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Tidal front at Osaka Bay, Japan, in winter
Continental Shelf Research,
Vol. 15, No. 14, pp. 1723-1735. 1995 Copyright 0 1995 Elsevier Science Ltd
Printed in Great Britain. All rights reserved 0278-4343/95 $9.SO+ 0.00
0278_4343(94)E0034-J
Tidal front at Osaka Bay, Japan, in winter Tetsuo Yanagi,* Shuzo Igawa* and Osamu Matsudat (Received 22 April 1992; in revised form 21 January 1994; accepted 17 March 1994)
Abstract-The detailed structure of an oceanic front at Osaka Bay, Japan was observed in January 1992. The front separates the stratified nearshore region from the well-mixed region of the central part of the bay. The front runs in the domain where the value of log(hlU3) (where h is the water depth in meters and U the amplitude of the M, tidal current in m SK’) ranges from 3.0 to 3.5, showing that tidal stirring plays an important role in its formation. We may consider this front as a tidal front in winter, because the large fresh water inflow generates the density stratification in spite of the strong sea surface cooling. The difference of positions of the tidal front at Osaka Bay between winter and summer is also discussed.
1. INTRODUCTION The classical tidal front is one generated in summer at a transition region between vertically well-mixed water where the tidal stirring is strong and the stratified water where the tidal mixing is relatively weak and the effect of surface heating is dominant (Simpson and Hunter, 1974). Yanagi and Yoshikawa (1987) found a marked tidal front at Osaka Bay, Japan in summer and Yanagi and Takahashi (1988a) investigated the detailed structure of this tidal front at Osaka Bay and they found that the effect of fresh water discharge was important in the formation of this tidal front at Osaka Bay. Such facts suggests that the tidal front may be generated at Osaka Bay even in winter if the density stratification is maintained by the large supply of buoyancy from rivers inflowing into the bay head. In this paper, we report the results of the detailed observations of the oceanic front at Osaka Bay conducted in January 1992. 2. OBSERVATION Intensive field observations of the front in Osaka Bay (Fig. 1) were carried out on board R. V. Toyoshio-Guru of Hiroshima University from 22 to 23 January 1992. The infrared image of NOAA 11 at 3:09 JMT on 17 January 1992 is shown in Fig. 2. A sharp temperature front running across the central part of the bay roughly from north to south can be seen in this picture, and it shows that a sharp front had already been formed during
‘*Department of Civil and Ocean Engineering, Ehime University, Matsuyama 790, Japan. -tFaculty of Applied Biological Science, Hiroshima University, Higashi-Hiroshima 724, Japan
1723
1724
Fig. 1.
T.
Observation
Yanagi
stations in Osaka Bay. Current measurement
et al
Numbered was carried
contours show out at Sta. 2.
the depth
in meters.
the time of our observations. The bottom topography and the site of the observations are shown in Fig. 1. The observations may be divided into the following two phases. In the first phase on 22 January, vertical profiles of temperature, salinity and fluorescence were measured at five stations along one east-west line shown in Fig. 1, by using conductivity, temperature, fluorescence and depth sensors (CTFD), Martek 6. Dissolved oxygen and pH were also measured at some depths using the water obtained by Van Dorn bottles. During the cruise, we also continuously measured the horizontal distributions of water temperature and salinity at a depth of ca 4 m along this line using a thermistor and a salinometer in order to find the exact position and sharpness of the front. In the second phase, current meters with temperature and salinity sensors (Aanderaa RCM-4) were set at three levels (5,lO and 15 m below the sea surface) at the station in the vicinity of the front (Sta. 2; water depth, 20 m). The measurements were conducted during the period 15:OOJMT, 22 January to 15:OOJMT, 23 January, and the current velocity, water temperature and salinity values were obtained at 1 min intervals. At the same time, the vertical profiles of temperature and salinity were observed every three hours from the research vessel using CTFD, Martek 6. 3. CROSS-SECTIONAL
STRUCTURE
OF THE FRONT
The horizontal distributions of temperature and salinity at ca 4 m depth are shown in the upper part of Fig. 3. The figure shows that water temperature and salinity suddenly vary
Tidal front at Osaka
Bay, Japan,
in winter
Fig. ’ 2. NOAA 11 infrared image, at 3:09 JMT on 17 January 1992, showing the position of the tidal front in Osaka Bay. The light areas on the satellite image indicate warmer water. The dark area in the central part of the bay is due to the cloud. The central vertical line means the 135”E la1titude. North is at the top of the image and the grid size on the satellite image is 1.38 km’.
