i
i,
t
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1
i
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. .
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IE
!
,
Fonds bocumentaireLORSTOM
Commission of the European Communities
I
First R8tD programme
in the field of climatology
1981-85
Edited by
R. Fantechi, A. Ghazi
Commission of the European Communities
200, rue de la Loi
6-1049 Brussels
Reports on research sponsored under
the first phase: 1981-83
INDIRECT ACTION
R&D programme 'Environment'
Directorate-General
Science, Research and Development
1985
EUR 9920 EN
C o n t r a c t o r : O R S T O I4
C o n t r a t N o : CLI-052- F
P r o j e c t l e a d e r ( s ) : F. JARRIGE, J . CITEAU, P . R U A L , J.M. VERSTRAETE
T i t l e of P r o j e c t :
CLIMATE VARIATIONS STUDY I N ATLANTIC INTERTROPICAL A R E A .
1. O b j e c t i v e s
T h e o b j e c t i v e s of t h i s program a r e t o test t h e f e a s a b i l i t y and
i n t e r e s t of a l o n g term m o n i t o r i n g of t h e t r o p i c a l A t l a n t i c .
Ocean and atmosphere are coupled i n a t h e r m a l e n g i n e , t h e energy
s o u r c e b e i n g i n t h e t r o p i c s . C l i m a t e v a r i a t i o n s a r e d i r e c t l y a f f e c t e d by
f l u c t u a t i o n i n a i r / s e a h e a t t r a n s f e r s and by the r e s p o n s e o f t h e ocean t o
a t m o s p h e r i c s o l i c i t a t i o n s . P r e v i o u s s t u d i e s and on-going I;'OCAl./SEOUAl.
(French/US) prograrn5have i d e n t i f i e d s e v e r a l major. f'c..:itr~rt::;,
i r i 1.lirt 1
1.4rlii.
c a l A t l a n t i c , t h a t may be o f importance t o g l o b a l cl i i i i a t . i c v n r i n 1 , i o t r s .
S h i p o f o p p o r t u n i t y , s a t e l l i t e s , a n d t i d e g,agc network a r e incxp e n s i v e means used t o monitor m e t e o r o l o g i c a l c o n d i t i o n s , h e a t c o n t e n t and
mean s e a l e v e l of t h e ocean. T h e i r r e l a t i o n w i t h t h e s e major f e a t u r e s and
the climate f l u c t u a t i o n s a r e being tested during these three y e a r s of s i m u l taneous programs. Then t h e l o n g term m o n i t o r i n g a l o n e may be a b l e t o f o l low t h e time v a r i a b i l i t y of t h e s e s f e a t u r e s and perhaps one day, could
predict the climate fluctuations !
2 . M a t e r i a l s and Methods
Three f i e l d o p e r a t i o n s a r e on-going t o c o l l e c t d a t a n e c e s s a r y to
a c o n t i n u o u s s u r v e y of h e a t c o n t e n t , w a t e r s motions and exchange between
a i r and sea ( f i g . 11. T h i s s u r v e y w i l l l a s t t h r e e y e a r s from
1982
in
order t o describe seasonnal s i g n a l s w i t h s u f f i c i e n t s t a t i s t i c a l s t a b i l i t y
a ) The s e a s u r f a c e t e m p e r a t u r e f i e l d is observed by remote sens i n g : mainly METEOSAT d a t a t o which a t m o s p h e r i c c o r r e c t i o n w i l l be added
e i t h e r w i t h TOVS ( N O A A ) d a t a , o r f o l l o w i n g MIEC ( E S O C ) p r o c e d u r e s . S i n c e
JUNE 1982, on weekly b a s i s , composite p i c t u r e s have been made and ground
t h r u t h from merchant s h i p s a r e used f o r c a l i b r a t i o n s and gap f i l l i n g .
