Current and temperature observations in the east Tsushima channel and the Sea of Genkai

I'/ok' Ot,',.I,,g . V o l 17. pp. 277 2~;5. I~lSh. I h m t c d tu (,l£.ll I{iH.Htl All It~2hl~ Ik'~t21"~L'd

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C u r r e n t and t e m p e r a t u r e o b s e r v a t i o n s in the east T s u s h i m a c h a n n e l and the Sea o f G e n k a i S. MIzuNo,* K. KAWATATE* and T. MIITAt (Received 3(I November 1985; in revised form 31 March 1986; accepted 111July 1986) A b s t r a c t - - D a t a on the Tsushima, C u r r e n t and its neighboring coastal current are analyzed to e x a m i n e short-term variability of the currents and storm events due to typhoons. A three currentmeter array was dcploycd in a strong current region of the cast T s u s h i m a channel during s u m m e r in 1983 and 1984, and other two c u r r e n t - m e t e r arrays in the eastern coastal area of the channel (the Sea of Gcnkai) in the s u m m c r and a u t u n m in 1983. The observations of coastal current show that the kinetic energy of the subtidal current c o m p o n e n t was larger in s u m m e r than in a u t u m n by a factor of about 2. A comparison of the wind stresses and thc estimated values of mixed layer depth in the s u m m e r and a u t u m n season suggest that this seasonal change is closely associated with that of the mixed layer depth rather than with that of the wind stress. The T s u s h i m a Current was greatly influenced by two storm events: its speed increased by a factor of 2 in one event and decreased to nearly zero in the other. Such a large variation of mean current during the storm was observed only for the Tsushima Current and not for the coastal current, suggesting that the T s u s h i m a Current m a y temporarily change its regular course as a result of a storm.

INTRODUCTION

BECAUSE the Tsushima strait is located on a shallow continental shelf of <200 m water depth, the Tsushima Current may be greatly influenced by storms. In spite of this interesting characteristic, very few moorings have been deployed in this coastal area until now because of the fishing activity. Before examining short-term variability of the Tsushima Current and its neighboring coastal current, we asked fishermen how long and where we could keep mooring current meters without them being damaged by the net fishing. According to thcir rccomnacndation, since 1983 wc have deployed few moorings at the east Tsushima channel and the Sea of Genkai (the eastern coastal arca of the east channel), during the summer season. During the study periods, we observed the response of the Tsushima Current and the coastal current to three storms. The observations have shown that the storm has a greater effect on the Tsushima Current than on the coastal current. In this paper we first describe the observational results of the coastal current and examine the response of subtidal current in the upper mixed layer to two storms by applying the POLLARD and MILLARD (1970) model to storm events. Second, we discuss a seasonal change found in the coastal current during the summer and autumn seasons.

* Research Institute for Applied Mechanics 87, Kyushu University, Kasuga 816, Japan. ¢ Fukuoka Prefectural Fisheries Experimental Station. Fukuoka 819-(11, Japan. 277

278

S. MizuNo el a/.

Third, we describe some features of a vertical profile of the Tsushima Current during summer, and linally discuss the response of the Tsushima Current to storms. DEPLOYMENT

AND DATA ANALYSIS

Stations for current and temperature measurements in 1983 and 1984 are T I , T2 and T4 shown in Fig. 1. Specific station and deployment information are given in Table 1. Although current measurements were also made at the other stations (T3 and T5) in Fig. 130'

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Location

Current-meter deployments attd meteorological data station Depth (m)

Start

Duration (days)