1725
Tidal front at Osaka Bay, Japan,
in winter
1727
across the front near Sta. 2, that is, water temperature changes from 10.5 to 11S”C and salinity from 31 psu to 32 psu. Cross-sectional distributions of temperature, salinity, density and chlorophyll a along the observation line are shown in the lower part of Fig. 3. It can be seen that the vertically well-mixed region exists westward of the front in the central deep water of the bay and the stratified region eastward of the front in the shallow nearshore water. Since the temperature difference of 1.2”C across the front induces the density difference of about 0.4 and the salinity difference of 1.0 psu induces the density difference of 0.9, the salinity difference overcomes the temperature difference in terms of density and low salinity plays a dominant role in the density difference in the case of the front at Osaka Bay in winter. The concentration of chlorophyll a is high in the stratified region and is low in the wellmixed region and such situation is different from the usual tidal fronts around the U.K. (e.g. Pingree and Griffiths, 1978) where the high concentration of chlorophyll a is found at the transition region from the well-mixed region to the stratified region. This difference may be due to the fact that the nutrient is rich in particular in the stratified region because much nutrient is supplied by fresh water inflow from the Yodo and Yamato Rivers (Matsuda, 1990) whose river mouths are located at the head of Osaka Bay (Fig. 1). The cross-frontal distributions of dissolved oxygen and pH are shown in Fig. 4. The dissolved oxygen is supersaturated in the upper layer of the stratified region, possibly due to active photosynthesis associated with high concentration of chlorophyll a but is a little undersaturated near the front. Yanagi et al. (1989) found that the dissolved oxygen is undersaturated near the thermohaline front at the mouth of Tokyo Bay in winter though the dissolved oxygen is supersaturated in the both sides of the front. They consider that such undersaturation of dissolved oxygen near the front may be due to the respiration of large biomass which are gathered to the front. A similar situation may happen near the tidal front at Osaka Bay in winter and may result in a little undersaturation of dissolved oxygen near the front. The pH is high in the stratified region and low in the well-mixed region. This may be also due to the fact that the photosynthesis of phytoplankton is very active in the stratified region. 4. CURRENT
STRUCTURE
NEAR THE FRONT
During our moored current meter observation, the front appeared to move southwestward and passed by the observation Sta. 2. The temporal variations of vertical profiles of temperature, salinity, density and chlorophyll a measured at Sta. 2 by CTFD at 3 h intervals are shown in Fig. 5. The well-mixed water existed at Sta. 2 at the first half of the observation but the stratified water occupied the observation site after 1:00 on 23 January owing to the southwestward migration of the front. The passing of the front was easily recognized by eye from the research vessel as the front was accompanied by a narrow band of foams and various kinds of drifting material on the sea surface. The temporal variations of temperature and salinity observed at the current meter levels (5, 10 and 15 m deep) at Sta. 2 are shown in Fig. 6. The variability of those parameters is large at the middle layer in the stratified region and small in the well-mixed region. The large oscillations in temperature and salinity at the middle layer in the stratified region in the period 4:00-10:00 on 23 January are suggestive of internal waves, which develop between the surface fresh water and the lower salty water. The period of this wave is about 6 min from the spectrum analysis and it nearly coincides with the period of internal wave of
1728
T. Yanagi
et al
(psul 133
________________----___________\\
32
\.-__ *.-___
31 30
“Cl
0
o
,
2km
stn.
1
Salinity(psu1
stn.5
4
3
0
2
IIw- 15
1
1 16
I
chl.a(pg/ll 30
22,Jan.,1992
Tidal front at Osaka
Bay, Japan,
1729
in winter
7 min observed in June 1991 near the tidal front in Osaka Bay (Yanagi et al., 1993). Internal waves may contribute to the mixing process in the stratified region of the front. The temporal variations of current velocity at three levels are shown in Fig. 7. In this figure, we show the velocity components roughly parallel to (north-south) and perpendicular (east-west) to the frontal line. The semi-diurnal variation is the most energetic and is dominant in the lower layer. The appearance of the front at 1:00 on 23 January roughly coincides with the end of the period of the northwestward tidal current, which is flood tidal current at this position. It should be noted that the strong southwestward current of about 24 cm s-l appeared from 1:00 till 4:00 on 23 January only in the surface layer. This means that the stratified water flows to the left of the current in the well-mixed region. This is not agreement with the geostrophic relation based on the density field. In the case of the summer front (Yanagi and Takahashi, 1988a), the strong southeastward current of about 10 cm s-’ appeared only at the surface layer in the well-mixed region and that is the opposite sense to this observational result.
5. DISCUSSION In the case of Osaka Bay in winter, the stratified region exists at the nearshore side of the front, and so the fresh water discharge from the rivers would generate the stratification. Osaka Bay introduces two large rivers, Yodo and Yamato, at the bay head, and the vital effect of the fresh water to the stratification, especially in the surface layer, is demonstrated from our detailed observations as discussed in Section 3. At first, we will check whether this observed front is an estuarine front or not. In the case of a estuarine front, the internal Froude number F,+ has to be 1 .O at the front (Yanagi, 1985). Fri
= +t
=
t
(2h-
w @- w
h(h + 1)
guphl,v P
f
where Vg denotes the moving speed of stratified water mass to the well-mixed region as a gravity current (Benjamin, 1968) and V, the speed of tidal current flowing in THE opposite direction to V,, h the water depth (20 m), hI the thickness of stratified region (10 m), g the gravitational acceleration (9.8 m s-‘), p the density of well-mixed region (1024.5 kg m-“), Ap the density difference between the stratified region and the well-mixed region (OS kg m-“). Calculated from Fig. 5 V,is 15 cm s-l and V,estimated from the lower panel in Fig. 7 is smaller than 10 cm s-l. Such facts suggest that Fri is not 1 .O at this front and the moving stratified region is always eroded by tidal mixing at the front, that is, this observed front is a tidal front. The position of tidal front under the effects of solar radiation, river discharge, wind mixing and tidal mixing is defined by the following equation (Czitrom et al., 1988).