b ) XBTs dropped by s h i p s of o p p o r t u n i t y a l o n g t r a c k s from Le
Havre t o South America p r o v i d e 30 t r a n s 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 s
-
191
'
every y e a r . T h i s w i l l be used t o e s t i m a t e heat c o n t e n t , t h e r m o c l i n e d e p t h , dynamic h e i g h t s and f l u x e s : t h e s l o p e o f t h e t h e r m o c l i n e i n a m e r i d i o n a l p l a n e
i s a s s o c i a t e d w i t h t h e i n t e n s i t y of t h e current a c r o s s i t . M e t e o r o l o g i c a l d a t a
such a s w i n d , m o i s t u r e , a i r t e m p e r a t u r e , a t m o s p h e r i c p r e s s u r e and water sample
f o r s a l i n i t y measurements a r e a l s o c o l l e c t e d i n t h e same way.
c ) Ten new i n s t r u m e n t s extend t h e e x i s t i n g t i d e gage network t o
t h e whole Tropical. A t l a n t i c
.
Mean s e a l e v e l f l u c t u a t i o n s h i g h l y c o r r e l a t e
with var-iation!; of' tlic h e a t c o n t e n t o f t h e s u p e r f i c i a l l a y e r s i n t r o p i c a l zon e s (20" N
-
20" S I . Observed d e v i a t i o n s from t h e l o n g term averaged s e a l e -
v e l s can then be used t o monitor i n t u r n v a r i a t i o n s o f o c e a n i c c i r c u l a t i o n .
I n t h e f u t u r e , s e a l e v e l s measured a t i s l a n d s and c o a s t a l s t a t i o n s w i l l a l s o
b e o f g r e a t v a l u e f o r t h e c a l i b r a t i o n o f s a t e l l i t e a l t i m e t e r s mapping t h e
ocean s u r f a c e
I
By combining t h e sea l e v e l o b s e r v a t i o n s and XBT d a t a , t h e v a r i a -
t i o n s of t h e main o c e a n i c flows w i l l be followed on a monthly b a s i s . From
Lhe:;e,
momentum exchanges w i l l be o b t a i n e d w i t h i n t h e s u p e r f i c i a l l a y e r o f
t h e o c e a n . Advection of h e a t i n t h e s u r f a c e l a y e r w i l l be e v a l u a t e d through
v a r i a t i o n s of t h e s p a t i a l d i s t r i b u t i o n o f t h e s e a s u r f a c e t e m p e r a t u r e from
s a t e l l i t e d a t a and of t h e h e a t c o n t e n t . The a i r s e a exchange o f h e a t f l u x e s can
be deduced from the v a r i a t i o n s o f t h e d i s t r i b u t i o n o f s e a s u r f a c e temperat u r e and a i r t e m p e r a t u r e . M e t e o r o l o g i c a l d a t a and s e a s u r f a c e t e m p e r a t u r e d i s t r i b u t i o n w i l l also be used t o i n d i c a t e zones where s t r o n g e v a p o r a t i o n occas i o n n a l l y occur and i n some c a s e t o e v a l u a t e t h e e v a p o r a t i o n r a t e .
-t .i w r e \ : T r o p i c a l f i t l a n t i c Ocean
Ø
,
Europe-South America s h i p of o p p o r t u n i t y l i n e
\, Europe-Cap Town
/I
Figure 2 : Thermal mapping ìn t h e South R t l a n t i c Ocean from 13 t o 19 J a n u a r y 1983
(Meteosat d a t a )
ITCZ Yeasono1 v;it-intioli zone
192
-
--!O70
Fi
h
rd 3
-
: I n t e r t r o p i c a l converaence zone (maximum c o n v e c t i v e c l o u d i n e s s by METEOSAT/COES-E
d Mean Sea S u r f a c e Temperature between 4 O N
5 O S around loo W , f o r t u o y e a r s w i t h
a weak e q u a t o r i a l u p w e l l i n g (1979-81) and t h r e e y e a r s o f strong e q u a t o r i a l u p w e l l i n g 197882-83)
F i g u r e 4 : T r a n s e q u a t o r i a l s e c t i o n o f temp e r a t u r e , a l o n g t h e “LE HAVRE-RIO” navigat i o n l i n e , from expandable B a t h y t h e m o g r a phes (XBT) launched by t h e S.S. EIFFEL (compagnie G é n é r a l e N a r i t i m e ) , 26-0-83/2.9.03.
!
