S o m e X B T - o b s e r v a t i o n s * o n t h e t h e r m a l
structure of t h e Warmwassersph~re in e q u a t o r i a l a n d l o w e r l a t i t u d e s of t h e e a s t e r n Atlantic
M i c h a e l a K n o l l , W a i t e r Z e n k a n d E v a B a u e r
UDC 551.465.42:551.465.435:551.465.45; Atlantic Ocean
S u m m a r y
T w o m e g a m e t e r l o n g XBT sections p a s s i n g t h r o u g h t h e A t l a n t i c b e t w e e n t h e I b e r i a n P e n i n s u l a a n d t h e e q u a t o r i a l Brazilian shelf, a n d b e t w e e n t h e P e n i n s u l a a n d t h e C a p e r e g i o n of S o u t h Africa w e r e o b t a i n e d d u r i n g s u p p l y cruises to t h e A n t a r c t i c i n N o v e m b e r 1980 a n d M a r c h 1981. Most of t h e i n d i v i - d u a l p r o b e s r e a c h e d a n o b s e r v a t i o n d e p t h of n e a r l y 800 m a l l o w i n g s t a t e m e n t s o n t h e t h e r m a l stratification a n d z o n a t i o n of t h e
Warmwassersphi~re
i n t h e o p e n o c e a n . O u r n e w d a t a are c o m p a r e d w i t h a n u m b e r of e a r l i e r l o n g sections. As a g e n e r a l r e s u l t w e i d e n t i f y five z o n e s d i v i d i n g different h y d r o g r a p h i c r e g i o n s . Z o n e 1 is c h a r a c t e r i z e d b y t h e c l e a r i n f l u e n c e of t h e M e d i t e r r a n e a n U n d e r c u r - r e n t o n t h e l o w e r b o u n d a r y of t h eWarmwassersphi~re
c r e a t i n g n e a r l y t h e r m o - static c o n d i t i o n s b e l o w 300 m off Portugal. T h e a d j a c e n t z o n e 2 is i d e n t i c a l w i t h the S u b t r o p i c a l C o n v e r g e n c e , w h e r e theWarmwassersphi~re
r e a c h e s its m a x i - m u m d e p t h (15 ~ i s o t h e r m at a b o u t 300 m). We call t h e e q u a t o r i a l r e g i o n z o n e 3 a n d p r e s e n t a n i m p r o v e d s c h e m e of its c u r r e n t s y s t e m a n d its t h e r m a l stratifi- cation. Z o n e 4 is m a r k e d b y t h e B e n g u e l a C u r r e n t a n d its n o r t h e r n e x t e n s i o n . In a final z o n e 5 w e s h o w t h e i n f l u e n c e of t h e A g u l h a s C u r r e n t e x t e n s i o n o n t h e stratification.X B T - B e o b a c h t u n g e n zur t h e r m i s c h e n Struktur der Warmwassersph~ire in ~iquatorialen und n i e d e r e n Breiten des 6stlichen Atlantiks
(Zusammenfassung)
Im N o v e m b e r 1980 u n d Mfirz 1981 w u r d e n auf V e r s o r g u n g s f a h r t e n z u r A n t a r k t i s z w e i l a n g e X B T - S c h n i t t e g e w o n n e n . Sie v e r l a u f e n z w i s c h e n d e r I b e r i s c h e n H a l b i n s e l u n d d e m f i q u a t o r i a l e n b r a s i l i a n i s c h e n Schelf b e z i e h u n g s - w e i s e d e r K a p r e g i o n v o n Siidafrika. Die m e i s t e n S o n d e n r e i c h t e n bis z u e i n e r Mel~fiefe v o n 800 m, so dal~ A u s s a g e n z u r t h e r m i s c h e n S c h i c h t u n g u n d h o r i z o n - t a l e n G l i e d e r u n g d e r W a r m w a s s e r s p h f i r e i m o f f e n e n O z e a n m 6 g l i c h w u r d e n . Wir v e r g l e i c h e n u n s e r e n e u e n D a t e n m i t e i n e r Reihe v o n frtiher g e w o n n e n e n S c h n i t t e n . AIs E r g e b n i s e r h a l t e n w i r f/inf u n t e r s c h i e d l i c h e h y d r o g r a p h i s c h e R e g i o n e n . Z o n e 1 ist g e k e n n z e i c h n e t d u t c h d e n k l a r e n Einflug d e s M i t t e l m e e r - a u s s t r o m e s , d e r a n d e r U n t e r s e i t e d e r W a r m w a s s e r s p h f i r e w e s t l i c h P o r t u g a l s u n t e r h a l b v o n 300 m n a h e z u t h e r m o s t a t i s c h e B e d i n g u n g e n schafft. Die a n - s c h l i e B e n d e Z o n e 2 ist i d e n t i s c h m i t d e r S u b t r o p e n k o n v e r g e n z , i n n e r h a l b d e r e r die W a r m w a s s e r s p h ~ i r e ihre M a x i m a l t i e f e (15-~ b e i e t w a 300 m) er- reicht. Die fiquatoriale R e g i o n n e n n e n w i r Z o n e 3. Es w i r d e i n e f i b e r a r b e i t e t e