$Q+g%
A(dpld~)~ - dk,p, p
= $
ek,p;
Fig. 3. Horizontal variations in water temperature and salinity 4 m below the sea surface (upper) and vertical distributions of water temperature, salinity, density and chlorophyll a (lower) along the observation line from Sta. 1 to Sta. 5 across the tidal front in Osaka Bay on 22 January 1992.
1730
T. Yanagi
et al
stn. 0 1
z
lo-
: P E 20-
22,Jan.,1992 Fig. 4.
Vertical distributions of dissolved oxygen and pH along the observation Sta. 5 across the tidal front in Osaka Bay on 22 January
line from Sta. 1 to
1992.
where 6 is the coefficient of linear thermal expansion of sea water (1.5 x low4 ‘C-l), Q is the heat flux at the air-sea interface in J mP2 s-l, c is the specific heat of sea water (4.0 x lo3 J kg-’ ‘C-l), gis the gravitational acceleration, h, and h2 are the upper and lower layer depths in meters, h = h, + h2, A is a coefficient relating velocity shear to the horizontal density gradient defined by an equation given by Czitrom et al. (1988) (600 kg-’ m5 s-l), aplax is the horizontal density gradient in kg m-4, 6 is the wind mixing efficiency (0.023), k, is the surface drag coefficient x y (6.4 x lo-‘), y is the ratio of the wind-induced surface current to the wind speed (0.03), pS is the air density (1.22 kg me3), IV3is the root mean cubed wind speed in m3 s-~, E is the tidal mixing efficiency, kb is the bottom friction coefficient (2.5 x 10P3), p is the water density and, U is the tidal current amplitude in m s -1 We will discuss the variation in the position of the front at Osaka Bay in summer and in winter with use of equation (2). The first term in the left-hand side of equation (2) represents the stratification due to solar radiation and we adopt Q as 100 J mP2 s-’ in summer and - 100 J me2 s-’ in winter at Osaka Bay from the results of field observation in Uwajima Bay (Yanagi, 1982) near Osaka Bay. The second term represents the stratification due to differential advection by the fresh water discharge and we adopt hl = h2 = 10 m in summer and winter and the horizontal density gradient (aplax) in summer as
Tidal front at Osaka
Bay, Japan,
1731
in winter
(PSU) 32.5
(“Cl 12 11
32.0
s____-_----------"._____, T
31.5
10 1
: 31.0
_
Salinity(psu1 201
> ,,,,,,,,,,,,,,,,
j ,,,,,,,,,,,,
o-
z E lo:
Sigma-t
I
15:oo
I
18:OO
I
21:oo
I
0:oo
I
3:oo
I
6:OO
I
I
9:oo
12:oo
J
15:oo
22-23,Jan.,1992
Fig. 5. Temporal variations of water temperature and salinity 4 m below the sea surface (upper) and vertical distributions of water temperature, salinity, density and chlorophyll a observed every three hours (lower) at Sta. 2 in Osaka Bay from 15:00 on 22 January to 15:OO on 23 January, 1992.
1732
T. Yanagi et al.
UPPER
II
12
MIDDLE
I
” I-----
-~-
1oL
LOWER
11
0:oo
Temperature(
15:oo
“C)
22-23,Jan..1992
32
UPPER 31 33
MIDDLE
32 I-311
---
---
15:oo
Salinity(psu) Fig. 6.
22-23,Jan.,1992
Temporal variations of water temperature and salinity in the upper, middle and lower layers at Sta. 2 from 15:OOon 22 January to 15:OOon 23 January, 1992.
1.0 x 1O-4 kg m-4from Fig. 6 of Yanagi and Takahashi (1988a) and that in winter as 5.0 x lop5 kg me4 from Fig. 3 of this paper. The third term represents the vertical mixing due to wind and we adopt Win summer as 1 m s-l and W in winter as 3 m s-l from Yanagi and Takahashi (1988b). The right-hand side of equation (2) represents the tidal mixing. Estimates in J m-s day-’ of the different terms in the left-hand side of equation (2) are given in Table 1. The stratification due to differential advection by the fresh water discharge (3.2) is larger than the destratification due to surface cooling (1.59) and wind
Tidal front at Osaka Bay, Japan, in winter
1733
UPPER
MIDDLE
0
LOWER
o I II”“’ 15:oo
0:oo
N-S
IO,
MIDDLE
Velocity
Comp.
15:oo
(cm/set)
22-23,Jan.,1992
E
0
LOWER
15:oo
0:oo
E-W
Velocity
Comp.
