- Email: [email protected]
Tsushima Currents Measured with Current Meters and Drifters
67
TSUSHIMA CURRENTS MEASURED WITH CURRENT METERS AND DRIFTERS
T. MIITA' and Y. OGAWA2 'Fukuoka P r e f e c t u r a l F i s h e r i e s E x p e r i m e n t a l S t a t i o n , Fukuoka 819-01 (Japan) 2Yamaguchi P r e f e c t u r a l Open-Sea F i s h e r i e s E x p e r i m e n t a l S t a t i o n , Nagato 759-41 (Japan)
ABSTRACT C u r r e n t measurements f o r a p e r i o d more t h a n one day were c a r r i e d o u t a t 431 s t a t i o n s between 1920 and 1974 i n t h e Tsushima S t r a i t o u t s i d e t h e c o a s t a l areas. The mean c u r r e n t i s determined f r o m t h e d a t a f o r each s t a t i o n l a s t i n g one day o r more. I t s h o r i z o n t a l d i s t r i b u t i o n i s shown f o r each season. The v e r t i c a l d i s t r i b u t i o n o f the mean c u r r e n t a t s e v e r a l s e c t i o n s i s sought. The volume t r a n s p o r t i s determined. Seasonal changes o f t h e c u r r e n t a t two p o i n t s i n t h e s t r a i t a r e a l s o presented. Experiments were p e r f o r m e d on t r a c k i n g c u r r e n t drogues by S e i k a i Regional F i s h e r i e s Research L a b o r a t o r y t o g e t h e r w i t h Yamaguchi, Fukuoka, Saga, and Nagasaki P r e f e c t u r a l F i s h e r i e s Experimental S t a t i o n s f r o m 1973 t o 1976 f o r s e v e r a l days each t i m e . The t r a j e c t o r i e s of d r i f t e r s a r e determined. Space and t i m e v a r i a t i o n s o f the d r i f t i n g v e l o c i t i e s are described. INTRODUCTION The Tsushima S t r a i t i s t h e o n l y passage c o n n e c t i n g t h e E a s t China Sea and t h e Japan Sea. I t occupies a key p o i n t t o d e t e c t t h e c h a r a c t e r i s t i c s o f t h e Tsushima Current f l o w i n g t h r o u g h i t . Some d i r e c t measurements o f t h e c u r r e n t have been i n t r o d u c e d u n t i l now a t t h e Tsushima S t r a i t . M i i t a r e p o r t e d i n 1976 on t h e s t r u c t u r e and t h e t r a n s p o r t o f t h e c u r r e n t depending on t h e r e s u l t s o f E u l e r i a n measurement. Ogawa e t a l . commented i n 1978 on t h e v a r i a t i o n o f t h e c u r r e n t on t h e b a s i s o f t h e r e s u l t s o b t a i n e d by Lagrangian measurement. I n t h e p r e s e n t r e p o r t t h e main p a r t o f these two papers w i l l be reviewed w i t h some m o d i f i c a t i o n s . MEASUREMENTS WITH CURRENT METERS M a t e r i a l s and methods C u r r e n t s were measured f o r a p e r i o d l o n g e r t h a n one day a t 431 s t a t i o n s between 1920 and 1974 i n t h e Tsushima S t r a i t o u t s i d e t h e c o a s t a l a r e a s . See F i g . 1 and Table 1. I n t h e t a b l e , A r e p r e s e n t s H y d r o g r a p h i c Department o f Japan I m p e r i a l Navy; B, H y d r o g r a p h i c Department o f Japan M a r i t i m e S a f e t y Agency; C, Nagasaki Marine M e t e o r o l o g i c a l Observatory; D, Fukuoka P r e f e c t u r a l F i s h e r i e s Experimental S t a t i o n ; E, S e i k a i Regional F i s h e r i e s Research L a b o r a t o r y ; F, F i s h e r i e s Research
68
34
32
F i g . 1. S t a t i o n s o f d i r e c t c u r r e n t measurements.
TABLE 1 S t a t i o n s o f d i r e c t c u r r e n t measurements Period
1920-1929 1930-1939 1940-1949 1950-1959 ditto ditto 1960-1969 ditto ditto ditto 1970-1974 ditto ditto ditto
Total
%
I
~
Or A A A
B
C D B E D F B C D F
Seasons
Stns
7 47 154 2 4 2 105 4 8 6 38 14 4 36
1
431 100
33 8
1 10 2
1 14 30
1
3 23 115 2 3 1 87
17 4 1 1
4 2
2 2 21
14 3 91 21
9
4
268 62
2 7 1
1 1 1 17
5
1
2
1
4 1 1
1 103 3 38
1
9
2
1 1 2
39 9
148 34
11 3
Number o f l a y e r s 3 4 5
3 14
13 13
1 1
1
3 1
5 4
1
9 4 3 4 2 2
6
39 9
62 14
12 64 1 2
86 20
6
7
4 58 1 2
16 4
65 15
20 5
69 and Development Agency, Korea. Most o f t h e measurements were made w i t h t h e EkmanMertz c u r r e n t m e t e r s . Moored s e l f - r e c o r d i n g c u r r e n t meters were deployed f o r several days a t some s t a t i o n s . The number o f o b s e r v a t i o n s r i s e s t o t h e h i g h e s t i n summer when m e t e o r o l o g i c a l c o n d i t i o n s a r e most f a v o u r i t e f o r measurement. R e s u l t s o f t h e a f o r e m e n t i o n e d measurements were processed by means o f a harmonic a n a l y s i s , assuming t h e c u r r e n t s c o n s i s t i n g o f s e m i - d i u r n a l t i d e , d i u r n a l t i d e , and t h e c o n s t a n t c u r r e n t . The l a t t e r was d e t e r m i n e d as an average f l o w o v e r t h e d u r a t i o n o f each measurement. Several c u r r e n t p a t t e r n s t o be shown l a t e r were o b t a i n e d on t h e b a s i s o f t h e c o n s t a n t c u r r e n t . The c o n s t a n t c u r r e n t d a t a were grouped i n t o f o u r c l a s s e s a c c o r d i n g t o seasons. Horizontal d i s t r i b u t i o n o f c u r r e n t vector I n o r d e r t o seek t h e h o r i z o n t a l d i s t r i b u t i o n o f t h e c u r r e n t we d i v i d e t h e d a t a i n t o t h r e e p a r t s a c c o r d i n g t o t h e depth: s u r f a c e , depth l e s s than 10 m; medium, between 25 and 7 5 m; and bottom, 10 m f r o m t h e bottom. The s u r f a c e d a t a i n summer are shown i n F i g . 2 , f o r i n s t a n c e . I n a r e g i o n f r o m t h e west o f t h e Goto I s l a n d s t o t h e Japan Sea t h r o u g h t h e Tsushima S t r a i t , c u r r e n t d i r e c t i o n s a r e f r o m west o r southwest t o e a s t o r n o r t h e a s t . T h i s d i s t r i b u t i o n i m p l i e s t h a t t h e n o r t h e a s t w a r d c u r r e n t s e x i s t a whole y e a r . The speed i s s t r o n g i n summer and weak i n w i n t e r . The c u r r e n t d i r e c t i o n d e v i a t e s s l i g h t l y t o n o r t h i n summer, b u t i t d e v i a t e s t o
34O
32
2
--L
126
0.5 K t > 0.5 -1.0 1.0 c
128
F i g . 2 . C u r r e n t v e c t o r s a t s u r f a c e i n summer.