* XBT Expendable Bathythermograph
74 Dt.hydrogr.Z. 35, 1982. H. 2. K n o 11 et al., Some XBT-observations
D a r s t e l l u n g d e r StrOmungs- u n d T e m p e r a t u r s c h i c h t u n g s v e r h & l t n i s s e d i e s e r Re- g i o n g e g e b e n . Die Z o n e 4 ist d u r c h d e n B e n g u e l a Strom u n d s e i n e n O r d l i c h e n A u s l ~ u f e r gepr/igt. In d e r l e t z t e n Z o n e 5 w i r d d e r EinfluB d e s A g u l h a s - S t r o m - A u s l ~ u f e r s auf die S c h i c h t u n g vorgestellt.
D e s o b s e r v a t i o n s XBT sur la structure t h e r m i q u e de la
Warmwassersphiire
dans l e s l a t i t u d e s 6 q u a t o r i a l e s et b a s s e s de l ' A t l a n t i q u e o r i e n t a l (R6sum6) E n n o v e m b r e 1980 et m a r s 1981 o n a o b t e n u d e u x l o n g u e s sections XBT p e n d a n t des v o y a g e s d ' a p p r o v i s i o n n e m e n t ~ l ' A n t a r c t i q u e . Les s e c t i o n s p a s s e n t e n t r e la p ~ n i n s u l e I b 6 r i q u e et le p l a t e a u 6 q u a t o r i a l b r a s i l i e n et e n t r e la p 6 n i n - sule I b 6 r i q u e et la r 6 g i o n d u C a p de l ' A f r i q u e d u S u d r e s p e c t i v e m e n t . La p h i - p a r t d e s s o n d e s d e s c e n d a i e n t j u s q u ' /i 800 m, ce q u i a p e r m i s d ' o b t e n i r des r e n s e i g n e m e n t s s u r l a stratification t h e r m i q u e et l a s t r u c t u r e h o r i z o n t a l e d e la
Warmwassersphiire
e n h a u t e mer. N o u s a v o n s c o m p a r 6 n o s n o u v e l l e s d o n n ~ e s avec u n e s6rie de sections o b t e n u e s p l u s t6t. N o u s e n a v o n s o b t e n u c o m m e r~sultat c i n q z o n e s h y d r o g r a p h i q u e s . La z o n e 1 est c a r a c t ~ r i s 6 e p a r r i n f l u e n c e 6 v i d e n t e d u c o u r a n t p r o f o n d de la M 6 d i t e r r a n 6 e sur la c6t6 i n f ~ r i e u r e de laWarmwassersphiire,
ce q u i cr6e des c o n d i t i o n s p r e s q u e t h e r m o s t a t i q u e s a u x p r o f o n d e u r s e x c ~ d a n t 300 m ~i l ' o u e s t d u Portugal. La z o n e 2 a d j a c e n t e est identiclue ~ la c o n v e r g e n c e s u b t r o p i c a l e , o~ ]aWarrnwassersphi~re
a t t e i n t sa p r o f o n d e u r m a x i m a l e (isotherme de 15 ~ h e n v i r o n 300 m). N o u s d ~ s i g n o n s la r~gion 6 q u a t o r i a l e la z o n e 3, et n o u s p r 6 s e n t o n s u n s c h 6 m a r6vis6 des syst~mes des c o u r a n t s et des stratifications t h e r m i q u e s de cette zone. La z o n e 4 est m a r - q u e e p a r le c o u r a n t B e n g u e l a et son e x t e n s i o n vers le nord. D a n s la z o n e 5 n o u s p r e s e n t o n s l ' i n f l u e n c e d e l ' e x t e n s i o n d u c o u r a n t A g u l h a s sur l a stratification.1 I n t r o d u c t i o n
O n the o c c a s i o n of t h e G e r m a n F i l c h n e r Ice Shelf E x p e d i t i o n 1980/81 w e h a d a n o p p o r t u n i t y to m a p l a r g e parts of t h e
Warmwassersphdre