Remarks

T1

33°46.10'N 130°26.50'E

16

19 July 1983 9 July 1984

89 49

Meteorological and oceanographic data station

T2

33"48.10'N 130°17.20'E

45

26 July 1983

79

2 current mctcrs

T4

34°02.14'N 129°25.02'E

130

27 July 1983 9 July 1984

43 49

3 current meters 2 currcnt mcters

()hscrvations in the Tstnshim;i ch~lnncl

279

1, we do not use their records here because of short duration or some other trouble. Station T I , which is Iocatcd 2 km offshorc lronl Kyushu lsl~md, is a nlalille I o w c r of tile Research Institute for Applied Mechanics, Kyushu University, where we acquire meteorological and oceanographic data: wind speed, wind direction, air temperature, atmospheric pressurc and illcoming solar radiation measured with Aanderaa weather instruments, and water temperature measured at 7 depths with 2 m spacing, with an Aanderaa 20 m thermistor string suspended from the top of the tower. In addition to these data, daily values of surface vapor pressure and fraction of sky covered by cloud, whose tables are published by the Fukuoka Meteorological Observatory about 20 km south of Sta. T I , were available to compute the air-sea heatexchange terms: Q.,, Qu, Q;:, and Q,., where Q, is heat absorbed from solar radiation. Q;~ is net loss of heat by long wave radiation, QE is loss of heat by evaporation, and Q,. is loss of heat by conduction to the atmosphere. The total surface exchange (QH) is then

e , , = Q , - e , ~ - Q E - O,.. Each of the heat budget terms was computed following BOWDEN (1970), except for which we used the following bulk formula (BUDYKO, 1973):

(1 a) for Qu,

QB = 0.985~[0~,(0.39 - 0.058 E~,)(1 - 0.64 C~) + 403, (T,,,- 7",,)] cal cm -2 min -I,

(lb)

where c~ is the Stefan-Boltzmann black body constant. 0,,, is the absolute temperature of the sea surface, E,, is the surface vapour pressure, C; is the cloudiness factor, T,,. is the sea surface temperature and T, is the atmospheric temperature. Although BOWDEN (1977) did not take into account the effect of air-water temperature difference (i.e. the second term in the right hand side of QB), it is very important in the present study because the temperature difference is large in the cooling season. The current meter depths were 16 and 24 m from the surface for Sta. T2, and 5(I, 80 and 110 m from the surface for Sta. T4. The deployment of the mooring lines was made by R.V. Genkai (140 ton) of Fukuoka Prefcctural Fishcrics Expcrimental Station (FPFES). Most of the current meters were A a n d e r a a RCM-4 instruments, equipped with temperature sensors; some of them also had pressure sensors. During the 1983 observations, full current records were obtained at both T2 and T4, but some troubles occurred at T4 in 1984: the current rotor in the top (50 m) meter did not work although the current direction was recorded, nor did the current direction in the middle (80 m) meter seem correct because this direction alone was systematically different from the others among a total of six current meters used in the east Tsushima channel in 1983 and 1984. So, in the 1984 data analysis the direction data at the 80 m level in Fig. 12 are replaced by those at the 50 m level to make the fullest possible use of the data at the upper two levels. On the first cruise, CTD observations were made along two lines (A and B in Fig. 1) across the east Tsushima strait. Figure 2 shows the vertical section of water temperature along line B on 28 July, 1983. It is interesting that a cold 15°C bottom water is observed over a wide area of the deep east channel. During the study periods the Tsushima strait was influenced by three typhoons, the tracks of which are shown in Fig. 3. The two typhoons in 1983 just approached the strait, but T y p h o o n 8410 passed over Tsushima Island during the measurement period. After obvious erroneous values had been edited, all values of the east (u) and north (v) components of the currents, water temperature, and meteorological data were averaged

280

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Temperature section across the eilst Tsushima channel along line B in Fig. 1 on 27 July, 1983.

116 t

No.8310\I

%N0.8410 "INo.8305

Fig. 3. The track of typhoons during the study periods. Typhoons are identified by 4 figurcs: the first two give the year of generation; the last two give the order of generation in the year.

to obtain hourly mean values, Tidal and higher frequency fluctuations were removed from the time series of Figs 4, 5, 8, 1 1 and 12, and Fig. 9 by applying A225A24 and A25 Godin low-pass filters (GoDIN, 1972), respectively. C O A S T A L C U R R E N T IN T I l E SEA OF G E N K A I

Figure 4 shows low-pass filtered time series of the coastal current and water temperature at two levels at Sta. T2, together with atmospheric pressure and wind velocity data from the marine tower T1. Both the atmospheric pressure and wind data show the influence of two typhoons. So we first examine the response of the coastal currents to the two storms.

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Fig. 4. Observations of coastal current and temperature at Sta. T2 during summer-autumn 1983, together with atmospheric pressure and wind data at Sta. TI. No. 8305 and No. 8310 indicate typhoon numbers. The water temperature at Sta. T4 is also plotted for comparison.