130'
70 e a s t i n w i n t e r . Such a seasonal d i f f e r e n c e o f t h e speed and d i r e c t i o n o f t h e current d i m i n i s h e s w i t h t h e depth o f t h e sea. I n t h e b o t t o m c u r r e n t seasonal changes are h a r d l y observed. S e c t i o n a l d i s t r i b u t i o n o f speed L e t us examine t h e d i s t r i b u t i o n o f speed normal t o s e v e r a l s e c t i o n s . I n t o t a l , t h i r t e e n s e c t i o n s were chosen i n t h e n o r t h w e s t r e g i o n o f Kyushu, as shown i n F i g . 1. Most o f t h e d a t a were o b t a i n e d i n summer o f 1942 and 1943 by t h e Japan I m p e r i a l Navy. To supplement these, t h e o t h e r d a t a , which were c o l l e c t e d by o t h e r i n s t i t u t i o n s a t d i f f e r e n t t i m e s , were a l s o used. Number o f l a y e r s , i n which measurements were made, ranged f r o m f i v e t o seven a t each s t a t i o n . KAWAZl R IMlSAKl
LG I
150
0
!
rn
50
100
150
,
2
p
,
w
D Y N A M I C CALCULATION
F i g . 3. D i s t r i b u t i o n o f speed a t s e c t i o n I . ( a ) E u l e r i a n measurement. ( b ) Dynamic calculation.
71
A r e s u l t on s e c t i o n I , f o r i n s t a n c e , i s shown i n t h e upper p a r t o f F i g . 3. The s e c t i o n I l o c a t e s a t t h e e n t r a n c e o f t h e Japan Sea, between Cape K a w a j i r i o f Japan and U l g i o f Korea. The b o t t o m topography i s c o m p a r a t i v e l y f l a t . The h o r i z o n t a l d i s t a n c e i s about 180 km (100 n a u t i c a l m i l e s ) . The c u r r e n t d i s t r i b u t i o n on t h i s s e c t i o n has c h a r a c t e r i s t i c s d i f f e r e n t f r o m t h o s e o f t h e o t h e r s e c t i o n s . Southward c u r r e n t spreads o v e r a c o n s i d e r a b l e wide range i n t h e m i d d l e o f s e c t i o n , and northward c u r r e n t e x i s t s on b o t h s i d e s o f t h e m i d d l e s e c t i o n . The maximum and the average speed o f t h e west s i d e a r e r e s p e c t i v e l y about 80.0 and 34.8 cm/s (0.41 and 0.18 k n o t ) . On t h e e a s t s i d e t h e y a r e 40.0 and 17.8 cm/s (0.21 and 0.092 k n o t ) , which a r e a p p r o x i m a t e l y a h a l f o f t h e speed on t h e west s i d e . We e v a l u a t e d t h e b a r o c l i n i c g e o s t r o p h i c c u r r e n t d i s t r i b u t i o n o f t h e s e c t i o n I . Dynamic c a l c u l a t i o n was c a r r i e d o u t by u s i n g t h e h y d r o g r a p h i c o b s e r v a t i o n d a t a o f August between 1937 and 1941, which were c o l l e c t e d by t h e F i s h e r i e s Experimental S t a t i o n of t h e Government-General o f Korea. A r e f e r e n c e l a y e r was taken a t t h e bottom. R e s u l t s a r e g i v e n i n t h e l o w e r p a r t o f F i g . 3. A c l o s e resemblance e x i s t s between t h e d i s t r i b u t i o n p a t t e r n s o f t h e measured and t h e c a l c u l a t e d speeds. However, magnitude o f speed a r e d i f f e r e n t . The c u r r e n t speed f r o m t h e measured data i s i n e v i t a b l y l a r g e r t h a n t h e one determined w i t h t h e dynamic c a l c u l a t i o n . Volume t r a n s p o r t The volume t r a n s p o r t i s used as an i n d e x f o r t h e i n t e n s i t y o f t h e c u r r e n t . I n t h e p r e s e n t paper i t i s sought f r o m t h e c u r r e n t d i s t r i b u t i o n i n t h i r t e e n sections according t o 36 O
34
Unit
126
Fig. 4. Volume t r a n s p o r t .
Sv
106 m3
128
130°
32
72
T=C ViSi, where summation i s c a r r i e d o u t on i, T i s t h e volume t r a n s p o r t , Vi speed, and Si
t h e area occupied by Vi.
the current
R e s u l t s a r e shown i n F i g . 4. Northward
t r a n s p o r t a t each s t a t i o n f r o m t h e w e s t o f t h e Goto I s l a n d s A t o t h e Tsushima Strait
H ranges f r o m 3.3 t o 3 . 7 Sv ( 1 Sv=lO6 m3/ s ) .