i n b o t h h e m i s p h e r e s of t h e e a s t e r n A t l a n t i c O c e a n . T h eWarmwassersphi~re
is d e f i n e d as t h e u p p e r p a r t of the o c e a n b o u n d e d t o w a r d s the poles b y the polar fronts a n d b e l o w b y t h e 10 ~ isotherm.T h e special i m p o r t a n c e of this r e l a t i v e l y t h i n w a r m w a t e r l a y e r c o m e s from t h e m e r i - d i o n a l h e a t t r a n s p o r t w h i c h t a k e s p l a c e w i t h i n this s t r a t u m a n d w h i c h f e e d s t h e a t m o - s p h e r e i n t h e m o d e r a t e a n d h i g h e r l a t i t u d e s . I n c o n t i n u a t i o n of a n e a r l i e r series of m e g a m e t e r l o n g t e m p e r a t u r e sections ( H e n k e [1978]; H e n k e a n d Z e n k [1980]), w e c o m p i l e d two n e w sections of e x p e n d a b l e b a t h y t h e r m o g r a p h (XBT) o b s e r v a t i o n s . In contrast to o u r e a r l i e r o b s e r v a t i o n s , most of our n e w profiles cover t h e w h o l e
Warm-
wassersphi~re,
s i n c e 760 m p r o b e s could b e used. W h i l e our activity o n t h e s o u t h b o u n d l e g (section K) o n t h e RV "Polarsirkel" i n D e c e m b e r 1980 h a d to b e l i m i t e d to t h e direct course of its d e s t i n a t i o n port i n S o u t h A m e r i c a , t h e r e t u r n l e g (section W) i n M a r c h 1981 d e v i a t e d from the shortest course a n d l e d t h r o u g h t h e e a s t e r n d e e p b a s i n s of the Atlantic (Fig. 1). Thus, shelf effects on the t h e r m a l stratification of t h eWarmwas-
sersph~ire
c o u l d b e avoided. As a n o m i n a l d i s t a n c e b e t w e e n XBT stations, 56 k m w a s c h o s e n . In a r e a s of s p e c i a l i n t e r e s t t h e s t a t i o n d i s t a n c e w a s r e d u c e d to 28 km. In total, 312 XBT o b s e r v a t i o n s w e r e o b t a i n e d , 133 on s e c t i o n K a n d 179 on s e c t i o n W. T h e rea- d e r is referred to G e o r g i , D e a n a n d C h a s e [1979] for i n f o r m a t i o n a b o u t t e m p e - r a t u r e a n d d e p t h a c c u r a c y of t h e XBT probes.Dt, hydrogr.Z.35, 1982. H.2. K n o l l et al., Some XBT-observations 75
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Fig. 1. Track lines of selected XBT sections across the Atlantic O c e a n (Sections K a n d W are s h o w n in Fig. 2 a n d 3)
76 Dt.hydrogr.Z.35, 1982. H. 2. K n o 11 et aL, Some XBT-observations
2 Temperature distribution a l o n g section K
This s e c t i o n (Fig. 2) w a s o b t a i n e d d u r i n g n o r t h w i n t e r a n d i n t e r s e c t s t h e a r e a b e t w e e n t h e e a s t e r n I b e r i a n Basin (42 ~ N) a n d t r o p i c a l l a t i t u d e s (10 ~ S) of t h e Brasilian shelf. T h e o b s e r v a t i o n s t o o k 13 days. H y d r o g r a p h i c a l l y t h e s e c t i o n c a n b e d i v i d e d into t h r e e zones.
F r o m n o r t h to south, t h e first z o n e follows r o u g h l y t h e I b e r i a n S h e l f off P o r t u g a l to a c h a i n of s e a m o u n t s s o u t h w e s t of Portugal. A m i x e d l a y e r d e p t h d o w n to 80 m is p r e - sent. In t h e h o r i z o n t a l , t h e t e m p e r a t u r e i n c r e a s e s g r a d u a l l y b y 5 ~ km. Also c h a - r a c t e r i s t i c of this r e g i o n is t h e w e a k t h e r m a l stratification in m i d d e p t h r a n g e , t h a t m a i n l y o r i g i n a t e s f r o m h e a t diffusion f r o m t h e u n d e r l y i n g M e d i t e r r a n e a n O u t f l o w ( Z e n k [1971]).
