifted isotherms is seen at depths of
30–100 m below the cold core of shelf water. This feature should indicate that colder shelf slope
water has upwelled between the KWF and the body of the Kuroshio. This is certainly consistent
with the results found by both Vukovich and Crissman (1980) and Bane et al. (1981) in the Gulf
Fig. 9. Horizontal (a) and vertical (b) structures of the KWFs.
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H. Akiyama and S. Saitoh
Fig. 10. As in Fig. 9 but for its thickness (b, dotted shading).
Stream front on the South Atlantic Bight (SAB).
The other cases associated with a KWF are shown in Fig. 10. The satellite IR images in Fig.
10(a) clearly take the enlarged KWFs which have already encountered the southwest coast of
Shikoku. The OPFES happened to observe the cross-section of the KWFs, each of which had a
depth of about 30 m in the north of this study area as shown in the dotted shading of Fig. 10(b).
6. Discussions
6.1 Unusual change of temperature at the mouth of Sukumo Bay
When the Kyucho occurred in the bay, the temperatures in and out of the bay at first increased
suddenly like a wall, followed by decreasing past the peak gradually as mentioned in Subsection
3.2. In the cases with the Kyucho on both 23 December 1990 and 9 February 1991 during the
cooling period, however, the temperature at the bay mouth experienced three times a sudden rise
and fall (see Fig. 2). We examine here why such the unusual change of temperature happened at
the bay mouth.
The satellite IR imagery on 21 December 1990 in Fig. 5(b) takes the characteristics that the
KWF closely approaches to the east coast of Kyushu while migrating northward along the coast,
then the coastal boundary of it becomes unstable due to the coastal friction, resulting in the
deformation of a few wave-like shape. Such characteristics are also seen in the imagery on 8
The Kyucho Induced by Kuroshio Warm Filament Intrusion
679
February 1991 in Fig. 6(b), but those are not very clear. The similar phenomena have been
reported in Kumano-nada Sea, east of the Kii Peninsula. Takeuchi (1989) found that a KWF
appeared off Cape Shiono-misaki, southern edge of the peninsula, approached to the east coast
of the peninsula, then migrated northward along the coast with the Kyucho in bays. This KWF
rapidly decreased its thickness with its northward migration. The cold coastal water became to
be confined between the KWF and the coast in the center and northern parts of Kumano-nada Sea
(see his Figs. 4, 6, and 7). Kimura and Sugimoto (1990) also found a KWF using satellite IR
images, which accompanied with the same characteristics as that found in Takeuchi (1989). The
images in their Figs. 5 and 6 clearly took the characteristic as the coastal boundary of the KWF
deformed to be three wave-like shape. The KWFs found in these studies are sure to approach to
and migrate along the coast. This suggests that the instability due to the coastal friction contribute
to the deformation of a KWF.
By the way, we found that there were two cases on the migrating path of a KWF in the study
area. One is that a KWF advects offshore to the northeast. The other is that a KWF approaches
to and migrates along the east coast of Kyushu. It is interesting that the Kyucho events with the
unusual change of temperature at the mouth of Sukumo Bay are sure to occur in the latter case.
This fact means that such the Kyucho events may be induced by the intrusion of the deformed
KWF passing near the east coast of Kyushu.
6.2 Three-dimensional structure of a Kuroshio warm filament
On the basis of the results mentioned in Sections 4 and 5, we make a bold try here to present
a three-dimensional structure of a KWF. Figure 11 gives a schematic diagram of a KWF in the
study area with reference to the conceptual model of a frontal eddy with a warm filament
proposed by Lee et al. (1981) in the SAB. We have taken both the shape of fully enlarged KWF
and the topography of the Hyuga Basin into careful consideration when drew this diagram. Solid
lines and arrows denote an isotherm and inferred flow direction, respectively.
An onshore meander of the Kuroshio front (A) is formed by an onshore migration of warm
Fig. 11. Schematic diagram of the three-dimensional structure of a KWF appearing in the sea off east of
Kyushu. Characters A–E see in the text.
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H. Akiyama and S. Saitoh
core of the Kuroshio. The KWF (B) pulls out of the Kuroshio water, extending southwestward
like a river from northern edge of the onshore meander, then wraps cyclonically around the cold
core of shelf water (C). This is probably due to the cyclonic circulation. This cyclonic circulation
appears to be in nearly geostrophic equilibrium with the uplifted density surfaces (herein
isotherms) (D) below the cold core. The alongshelf dimension of the KWF is approximately 50–
100 km, and the cross-shelf distance from the western edge of the KWF to that of the body of the
Kuroshio is about 30–50 km.
