i
I
i
!
t
3
:.
I
Honday
Rosshy-gravity
:cial references
IP
I
l
t
I
i
i
,
.50'
7
,/@
Shelf station
I
L --'
i
.. ..' .
'
-----
. .
,//A-.
.
f
I
l
\
8'30'
35'
17'30'
.
25'
\
20
I
'-Elo
15'
Fig. la The Cap-Vert Penisnula; isobaths and locakion of the
coastal and shelf stations.
(
BAMURS
,
Fig. lb Directions of southern, northern coasts
and typical distribution of surface isotherms, i n
winter, w i t h respect t o wind.
144
global
mg/m3
and Qf
monthly total chlorophyll
m/month
vourab
A.
73- 79
corre1
equal
I
smalle.
compri:
puted
c
doublec
L
I
'
.a
d
pectivr
means
(
Wlf
I
DIRECTI[
Fig. 2 T o t a l chlorophyll "a" c o n t e n t and upwellings : c o a s t a l and oceanic.
(
C
PLANT-BIOMASS AND UPWELLING AROUND THE PENINSULA : LOCAL AND LARGE SCALE EFFECTS
The t o t a l chlorophyll "a" c o n c e n t r a t i o n s of t h e northern c o a s t a l s t a t i o n a r e less
abundant, about 3 mg/m 3 P e r year (Fig. 3 )
; however
t h e s e v a l u e s are r e l a t i v e l y high
.c
;1
H
and r e s u l t s u r e l y o f e i t h e r l o c a l v e r t i c a l motions o r advected c o l d c o a s t a l waters
from t h e North because of t h e southern b a r r i e r and of t h e dominant southward s u r f a c e
current.
An i d e n t i c a l wind d r i v e s t h e waters washing both s i d e s of the Peninsula
b u t i t s d i r e c t i o n i s o f t e n favourable t o a higher Ekman o f f s h o r e t r a n s p o r t from t h e
southern c o a s t (see t h e upper graphics o f Fig. 3 ) .
f o r t h e period of A p r i l t o September, i.e.
--
---
----"-
T h i s remark i s v a l i d p r i n c i p a l l y
s p r i n g and summer.
c e n t r a t i o n s a r e w e l l s u s t a i n e d a t t h e "Dakar-South"
Also chlorophyll con-
s t a t i o n b u t n o t a t t h e "Dakar-
North" s t a t i o n where t h i s p e r i o d corresponds t o a minimum.
Consequently, t h i s period
6
must be, with r e s p e c t t o t h e wind d i r e c t i o n , t h e most p r o p i t i o u s t o t h e hydrodynamic
Separation between the two ecosystems.
Moreover the comparison o f t h e w i n t e r s and s p r i n g s of 1979 and 1980 (Fig. 3) indi-
4-
c a t e s a p o s i t i v e c o r r e l a t i o n between t h e wind v e l o c i t y and t h e southern chlorophyll
biomass
:
while v e l o c i t i e s changed, from about 4.7
m/s
(1979) t o 6.0 m/s
(198Q),
winter and s p r i n g chlorophyll c o n c e n t r a t i o n s approximately doubled from 10.5 t o 19 mg/
m3 a s t h e wind str.esses
:
t h e squares o f wind speeds 4.7 and 6.0 are r e s p e c t i v e l y 21
and 36.
F i n a l l y , when a l o c a l upwelling i s n o t p o s s i b l e , t h a t i s t o say, d u r i n g December
1979 a t t h e Thiaroye-station and during t h e "15 May-15 Septemb'er" p e r i o d a t t h e Yoff-
2
.-
i
o
1
s t a t i o n , w e remark a good c o r r e l a t i o n t o o with an annual minimum of chlorophyll.
On the.whole, t h e s e r e s u l t s confirm t h e real e f f e c t s o f t h e l o c a l upwelling on the
Fig. 3 Cirespect t o
f i v e days
146
r
global v a r i a t i o n s o f c h l o r o p h y l l : concordance between the'maximums o f wind v e l o c i t i e s
and of chlorophxll on the one hand, minimums of plant-biomass i n t h e case o f non f a vourable wind d i r e c t i o n s on the o t h e F hand.