T h e a d j a c e n t s e c o n d z o n e of Section K c o v e r s t h e S u b t r o p i c a l C o n v e r g e n c e Z o n e . It b e g i n s w i t h a s i g n i f i c a n t drop of t h e 15 ~ i s o t h e r m (stations 15 to 20) s o u t h w e s t of t h e J o s e p h i n e / G e t t y s b u r g B a n k r e g i o n at a b o u t 37 ~ N. This d r o p p r o b a b l y c a u s e d b y t o p o g r a p h y c a n b e o b s e r v e d in s e v e r a l X B T s e c t i o n s (Tab. 1). T y p i c a l of this d i v i s i o n w h i c h r e a c h e s its s o u t h e r n b o u n d a r y n e a r 16 ~ N is t h e w e l l m a r k e d d e e p t h e r m o c l i n e . C o n c u r r e n t l y w i t h t h e n e a r l y h o r i z o n t a l o r i e n t a t i o n of t h e d e e p t h e r m o c l i n e , w e o b - s e r v e a d e c r e a s e of t h e v e r t i c a l e x t e n t of t h e
Warmwassersph~re
f r o m g r e a t e r t h a n 800 m to a p p r o x i m a t e l y 400 m w i t h i n 1600 km. It is r e p r e s e n t e d b y t h e c o n t i n o u s s l o p e of t h e 10 ~ i s o t h e r m .A l t h o u g h t h e XBT s e c t i o n is n o t totally f r e e of a z o n a l c o m p o n e n t (K a t z [1977]), t h e 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 h o w s t h e p r e d o m i n e n t i y m e r i d i o n a l i s o t h e r m a l slopes of t h e e q u a t o r i a l r e g i o n . In t h e t h i r d z o n e , t h e
Warmwassersph~re
r e p r e s e n t e d b y t h e 10 ~ s t r a t u m rises in s t e p s f r o m 400 m n e a r t h e C a p e V e r d e s to 280 m n o r t h of F e r n a n d o d e N o r o n h a . F u r t h e r s o u t h t o w a r d s t h e Brasilian Shelf, it s l o p e s d o w n to 380 m again. T h e t y p i c a l d o m i n g of t h e 11 ~ w a t e r n e a r 9 ~ N, t h e G u i n e a D o m e , is f o u n d r e p e a t e d l y in this a r e a as a n e x p r e s s i o n of a c y c l o n i c g y r e in c o n j u n c t i o n w i t h t h e N o r t h E q u a t o r i a l C o u n t e r C u r r e n t . T h e a r e a of h i g h e s t s u r f a c e t e m p e r a t u r e s , t h e t h e r m a l e q u a t o r , o b s e r v e d at 4.5 ~ N c o i n c i d e s w i t h c o m p a r a b l e o b s e r v a t i o n s b y B e c k e r , H e n n i n g s a n d M e t z n e r [1981] a n d H u b e r , M i l l e r , W e i s b e r g et al. [1976]. T h e t e m p e r a t u r e d i f f e r e n c e s b e t w e e n t h o s e s e c t i o n s c o r r e s p o n d to t h e s e a s o n a l v a r i a t i o n s of t h e s u r f a c e t e m p e r a t u r e in t h a t r e g i o n as s h o w n b y M e r 1 e , F i e u x a n d H i s a r d [1980]. O t h e r c h a r a c t e r i s t i c f e a t u r e s a r e t h e e q u a t o r i a l t h e r m o - s t a d e x t e n d i n g f r o m 4 ~ S to 4 ~ N a n d t h e s p r e a d i n g of t h e t h e r m o c l i n e at t h e e q u a t o r . Both f e a t u r e s a r e p a r t of t h e S o u t h (SEUC), t h e N o r t h (NEUC) a n d 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 (EUC) f l o w i n g e a s t w a r d at a b o u t 240 m (SEUC, N E U C ) a n d 90 m (EUC) depth. D u e to t h e l o c a t i o n s of t h e u n d e r c u r r e n t s a n d t h e e q u a t o r i a l d i v e r g e n c e , a m i n i - m u m of t h e t h e r m o s