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The Water Circulation in the North-Western Mediterranean Sea, its Relations with Wind and Atmospheric Pressure
129
THE WATER CIRCULATION I N THE NORTH-WESTERN
MEDITERRANEAN SEA ,
ITS RELATIONS W I T H W I N D AND ATMOSPHERIC PRESSURE
J.P.
Bethoux(x!
L.
Prieur(x!
F. N y f f e l e r ( x x )
ABSTRACT Mean v a l u e s o f h y d r o l o g i c a l d a t a ( , r e l a t i n g t o t h e p e r i o d 1950-1973) are u s e d t o e v a l u a t e t h e c y c l o n i c f l u x e s i n t h e L i g u r i a n S e a , i . e . , through the Nice-Calvi
s e c t i o n a n d t h e C o r s i c a n c h a n n e l . A marked
s e a s o n a l c y c l e a p p e a r s o f f Nice a n d a l s o t h r o u g h t h e C o r s i c a n c h a n n e l w h e r e are c o n f i r m e d some p r e v i o u s d i r e c t m e a s u r e m e n t s o f t h e f l o w . The s t u d y o f wind a n d a t m o s p h e r i c p r e s s u r e shows t h a t t h e s e e x t e r n a l f o r c e s o n l y e x e r t a moderate e f f e c t on t h e w a t e r c i r c u l a t i o n w h i c h t h e r e f o r e s h o u l d be c h i e f l y t h e r m o h a l i n e . 1
INTRODUCTION The d i f f e r e n t b a s i n s of t h e N o r t h - W e s t e r n
Yediterranean, t h e Tyrrhe-
n i a n S e a , t h e L i q u r i a n S e a , t h e G u l f o f L i o n s and t h e C a t a l a n S e a a r e t h e s i t e o f g r e a t c y c l o n i c c i r c u i t s whose g e n e r a l o u t l i n e w a s s u g g e s t e d by N i e l s e n i n 1 9 1 2 . Most s t u d i e s o f t h e m a r i n e e n v i r o n m e n t a n d t h e n o d e l l i n g o f o b s e r v e d phenomena r e q u i r e a q u a n t i t a t i v e k n o w l e d g e of
w a t e r c i r c u l a t i o n . The w a t e r f l o w of t h e L i g u r i a n S e a f o r m s a n i m p o r t a n t l i n k i n t h e c i r c u l a t i o n of t h e W e s t e r n b a s i n , s i n c e it conn e c t s t h e Tyrrhenian Sea, t h e Algero-provenqal
b a s i n and t h e Gulf
of L i o n s . I n t h i s s t u d y , w e p r e s e n t f i r s t a d y n a m i c a l e v a l u a t i o n o f t h e water f l u x e s i n t h e L i q u r i a n Sea and o f t h e i r s e a s o n a l v a r i a t i o n s . B u t t h e d y n a m i c a l m e t h o d does n o t g i v e a n y i n f o r m a t i o n upon t h e ext e r n a l or i n t e r n a l f o r c e s w h i c h c r e a t e t h e m e a s u r e d d e n s i t y g r a d i e n t s . So, a f t e r , w e e x a m i n e w h a t may b e t h e r e s p e c t i v e e f f e c t s o f t h e ex-
t e r n a l f o r c e s , wind and a t m o s p h e r i c p r e s s u r e , on t h e d i f f e r e n t calculated fluxes. L a b o r a t o i r e de P h y s i q u e e t C h i m i e M a r i n e s , E r a CNRS, S t a t i o n M a r i n e , BP 8 , 06230 V i l l e f r a n c h e s u r M e r , F r a n c e . ( X x ) U n i v e r s i t 6 d e N e u c h a t e l , I n s t i t u t d e G E o l o g i e , 11 r u e E . A r q a u d , CH 2 0 0 0 N e u c h a t e l , S u i s s e .
130
44"
Ligurian Sea
43'
-
-
Strperficial Layer
,
.-...- P intermediate 7"
Layer
8"
9"
6 3 F i g . 1. Mean v a l u e s of f l u x e s ( i n 1 0 m / s e c , o r Sv) i n t h e s u p e r f i c i a l (0-200m) and i n t e r m e d i a t e (200-800m) l a y e r s , t h r o u g h t h e N i c e C a l v i s e c t i o n and t h e C o r s i c a n c h a n n e l .
131 2
FLUXES AND T H E I R SEASONAL VARIATIONS The p r e s e n c e of a M a r i n e S t a t i o n a t V i l l e f r a n c h e s u r M e r h a s a l l o w e d
t h e f r e q u e n t t a k i n g of m e a s u r e m e n t s o f f Nice and a l o n g t h e N i c e - C a l v i s e c t i o n , t h r o u g h t h e permanent c y c l o n i c c i r c u l a t i o n . T h i s c i r c u l a t i o n c o n c e r n s t h e s u p e r f i c i a l w a t e r ( a b o u t 0-200m)
and i n t e r m e d i a t e w a t e r
(200-800rn). Two s t r e a m s o f m e r i d i o n a l waters f l g w up b o t h s i d e s of Cor-
s i c a , j o i n a t t h e N o r t h o f Cap C o r s e and form t h e L i g u r i a n c u r r e n t which l a p s t h e s h o r e s o f t h e I t a l i a n R i v i e r a and t h e F r e n c h C 6 t e d'Azur. I n t h e case o f l a c k of d i r e c t m e a s u r e m e n t s , t h e knowledge of t h e c i r c u l a t i o n i s b a s e d on t h e c a l c u l a t i o n o f t h e g e o s t r o p h i c c u r r e n t a c r o s s t h e Nice-Calvi
s e c t i o n . D u r i n g t h e p a s t y e a r s , a growing number of
h y d r o l o g i c a l d a t a h a v e been e x p l o i t e d , ; ' and r e c e n t l y h y d r o l o g i c a l aver a g e s h a v e been c a l c u l a t e d , b a s e d on t h e c o m p l e t e m e a s u r e m e n t s c a r r i e d o u t d u r i n g t h e p e r i o d o f 1950-1973 and s t o r e d i n t h e a r c h i v e s of t h e d a t a bank o f t h e BNDO-COB
i n B r e s t ( N y f f e l e r e t a l . , 1980). Among 2 2 0 0
h y d r o l o g i c a l s t a t i o n s i n t h e L i g u r i a n S e a , m o r e t h a n 9 0 0 , on t h e N i c e C a l v i s e c t i o n , w e r e t h u s u s e d . The a v e r a g e a n n u a l f l o w s on t h i s sect i o n h a v e been c a l c u l a t e d a n d , t a k i n g i n t o a c c o u n t t h e w a t e r b u d g e t of t h e b a s i n , t h e f l u x e s t h r o u g h t h e C o r s i c a n c h a n n e l have been deduct e d and compared t o p r e v i o u s e s t i m a t i o n s (Bethoux e t a l . , 1 9 8 0 ) . I n f i g u r e 1 and i n t a b l e 1 are p r e s e n t e d t h e mean f l o w s r e l a t i n g t o t h e s u p e r f i c i a l (SL) and i n t e r m e d i a t e (IL) l a y e r s , t h e
+
sign indicates
a SW/NE f l o w ( o f f C a l v i ) o r a S/N f l o w ( C o r s i c a n c h a n n e l ) , and t h e s i g n i n d i c a t e s a f l o w i n a NE/SW d i r e c t i o n ( 1 Sv = 1 0 6m 3 /set).
-
TABLE 1
Mean f l o w s i n t h e s u p e r f i c i a l (SL) and i n t e r m e d i a t e ( I L ) l a y e r s i n t h e L i q u r i a n Sea N i c e side
SL IL
-
-
Calvi side
1.4 s v
0.4
+ +
0.7 0.2
sv
Corsican channel
+
+
0 . 7 Sv 0.2
T h e s e f l o w s a r e l a r g e , e s p e c i a l l y o f f N i c e , where t h e s t r o n g e s t c u r r e n t s s t a y c h a n n e l l e d i n t o a c o a s t a l band 2 0 t o 3 0 m i l e s wide. The s u r f a c e s p e e d s a r e between 2 0 t o 3 0 cm/sec and are below 5 cm/sec from 250m downwards (see F i g . 2 ) .
I n t h e c e n t r a l z o n e t h e a v e r a g e geo-
s t r o p h i c f l o w s are a l m o s t n u l l . T h e f l o w s on b o t h s i d e s of Cap Corse a r e , on an a v e r a g e , e q u a l , which shows t h e i m p o r t a n c e of t h e T y r r h e n i a n S e a o u t f l o w , p e r h a p s u n d e r e s t i m a t e d u p t o now.
132
500
800 .rn
i
10amB
1I
ZONE
14
NICE
15
10
\i
16
30
18
17
60
19
20
8G
miles
CALVI
F i g . 2. V e r t i c a l p r o f i l e s of c u r r e n t s t h r o u g h t h e Nice-Calvi (90 m i l e s , d i v i d e d i n t o 7 z o n e s numbered 1 4 t o 20).