However, as mentiqned above, there i s no
c o r r e l a t i o n "chlorophyll-coastal upwelling" during th@ summer, when the wind v e l o c i t i e s
equal only 3-4 m / s and g i v e , consequently, v e r t i c a l v e l o c i t i e s two or three time
smaller while chlorophyll c o n c e n t r a t i o n s s t i l l remain r e l a t i v e l y high and f r e q u e n t l y
comprised between 8 and 9 mg/m 3
.
As a matter of fact, the standard d e v i a t i o n s com-
puted on s e r i e s of d i f f e r e n t averaged months f o r the y e a r s 1973-1979 i n d i c a t e a
doubled v a r i a b i l i t y during t h e s p r i n g months with r e s p e c t t o t h e summer months, resp e c t i v e l y about 4 and 2 mg/m3, t h a t is t o say 30 % and 20 0 of t h e s p r i n g and sim e r
means (see t a b l e I).
î
less
high
Z r s
rface
La
CHLOAOPHYLL CON TENT
the
>ally
conir-
ieriod
iamic
indiI
1yll
15-
2-1
19
ms/
I
I
CHLOROPHYLL CONTENT
Ly 21
'
5
O
O
Yoff-
loi1
'
I .
nber
an thc
I
1979
1980
I
Chlorophyllian comparison of northern and southern s i d e s of t h e peninsula
t o the wind v e l o c i t y and d i r e c t i o n i
f i v e days averages.
Fig. 3
W i t h
146
The series 73-79 i n d i c a t e t o o a low v a r i a b i l i t y for January and November whereas
October and December are more v a r i a b l e
i
l o g i c a l l y , g r e a t v a r i a b i l i t y arises from up-
welling b u
*t a glance a t t h e monthly l o c a l winds o f t h e series 73-79 shows t h e imposs i b i l i t y of an Ekman c o a s t a l upwelling i n c r e a s i n g t h e chlorophyll i n October 1976 and
December 1977 ( s e e t a b l e I).
As a m a t t e r of f a c t ,
the v i r i a b l e r e l a t i o n s h i p between
1
2
'i
i
f
t h e chlorophyll c Q n c e n t r a t i o n s and l o c a l c o a s t a l upwelling is corroborated by t h e
On
tì-
further
Jan coas
ther
BUT
either w
1974 a r e i n coincidence.
forcemen
in the A
accelera
TABLE I
ring
small c o r r e l a t i o n between tables I and I1 : only January 1977, February 1977 and b y
Phyto
Jun
and Nove
Mean monthly concentrations o f t o t a l chlorophyll "a" (mg/m3
1973-79 i p o s i t i v e anomalies are underlined.
a t t h e Thiaroye T-Statiorl
Month
I
II
III
IV
v
VI
VI1
VI11
Final
cumber 1
x
IX
XI
XII
3.4
3.4
5.1
7.3
1.7
4.2
4.5
7.1
-
t h souk
Year
1972
1973
74
75
76
77
78
79
5.9
7.4
3.3
4.7
6.2
8.0
5.0
7.2
9.9 29.6
7.5 11.5
10.8
7.3
-
4.1
11.6
12.0
17.5
6.7
8.9
10.3
18.8
7.7
20.4
7.5
7.3
18.0
8:l
15.3
14.0
8.3
11.6
9.1
10.5 10.2
10.7
6.9
14.2 13.2
9.2
12.6
9.4
-
-
7.8
10.6
14.3
9.0
8.0
11.1
7.8
8.8
11.7
3.5
8.0
7.4
3.2
7.0
17.5
8.5
9.0
12.3
13.1
12.4
10.1
8.0
8.4
*
2.5
4.1
7.6
9.4 12.3
15.0 8.7
'l'ABLE LI
Monthly
Month
3.0
3.6
Yoar
1973
mean
6.9
10.8
11.0
11.4
11.3
11.1
10.7
9.9
9.5
7.5
5.5
6.1
Standard
deviation
2.7
8.5
4.3
4.0
4.6
4.2
. 2.2
2.3
1.8
4.6
2.3
5.0
74
75
7G
77
70
79
TABLE II
TABLE IV
Mean monthly s u r f a c e t h e m a l anomalies a t the Thiarove T- t a t i o n 1973-79 i negative
anomalies a r e underlined ( t e n t h o f degrees a n d ' a f t e r series 60-62, 65-79).
