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Seasonal Observations in the Western Indian Ocean
17
SEASONAL OBSERVATIONS IN THE WESTERN INDIAN OCEAN Michele FIEUX and CLAIRE LEVY L.O.P. Museum Paris
-
LA 175 CNRS
Abstract XBT and surface salinity observations which are part of the SINODE (Surface INdian Ocean Dynamics Experiment) program are presented to describe the variations in the surface layers of the western Indian ocean between Cape Guardafui and Reunion island. In the Somali area, the temperature structure presents a high variability related to a strong eddy structure; the surface salinities are a good indicator of the circulation. At the equator, during the two transition periods, after the two monsoons, when the eastward jet is present, the temperature structures are different :the thermocline is broader after the SW monsoon. South of the equator, the South Equatorial Current northern limit wanders between 6 ' s and 9 ' s .
1.
INTRODUCTION The "MARION DUFRESNE" who supplies the "Terres Australes
et Antarctiques FranGaises" in the southern Indian Ocean crosses the tropical and equatorial western region of the Indian ocean four times a year.To reach the seasonal variations, since the SINODE cruises of 1979 and 1980, this opportunity is used to make XBT sections, surface temperature and salinity measurements, ship drift estimations along the route between Qpe
Guardafui and la Reunion,
and to launch satellite tracked drifting buoys at the equator. Here are presented the preliminary results from the first eight SINODE cruises except the drifting buoys ones which are presented in this volume by REVERDIN et al.. The routes are somehow different from one cruise to another (Fig.1 and 2 ) . During SINODE 1 , SINODE 2 and SINODE 6 zig-zag sections were done in the Somali basin. I t is only during SINODE 4 , SINODE 7 and SINODE 8 that the direct meridional route had to be followed because no ship time was available. Surface temperature and salinity were recorded continuously except during SINODE 7. The ship drifts, estimated from the difference between dead reckoning
18 and satellite positioning, were obtained only for SINODE 4 , 5 , 7 , 8. The X B T probes
(T 7 type) gave the temperature down to at least
850 meters. Due to the ship program constraints, the cruises are not evenly spread over the seasons
;
the months sampled are May,
June, July, October, November and December i.e. at the beginning and during the S w monsoon, during the transition period of October and during the beginning of the NE monsoon.
Fig. 1. Routes and ship drift of cruises : SINODE 1, SINODE 2, SINODE 4 and SINODE 7.
19
,, ,.,
,,
,2;i 10.E
Fig. 2. Routes and ship drift of cruises :SINODE 3, SINODE 5, SINODE 6 and SINODE 8 .
2.
2.1
PRELIMINARY RESULTS Somali basin The temperature structure in that area is similar to the
density structure because the density variations are more dependent o n the temperature than on the salinity. The slope of the isopycnes along a section gives the component of the geostrophic current perpendicular to the section. Thus the geostrophic current perpendicular to the section can be guessed from the slope of the isotherms.
20 I n t h e Somali b a s i n t h e lominant f e a t u r e of m o s t of temperature
( e x c e p t i n J u n e 1 9 7 9 a n d May 1 9 8 0 ) i s a t r o u g h
sections
i n t h e t h e r m o c l i n e w i t h a s t e e p e r f r o n t on t h e n o r t h e r n on t h e s o u t h e r n one of
the
(fig.3,
side than
t h e 2OoC isotherm l i e s i n t h e middle
t h e thermocline t h u s can be taken as t h e thermocline index
(QUADFASEL,
1982)).
10' N l
5' N 4
,
,
,
00 i
,
,
~
......
. ___
-
SINODE 3 SINODE 4
Oct 80 D e c 80
SINODE 5
May 81 J u l y 81 O c t 81 Nov 81
- SINODE 6
__
SINODE 7 SINODE 8
_._ F i g . 3.
20' C i s o t h e r m d e p t h .
T h i s t r o u g h c o r r e s p o n d s t o a n a n t i c y c l o n i c eddy p r i n t , northern
al.
Somali eddy
1966,
(Bruce,
1982) which
stress c u r l
1968,
the
1 9 7 8 , SWALLOW e t
1979, DOING,
s e e m s t o b e d r i v e n b y t h e SW m o n s o o n w i n d
(SCHOTT e t a l .
t o a p p e a r , ANDERSON 1 9 8 0 ) . I n J u n e 7 9
a n d May 8 0 , t h e s e c t i o n s w e r e m a d e j u s t o n e o r t w o d a y s a f t e r t h e t h e SW m o n s o o n s o t h e e d d y w a s n o t f o r m e d y e t .
onset of vations
show t h e b e g i n n i n g o f
a cooling
(AT
1
2°C)
The o b s e r -
i n the surface
l a y e r close t o t h e c o a s t which i s a r a p i d r e s p o n s e t o t h e o n s e t (SCHOTT e t a 1 sation of 7'
,
1980). I n October of
t h e s a m e y e a r a f t e r t h e ces-
t h e SW m o n s o o n t h e t r o u g h w a s b r o a d w i t h a n a x i s a r o u n d
N and a n o r t h e r n
f r o n t a r o u n d 9'30
N
I n December 8 0 ,
(fig.4).
a t t h e b e g i n n i n g o f t h e N E m o n s o o n t h e t r o u g h w a s much r e d u c e d a n d the northern
f r o n t h a s m i g r a t e d t o a r o u n d 11'30
N.