t a d o c c u r s at t h e e q u a t o r . W h i l e t h e S E U C e x t e n d s f r o m Brazil to t h e Golf of G u i n e a ( M o l i n a r i , V o i t u r i e z a n d D u n c a n [1981]), t h e N E U C b r e a k s u p e a s t of 28 ~ W (C o c h r a n e , K e 11 e y a n d O 11 i n g [1979]). T h e r e f o r e , o n l y a w e e k i n d i c a t i o n of t h e N E U C is f o u n d in this section. T h e s h a r p e s t t h e r m o c l i n e occurs a b o v e t h e S E U C a n d N E U C w i t h a v e r t i c a l t e m p e r a t u r e g r a d i e n t of 12 ~ m at 3 ~ S a n d 12 ~ m at 4 ~ N. T h e l o c a t i o n of t h e e q u a t o r i a l c u r r e n t s y s t e m of s e v e r a l s e l e c t e d XBT s e c t i o n s is s u m m a r i z e d in Tab. 1.3 Temperature distribution along section W
A s e c o n d , l o n g X B T s e c t i o n c o n n e c t i n g t h e A g u l h a s C u r r e n t e x t e n s i o n a r e a (31 ~ S) w i t h t h e s o u t h e r n I b e r i a n Basin (36 ~ N) w a s o b t a i n e d d u r i n g 18 d a y s in n o r t h e r n s p r i n g (Fig. 3). A g a i n , f r o m t h e t e m p e r a t u r e distributions, t h e a r e a c a n b e d i v i d e d into h y d r o g r a p h i c z o n e s . In a d d i t i o n to t h e t h r e e d i v i s i o n s a l r e a d y s e e n in s e c t i o n K, w e h a v e to a d d m o r e for t h e s o u t h e r n Atlantic.
C o m p a r e d to t h e first z o n e of s e c t i o n K, t e m p e r a t u r e s at t h e s u r f a c e n e a r t h e I b e -
Dt.hydrogr.Z.35, 1982. H.2. K n o l l et al., S o m e XBT-observations 77
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78 Dt.hydrogr.Z.35, 1982. H.2. K n o l 1 et al., Some XBT-observations
r i a n P e n i n s u l a a r e 1 or 2 d e g r e e s lower. A s s o c i a t e d w i t h t h e o b s e r v e d s u r f a c e c o o h n g is t h e m a x i m u m of t h e d e e p w i n t e r c o n v e c t i o n . It c l e a r l y c a n b e s e e n f r o m t h e w e a k e r t h e r m a l stratification a n d a p r o n o u n c e d d e e p e n i n g of t h e 15 ~ i s o t h e r m . Of course, t h e i n f l u e n c e of t h e M e d i t e r r a n e a n o u t f l o w w a t e r is p r e s e n t in t h e U p p e r D e e p W a t e r of t h e first z o n e b e l o w 650 m. S o u t h of t h e s e a m o u n t c h a i n J o s e p h i n e / G e t t y s b u r g Bank, w h i c h r e p r e s e n t s t h e t r a n s i t i o n to t h e s e c o n d z o n e , n e a r M a d e i r a , w e o b s e r v e t h e c h a r a c t e r i s t i c d r o p of t h e 15 ~ i s o t h e r m a l r e a d y s e e n in s e c t i o n K f r o m 180 m d o w n to 290 m w i t h i n 150 kin. T h e s y s t e m a t i c s o u t h b o u n d d e c r e a s e of t h e
Warmwas- sersphOre,
r e p r e s e n t e d b y t h e 10 ~ i s o t h e r m , c o i n c i d e s w i t h s e c t i o n K. T o w a r d s t h e C a p e V e r d e Island, a n u p p e r t h e r m o c l i n e is f o r m i n g , w h i c h g r a d u a l l y s h a r p e n s .In t h e e q u a t o r i a l r e g i o n , t h e t h i r d z o n e , t h e t h e r m o c l i n e c o n t i n u e s s h a r p e n i n g a n d t h e d e p t h of t h e 10 ~ i s o t h e r m shoals t o w a r d s t h e e q u a t o r u p to 300 m. D u e to t h e dif- f e r e