MU11thly :
Month
I
II
III
IV
v
VI
VI1
VIZI
+16
+23
+ 1
- 1
+4
+ 2
- 9
x
Month
XI
XX'
-1- - 7
-15
- 2
-1s'
+5
-3
-.lb
- 4
- 1
+12
*&
+20
iIr,
15
IX
year
1973
74
75
76
77
78
79
devia t i o n
-14
- 6
+15
+1
- 4
- 2
-2
-16
-18 -14
+ 4
+ 5
12
- 2
+ 1
10
- 4
-11
-12
-12
+ 7
- 6
- 6
+ 1
-12
- 7
+ 2
- 3
+ 3
-23
-11
-12
+ 5
+ 6
- 5
- 5
+I7
+21
- 2
- 6
1
+ 1
+ 3
+ 5
+ 2
13
11
16
19
8
-19 -23 -22
-
-
Ohkbhur,
-16
-
5
+ 4
+E -T -16
o - 7 -16
+y
$ 3
-3
- 1
o
+6
+20
+9
7
7
10
16
S&&&*h"&
L
r
*
-
- 8
4-3
d%*k*3-5-
Year
-il
- 5 ~
1973
74
75
76 .
77
78
,I
whereas
i
-
le i m p o s -
jan c o a s t a l s t a t i o n , which i s r e p r e s e n t a t i v e of t h e e q u a t o r i a l upwelling, g i v e s ra-
. 1976 and
)
ther surprising results
i
a g r e a t p a r t of positive chlorophyll anomalies c o i n c i d e s
e i t h e r w i t h , t o our mind, l a r g e p u l s a t i o n s of the Mauritanian upwelling or w i t h r e i n -
between
I
'y t h e
'
On t h e contrary. an examination of long temperature series (see t a b l e s III, IV)
f u r t h e r North a s a t t h e Saint-Louis c o a s t a l s t a t i o n , o r f u r t h e r South a s a t the Abid-
from up-
forcements of t h e e q u a t o r i a l upwelling. Such events occur namely i n 1976-77 n o t o n l y
i n t h e A t l a n t i c b u t a l s o i n t h e e q u a t o r i a l P a c i f i c (Wyrtki, 1979) and arise from an
and May
a c c e l e r a t i o n of t h e t r a d e s along t h e e q u a t o r i a l band.
I
Phytoplanktonic blooms may r e s u l t from the mauritanian upwelling p r i n c i p a l l y du??
r i n g June 1975, February and October 1977 ; from e q u a t o r i a l upwelling d u r i n g October'
and November 1976, J
$e and December
1977.
F i n a l l y , it i s l i k e l y t h a t d u r i n g more than two years, s i n c e June 1975 u n t i l De-
T-St a t i o n
Ø
cember 1977, l a r g e - s c a l e e f f e c t s are dominant on the c h l o r o p h y l l c o n c e n t r a t i o n s o f
the southern coast.
c
XII
7.3
1.7
4.2
4.5
7.1
7.5
3 .O
3.6
6.1
l
5 .o
i
1973
74
75
76
77
78
79
153
169
160
156
172
176
162
166
162
156
158
- 169
165
164
161
160
162
169
177
176
175
176
182
-
196
193
240
241
269
267
269
2 .
268
208
264
273
203
260
232
191 233
278
272
746
3nQ
272
497
278
243
2
202
-
285
281
284
- ___
280
283
274
___
I
170
178
---
-i 77
-
TABLE IV
jative
I
Monthly sea-surface temperatures a t the Abidjan (Ivory c o a s t ) s t a t i o n .
Month
.5
3
16
16
1
12
20
'
¿'>
-I,
<
'
I
\
c
#
I
I
II
275
264
273
259
256
274
273
273
271
257
269
III
IV
285
279
273
282
270
v
VI
VI1
VI11
IX
x
XI
XII
qcm
Year
1973
74
75
76
77
78
- -
-
769
766
347
77s
m c
xc.