However, on t h e s e c t i o n I between Cape K a w a j i r i and U l g i , on a c c o u n t o f l a r g e t r a n s p o r t a t t h e west s i d e c u r r e n t a x i s , t o t a l volume t r a n s p o r t i s 4.2 Sv, which i s a p p r e c i a b l y g r e a t e r t h a n t h e v a l u e s o b t a i n e d a t t h e o t h e r s e c t i o n s f r o m A t o H.
A t p r e s e n t a u t h o r s do n o t g e t any d e f i n i t e c o n c l u s i o n s as t o whether t h i s discrepancy i n d i c a t e s a c h a r a c t e r i s t i c s of t h e r e g i o n o r i s b r o u g h t by t h e process o f t h e
WINTER
SPRING
SUMMER
AUTUMN
5m 20 m 50 m
100 m
-F WINTER
SPRING
SUMMER
0
0.5
1.0
1.5 Kt
F i g . 5 . Seasonal change o f c u r r e n t v e c t o r s .
AUTUMN
73
d a t a h a n d l i n g . The dynamic c a l c u l a t i o n s (Suda, 1938; L i , 1966) were c a r r i e d o u t b e f o r e . A l l o f t h e p r e v i o u s and t h e p r e s e n t r e s u l t s o f t h e dynamic c a l c u l a t i o n produced t h e volume t r a n s p o r t and t h e speed s m a l l e r t h a n t h o s e determined f r o m t h e d i r e c t measurements. A c t u a l l y a b a r o t r o p i c e f f e c t may e x i s t . Seasonal change o f c u r r e n t i n t h e e a s t and t h e west channels Up t o now t h e steady s t a t e o f t h e Tsushima C u r r e n t i s c o n s i d e r e d . We know t h a t a r e a l s t a t e o f t h e Tsushima C u r r e n t changes i n t i m e and space. Our f i n a l goal i s t o c l a r i f y t h e v a r i a t i o n o f t h e Tsushima C u r r e n t and t o e s t a b l i s h models The measured d a t a a r e n o t s u f f i c i e n t t o d i s c u s s v a r i a t i o n s o f t h e c u r r e n t as a whole. Thus v a r i a t i o n s o f t h e c u r r e n t w i l l be d i s c u s s e d a t two s t a t i o n s , where the d a t a o f f o u r seasons e x i s t . One s t a t i o n i s l o c a t e d a t t h e m i d d l e o f t h e s e c t i o n FE (34'04"
139'32'E)
of
the e n t r a n c e o f t h e e a s t channel and l i e s on t h e a x i s o f t h e c u r r e n t . As i s seen from t h e upper p a r t o f F i g . 5, t h e c u r r e n t d i r e c t i o n i s a l m o s t n o r t h e a s t w a r d . I t s h i f t s t o n o r t h a t t h e deep l a y e r . Seasonal v a r i a t i o n i s remarkable a t t h e upper l a y e r and becomes weak a t t h e deep l a y e r . Another s t a t i o n i s l o c a t e d on t h e m i d d l e o f t h e s e c t i o n HW (34'52"
129'15'E)
of t h e west channel and l i e s on t h e a x i s which shows t h e s t r o n g e s t c u r r e n t . S i m i l a r t o t h e e a s t channel t h e c u r r e n t d i r e c t i o n i s a l m o s t n o r t h e a s t w a r d . The speed i s s t r o n g a t t h e upper l a y e r and weak a t t h e bottom. Seasonal v a r i a t i o n i s remarkable a t t h e upper l a y e r . MEASUREMENT WITH DRIFTERS M a t e r i a l s and methods C u r r e n t v e l o c i t i e s were measured w i t h drogues f r o m 1973 t o 1976 around t h e Tsushima S t r a i t i n c o - o p e r a t i o n w i t h f i v e r e g i o n a l f i s h e r i e s r e s e a r c h i n s t i t u t i o n s . The l o n g e s t t r a c k i n g was p e r f o r m e d f o r 98 hours i n summer. The s h o r t e s t t r a c k i n g h a r d l y reached 24 hours i n w i n t e r because o f a bad weather c o n d i t i o n . C o n s t r u c t i o n o f t h e c u r r e n t drogue system i s shown i n F i g . 6. The drogue used was made f r o m wood. Most o f t h e drogues were p l a c e d a t a nominal depth o f 5 m. D u r i n g each r u n o f t h e t r a c k i n g , t h e p o s i t i o n o f t h e drogue was determined a t an i n t e r v a l o f 1 hour between l a u n c h i n g and r e c o v e r y o f t h e drogue. The p o s i t i o n o f t h e drogue on each r u n was f i x e d w i t h Oecca, Loran, and Radar w i t h accuracy o f 0.05 n a u t i c a l m i l e s ( 9 3 m). R e s u l t s on t h e a r e a n o r t h o f t h e Goto I s l a n d s Four drogues launched on t h e 1 s t o f June i n 1975 were t r a c k e d f o r 67 hours t i l l bad weather c o n d i t i o n s began t o a b o r t t h e experiment. As i s shown i n F i g . 7, t h e movement o f t h e drogues was t o n o r t h e a s t o r n o r t h w i t h a mean speed f r o m 0.09 t o 0.65 k n o t (4.63 t o 33.4 cm/s). There i s a tendency t h a t v e l o c i t i e s o f
74
p FLAG f
LIGHT
FLOAT
n
V
I7
'
1
U
TSUSHIM,
*12 cm
n F i g . 6. C o n s t r u c t i o n o f a drogue.