15:oo
(cm Isec)
22-23,Jan.,1992
Fig. 7. Temporal variations of alongfront velocity (N-S) and crossfront velocity (E-W) in the upper, middle and lower layers at Sta. 2 from 15:00 on 22 January to 15:OOon 23 January, 1992.
mixing (0.21) at Osaka Bay in winter from Table 1. As for the right-hand side term in equation (2), the numerical simulation of the barotropic M2 tide in Osaka Bay was already carried out (Yanagi and Takahashi, 1988b) as the M, tide is predominant in Osaka Bay (Yanagi et al., 1982), and the distribution of log (h / U3) was calculated. The result is
1734
T. YANAGI et al. Table 1.
Q (J m-2 s-9 Summer Winter
100 -100
Estimates of left-hand side terms in equation (2)
aplax 0% m-“)
(m?‘)
1.0 x 10-4
1.0
5.0 x 10-s
3.0
agl2c Q (J mm3 day-‘)
3
ghlhzA(ap/ax)2 (J rnh3 day-‘)
1.59
(J!?‘Lihl)
12.7
-1.59
0.0078
3.2
0.21
I-
O
IO
2'0
km Fig. 8.
Calculated
log (hlU3) with the barotropic numerical model and positions observed by NOAA 9 and 11 infrared image in Osaka Bay.
of the tidal front
shown in Fig. 8 with the observed positions of the fronts in summer (Yanagi and Takahashi, 1988a) and in winter. It should be noted that the front in winter is located along the contour of log (h / U”) = 3.0 and 3.5 though the front in summer is located between the contours of log (h / U3) = 2.5 and 3.0, except for the southern portion. Some modification of the front configuration may occur in this portion but we have no data to investigate this at present. The difference of the positions of the front at Osaka Bay in summer and in winter may be explained by the difference of left-hand side terms in equation (2) and that of tidal mixing efficiency in summer and in winter. The tidal mixing efficiency E in summer
Tidal front at Osaka Table 2.
Bay, Japan,
1735
in winter
Estimates of right-hand side term in equation (2)
P (kg me3)
U3/h (m* se3)
4/(3x) kbp .!J3/h (J mm3 day-‘)
Summer
1022.0
1.0 x 10-j - 3.16 x lO-3
93.3 - 29.6
0.048 - 0.15
Winter
1024.3
3.16 x lO-4 - 1.0 x 10-3
29.6 - 94.2
0.015 - 0.047
E
from equation (2) is 0.048-0.15 and that in winter is 0.015-0.047 from Table 2. As for the absolute values of E in summer and in winter, they are much greater than those in the European and North American shelfs (e.g. 0.0028 by Garrett etal., 1978). The value of E in equation (2) depends on that of A, which was empirically obtained in the eastern Irish Sea by Czitrom et al. (1988). The value A in Osaka Bay may be different from that in the eastern Irish Sea, though we have no field data to determine the value A in Osaka Bay now. The exact values E, the reason for their difference between summer and winter at Osaka Bay and the comparison of E values between Osaka Bay and other shelf seas may be an interesting theme to be studied in the near future. Acknowledgements-The authors express their sincere thanks to Dr H. Takeoka of Ehime University for his helpful discussion, to the officers and crew of R.V. Toyoshio-Maru of Hiroshima University for their cooperation during the observations and the anonymous reviewers for their helpful comments. This study was partially supported by the fund defrayed from the Ministry of Education, Culture and Science, Japan.
REFERENCES Benjamin T. B. (1968) Gravity currents and related phenomena. Journal of Fluid Mechanics, 31, 209-248. Czitrom S. P. R., G. Budeus and G. Krause (1988) A tidal mixing front in an area influenced by land runoff. Continental Shelf Research, 8,225237. Garrett C. J. R., J. R. Keeley and D. A. Greenberg (1978) Tidal mixing versus thermal stratification in the Bay of Fundy and Gulf of Maine. Atmosphere-Ocean, 16,403423. Matsuda 0. (1990) Chemical aspects of tidal front. In: The science of Some, T. Yanagi, editor, KoseishaKoseikaku, Tokyo, pp. 37-50. Pingree R. D. and D. K. Griffiths (1978) Tidal fronts on the shelf seas around the British Isles. Journal of Geophysical Research, 83,4615--1622. Simpson J. H. and J. R. Hunter (1974) Fronts in the Irish Sea. Nature, 250,404-406. Yanagi T. (1982) Heat budget in Uwajima Bay. Umi to Sora, 58, 13-20. Yanagi T., H. Takeoka and H. Tsukamoto (1982) Tidal energy balance in the Seto Inland Sea. Journal of the Oceanography Society of Japan, 38,293-299. Yanagi T. (1985) Estuarine front at the Hiji River-Tidal variation of the front. Memoirs of the Faculty of Engineering, Ehime University, lo-4,253-261, Yanagi T. and K. Yoshikawa (1987) Tidal fronts in Hiuchi-Nada and Osaka Bay. Journal of the Fisheries and Oceanography Society, 51, 115-118. Yanagi T. and S. Takahashi (1988a) A tidal front influenced by river discharge. Dynamics of Atmosphere and Oceans, 12, 191-206. Yanagi T. and S. Takahashi (1988b) Variability of the residual flow in Osaka Bay. Bulletin on Coastal Oceanography, 26,66-70. Yanagi T., A. Isobe, T. Saino and T. Ishimaru (1989) Thermohaline front at the mouth of Tokyo Bay in winter. Continental Shelf Research, 9,77-91. Yanagi T., T. Yamamoto, T. Ishimaru and T. Saino (1993) Short-period internal wave observed near a tidal front in Osaka Bay. Bulletin on Coastal Oceanography, 30,201-207.