section
133
For t h e s t u d y of t h e s e a s o n a l c y c l e s , based on h y d r o l o g i c a l c r i t e r i a and t a k i n g i n t o a c c o u n t t h e number of h y d r o l o g i c a l s t a t i o n s a v a i l a b l e e a c h month, t h e y e a r h a s been d i v i d e d i n t o 7 p e r i o d s which c o r respond t o t h e months : period : month :
1 I
2 I1
3
4 IV,V
I11
5
V1,VII
6 VIII;IX,X
7 X1,XII
The s e a s o n a l c y c l e s of t h e w a t e r f l o w s o f f Nice and C a l v i , and, f o r comparison, t h r o u g h t h e C o r s i c a n c h a n n e l , a r e shown i n f i g u r e 3 , f o r b o t h w a t e r l a y e r s examined. The flow on t h e h a l f s e c t i o n on t h e Nice s i d e shows a v e r y marked c y c l e , w i t h a n o t i c e a b l y h i g h r i s e between p e r i o d s 6 (August, September and O c t o b e r ) and 7 (November and December) from 1 . 4 t o 2.3 Sv. Near C a l v i , t h e " s e a s o n a 1 c y c l e of f l o w i s less marked and seems more i r r e g u l a r . I n comparison w i t h t h e two p r e c e d i n g c y c l e s , a c y c l e comparable t o t h a t found o f f N i c e i s found i n t h e Cors i c a n c h a n n e l , w i t h a v e r y marked f l u x i n c r e a s e between p e r i o d s 6 and 7 , when t h e t o t a l f l o w ( 0 - 4 0 0 m )
g o e s from 0 . 2 t o 1 . 8 Sv. On t h i s
graph concerning t h e flow through t h e Corsican channel, t h e d i r e c t measurements of S t o c c h i n o and T e s t o n i (1969) c a r r i e d o u t i n J u n e - J u l y 1966
(0.7 Sv) a r e shown t o g e t h e r w i t h t h o s e of LeFloch ( 1 9 6 3 ) performed
i n F e b r u a r y and i n August 1960 (1.5 and 0.4 Sv, r e s p e c t i v e l y ) . These d i r e c t f l o w measurements a g r e e w i t h o u r c a l c u l a t e d c y c l e and t h e geost r o p h i c c a l c u l a t i o n s . They a l s o c o n f i r m t h e c l e a r d e c l i n e of t h e f l o w between p e r i o d s 2 and 6. During p e r i o d 7, up t o about 30 m i l e s o f f N i c e , t h e a v e r a g e speed c a l c u l a t e d i s 2 8 cm/sec on t h e s u r f a c e and 3 cm/sec a t 3 0 0 m d e p t h . With r e g a r d t o p e r i o d 6 , t h e s p e e d s i n c r e a s e on a v e r a g e by 5 cm/sec between t h e s u r f a c e and 150m d e p t h . Moreover, o f f N i c e , a d e c r e a s e a p p e a r s i n t h e s a l i n i t y of t h e s u r f a c e up t o 300m, on a v e r a g e e q u a l t o 0.07°/oo
(see f i g u r e 4 ) . Such a v a r i a t i o n i n t h e s a l i n i t y cannot
be a r e s u l t of t h e s u r f a c e w a t e r budget
( t h e months of November and
December a r e r e l a t i v e l y r a i n y , b u t a t t h a t t i m e heavy e v a p o r a t i o n a l s o o c c u r s ) and t h e r e f o r e i t l e a d s t o t h e h y p o t h e s i s of t h e i n f l u x of l e s s s a l i n e w a t e r , o r i g i n a t i n g from t h e S o u t h . T h i s d e c r e a s e i n s a l i n i t y ,
i n e f f e c t , between p e r i o d s 6 and 7, can be e q u a l l y o b s e r v e d , i n t h e 0-1OOm
l a y e r , on b o t h s i d e s o f Cap Corse and even between weriod 5
and 6 off C a l v i . D i f f e r e n t a u t h o r s have s t u d i e d t h e e x t e r n a l f o r c e s e f f e c t s (wind, atmospheric p r e s s u r e
,
t i d e ) upon t h e s u p e r f i c i a l f l u x e s i n t h e Mediter-
r a n e a n . Among them, Crepon (1965) examined t h e e f f e c t s of t h e atmosp h e r i c p r e s s u r e v a r i a t i o n s on t h e s e a l e v e l , and E l l i o t t (1979) s t u d i e d
134
m3/s
f
lo6
CALVI
p l I
0 m3A
2.lo' CORSICA CHANNEL
Id
0
' 1 ' 2 ' 3 '
4
I
5
l
6
I
7
' 1 ' 2 ' 3 -
F i g . 3 . S e a s o n a l c y c l e s of f l u x e s o f f N i c e , o f f C a l v i and t h r o u g h t h e C o r s i c a n c h a n n e l . Direct m e a s u r e m e n t s of L e F l o c h (LF) and t h o s e of S t o c c h i n o and T e s t o n i (ST) t h r o u g h t h e C o r s i c a n c h a n n e l .
135
current speed
cmh
3
10
fi section
periods 6and 7
20
30
m
F i g . 4. V e r t i c a l p r o f i l e s of c u r r e n t s and s a l i n i t i e s , o f f N i c e , a t :’ p e r i o d 6 ( A u g u s t , S e p t e m b e r and O c t o b e r ) and 7 (November and December)
.
136 t h e wind e f f e c t on t h e sea l e v e l and c u r r e n t s i n t h e Gulf o f Genova, whereas Laevastu
( 1 9 7 2 ) shown t h a t a b o u t 1/4 o f t h e i n f l o w i n g A t l a n t i c
w a t e r s r e s u l t from t h e t i d a l e f f e c t s ( a m p l i t u d e d i f f e r e n c e a t o p p o s i t e e n d s of t h e s t r a i t s ) t h r o u g h t h e S t r a i t of G i b r a l t a r . S i n c e V i l l a i n ( 1 9 4 9 , 1 9 5 2 ) v e r y f e w s t u d i e s a b o u t M e d i t e r r a n e a n t i d e s h a v e been c a r r i e d o u t . So it i s n o t p o s s i b l e t o e s t i m a t e t i d e e f f e c t s upon t h e c i r c u l a t i o n i n s i d e t h e M e d i t e r r a n e a n . However, owing t o P u r g a e t a l . (19791, t i d e i s s y n c h r o n o u s i n t h e T y r r h e n i a n S e a , and s o , it may have a s t a -
t i c e f f e c t ( s u c h a s a t m o s p h e r i c p r e s s u r e ) on t h e sea l e v e l and on t h e superficial circulation. W e have l o o k e d f o r t h e p o s s i b l e c l i m a t i c e x t e r n a l c a u s e s of t h e i m -
p o r t a n t mean c a l c u l a t e d f l u x e s f i r s t , and t h e n ,of t h e s e a s o n a l c y c l e , by e x a m i n i n g t h e e f f e c t s of wind and o f a t m o s p h e r i c p r e s s u r e v a r i a t i o n s on t h e w a t e r c i r c u l a t i o n , on a m o n t h l y scale. 3
WINDS I N THE NORTH-WESTERN MEDITERRANEAN SEA
I n o r d e r t o make a c o m p a r i s o n w i t h h y d r o l o g i c a l a v e r a g e s ( r e f e r r i n g t o t h e y e a r s 1953-1973) w e have u s e d t h e wind a v e r a g e s r e l a t i v e t o t h e p e r i o d 1951-1960,
o b t a i n e d from t h e m e t e o r o l o g i c a l s t a t i o n s of
Cap BBar, S P t e , PomPgues, Cap C a m a r a t , Cap F e r r a t and Cap Corse (Darchen and DeBlock, 1 9 6 8 ) , which g i v e t h e s p e e d , t h e o r i g i n and t h e
f r e q u e n c y of wind f o r e a c h month.
I t i s p o s s i b l e t o e v a l u a t e t h e con-
s e q u e n t d r i f t f l u x , F , a t 90° t o t h e w i n d , l i n k e d t o t h e s q u a r e of t h e wind s p e e d , V , by t h e e m p i r i c a l e q u a t i o n : F =
2
K pa V 2 w s i n e pw
where p a and p w a r e a i r and w a t e r d e n s i t i e s . I f w e assume t h e adimens i o n a l f r i c t i o n c o e f f i c i e n t , K , be e q u a l t o 2 m3/sec/m
--
2.7
10-2
v2
t h e equation is:
m/sec
3 . 1 Wind a t N i c e (Cap F e r r a t ) T h e a v e r a g e s p e e d of wind a t Cap F e r r a t , from a l l d i r e c t i o n s , i s 2 . 1 m/sec.
However, a s a r e s u l t of d i f f e r e n t s e c t o r s , f r e q u e n c i e s and
s p e e d s of w i n d , t h e r e s u l t i n g a v e r a g e wind s p e e d , V , f a v o u r a b l e t o t h e f l o w (NE/SW) i s o n l y a b o u t 1 m/sec
( s e e F i g . 5 ) . On a h a l f s e c t i o n
4 5 m i l e s o f f N i c e , t h e a v e r a g e f l o w r e s u l t i n g from t h e wind i s a b o u t 2.2
103m3/sec,
t r o p h i c flow.
t h a t i s around one t h o u s a n d t h of t h e c a l c u l a t e d geosI n a d d i t i o n , t h e s e a s o n a l wind c y c l e e x p l a i n s n e i t h e r
q u a l i t a t i v e l y nor q u a n t i t a t i v e l y t h e f l o w c y c l e o f f N i c e .
137
4P
-rrl iI cap
-4
Come
-6
Calvi
12
; : l4 20-
I I
cap BBar
1
V
VI
VII
Vlll
L
IX
I
X
XI
1
XI1 month-
F i g . 5. S e a s o n a l c y c l e of mean r e s u l t i n g w i n d , owing t o i t s s p e e d , f r e q u e n c y , o r i g i n and t o t h e mean l o c a l water c i r c u l a t i o n .
138 3 . 2 Wind o n e i t h e r s i d e o f Cap Corse
T h e c o n s e q u e n t average wind a t Cap Corse i s a l w a y s f r o m t h e N o r t h
o r West s e c t o r s , and it i s t h e r e f o r e a l w a y s u n f a v o u r a b l e t o t h e SW/NE c i r c u l a t i o n o f f C a l v i , a s w e l l a s t o t h e S/N c i r c u l a t i o n t h r o u g h t h e Corsican channel.
