Journal of Oceanography, Vol. 53, pp. 403 to 409. 1997

Short Contribution

Thermohaline Front at the Mouth of Ise Bay
TETSUO YANAGI 1, XINYU GUO1, TOSHIRO SAINO2, TAKASHI ISHIMARU3 and SINICHIRO NORIKI4
1Department of Civil and Ocean Engineering, Ehime University, Matsuyama 790, Japan
2Institute for Hydrospheric-Atmospheric Science, Nagoya University,

Furocho, Chigusa-ku, Nagoya 464-01, Japan
3Tokyo University of Fisheries, Konan 4-5-7, Minato-ku 108, Japan
4Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan(Received 7 November 1996; in revised form 8 March 1997; accepted 13 March 1997)

The detailed structure of the thermohaline front at the mouth of Ise Bay during winter has
been investigated by intensive field observation. The transport of suspended matter fromIse Bay to the Pacific Ocean through the thermohaline front has also been investigatedusing the data from a time-series sediment trap, a current meter and a nephelometermoored at the bay mouth station. The mixed coastal and offshore water at the surface ofthe thermohaline front sinks to a depth of about 200 m in the offshore area.

1. Introduction
It is well known that quasi-steady thermohaline fronts
develop in the shelf sea area of the northwestern Pacific
Ocean during winter (e.g. Yanagi, 1980; Lie, 1985; Yoshioka,
1988). Such a thermohaline front is generated in a transitionzone between colder coastal water and warmer oceanic
water. Due to cooling through the sea surface during winter,water of moderate salinity in the central region of the shelfsea becomes heavier and sinks there. The colder coastal
water cannot become heavy enough to sink due to the inflow
of fresh water from the land. Oceanic saline water cannot
become heavy enough to sink due to mixing with warm
water from the open sea. As a consequence there is little
density difference between both sides of the thermohaline
front because the salinity difference compensates the temperature difference. The thermohaline fronts exist at nearly
the same position in Iyo-Nada and Kii Channel from lateautumn to early spring, when the cooling through the sea
surface is dominant (Yanagi and Koike, 1987; Yoshioka,
1988).
A remarkable thermohaline front was observed at themouth of Ise Bay, Japan, during winter (Unoki, 1985;
Sekine et al., 1992) but its detailed structure and its role inthe material transport from Ise Bay to the Pacific Ocean have
yet to be disclosed. We report here the detailed structure ofa thermohaline front at the mouth of Ise Bay during winter.
The role of such a thermohaline front in the transport ofsuspended matter from Ise Bay to the Pacific Ocean is also
discussed, using data from a time-series sediment trap, acurrent meter and a nephelometer moored at the bay mouth
station.

2. Observations
The field observation was carried out in Ise Bay (Fig.
1) by R.V. Tansei-Maru of the Ocean Research Institute,
University of Tokyo, from 8 to 12 February 1995. The
vertical profiles of water temperature, salinity, chlorophylla (Chl.a) and dissolved oxygen (D.O.) from the head of Ise
Bay (Stn. I-1) to the offshore region of Ise Bay (Stn. I-10)
were observed on 8 and 9 February 1995. The verticalsections of water temperature, salinity, Chl.a and D.O. withhigh horizontal-scale resolution were observed across athermohaline front (from Stn. IF-2 to Stn. IF-10) from 14:58
to 17:33 on 11 February 1995. Water temperature andsalinity were observed by the CTD (Neil Brown Mark III)and Chl.a and D.O. were measured by the fluorometer(Turner Design) and by Winkler titration analysis, respectively, after water sampling using a Rosette Multi Sampler
which was mounted on the CTD. All of the analyses wereconducted on board with no delay after the sampling.
A time-series sediment trap (WHOI Mark V) with anephelometer (Alec MTB-8M, 940 nm) and a current meter(Aanderaa RCM 4) were set at 118 m and 112 m (shown by
a full star in Fig. 3) below the sea surface, respectively, andKnauer-type sediment traps were set at 97 m, 107 m, 128 m,
178 m and 228 m (shown by full triangles in Fig. 3) below
the sea surface of Stn. I-9 (the depth was 271 m) above theshelf slope. Data on water temperature, salinity, current
velocity and direction were obtained every 5 minutes andturbidity data every 10 minutes. The cup of the time-series
sediment trap rotated at the slack water of Irago Strait which
was predicted by the Hydrographic Department (see the full
triangle in Fig. 1).
403

Copyright  The Oceanographic Society of Japan.