128
129"
F i g . 7. T r a j e c t o r i e s o f drogues i n t h e area n o r t h o f t h e Goto I s l a n d s i n t h e west channel o f t h e Tsushima S t r a i t i n summer. t h e drogues i n c r e a s e as t h e y l e a v e t h e Goto I s l a n d s . T h i s i n d i c a t e s t h e presence o f a h o r i z o n t a l shear. The shear i s e s t i m a t e d a t about 0.01 k n o t (0.51 cm/s) p e r n a u t i c a l m i l e on an average. T r a j e c t o r i e s o f t h e drogues i n d i c a t e a k i n d o f "trochoidal curves", suggesting t h a t f l u c t u a t i o n s o f i n e r t i a o s c i l l a t i o n s are superimposed on t h e mean f l o w . The p a t t e r n of t h e t r a j e c t o r i e s , however, i s n o t always s i m i l a r t o each o t h e r . R e s u l t s on t h e e a s t channel o f t h e Tsushima S t r a i t A f t e r t h e t r a c k i n g s a t t h e n o r t h o f t h e Goto I s l a n d s , t h r e e drogues were s e t i n t h e e a s t channel o f t h e Tsushima S t r a i t on t h e 7 t h o f June i n 1975. As i s shown i n F i g . 8, t h e movement of t h e drogues was t o e a s t o r n o r t h e a s t d u r i n g 48 hours when t r a c k i n g s were c o n t i n u e d w i t h a mean speed f r o m 0.54 t o 0.90 k n o t (27.8
t o 46.3 cm/s). S i m i l a r t e n d e n c i e s t o those observed a t t h e n o r t h o f t h e
Goto I s l a n d s were a l s o r e c o g n i z e d . On t h e 2 0 t h of November i n 1975, a drogue was launched a t about m i d d l e p o i n t o f t h e e a s t channel and was f o l l o w e d f o r 51 h o u r s . As i s shown i n F i g . 9, t h e drogue moved t o e a s t o r n o r t h e a s t w i t h a mean speed o f 0.48 k n o t (24.7 cm/s). The speed i s l o w e r t h a n i n summer. The d i r e c t i o n i s a l m o s t t h e same as i n summer,
75
-..
70h 3 5"
T S U S H l MA
34
130
131'
Fig. 8. T r a j e c t o r i e s of drogues i n t h e Tsuahima S t r a i t and i n t h e area e a s t o f the s t r a i t i n summer.
I : 35"
34
130
131'
F i g . 9. T r a j e c t o r i e s o f drogues i n t h e Tsushima S t r a i t and i n t h e area e a s t o f the s t r a i t i n w i n t e r . Results on t h e e a s t e r n area o f t h e Tsushima S t r a i t I n t h e a r e a e a s t o f t h e Tsushima S t r a i t , a s i n g l e measurement was r e p e a t e d i n t e r m i t t e n t l y f r o m 1973 t o 1976. See F i g s . 8 and 9. Behaviour o f t h e drogues seems t o show a f a i r l y d i f f e r e n t f l o w p a t t e r n f o r each r u n e s p e c i a l l y i n t h e nearshore waters. Most o f r u n s were made f o r t o o s h o r t a d u r a t i o n t o r e v e a l d e t a i l s . However, i t seems n o t e w o r t h y t h a t a c o n t i n u o u s southwestward movement o v e r 23 hours occured
i n w i n t e r , which i m p l i e s a n o t h e r change i n f l o w from t i d a l c u r r e n t .
76
TSUSHlMA
Srn
K
Y
129
34.
51h
130'
F i g . 10. T r a j e c t o r i e s o f deep c u r r e n t drogues i n t h e Tsushima S t r a i t i n comparison w i t h those o f s u r f a c e drogues. V a r i a t i o n o f c u r r e n t v e l o c i t y w i t h depth I n t h e e a s t channel o f t h e Tsushima S t r a i t t h e R / V Yokomaru t r a c k e d a drogue launched a t a d e p t h o f 80 m c l o s e t o t h e b o t t o m i n November o f 1975, when t h e R / V Kuroshiomaru f o l l o w e d t h e s u r f a c e drogue launched a t a d e p t h o f 5 m. As i s
shown i n F i g . 10, c u r r e n t v e l o c i t i e s a t t h e deep d e p t h a r e s l i g h t l y l o w e r t h a n those o f t h e s u r f a c e . One o f two r u n s made i n t h i s a r e a i n d i c a t e s a c o n s i d e r a b l e d i f f e r e n c e i n d i r e c t i o n s o f movements o f t h e two drogues. The d i r e c t i o n o f t h e deep drogue d e f l e c t e d a b o u t 22 degrees n o r t h w a r d f r o m t h a t o f t h e s u r f a c e . However, i n t h e o t h e r r u n which was made a t t h e n o r t h e a s t o f t h e f o r m e r , t h e deep drogue showed t h e same f l o w p a t t e r n as t h a t o f t h e s u r f a c e drogue. CONCLUDING REMARKS We showed t h e d a t a o f t h e Tsushima C u r r e n t measured w i t h c u r r e n t meters and d r i f t e r s . The d a t a used so f a r were c o l l e c t e d w i t h c o m p a r a t i v e l y s h o r t d u r a t i o n s , which were t h e o r d e r o f s e v e r a l days a t t h e l o n g e s t . Long range v a r i a t i o n and s y n o p t i c s t r u c t u r e o f t h e Tsushima C u r r e n t a r e n o t y e t c l a r i f i e d . We hope t h a t w e l l - p l a n n e d c o n t i n u o u s measurement w i l l be made i n f u t u r e o v e r l o n g range o f t i m e i n wide a r e a . REFERENCES L i , Sok-U, 1966. Seasonal and S e c u l a r V a r i a t i o n s o f t h e Water Volume T r a n s p o r t across t h e Korean S t r a i t , J o u r . Oceanol. SOC. Korea 1 ( 1 - 2 ) . M i i t a , T., 1976. C u r r e n t c h a r a c t e r i s t i c s measured w i t h c u r r e n t m e t e r a t t h e f i x e d s t a t i o n ( i n Japanese), B u l l e t i n Japan SOC. F i s h . Oceanogr. 28: 33-58. Ugawa, Y . - M i i t a , T., I c h i h a r a , A., Hasegawa, Y . , and Inoue, N., 1978. F l u c t u a t i o n o f t h e Tsushima C u r r e n t Measured w i t h t h e C u r r e n t Drogue, B u l l e t i n o f t h e S e i k a i Regional F i s h e r i e s Research L a b o r a t o r y 51: 13-44. Suda, K., 1938. Annual V a r i a t i o n o f t h e Tsushima C u r r e n t ( i n Japanese), Limnol. Mag. 8: 205-215.