n t l o c a t i o n of s e c t i o n W (Fig. 1), t h e G u i n e a D o m e c a n n o t b e o b s e r v e d . T h e m i x e d l a y e r d e p t h at t h e e q u a t o r d i m i n i s h e s f r o m 80 m in s e c t i o n K to 20 m in s e c t i o n W in a c c o r d a n c e w i t h t h e e a s t - w e s t s l o p e of t h e i s o t h e r m s s h o w n b y M e r 1 e [1980]. Also, t h e m i n i m a l m i x e d l a y e r d e p t h at t h e n o r t h e q u a t o r i a l d i v e r g e n c e z o n e is g e t t i n g d e e - p e r t o w a r d s t h e w e s t (cf. Tab. 1). At t h e t h e r m a l e q u a t o r , 5.5 ~ N, t h e s u r f a c e t e m p e r a - t u r e e x c e e d s 29 ~ c o i n c i d i n g w i t h t h e s e a s o n a l v a r i a t i o n s in t h e e q u a t o r i a l A t l a n t i c d e s c r i b e d b y M e r 1 e [1978]. T h e e q u a t o r i a l t h e r m o s t a d , p r e s e n t e d b y a s p r e a d i n g of t h e 13 ~ a n d 15 ~ i s o t h e r m s is f o u n d b e t w e e n 4 ~ S a n d 3 ~ N in a d e p t h of 80 to 260 m w i t h t h e s h a r p e s t t h e r m o c l i n e at 2 ~ N (12 ~ m ) a n d 3 ~ S (12 ~ m).
In t h e fourth z o n e , s o u t h of 10 ~ S, t h e s e c t i o n e n c o u n t e r s t h e n o r t h w e s t w a r d e x t e n - sions of t h e B e n g u e l a C u r r e n t . This p a r t is c h a r a c t e r i z e d b y a s m o o t h 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 t h i n t h e
Warmwassersph~re.
Its v e r t i c a l e x t e n t , l i m i t e d b y t h e 10 ~ iso- t h e r m , e x p a n d s f r o m 280 to 320 m d e p t h . T h e u p p e r p a r t of t h e m a i n t h e r m o c l i n e w e a - k e n s g r a d u a l l y . W i t h i n t h e m i x e d layer, w e find a 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 of a p p r o x i m a t e l y 3 ~ kin.T h e a d j a c e n t fifth z o n e c o v e r s t h e a r e a of t h e W a l v i s - R i d g e at 23 ~ S a n d t h e C a p e B a s i n t o w a r d s t h e S o u t h A f r i c a n Shelf. This p a r t is g o v e r n e d b y a n e d d y - h k e f e a t u r e c a u s e d b y t h e e x t e n s i o n of t h e A g u l h a s C u r r e n t . H a r r i s, L e g e c k i s a n d F o r e s t [1978] s h o w e d t h a t d u e to t h e s o u t h e a s t e r l y w i n d s , a p p e a r i n g e s p e c i a l l y in t h e sou- t h e r n s u m m e r , a b r a n c h of t h e A g u l h a s C u r r e n t s o m e t i m e s p a s s e s into t h e S o u t h At- lantic. E v e n at 19 ~ S, D a r b y s h i r e [1966] o b s e r v e d a T / S - r e l a t i o n s h i p c o r r e s p o n d i n g to A g u l h a s w a t e r . In t w o e a r l i e r s e c t i o n s m a d e in J u l y / A u g u s t 1978 a n d M a r c h 1980 (H e n k e [1978]; H e n k e a n d Z e n k [1980]), a n e d d y - l i k e f e a t u r e w a s also f o u n d at e x a c t l y t h e s a m e l o c a t i o n . A t t h e e n d of this z o n e , t h e r e is a n i n d i c a t i o n of t h e shelf e d g e d i v e r g e n c e b e l t s e p a r a t i n g t h e w e a t h e r d o m i n a t e d B e n g u e l a C u r r e n t , f l o w i n g a l o n g t h e coast, f r o m t h e c l i m a t e f o r c e d T r a d e W i n d Drift (D e f a n t [1936 a]; H a r t a n d C u r r i e [1960]; B a n g [1971]).