Its r e s u l t i n g annual averaqe i s -4.8
f l o w o f f C a l v i , and - 4 . 6
m/sec
m/sec f o r t h e f o r t h e f l o w coming f r o m tce T y r r h e n i a n
S e a . I n a d d i t i o n , t h e s e a s o n a l wind c y c l e h a s n o a p p a r e n t c o n n e c t i o n w i t h t h a t of t h e flows. 3 . 3 Wind i n t h e G u l f o f L i o n s
I n o r d e r t o s t u d y a p o s s i b l e e f f e c t o f t h e wind downstream o f t h e c u r r e n t s , w e e x a m i n e d t h e d a t a o n wind r e l a t i n g t o t h e s t a t i o n s o f t h e G u l f of L i o n s . A t Cap B & a r , S B t e and Porni5gueG t h e d o m i n a n t w i n d ,
a n o r t h w e s t e r l y w i n d , i s t h e r e f o r e f a v o u r a b l e to t h e c i r c u l a t i o n i n t h e o p e n sea (NE/SW). T h e r e s u l t i n g a v e r a g e wind s p e e d s a r e 6 m/sec
a t PomGgues, 1 . 9 m/sec a t S G t e and 9 m / s e c
a t Cap B B a r . F o r t h e l a t t e r
s t a t i o n , w h i c h c o r r e s p o n d s t o t h e m o s t f a v o u r a b l e case ( s e e f i g u r e 5 )
,
t h e NE/SW f l o w g e n e r a t e d b y t h e wind a l o n g a w i d t h o f 4 5 m i l e s i s about 0.2 Sv, which s t i l l o n l y r e p r e s e n t s n e a r l y a t e n t h of t h e geost r o p h i c water f l o w c a l c u l a t e d o f f N i c e . F u r t h e r m o r e a g a i n , t h e s e a s o n a l wind c y c l e d o e s n o t s e e m t o a c c o u n t f o r t h e w a t e r f l o w c y c l e . G a l e s p l a y a n i m p o r t a n t r o l e a s much o n t h e s u r f a c e f l o w ( a d v e c t i o n phenomena) a s o n t h e o c e a n - a t m o s p h e r e
interchanges,particularly i n
t h e Gulf o f L i o n s . But w i t h r e g a r d s t o t h e a v e r a g e c i r c u l a t i o n o f a c y c l o n i c t y p e i n t h e North-Western M e d i t e r r a n e a n , w i n d s c a n o n l y gene-
r a t e a b o u t a t e n t h of t h e c a l c u l a t e d f l o w s , i n t h e m o s t f a v o u r a b l e c a s e , i . e . i n t h e G u l f o f L i o n s . I n t h e Cap Corse s e c t o r , wind i s countrary t o t h e circulation, while off N i c e ,
it i s o n l y s l i g h t l y
f a v o u r a b l e . T h e r e f o r e t h e r e i s d e f i n i t e l y n o d i r e c t r e l a t i o n between t h e w a t e r f l u x e s and t h e w i n d s , on a m o n t h l y o r s e a s o n a l s c a l e , i n t h e North-Western Mediterranean. 4
ATMOSPHERIC PRESSURE I N THE WESTERN MEDITERRANEAN The a t m o s p h e r i c d i s t u r b a n c e s a b o v e . t h e M e d i t e r r a n e a n h a v e been
s t u d i e d i n p a r t i c u l a r by B e r e n g e r ( 1 9 5 5 ) . One p e c u l i a r i t y , d u e t o t h e g e o g r a p h i c p o s i t i o n and t h e m o u n t a i n o u s s u r r o u n d i n g s , i s t h a t 73% o f t h e d e p r e s s i o n s i n t h e M e d i t e r r a n e a n are formed a b o v e t h i s
sea, c h i e f l y i n t h e W e s t e r n b a s i n . I n t h i s b a s i n , t h e most f r e q u e n t a r e t h e G u l f o f Genova ( 5 0 % ) , t h e B a l e a r i c ( 2 5 % ) a n d t h e S a h a r a n d e p r e s s i o n s . T h e s e d i s t u r b a n c e s o c c u r p r i n c i p a l l y i n autumn
,
winter
139 and s p r i n g . T h e r e i s t h e r e f o r e a marked c o n t r a s t between summer and t h e other seasons. V a r i o u s a u t h o r s h a v e d e a l t w i t h t h e q u a l i t a t i v e l i n k a g e between a t m o s p h e r i c p r e s s u r e and sea l e v e l i n t h e M e d i t e r r a n e a n . L i s i t z i n (1954) s t u d i e d t h e s e a s o n a l v a r i a t i o n s i n t h e sea l e v e l i n d i f f e r e n t s p o r t s , where t h e s e a l e v e l u n d e r g o e s a g r e a t i n c r e a s e , i n O c t o b e r and November b e f o r e d e c r e a s i n g a t t h e beginning of t h e y e a r . T h i s c y c l e t h e r e f o r e p r e s e n t s remarkable analogies with t h e c u r r e n t wariations off N i c e , but, according t o P a t t u l l o et al.
(1955). a high sea l e v e l during t h e
f a l l ( a low s e a l e v e l d u r i n g s p r i n g ) i s a s s o c i a t e d w i t h t e m p e r a t u r e and s a l i n i t y f l u c t u a t i o n s i n . t h e u p p e r 1OOm ( s p e c i f i c volume f l u c t u a tions)
.
However, a t Monaco, M a r s e i l l e , , , and P o r t o - M a u r i z i o ,
Lisitzin
found q u i t e a good c o r r e l a t i o n between t h e m o n t h l y v a r i a t i o n s i n t h e s e a l e v e l and t h o s e o f t h e a t m o s p h e r i c p r e s s u r e . A c c o r d i n g t o Crepon (19651, t h e v a r i a t i o n s i n a t m o s p h e r i c p r e s s u r e , i n e f f e c t , a r e t h e e s s e n t i e l c a u s e of t h e v a r i a t i o n s i n t h e a v e r a g e l e v e l o f v a r i o u s p o r t s i n t h e W e s t e r n b a s i n . On t h e o t h e r h a n d , a c c o r d i n g t o Lacombe ( 1 9 6 0 ) , a l o w e r i n g o f t h e a t m o s p h e r i c p r e s s u r e upon t h e M e d i t e r r a n e a n i n v o l v e s an i n c r e a s e o f t h e i n f l o w (0-200111) t h r o u g h t h e S t r a i t s of G i b r a l t a r , i n c o u n t e r p a r t , a r i s e o f t h e o u t f l o w ( a t more t h a n 180m d e p t h ) w i l l accompany a rise o f t h e a t m o s p h e r i c p r e s s u r e . In order t o estimate t h e f l u x e s due t o t h e atmospheric v a r i a t i o n s of s e a l e v e l , w e s t u d i e d t h e M e d i t e r r a n e a n a t m o s p h e r i c p r e s s u r e v a r i a t i o n s a s a f u n c t i o n o f t h e s e a s o n s , whose e x a m p l e s a r e g i v e n i n f i g u r e 6.
-
The Gulf o f Genova b e i n g a low p r e s s u r e c e n t e r a l l a l o n g t h e y e a r ,
a h o r i z o n t a l g r a d i e n t o f p r e s s u r e e x i s t s from G i b r a l t a r t o Genova-The s e a s o n a l c y c l e of t h e f i g u r e 6 a , e s t i m a t e d from t h e d a t a i n 'Weather i n the Mediterranean'
-
(1962)
,
shows a jump between p e r i o d 6 and 7.
The f i g u r e 6 b i s a r e p r o d u c t i o n o f t h e a v e r a g e s e a s o n a l c y c l e of
t h e a t m o s p h e r i c p r e s s u r e a t N i c e , f o r t h e y e a r s 1951-1960
(Garnier,
1 9 6 6 ) . The a v e r a g e p r e s s u r e i s maximum d u r i n g p e r i o d 6 and minimum i n
F e b r u a r y o r March.
-
The l o c a l a t m o s p h e r i c p r e s s u r e shows a marked s e m i - d i u r n a l c y c l e ,
whose a m p l i t u d e a t N i c e ( f i g u r e 6 c ) v a r i e s from 0.7mb i n p e r i o d 6 t o a b o u t 0.9mb i n p e r i o d 7 . Owing t o t h e s e a s o n a l c y c l e s o f t h e a t m o s p h e r i c p r e s s u r e
(figures 6
a , b and c ) it i s p o s s i b l e t o f i n d q u a l i t a t i v e c o r r e l a t i o n s between f l u x e s and a t m o s p h e r i c p r e s s u r e v a r i a t i o n s o f f N i c e .
However, o n l y
t h e s e m i - d i u r n a l a t m o s p h e r i c p r e s s u r e v a r i a t i o n ( F i g . 6 c ) can p r o d u c e
140
m o r e o r l e s s p e r m a n e n t f l u x e s , on a m o n t h l y o r s e a s o n a l s c a l e . F o r t h e whole M e d i t e r r a n e a n ( a r e a a b o u t 2 . 5 1 0 1 2 r n 2 ) , a s e m i - d i u r n a l a m p l i t u d e of t h e a t m o s p h e r i c p r e s s u r e of a b o u t 0.8mb i n v o l v e s a maximum i n - o r - o u t
f l o w i n g f l u x e s o f 0 . 4 6 S v , t h a t i s a b o u t 3 0 % of t h e e s t i -
mated f l u x e s t h r o u g h t h e S t r a i t s of G i b r a l t a r ( B e t h o u x , l 9 7 9 ) . The f l u x e s o f f N i c e f o r m i n g a n i m p o r t a n t b r a n c h of t h e c i r c u l a t i o n i n t h e W e s t e r n b a s i n , it i s d i f f i c u l t t o know t h e i n f l u e n c e a r e a of t h e a t m o s p h e r i c p r e s s u r e v a r i a t i o n s . A s a rough e s t i m a t e , i f w e suppose t h a t t h e s t a t i c v a r i a t i o n s of t h e s e a l e v e l o f a b o u t h a l f of t h e Wes12 2 t e r n b a s i n ( 0 . 4 1 0 m ) a c t on t h e f l u x e s o f f N i c e , t h e a t m o s p h e r i c s e m i - d i u r n a l v a r i a t i o n s may p r o d u c e a b o u t 10% of t h e c a l c u l a t e d f l u x e s . S e m i - d i u r n a l v a r i a t i o n s o f t h e a t m o s p h e r i c p r e s s u r e may be an i m p o r t a n t cause of t h e Mediterranean c i r c u l a t i o n , s p e c i a l y through t h e S t r a i t s of G i b r a l t a r
(and S i c i l y ) . B u t , i n t h e L i g u r i a n S e a , t h e y d o n o t seem
t o i n v o l v e more t h a n 1 0 % o f t h e f l u x e s , and d o n o t e x p l a i n t h e i n c r e a s e o f t h e f l u x e s between p e r i o d 6 and 7.
A P(GIBRALTAR 3-
- GENOVA)
6a
2
c
P(NICE)
15t
6b 1 -
--
-
0.5
- 1
' 1 ' 2 ' 3 '
4
IV ' V
I
I
Vl
5 I
A P(NICE)
65
I
Vlf
I
VIiI
'
6 IX
1
7
I
period
X ' XI ' XI1 ' month
F i g . 6 . S e a s o n a l c y c l e of a t m o s p h e r i c p r e s s u r e d i f f e r e n c e s between G i b r a l t a r and Genova ( 6 a ) , o f mean a t m o s p h e r i c p r e s s u r e i n Nice ( 6 b ) and of i t s s e m i - d i u r n a l v a r i a t i o n s ( 6 c ) .