.Figure 5 : Time-Space diagram o f t h e d e p t h ( m e t r e s ) o f t h e 20° C i s o t h e r m on Lhe n a v l g a t l o n
l í n e , Europe-South America. XBTs launched f r o m CCM s h i p s of o p p o r t u n i t y .
-
193
-
As a m a t t e r
or
fact
,
t h e b e g i n n i n g of o u r g l o b a l watch of t h e t r o -
p i c a l A t l a n t i c , lias been a l s o t h e b e g i n n i n g of a s e r i e of anomalous warm
arid c o l d e v e n t s , summarized as f o l l o w s :
I n terms of e q u a t o r i a l c o o l i n p , 1 9 8 1 r e c a l l s a c l a s s i c a l y e a r , i n
c o n t r a s t with 1982 and 1983 which have been "cold y e a r s " . Furthermore, tieekl y composite infrai-ed s a t e l l i t e
images have a l s o r e v e a l e d :
a ) t h e weakness o r t h e d e l a y o f t h e s e n e g a l e s e u p w e l l i n g ( w i n t e r
81/02 and 8 2 / 8 3 ) .
b ) t h e e a r l i n e s s o f t h e gabonese u p w e l l i n g and t h e s t r e n g t h o f t h e
equatoi-ia1 cooling
i n summers 82 and 8 3
c )t h e anonlalous e x t e n t o f warm c e n t r a l A t l a n t i c w a t e r s towards
South A f r i c a n c o a s t i n J a n u a r y 8 3 ( f i g . 2) and January 8 4 , s u g g e s t i n g a
"Sou t h Atlan t i c E l Nino"
I
I n o r d e r Lo r e l a t e some o f t h e s e e v e n t s t o wind f i e l d o r a t least
t o wind limits, one of t h e s e , t h e I n t e r t r o p i c a l Convergence Zone h a s been
f ' o l l o w e d a l o n e 2fi0 W f o r f i v e years and compared t o t h e mean v a l u e of t h e
sea s u r f a c e t e m p e r a t u r e
(4oN
-
50sn e a r l o o W )
-
t h i s area well d e s c r i b e d
by v e s s e l l i n e gives a l s o a good i n d i c a t i o n f o r e q u a t o r i a l c o o l i n g . As d i s played on f i g u r e 3 , t h e n o r t h e r n m o s t p o s i t i o n of ITCZ (lOON
-
12"N) i s r e a -
ched i n "cold y e a r s " w i t h mean SST n e a r 23' C I i n c o n t r a s t w i t h "warm y e a r s , ,
where t h e northernmost ITCZ p o s i t i o n l i e s between T'and
SST near 24O
C . Furthermore
gON and lowest mean
i t i s s u g g e s t e d t h a t a t t h e b e g i n n i n g of each
c o l d y e a r , t h e I T C Z is y e t t o i t s southernmost p o s i t i o n and moves n o r t h wai.ds a s soon as e n d o f J a n u a r y o r m i d F e b r u a r y . On t h e o t h e r hand, t h e
l'i
r.71,
rnorit.lin
rir
,rw;tt*iiis
y c n ~ sd i~s p~l a y southwards movement t i l l end of March
wtlc!m riiovinE d i r * e c t i o n change ( c l o s e t o t h e t y p i c a l movement t h a t can be
e x t r a c t e d from HASTENRATFI & LAMB Atlas n o t d e s c r i b e d h e r e ) .
T h i s time d e l a y , a b o u t two months,need
o f c o u r s e t o b e confirmed
( o n - g o i n g work) i n o r d e r t o s u g g e s t an a t t r a c t i v e p r e d i c t i o n scheme, and
improve t h e c o n n e c t i o n between e q u a t o r i a l c o o l i n g and t h e S t e Helene Anticyclone a c t i v i t y .
I n s u b s u r f a c e , some of t h e anomalies and cold/warm y e a r c o n t r a s t s
d e s c r i b e d a b o v e , have been evidenced by XBT d a t a .