The s u r f a c e
d r i f t s are i n agreement with t h e subsurface temperature s t r u c t u r e w i t h a s t r o n g n o r t h e a s t w a r d d r i f t i n t h e f r o n t area and a southwest w a r d d r i f t f r o m 1 0 ° N t o a r o u n d 4"N w h i c h b r i n g s h i g h s a l i n i t y w a -
t e r s from t h e Arabian Sea of
(S%.,,
3 6 % , ) . I n May 8 1 ,
after the onset
t h e S W m o n s o o n w i t h n o o b s e r v a t i o n b e t w e e n D e c e m b e r a n d May,
a n o r t h e r n eddy w i t h a n o r t h e r n f r o n t around
there
i s t h e p r i n t of
ll"N,
l e s s s t e e p t h a n i n t h e p r e v i o u s December
drifts directions
(Fig.2)
( F i g . 3 ) . The s h i p
are coherent with t h e subsurface struc-
21 400 E
60°
E
00' E
60. E
I
I
I
I
SEC
i I
I 00.E
60' E
60.E
F i g . 4. P o s i t i o n s of t h e n o r t h e r n f r o n t s , of t h e buoy d r i f t s a t t h e e q u a t o r and o f t h e t h e r m o c l i n e r i d g e n o r t h o f t h e S o u t h E q u a t o r i a l C u r r e n t , f o r e a c h c r u i s e when a v a i l a b l e .
ture
: t h e s u r f a c e s a l i n i t y i s h i g h i n t h e eddy
o f 5"N t h e s u r f a c e s a l i n i t y d e c r e a s e s southern origin
(LEETMAA
et al.
( s > 35.6%,)
( S < 35.3%,) which
eddy between of
f r o n t a r o u n d 8'45N.
10'N
and
12'N.
south
1972). A t t h e beginning of J u l y 81,
t h e t r o u g h i s m u c h more s o u t h w i t h t h e a x i s a r o u n d 7'30N and a n o r t h e r n
;
shows i t s
T h e r e i s t h e s i g n of
The s a l i n i t y i s h i g h e r
(Flg.3)
a smaller
t h a n 36%, n o r t h
10°N ( w a t e r f r o m A r a b i a n S e a ) a n d h i g h e r t h a n 3 5 . 6 % " b e t w e e n
22 3'N
and
10'N. 8 1 , t h e t r a c e o f a n a n t i c y c l o n i c eddy i s p r e -
I n October s e n t b e t w e e n 3'30
a n d 7'30
N
which i s q u i t e s o u t h
N with t h e northern f r o n t around l o N
( f i g . 3 ) . North of
shows t h e s o u t h e r n p a r t o f
it, the temperature section
a n o t h e r a n t i c y c l o n i c eddy.
The s h i p
d r i f t d i r e c t i o n s a r e i n agreement w i t h a d o u b l e eddy s t r u c t u r e ( F i g . 1). Between
and 2 ' N
1'N
i n May 81, 1'30
N a n d 2'30
N in July
8 1 and 1"N and 3 " 3 0 N i n O c t o b e r 8 1 , t h e r e a r e c o o l e r a n d f r e s h e r waters a t the surface ward d r i f t d i r e c t i o n s . s i o n of
o r 2"
(1'
C c o o l e r ) which correspond
This can be r e l a t e d t o t h e eastward exten-
t h e s o u t h e r n l o o p which l e a v e s t h e c o a s t around 3'N
b r i n g s w a t e r s from t h e S o u t h E q u a t o r i a l SWALLOW e t a l .
e t al.,
(SC
t o appear i n J . P . O . ) .
81, 6 w e e k s l a t e r t h e r e was o n l y o n e t r o u g h b e t w e e n 6'N with a n o r t h e r n f r o n t around
10'30
N
and
C u r r e n t and E a s t A f r i c a
c o a s t a l c u r r e n t w i t h low s a l i n i t y c h a r a c t e r i s t i c s (LEETMAA
t o east-
35.3%0). I n November and
llON
( F i g . 3 ) . Again t h e s u r f a c e
d r i f t d i r e c t i o n s a r e i n agreement with t h e subsurface s t r u c t u r e ( F i g . 5 ) a n d w i t h t h e s u r f a c e s a l i n i t y w h i c h i s maximum b e t w e e n a n d 10'N of
c o r r e s p o n d i n g t o t h e w e s t w a r d d r i f t of
the northern
ward d r i f t ,
eddy.
I n c o n t r a s t north of
t h e s u r f a c e s a l i n i t y i s lower
t o t h e v a l u e s b e t w e e n 3'N pear
south of
0'30
and
?ON.
1O"N i n t h e n o r t h e a s t (35.6 < S < 3 6 % 0 ) s i m i l a r
The v a l u e s l e s s t h a n 3 5 . 3 % 0 a p -
N i n the eastward equatorial j e t .
To s u m m a r i z e t h e s e o b s e r v a t i o n s seems t h a t t h e p r i n t o f a f t e r t h e o n s e t of
1°N
the southern p a r t
the northern
i n t h e Somali b a s i n ,
it
Somali eddy i s always t h e r e
t h e SW monsoon w i t h a t r o u g h a s d e e p a s 1 5 0 me-
t e r s and w i t h a s t e e p e r f r o n t i n t h e n o r t h which c o u l d be due t o t h e p r e s e n c e of
the coast.
The s u r f a c e s a l i n i t y i s a g o o d i n d e x o f
t h e c i r c u l a t i o n : westward d r i f t i n t h e s o u t h e r n b r a n c h of b r i n g s h i g h Arabian Sea s a l i n i t y w a t e r s low s a l i n i t y w a t e r s Somali c u r r e n t l o o p .
(S
ri
35.380)
Drift,
(S>
a r e brought
f r o m t h e SEC by t h e
s u r f a c e s a l i n i t y and s u b s u r f a c e
t u r e a r e u s u a l l y i n good agreement
S t r U C -
( F i g . 5 ) . The o b s e r v a t i o n s made
o n d i f f e r e n t y e a r s a t t h e same s e a s o n a r e n o t s i m i l a r , i n October
t h e eddy
36%,),on the contrary,
f o r example
81, a s h a r p t e m p e r a t u r e r i d g e i s p r e s e n t around 8"N,
much f u r t h e r s o u t h t h a n i n O c t o b e r 8 0 when t h e s t r u c t u r e was a s i n gle-eddy
one w i t h n o r t h e r n
f r o n t a r o u n d 9 O 3 0 N.
t h e s e o b s e r v a t i o n s , t h a t a t t h e end of
But i t seems,
from
t h e eddy supposed l i f e t h e
23 northern subsurface front gets steeper
(cf. Nov.