141 5
CONCLUSION
The u s e of a v e r a g e s , c a l c u l a t e d from a g r e a t number o f h y d r o l o g i c a l s t a t i o n s between N i c e and C a l v i , a l l o w s n o t o n l y t h e a v e r a g e a n n u a l v a l u e o f t h e f l o w s b u t a l s o t h e i r s e a s o n a l v a r i a t i o n s t o be o b t a i n e d . These r e s u l t s s h o u l d be u s e f u l i n d i f f e r e n t s t u d i e s i n dynamics, chem i s t r y o r marine biology. ,.
The L i g u r i a n Sea i s a n i m p o r t a n t l i n k i n t h e c i r c u l a t i o n o f w a t e r i n t h e N o r t h Western M e d i t e r r a n e a n , and i t s e e m s t o be n e c e s s a r y t o p r o v e t h e c a l c u l a t e d f l o w s by d i r e c t measurements on t h e s e c t i o n s o f t h e R i v i e r a - C o r s i c a and E l b a - C o r s i c a . The C o r s i c a n c h a n n e l a p p e a r s t o be one o f t h e key p o i n t s i n t h e u n d e r s t a n d i n g of t h e dynamics o f t h e Mediterranean. I
The s t u d y o f m e t e o r o l o g i c a l d a t a o b t a i n e d on t h e North-West c o a s t s
l e a d t o a t t r i b u t e a r e d u c e d e f f e c t on t h e a v e r a g e water f l o w s and on t h e i r s e a s o n a l v a r i a t i o n s by t h e m e c h a n i c a l a c t i o n o f wind. On t h e o t h e r hand, t h e s e m i - d i u r n a l v a r i a t i o n o f a t m o s p h e r i c p r e s s u r e may o n l y p r o d u c e a b o u t 10% o f t h e f l u x e s , and c a n n o t e x p l a i n t h e a n n u a l c y c l e of t h e c i r c u l a t i o n o f f N i c e . S i n c e t h e s e two e x t e r n a l f o r c e s , wind and a t m o s p h e r i c p r e s s u r e ,
e x e r t moderate e f f e c t s , t h e c i r c u l a t i o n should be c h i e f l y thermohaline. The i n i t i a l c y c l o n i c g y r e , c e r t a i n l y produced by t h e water d e f i c i t (Bethoux,l980)
,
i s a m p l i f i e d by t h e a r r i v a l a t t h e p e r i p h e r y o f t h e
b a s i n of waters coming from t h e s o u t h e r n r e g i o n s . These w a t e r s a r e r e l a t i v e l y warmer and l e s s s a l i n e t h a n t h o s e o f t h e c e n t r a l a r e a . S u c h a t h e r m a l a d v e c t i o n by t h e s u r f a c e w a t e r s i s s u e d from t h e T y r r h e n i a n S e a i s c l e a r l y v i s i b l e on t h e i n f r a - r e d
et a1.,1979).
s a t e l l i t e p i c t u r e s (Bethoux
The i n c r e a s e o f t h e c i r c u l a t i o n between p e r i o d 6 and 7
is d u e t o t h e d e n s i t y c o n t r a s t between t h e c o a s t a l and c e n t r a l waters. I n t h e c e n t r a l a r e a , t h e sea u n d e r g o e s t h e e f f e c t s o f t h e n e g a t i v e h e a t and w a t e r b u d g e t s w i t h t h e a t m o s p h e r e , w h i l e , i n t h e p e r i p h e r y a r e a , s u c h e f f e c t s a r e lowered by t h e r m a l a d v e c t i o n o f s o u t h e r n waters and by c o a s t a l r u n o f f .
Acknowledgements- T h i s work w a s s u p p o r t e d by CNEXO ( c o n t r a c t 79/2084) CNRS (GRECO 3 4 ) and Fonds N a t i o n a l S u i s s e d e l a Recherche S c i e n t i f i q u e [ p r o j e c t 2 . 2 8 0 . 0 7 9 ) . T h a n k s a r e due t o F. L o u i s , who d r a f t e d t h e f i g u r e s .
142 REFERENCES B e r e n g e r , M. ,1955. E s s a i d ' b t u d e m S t 6 o r o l o g i q u e d u b a s s i n m s d i t e r r a ngen. Mgmorial d e l a M 6 t C o r o l o g i e N a t i o n a l e , 4 0 , 42pp. Bethoux , J . P . , 1 9 7 9 . B u d g e t s o f t h e M e d i t e r r a n e a n S e a . T h e i r dependance on t h e l o c a l c l i m a t e and on t h e c h a r a c t e r i s t i c s of t h e A t l a n t i c waters. O c e a n o l . A c t a , 2 , 2 , 1 5 7 - 1 6 3 . Bethoux , J . P . ,1960. Mean w a t e r f l u x e s a c r o s s s e c t i o n s i n t h e M e d i t e r r a n e a n S e a , e v a l u a t e d on t h e b a s i s of water and s a l t b a g e t s and of o b s e r v e d s a l i n i t i e s . Oceanol. Acta,3,1,79-88. B e t h o u x , J . P . , P r i e u r , L. and A l b u i s s o n , M. ,1979. A p p o r t s d e l a t ' e l 8 d e t e c t i o n i n f r a - r o u g e Fi l a c o n n a i s s a n c e d e l a c i r c u l a t i o n s u p e r f ic i e l l e d a n s l a p a r t i e Nord-Est du b a s s i n O c c i d e n t a l . Rapp.Com. i n t . Mer M g d i t . , 25-26. B e t h o u x , J . P . , N y f f e l e r , F. and P r i e u r , L. ,1960. U t i l i s a t i o n d e moyennes hydrologiques pour l e c a l c u l d e s f l u x d'eau dans l e bassin Lig u r o - p r o v e n q a l , X X V I I C o n g r e s C I E S M , C a g l i a r i , 9 - 1 8 o c t o b r e 1980,4pp. C r e p o n , M.,1965. I n f l u e n c e d e l a p r e s s i o n a t m o S p h e r i q u e s u r l e n i v e a u moyen d e l a M g d i t e r r a n g e O c c i d e n t a l e e t s u r l e f l u x a t r a v e r s l e d 6 t r o i t d e G i b r a l t a r . C a h i e r s O c b a n o g r a p h i q u e s , XVII,1,15-32. D a r c h e n , J. and d e B l o c k , A. ,1968. Le v e n t s u r l e s c 6 t e s d e l a F r a n c e M 6 t r o p o l i t a i n e , M 6 d i t e r r a n G e . Monographies d e l a M E t 6 o r o l o g i e N a t i o n a l e , 6 2 ,2 ,9 7pp. E l l i o t t , A . J . , 1 9 7 9 . The e f f e c t of l o w f r e q u e n c y w i n d s on s e a l e v e l and c u r r e n t s i n t h e G u l f o f Genova. O c e a n o l . A c t a , 2 , 4 , 4 2 9 - 4 3 3 . G a r n i e r , M.,1966. Climatologie d e l a France, Elements d e l a v a r i a t i o n d i u r n e . Memorial d e l a M G t g o r o l o g i e N a t i o n a l e , 5 1 , 1 4 8 p p . Lacombe, H.,lY60. Note s u r l e r e g i m e du d d t r o i t d e G i b r a l t a r . Memoires e t T r a v a u x d e l a S.H.F.,11,136-143. L a e v a s t u , T. ,1972. R e p r o d u c t i o n o f c u r r e n t s and w a t e r e x c h a n g e i n t h e S t r a i t of G i b r a l t a r w i t h h y d r o d y n a m i c a l n u m e r i c a l model of Walter Hansen. I n : S t u d i e s i n P h y s i c a l Oceanography. A r n o l d L . G o r d o n , e d i t o r , Gordon and B r e a c h , 2 , 2 1 9 - 2 3 2 . L e F l o c h , J . ,1963. S u r l e s v a r i a t i o n s s a i s o n n i e r e s d e l a c i r c u l a t i o n s u p e r f i c i e l l e d a n s l e s e c t e u r Nord-Est d e l a M 6 d i t e r r a n 6 e Occident a l e . CRE0,5,1,5-10. L i s i t z i n , E . ,1954. L e s v a r i a t i o n s du n i v e a u d e l a m e r Fi Monaco,Compar a i s o n avec q u e l q u e s a u t r e s s t a t i o n s m a r 6 g r a p h i q u e s d e l a c6te f r a n qaise e t i t a l i e n n e . B u l l . Inst.Oc6an.Monaco,1040 ,24pp. Nielsen, J.N.,1912. Hydrography of t h e M e d i t e r r a n e a n and a d j a c e n t wat e r s . D a n i s h Oceanog.Exp.1908-1910,Rep.VI177-191. N y f f e l e r , F. , R a i l l a r d , J. and P r i e u r , L. ,1960. L e b a s s i n L i g u r o - p r o v e n q a l , E t u d e s t a t i s t i q u e des d o n n 6 e s h y d r o l o g i q u e s 1950-1973. Rapp o r t s s c i e n t i f i q u e s e t t e c h n i c y e s CNEX0,42,163pp. P a t t u l l o , J.,Munk, W . , R e v e l l e , R. and S t r o n g , E.,1955. The s e a s o n a l o s c i l l a t i o n s i n s e a l e v e l . J . o f Marine Research,14,1,88-123. P u r g a , N . , M o s e t t i , F . and A c c e r b o n i , E . ,1979. T i d a l harmonic c o n s t a n t s f o r some M e d i t e r r a n e a n h a r b o u r s . B o l l e t i n o D i G e o f i s i c a T e o r i c a Ed A p p l i c a t a , XXI,81,72-81. S t o c c h i n o , C. and T e s t o n i , A . , 1 9 6 9 . Le c o r r e n t i n e l canale d i Corsica e n e l l ' a r c h i p e l a g o Toscan0,C.N. R. , S e r . A,19,26pp. V i l l a i n , M . C . , 1 9 4 9 . S u r l a maree Fi A l g e r e t en M b d i t e r r a n b e Occident a l e . COEC ,9 ,1 6 - 1 9 . V i l l a i n , M.C. ,1952. L e s m a r 6 e s d e l a M b d i t e r r a n g e O r i e n t a l e . COEC, IV,3,92-103. Weather i n t h e Mediterranean,Meteorological o f f i c e , l 9 6 2 . H e r M a j e s t y ' s S t a t i o n e r y Off i c e , London, 3 62pp.