TSUSHIMA CURRENTS MEASURED WITH CURRENT METERS AND DRIFTERS
T. MIITA' and Y. OGAWA2 'Fukuoka P r e f e c t u r a l F i s h e r i e s E x p e r i m e n t a l S t a t i o n , Fukuoka 819-01 (Japan) 2Yamaguchi P r e f e c t u r a l Open-Sea F i s h e r i e s E x p e r i m e n t a l S t a t i o n , Nagato 759-41 (Japan)
ABSTRACT C u r r e n t measurements f o r a p e r i o d more t h a n one day were c a r r i e d o u t a t 431 s t a t i o n s between 1920 and 1974 i n t h e Tsushima S t r a i t o u t s i d e t h e c o a s t a l areas. The mean c u r r e n t i s determined f r o m t h e d a t a f o r each s t a t i o n l a s t i n g one day o r more. I t s h o r i z o n t a l d i s t r i b u t i o n i s shown f o r each season. The v e r t i c a l d i s t r i b u t i o n o f the mean c u r r e n t a t s e v e r a l s e c t i o n s i s sought. The volume t r a n s p o r t i s determined. Seasonal changes o f t h e c u r r e n t a t two p o i n t s i n t h e s t r a i t a r e a l s o presented. Experiments were p e r f o r m e d on t r a c k i n g c u r r e n t drogues by S e i k a i Regional F i s h e r i e s Research L a b o r a t o r y t o g e t h e r w i t h Yamaguchi, Fukuoka, Saga, and Nagasaki P r e f e c t u r a l F i s h e r i e s Experimental S t a t i o n s f r o m 1973 t o 1976 f o r s e v e r a l days each t i m e . The t r a j e c t o r i e s of d r i f t e r s a r e determined. Space and t i m e v a r i a t i o n s o f the d r i f t i n g v e l o c i t i e s are described. INTRODUCTION The Tsushima S t r a i t i s t h e o n l y passage c o n n e c t i n g t h e E a s t China Sea and t h e Japan Sea. I t occupies a key p o i n t t o d e t e c t t h e c h a r a c t e r i s t i c s o f t h e Tsushima Current f l o w i n g t h r o u g h i t . Some d i r e c t measurements o f t h e c u r r e n t have been i n t r o d u c e d u n t i l now a t t h e Tsushima S t r a i t . M i i t a r e p o r t e d i n 1976 on t h e s t r u c t u r e and t h e t r a n s p o r t o f t h e c u r r e n t depending on t h e r e s u l t s o f E u l e r i a n measurement. Ogawa e t a l . commented i n 1978 on t h e v a r i a t i o n o f t h e c u r r e n t on t h e b a s i s o f t h e r e s u l t s o b t a i n e d by Lagrangian measurement. I n t h e p r e s e n t r e p o r t t h e main p a r t o f these two papers w i l l be reviewed w i t h some m o d i f i c a t i o n s . MEASUREMENTS WITH CURRENT METERS M a t e r i a l s and methods C u r r e n t s were measured f o r a p e r i o d l o n g e r t h a n one day a t 431 s t a t i o n s between 1920 and 1974 i n t h e Tsushima S t r a i t o u t s i d e t h e c o a s t a l a r e a s . See F i g . 1 and Table 1. I n t h e t a b l e , A r e p r e s e n t s H y d r o g r a p h i c Department o f Japan I m p e r i a l Navy; B, H y d r o g r a p h i c Department o f Japan M a r i t i m e S a f e t y Agency; C, Nagasaki Marine M e t e o r o l o g i c a l Observatory; D, Fukuoka P r e f e c t u r a l F i s h e r i e s Experimental S t a t i o n ; E, S e i k a i Regional F i s h e r i e s Research L a b o r a t o r y ; F, F i s h e r i e s Research
68
34
32
F i g . 1. S t a t i o n s o f d i r e c t c u r r e n t measurements.
TABLE 1 S t a t i o n s o f d i r e c t c u r r e n t measurements Period
1920-1929 1930-1939 1940-1949 1950-1959 ditto ditto 1960-1969 ditto ditto ditto 1970-1974 ditto ditto ditto
Total
%
I
~
Or A A A
B
C D B E D F B C D F
Seasons
Stns
7 47 154 2 4 2 105 4 8 6 38 14 4 36
1
431 100
33 8
1 10 2
1 14 30
1
3 23 115 2 3 1 87
17 4 1 1
4 2
2 2 21
14 3 91 21
9
4
268 62
2 7 1
1 1 1 17
5
1
2
1
4 1 1
1 103 3 38
1
9
2
1 1 2
39 9
148 34
11 3
Number o f l a y e r s 3 4 5
3 14
13 13
1 1
1
3 1
5 4
1
9 4 3 4 2 2
6
39 9
62 14
12 64 1 2
86 20
6
7
4 58 1 2
16 4
65 15
20 5
69 and Development Agency, Korea. Most o f t h e measurements were made w i t h t h e EkmanMertz c u r r e n t m e t e r s . Moored s e l f - r e c o r d i n g c u r r e n t meters were deployed f o r several days a t some s t a t i o n s . The number o f o b s e r v a t i o n s r i s e s t o t h e h i g h e s t i n summer when m e t e o r o l o g i c a l c o n d i t i o n s a r e most f a v o u r i t e f o r measurement. R e s u l t s o f t h e a f o r e m e n t i o n e d measurements were processed by means o f a harmonic a n a l y s i s , assuming t h e c u r r e n t s c o n s i s t i n g o f s e m i - d i u r n a l t i d e , d i u r n a l t i d e , and t h e c o n s t a n t c u r r e n t . The l a t t e r was d e t e r m i n e d as an average f l o w o v e r t h e d u r a t i o n o f each measurement. Several c u r r e n t p a t t e r n s t o be shown l a t e r were o b t a i n e d on t h e b a s i s o f t h e c o n s t a n t c u r r e n t . The c o n s t a n t c u r r e n t d a t a were grouped i n t o f o u r c l a s s e s a c c o r d i n g t o seasons. Horizontal d i s t r i b u t i o n o f c u r r e n t vector I n o r d e r t o seek t h e h o r i z o n t a l d i s t r i b u t i o n o f t h e c u r r e n t we d i v i d e t h e d a t a i n t o t h r e e p a r t s a c c o r d i n g t o t h e depth: s u r f a c e , depth l e s s than 10 m; medium, between 25 and 7 5 m; and bottom, 10 m f r o m t h e bottom. The s u r f a c e d a t a i n summer are shown i n F i g . 2 , f o r i n s t a n c e . I n a r e g i o n f r o m t h e west o f t h e Goto I s l a n d s t o t h e Japan Sea t h r o u g h t h e Tsushima S t r a i t , c u r r e n t d i r e c t i o n s a r e f r o m west o r southwest t o e a s t o r n o r t h e a s t . T h i s d i s t r i b u t i o n i m p l i e s t h a t t h e n o r t h e a s t w a r d c u r r e n t s e x i s t a whole y e a r . The speed i s s t r o n g i n summer and weak i n w i n t e r . The c u r r e n t d i r e c t i o n d e v i a t e s s l i g h t l y t o n o r t h i n summer, b u t i t d e v i a t e s t o
34O
32
2
--L
126
0.5 K t > 0.5 -1.0 1.0 c
128
F i g . 2 . C u r r e n t v e c t o r s a t s u r f a c e i n summer.