81 a n d Dec.80)
w h i c h c o u l d b e d u e t o t h e c o n c e n t r a t i o n of t h e e d d y e n e r g y i n a much restricted coastal zone.
Fig. 5. Surface salinity, surface drift and depth of SINODE 8 , November 1981.
2 O o C isotherm during
24
F i g . 6. E q u a t o r i a l t e m p e r a t u r e s e c t i o n from ( i n May 1 9 8 0 ) .
2.2
to
62' 3 0 ' E
Equatorial region t h e e q u a t o r t h e e a s t w a r d j e t which a p p e a r s e a s t of
A t
r o u n d 49'
et al.
April-May
1981,
LUYTEN,
and October-November
e t al.
(with speed around l m / s ) a n d i n November section
a-
- 50° E , between t h e monsoons, d u r i n g t h e t r a n s i t i o n
E
periods of
(WYRTKI,
1 9 7 3 , CRESSWELL,
1980) w a s p r e s e n t d u r i n g e a c h c r o s s i n g
e x c e p t i n J u l y 8 1 d u r i n g t h e SW m o n s o o n
8 1 when t h e j e t w a s p r e s e n t f u r t h e r e a s t o f
the
(Fig.4). The t h e r m o c l i n e s t r u c t u r e
with
46' 3 0 ' E
longitude along the equator
May 8 0 w h e r e
d o e s n o t s h o w much v a r i a t i o n s
(Fig.6,8)
the thermocline is thinner.
except w e s t of
49'E
in
This western equatorial
a r e a , where t h e j e t d o e s n o t e x i s t , p r e s e n t s low s a l i n i t y c h a r a c teristics (fig.7).
(S < 35.20%,)i.e.waters The
a r o u n d 49'E.
of
southern hemisphere o r i g i n
s a l i n i t y f r o n t e x t e n d s from 0 t o 9 0 meters and s t a n d s As
in
1980,
t h e 49'E
July 81 temperature section pre-
s e n t s a t h i n n e r t h e r m o c l i n e t h a n a t 53O ponds t o t h e r e t u r n o f
-
54OE
: i t s s l o p e corres-
Somali c u r r e n t f r e s h e r flow from t h e coast.
25 62'30E
46'30E
Om
m m
Fig. 7. E q u a t o r i a l s a l i n i t y s e c t i o n from (May 1 9 8 0 ) .
46' 30' E
to
62'
30'E
C o m p a r i n g now t h e t w o t r a n s i t i o n p e r i o d s c r o s s - e q u a t o r i a l
sections
it appears t h a t the
t h e r m o c l i n e i s much b r o a d e r d u r i n g t h e s e c o n d
period
Dec.)
DIN
(0ct.-
e t al.)
ses o f
Nov.-
when t h e
j e t speeds are higher
(cf.REVER-
( F i g . 9 ) . This difference must r e f l e c t d i f f e r e n t proces-
formation of
the
j e t and/or
d i f f e r e n t water masses involved.
The two d i f f e r e n t c i r c u l a t i o n s y s t e m s a t t h e e n d o f
t h e two monso-
o n s seem t o a f f e c t d i f f e r e n t l y t h e e q u a t o r i a l s t l r u c t u r e d u r i n g t h e two e a s t w a r d j e t t r a n s i t i o n p e r i o d s . The c r o s s - e q u a t o r i a l
temperature
e a s t w a r d j e t p e r i o d d o n o t show a
s e c t i o n s made d u r i n g t h e
s t r o n g convergence.
The j e t m u s t
be f e d p a r t l y by w a t e r s f r o m t h e w e s t a n d p a r t l y t h r o u g h h o r i z o n t a l convergence.
26
E5'
18.
10.
10.
1szoo
62. E
debs* 30 May 79 4 Junr 79 62OE O P E
03*30 E
Fig. 8. Cross e q u a t o r i a l temperature s e c t i o n s a t d i f f e r e n t times and l o n g i t u d e s during t h r e e c r u i s e s .
Fig. 9. year.
Cross e q u a t o r i a l temperature s e c t i o n s a t d i f f e r e n t p e r i o d s o f t h e
21
2.3
South equatorial current and counter-current region T h e XBT sections have been expanded down to Reunion island.
The p r o m i n e n t feature i s the slope of the isotherms between Reunion and a thermocline ridge which varies with the cruises from 9 ' s 6's
to
(Fig.4 and 10). This slope is the mark of the south equatorial
current ( S E C ) which i s steadier and stronger during the SW monsoon period w h e n the SE trade winds are also stronger and when the currents are weak and variable in the region between the thermocline ridge ( l i m i t o f the SEC) and the equator. During the NE monsoon the SE trade winds decrease and the SEC retreats towards the South like
in December 8 0 . increases
Simultaneously the South Equatorial Counter current
( b etween 2 ' 5
and 8 ' 3 0
S in December 80 - Fig.1). Until
now, due to ship program constraints, the sections have not been carried o u t during characteristic period soon). D u r i n g the transition periods mocline r i d g e lies around 6 " -
7 O
(i.e. full SW or NE mon-
(May and Oct - Nov) the ther-
S exc ept in May 81 (Fig.4). In
December 80 a t the beginning of the NE monsoon the ridge had retracted down t o 8 " 3 0
S.
A t depth, in the central water, the ridge o f
the d e e p isotherms i s somewhat displaced towards the south and could be a s far asll's
- 12"s. I n the SECC region the depth varia-
tions o f the thermocline is very small.
0
100
100
100
.oo
¶OO
600
I00
-00
Fig. 10.
South Equatorial Current sections (SINODE 5).
28 3.