4 S u m m a r y
W e h a v e p r e s e n t e d t w o n e w q u a s i - m e r i d i o n a l a n d q u a s i - s y n o p t i c X B T s e c t i o n s f r o m t h e e a s t e r n N o r t h a n d S o u t h Atlantic. D u e to n a t u r a l v a r i a t i o n s in t i m e a n d space, o n e n e e d s s y n o p t i c d a t a sets to d e s c r i b e the m e a n p r o p e r t i e s of t h e
Warmwassersph~-
re. W h i l e a n u m b e r of p e r m a n e n t f e a t u r e s c a n a l r e a d y b e e x t r a c t e d f r o m a v a i l a b l e d a t a collections, o t h e r s of a m o r e t r a n s i e n t n a t u r e , s u c h as t h e v e r t i c a l scales, t h e hori- z o n t a l z o n a t i o n a n d v a r i a b i l i t y of e d d y size a n d f r e q u e n c y , n e e d a statistical t r e a t m e n t of d a t a h o m o g e n e o u s in t i m e a n d space. In T a b l e 1, w e p r e s e n t a n u m b e r of c h a r a c t e - ristic p r o p e r t i e s of t h e
Warmwassersph~re
in e q u a t o r i a l a n d l o w e r l a t i t u d e s as d e r i v e d f r o m sections K a n d W a n d f r o m o t h e r XBT s e c t i o n s in t h e h t e r a t u r e . T h e t r a c k l i n e s of t h e d i s c u s s e d cross s e c t i o n s are d e p i c t e d in Fig. 1. In a d d i t i o n to T a b l e 1, w e p r e s e n t a n u p d a t e d p i c t u r e (D e f a n t [1936 b]) of t h e t e m p e r a t u r e a n d c u r r e n t s y s t e m s at t h e e q u a t o r (Fig. 4), w h e r e o u r d a t a c o v e r a g e w a s best. R e c e n t f i n d i n g s on v a r i o u s e q u a t o -D t . h y d r o g r . Z . 3 5 , 1 9 8 2 . H . 2 . K n o l l e t a l . , S o m e X B T - o b s e r v a t i o n s 7 9
a:
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' ~ ~ : ~ ~ Z ~o m ~ o Z ~ ~ o
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80 Dt.hydrogr.Z.35, 1982. H.2. K n o l l et al., Some XBT-observations
O-- lO0-- I ZOO--
200 - -
I
EQUATORIAL NORTH EQUATORIAL DIVERGENCE
i0
4 ~. DIVERGENCE (MINIMAL MIXED LAYER DEPTH)0 2 ~ l NORTH EQUATORIAL I
m ~ i CONVERGENCE I
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THERMOSTAO
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THERMOELINE
l i I i I I I I I I I i I I i i
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Fig. 4. Generalized graph of the equatorial temperature stratification and current system in the Warmwassersph~re according to D e f a n t [1936 b]. Several XBT sections across the Atlantic Ocean ( B e c k e r , H e n n i n g s and M e t z n e r [1981]; B r o c k m a n n , F a h r - b a c h and M e i n c k e [1980]; H e n k e and Z e n k [19801; H e n k e [1978]; H u b e r , M i l l e r , W e i s b e r g et al. [19761 and current measurements ( M o l i n a r i , V o i t u r e z and D u n c a n [1981]; D t i i n g , O s t a p o f f and M e r l e [1980]; C o c h r a n e , K e l l e y and O 11 i n g [1979] are included. Due to the variations of depth and latitude of the illustrated fea-
tures with season and longitude, their positions can only be given approximately
rial c u r r e n t b r a n c h e s b y D / i i n g , O s t a p o f f a n d M e r l e [1980]; C o c h r a n e , K e l l e y a n d O i l i n g [1979] a n d M o l i n a r i , V o i t u r i e z a n d D u n c a n [1981], a r e i n c l u d e d in Fig. 4. This n o t e is a p r e s e n t a t i o n of s o m e p r e l i m i n a r y results of o u r o n g o i n g work.
5 Acknowledgements
This w o r k w a s s u p p o r t e d b y t h e D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t , B o n n - B a d G o d e s b e r g . W e t h a n k t h e D e u t s c h e s H y d r o g r a p h i s c h e s Institut, H a m b u r g , for p r o v i - d i n g t h e XBT r e c o r d e r o n t h e RV " P o l a r s i r k e l " . R. Dickson, L o w e s t o f t , h e l p e d us w i t h a n i m m e d i a t e , n o n - b u r e a u c r a t i c t r a n s f e r of XBT p r o b e s for s e c t i o n K. W e also t h a n k t h e c r e w of t h e ,,Polarsirkel" w h i c h w e r e a c t i v e l y i n v o l v e d in o b t a i n i n g t h e X B T data.
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