Vol. 15, No. 14, pp. 1723-1735. 1995 Copyright 0 1995 Elsevier Science Ltd
Printed in Great Britain. All rights reserved 0278-4343/95 $9.SO+ 0.00
0278_4343(94)E0034-J
Tidal front at Osaka Bay, Japan, in winter Tetsuo Yanagi,* Shuzo Igawa* and Osamu Matsudat (Received 22 April 1992; in revised form 21 January 1994; accepted 17 March 1994)
Abstract-The detailed structure of an oceanic front at Osaka Bay, Japan was observed in January 1992. The front separates the stratified nearshore region from the well-mixed region of the central part of the bay. The front runs in the domain where the value of log(hlU3) (where h is the water depth in meters and U the amplitude of the M, tidal current in m SK’) ranges from 3.0 to 3.5, showing that tidal stirring plays an important role in its formation. We may consider this front as a tidal front in winter, because the large fresh water inflow generates the density stratification in spite of the strong sea surface cooling. The difference of positions of the tidal front at Osaka Bay between winter and summer is also discussed.
1. INTRODUCTION The classical tidal front is one generated in summer at a transition region between vertically well-mixed water where the tidal stirring is strong and the stratified water where the tidal mixing is relatively weak and the effect of surface heating is dominant (Simpson and Hunter, 1974). Yanagi and Yoshikawa (1987) found a marked tidal front at Osaka Bay, Japan in summer and Yanagi and Takahashi (1988a) investigated the detailed structure of this tidal front at Osaka Bay and they found that the effect of fresh water discharge was important in the formation of this tidal front at Osaka Bay. Such facts suggests that the tidal front may be generated at Osaka Bay even in winter if the density stratification is maintained by the large supply of buoyancy from rivers inflowing into the bay head. In this paper, we report the results of the detailed observations of the oceanic front at Osaka Bay conducted in January 1992. 2. OBSERVATION Intensive field observations of the front in Osaka Bay (Fig. 1) were carried out on board R. V. Toyoshio-Guru of Hiroshima University from 22 to 23 January 1992. The infrared image of NOAA 11 at 3:09 JMT on 17 January 1992 is shown in Fig. 2. A sharp temperature front running across the central part of the bay roughly from north to south can be seen in this picture, and it shows that a sharp front had already been formed during
‘*Department of Civil and Ocean Engineering, Ehime University, Matsuyama 790, Japan. -tFaculty of Applied Biological Science, Hiroshima University, Higashi-Hiroshima 724, Japan
1723
1724
Fig. 1.
T.
Observation
Yanagi
stations in Osaka Bay. Current measurement
et al
Numbered was carried
contours show out at Sta. 2.
the depth
in meters.
the time of our observations. The bottom topography and the site of the observations are shown in Fig. 1. The observations may be divided into the following two phases. In the first phase on 22 January, vertical profiles of temperature, salinity and fluorescence were measured at five stations along one east-west line shown in Fig. 1, by using conductivity, temperature, fluorescence and depth sensors (CTFD), Martek 6. Dissolved oxygen and pH were also measured at some depths using the water obtained by Van Dorn bottles. During the cruise, we also continuously measured the horizontal distributions of water temperature and salinity at a depth of ca 4 m along this line using a thermistor and a salinometer in order to find the exact position and sharpness of the front. In the second phase, current meters with temperature and salinity sensors (Aanderaa RCM-4) were set at three levels (5,lO and 15 m below the sea surface) at the station in the vicinity of the front (Sta. 2; water depth, 20 m). The measurements were conducted during the period 15:OOJMT, 22 January to 15:OOJMT, 23 January, and the current velocity, water temperature and salinity values were obtained at 1 min intervals. At the same time, the vertical profiles of temperature and salinity were observed every three hours from the research vessel using CTFD, Martek 6. 3. CROSS-SECTIONAL
STRUCTURE
OF THE FRONT
The horizontal distributions of temperature and salinity at ca 4 m depth are shown in the upper part of Fig. 3. The figure shows that water temperature and salinity suddenly vary
Tidal front at Osaka
Bay, Japan,
in winter
Fig. ’ 2. NOAA 11 infrared image, at 3:09 JMT on 17 January 1992, showing the position of the tidal front in Osaka Bay. The light areas on the satellite image indicate warmer water. The dark area in the central part of the bay is due to the cloud. The central vertical line means the 135”E la1titude. North is at the top of the image and the grid size on the satellite image is 1.38 km’.