S. MlZtiNOct

282

al.

Application of the Pollard-Millard model POLLARD and MILLARD (1970) put forward a model to represent the wind stress induced current in the upper mixed layer and succeeded in reproducing most of the major features of the observed inertial currents. The basic equations are: (911 - -

Tx --[V

--

Ot

Ot

(2a)

CU

p,,.h

Ov --

- -

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fu

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(2b)

cv.

where "~ = ('r.,, Zv) is the wind stress, p,, is tile water density, h is the mixed layer depth. and c -I is the e:folding time, which represents the dispersion effect by introducing a damping factor of the form exp(-ct). These equations were integrated forward in time by using a R u n g e - K u t t a 4th-order scheme and updating the wind stress every hour. The resulting current data were processed using the same low-pass filter used for the observed current data. Figure 5 compares the model current with the upper (16 m) current. Numerical values of the parameters used are as follows: h = 20 m, c-~ = 4 days, and f = 8.2 × 10-5 s-~. Further, the drag coefficient (CD) of the wind stress is taken to be 1.3 × 10-3. A comparison of the model current with the observed wind vector in Fig. 4 shows that the subinertial model current veers approximately 90 ° to the right of the wind direction, i.e. to the direction of the Ekman transport, as expected. The response of the model current to the two storms reproduces well the major features of the upper current. That is, in the first storm on 16 August the model current agrees well with the upper (16 m) current but the lower (24 m) current does not respond to the storm at all, suggesting that the mixed layer is approximately 20 m deep, as assumed in the model. In the second storm on 28 September, however, the model current

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OCT.

Fig. 5. A comparison of the observed current at Sta. T2 (16 m) (solid line) and lhc PollardMillard model currcnt (dotted line).

Ohscrv~ltions in thc Tsushima channel

283

is about twice as fast as the obscrvcd one. Further, there is an apparent response to thc 28 September storm at the 24 m level. This suggests that the mixed layer becomes much deeper in autumn because of cooling. Thus, although the model with a fixed /1 can illustrate the major qualitative features of the storm response, we need to know the variation of h to predict the current in the upper mixed layer over a long time. Seasonal change in the coastal current Another interesting feature of the coastal current is the noticeable seasonal change in the records, particularly at the 16 m level meter. That is, the magnitude of the current markedly decreased after early September, as is evident from the stick diagram of current records in Fig. 4. To examine the seasonal change quantitatively, we divide the whole period into summer and autumn periods and compare statistics and power spectra of coastal current and wind stress for each period. The major power spectra of coastal current consist of three frequency bands: a semidiurnal tidal band, a diurnal tidal and/or inertial oscillation band, and a subtidal band with periods longer than 5 days. More than 80% of the total kinetic energy is occupied by the diurnal and semidiurnal bands. The seasonal change is found in the latter two bands but not in the semidiurnal band (Fig. 6). For each of the diurnal and subtidal coastal currents, the kinetic energy was larger in the summer period than in the autumn period by a factor of about 2, while the corresponding seasonal ratio for wind stress was ap,proximately unity (Table 2). This suggests that the seasonal change observed in the coastal current is nearly independent of wind stress. Incidentally, although the variance of the wind stress did not vary with season, a great seasonal change is seen between the two power spectra of wind stress in the low-frequency range (Fig. 7). This reflects the stability of the weather system; it was more stable during the summer than during the autumn. We now examine the air-sea heat budget because its seasonal change may cause the change of mixed layer depth. The inversion of air and sea surface temperature occurred in early September when the coastal current decreased, and subsequently the sea water

0.3 >___

Q0.2 U

zz tEL_

0 0 Fig. 6.

1 2 FREOUENCT ( C / B A T )

"~ 3

Seasonal change in the power spectrum of the coastal current between summer (solid line) and autumn (dash-dottcd line).

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lilt)

u (I21n S i)

Kim,matics o f coaslal ('l~rretttx altd wiml stress at Sin. 7"2_

v

Subtidal

Diurnal

Scmidiurnal

Total

(cnt s ')

(K.F.) (%)

(K.E.) ('Y,,)

(K.E.) ('Y,,)

(K.E.)