Keywords:⋅ Thermohaline front,⋅ material transport,⋅ density-driven
current,⋅ sediment trap.

3. Results
Figure 2 is a NOAA 12 infrared image, collected at 8:00
JMT on 11 February 1995, which shows the position of thethermohaline front at the mouth of Ise Bay. The thermohaline
front, represented by the discontinuity line between cooler
coastal water (shown in a gray color) and warmer offshore
water (shown in blue and green), runs across the mouth ofIse Bay.
The vertical distributions of water temperature, salinity, density (sigma-t), Chl.a and D.O. from the head to theoffshore region of Ise Bay are shown in Fig. 3. No data on
water temperature, salinity and D.O. were obtained at Stn.
I-10 due to instrument malfunction. The water temperature
is low in Ise Bay and high out of the bay, and an abruptchange of water temperature from 10°C to 14°C occurs at
the mouth of the bay near Stn. I-7. This abrupt change ofwater temperature corresponds to the thermohaline front at
the mouth of Ise Bay. A weak salinity stratification exists in
Ise Bay due to the large river discharge from the head of thebay (about 200 m 3s –1 in this season) and a such structure
affects the density distribution in the bay. The concentrationof Chl.a is high in Ise Bay and low out of the bay, and an
abrupt change occurs at the thermohaline front. It is noteworthy that the coastal water with high Chl.a and high D.O.
in Ise Bay seems to sink at the thermohaline front andreaches to a depth of 200 m at Stn. I-9. D.O. is supersaturated
in the surface layer of Ise Bay, which suggests high activityof photosynthesis by phytoplankton, and is undersaturated
out of the bay. The vertical flux of sinking particles is highbelow 175 m and this corresponds to the vertical Chl.a
distribution at Stn. I-9 mentioned above.
Fig. 1. Observation stations in Ise Bay. Contour numbers show thedepth in meters.

Fig. 2. NOAA 12 infrared image collected 8:00 JMT on 11 February 1995. Grey color denotes the lower water temperature, blue andgreen higher water temperatures and white the cloud.
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Fig. 3. Vertical distributions of water temperature, salinity, sigma-t, Chl.a and D.O. from the head of Ise Bay (Stn. I-1) out of Ise Bay(Stn. I-10) and vertical flux of sinking particles at Stn. I-9. Full triangles in the density section show the positions of Knauer-type
sediment traps and full star that of time-series sediment trap.

Figure 4 shows the vertical distributions of watertemperature, salinity, density, Chl.a and D.O. at high horizontal-scale resolution across the thermohaline front, which
is located at Stn. IF-5. Cold and fresh water exists on thenearshore side of the thermohaline front, and warm andsaline water lies on the offshore side, but there is little
density difference between both sides and the heaviest water
exists just below the thermohaline front. The maximum
horizontal gradients of water temperature and salinity are

2.5°C/100 m and 0.6 psu/100 m, respectively, across thethermohaline front. The cold-fresh coastal water and thewarm-saline offshore water seem to converge in the surface
layer at the thermohaline front and the mixed water diverges
like a skirt in the bottom layer below the thermohaline front.
The concentration of Chl.a is high in the nearshore water andlow in the offshore water. D.O. is supersaturated in the
coastal water and mixed water, and is undersaturated in theoffshore water.
Thermohaline Front at the Mouth of Ise Bay405

Fig. 4. Vertical distributions of water temperature, salinity, density (sigma-t), Chl.a and D.O. across the thermohaline front at the mouthof Ise Bay.

The results from the deployment of the time-series
sediment trap at a depth of 118 m at Stn. I-9 are shown in Fig.
5. The vertical flux of sinking particles in the odd-numberedcups is obtained at flood tidal current and that in the evennumbered cups at ebb tidal current, but the tidal signal is not
dominant in the current record shown in Fig. 5. The verticalfluxes of sinking particles at cups numbered from 2 to 9,406

T. Yanagi et al.

when the northward current prevails, are larger than those atcups numbered from 10 to 11, when the southward current
prevails. When the southward current begins around 3 A.M.
on 11 February, water temperature suddenly increases andthe density decreases. Temporal variation in salinity is very
low during this observation period and the turbidity variation does not well correspond to that of particle flux.

Fig. 5. Results of deployments of time-series sediment trap, current meter and nephelometer 112–118 m below the sea surface at Stn.
I-9.