?cm
273
288
273
289
278
279
275
280
251
253
229
238
210
222
?in
777
76.6
780
797
351
714
268 267
769
76.7
-
In order to obtain a schematic representation of the very different upwelling
effects around the peninsula, it is useful to reproduce (Fig. 4 and 5) first a conceptual ,node1 of the hydrological structure (isothens and currents) on both coasta
according to Rebert (unpublished manuscript, 1979).
To our mind, two essential fea-
tures result from the difference of the shelfs.
x
O'
Fig. 4 Weak upwelling of the north coast starting from 50 m depth only.
Fig. 5 Upwelling of the south coast ; destabilization of the thermocline and
vertical flow starting from 100 m depth.
\
,
The first one is the opposite cross-shelf flow, offshore qt the n o r a
14.0'
and onshore at the southern coast.
The second one is the destabilization of the thermocline in the case
Of
the gen6lY
inclined southern slope. As a reslllt, strong tridimensional turbdewae may @GelOP
above the shelf-edge, in the same time as a,divergence of surface currents (Pig, 6).
The resulting doming of isotherms is often observed along the axis of the
shelf in winter in the vicinity of the 30-50 m isobaths.
30'
SOUwern
(Recall too the figure 1b
giving the same isothermal configuration than figure 6).
The doming structure of the southern upwelling enables it to trap at the Coast#
on the left side of the divergence, high nutrients which upwelled along its right:
side.
On the contrary, the pure Ekman offshore transport off the soqthern C W S t re*
moves quickly the upwelled nutrients away from the Yoff coastal station.
-13'0'
ChloroPhYll
concentrations are consequently higher at the Thiaroye coastal station (or Dakar-Soutb
statioq).
In the next chapter we shall try to "model" variations of chlorophyll
On
both sides of the peninsula with the aid of current and temperature gradients mea-
i
surements above the shelf (50 m bottom) and at the coast.
-30'
i
5
4
149
felling
1
AlTEMPT TQMODEL CHLOROPHYLL VARIATIONS ON BOTH SIDES O F CAP-VERT
'st a conk h coasts
I n a d d i t i o n t o t h e d a i l y c o a s t a l measurement, hydrological and c u r r e n t observat i o n s were made on the s h e l f as mentioned above (Fig. l a ) .
i n t i a l fea-
The p e r i o d s of measure-
ments were always s e l e c t e d t o c o i n c i d e with Neap t i d e s . Thus, f o u r t e e 9 sequences
I
including g e n e r a l l y t h r e e northern and t h r e e southern hydrological s t a t i o n s were made
i n A p r i l , July, August, September, November and December of t h e year 1979 and t h e
first t h r e e months
1
Of
t h e year 1980.
These sequences were s h o r t and c u r r e n t , depth,
temperature and conductivity r e c o r d i n g s with Aanderaa instruments permit o n l y elementary s t a t i s t i c s on t h e r e s u l t s .
I n a d d i t i o n t o t h a t , high c o n c e n t r a t i o n s o f n i t r a $ e s and often:'too o f chlorophyll
were observed e i t h e r on t h e northern or t h e southern s h e l f from t h e s u r f a c e t o
30'
..
..
!
and l a r g e
n coast
?
14'0'
t h e gently
y develop
(Fig. 6 ) .
southern
O'
Eigure l b
2
coast,
;r i g h t
c o a s t re-
3'0'
:hlorophyll
Dakar-South
> p h y l l on
its meaI'
?
Fig. 6 The doming s t r u c t u r e of isotherms above t h e southern s h e l f of Senegal and
the remarkable divergence of s u r f a c e c u r r e n t s ; March 1974. ( A f t e r J.P. Rebert).
I
160
r
t h e bottom.
The same remark however cannot b e made f o r t h e summer when n i t r a t e data
onshor
are absent.
However, because of t h e e x i s t e n c e of domings and r i d g i n g s of t h e thermo-
wellin
1
c l i n e o n l y o f f t h e southern s h e l f , we may presume t h a t ++e n i t r a t e maximum disappears
from t h e northern c o a s t i n summer.