(a) Sulnnlcr (1024 h, 27 .luly-7 September. 1983) 16 3.0 2.6 41.5 24 2.2 (1.9 18.1 Wind -11.112 -0. I0 0.20

18 10 72

40 94.3 6%11 37 0.114 14

95.4 90.4 0.02

41) 51 7

236.11 18S.3 U.2S

(b) A u t u m n (11124 h, 311 A u g u s t - l ( ) October, 1983) 16 2.4 0.6 16.8 24 1.4 -11.3 9.4 Wind -0.14 -(1.25 0.18

I1 7 64

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89.4 811.2 0.tl3

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31) 29 211

The auluntn period of wind stress is 1024 h front 14 September to 26 October. because wind data were not obtained for several days early September. Mean, subtidal, diurnal, and scmidiurnal (K.E.] represent the kinetic energy of record m e a n ~,nd that of the frequency band <0.5 c day i, between 0.5 and 1.5 c day ~ and between 1.5 and 4.0 c day ~, respectively, and the ratio of each kinetic energy to the total kinetic energy is represented as percent. Units are cm s J for currents and dyn cm " for wind stress.

10 >--< C3 "N t_)

CD laJ Z C3

0 Fig. 7.

1 FREOUENC'r [ C / D A Y }

2

A comparison of the power spectrum of the wind stress in summer (solid line) and that in autumn ( & , s h e d - d o n e d line).

was cooled gradually (Fig. 8a). The relationship between QIt and OH,/Ot is shown in Fig. 8b, where H, is the heat content given by r

H. = 9,. Ct, I

0

d -D

Tdz

(3)

where Tis water temperature measured with 7 thermistors at T1. C, is the specific heat at constant pressure and D is the water depth at Tl. During the summer season OH,JOt, which is the daily rate of change of H,,, is mostly independent of QH, suggesting that horizontal advection of heat is much more active than the local heat input from the atmosphere through the surface, while after 7 September, when QIt becomes negative, the local air-sea heat exchange determines the heat content in the coastal area. Hence, we can assume that the summer and autumn seasons separate on 7 September. Figure 8c gives time series of the four heat exchange terms: Q.,, QB, QE and Qc- In summer Q.,. is overwhelmingly dominant, while in autumn QE becomes more significant than Q.,. This is primarily the result of a more rapid cooling of the atmosphere than of the sea surface.

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Fig. 8. Observations of heat budget through the sea surface during summer-autumn 1983 at the marine tower TI. Data are lacking between 11 and 13 September. (a) T,,, Atmospheric tcml')craturc; T,,., sea surface temperature. (b) Qtt, Net air-sea heat budget; i~tt,JOt, the daily rate of change of heat content. (c) Daily values of the four primary heat budget terms. Q,, incoming solar radiation; Qm back radiation; QI~, heat loss hy evaporation; Q,, heat loss by conduction.

Time variation of isothermal contours shows that the thermal structure is rather different before and after 7 September (Fig. 9), when vertical convection began to occur as a result of the inversion of air and water temperature. Also note that vertical mixing occurs very rapidly through the upper layer by strong wind of the two typhoons. From daily values of the heat exchange terms in Fig. 8c, the mean daily values during the summer and autumn seasons are computed and summarized in Table 3, in which the friction velocity of water (W.) and the time rate of change of the water temperature at the 16 m level of Sta. T2 are included to estimate the mixed layer depth later. W. was determined by equating the wind and water stresses (i.e. p,, u~ = p,, WL where p,, is the air density and u. is the friction velocity of air).

Estimation of mixed layer depth One-dimensional models based on the three conservation equations of heat, momentum, and turbulent kinetic energy are commonly used in modeling the variability of the upper mixed layer (NHLER and KRAUSE, 1977). Using the results in previous section, we shall apply the Niiler-Krause model to the estimation of the mixed layer depth. The

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Observations in tile Tsushima channel

Table 3.

Daily average values of heat budget terms and wind stress Q,

Qn

Ot

Q,.