THE WATER CIRCULATION I N THE NORTH-WESTERN
MEDITERRANEAN SEA ,
ITS RELATIONS W I T H W I N D AND ATMOSPHERIC PRESSURE
J.P.
Bethoux(x!
L.
Prieur(x!
F. N y f f e l e r ( x x )
ABSTRACT Mean v a l u e s o f h y d r o l o g i c a l d a t a ( , r e l a t i n g t o t h e p e r i o d 1950-1973) are u s e d t o e v a l u a t e t h e c y c l o n i c f l u x e s i n t h e L i g u r i a n S e a , i . e . , through the Nice-Calvi
s e c t i o n a n d t h e C o r s i c a n c h a n n e l . A marked
s e a s o n a l c y c l e a p p e a r s o f f Nice a n d a l s o t h r o u g h t h e C o r s i c a n c h a n n e l w h e r e are c o n f i r m e d some p r e v i o u s d i r e c t m e a s u r e m e n t s o f t h e f l o w . The s t u d y o f wind a n d a t m o s p h e r i c p r e s s u r e shows t h a t t h e s e e x t e r n a l f o r c e s o n l y e x e r t a moderate e f f e c t on t h e w a t e r c i r c u l a t i o n w h i c h t h e r e f o r e s h o u l d be c h i e f l y t h e r m o h a l i n e . 1
INTRODUCTION The d i f f e r e n t b a s i n s of t h e N o r t h - W e s t e r n
Yediterranean, t h e Tyrrhe-
n i a n S e a , t h e L i q u r i a n S e a , t h e G u l f o f L i o n s and t h e C a t a l a n S e a a r e t h e s i t e o f g r e a t c y c l o n i c c i r c u i t s whose g e n e r a l o u t l i n e w a s s u g g e s t e d by N i e l s e n i n 1 9 1 2 . Most s t u d i e s o f t h e m a r i n e e n v i r o n m e n t a n d t h e n o d e l l i n g o f o b s e r v e d phenomena r e q u i r e a q u a n t i t a t i v e k n o w l e d g e of
w a t e r c i r c u l a t i o n . The w a t e r f l o w of t h e L i g u r i a n S e a f o r m s a n i m p o r t a n t l i n k i n t h e c i r c u l a t i o n of t h e W e s t e r n b a s i n , s i n c e it conn e c t s t h e Tyrrhenian Sea, t h e Algero-provenqal
b a s i n and t h e Gulf
of L i o n s . I n t h i s s t u d y , w e p r e s e n t f i r s t a d y n a m i c a l e v a l u a t i o n o f t h e water f l u x e s i n t h e L i q u r i a n Sea and o f t h e i r s e a s o n a l v a r i a t i o n s . B u t t h e d y n a m i c a l m e t h o d does n o t g i v e a n y i n f o r m a t i o n upon t h e ext e r n a l or i n t e r n a l f o r c e s w h i c h c r e a t e t h e m e a s u r e d d e n s i t y g r a d i e n t s . So, a f t e r , w e e x a m i n e w h a t may b e t h e r e s p e c t i v e e f f e c t s o f t h e ex-
t e r n a l f o r c e s , wind and a t m o s p h e r i c p r e s s u r e , on t h e d i f f e r e n t calculated fluxes. L a b o r a t o i r e de P h y s i q u e e t C h i m i e M a r i n e s , E r a CNRS, S t a t i o n M a r i n e , BP 8 , 06230 V i l l e f r a n c h e s u r M e r , F r a n c e . ( X x ) U n i v e r s i t 6 d e N e u c h a t e l , I n s t i t u t d e G E o l o g i e , 11 r u e E . A r q a u d , CH 2 0 0 0 N e u c h a t e l , S u i s s e .
130
44"
Ligurian Sea
43'
-
-
Strperficial Layer
,
.-...- P intermediate 7"
Layer
8"
9"
6 3 F i g . 1. Mean v a l u e s of f l u x e s ( i n 1 0 m / s e c , o r Sv) i n t h e s u p e r f i c i a l (0-200m) and i n t e r m e d i a t e (200-800m) l a y e r s , t h r o u g h t h e N i c e C a l v i s e c t i o n and t h e C o r s i c a n c h a n n e l .
131 2
FLUXES AND T H E I R SEASONAL VARIATIONS The p r e s e n c e of a M a r i n e S t a t i o n a t V i l l e f r a n c h e s u r M e r h a s a l l o w e d
t h e f r e q u e n t t a k i n g of m e a s u r e m e n t s o f f Nice and a l o n g t h e N i c e - C a l v i s e c t i o n , t h r o u g h t h e permanent c y c l o n i c c i r c u l a t i o n . T h i s c i r c u l a t i o n c o n c e r n s t h e s u p e r f i c i a l w a t e r ( a b o u t 0-200m)
and i n t e r m e d i a t e w a t e r
(200-800rn). Two s t r e a m s o f m e r i d i o n a l waters f l g w up b o t h s i d e s of Cor-
s i c a , j o i n a t t h e N o r t h o f Cap C o r s e and form t h e L i g u r i a n c u r r e n t which l a p s t h e s h o r e s o f t h e I t a l i a n R i v i e r a and t h e F r e n c h C 6 t e d'Azur. I n t h e case o f l a c k of d i r e c t m e a s u r e m e n t s , t h e knowledge of t h e c i r c u l a t i o n i s b a s e d on t h e c a l c u l a t i o n o f t h e g e o s t r o p h i c c u r r e n t a c r o s s t h e Nice-Calvi
s e c t i o n . D u r i n g t h e p a s t y e a r s , a growing number of
h y d r o l o g i c a l d a t a h a v e been e x p l o i t e d , ; ' and r e c e n t l y h y d r o l o g i c a l aver a g e s h a v e been c a l c u l a t e d , b a s e d on t h e c o m p l e t e m e a s u r e m e n t s c a r r i e d o u t d u r i n g t h e p e r i o d o f 1950-1973 and s t o r e d i n t h e a r c h i v e s of t h e d a t a bank o f t h e BNDO-COB
i n B r e s t ( N y f f e l e r e t a l . , 1980). Among 2 2 0 0
h y d r o l o g i c a l s t a t i o n s i n t h e L i g u r i a n S e a , m o r e t h a n 9 0 0 , on t h e N i c e C a l v i s e c t i o n , w e r e t h u s u s e d . The a v e r a g e a n n u a l f l o w s on t h i s sect i o n h a v e been c a l c u l a t e d a n d , t a k i n g i n t o a c c o u n t t h e w a t e r b u d g e t of t h e b a s i n , t h e f l u x e s t h r o u g h t h e C o r s i c a n c h a n n e l have been deduct e d and compared t o p r e v i o u s e s t i m a t i o n s (Bethoux e t a l . , 1 9 8 0 ) . I n f i g u r e 1 and i n t a b l e 1 are p r e s e n t e d t h e mean f l o w s r e l a t i n g t o t h e s u p e r f i c i a l (SL) and i n t e r m e d i a t e (IL) l a y e r s , t h e
+
sign indicates
a SW/NE f l o w ( o f f C a l v i ) o r a S/N f l o w ( C o r s i c a n c h a n n e l ) , and t h e s i g n i n d i c a t e s a f l o w i n a NE/SW d i r e c t i o n ( 1 Sv = 1 0 6m 3 /set).
-
TABLE 1
Mean f l o w s i n t h e s u p e r f i c i a l (SL) and i n t e r m e d i a t e ( I L ) l a y e r s i n t h e L i q u r i a n Sea N i c e side
SL IL
-
-
Calvi side
1.4 s v
0.4
+ +
0.7 0.2
sv
Corsican channel
+
+
0 . 7 Sv 0.2
T h e s e f l o w s a r e l a r g e , e s p e c i a l l y o f f N i c e , where t h e s t r o n g e s t c u r r e n t s s t a y c h a n n e l l e d i n t o a c o a s t a l band 2 0 t o 3 0 m i l e s wide. The s u r f a c e s p e e d s a r e between 2 0 t o 3 0 cm/sec and are below 5 cm/sec from 250m downwards (see F i g . 2 ) .
I n t h e c e n t r a l z o n e t h e a v e r a g e geo-
s t r o p h i c f l o w s are a l m o s t n u l l . T h e f l o w s on b o t h s i d e s of Cap Corse a r e , on an a v e r a g e , e q u a l , which shows t h e i m p o r t a n c e of t h e T y r r h e n i a n S e a o u t f l o w , p e r h a p s u n d e r e s t i m a t e d u p t o now.
132
500
800 .rn
i
10amB
1I
ZONE
14
NICE
15
10
\i
16
30
18
17
60
19
20
8G
miles
CALVI
F i g . 2. V e r t i c a l p r o f i l e s of c u r r e n t s t h r o u g h t h e Nice-Calvi (90 m i l e s , d i v i d e d i n t o 7 z o n e s numbered 1 4 t o 20).