130'
70 e a s t i n w i n t e r . Such a seasonal d i f f e r e n c e o f t h e speed and d i r e c t i o n o f t h e current d i m i n i s h e s w i t h t h e depth o f t h e sea. I n t h e b o t t o m c u r r e n t seasonal changes are h a r d l y observed. S e c t i o n a l d i s t r i b u t i o n o f speed L e t us examine t h e d i s t r i b u t i o n o f speed normal t o s e v e r a l s e c t i o n s . I n t o t a l , t h i r t e e n s e c t i o n s were chosen i n t h e n o r t h w e s t r e g i o n o f Kyushu, as shown i n F i g . 1. Most o f t h e d a t a were o b t a i n e d i n summer o f 1942 and 1943 by t h e Japan I m p e r i a l Navy. To supplement these, t h e o t h e r d a t a , which were c o l l e c t e d by o t h e r i n s t i t u t i o n s a t d i f f e r e n t t i m e s , were a l s o used. Number o f l a y e r s , i n which measurements were made, ranged f r o m f i v e t o seven a t each s t a t i o n . KAWAZl R IMlSAKl
LG I
150
0
!
rn
50
100
150
,
2
p
,
w
D Y N A M I C CALCULATION
F i g . 3. D i s t r i b u t i o n o f speed a t s e c t i o n I . ( a ) E u l e r i a n measurement. ( b ) Dynamic calculation.
71
A r e s u l t on s e c t i o n I , f o r i n s t a n c e , i s shown i n t h e upper p a r t o f F i g . 3. The s e c t i o n I l o c a t e s a t t h e e n t r a n c e o f t h e Japan Sea, between Cape K a w a j i r i o f Japan and U l g i o f Korea. The b o t t o m topography i s c o m p a r a t i v e l y f l a t . The h o r i z o n t a l d i s t a n c e i s about 180 km (100 n a u t i c a l m i l e s ) . The c u r r e n t d i s t r i b u t i o n on t h i s s e c t i o n has c h a r a c t e r i s t i c s d i f f e r e n t f r o m t h o s e o f t h e o t h e r s e c t i o n s . Southward c u r r e n t spreads o v e r a c o n s i d e r a b l e wide range i n t h e m i d d l e o f s e c t i o n , and northward c u r r e n t e x i s t s on b o t h s i d e s o f t h e m i d d l e s e c t i o n . The maximum and the average speed o f t h e west s i d e a r e r e s p e c t i v e l y about 80.0 and 34.8 cm/s (0.41 and 0.18 k n o t ) . On t h e e a s t s i d e t h e y a r e 40.0 and 17.8 cm/s (0.21 and 0.092 k n o t ) , which a r e a p p r o x i m a t e l y a h a l f o f t h e speed on t h e west s i d e . We e v a l u a t e d t h e b a r o c l i n i c g e o s t r o p h i c c u r r e n t d i s t r i b u t i o n o f t h e s e c t i o n I . Dynamic c a l c u l a t i o n was c a r r i e d o u t by u s i n g t h e h y d r o g r a p h i c o b s e r v a t i o n d a t a o f August between 1937 and 1941, which were c o l l e c t e d by t h e F i s h e r i e s Experimental S t a t i o n of t h e Government-General o f Korea. A r e f e r e n c e l a y e r was taken a t t h e bottom. R e s u l t s a r e g i v e n i n t h e l o w e r p a r t o f F i g . 3. A c l o s e resemblance e x i s t s between t h e d i s t r i b u t i o n p a t t e r n s o f t h e measured and t h e c a l c u l a t e d speeds. However, magnitude o f speed a r e d i f f e r e n t . The c u r r e n t speed f r o m t h e measured data i s i n e v i t a b l y l a r g e r t h a n t h e one determined w i t h t h e dynamic c a l c u l a t i o n . Volume t r a n s p o r t The volume t r a n s p o r t i s used as an i n d e x f o r t h e i n t e n s i t y o f t h e c u r r e n t . I n t h e p r e s e n t paper i t i s sought f r o m t h e c u r r e n t d i s t r i b u t i o n i n t h i r t e e n sections according t o 36 O
34
Unit
126
Fig. 4. Volume t r a n s p o r t .
Sv
106 m3
128
130°
32
72
T=C ViSi, where summation i s c a r r i e d o u t on i, T i s t h e volume t r a n s p o r t , Vi speed, and Si
t h e area occupied by Vi.
the current
R e s u l t s a r e shown i n F i g . 4. Northward
t r a n s p o r t a t each s t a t i o n f r o m t h e w e s t o f t h e Goto I s l a n d s A t o t h e Tsushima Strait
H ranges f r o m 3.3 t o 3 . 7 Sv ( 1 Sv=lO6 m3/ s ) .