Thu
the se
F i r s t , it i s u s e f u l t o r e c a l l (Fig. 2 and 7) t h e i n t e r e s t i n g concordance between
t h i s ml
t h e annual evolution of t h e southern c o a s t chlorophyll and an oceanic mesoscale up-
s i n g o.
welling driven by t h e curl o f t h e s t a t i s t i c a l wind stress i n t h e v i c i n i t y of the
t i e s ec
The monthly averages computed by the author a f t e r meteorological d a t a
s h e l f edge.
lings <
supplied by the U.S. National Oceanic and Atmospheric Administration are grouped i n
modelle
two degrees-side subsquare Marsden and, consequently, g i v e o n l y a poor image o f t h e
wind ar
i n t e n s i t y of mesoscale e d d i e s which may develop t o g e t h e r i n both Atmosphere and Ocean.
condi ti
The improvement i n f i g u r e 7 i s o b t a i n e d by averaging t h e s d e x " b p w e 1 l i n g o f f river
generat
Casamance, a t the southern e x t r e m i t y o f Senegal, w i t h t h a t of t h e south-Dakar s h e l f ,
1979).
t h a t i s t o say, subsquares c e n t r e d r e s p e c t i v e l y a t t h e l a t i t u d e s 11.5 N and 13.5 N
along 17.7 W.
This averaging i s n o t a r b i t r a r y s i n c e t h e summer r e v e r s a l of t h e shelf
If w
of a ce
c i r c u l a t i o n from south to n o r t h may advect t h e n u t r i e n t s upwelled off Casamance as
include
f a r as t h e s o u t h e m s i d e o f t h e Peninsula.
w e obtn
12-
I
OFF CASAMANCE 111.5 N )
a-1
4-
o'
Y
J
F M
AMaJrJCArS
I
I
I
O N O
1
e
J
Fig. 7 Modelling o f t h e annual chlorophyll w i t h mesoscale upwellings d r i v e n by the
c u r l s of s t a t i s t i c a l wind stress.
We emphasize a l s o t h a t excess of chlorophyll with r e s p e c t t o mesoscale upwelling dur i n g t h e summer months (June, J u l y , August and September) may arise from continental
f e r t i l i z a t i o n as summer i s t h e r a i n y season.
A t last, i n o r d e r t o
p o i n t out the slow
*
151
:en n i t x a t e d a t a
onshore advection o f n u t r i e n t s along t h e slope, t h r e e months running means o f up-
9s of t h e thenno-
welling were used and centred on t h e t h i r d month.
ximwn disappears
Thus w e have obtained a simple s t a t i s t i c a l model which g i v e s t h e broad f e a t u r e s of
t h e seasonal evoLution of chlorophyll.
rdance between
mesoscale up-
However, t h e v e r t i c a l v e l o c i t i e s computed with
t h i s model a r e smaller than 12 m/month o r khan 5.10-4 c m / s , whereas t h e observed rising of c o a s t a l isotherms and r a p i d growing of chlorophyll r e q u i r e v e r t i c a l velocit i e s e q u a l o f t e n t o 10-2 cm/s o r 10 m/day.
ulogkcal d n t a
l i n g s o f west Africa (Hagen, 1974) and it i s obvious t h a t upwelling e f f e c t s should be
These values are f r e q u e n t i n t h e upwel-
,ire grouped i n
modelled a t t h e scales (day-10 km), because o f l i k e l y resonance phenomena between t h e
r image of t h e
wind and the c u r v a t u r e s of i s o b a t h s around t h e peninsula
:
mesoscale hydrographic
qphere and Ocean.
c o n d i t i o n s may be caused by t h e modal s t r u c t u r e o f b a r o t r o p i c c o n t i n e n t a l s h e l f waves
ling off r i v e r
generated by wave f l u c t u a t i o n s i n t h e meridional component of NE-trade winds (Hagen,
th-Dakar s h e l f ,
1979).
N an8 13.5 N
Casamance a s
If we follow Arthur (19651, w e are a b l e t o e s t i m a t e v e r t i c a l motions a t t h e south
o f a cape, s t a r t i n g from t h e v o r t i c i t y equation and n e g l e c t i n g a l l t h e terms ( f r i c t i o n
included) which do n o t comprise t h e r e l a t i v e v o r t i c i t y .
With t h e s e approximations,
we obtain
!,
%
= (5 .t
Dt
t\;
f)
-\
.6
-2
".