CONCLUSION The p r e l i m i n a r y
r e s u l t s of
the
f i r s t e i g h t S I N O D E XBT
s e c t i o n s i n t h e w e s t e r n I n d i a n o c e a n show t h e h i g h v a r i a b i l i t y o f t h e t e m p e r a t u r e s t r u c t u r e i n t h e Somali b a s i n which c a n be r e l a t e d t o the circulation variations. t i e s a r e a good i n d i c a t o r o f perature sections.
In t h a t region,
the
surface salini-
t h e c i r c u l a t i o n t o g e t h e r w i t h t h e tem-
The n o r t h e r n
f r o n t of
t h e Somali eddy i s always
s t e e p e r than t h e southern one. A t
t h e e q u a t o r , during t h e e x i s t e n c e of
the eastward j e t ,
t h e two t r a n s i t i o n p e r i o d s p r e s e n t d i f f e r e n t t e m p e r a t u r e s t r u c t u r e s with a broader thermocline during the Nov.Dec.)
with the northern
(Oct.
jet is faster.
when t h e South of
second t r a n s i t i o n p e r i o d
the equator,
the thermocline ridge associated
l i m i t of
South E q u a t o r i a l Current,
the
varies
between 6 O S and 9OS. Sections during the characteristic periods soons) w i l l permit a b e t t e r d e s c r i p t i o n of
( N E a n d SW mon-
the annual cycle.
The g e o s t r o p h i c c a l c u l a t i o n s u n d e r t a k e n ,
u s i n g mean T - S
w i l l allow t o quantify the circulation.
relationships, A s the
o b s e r v a t i o n s made o n d i f f e r e n t y e a r s a t t h e same
season a r e not similar,
a c a r e f u l study of
the correlation with the
wind s t r e s s v a r i a t i o n s h a s t o be done t o f u r t h e r u n d e r s t a n d t h e s e different features.
Acknowledgments T h i s s t u d y i s i n c l u d e d i n t h e SINODE p r o g r a m s u p p o r t e d by t h e "Terres A u s t r a l e s e t Antarctiques FranGaises" r i e d o u t on b o a r d t h e i r t o i r e d'Ocdanographie relle,
the
"MARION
DUFRESNE", by t h e L a b o r a -
P l i y s i q u e du Museum N a t i o n a l
Laboratoire Associd
seph GONELLA. probes,
ship,
The F r e n c h
(TAAF) a n d c a r d ' H i s t o i r e Natu-
1 7 5 d u CNRS, u n d e r t h e d i r e c t i o n o f J o -
"Marine N a t i o n a l e " p r o v i d e d t h e
t h e o t h e r o n e s w e r e p r o v i d e d by t h e T A A F .
1 9 7 9 XBT
29 REFERENCES
.
The Somali Current. Ocean Modelling, No.34 Anderson,D.L.T.,1980 (unpublished manuscript) . Bruce, J.G. 1968 . Comparison o f near surface dynamic topography during the two monsoons i n the western Indian Ocean. Deep-sea Res., 1 5 , 665-677. Bruce, J.G., 1979 . Eddies o f f the Somali coast during the southwest monsoon. J. Geophys. Res., 8 4 , 7742-7748. Cresswell, G., M.Fieux and J.Gonella . The Wyrtki equatorial jet, MayIJune 1980. Tropical ocean-Atmosphere Newsletter - January 1981. Unpublished manuscript. Duing, W.,1978 . The Somali Current. P a s t and recent observations. I n : Review Papers o f Equatorial Oceanography, Fine Workshop Proceedings. Nova/Nyit Univ. Press (unpublished manuscript). Fieux,M., J.F. Murail, G.Soares, 1980 . Navigation et sections bathythermiques (XBT). Recueil des donnees collectees, campagne SINODE - MD 18 - P E M G . Vol.1. Rapp. Int. d u Laboratoire d'OcBanographie Physique d u Museum Paris. (Unpublished manuscript). F i e u x , ~ . ,~ . ~ r e m o n tA,. ~ a r t a v t s e f f ,J.F. ~ u r a i l ,1982. Navigations sections bathythermiques (XBT) et bouees derivantes. Recueil des donnees collectees, campagne SINODE 11-MD 22 - Vol.1. Rapp. Int. d u Laboratoire d'oceanographie Physique d u Museum Paris. (Unpublished manuscript) . Leetmaa,A.and.V. Truesdale, 1972. Changes in the currents i n 1970 o f f the E a st African coast with the onset o f the southeast mo1.s o o n . J. Geophys.Res., 7 7 , 3281-3283. Leetmaa, A., D.R. Quadfasel, and D.Wilson, 1982 : Development o f the f l o w field during the onset o f the Somali Current, 1979 in J . Phys. Oceanogr. Luyten, J.R., M.Fieux, and J.Gonella, 1980 . Equatorial currents i n the w e s tern Indian Ocean. Scienc e, 209, 600-603. Quadfasel, D.R. 1982. L o w frequency variability o f the 20'C isotherm topography i n the western Indian Ocean. J.Geophys. Res., 8 7 , C 3 , pp. 1990-1996. Schott,F. a n d D.R. Quadfasel, 1980 . Development o f the Northern S o m a l i current gyre i n 1979. Science. Vol, 209. Aug.1980. pp.593-595. Schott,F., a n d D.R. Quadfasel, 1982. Variability o f t h e Somali CUrr e n t system during the monsoon o n s e t , 1979 (to appear in J . P h y s Oceanogr 1 Swallow,J.C. and J.G. Bruce, 1966 . Cu rrent measurements off the S o m a l i coast during the Southwest Monsoon o f 1964. Deep Sea Res. 13-5, 861-888. Sw a l l o w , J.C. and M.Fieux, 1982 . Historical evidence for two gyres i n the S o m ali Current. J. Mar. Res. V01.40, Suppl.pp.747-755. Swallow, J.C., R.Molinari, J.G.Bruce, O.B. Brown. Development o f near-surface flow pattern and water mass distribution i n the Somali b a s in, i n response t o the southwest monsoon o f 1979. T o appear in J. Phys. Oceanogr. Wyrtki, K. 1973. An equatorial jet i n the Indian Ocean. Science,l81 262-264.