However, on t h e s e c t i o n I between Cape K a w a j i r i and U l g i , on a c c o u n t o f l a r g e t r a n s p o r t a t t h e west s i d e c u r r e n t a x i s , t o t a l volume t r a n s p o r t i s 4.2 Sv, which i s a p p r e c i a b l y g r e a t e r t h a n t h e v a l u e s o b t a i n e d a t t h e o t h e r s e c t i o n s f r o m A t o H.
A t p r e s e n t a u t h o r s do n o t g e t any d e f i n i t e c o n c l u s i o n s as t o whether t h i s discrepancy i n d i c a t e s a c h a r a c t e r i s t i c s of t h e r e g i o n o r i s b r o u g h t by t h e process o f t h e
WINTER
SPRING
SUMMER
AUTUMN
5m 20 m 50 m
100 m
-F WINTER
SPRING
SUMMER
0
0.5
1.0
1.5 Kt
F i g . 5 . Seasonal change o f c u r r e n t v e c t o r s .
AUTUMN
73
d a t a h a n d l i n g . The dynamic c a l c u l a t i o n s (Suda, 1938; L i , 1966) were c a r r i e d o u t b e f o r e . A l l o f t h e p r e v i o u s and t h e p r e s e n t r e s u l t s o f t h e dynamic c a l c u l a t i o n produced t h e volume t r a n s p o r t and t h e speed s m a l l e r t h a n t h o s e determined f r o m t h e d i r e c t measurements. A c t u a l l y a b a r o t r o p i c e f f e c t may e x i s t . Seasonal change o f c u r r e n t i n t h e e a s t and t h e west channels Up t o now t h e steady s t a t e o f t h e Tsushima C u r r e n t i s c o n s i d e r e d . We know t h a t a r e a l s t a t e o f t h e Tsushima C u r r e n t changes i n t i m e and space. Our f i n a l goal i s t o c l a r i f y t h e v a r i a t i o n o f t h e Tsushima C u r r e n t and t o e s t a b l i s h models The measured d a t a a r e n o t s u f f i c i e n t t o d i s c u s s v a r i a t i o n s o f t h e c u r r e n t as a whole. Thus v a r i a t i o n s o f t h e c u r r e n t w i l l be d i s c u s s e d a t two s t a t i o n s , where the d a t a o f f o u r seasons e x i s t . One s t a t i o n i s l o c a t e d a t t h e m i d d l e o f t h e s e c t i o n FE (34'04"
139'32'E)
of
the e n t r a n c e o f t h e e a s t channel and l i e s on t h e a x i s o f t h e c u r r e n t . As i s seen from t h e upper p a r t o f F i g . 5, t h e c u r r e n t d i r e c t i o n i s a l m o s t n o r t h e a s t w a r d . I t s h i f t s t o n o r t h a t t h e deep l a y e r . Seasonal v a r i a t i o n i s remarkable a t t h e upper l a y e r and becomes weak a t t h e deep l a y e r . Another s t a t i o n i s l o c a t e d on t h e m i d d l e o f t h e s e c t i o n HW (34'52"
129'15'E)
of t h e west channel and l i e s on t h e a x i s which shows t h e s t r o n g e s t c u r r e n t . S i m i l a r t o t h e e a s t channel t h e c u r r e n t d i r e c t i o n i s a l m o s t n o r t h e a s t w a r d . The speed i s s t r o n g a t t h e upper l a y e r and weak a t t h e bottom. Seasonal v a r i a t i o n i s remarkable a t t h e upper l a y e r . MEASUREMENT WITH DRIFTERS M a t e r i a l s and methods C u r r e n t v e l o c i t i e s were measured w i t h drogues f r o m 1973 t o 1976 around t h e Tsushima S t r a i t i n c o - o p e r a t i o n w i t h f i v e r e g i o n a l f i s h e r i e s r e s e a r c h i n s t i t u t i o n s . The l o n g e s t t r a c k i n g was p e r f o r m e d f o r 98 hours i n summer. The s h o r t e s t t r a c k i n g h a r d l y reached 24 hours i n w i n t e r because o f a bad weather c o n d i t i o n . C o n s t r u c t i o n o f t h e c u r r e n t drogue system i s shown i n F i g . 6. The drogue used was made f r o m wood. Most o f t h e drogues were p l a c e d a t a nominal depth o f 5 m. D u r i n g each r u n o f t h e t r a c k i n g , t h e p o s i t i o n o f t h e drogue was determined a t an i n t e r v a l o f 1 hour between l a u n c h i n g and r e c o v e r y o f t h e drogue. The p o s i t i o n o f t h e drogue on each r u n was f i x e d w i t h Oecca, Loran, and Radar w i t h accuracy o f 0.05 n a u t i c a l m i l e s ( 9 3 m). R e s u l t s on t h e a r e a n o r t h o f t h e Goto I s l a n d s Four drogues launched on t h e 1 s t o f June i n 1975 were t r a c k e d f o r 67 hours t i l l bad weather c o n d i t i o n s began t o a b o r t t h e experiment. As i s shown i n F i g . 7, t h e movement o f t h e drogues was t o n o r t h e a s t o r n o r t h w i t h a mean speed f r o m 0.09 t o 0.65 k n o t (4.63 t o 33.4 cm/s). There i s a tendency t h a t v e l o c i t i e s o f
74
p FLAG f
LIGHT
FLOAT
n
V
I7
'
1
U
TSUSHIM,
*12 cm
n F i g . 6. C o n s t r u c t i o n o f a drogue.