-
-
194
-
Along t h e Europa-South America sea r o u t e , t h r e e t h e r m a l zones can be d e f i ned, t h e Guinea dome, o f f t h e A f r i c a n c o a s t (10" N), t h e e q u a t o r i a l a r e a ,
t h e Brasil d e e p warm l a y e r (10" S
-
15" S ) ( f i g . 4 ) . The h e a t c o n t e n t o f
t h e s e t h r e e areas is v e r y d i f f e r e n t mainly due t o t h e d i f f e r e n t t h i c k n e s s of
t h e warm u p p e r mixed l a y e r and t h e r m o c l i n e . T h e i r time v a r i a t i o n s a r e a l s o
very d i f f e r e n t ( f i g . 5 ) . The Guinea dome and t h e B r a s i l i a n warm w a t e r s seem
t o have.an a n n u a l c y c l e , when t h e f i r s t rises t h e o t h e r d e e p e n s , w h i l e t h e
e q u a t o r i a l area h a s a much more c h a o t i c behaviour. U n t i l mid-82 i t seems t o
behave a s p r e v i o u s l y t h o u g h t : an a n n u a l c y c l e w i t h a r i s e when t h e wind i s
weak and t h e ITCZ a t t h e e q u a t o r , and a deepenning when t h e SE T r a d e s a r e
s t r o n g and t h e Gulf of Guinea u p w e l l i n g s a r e a c t i v e . But from mid-82 u n t i l
now t h e r e seems t o b e a more complex s t r u c t u r e w i t h a maximum around 5" N
s e v e r a l minima a t t h e e q u a t o r and o f t e n a second maximum s o u t h o f t h e equat o r , a l t o g e t h e r w i t h a b r u p t changes from one s e c t i o n t o t h e o t h e r a s if wave
f r o n t s were p r o p a g a t i n g .
H o r i z o n t a l t e m p e r a t u r e g r a d i e n t s a r e t h e major component of dynamic
h e i g h t g r a d i e n t s and t h e i s o t h e r m s s l o p e s around 7" N and 7" S a r e an i n d i c a t i o n o f t h e North E q u a t o r i a l
Counter C u r r e n t
( N E C C ) and of' t h e S o u t h Equa-
t o r i a l C u r r e n t (SEC). The Pirst one i s s t r o n g e r i n 82-83 than i n 81. I t m u s t
be very weak,if
p r e s e n t , i n t h e f i r s t h a l f o f 82. For t h e f i r s t f i v e months of
83 a bad XBT b a t c h produced a gap i n t h e d a t a which i s s p e c i a l y u n f o r t u n a t e
a s 82/83 i s a very abnormal climatic p e r i o d i n many s e n s e s . The E q u a t o r i a l
'
C u r r e n t (ES) and t h e E q u a t o r i a l U n d e r c u r r e n t ( E U C ) do n o t show up very well
on thermal s e c t i o n s . The s p r e a d i n g o f t h e i s o t h e r m s i n t h e t h e r m o c l i n e a t
t h e e q u a t o r i s an i n d i c a t i o n o f t h e EUC b u t n o t o f i t s s t r e n g t h .
Using h i s t o r i c a l da t a , we have
shown t h a t t h e a n n u a l c y c l e s o f Mean Sea
L e v e l ( M S L ) , Dynamic Height ( D l I ) and Heat
Content (HC) are s i g n i f i c a n t l y c o r r e l a t e d
i n t h e t r o p i c a l e a s t e r n A t l a n t i c . The
main u p w e l l i n g e v e n t s i n t h e Dakar, Abidjan
and P o i n t e Noire c o a s t a l a r e a s a r e d e t e c t e d
by 3 d r o p o f MSL s i m u l t a n e o u s w i t h a d e c r e a re 6
Hean Sea Level (HSL), Dynamic
r eFi g-h t 7DW),
: H e a t Content (HC) a t P o i n t e
Noire (Congot from historical data.
Fi
se of DH and HC. Annual v a r i a t i o n s of MSL,
DI1 and HC a r e p r e s e n t e d f o r P o i n t e N o i r e
i n F i g . 6 . I t s h o u l d be n o t i c e d t h a t MSL
d e c r e a s e i n a p r i 1 l e a d s HC d e c r e a s e by
a b o u t one month. These r e s u l t s c l e a r l y
- 195 -
show t h a t Sea S u r f a c e Temperature (SST) i s n o t a good i n d i c a t o r of t h e v e r -
t i c a l thermal s t r u c t u r e whereas MSL is s e n s i t i v e t o s u b s u r f a c e t h e r m a l
s t r u c t u r e changes.