.
SEASONAL OBSERVATIONS IN THE WESTERN INDIAN OCEAN Michele FIEUX and CLAIRE LEVY L.O.P. Museum Paris
-
LA 175 CNRS
Abstract XBT and surface salinity observations which are part of the SINODE (Surface INdian Ocean Dynamics Experiment) program are presented to describe the variations in the surface layers of the western Indian ocean between Cape Guardafui and Reunion island. In the Somali area, the temperature structure presents a high variability related to a strong eddy structure; the surface salinities are a good indicator of the circulation. At the equator, during the two transition periods, after the two monsoons, when the eastward jet is present, the temperature structures are different :the thermocline is broader after the SW monsoon. South of the equator, the South Equatorial Current northern limit wanders between 6 ' s and 9 ' s .
1.
INTRODUCTION The "MARION DUFRESNE" who supplies the "Terres Australes
et Antarctiques FranGaises" in the southern Indian Ocean crosses the tropical and equatorial western region of the Indian ocean four times a year.To reach the seasonal variations, since the SINODE cruises of 1979 and 1980, this opportunity is used to make XBT sections, surface temperature and salinity measurements, ship drift estimations along the route between Qpe
Guardafui and la Reunion,
and to launch satellite tracked drifting buoys at the equator. Here are presented the preliminary results from the first eight SINODE cruises except the drifting buoys ones which are presented in this volume by REVERDIN et al.. The routes are somehow different from one cruise to another (Fig.1 and 2 ) . During SINODE 1 , SINODE 2 and SINODE 6 zig-zag sections were done in the Somali basin. I t is only during SINODE 4 , SINODE 7 and SINODE 8 that the direct meridional route had to be followed because no ship time was available. Surface temperature and salinity were recorded continuously except during SINODE 7. The ship drifts, estimated from the difference between dead reckoning
18 and satellite positioning, were obtained only for SINODE 4 , 5 , 7 , 8. The X B T probes
(T 7 type) gave the temperature down to at least
850 meters. Due to the ship program constraints, the cruises are not evenly spread over the seasons
;
the months sampled are May,
June, July, October, November and December i.e. at the beginning and during the S w monsoon, during the transition period of October and during the beginning of the NE monsoon.
Fig. 1. Routes and ship drift of cruises : SINODE 1, SINODE 2, SINODE 4 and SINODE 7.
19
,, ,.,
,,
,2;i 10.E
Fig. 2. Routes and ship drift of cruises :SINODE 3, SINODE 5, SINODE 6 and SINODE 8 .
2.
2.1
PRELIMINARY RESULTS Somali basin The temperature structure in that area is similar to the
density structure because the density variations are more dependent o n the temperature than on the salinity. The slope of the isopycnes along a section gives the component of the geostrophic current perpendicular to the section. Thus the geostrophic current perpendicular to the section can be guessed from the slope of the isotherms.
20 I n t h e Somali b a s i n t h e lominant f e a t u r e of m o s t of temperature
( e x c e p t i n J u n e 1 9 7 9 a n d May 1 9 8 0 ) i s a t r o u g h
sections
i n t h e t h e r m o c l i n e w i t h a s t e e p e r f r o n t on t h e n o r t h e r n on t h e s o u t h e r n one of
the
(fig.3,
side than
t h e 2OoC isotherm l i e s i n t h e middle
t h e thermocline t h u s can be taken as t h e thermocline index
(QUADFASEL,
1982)).
10' N l
5' N 4
,
,
,
00 i
,
,
~
......
. ___
-
SINODE 3 SINODE 4
Oct 80 D e c 80
SINODE 5
May 81 J u l y 81 O c t 81 Nov 81
- SINODE 6
__
SINODE 7 SINODE 8
_._ F i g . 3.
20' C i s o t h e r m d e p t h .
T h i s t r o u g h c o r r e s p o n d s t o a n a n t i c y c l o n i c eddy p r i n t , northern
al.
Somali eddy
1966,
(Bruce,
1982) which
stress c u r l
1968,
the
1 9 7 8 , SWALLOW e t
1979, DOING,
s e e m s t o b e d r i v e n b y t h e SW m o n s o o n w i n d
(SCHOTT e t a l .
t o a p p e a r , ANDERSON 1 9 8 0 ) . I n J u n e 7 9
a n d May 8 0 , t h e s e c t i o n s w e r e m a d e j u s t o n e o r t w o d a y s a f t e r t h e t h e SW m o n s o o n s o t h e e d d y w a s n o t f o r m e d y e t .
onset of vations
show t h e b e g i n n i n g o f
a cooling
(AT
1
2°C)
The o b s e r -
i n the surface
l a y e r close t o t h e c o a s t which i s a r a p i d r e s p o n s e t o t h e o n s e t (SCHOTT e t a 1 sation of 7'
,
1980). I n October of
t h e s a m e y e a r a f t e r t h e ces-
t h e SW m o n s o o n t h e t r o u g h w a s b r o a d w i t h a n a x i s a r o u n d
N and a n o r t h e r n
f r o n t a r o u n d 9'30
N
I n December 8 0 ,
(fig.4).
a t t h e b e g i n n i n g o f t h e N E m o n s o o n t h e t r o u g h w a s much r e d u c e d a n d the northern
f r o n t h a s m i g r a t e d t o a r o u n d 11'30
N.