128
129"
F i g . 7. T r a j e c t o r i e s o f drogues i n t h e area n o r t h o f t h e Goto I s l a n d s i n t h e west channel o f t h e Tsushima S t r a i t i n summer. t h e drogues i n c r e a s e as t h e y l e a v e t h e Goto I s l a n d s . T h i s i n d i c a t e s t h e presence o f a h o r i z o n t a l shear. The shear i s e s t i m a t e d a t about 0.01 k n o t (0.51 cm/s) p e r n a u t i c a l m i l e on an average. T r a j e c t o r i e s o f t h e drogues i n d i c a t e a k i n d o f "trochoidal curves", suggesting t h a t f l u c t u a t i o n s o f i n e r t i a o s c i l l a t i o n s are superimposed on t h e mean f l o w . The p a t t e r n of t h e t r a j e c t o r i e s , however, i s n o t always s i m i l a r t o each o t h e r . R e s u l t s on t h e e a s t channel o f t h e Tsushima S t r a i t A f t e r t h e t r a c k i n g s a t t h e n o r t h o f t h e Goto I s l a n d s , t h r e e drogues were s e t i n t h e e a s t channel o f t h e Tsushima S t r a i t on t h e 7 t h o f June i n 1975. As i s shown i n F i g . 8, t h e movement of t h e drogues was t o e a s t o r n o r t h e a s t d u r i n g 48 hours when t r a c k i n g s were c o n t i n u e d w i t h a mean speed f r o m 0.54 t o 0.90 k n o t (27.8
t o 46.3 cm/s). S i m i l a r t e n d e n c i e s t o those observed a t t h e n o r t h o f t h e
Goto I s l a n d s were a l s o r e c o g n i z e d . On t h e 2 0 t h of November i n 1975, a drogue was launched a t about m i d d l e p o i n t o f t h e e a s t channel and was f o l l o w e d f o r 51 h o u r s . As i s shown i n F i g . 9, t h e drogue moved t o e a s t o r n o r t h e a s t w i t h a mean speed o f 0.48 k n o t (24.7 cm/s). The speed i s l o w e r t h a n i n summer. The d i r e c t i o n i s a l m o s t t h e same as i n summer,
75
-..
70h 3 5"
T S U S H l MA
34
130
131'
Fig. 8. T r a j e c t o r i e s of drogues i n t h e Tsuahima S t r a i t and i n t h e area e a s t o f the s t r a i t i n summer.
I : 35"
34
130
131'
F i g . 9. T r a j e c t o r i e s o f drogues i n t h e Tsushima S t r a i t and i n t h e area e a s t o f the s t r a i t i n w i n t e r . Results on t h e e a s t e r n area o f t h e Tsushima S t r a i t I n t h e a r e a e a s t o f t h e Tsushima S t r a i t , a s i n g l e measurement was r e p e a t e d i n t e r m i t t e n t l y f r o m 1973 t o 1976. See F i g s . 8 and 9. Behaviour o f t h e drogues seems t o show a f a i r l y d i f f e r e n t f l o w p a t t e r n f o r each r u n e s p e c i a l l y i n t h e nearshore waters. Most o f r u n s were made f o r t o o s h o r t a d u r a t i o n t o r e v e a l d e t a i l s . However, i t seems n o t e w o r t h y t h a t a c o n t i n u o u s southwestward movement o v e r 23 hours occured
i n w i n t e r , which i m p l i e s a n o t h e r change i n f l o w from t i d a l c u r r e n t .
76
TSUSHlMA
Srn
K
Y
129
34.
51h
130'
F i g . 10. T r a j e c t o r i e s o f deep c u r r e n t drogues i n t h e Tsushima S t r a i t i n comparison w i t h those o f s u r f a c e drogues. V a r i a t i o n o f c u r r e n t v e l o c i t y w i t h depth I n t h e e a s t channel o f t h e Tsushima S t r a i t t h e R / V Yokomaru t r a c k e d a drogue launched a t a d e p t h o f 80 m c l o s e t o t h e b o t t o m i n November o f 1975, when t h e R / V Kuroshiomaru f o l l o w e d t h e s u r f a c e drogue launched a t a d e p t h o f 5 m. As i s
shown i n F i g . 10, c u r r e n t v e l o c i t i e s a t t h e deep d e p t h a r e s l i g h t l y l o w e r t h a n those o f t h e s u r f a c e . One o f two r u n s made i n t h i s a r e a i n d i c a t e s a c o n s i d e r a b l e d i f f e r e n c e i n d i r e c t i o n s o f movements o f t h e two drogues. The d i r e c t i o n o f t h e deep drogue d e f l e c t e d a b o u t 22 degrees n o r t h w a r d f r o m t h a t o f t h e s u r f a c e . However, i n t h e o t h e r r u n which was made a t t h e n o r t h e a s t o f t h e f o r m e r , t h e deep drogue showed t h e same f l o w p a t t e r n as t h a t o f t h e s u r f a c e drogue. CONCLUDING REMARKS We showed t h e d a t a o f t h e Tsushima C u r r e n t measured w i t h c u r r e n t meters and d r i f t e r s . The d a t a used so f a r were c o l l e c t e d w i t h c o m p a r a t i v e l y s h o r t d u r a t i o n s , which were t h e o r d e r o f s e v e r a l days a t t h e l o n g e s t . Long range v a r i a t i o n and s y n o p t i c s t r u c t u r e o f t h e Tsushima C u r r e n t a r e n o t y e t c l a r i f i e d . We hope t h a t w e l l - p l a n n e d c o n t i n u o u s measurement w i l l be made i n f u t u r e o v e r l o n g range o f t i m e i n wide a r e a . REFERENCES L i , Sok-U, 1966. Seasonal and S e c u l a r V a r i a t i o n s o f t h e Water Volume T r a n s p o r t across t h e Korean S t r a i t , J o u r . Oceanol. SOC. Korea 1 ( 1 - 2 ) . M i i t a , T., 1976. C u r r e n t c h a r a c t e r i s t i c s measured w i t h c u r r e n t m e t e r a t t h e f i x e d s t a t i o n ( i n Japanese), B u l l e t i n Japan SOC. F i s h . Oceanogr. 28: 33-58. Ugawa, Y . - M i i t a , T., I c h i h a r a , A., Hasegawa, Y . , and Inoue, N., 1978. F l u c t u a t i o n o f t h e Tsushima C u r r e n t Measured w i t h t h e C u r r e n t Drogue, B u l l e t i n o f t h e S e i k a i Regional F i s h e r i e s Research L a b o r a t o r y 51: 13-44. Suda, K., 1938. Annual V a r i a t i o n o f t h e Tsushima C u r r e n t ( i n Japanese), Limnol. Mag. 8: 205-215.