1725
Tidal front at Osaka Bay, Japan,
in winter
1727
across the front near Sta. 2, that is, water temperature changes from 10.5 to 11S”C and salinity from 31 psu to 32 psu. Cross-sectional distributions of temperature, salinity, density and chlorophyll a along the observation line are shown in the lower part of Fig. 3. It can be seen that the vertically well-mixed region exists westward of the front in the central deep water of the bay and the stratified region eastward of the front in the shallow nearshore water. Since the temperature difference of 1.2”C across the front induces the density difference of about 0.4 and the salinity difference of 1.0 psu induces the density difference of 0.9, the salinity difference overcomes the temperature difference in terms of density and low salinity plays a dominant role in the density difference in the case of the front at Osaka Bay in winter. The concentration of chlorophyll a is high in the stratified region and is low in the wellmixed region and such situation is different from the usual tidal fronts around the U.K. (e.g. Pingree and Griffiths, 1978) where the high concentration of chlorophyll a is found at the transition region from the well-mixed region to the stratified region. This difference may be due to the fact that the nutrient is rich in particular in the stratified region because much nutrient is supplied by fresh water inflow from the Yodo and Yamato Rivers (Matsuda, 1990) whose river mouths are located at the head of Osaka Bay (Fig. 1). The cross-frontal distributions of dissolved oxygen and pH are shown in Fig. 4. The dissolved oxygen is supersaturated in the upper layer of the stratified region, possibly due to active photosynthesis associated with high concentration of chlorophyll a but is a little undersaturated near the front. Yanagi et al. (1989) found that the dissolved oxygen is undersaturated near the thermohaline front at the mouth of Tokyo Bay in winter though the dissolved oxygen is supersaturated in the both sides of the front. They consider that such undersaturation of dissolved oxygen near the front may be due to the respiration of large biomass which are gathered to the front. A similar situation may happen near the tidal front at Osaka Bay in winter and may result in a little undersaturation of dissolved oxygen near the front. The pH is high in the stratified region and low in the well-mixed region. This may be also due to the fact that the photosynthesis of phytoplankton is very active in the stratified region. 4. CURRENT
STRUCTURE
NEAR THE FRONT
During our moored current meter observation, the front appeared to move southwestward and passed by the observation Sta. 2. The temporal variations of vertical profiles of temperature, salinity, density and chlorophyll a measured at Sta. 2 by CTFD at 3 h intervals are shown in Fig. 5. The well-mixed water existed at Sta. 2 at the first half of the observation but the stratified water occupied the observation site after 1:00 on 23 January owing to the southwestward migration of the front. The passing of the front was easily recognized by eye from the research vessel as the front was accompanied by a narrow band of foams and various kinds of drifting material on the sea surface. The temporal variations of temperature and salinity observed at the current meter levels (5, 10 and 15 m deep) at Sta. 2 are shown in Fig. 6. The variability of those parameters is large at the middle layer in the stratified region and small in the well-mixed region. The large oscillations in temperature and salinity at the middle layer in the stratified region in the period 4:00-10:00 on 23 January are suggestive of internal waves, which develop between the surface fresh water and the lower salty water. The period of this wave is about 6 min from the spectrum analysis and it nearly coincides with the period of internal wave of
1728
T. Yanagi
et al
(psul 133
________________----___________\\
32
\.-__ *.-___
31 30
“Cl
0
o
,
2km
stn.
1
Salinity(psu1
stn.5
4
3
0
2
IIw- 15
1
1 16
I
chl.a(pg/ll 30
22,Jan.,1992
Tidal front at Osaka
Bay, Japan,
1729
in winter
7 min observed in June 1991 near the tidal front in Osaka Bay (Yanagi et al., 1993). Internal waves may contribute to the mixing process in the stratified region of the front. The temporal variations of current velocity at three levels are shown in Fig. 7. In this figure, we show the velocity components roughly parallel to (north-south) and perpendicular (east-west) to the frontal line. The semi-diurnal variation is the most energetic and is dominant in the lower layer. The appearance of the front at 1:00 on 23 January roughly coincides with the end of the period of the northwestward tidal current, which is flood tidal current at this position. It should be noted that the strong southwestward current of about 24 cm s-l appeared from 1:00 till 4:00 on 23 January only in the surface layer. This means that the stratified water flows to the left of the current in the well-mixed region. This is not agreement with the geostrophic relation based on the density field. In the case of the summer front (Yanagi and Takahashi, 1988a), the strong southeastward current of about 10 cm s-’ appeared only at the surface layer in the well-mixed region and that is the opposite sense to this observational result.
5. DISCUSSION In the case of Osaka Bay in winter, the stratified region exists at the nearshore side of the front, and so the fresh water discharge from the rivers would generate the stratification. Osaka Bay introduces two large rivers, Yodo and Yamato, at the bay head, and the vital effect of the fresh water to the stratification, especially in the surface layer, is demonstrated from our detailed observations as discussed in Section 3. At first, we will check whether this observed front is an estuarine front or not. In the case of a estuarine front, the internal Froude number F,+ has to be 1 .O at the front (Yanagi, 1985). Fri
= +t
=
t
(2h-
w @- w
h(h + 1)
guphl,v P
f
where Vg denotes the moving speed of stratified water mass to the well-mixed region as a gravity current (Benjamin, 1968) and V, the speed of tidal current flowing in THE opposite direction to V,, h the water depth (20 m), hI the thickness of stratified region (10 m), g the gravitational acceleration (9.8 m s-‘), p the density of well-mixed region (1024.5 kg m-“), Ap the density difference between the stratified region and the well-mixed region (OS kg m-“). Calculated from Fig. 5 V,is 15 cm s-l and V,estimated from the lower panel in Fig. 7 is smaller than 10 cm s-l. Such facts suggest that Fri is not 1 .O at this front and the moving stratified region is always eroded by tidal mixing at the front, that is, this observed front is a tidal front. The position of tidal front under the effects of solar radiation, river discharge, wind mixing and tidal mixing is defined by the following equation (Czitrom et al., 1988).