QH

(cal cm : day) Summer (27 July-6 Sop. 1983) Autumn (14 Scp.-25 Oct. 1983)

351 218

35 103

102 218

-19 39

dT,,Idt

W,

(°C day I)

(crn s J)

-0.10

0.70 0.66

233 -143

equations describing the upper mixed layer on the seasonal scale are given by

W,,AT

dT., - -

-

-

dt 2m W~ we

r -

-

-

QII +

h

-

-

,

(4)

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-

2Q.,. +

ugh

Pw CI,

-

-

,

9w CpTh

(5)

where We represents the rate with which colder water is entrained across the base of the mixed layer, AT represents the temperature difference across the base of the mixed layer, ct is the thermal expansion coefficient, g is the acceleration of gravity, 7 is the extinction coefficient representing penetration of solar radiation, and m and n are parameters which take into account the effects of dissipation of turbulent kinetic energy associated with the wind stirring on the surface and the convection (n = 1, during a term of no convection). Equation (4) describes the change in the sea surface temperature, which is caused by the entrainment of colder water below the mixed layer and the heat flux through the sea surface, and equation (5) is a turbulent kinetic energy equation integrated over the mixed layer. Substitution of (5) into (4) gives

----

at

h2 L \ ug + P,; ?pY t

L

owc,,

pwc,,

]J

In the cooling period, we must solve the coupled equations (4) and (5) numerically. In the present case, however, the average value of d T d d t during the autumn period is known from the observation at Sta. T2, so that the average depth of the mixed layer during the autumn period can be estimated from (6). In the heating period, the mixed layer becomes shallower and the entrainment must cease, so that IV,, =- O. Then, by setting We = 0 and n = 1 in (5), we obtain the following equation: h =

2(mW3"lctg + Q.,.Ip,v C/,7) Q,flp,,, Cp

(7)

The Niiler-Krause model includes two empirical parameters, m and n, which must be determined by laboratory or field experiments. They selected m = 1.25 and n = 0.7 as typical values in a model test. The KRAUSE and TURNER (1967) model corresponds to In = n = 1. Using the results of observation in Table 3, we estimated the mixed layer depth during the summer and autumn period. Table 4 summarizes the estimated mixed layer depth as

.~S

S. MIZt;NO e/ al.

Tabh" 4.

Mi.red layer d(Tth (hKr and h~,h-) during the summer and IlIIIIIDlll 3"('aSOIL~"

Sulnnlcr

Autumn

y

hl
lINK

hKI

(m ')

(m)

(,]1)

(m)

(m)

11.2 11.3 11.4 11.5

28.1 23.1 20.6 19.1

31.3 26.3 23.8 22.3

4(I.2 38.8 38.11 37.5

33.6 32.8 31.2 311.8

lINK

* hKrand hNh. indicate the mixed layer depth of the Krausc-'lunlcr and Niilcr-Krause models, respectively.

a function of y for the above two models. The extinction coefficient y depends on the turbidity of water. According to IVANOFF(1977), values of 0.3-0.4 m-~ are appropriate for turbid coastal water. The Niiler-Krause and Krause-Turner models then give a mixed layer depth of approximately 20 m for the summer, and of 30 and 40 m for the autumn, respectively. The depth difference in the autumn depends on the parameter n. The FPFES makes monthly CTD observations along two fixed lines across the cast Tsushima strait. In August and even in early September the variability of the vertical profiles is very large (Fig. 10), so that it is difficult to compare the observed mixed layer depth with the results of the one-dimensional models. On the other hand, in early October the variability decreases. Furthermore, except for one profile, the estimated layer thickness agrees well with the observed ones, although it is difficult to determine which model is better. Also note from Fig. 8 that at the marine tower OHo/Otwell followed Qn in the cooling season, suggesting that the one-dimensional model is useful in the cooling period. Although there is some ambiguity in the mixed layer depth in summer, its estimated values are not inconsistent with the observed results. Because of cooling, the mixed layer

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,

,

Vertical temperature proliles in the Sea of Ocnkai during summer-aulumn 1983.