The s u r f a c e
d r i f t s are i n agreement with t h e subsurface temperature s t r u c t u r e w i t h a s t r o n g n o r t h e a s t w a r d d r i f t i n t h e f r o n t area and a southwest w a r d d r i f t f r o m 1 0 ° N t o a r o u n d 4"N w h i c h b r i n g s h i g h s a l i n i t y w a -
t e r s from t h e Arabian Sea of
(S%.,,
3 6 % , ) . I n May 8 1 ,
after the onset
t h e S W m o n s o o n w i t h n o o b s e r v a t i o n b e t w e e n D e c e m b e r a n d May,
a n o r t h e r n eddy w i t h a n o r t h e r n f r o n t around
there
i s t h e p r i n t of
ll"N,
l e s s s t e e p t h a n i n t h e p r e v i o u s December
drifts directions
(Fig.2)
( F i g . 3 ) . The s h i p
are coherent with t h e subsurface struc-
21 400 E
60°
E
00' E
60. E
I
I
I
I
SEC
i I
I 00.E
60' E
60.E
F i g . 4. P o s i t i o n s of t h e n o r t h e r n f r o n t s , of t h e buoy d r i f t s a t t h e e q u a t o r and o f t h e t h e r m o c l i n e r i d g e n o r t h o f t h e S o u t h E q u a t o r i a l C u r r e n t , f o r e a c h c r u i s e when a v a i l a b l e .
ture
: t h e s u r f a c e s a l i n i t y i s h i g h i n t h e eddy
o f 5"N t h e s u r f a c e s a l i n i t y d e c r e a s e s southern origin
(LEETMAA
et al.
( s > 35.6%,)
( S < 35.3%,) which
eddy between of
f r o n t a r o u n d 8'45N.
10'N
and
12'N.
south
1972). A t t h e beginning of J u l y 81,
t h e t r o u g h i s m u c h more s o u t h w i t h t h e a x i s a r o u n d 7'30N and a n o r t h e r n
;
shows i t s
T h e r e i s t h e s i g n of
The s a l i n i t y i s h i g h e r
(Flg.3)
a smaller
t h a n 36%, n o r t h
10°N ( w a t e r f r o m A r a b i a n S e a ) a n d h i g h e r t h a n 3 5 . 6 % " b e t w e e n
22 3'N
and
10'N. 8 1 , t h e t r a c e o f a n a n t i c y c l o n i c eddy i s p r e -
I n October s e n t b e t w e e n 3'30
a n d 7'30
N
which i s q u i t e s o u t h
N with t h e northern f r o n t around l o N
( f i g . 3 ) . North of
shows t h e s o u t h e r n p a r t o f
it, the temperature section
a n o t h e r a n t i c y c l o n i c eddy.
The s h i p
d r i f t d i r e c t i o n s a r e i n agreement w i t h a d o u b l e eddy s t r u c t u r e ( F i g . 1). Between
and 2 ' N
1'N
i n May 81, 1'30
N a n d 2'30
N in July
8 1 and 1"N and 3 " 3 0 N i n O c t o b e r 8 1 , t h e r e a r e c o o l e r a n d f r e s h e r waters a t the surface ward d r i f t d i r e c t i o n s . s i o n of
o r 2"
(1'
C c o o l e r ) which correspond
This can be r e l a t e d t o t h e eastward exten-
t h e s o u t h e r n l o o p which l e a v e s t h e c o a s t around 3'N
b r i n g s w a t e r s from t h e S o u t h E q u a t o r i a l SWALLOW e t a l .
e t al.,
(SC
t o appear i n J . P . O . ) .
81, 6 w e e k s l a t e r t h e r e was o n l y o n e t r o u g h b e t w e e n 6'N with a n o r t h e r n f r o n t around
10'30
N
and
C u r r e n t and E a s t A f r i c a
c o a s t a l c u r r e n t w i t h low s a l i n i t y c h a r a c t e r i s t i c s (LEETMAA
t o east-
35.3%0). I n November and
llON
( F i g . 3 ) . Again t h e s u r f a c e
d r i f t d i r e c t i o n s a r e i n agreement with t h e subsurface s t r u c t u r e ( F i g . 5 ) a n d w i t h t h e s u r f a c e s a l i n i t y w h i c h i s maximum b e t w e e n a n d 10'N of
c o r r e s p o n d i n g t o t h e w e s t w a r d d r i f t of
the northern
ward d r i f t ,
eddy.
I n c o n t r a s t north of
t h e s u r f a c e s a l i n i t y i s lower
t o t h e v a l u e s b e t w e e n 3'N pear
south of
0'30
and
?ON.
1O"N i n t h e n o r t h e a s t (35.6 < S < 3 6 % 0 ) s i m i l a r
The v a l u e s l e s s t h a n 3 5 . 3 % 0 a p -
N i n the eastward equatorial j e t .
To s u m m a r i z e t h e s e o b s e r v a t i o n s seems t h a t t h e p r i n t o f a f t e r t h e o n s e t of
1°N
the southern p a r t
the northern
i n t h e Somali b a s i n ,
it
Somali eddy i s always t h e r e
t h e SW monsoon w i t h a t r o u g h a s d e e p a s 1 5 0 me-
t e r s and w i t h a s t e e p e r f r o n t i n t h e n o r t h which c o u l d be due t o t h e p r e s e n c e of
the coast.
The s u r f a c e s a l i n i t y i s a g o o d i n d e x o f
t h e c i r c u l a t i o n : westward d r i f t i n t h e s o u t h e r n b r a n c h of b r i n g s h i g h Arabian Sea s a l i n i t y w a t e r s low s a l i n i t y w a t e r s Somali c u r r e n t l o o p .
(S
ri
35.380)
Drift,
(S>
a r e brought
f r o m t h e SEC by t h e
s u r f a c e s a l i n i t y and s u b s u r f a c e
t u r e a r e u s u a l l y i n good agreement
S t r U C -
( F i g . 5 ) . The o b s e r v a t i o n s made
o n d i f f e r e n t y e a r s a t t h e same s e a s o n a r e n o t s i m i l a r , i n October
t h e eddy
36%,),on the contrary,
f o r example
81, a s h a r p t e m p e r a t u r e r i d g e i s p r e s e n t around 8"N,
much f u r t h e r s o u t h t h a n i n O c t o b e r 8 0 when t h e s t r u c t u r e was a s i n gle-eddy
one w i t h n o r t h e r n
f r o n t a r o u n d 9 O 3 0 N.
t h e s e o b s e r v a t i o n s , t h a t a t t h e end of
But i t seems,
from
t h e eddy supposed l i f e t h e
23 northern subsurface front gets steeper
(cf. Nov.