Records from f i v e s t a t i o n s have been a l r e a d y r e t r i e v e d . The r e s u l t s
o b t a i n e d a t t h e Dakar s t a t i o n are p r e s e n t e d h e r e ( F i g . 7 ) . The t i d e gauge
was deployed on december 2 3 t h 1982, i n a b o u t 8 meters of water. During t h e
f i r s t month, " i n s i t u " t e m p e r a t u r e i n c r e a s e s from 18.5"C t o 19.5OCI d u r i n g
t h e same p e r i o d MSL increases from 8.35111. t o 8.45111.~ t h e n o c c u r s a n upwell i n g e v e n t : from f e b r u a r y 3 r d t o 1 7 t h , t h e t e m p e r a t u r e d e c r e a s e s from
n e a r l y 21°C t o l5"C, and MSL
from 8.50m. t o n e a r l y 8.30m.. I n 1903, t h e
u p w e l l i n g o n s e t i n Dakar area was l a t e by a b o u t one month ( n o u p w e l l i n g
i n j a n u a r y l 9 8 3 ) . From f e b r u a r y 1 7 t h t o may 1 7 t h , t h e u p w e l l i n g i s a c t i v e
w i t h t e m p e r a t u r e l e s s t h a n 19°C and MSL l e s s t h a n 8.40111.. A f t e r may 1 7 t h
t e m p e r a t u r e i n c r e a s e s r e g u l a r l y t o r e a c h 28°C whereas MSL s l o w l y o s c i l l a t e s
between 8.40m. and 8.50111..
T h i s d i v e r g e n c e between t h e two p a r a m e t e r s may
be a n i n d i c a t i o n t h a t , w h i l e t h e s u r f a c e l a y e r i s warmed up by t h e s u n ,
t h e t h e r m o c l i n e s t a y s v e r y close t o t h e s u r f a c e and t h e DH o r MSL a r e t h o s e
of a dome. A closer l o o k a t F i g . 7 shows t h a t i n f e b r u a r y , MSL d r o p s b e f o r e
t h e sea s u r f a c e t e m p e r a t u r e i n d i c a t i n g a s u b s u r f a c e f o r c i n g of t h e upwell i n g whereas, i n m a r c h - a p r i l ,
t h e r i s e i n sea s u r f a c e t e m p e r a t u r e o c c u r s
b e f o r e t h e r i s e of M S L , i n d i c a t i n g a n e v e n t o r i g i n a t e d from t h e surface
(warming by t h e s u n ) .
- D A K A R (1983) -
T-C I
I
Dcc '
IAH.
I
fC8.
YAR,
'
APR.
Y AI
Jun.
'
F i r r e 7 : N e m Sea L e v e l and s e a temperature a t 8 m. a t Dakar (Senegal) from december
19 2 t o J u l y 1983. Both.parameter3 are f i l t e r e d for the t l d e up t o t h e 1 day3 periods
(DMERLIAC Cilter).
- 196
-
1
JUL.
4. Conclusions
The program i s i n i t s second y e a r o n l y and most o f t h e d a t a acq u i s i t i o n is g o i n g smoothly. Its p r o c e s s i n g and i n t e r p r e t a t i o n i s o n l y a t
a p r e l i m i n a r y s t a g e . Data from t h e FOCAL/SEQUAL s i m u l t a n e o u s programs have
n o t y e t been compared t o o u r r e s u l t s . But t h e f i r s t r e s u l t s a r e encouraging
S a t e l l i t e thermography and c l o u d i n t e r p r e t a t i o n compare very well w i t h sur-
face and XBT d a t a , s c a l i n g
t h e h o r i z o n t a l v a l i d i t y of t h e s e s p u n c t u a l
measurements. Mean sea l e v e l and dynamic h e i g h t f l u c t u a t i o n s compare a l s o
very well and o c e a n i c c i r c u l a t i o n changes can b e monitored. The h i g h l y abnormal ocean-atmosphere c o n d i t i o n s i n 1982/83 can be t r a c e d t h r o u g h t w i t h
-
t h e monitored p a r a m e t e r s . H i s t o r i c a 1 d a t a a n a l y s i s is h e l p f u l i n d e t e r m i n i n g
t h e mean s i t u a t i o n or t h e d i f f e r e n t s i t u a t i o n t y p e s : f o r i n s t a n c e , t h e s t r o n g
e q u a t o r i a l u p w e l l i n g y e a r s and t h e weak o n e s , and t h e i r r e l a t i o n w i t h t h e
ITCZ p o s i t i o n , t h e nebu1,osìty maximum o r t h e c o n f l u e n c e of t h c trade-winds.