section
133
For t h e s t u d y of t h e s e a s o n a l c y c l e s , based on h y d r o l o g i c a l c r i t e r i a and t a k i n g i n t o a c c o u n t t h e number of h y d r o l o g i c a l s t a t i o n s a v a i l a b l e e a c h month, t h e y e a r h a s been d i v i d e d i n t o 7 p e r i o d s which c o r respond t o t h e months : period : month :
1 I
2 I1
3
4 IV,V
I11
5
V1,VII
6 VIII;IX,X
7 X1,XII
The s e a s o n a l c y c l e s of t h e w a t e r f l o w s o f f Nice and C a l v i , and, f o r comparison, t h r o u g h t h e C o r s i c a n c h a n n e l , a r e shown i n f i g u r e 3 , f o r b o t h w a t e r l a y e r s examined. The flow on t h e h a l f s e c t i o n on t h e Nice s i d e shows a v e r y marked c y c l e , w i t h a n o t i c e a b l y h i g h r i s e between p e r i o d s 6 (August, September and O c t o b e r ) and 7 (November and December) from 1 . 4 t o 2.3 Sv. Near C a l v i , t h e " s e a s o n a 1 c y c l e of f l o w i s less marked and seems more i r r e g u l a r . I n comparison w i t h t h e two p r e c e d i n g c y c l e s , a c y c l e comparable t o t h a t found o f f N i c e i s found i n t h e Cors i c a n c h a n n e l , w i t h a v e r y marked f l u x i n c r e a s e between p e r i o d s 6 and 7 , when t h e t o t a l f l o w ( 0 - 4 0 0 m )
g o e s from 0 . 2 t o 1 . 8 Sv. On t h i s
graph concerning t h e flow through t h e Corsican channel, t h e d i r e c t measurements of S t o c c h i n o and T e s t o n i (1969) c a r r i e d o u t i n J u n e - J u l y 1966
(0.7 Sv) a r e shown t o g e t h e r w i t h t h o s e of LeFloch ( 1 9 6 3 ) performed
i n F e b r u a r y and i n August 1960 (1.5 and 0.4 Sv, r e s p e c t i v e l y ) . These d i r e c t f l o w measurements a g r e e w i t h o u r c a l c u l a t e d c y c l e and t h e geost r o p h i c c a l c u l a t i o n s . They a l s o c o n f i r m t h e c l e a r d e c l i n e of t h e f l o w between p e r i o d s 2 and 6. During p e r i o d 7, up t o about 30 m i l e s o f f N i c e , t h e a v e r a g e speed c a l c u l a t e d i s 2 8 cm/sec on t h e s u r f a c e and 3 cm/sec a t 3 0 0 m d e p t h . With r e g a r d t o p e r i o d 6 , t h e s p e e d s i n c r e a s e on a v e r a g e by 5 cm/sec between t h e s u r f a c e and 150m d e p t h . Moreover, o f f N i c e , a d e c r e a s e a p p e a r s i n t h e s a l i n i t y of t h e s u r f a c e up t o 300m, on a v e r a g e e q u a l t o 0.07°/oo
(see f i g u r e 4 ) . Such a v a r i a t i o n i n t h e s a l i n i t y cannot
be a r e s u l t of t h e s u r f a c e w a t e r budget
( t h e months of November and
December a r e r e l a t i v e l y r a i n y , b u t a t t h a t t i m e heavy e v a p o r a t i o n a l s o o c c u r s ) and t h e r e f o r e i t l e a d s t o t h e h y p o t h e s i s of t h e i n f l u x of l e s s s a l i n e w a t e r , o r i g i n a t i n g from t h e S o u t h . T h i s d e c r e a s e i n s a l i n i t y ,
i n e f f e c t , between p e r i o d s 6 and 7, can be e q u a l l y o b s e r v e d , i n t h e 0-1OOm
l a y e r , on b o t h s i d e s o f Cap Corse and even between weriod 5
and 6 off C a l v i . D i f f e r e n t a u t h o r s have s t u d i e d t h e e x t e r n a l f o r c e s e f f e c t s (wind, atmospheric p r e s s u r e
,
t i d e ) upon t h e s u p e r f i c i a l f l u x e s i n t h e Mediter-
r a n e a n . Among them, Crepon (1965) examined t h e e f f e c t s of t h e atmosp h e r i c p r e s s u r e v a r i a t i o n s on t h e s e a l e v e l , and E l l i o t t (1979) s t u d i e d
134
m3/s
f
lo6
CALVI
p l I
0 m3A
2.lo' CORSICA CHANNEL
Id
0
' 1 ' 2 ' 3 '
4
I
5
l
6
I
7
' 1 ' 2 ' 3 -
F i g . 3 . S e a s o n a l c y c l e s of f l u x e s o f f N i c e , o f f C a l v i and t h r o u g h t h e C o r s i c a n c h a n n e l . Direct m e a s u r e m e n t s of L e F l o c h (LF) and t h o s e of S t o c c h i n o and T e s t o n i (ST) t h r o u g h t h e C o r s i c a n c h a n n e l .
135
current speed
cmh
3
10
fi section
periods 6and 7
20
30
m
F i g . 4. V e r t i c a l p r o f i l e s of c u r r e n t s and s a l i n i t i e s , o f f N i c e , a t :’ p e r i o d 6 ( A u g u s t , S e p t e m b e r and O c t o b e r ) and 7 (November and December)
.
136 t h e wind e f f e c t on t h e sea l e v e l and c u r r e n t s i n t h e Gulf o f Genova, whereas Laevastu
( 1 9 7 2 ) shown t h a t a b o u t 1/4 o f t h e i n f l o w i n g A t l a n t i c
w a t e r s r e s u l t from t h e t i d a l e f f e c t s ( a m p l i t u d e d i f f e r e n c e a t o p p o s i t e e n d s of t h e s t r a i t s ) t h r o u g h t h e S t r a i t of G i b r a l t a r . S i n c e V i l l a i n ( 1 9 4 9 , 1 9 5 2 ) v e r y f e w s t u d i e s a b o u t M e d i t e r r a n e a n t i d e s h a v e been c a r r i e d o u t . So it i s n o t p o s s i b l e t o e s t i m a t e t i d e e f f e c t s upon t h e c i r c u l a t i o n i n s i d e t h e M e d i t e r r a n e a n . However, owing t o P u r g a e t a l . (19791, t i d e i s s y n c h r o n o u s i n t h e T y r r h e n i a n S e a , and s o , it may have a s t a -
t i c e f f e c t ( s u c h a s a t m o s p h e r i c p r e s s u r e ) on t h e sea l e v e l and on t h e superficial circulation. W e have l o o k e d f o r t h e p o s s i b l e c l i m a t i c e x t e r n a l c a u s e s of t h e i m -
p o r t a n t mean c a l c u l a t e d f l u x e s f i r s t , and t h e n ,of t h e s e a s o n a l c y c l e , by e x a m i n i n g t h e e f f e c t s of wind and o f a t m o s p h e r i c p r e s s u r e v a r i a t i o n s on t h e w a t e r c i r c u l a t i o n , on a m o n t h l y scale. 3
WINDS I N THE NORTH-WESTERN MEDITERRANEAN SEA
I n o r d e r t o make a c o m p a r i s o n w i t h h y d r o l o g i c a l a v e r a g e s ( r e f e r r i n g t o t h e y e a r s 1953-1973) w e have u s e d t h e wind a v e r a g e s r e l a t i v e t o t h e p e r i o d 1951-1960,
o b t a i n e d from t h e m e t e o r o l o g i c a l s t a t i o n s of
Cap BBar, S P t e , PomPgues, Cap C a m a r a t , Cap F e r r a t and Cap Corse (Darchen and DeBlock, 1 9 6 8 ) , which g i v e t h e s p e e d , t h e o r i g i n and t h e
f r e q u e n c y of wind f o r e a c h month.
I t i s p o s s i b l e t o e v a l u a t e t h e con-
s e q u e n t d r i f t f l u x , F , a t 90° t o t h e w i n d , l i n k e d t o t h e s q u a r e of t h e wind s p e e d , V , by t h e e m p i r i c a l e q u a t i o n : F =
2
K pa V 2 w s i n e pw
where p a and p w a r e a i r and w a t e r d e n s i t i e s . I f w e assume t h e adimens i o n a l f r i c t i o n c o e f f i c i e n t , K , be e q u a l t o 2 m3/sec/m
--
2.7
10-2
v2
t h e equation is:
m/sec
3 . 1 Wind a t N i c e (Cap F e r r a t ) T h e a v e r a g e s p e e d of wind a t Cap F e r r a t , from a l l d i r e c t i o n s , i s 2 . 1 m/sec.
However, a s a r e s u l t of d i f f e r e n t s e c t o r s , f r e q u e n c i e s and
s p e e d s of w i n d , t h e r e s u l t i n g a v e r a g e wind s p e e d , V , f a v o u r a b l e t o t h e f l o w (NE/SW) i s o n l y a b o u t 1 m/sec
( s e e F i g . 5 ) . On a h a l f s e c t i o n
4 5 m i l e s o f f N i c e , t h e a v e r a g e f l o w r e s u l t i n g from t h e wind i s a b o u t 2.2
103m3/sec,
t r o p h i c flow.
t h a t i s around one t h o u s a n d t h of t h e c a l c u l a t e d geosI n a d d i t i o n , t h e s e a s o n a l wind c y c l e e x p l a i n s n e i t h e r
q u a l i t a t i v e l y nor q u a n t i t a t i v e l y t h e f l o w c y c l e o f f N i c e .
137
4P
-rrl iI cap
-4
Come
-6
Calvi
12
; : l4 20-
I I
cap BBar
1
V
VI
VII
Vlll
L
IX
I
X
XI
1
XI1 month-
F i g . 5. S e a s o n a l c y c l e of mean r e s u l t i n g w i n d , owing t o i t s s p e e d , f r e q u e n c y , o r i g i n and t o t h e mean l o c a l water c i r c u l a t i o n .
138 3 . 2 Wind o n e i t h e r s i d e o f Cap Corse
T h e c o n s e q u e n t average wind a t Cap Corse i s a l w a y s f r o m t h e N o r t h
o r West s e c t o r s , and it i s t h e r e f o r e a l w a y s u n f a v o u r a b l e t o t h e SW/NE c i r c u l a t i o n o f f C a l v i , a s w e l l a s t o t h e S/N c i r c u l a t i o n t h r o u g h t h e Corsican channel.