37
--,
?y
- 1
i:
7>
-s
1
c i t y , p o s i t i v e i n cage o f upwelling.
v e r t i c a l g r a d i e n t o f the v e r t i c a l v e l o c i t y by t h r e e d i f f e r e n t ! methods :
use t h e d a t a of f i g u r e 6, x a d i i of curvature o f isotherms and v e l o c i t i e s o f
(i)
c u r r e n t s ( w i t h Arthur, we n e g l e c t the v e l o c i t y g r a d i e n t normal to t h e stxeamline) ;
(ii) c a l c u l a t e , using the c u r r e n t measurements a t t h e s h e l f - s t a t i o n s , t h e term
avhx
-
au/ay on t h e same f i g u r e :
(iii) estimate
-
W/ay f o r the s t a t i o n s i n 1979-1980 l o c a t e d o f f Yoff and o f f
Thiaroye.
I n o u r a r e a of i n t e r e s t , ne& t h e peninsula, w e g e t t h e following values :
. , .*~
1)
2.4 IO-'
2)
5
31
23 values, a l l p o s i t i v e , of which 22 a r e comprised between t h e values 4.2 10-5
s-'
(from Fig. 6 ) with a r a d i u s o f c u r v a t u r e equal t o 24 km;
lom5 s - l , of
which 3.10-5 for t h e term av/ax (from Fig. 6 ) ;
and 0.3 10-5 s-1.
upwelling du-
m continental
It out the slow
?
)
n i t y of t h e
; a l of t h e s h e l f
A
The most f r e q u e n t value is
equal t o 4.10-'
and t h e mean i s 1.6
:
t h a t gives a v o r t i c i t y
by e x t r a p o l a t i o n from 2.
Wit3 t h i s v a l u e of t h e v o r t i c i t y , t h e c h a r a c t e r i s t i c time o f change-around t h e peninsula i s 3.10''
and by i n t e g r a t i o n between t h e depths O and 100 m, we o b t a i n an
i)
7
152
P
upwelling o f 4.10-2
*
cm/s-l
or 33 m p e r day.
i
T h i s v e r t i c a l v e l o c i t y appears too high,
doubt1ess"because o f t h e numerous approximations ; b u t "10 m p e r day" i s a very plaus i b l e value.
i
An i n t e r e s t i n g r e s u l t a r i s e s from t h e t h i ~ d m e t h o dand concerns t h e va-
r i a b i l i t y of t h e upwel1ing:the h i g h e s t v e l o c i t i e s may be' t w o t o f o u r t i m e s t h e average
or t h e most f r e q u e n t value. Thus, f o r a y e a r l y averaged chlorophyll c o n c e n t r a t i o n o f
3
about 10 mg/n , it w i l l be n o t s u r p r i s i n g t o observe some maxima a t 30 o r even
3
50 mg/m
Moreover t h e r a t i o of t h e extremes v a l u e s of chlorophyll, equal t o a b o u t
R
1
h
.
1/30 i s of t h e same o r d e r as t h e r a t i o o f extreme v o r t i c i t i e s , 1/14.
1
Comparison o f t h e local winds and t h e i n d i v i d u a l v o r t i c i t i e s estimated by t h e t h i r d
t
method shows an absence of c o r r e l a t i o n which may be r e l a t e d t o the mesoscale and lar-
1
30-
ge s c a l e e f f e c t s suggested above : t h e local v o r t i c i t y depends o11 the mauritanian and
maybe too on t h e e q u a t o r i a l winds ; it must vary moeeover (Rebert, p e r s o n a l communi-
i
a3
c a t i o n ) with t h e c o a s t a l thermohaline c i r c u l a t i o n .
The l a s t attempt i n modelling (Fig. 8) i s t o use the north-south g r a d i e n t of s u r f a c e bimonthly averaged temperatures between the two c o a s t a l s t a t i o n s , namely a n i n dex o f c i r c u l a t i o n , i n o r d e r t o e s t i m a t e t h e annual v a r i a t i o n of t h e r e l a t i v e abundance o f chlorophyll between both s i d e s o f t h e peninsula.