Observations in the Tsushima channel

289

depth in autumn tends to be greater than that in summer by a factor of 1.3-1.8, as suggested previously for 28 September storm event. On the other hand, the amplitude of the subtidal coastal current component corresponding to the same two periods decreases down to 0.6-0.7 (Table 2). It follows that tile mixed layer depth is approximately inversely proportional to the amplitude of the subtidal coastal current. This fact suggests that the seasonal change observed in the coastal currents may begin with the occurrence of vertical mixing due to convection that simultaneously deepens the upper mixed layer. In addition to wind stress, coastal currents are also influenced by several other factors: for example, the surface slope, horizontal density gradient and bottom friction. Since the surface slope current makes the greatest contribution to the coastal currents, we examined the seasonal variation of coastal sea level. The coastal sea level difference between Hakata and Izuhara yielded the fluctuation amplitude of 5 cm in sea level for the subtidal frequency range, which corresponds to a surface slope current of about 4 cm s-~. This implies that 40% or more of the observed coastal currents is contributed by the surface slope current. However, we could not find a seasonal variation corresponding to the observed coastal currents in the sea level difference. It seems therefore unlikely that the sea level fluctuations make a significant contribution to the seasonal change of the observed coastal currents.

OBSERVATIONS

OF T I t E T S U S t I I M A C U R R E N T

Figures 11 and 12 show low-pass filtered time series of the Tsushima Current and water temperature at Sta. T4 in 1983 and 1984, respectively, together with those of air pressure and wind velocity at the marine tower T1 and of sea surface temperature at lzuhara. During the study periods two typhoons had great influence on the Tsushima Current in very different ways. Before discussing the storm response, we describe a general feature of the Tsushima Current during the summer season. Table 5 summarizes the statistics of the Tsushima Current and Fig. 13 shows the power spectra of the Tsushima Current in 1983. The east Tsushima Current is of the order of 30 cm s-~ and flows consistently east northeastward or northeastward. The major kinetic energy of the current consists of the record-mean, diurnal and semidiurnal tide, each of which occupies approximately 30% of the total kinetic energy, the remaining 10% being in the subtidal current. The strongest mean current in 1983 was observed at the 80 m level rather than at the upper 50 m level, although the sum of the record-mean and subtidal kinetic energy was nearly equal at the two depths. Vertical shear of the Ecomponent was, on an average, 0.4 x 10-2 s-l between 80 and 110 m levels, but nearly negligible between the 50 and 80 m levels, while the N-component of the current was nearly constant with depth. The Tsushima Current measured at the 80 m level was somewhat weaker in 1984 than in 1983, but the one at the 110 m level was nearly the same for the two summer periods, indicating that the year-to-year variability in the nearbottom current is relatively small. The results for 1983 agree well with current measurements made near Sta, T4 of the east channel in autumn period by SlaM et al. (1984), who obtained a vertical shear of 0.36 x 10-2 s-t between 66 and 108 m and much smaller shear between 25 and 66 m depths. The CTD observations at the deployment cruise showed the existence of cold 15°C bottom water north of Sta. T4 (Fig. 2). Since vertical shear is balanced by horizontal temperature gradient in the thermal wind relation, this

,',;. MlztINl> ~'! a/.

290

1020 I010 i000 990 NIND

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'83

1 AUG.

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Fig. 11. O b s e r v a t i o n s o f t h e T s u s h i m a C u r r e n t a n d t e t r a p e r a t u r e at S t a . T 4 d u r i n g s u m m e r 1983, t o g e t h e r w i t h a t m o s p h e r i c p r e s s u r e a n d w i n d d a t a at Sta. T I . T h e w a t e r t e m p e r a t u r e at I z u h a r a ( T s u s h i m a I s l a n d ) is a l s o p l o t t e d as a m e a s u r e o f the s e a s u r f a c e t e m p c r a t u r c .

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Same as in Fig. 11 except for 1984.

Table 5.

252.1 176.4

370.0 462.0 190.2 29 28

33 42 33

Mean (K.E.) (%)

122.4 39.8

126.0 68.8 34.0 14 6

11 6 6

Subtidal (K.E.) (%)

326.5 203.4

315.1 309.4 171.7

37 33

28 28 29

Diurnal (K.E.) (%)

Statistics of Ts.shima Current at Sta. T4

174.0 199.7

305.6 261.3 181.9

211 32

_,"" 24 31

Semidiurnal (K.E.) I°o)

879.9 62(I.7

11_4.5 1106.7 583.5

Total (K.E.)

* This current meter has some trouble. For the details see section on Deployment and Data Analysis. Frequency bands and units are the same as those in Table 2.