$Q+g%
A(dpld~)~ - dk,p, p
= $
ek,p;
Fig. 3. Horizontal variations in water temperature and salinity 4 m below the sea surface (upper) and vertical distributions of water temperature, salinity, density and chlorophyll a (lower) along the observation line from Sta. 1 to Sta. 5 across the tidal front in Osaka Bay on 22 January 1992.
1730
T. Yanagi
et al
stn. 0 1
z
lo-
: P E 20-
22,Jan.,1992 Fig. 4.
Vertical distributions of dissolved oxygen and pH along the observation Sta. 5 across the tidal front in Osaka Bay on 22 January
line from Sta. 1 to
1992.
where 6 is the coefficient of linear thermal expansion of sea water (1.5 x low4 ‘C-l), Q is the heat flux at the air-sea interface in J mP2 s-l, c is the specific heat of sea water (4.0 x lo3 J kg-’ ‘C-l), gis the gravitational acceleration, h, and h2 are the upper and lower layer depths in meters, h = h, + h2, A is a coefficient relating velocity shear to the horizontal density gradient defined by an equation given by Czitrom et al. (1988) (600 kg-’ m5 s-l), aplax is the horizontal density gradient in kg m-4, 6 is the wind mixing efficiency (0.023), k, is the surface drag coefficient x y (6.4 x lo-‘), y is the ratio of the wind-induced surface current to the wind speed (0.03), pS is the air density (1.22 kg me3), IV3is the root mean cubed wind speed in m3 s-~, E is the tidal mixing efficiency, kb is the bottom friction coefficient (2.5 x 10P3), p is the water density and, U is the tidal current amplitude in m s -1 We will discuss the variation in the position of the front at Osaka Bay in summer and in winter with use of equation (2). The first term in the left-hand side of equation (2) represents the stratification due to solar radiation and we adopt Q as 100 J mP2 s-’ in summer and - 100 J me2 s-’ in winter at Osaka Bay from the results of field observation in Uwajima Bay (Yanagi, 1982) near Osaka Bay. The second term represents the stratification due to differential advection by the fresh water discharge and we adopt hl = h2 = 10 m in summer and winter and the horizontal density gradient (aplax) in summer as
Tidal front at Osaka
Bay, Japan,
1731
in winter
(PSU) 32.5
(“Cl 12 11
32.0
s____-_----------"._____, T
31.5
10 1
: 31.0
_
Salinity(psu1 201
> ,,,,,,,,,,,,,,,,
j ,,,,,,,,,,,,
o-
z E lo:
Sigma-t
I
15:oo
I
18:OO
I
21:oo
I
0:oo
I
3:oo
I
6:OO
I
I
9:oo
12:oo
J
15:oo
22-23,Jan.,1992
Fig. 5. Temporal variations of water temperature and salinity 4 m below the sea surface (upper) and vertical distributions of water temperature, salinity, density and chlorophyll a observed every three hours (lower) at Sta. 2 in Osaka Bay from 15:00 on 22 January to 15:OO on 23 January, 1992.
1732
T. Yanagi et al.
UPPER
II
12
MIDDLE
I
” I-----
-~-
1oL
LOWER
11
0:oo
Temperature(
15:oo
“C)
22-23,Jan..1992
32
UPPER 31 33
MIDDLE
32 I-311
---
---
15:oo
Salinity(psu) Fig. 6.
22-23,Jan.,1992
Temporal variations of water temperature and salinity in the upper, middle and lower layers at Sta. 2 from 15:OOon 22 January to 15:OOon 23 January, 1992.
1.0 x 1O-4 kg m-4from Fig. 6 of Yanagi and Takahashi (1988a) and that in winter as 5.0 x lop5 kg me4 from Fig. 3 of this paper. The third term represents the vertical mixing due to wind and we adopt Win summer as 1 m s-l and W in winter as 3 m s-l from Yanagi and Takahashi (1988b). The right-hand side of equation (2) represents the tidal mixing. Estimates in J m-s day-’ of the different terms in the left-hand side of equation (2) are given in Table 1. The stratification due to differential advection by the fresh water discharge (3.2) is larger than the destratification due to surface cooling (1.59) and wind
Tidal front at Osaka Bay, Japan, in winter
1733
UPPER
MIDDLE
0
LOWER
o I II”“’ 15:oo
0:oo
N-S
IO,
MIDDLE
Velocity
Comp.
15:oo
(cm/set)
22-23,Jan.,1992
E
0
LOWER
15:oo
0:oo
E-W
Velocity
Comp.