Y
The c y c l e o f t h e r a t i o
i
southern/northern chlorophyll, which may d i f f e r l a r g e l y from one year t o another
seems t o be p a r t i a l l y c o n t r o l l e d by t h e south-north g r a d i e n t of s u r f a c e temperature
m
-
and p e c u l i a r l y by t h e r e v e r s a l of s u r f a c e c u r r e n t s .
On t h e whole, t h e r a t i o follows an e v o l u t i o n along a schematic c y c l e c o n s i s t i n g o f
f o u r branchs which coincide w e l l with t h e f o u r seasons ; i n w i n t e r the r a t i o i n c r e a ses slowly whereas t h e s u r f a c e southern waters are r a p i d l y c o l d e r ; i n s p r i n g the
temperature g r a d i e n t reaches i t s maximum and, i n the same time, t h e ratio i n c r e a s e s
$5
f
f
.I
much, e s p e c i a l l y i f t h e winds a r e s t r o n g and enduring, l i k e i n 1980 u n t i l the end o f
June.
This process of n u t r i e n t and c h l o r o p h y l l accumulation on t h e southern s h e l f
must be l o g i c a l l y dependent on t h e i n t e n s i t y of t h e mesoscale wind v o r t i c i t y which
advects waters both warmer and r i c h e r .
I n summer a high r a t i o i s s u s t a i n e d b u t t h e
a b s o l u t e values of chlorophyll c o n c e n t r a t i o n decrease on both s i d e s because of t h e
growing grazing by numerous young f i s h .
Since October, with t h e r o t a t i o n of winds
5
1,
20
-
4
Change
scale
10
Eo
4
3
2
1 -
and t h e north-south r e v e r s a l of c u r r e n t s , t h e r e is a n e t decrease of t h e south-north
O
chlorophyll r a t i o , from t e n o r more, t o one.
-1
SUMMARY
Since a long t i m e f i s h e r y b i o l o g i s t s s u s p e c t d i f f e r e n c e s i n t h e abundance and d i s t r i b u t i o n ( l o c a t i o n , s i z e ) of f i s h on both s i d e s o f t h e peninsula. This d i f f e r e n c e
+f
i s f i r s t m a t e r i a l i z e d by an oceanic f r o n t l o c a t e d a t t h e "Pointe d e s Almadies", with
i
s t r o n g l y s t r a t i f i e d waters on i t s n o r t h e r n f l a n k . Northern waters are o f t e n two o r
t h r e e degrees warmer than southern waters.
Chlorophyll observations p o i n t o u t a
g r e a t e r d i f f e r e n c e between both s i d e s than s u r f a c e p h y s i c a l c o n d i t i o n s : i n f a c t ,
6
ia
Ph
f
t
Fig. 6 I
south-no
ppears too high,
'
is a very plauconcerns the va-
times the average
concentration o f
3
RATIO SOUTHERN CHLOROPHYLL
NORTHERN
o r even
?qual t o about
t (eo)
Ju 1
i t e d by t h e t h i r d
ioscale and l a r mauritanian and
l
30
sona1 comuni'\
a d i e n t of s u r namely an i n e l a t i v e abun-
I: t h e r a t i o
another
.1
"1
Change of
scale
\
-\
A1 (80)
3c
\
,
Maximums
OF
\
I
\
temperature
i'
c o n s i s t i n g of
ri
ratio i n c r e a spring t h e
io i n c r e a s e s
'
il t h e end o f
lthern s h e l f
ì c i t y which
lined b u t t h e
:I
i *
l M 2 (80)
I
REVERSALOF
ause o f t h e
m of winds
i
'
south-north
-
9
OIFFERENCÊ NORTHERN SOUTHERN
COASTAL SURFACE TEMP.
lance and d i s i
difference
nadies", with
"ten two o r
n t out a
:
i n fact,
v e r t i c a l v e l o c i t i e s , threedimensional turbulence and mesoscale eddies p l a y an impor-
t a n t r o l e i n t h e space-time v a r i a b i l i t y o f t h e n u t r i e n t s and t h u s of chlorophyll.