81 a n d Dec.80)
w h i c h c o u l d b e d u e t o t h e c o n c e n t r a t i o n of t h e e d d y e n e r g y i n a much restricted coastal zone.
Fig. 5. Surface salinity, surface drift and depth of SINODE 8 , November 1981.
2 O o C isotherm during
24
F i g . 6. E q u a t o r i a l t e m p e r a t u r e s e c t i o n from ( i n May 1 9 8 0 ) .
2.2
to
62' 3 0 ' E
Equatorial region t h e e q u a t o r t h e e a s t w a r d j e t which a p p e a r s e a s t of
A t
r o u n d 49'
et al.
April-May
1981,
LUYTEN,
and October-November
e t al.
(with speed around l m / s ) a n d i n November section
a-
- 50° E , between t h e monsoons, d u r i n g t h e t r a n s i t i o n
E
periods of
(WYRTKI,
1 9 7 3 , CRESSWELL,
1980) w a s p r e s e n t d u r i n g e a c h c r o s s i n g
e x c e p t i n J u l y 8 1 d u r i n g t h e SW m o n s o o n
8 1 when t h e j e t w a s p r e s e n t f u r t h e r e a s t o f
the
(Fig.4). The t h e r m o c l i n e s t r u c t u r e
with
46' 3 0 ' E
longitude along the equator
May 8 0 w h e r e
d o e s n o t s h o w much v a r i a t i o n s
(Fig.6,8)
the thermocline is thinner.
except w e s t of
49'E
in
This western equatorial
a r e a , where t h e j e t d o e s n o t e x i s t , p r e s e n t s low s a l i n i t y c h a r a c teristics (fig.7).
(S < 35.20%,)i.e.waters The
a r o u n d 49'E.
of
southern hemisphere o r i g i n
s a l i n i t y f r o n t e x t e n d s from 0 t o 9 0 meters and s t a n d s As
in
1980,
t h e 49'E
July 81 temperature section pre-
s e n t s a t h i n n e r t h e r m o c l i n e t h a n a t 53O ponds t o t h e r e t u r n o f
-
54OE
: i t s s l o p e corres-
Somali c u r r e n t f r e s h e r flow from t h e coast.
25 62'30E
46'30E
Om
m m
Fig. 7. E q u a t o r i a l s a l i n i t y s e c t i o n from (May 1 9 8 0 ) .
46' 30' E
to
62'
30'E
C o m p a r i n g now t h e t w o t r a n s i t i o n p e r i o d s c r o s s - e q u a t o r i a l
sections
it appears t h a t the
t h e r m o c l i n e i s much b r o a d e r d u r i n g t h e s e c o n d
period
Dec.)
DIN
(0ct.-
e t al.)
ses o f
Nov.-
when t h e
j e t speeds are higher
(cf.REVER-
( F i g . 9 ) . This difference must r e f l e c t d i f f e r e n t proces-
formation of
the
j e t and/or
d i f f e r e n t water masses involved.
The two d i f f e r e n t c i r c u l a t i o n s y s t e m s a t t h e e n d o f
t h e two monso-
o n s seem t o a f f e c t d i f f e r e n t l y t h e e q u a t o r i a l s t l r u c t u r e d u r i n g t h e two e a s t w a r d j e t t r a n s i t i o n p e r i o d s . The c r o s s - e q u a t o r i a l
temperature
e a s t w a r d j e t p e r i o d d o n o t show a
s e c t i o n s made d u r i n g t h e
s t r o n g convergence.
The j e t m u s t
be f e d p a r t l y by w a t e r s f r o m t h e w e s t a n d p a r t l y t h r o u g h h o r i z o n t a l convergence.
26
E5'
18.
10.
10.
1szoo
62. E
debs* 30 May 79 4 Junr 79 62OE O P E
03*30 E
Fig. 8. Cross e q u a t o r i a l temperature s e c t i o n s a t d i f f e r e n t times and l o n g i t u d e s during t h r e e c r u i s e s .
Fig. 9. year.
Cross e q u a t o r i a l temperature s e c t i o n s a t d i f f e r e n t p e r i o d s o f t h e
21
2.3
South equatorial current and counter-current region T h e XBT sections have been expanded down to Reunion island.
The p r o m i n e n t feature i s the slope of the isotherms between Reunion and a thermocline ridge which varies with the cruises from 9 ' s 6's
to
(Fig.4 and 10). This slope is the mark of the south equatorial
current ( S E C ) which i s steadier and stronger during the SW monsoon period w h e n the SE trade winds are also stronger and when the currents are weak and variable in the region between the thermocline ridge ( l i m i t o f the SEC) and the equator. During the NE monsoon the SE trade winds decrease and the SEC retreats towards the South like
in December 8 0 . increases
Simultaneously the South Equatorial Counter current
( b etween 2 ' 5
and 8 ' 3 0
S in December 80 - Fig.1). Until
now, due to ship program constraints, the sections have not been carried o u t during characteristic period soon). D u r i n g the transition periods mocline r i d g e lies around 6 " -
7 O
(i.e. full SW or NE mon-
(May and Oct - Nov) the ther-
S exc ept in May 81 (Fig.4). In
December 80 a t the beginning of the NE monsoon the ridge had retracted down t o 8 " 3 0
S.
A t depth, in the central water, the ridge o f
the d e e p isotherms i s somewhat displaced towards the south and could be a s far asll's
- 12"s. I n the SECC region the depth varia-
tions o f the thermocline is very small.
0
100
100
100
.oo
¶OO
600
I00
-00
Fig. 10.
South Equatorial Current sections (SINODE 5).
28 3.
CONCLUSION The p r e l i m i n a r y
r e s u l t s of
the
f i r s t e i g h t S I N O D E XBT
s e c t i o n s i n t h e w e s t e r n I n d i a n o c e a n show t h e h i g h v a r i a b i l i t y o f t h e t e m p e r a t u r e s t r u c t u r e i n t h e Somali b a s i n which c a n be r e l a t e d t o the circulation variations. t i e s a r e a good i n d i c a t o r o f perature sections.