5 . Additionna1 comments and s u g g e s t i o n s f o r f u r t h e r r e s e a r c h
By t h e end of 1984, end o f t h i s c o n t r a c t , t h e f'ictd studies ul'
t h e FOCAL/SEQUAL programs w i l l b e a l s o t e r m i n a t e d . No more i n t e n s i v e t'iekls
programs w i l l b e sponsored a t l e a s t f o r s e v e r a l y e a r s . I f it: i s d e s i r a b l e t o
c c n t i n u e t o follow t h e i n t e r a n n u a l v a r i a t i o n of t h e . d e c i s i v e climate param e t e r s , t h e o n l y mean w i l l b e t o c o n t i n u e and expend m o n i t o r i n g programs a s
t h i s one,which i s r e l a t i v e l y i n e x p e n s i v e i n comparison w i t h t r a d i t i o n a l ocean i c d a t a a c q u i s i t i o n programs, T h i s i s a l r e a d y done for t h e P a c i f i c ocean on
a n i n t e r n a t i o n a l b a s i s . I n f a c t i n t e r a n n u a l c l i m a t i c anomalies have v e r y o f t e n a worldwide i n f l u e n c e s o t h e m o n i t o r i n g of t h e world ocean i s t h e f i n a l
goal i n o r d e r t o u n d e r s t a n d a n d , may be p r e d i c t , t h e d r a m a t i c climate anomamies a s t h e 1982/83 climate d i s t o r s i o n !
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THEPENIER R.M.
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n i c e v e n t s i n t h e t r o p i c a l A t l a n t i c and t h e i r r e l a t i o n s h i p w i t h t h e
I n t e r t r o p i c a l Convergence Zone. T r o p i c a l Ocean Atmosphere Newsletter
n o 23 January 1984, p 19-20.
CITEAU J., GUILLOT B . , LAE R . , THEPENIER R . M .
1984
.- Warm and
c o l d ocea-
n i c e v e n t s i n t h e t r o p i c a l A t l a n t i c and t h e i r r e l a t i o n s h i p w i t h t h e
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SAT. Clermont-Ferrand
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ICCAT f o r p u b l i c a t i o n ) ,
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CITEAU J., GUILLOT B. 1983
d e s donnGes s a t e l l i t a i r e s , METEOSAT 2 e t NOAA 7 , u t i l i s é e s pour l a
p r é v i s i o n d e s zones d e pêche du thon b l a n c dans l ' A t l a n t i q u e Nord-
E s t . 8ieme Symposium Canadien d e T 6 1 é d é t e c t i o n . M o n t r e a l , 3-6 May
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FOCAL-SEQUAL m e e t i n g . New-York J u l y 1982.
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s t r u c t u r e (20" N
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ItEurope-South Americat1 sea r o u t e . S u b m i t t e d t o p u b l i c a t i o n i n Geophy-
s i c a l Research L e t t e r , s p e c i a l i s s u e " F i r s t FOCAL-SEQUAL r e s u l t s t t .
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Wind and s e a t e m p e r a t u r e s . 3 rd FOCAL-SEQUAL m e e t i n g . P a r i s , Feb. 1984
CITEAU ?J., JARRIGE F . , VERSTRAETE J.M., 1981.- C l i m a t e V a r i a t i o n s Study
i n A t l a n t i c L n t e r t r o p i c a l Area. l b r e Réunion du Groupe d e T r a v a i l
" M o d é l i s a t i o n du Climat" d e l a D i r e c t i o n d e l a Recherche d e la Comm i s s i o n d e s Communautés Europgennes. B r u x e l l e s , Mai 1981.
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