4. Discussion
The T-S diagram of observed water temperature andsalinity at Stns. IF-2 to IF-10 and I-9 is shown in Fig. 6. The
offshore surface water having high water temperature andhigh salinity (Stns. IF-6, 7, 8, 9 and 10), and the nearshore
surface water having low water temperature and low salinity(Stns. IF-2, 3, 4 and 5) mix at the thermohaline front, andform the heaviest bottom water shown by triangles in Fig. 6

just below the thermohaline front. The nature of this heavy
bottom water is nearly the same as that of the water 200 m
below the sea surface of Stn. I-9; however, the water 200 m
below the sea surface of Stn. I-9 is not generated by verticalmixing of the waters 150 m and 250 m below the sea surfaceof Stn. I-9 from Fig. 6. This implies that the bottom heavy
water at the thermohaline front sinks and intrudes to 200 m
depth in the offshore region, as suggested in the Chl.a andThermohaline Front at the Mouth of Ise Bay407

D.O. sections shown in Fig. 3.
The bottom heavy water at the thermohaline front in theKii Channel (Yoshioka, 1988) and that at the mouth ofTokyo Bay (Yanagi et al., 1989) sink to about 80 m depth inthe offshore region. The bottom water at the thermohaline
front of Ise Bay is heavier than the bottom waters in the Kii
Channel and at the mouth of Tokyo Bay and sinks to about
200 m depth. This difference may be due to the salinity
stratification remaining throughout the winter only in Ise
Bay due to large river discharge, as shown in Fig. 3, althoughsalinity in the nearshore region is nearly homogeneous
during winter in the Kii Channel (Yoshioka, 1988) andTokyo Bay (Yanagi et al., 1989) due to a small river discharge. The sinking water from the surface layer at thethermohaline front of Ise Bay may become heavy at theFig. 6. T-S diagram of observed water temperature and salinity at
Stns. IF-2–IF-10 and Stn. I-9.

bottom due to mixing with the surrounding saline water
existing below the thermohaline front, as shown in Figs. 3
and 4.
The temporal variation in hourly wind vector at the
Irago peninsula (see full square in Fig. 1), which wasobserved by the Japan Meteorological Agency during ourobservation period is shown in Fig. 7. A strong southeastward wind with a speed of about 10 m s–1 blew on 6, 7 and
10, 11 February and the southeastward current occurred 112
m below the sea surface at Stn. I-9 on 8 and 11 February, as
shown in Fig. 5, about a day after such a strong southeastward wind blew. When the wind was weak, a weak northward flow appeared 112 m below the sea surface at Stn. I-9.
After the beginning of southeastward flow at about 3:00 on
11 February, water temperature increased, density decreased
and the vertical flux of sinking particles decreased, as shownin Fig. 5. On the other hand, after the northeastward current
stopped at about 0:00 on 12 February, water temperature
decreased, density increased and the vertical flux of sinking
particles increased at the depth of about 110 m of Stn. I-9.
These facts suggest that the current and water characteristics at the depth of about 110 m of Stn. I-9 were affectedby the wind blowing and that the intrusion of mixed water at
the thermohaline front to the depth of about 200 m was notsteady but rather an intermittent phenomenon. The strong
southeastward wind may be related to the development ofgravitational circulation in Ise Bay, and may result in the
supply of saline offshore water to the bottom layer of thethermohaline front and the formation of a heavy bottom
water mass below the thermohaline front. On the other hand,during the weak wind, the gravitational circulation in Ise
Bay may not be not developed, the heavy water mass may
not be formed at the bottom of thermohaline front and the
deep intrusion of mixed water to the depth of 200 m may notoccur. Both observations at the mouth of Ise Bay shown in
Figs. 3 and 4 were carried out on 8 and 11 March, respectively, just after the strong southeastward wind blew.
It remains for us to conduct consecutive field observations around the thermohaline front at the mouth of Ise
Bay and three-dimensional numerical experiments to give aquantitative clarification of the relation among the wind

Fig. 7. Temporal variation in hourly wind vector at the Irago peninsula (observation station is shown by full square in Fig. 1).
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blowing, the development of gravitational circulation, theformation of heavy water mass at the bottom of thermohaline front, and the intrusion of mixed water to the depth ofabout 200 m out of the mouth of Ise Bay in the near future.
Acknowledgements
The authors express their sincere thanks to the officers
and crew of R.V. Tansei-Maru for their help with the fieldobservation and Dr. H. Takeoka of Ehime University for
fruitful discussions. This study was partly supported by the
research fund of the Ministry of Education, Science andCulture, Japan.
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