In t h a t region,
the
surface salini-
t h e c i r c u l a t i o n t o g e t h e r w i t h t h e tem-
The n o r t h e r n
f r o n t of
t h e Somali eddy i s always
s t e e p e r than t h e southern one. A t
t h e e q u a t o r , during t h e e x i s t e n c e of
the eastward j e t ,
t h e two t r a n s i t i o n p e r i o d s p r e s e n t d i f f e r e n t t e m p e r a t u r e s t r u c t u r e s with a broader thermocline during the Nov.Dec.)
with the northern
(Oct.
jet is faster.
when t h e South of
second t r a n s i t i o n p e r i o d
the equator,
the thermocline ridge associated
l i m i t of
South E q u a t o r i a l Current,
the
varies
between 6 O S and 9OS. Sections during the characteristic periods soons) w i l l permit a b e t t e r d e s c r i p t i o n of
( N E a n d SW mon-
the annual cycle.
The g e o s t r o p h i c c a l c u l a t i o n s u n d e r t a k e n ,
u s i n g mean T - S
w i l l allow t o quantify the circulation.
relationships, A s the
o b s e r v a t i o n s made o n d i f f e r e n t y e a r s a t t h e same
season a r e not similar,
a c a r e f u l study of
the correlation with the
wind s t r e s s v a r i a t i o n s h a s t o be done t o f u r t h e r u n d e r s t a n d t h e s e different features.
Acknowledgments T h i s s t u d y i s i n c l u d e d i n t h e SINODE p r o g r a m s u p p o r t e d by t h e "Terres A u s t r a l e s e t Antarctiques FranGaises" r i e d o u t on b o a r d t h e i r t o i r e d'Ocdanographie relle,
the
"MARION
DUFRESNE", by t h e L a b o r a -
P l i y s i q u e du Museum N a t i o n a l
Laboratoire Associd
seph GONELLA. probes,
ship,
The F r e n c h
(TAAF) a n d c a r d ' H i s t o i r e Natu-
1 7 5 d u CNRS, u n d e r t h e d i r e c t i o n o f J o -
"Marine N a t i o n a l e " p r o v i d e d t h e
t h e o t h e r o n e s w e r e p r o v i d e d by t h e T A A F .
1 9 7 9 XBT
29 REFERENCES
.
The Somali Current. Ocean Modelling, No.34 Anderson,D.L.T.,1980 (unpublished manuscript) . Bruce, J.G. 1968 . Comparison o f near surface dynamic topography during the two monsoons i n the western Indian Ocean. Deep-sea Res., 1 5 , 665-677. Bruce, J.G., 1979 . Eddies o f f the Somali coast during the southwest monsoon. J. Geophys. Res., 8 4 , 7742-7748. Cresswell, G., M.Fieux and J.Gonella . The Wyrtki equatorial jet, MayIJune 1980. Tropical ocean-Atmosphere Newsletter - January 1981. Unpublished manuscript. Duing, W.,1978 . The Somali Current. P a s t and recent observations. I n : Review Papers o f Equatorial Oceanography, Fine Workshop Proceedings. Nova/Nyit Univ. Press (unpublished manuscript). Fieux,M., J.F. Murail, G.Soares, 1980 . Navigation et sections bathythermiques (XBT). Recueil des donnees collectees, campagne SINODE - MD 18 - P E M G . Vol.1. Rapp. Int. d u Laboratoire d'OcBanographie Physique d u Museum Paris. (Unpublished manuscript). F i e u x , ~ . ,~ . ~ r e m o n tA,. ~ a r t a v t s e f f ,J.F. ~ u r a i l ,1982. Navigations sections bathythermiques (XBT) et bouees derivantes. Recueil des donnees collectees, campagne SINODE 11-MD 22 - Vol.1. Rapp. Int. d u Laboratoire d'oceanographie Physique d u Museum Paris. (Unpublished manuscript) . Leetmaa,A.and.V. Truesdale, 1972. Changes in the currents i n 1970 o f f the E a st African coast with the onset o f the southeast mo1.s o o n . J. Geophys.Res., 7 7 , 3281-3283. Leetmaa, A., D.R. Quadfasel, and D.Wilson, 1982 : Development o f the f l o w field during the onset o f the Somali Current, 1979 in J . Phys. Oceanogr. Luyten, J.R., M.Fieux, and J.Gonella, 1980 . Equatorial currents i n the w e s tern Indian Ocean. Scienc e, 209, 600-603. Quadfasel, D.R. 1982. L o w frequency variability o f the 20'C isotherm topography i n the western Indian Ocean. J.Geophys. Res., 8 7 , C 3 , pp. 1990-1996. Schott,F. a n d D.R. Quadfasel, 1980 . Development o f the Northern S o m a l i current gyre i n 1979. Science. Vol, 209. Aug.1980. pp.593-595. Schott,F., a n d D.R. Quadfasel, 1982. Variability o f t h e Somali CUrr e n t system during the monsoon o n s e t , 1979 (to appear in J . P h y s Oceanogr 1 Swallow,J.C. and J.G. Bruce, 1966 . Cu rrent measurements off the S o m a l i coast during the Southwest Monsoon o f 1964. Deep Sea Res. 13-5, 861-888. Sw a l l o w , J.C. and M.Fieux, 1982 . Historical evidence for two gyres i n the S o m ali Current. J. Mar. Res. V01.40, Suppl.pp.747-755. Swallow, J.C., R.Molinari, J.G.Bruce, O.B. Brown. Development o f near-surface flow pattern and water mass distribution i n the Somali b a s in, i n response t o the southwest monsoon o f 1979. T o appear in J. Phys. Oceanogr. Wyrtki, K. 1973. An equatorial jet i n the Indian Ocean. Science,l81 262-264.
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