A
d e s c r i p t i v e review, axed on the d i f f e r e n t s c a l e s of motions d r i v i n g t h e upwellings
o f f Senegal, shows :
1)
DIFFER
the r e a l e f f e c t s of t h e local upwelling by Ekman offshore t r a n s p o r t e s p e c i a l l y
during w i n t e r w i t h a well s u s t a i n e d divergence on t h e southern s h e l f (Fig. 6 ) ;
2)
the g r e a t negative anomalies o f temperature i n s p r i n g which suggest a double ac-
3)
the p o s s i b l e e f f e c t s during s p e c i a l y e a r s o f t h e l a r g e - s c a l e c i r c u l a t i o n w i t h an
t i o n of b o t h mesoscale and l o c a l upwelling;
i
I
t
T e n t a t i v e models show t h e i n f l u e n c e of t h e slope and width o f t h e s h e l f (Fig. 4
2
Cyclonic v o r t i c i t y i s t h e most f r e q u e n t and i t s v a r i a b i l i t y may e x p l a i n t h a t
il
of t h e chlorophyll by upwelling.
The most a p p l i c a b l e r e s u l t seems t o be, on t h e whole, t h e c l o s e d c o r r e l a t i o n exis-
i
t i n g between t h e s t a t i s t i c a l annual e v o l u t i o n s of t h e chlorophyll and o f the c u r l s of
.t
1
A convincing demonstration should experiment the
real synoptic winds from moored s h i p s o r buoys
:
balance t h e g e n e r a l oceanic c i r c u l a t i o n .
Arthur, R.S., 1965. On t h e c a l c u l a t i o n of v e r t i c a l motion i n e a s t e r n boundary curr e n t s from determinations of h o r i z o n t a l motion. J. Geophys. Res., 70, 12:323-327.
'
Hagen, E.,
1974.
Migrations d e s poissons démersaux l e long des
de l a t i t u d e nord. Cah. Q.R.S.T.O.M., S é r .
A simple scheme o f t h e development o f c o l d water upwelling c i r c u B i t r . Meereskunde, 33:115-125.
l a t i o n cells along t h e northwest a f r i c a n c o a s t .
Hagen, E. and Weiss, R., 1979. Mesoscale c o a s t a l upwelling dynamics o f f t h e N-W
a f r i c a n coast and hypothetic r e l a t i o n s t o the chub mackerel concentrations.
I.C.E.S.
C.M.
1979/C:21:1-8.
Margalef, R., 1978. Life-forms of phytoplankton a s s u r v i v a l a l t e r n a t i v e s i n an uns t a b l e environment. Ocean. A c t a , 1, 4:493-509.
Wyrtki, K.,
1979.
E l Nino.
La Recherche, 10, 106:1212-1220.
Muc
1
7
i
I
REFERENCES
Champagnat, C. and Domain, F., 1978.
c ô t e s o u e s t a f r i c a i n e s de 10 ZI 24'
Qcéanogr., 16, 3-4:239-261.
INTROD
a p o s i t i v e conclusion could suggest
t h a t t h e p r i n c i p a l r o l e of mesoscale e d d i e s i s t o damp t h e g r e a t f l u c t u a t i o n s and t o
'
and fj
of Per
ammoni
linear
fixed
plankt
nutrie
I
temperature and the r e l a t i v e chlorophyll on both s i d e s , suggesting a f o u r seasons
the mesoscale wind stress (Fig. 7 ) .
D if
c
and 5 ) , a p l a u s i b l e c o r r e l a t i o n (Fig. 8 ) between t h e north-south g r a d i e n t o f s u r f a c e
cycle.
ABSTRA
t
i n s i g h t i n t o t h e e q u a t o r i a l upwelling.
i
,
the ei
action
t o cou
place
computq
large
I
t
Succes
i
devote#
It
1
i
i
I
solve
I
prohib
cost, 1
chemic.
only 1
contrii
Tux1
measurt
and Okt
i
I
)
t h e dir
portinr
slight1
Elsevier Oceanography Series, 32
*cr
-/-
-
-....
ECOHYDRODYNAMI
\ri,
PROCEEDINGS OF THE 12th INTERNATIONAL
ON OCEAN HYDRODYNAMICS
6
Edited by
LIÈGE COLLOQUIUM
'
JACQUES C.J. NIHOUL
Professor of Ocean Hydrodynamics,
University of Li2ge
Liège, Belgium
..
ELSEVIER SCIENTIFIC PUBLISHING COMPANY
Amsterdam
- Oxford -
New York
1981