24 40

0 (o)

(b) 1024 h (14 July-25 August, 1984) 80* 20.6 9.0 110 14.4 12.0

v (cms-i) 24 27 40

u ( c m s i)

(a) 1024 h (27 July-7 September, 1983) 50 24.8 I1.1 80 27.1 13.7 110 14.9 12.6

Depth (m)

293

Observations in the T s u s h i m a channel @ .--.

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Power spectra of the T s u s h i m a Current in s u m m e r 1983 at three levels of Sta. T4: 50 m. dotted Fine; 80 m, solid line; 110 m, dotted-broken line.

cold bottom water may play an important role in the formation of the strong shear near the bottom. The current magnitude obtained in summer by MIITA and OGAWA (1984), who used current data measured over 1 or 2 days, is larger than the present one-month average values by a factor of 1.3-1.6. Since the current measurements were made at almost the same location, the difference may be attributed to the variability of the Tsushima Current. The current direction rotated anticlockwise with depth from eastnortheast near the surface to northeast in deeper layers, and the near-bottom current was approximately aligned along the local depth contours. MIITA and OGAWA(1984) obtained similar results by the tracking of drifters set at two different depths. It is worthwhile to note that the anticlockwise rotation of the current direction with depth is associated with northward transport of heat, as is evident from Fig. 7.10 of GILL (1982). STORM

RESPONSE

As clearly visible in the current-vector plots, there are abrupt changes in the Tsushima Current in response to storms. The most remarkable event occurred with approach of Typhoon 8305, which caused a strong northerly wind over the Tsushima strait. Due to this northerly wind, the E-component of the Tsushima Current increased by a factor of 2 from the top to the bottom level during the stormy weather. Note that the storm response of the Tsushima Current is much greater than that of the coastal current observed in the Sea of Genkai (Figs 4 and 11). After the storm passed the middle and bottom currents returned to their original values, but the east component of the uppermost current transiently dropped, as shown in Fig. 11. The time-varying pattern of the Tsushima Current under the influence of Typhoon 8305 is more clearly displayed by a progressive vector diagram in Fig. 14, which shows that the uppermost current deviates from its regular course for about 5 days after the storm left. Another noticeable response to the 8305 storm is the abrupt change in the water temperature at the 50 m level (i.e. the temperature increased by about 5°C during the

2(;4

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0

D=50M

E 40 ]KM

200KM

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storm, Fig. 11). By comparing the temperature at T4 (50 m) with the N-component of the current at the same level, we conclude that this warm water was carried by a southward current. Accordingly, it must come from the coastal region of Tsushima Island, suggesting that the northerly wind caused the net Ekman transport toward Tsushima Island and that downwelling of warm water occurred along the coast of the island. It follows from the above arguments that the current drop occurring after the storm at the 50 m level of T4 was caused by the presence of coastal warm water. A n o t h e r storm occurred on 21 August 1984, when Typhoon 8410 passed over the Tsushima strait. The Tsushima Current rapidly decreased to nearly zero with the passage of the typhoon (Fig. 12), a situation which remained for several days after the typhoon left. The temperature at the 50 m level and the sea surface temperature at Izuhara both decreased by 2-3°C after the passage of the typhoon. It is well known that such a temperature drop can occur as a result of the baroclinic response of the ocean to a moving s t o r m (GEISLER, 1970; PRICE, 1981; G I L L , 1982). Thus, the Tsushima Current was apparently accelerated in the first storm but decelerated during the second typhoon. According to CSANADY (1981), a primary driving mechanism for the current field over the shelf in a storm event is the longshore wind-stress impulse, defined by ly = f~) Xy dt/ p,,,, where y is the coordinate parallel to the shoreline. If bottom friction is negligible, it equals the depth-integrated m o m e n t u m of the water column. By assuming the northeast direction to be nearly parallel to the shoreline, we estimated /v from the wind data (counting northeastward impulse as positive)

ly

=

-2.0 x 105 cm 2 s-t

for the first event,

ly

=

1.6 X 105 cm 2 s-1

for the second event.