15:oo
(cm Isec)
22-23,Jan.,1992
Fig. 7. Temporal variations of alongfront velocity (N-S) and crossfront velocity (E-W) in the upper, middle and lower layers at Sta. 2 from 15:00 on 22 January to 15:OOon 23 January, 1992.
mixing (0.21) at Osaka Bay in winter from Table 1. As for the right-hand side term in equation (2), the numerical simulation of the barotropic M2 tide in Osaka Bay was already carried out (Yanagi and Takahashi, 1988b) as the M, tide is predominant in Osaka Bay (Yanagi et al., 1982), and the distribution of log (h / U3) was calculated. The result is
1734
T. YANAGI et al. Table 1.
Q (J m-2 s-9 Summer Winter
100 -100
Estimates of left-hand side terms in equation (2)
aplax 0% m-“)
(m?‘)
1.0 x 10-4
1.0
5.0 x 10-s
3.0
agl2c Q (J mm3 day-‘)
3
ghlhzA(ap/ax)2 (J rnh3 day-‘)
1.59
(J!?‘Lihl)
12.7
-1.59
0.0078
3.2
0.21
I-
O
IO
2'0
km Fig. 8.
Calculated
log (hlU3) with the barotropic numerical model and positions observed by NOAA 9 and 11 infrared image in Osaka Bay.
of the tidal front
shown in Fig. 8 with the observed positions of the fronts in summer (Yanagi and Takahashi, 1988a) and in winter. It should be noted that the front in winter is located along the contour of log (h / U”) = 3.0 and 3.5 though the front in summer is located between the contours of log (h / U3) = 2.5 and 3.0, except for the southern portion. Some modification of the front configuration may occur in this portion but we have no data to investigate this at present. The difference of the positions of the front at Osaka Bay in summer and in winter may be explained by the difference of left-hand side terms in equation (2) and that of tidal mixing efficiency in summer and in winter. The tidal mixing efficiency E in summer
Tidal front at Osaka Table 2.
Bay, Japan,
1735
in winter
Estimates of right-hand side term in equation (2)
P (kg me3)
U3/h (m* se3)
4/(3x) kbp .!J3/h (J mm3 day-‘)
Summer
1022.0
1.0 x 10-j - 3.16 x lO-3
93.3 - 29.6
0.048 - 0.15
Winter
1024.3
3.16 x lO-4 - 1.0 x 10-3
29.6 - 94.2
0.015 - 0.047
E
from equation (2) is 0.048-0.15 and that in winter is 0.015-0.047 from Table 2. As for the absolute values of E in summer and in winter, they are much greater than those in the European and North American shelfs (e.g. 0.0028 by Garrett etal., 1978). The value of E in equation (2) depends on that of A, which was empirically obtained in the eastern Irish Sea by Czitrom et al. (1988). The value A in Osaka Bay may be different from that in the eastern Irish Sea, though we have no field data to determine the value A in Osaka Bay now. The exact values E, the reason for their difference between summer and winter at Osaka Bay and the comparison of E values between Osaka Bay and other shelf seas may be an interesting theme to be studied in the near future. Acknowledgements-The authors express their sincere thanks to Dr H. Takeoka of Ehime University for his helpful discussion, to the officers and crew of R.V. Toyoshio-Maru of Hiroshima University for their cooperation during the observations and the anonymous reviewers for their helpful comments. This study was partially supported by the fund defrayed from the Ministry of Education, Culture and Science, Japan.
REFERENCES Benjamin T. B. (1968) Gravity currents and related phenomena. Journal of Fluid Mechanics, 31, 209-248. Czitrom S. P. R., G. Budeus and G. Krause (1988) A tidal mixing front in an area influenced by land runoff. Continental Shelf Research, 8,225237. Garrett C. J. R., J. R. Keeley and D. A. Greenberg (1978) Tidal mixing versus thermal stratification in the Bay of Fundy and Gulf of Maine. Atmosphere-Ocean, 16,403423. Matsuda 0. (1990) Chemical aspects of tidal front. In: The science of Some, T. Yanagi, editor, KoseishaKoseikaku, Tokyo, pp. 37-50. Pingree R. D. and D. K. Griffiths (1978) Tidal fronts on the shelf seas around the British Isles. Journal of Geophysical Research, 83,4615--1622. Simpson J. H. and J. R. Hunter (1974) Fronts in the Irish Sea. Nature, 250,404-406. Yanagi T. (1982) Heat budget in Uwajima Bay. Umi to Sora, 58, 13-20. Yanagi T., H. Takeoka and H. Tsukamoto (1982) Tidal energy balance in the Seto Inland Sea. Journal of the Oceanography Society of Japan, 38,293-299. Yanagi T. (1985) Estuarine front at the Hiji River-Tidal variation of the front. Memoirs of the Faculty of Engineering, Ehime University, lo-4,253-261, Yanagi T. and K. Yoshikawa (1987) Tidal fronts in Hiuchi-Nada and Osaka Bay. Journal of the Fisheries and Oceanography Society, 51, 115-118. Yanagi T. and S. Takahashi (1988a) A tidal front influenced by river discharge. Dynamics of Atmosphere and Oceans, 12, 191-206. Yanagi T. and S. Takahashi (1988b) Variability of the residual flow in Osaka Bay. Bulletin on Coastal Oceanography, 26,66-70. Yanagi T., A. Isobe, T. Saino and T. Ishimaru (1989) Thermohaline front at the mouth of Tokyo Bay in winter. Continental Shelf Research, 9,77-91. Yanagi T., T. Yamamoto, T. Ishimaru and T. Saino (1993) Short-period internal wave observed near a tidal front in Osaka Bay. Bulletin on Coastal Oceanography, 30,201-207.