Its r e s u l t i n g annual averaqe i s -4.8
f l o w o f f C a l v i , and - 4 . 6
m/sec
m/sec f o r t h e f o r t h e f l o w coming f r o m tce T y r r h e n i a n
S e a . I n a d d i t i o n , t h e s e a s o n a l wind c y c l e h a s n o a p p a r e n t c o n n e c t i o n w i t h t h a t of t h e flows. 3 . 3 Wind i n t h e G u l f o f L i o n s
I n o r d e r t o s t u d y a p o s s i b l e e f f e c t o f t h e wind downstream o f t h e c u r r e n t s , w e e x a m i n e d t h e d a t a o n wind r e l a t i n g t o t h e s t a t i o n s o f t h e G u l f of L i o n s . A t Cap B & a r , S B t e and Porni5gueG t h e d o m i n a n t w i n d ,
a n o r t h w e s t e r l y w i n d , i s t h e r e f o r e f a v o u r a b l e to t h e c i r c u l a t i o n i n t h e o p e n sea (NE/SW). T h e r e s u l t i n g a v e r a g e wind s p e e d s a r e 6 m/sec
a t PomGgues, 1 . 9 m/sec a t S G t e and 9 m / s e c
a t Cap B B a r . F o r t h e l a t t e r
s t a t i o n , w h i c h c o r r e s p o n d s t o t h e m o s t f a v o u r a b l e case ( s e e f i g u r e 5 )
,
t h e NE/SW f l o w g e n e r a t e d b y t h e wind a l o n g a w i d t h o f 4 5 m i l e s i s about 0.2 Sv, which s t i l l o n l y r e p r e s e n t s n e a r l y a t e n t h of t h e geost r o p h i c water f l o w c a l c u l a t e d o f f N i c e . F u r t h e r m o r e a g a i n , t h e s e a s o n a l wind c y c l e d o e s n o t s e e m t o a c c o u n t f o r t h e w a t e r f l o w c y c l e . G a l e s p l a y a n i m p o r t a n t r o l e a s much o n t h e s u r f a c e f l o w ( a d v e c t i o n phenomena) a s o n t h e o c e a n - a t m o s p h e r e
interchanges,particularly i n
t h e Gulf o f L i o n s . But w i t h r e g a r d s t o t h e a v e r a g e c i r c u l a t i o n o f a c y c l o n i c t y p e i n t h e North-Western M e d i t e r r a n e a n , w i n d s c a n o n l y gene-
r a t e a b o u t a t e n t h of t h e c a l c u l a t e d f l o w s , i n t h e m o s t f a v o u r a b l e c a s e , i . e . i n t h e G u l f o f L i o n s . I n t h e Cap Corse s e c t o r , wind i s countrary t o t h e circulation, while off N i c e ,
it i s o n l y s l i g h t l y
f a v o u r a b l e . T h e r e f o r e t h e r e i s d e f i n i t e l y n o d i r e c t r e l a t i o n between t h e w a t e r f l u x e s and t h e w i n d s , on a m o n t h l y o r s e a s o n a l s c a l e , i n t h e North-Western Mediterranean. 4
ATMOSPHERIC PRESSURE I N THE WESTERN MEDITERRANEAN The a t m o s p h e r i c d i s t u r b a n c e s a b o v e . t h e M e d i t e r r a n e a n h a v e been
s t u d i e d i n p a r t i c u l a r by B e r e n g e r ( 1 9 5 5 ) . One p e c u l i a r i t y , d u e t o t h e g e o g r a p h i c p o s i t i o n and t h e m o u n t a i n o u s s u r r o u n d i n g s , i s t h a t 73% o f t h e d e p r e s s i o n s i n t h e M e d i t e r r a n e a n are formed a b o v e t h i s
sea, c h i e f l y i n t h e W e s t e r n b a s i n . I n t h i s b a s i n , t h e most f r e q u e n t a r e t h e G u l f o f Genova ( 5 0 % ) , t h e B a l e a r i c ( 2 5 % ) a n d t h e S a h a r a n d e p r e s s i o n s . T h e s e d i s t u r b a n c e s o c c u r p r i n c i p a l l y i n autumn
,
winter
139 and s p r i n g . T h e r e i s t h e r e f o r e a marked c o n t r a s t between summer and t h e other seasons. V a r i o u s a u t h o r s h a v e d e a l t w i t h t h e q u a l i t a t i v e l i n k a g e between a t m o s p h e r i c p r e s s u r e and sea l e v e l i n t h e M e d i t e r r a n e a n . L i s i t z i n (1954) s t u d i e d t h e s e a s o n a l v a r i a t i o n s i n t h e sea l e v e l i n d i f f e r e n t s p o r t s , where t h e s e a l e v e l u n d e r g o e s a g r e a t i n c r e a s e , i n O c t o b e r and November b e f o r e d e c r e a s i n g a t t h e beginning of t h e y e a r . T h i s c y c l e t h e r e f o r e p r e s e n t s remarkable analogies with t h e c u r r e n t wariations off N i c e , but, according t o P a t t u l l o et al.
(1955). a high sea l e v e l during t h e
f a l l ( a low s e a l e v e l d u r i n g s p r i n g ) i s a s s o c i a t e d w i t h t e m p e r a t u r e and s a l i n i t y f l u c t u a t i o n s i n . t h e u p p e r 1OOm ( s p e c i f i c volume f l u c t u a tions)
.
However, a t Monaco, M a r s e i l l e , , , and P o r t o - M a u r i z i o ,
Lisitzin
found q u i t e a good c o r r e l a t i o n between t h e m o n t h l y v a r i a t i o n s i n t h e s e a l e v e l and t h o s e o f t h e a t m o s p h e r i c p r e s s u r e . A c c o r d i n g t o Crepon (19651, t h e v a r i a t i o n s i n a t m o s p h e r i c p r e s s u r e , i n e f f e c t , a r e t h e e s s e n t i e l c a u s e of t h e v a r i a t i o n s i n t h e a v e r a g e l e v e l o f v a r i o u s p o r t s i n t h e W e s t e r n b a s i n . On t h e o t h e r h a n d , a c c o r d i n g t o Lacombe ( 1 9 6 0 ) , a l o w e r i n g o f t h e a t m o s p h e r i c p r e s s u r e upon t h e M e d i t e r r a n e a n i n v o l v e s an i n c r e a s e o f t h e i n f l o w (0-200111) t h r o u g h t h e S t r a i t s of G i b r a l t a r , i n c o u n t e r p a r t , a r i s e o f t h e o u t f l o w ( a t more t h a n 180m d e p t h ) w i l l accompany a rise o f t h e a t m o s p h e r i c p r e s s u r e . In order t o estimate t h e f l u x e s due t o t h e atmospheric v a r i a t i o n s of s e a l e v e l , w e s t u d i e d t h e M e d i t e r r a n e a n a t m o s p h e r i c p r e s s u r e v a r i a t i o n s a s a f u n c t i o n o f t h e s e a s o n s , whose e x a m p l e s a r e g i v e n i n f i g u r e 6.
-
The Gulf o f Genova b e i n g a low p r e s s u r e c e n t e r a l l a l o n g t h e y e a r ,
a h o r i z o n t a l g r a d i e n t o f p r e s s u r e e x i s t s from G i b r a l t a r t o Genova-The s e a s o n a l c y c l e of t h e f i g u r e 6 a , e s t i m a t e d from t h e d a t a i n 'Weather i n the Mediterranean'
-
(1962)
,
shows a jump between p e r i o d 6 and 7.
The f i g u r e 6 b i s a r e p r o d u c t i o n o f t h e a v e r a g e s e a s o n a l c y c l e of
t h e a t m o s p h e r i c p r e s s u r e a t N i c e , f o r t h e y e a r s 1951-1960
(Garnier,
1 9 6 6 ) . The a v e r a g e p r e s s u r e i s maximum d u r i n g p e r i o d 6 and minimum i n
F e b r u a r y o r March.
-
The l o c a l a t m o s p h e r i c p r e s s u r e shows a marked s e m i - d i u r n a l c y c l e ,
whose a m p l i t u d e a t N i c e ( f i g u r e 6 c ) v a r i e s from 0.7mb i n p e r i o d 6 t o a b o u t 0.9mb i n p e r i o d 7 . Owing t o t h e s e a s o n a l c y c l e s o f t h e a t m o s p h e r i c p r e s s u r e
(figures 6
a , b and c ) it i s p o s s i b l e t o f i n d q u a l i t a t i v e c o r r e l a t i o n s between f l u x e s and a t m o s p h e r i c p r e s s u r e v a r i a t i o n s o f f N i c e .
However, o n l y
t h e s e m i - d i u r n a l a t m o s p h e r i c p r e s s u r e v a r i a t i o n ( F i g . 6 c ) can p r o d u c e
140
m o r e o r l e s s p e r m a n e n t f l u x e s , on a m o n t h l y o r s e a s o n a l s c a l e . F o r t h e whole M e d i t e r r a n e a n ( a r e a a b o u t 2 . 5 1 0 1 2 r n 2 ) , a s e m i - d i u r n a l a m p l i t u d e of t h e a t m o s p h e r i c p r e s s u r e of a b o u t 0.8mb i n v o l v e s a maximum i n - o r - o u t
f l o w i n g f l u x e s o f 0 . 4 6 S v , t h a t i s a b o u t 3 0 % of t h e e s t i -
mated f l u x e s t h r o u g h t h e S t r a i t s of G i b r a l t a r ( B e t h o u x , l 9 7 9 ) . The f l u x e s o f f N i c e f o r m i n g a n i m p o r t a n t b r a n c h of t h e c i r c u l a t i o n i n t h e W e s t e r n b a s i n , it i s d i f f i c u l t t o know t h e i n f l u e n c e a r e a of t h e a t m o s p h e r i c p r e s s u r e v a r i a t i o n s . A s a rough e s t i m a t e , i f w e suppose t h a t t h e s t a t i c v a r i a t i o n s of t h e s e a l e v e l o f a b o u t h a l f of t h e Wes12 2 t e r n b a s i n ( 0 . 4 1 0 m ) a c t on t h e f l u x e s o f f N i c e , t h e a t m o s p h e r i c s e m i - d i u r n a l v a r i a t i o n s may p r o d u c e a b o u t 10% of t h e c a l c u l a t e d f l u x e s . S e m i - d i u r n a l v a r i a t i o n s o f t h e a t m o s p h e r i c p r e s s u r e may be an i m p o r t a n t cause of t h e Mediterranean c i r c u l a t i o n , s p e c i a l y through t h e S t r a i t s of G i b r a l t a r
(and S i c i l y ) . B u t , i n t h e L i g u r i a n S e a , t h e y d o n o t seem
t o i n v o l v e more t h a n 1 0 % o f t h e f l u x e s , and d o n o t e x p l a i n t h e i n c r e a s e o f t h e f l u x e s between p e r i o d 6 and 7.
A P(GIBRALTAR 3-
- GENOVA)
6a
2
c
P(NICE)
15t
6b 1 -
--
-
0.5
- 1
' 1 ' 2 ' 3 '
4
IV ' V
I
I
Vl
5 I
A P(NICE)
65
I
Vlf
I
VIiI
'
6 IX
1
7
I
period
X ' XI ' XI1 ' month
F i g . 6 . S e a s o n a l c y c l e of a t m o s p h e r i c p r e s s u r e d i f f e r e n c e s between G i b r a l t a r and Genova ( 6 a ) , o f mean a t m o s p h e r i c p r e s s u r e i n Nice ( 6 b ) and of i t s s e m i - d i u r n a l v a r i a t i o n s ( 6 c ) .