If a water depth is taken to be 100 m, we obtain the longshore current of v = -20 and 16 cm s-I, for the first and second events, respectively, which are of the same order of magnitude as the variation of the current observed during the two storm events. However, the longshore current observed during the first event increased against the

Observations in the Tsushima channel

295

direction of the local wind, so that it would be difficult to explain the storm response of the Tsushima Current simply by the wind stress impulse. A n o t h e r candidate is a compensating return current of the Ekman transport, given by u = zs/p,,.fH. Using the same water depth as above, we find a cross-shore current estimate o f u ~ 3 cm s -~, which is one order of magnitude less than the observed currents. A n o t h e r possibility is that atmospheric disturbances may cause the Tsushima Current to change its regular course, and that most of the Tsushima Current may flow temporarily through either of two channels east and west of Tsushima Island. H o w e v e r , it is difficult to examine this from a single point mooring. At the present stage of investigation, it is found that the Tsushima Current is unexpectedly influenced by a storm, but the response of the Tsushima Current is too complicated to be understood from the current data of a single mooring station. As a next step, it is desirable to make simultaneous measurements in the east and west channels. Acknowledgements--We wish to thank Prof. H. Mitsuyasu, chairman of a special project entitled "'Observations and Simulations studies of Marine Environments", for his encouragement and critical reading of the first draft, and also thank both Prof, K. Takano and a referee for critical reading and valuable commments to the draft. We arc also indebted to many members of Physical Oceanography and Ocean Technology groups of the Research Institute for Applied Mechanics, especially to Messrs T. Nagahama, M. Ishibashi, T. Shinozaki and A. Tashiro for much help in the data analysis as well as through various phases of the field work. Special ~cknowlcdgcmcnt is given to the co-operation of the staffs of Fukuoka Prefectural Fisheries Experimental Swtion for providing data used in the research and allowing its R.V. Genkai to be used for the deployment and rccovcry of the current meters and to the ofliccrs and crew of Genkai for their assistance on board. This work is part of the above special project supported by the Ministry of Education, Japan. REFERENCES BOWDEN K. F. (1977) Heat budget considerations in the study of upwelling. A Voyage of Discoveo,, George Deacon 70th Anniversary Volume, Supplement to Deep-Sex Research, M. ANGEL, editor, pp. 277-29(I. BUDYKO M. I. (1973) Clbnate and life, Japanese edition translated by Z. UCttIJ]MA and B. IWAKIRI, Tokyo University Press, 488 pp. CSANAD'¢ G. T. (1981) Circulation in the coastal ocean. In: Advances in geophysics, Vol. 23, B. S/,t,'rZMANN, editor, pp. 1{11-183. GHSLER J. E. (1970) Linear theory of the response of a two-layer ocean to a moving hurricane. Geophysical Fhdd Dynamics, !, 249-272. GILL A. E. (1982) Atmosphere-Ocean dynamics, Academic Press, New York, 662 pp. GODIN G. (1972) The analysis of tides. University of Toronto Press, Canada, 264 pp. IVANOFF A. (1977) Oceanic absorption of solar energy. In: Modelling and prediction of the upper layers of the ocean, E. B. KRAUS, editor, Pergamon Press, Oxford, pp. 47-71. KRAUSE E. B. and J. S. TURNER (1967) A one-dimensional model of the seasonal thermoclinc--ll. 3"he general theory and its consequences. Tellus, 19, 98-106. MIrIA T. and Y. O(;AWA (1984) "l'sushinla Currents measured with current mctcr~ and drifters. In: Ocean hydrodynami~w oJ" the Japall ~md East ('himl seas, "F. ]('IIIE, editor. Elsevier Oceanography Series, pp. 67-76. NIII,I~R P. P. and E. B. KRAUSl- (1977) One-dimensional models of the upper ocean. In: Modelling and prediction of the upper layers of the ocean, E. B. KRAUS,editor, Pergamon Press, Oxford, 143-172. POI,I_ARD R. T. and R. C. MILLARD, Jr (1970) Comparison between observed and simulated wind-generated inertial oscillations. Deep-Sea Research, 17,813-821. Pl{IcL J. F. (1981) Upper ocean response to a hurricane. Journal of Physical Oceanography, 1I, 153-175. SlIIM T., W. J. WISEMAN,Jr, O. K. HUH and W.-S. CIIUNG (1984) A test of the gcostrophic approximation in the Western channel of the Korea strait. In: Ocean hydrodynamics of the Japan and East China seas, T. to'HIE, editor, Elsevier Oceanography Series, pp. 263-272.