Colder shelf slope water found under the cold core of shelf water may result from upwelling
(E) induced by the interaction of the Kuroshio and bottom topography. That shelf slope water is
originated from deep North Pacific Central Water (e.g., Kawai, 1972).
Comparison of a warm filament in the study area with that in the SAB is virtually identical.
An enlarged warm filament derived from the Kuroshio (KWF) is thought to be kinematically
similar to that derived from the Gulf Stream. However, we have no information on the flow
structure around a KWF up to the present. Supporting intensive CTD observations including
current measurements should be carried out in the study area to verify our schematic representation of a KWF in near future.
In addition, the flow pattern in a KWF is not led to clarify yet. Pietrafesa (1983) presented
a view concerning the inside flow structure of a warm filament in the SAB. He found that the
water in a warm filament flowed clockwise. This flow pattern is different to that found by Lee
et al. (1981). This means that the precise flow structure in a warm filament is not obtained even
in the SAB. We must investigate its structure by either direct measurements using a high
frequency radar (e.g., Akiyama et al., 1992) and an ADCP, or indirect objective methods using
sequential satellite IR images (e.g., Emery et al., 1986).
7. Summary and Conclusion
We investigated both the synoptic scale phenomena, such as the Kyucho in Sukumo Bay and
a KWF in the sea off east of Kyushu, and those interaction based on the analysis of the field and
satellite observation data obtained from March 1990 through February 1991.
The Kyucho occurs in Sukumo Bay periodically when the Kuroshio axis is located near Cape
Ashizuri-misaki. An average onset period is both about 10 days in the warming (March through
June) and about 8 days in the cooling (November through February) periods, respectively. These
onset periods agree well with the generation one of the Kuroshio frontal eddy at the east side of
Cape Ashizuri-misaki found by Toda (1993). It is very interesting to investigate the relation
between those two events which occurred on both sides of the cape.
A KWF frequently appears off east of Kyushu. Its alongshelf dimension is approximately
50–100 km, and the cross-shelf distance from the western edge of it to that of the body of the
Kuroshio is about 30–50 km. The along- and cross-shelf scales of a KWF are roughly twice as
long during the warming period as during the cooling one.
It is quantitatively clarified that a KWF advects northeast to Cape Ashizuri-misaki along the
Kuroshio, then encounters the southwest coast of Shikoku, followed by inducing the Kyucho in
Sukumo Bay by the warm water intrusion. A KWF sometimes closely approaches to the east coast
of Kyushu.
As mentioned above, difference exists during the two observation periods as to both the
onset period of the Kyucho in Sukumo Bay and the scales of a KWF in the sea off east of Kyushu.
The vertical structure of the Kuroshio, especially on the coastal side of it, varies between the
warming period and the cooling one (e.g., Akiyama and Yanagi, 1990). It is likely that the degree
The Kyucho Induced by Kuroshio Warm Filament Intrusion
681
of stratification intensity around the Kuroshio front significantly affects the formation process
of a KWF in the study area.
Furthermore, the appearance and development processes and the three-dimensional structure of a KWF are roughly summarized by the event analysis using some fragmentary hydrographic
and satellite observation data. However, it is impossible for us to verify those results in detail due
to lack of field observation data. We hope to investigate the definite structure of a KWF in the
study area, especially paying our attention to its generation mechanism, in near future. For that
purpose, we will examine the instability process in the Kuroshio front and the coupling effect of
the topography of the capes with the Kuroshio in the next study, using numerical simulation
method available to verify those (e.g., Oey, 1988; Awaji et al., 1991). Quantitative assessment
of numerical model performance will be necessary to clarify the physical structure of a KWF.
By the way, the Kyucho events have considerable influence on a nursery environment of
pearls and yellowtails in bays such as Sukumo Bay and Uwajima Bay (Takeoka, 1990).
Therefore, it is very important to study generation mechanisms and behaviors of the Kyucho
events in coastal seas on a viewpoint of physical and fisheries oceanography. Moreover, it may
be a close connection between an appearance of a KWF and a fishing ground formation of pelagic
fishes on the shelf/slope region as pointed out by Kawai (1991) and Miyaji (1991). We will try
to investigate the relation between the physical structure of a KWF and the formation and
dispersion processes of the fishing ground.
Acknowledgements
The authors sincerely thank Drs. T. Yanagi, H. Takeoka, and Y. Isoda at Ehime University
for their helpful discussions and comments. Sincere thanks are also extended to the Sukumo City
Agency, the Ooita and Ehime Prefectural Fisheries Experimental Stations for the supply of the
field observation data. The authors are grateful to Prof. N. Imasato, one of the editors, and the
anonymous referees for their useful comments on the manuscript. The data processing was
carried out using the FACOM M-770/6 system at the Data Processing Center of Ehime
University with the assistance of Mr. M. Ookubo.
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