141 5
CONCLUSION
The u s e of a v e r a g e s , c a l c u l a t e d from a g r e a t number o f h y d r o l o g i c a l s t a t i o n s between N i c e and C a l v i , a l l o w s n o t o n l y t h e a v e r a g e a n n u a l v a l u e o f t h e f l o w s b u t a l s o t h e i r s e a s o n a l v a r i a t i o n s t o be o b t a i n e d . These r e s u l t s s h o u l d be u s e f u l i n d i f f e r e n t s t u d i e s i n dynamics, chem i s t r y o r marine biology. ,.
The L i g u r i a n Sea i s a n i m p o r t a n t l i n k i n t h e c i r c u l a t i o n o f w a t e r i n t h e N o r t h Western M e d i t e r r a n e a n , and i t s e e m s t o be n e c e s s a r y t o p r o v e t h e c a l c u l a t e d f l o w s by d i r e c t measurements on t h e s e c t i o n s o f t h e R i v i e r a - C o r s i c a and E l b a - C o r s i c a . The C o r s i c a n c h a n n e l a p p e a r s t o be one o f t h e key p o i n t s i n t h e u n d e r s t a n d i n g of t h e dynamics o f t h e Mediterranean. I
The s t u d y o f m e t e o r o l o g i c a l d a t a o b t a i n e d on t h e North-West c o a s t s
l e a d t o a t t r i b u t e a r e d u c e d e f f e c t on t h e a v e r a g e water f l o w s and on t h e i r s e a s o n a l v a r i a t i o n s by t h e m e c h a n i c a l a c t i o n o f wind. On t h e o t h e r hand, t h e s e m i - d i u r n a l v a r i a t i o n o f a t m o s p h e r i c p r e s s u r e may o n l y p r o d u c e a b o u t 10% o f t h e f l u x e s , and c a n n o t e x p l a i n t h e a n n u a l c y c l e of t h e c i r c u l a t i o n o f f N i c e . S i n c e t h e s e two e x t e r n a l f o r c e s , wind and a t m o s p h e r i c p r e s s u r e ,
e x e r t moderate e f f e c t s , t h e c i r c u l a t i o n should be c h i e f l y thermohaline. The i n i t i a l c y c l o n i c g y r e , c e r t a i n l y produced by t h e water d e f i c i t (Bethoux,l980)
,
i s a m p l i f i e d by t h e a r r i v a l a t t h e p e r i p h e r y o f t h e
b a s i n of waters coming from t h e s o u t h e r n r e g i o n s . These w a t e r s a r e r e l a t i v e l y warmer and l e s s s a l i n e t h a n t h o s e o f t h e c e n t r a l a r e a . S u c h a t h e r m a l a d v e c t i o n by t h e s u r f a c e w a t e r s i s s u e d from t h e T y r r h e n i a n S e a i s c l e a r l y v i s i b l e on t h e i n f r a - r e d
et a1.,1979).
s a t e l l i t e p i c t u r e s (Bethoux
The i n c r e a s e o f t h e c i r c u l a t i o n between p e r i o d 6 and 7
is d u e t o t h e d e n s i t y c o n t r a s t between t h e c o a s t a l and c e n t r a l waters. I n t h e c e n t r a l a r e a , t h e sea u n d e r g o e s t h e e f f e c t s o f t h e n e g a t i v e h e a t and w a t e r b u d g e t s w i t h t h e a t m o s p h e r e , w h i l e , i n t h e p e r i p h e r y a r e a , s u c h e f f e c t s a r e lowered by t h e r m a l a d v e c t i o n o f s o u t h e r n waters and by c o a s t a l r u n o f f .
Acknowledgements- T h i s work w a s s u p p o r t e d by CNEXO ( c o n t r a c t 79/2084) CNRS (GRECO 3 4 ) and Fonds N a t i o n a l S u i s s e d e l a Recherche S c i e n t i f i q u e [ p r o j e c t 2 . 2 8 0 . 0 7 9 ) . T h a n k s a r e due t o F. L o u i s , who d r a f t e d t h e f i g u r e s .
142 REFERENCES B e r e n g e r , M. ,1955. E s s a i d ' b t u d e m S t 6 o r o l o g i q u e d u b a s s i n m s d i t e r r a ngen. Mgmorial d e l a M 6 t C o r o l o g i e N a t i o n a l e , 4 0 , 42pp. Bethoux , J . P . , 1 9 7 9 . B u d g e t s o f t h e M e d i t e r r a n e a n S e a . T h e i r dependance on t h e l o c a l c l i m a t e and on t h e c h a r a c t e r i s t i c s of t h e A t l a n t i c waters. O c e a n o l . A c t a , 2 , 2 , 1 5 7 - 1 6 3 . Bethoux , J . P . ,1960. Mean w a t e r f l u x e s a c r o s s s e c t i o n s i n t h e M e d i t e r r a n e a n S e a , e v a l u a t e d on t h e b a s i s of water and s a l t b a g e t s and of o b s e r v e d s a l i n i t i e s . Oceanol. Acta,3,1,79-88. B e t h o u x , J . P . , P r i e u r , L. and A l b u i s s o n , M. ,1979. A p p o r t s d e l a t ' e l 8 d e t e c t i o n i n f r a - r o u g e Fi l a c o n n a i s s a n c e d e l a c i r c u l a t i o n s u p e r f ic i e l l e d a n s l a p a r t i e Nord-Est du b a s s i n O c c i d e n t a l . Rapp.Com. i n t . Mer M g d i t . , 25-26. B e t h o u x , J . P . , N y f f e l e r , F. and P r i e u r , L. ,1960. U t i l i s a t i o n d e moyennes hydrologiques pour l e c a l c u l d e s f l u x d'eau dans l e bassin Lig u r o - p r o v e n q a l , X X V I I C o n g r e s C I E S M , C a g l i a r i , 9 - 1 8 o c t o b r e 1980,4pp. C r e p o n , M.,1965. I n f l u e n c e d e l a p r e s s i o n a t m o S p h e r i q u e s u r l e n i v e a u moyen d e l a M g d i t e r r a n g e O c c i d e n t a l e e t s u r l e f l u x a t r a v e r s l e d 6 t r o i t d e G i b r a l t a r . C a h i e r s O c b a n o g r a p h i q u e s , XVII,1,15-32. D a r c h e n , J. and d e B l o c k , A. ,1968. Le v e n t s u r l e s c 6 t e s d e l a F r a n c e M 6 t r o p o l i t a i n e , M 6 d i t e r r a n G e . Monographies d e l a M E t 6 o r o l o g i e N a t i o n a l e , 6 2 ,2 ,9 7pp. E l l i o t t , A . J . , 1 9 7 9 . The e f f e c t of l o w f r e q u e n c y w i n d s on s e a l e v e l and c u r r e n t s i n t h e G u l f o f Genova. O c e a n o l . A c t a , 2 , 4 , 4 2 9 - 4 3 3 . G a r n i e r , M.,1966. Climatologie d e l a France, Elements d e l a v a r i a t i o n d i u r n e . Memorial d e l a M G t g o r o l o g i e N a t i o n a l e , 5 1 , 1 4 8 p p . Lacombe, H.,lY60. Note s u r l e r e g i m e du d d t r o i t d e G i b r a l t a r . Memoires e t T r a v a u x d e l a S.H.F.,11,136-143. L a e v a s t u , T. ,1972. R e p r o d u c t i o n o f c u r r e n t s and w a t e r e x c h a n g e i n t h e S t r a i t of G i b r a l t a r w i t h h y d r o d y n a m i c a l n u m e r i c a l model of Walter Hansen. I n : S t u d i e s i n P h y s i c a l Oceanography. A r n o l d L . G o r d o n , e d i t o r , Gordon and B r e a c h , 2 , 2 1 9 - 2 3 2 . L e F l o c h , J . ,1963. S u r l e s v a r i a t i o n s s a i s o n n i e r e s d e l a c i r c u l a t i o n s u p e r f i c i e l l e d a n s l e s e c t e u r Nord-Est d e l a M 6 d i t e r r a n 6 e Occident a l e . CRE0,5,1,5-10. L i s i t z i n , E . ,1954. L e s v a r i a t i o n s du n i v e a u d e l a m e r Fi Monaco,Compar a i s o n avec q u e l q u e s a u t r e s s t a t i o n s m a r 6 g r a p h i q u e s d e l a c6te f r a n qaise e t i t a l i e n n e . B u l l . Inst.Oc6an.Monaco,1040 ,24pp. Nielsen, J.N.,1912. Hydrography of t h e M e d i t e r r a n e a n and a d j a c e n t wat e r s . D a n i s h Oceanog.Exp.1908-1910,Rep.VI177-191. N y f f e l e r , F. , R a i l l a r d , J. and P r i e u r , L. ,1960. L e b a s s i n L i g u r o - p r o v e n q a l , E t u d e s t a t i s t i q u e des d o n n 6 e s h y d r o l o g i q u e s 1950-1973. Rapp o r t s s c i e n t i f i q u e s e t t e c h n i c y e s CNEX0,42,163pp. P a t t u l l o , J.,Munk, W . , R e v e l l e , R. and S t r o n g , E.,1955. The s e a s o n a l o s c i l l a t i o n s i n s e a l e v e l . J . o f Marine Research,14,1,88-123. P u r g a , N . , M o s e t t i , F . and A c c e r b o n i , E . ,1979. T i d a l harmonic c o n s t a n t s f o r some M e d i t e r r a n e a n h a r b o u r s . B o l l e t i n o D i G e o f i s i c a T e o r i c a Ed A p p l i c a t a , XXI,81,72-81. S t o c c h i n o , C. and T e s t o n i , A . , 1 9 6 9 . Le c o r r e n t i n e l canale d i Corsica e n e l l ' a r c h i p e l a g o Toscan0,C.N. R. , S e r . A,19,26pp. V i l l a i n , M . C . , 1 9 4 9 . S u r l a maree Fi A l g e r e t en M b d i t e r r a n b e Occident a l e . COEC ,9 ,1 6 - 1 9 . V i l l a i n , M.C. ,1952. L e s m a r 6 e s d e l a M b d i t e r r a n g e O r i e n t a l e . COEC, IV,3,92-103. Weather i n t h e Mediterranean,Meteorological o f f i c e , l 9 6 2 . H e r M a j e s t y ' s S t a t i o n e r y Off i c e , London, 3 62pp.