An intensive study of the southwestern Atlantic

Eos, V o l . 7 1 , N o . 4 1 , October 9, 1990

CONFLUENCE 1988-1990 An Intensive Study of the Southwestern Atlantic Confluence Principal

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Investigators

cruises c o m p l e t e d d u r i n g t h e following t i m e s : N o v e m b e r 1988, S e p t e m b e r 1989, a n d F e b r u a r y 1990, r e p r e s e n t i n g t h e a u s t r a l seasons of s p r i n g , w i n t e r a n d s u m m e r , respectively. D u r i n g t h e cruises, t h e following activities were undertaken. • T h e vertical d i s t r i b u t i o n of physical a n d c h e m i c a l characteristics of t h e w a t e r masses w e r e s t u d i e d t h r o u g h h y d r o g r a p h i c casts. A total of 125 c o n d u c t i v i t y - t e m p e r a t u r e - d e p t h ( C T D ) casts ( T , S, O , n u t r i e n t s ) w e r e p e r f o r m e d : 3 5 stations in 1988, 2 9 in 1989 a n d 61 in 1990. N u t r i e n t s w e r e m e a s u r e d at e a c h cast, a n d c h l o r o p h y l l was m e a s u r e d for t h e u p p e r layers of C o n f l u e n c e 2 a n d 3. C 0 a n d p C 0 s a m p l e s w e r e also collected d u r i n g t h e w i n t e r c r u i s e ( C o n f l u e n c e 2). 74 e x p e n d a b l e bathythermographs (XBT) were launched • A n a r r a y of 10 i n v e r t e d e c h o s o u n d e r s (IES) was d e p l o y e d across t h e s o u t h e r n ex t e n s i o n of t h e Brazil C u r r e n t (35°S) a n d at t h e frontal r e g i o n . • A n a r r a y of t h r e e c u r r e n t m e t e r m o o r i n g s ( C M M ) with a total of 14 c u r r e n t m e t e r s was d e p l o y e d at 42°S in t h e b o u n d a r y c u r r e n t s . B o t h a r r a y s (IES a n d C M M ) w e r e d e p l o y e d d u r i n g t h e first cruise a n d r e c o v e r e d after 16 m o n t h s d u r i n g t h e last. T h e i n s t r u m e n t loca tions a r e s h o w n o n F i g u r e 1. • Satellite-derived sea s u r f a c e t e m p e r a t u r e fields w e r e p r o d u c e d at 5-day intervals for t h e d u r a t i o n of t h e e x p e r i m e n t . 2

2

P A G E S 1 1 3 1 - 1 1 3 3 , 1137 T h e C O N F L U E N C E P r o g r a m is a j o i n t A r g e n t i n e , F r e n c h , a n d U . S . effort to s t u d y t h e d y n a m i c s of t h e S o u t h w e s t e r n Atlantic Ocean. T h e p r o g r a m concentrates on the Brazil a n d Malvinas (or F a l k l a n d ) c u r r e n t s a l o n g t h e coast of A r g e n t i n a a n d U r u g u a y . T h i s article is a n o v e r v i e w of t h e 1 9 8 8 - 1 9 9 0 field w o r k , with a few p r e l i m i n a r y results. T h e u p p e r c i r c u l a t i o n of t h e w e s t e r n b o u n d a r y of t h e S o u t h Atlantic O c e a n is c h a r a c t e r i z e d by a s o u t h w a r d flow of w a r m salty w a t e r t h a t r e p r e s e n t s t h e w e s t e r n l i m b of t h e S o u t h Atlantic s u b t r o p i c a l g y r e . T h i s s o u t h w a r d flow a p p e a r s to e x t e n d well below the thermocline into d e e p water, but the u p p e r 1000 m a r e g e n e r a l l y r e f e r r e d to as t h e Brazil C u r r e n t . N e a r 38°S, t h e Brazil C u r r e n t m e e t s t h e Malvinas C u r r e n t , a n o r t h w a r d flow of cold s u b a n t a r c t i c w a t e r . T h e Malvinas C u r r e n t o r i g i n a t e s as a b r a n c h of t h e A n t a r c tic C i r c u m p o l a r C u r r e n t b i f u r c a t i n g f r o m t h e m a i n flow d o w n s t r e a m of t h e D r a k e Passage. After e n c o u n t e r i n g t h e Malvinas flow, t h e Brazil C u r r e n t s e p a r a t e s f r o m t h e c o n t i n e n t a l slope a n d p e n e t r a t e s f u r t h e r s o u t h , r e a c h i n g ~ 4 3 ° S . At s e p a r a t i o n t h e Malvinas C u r r e n t m a k e s a n a b r u p t cyclonic l o o p a n d r e t u r n s s o u t h w a r d with t h e Brazil C u r r e n t . T h e strong thermohaline frontal region formed after t h e s e p a r a t i o n of t h e t w o c u r r e n t s is r e f e r r e d to as t h e Brazil/Malvinas C o n f l u e n c e . T h e C o n f l u e n c e t u r n s n o r t h w a r d after r e a c h ing ~43°S, a n d then eastward toward the c e n t r a l S o u t h Atlantic, w h e r e it c o n t i n u e s as the Subtropical Convergence. T h e s e features can b e s e e n in F i g u r e 1, w h i c h shows t h e dis t r i b u t i o n of sea s u r f a c e t e m p e r a t u r e d u r i n g t h e t h i r d C o n f l u e n c e h y d r o g r a p h i c cruise in F e b r u a r y 1990. T h e i n f r a r e d satellite i m a g e is a c o m p o s i t e of all i m a g e s available f r o m F e b r u a r y 2 1 - 2 5 , 1990. T h e satellite d a t a w e r e r e c o r d e d by t h e A r g e n t i n e M e t e o r o l o g i c a l Ser vice a n d p r o c e s s e d at t h e University of Mi ami. T h e C o n f l u e n c e is a highly d y n a m i c r e gion. T h e c u r r e n t s a n d t h e i r associated t e m !

T h e Confluence principal investigators are identified at t h e e n d of this article.

Cover. I n s t r u m e n t s u s e d to g a t h e r d a t a o n t h e C o n f l u e n c e cruises to s t u d y t h e s o u t h w e s t e r n Atlantic O c e a n . ( T o p left) D e p l o y m e n t of t h e c o n d u c t i v i t y - t e m p e r a ture-depth (CTD), (top right) deployment of a n I n v e r t e d E c h o S o u n d e r (IES), a n d ( b o t t o m ) flotation of o n e c u r r e n t m e t e r

p e r a t u r e a n d salinity p a t t e r n s h a v e h o r i z o n t a l excursions reaching h u n d r e d s of kilometers, with t i m e scales as s h o r t as a m o n t h . T h i s t i m e d e p e n d e n c e involves b o t h shifts in t h e location of t h e c u r r e n t s t h e m s e l v e s a n d t h e f o r m a t i o n of small closed circulations o r e d dies [Legeckis and Gordon, 1982]. Little is k n o w n a b o u t t h e d y n a m i c s of t h e s e c o m p l e x f e a t u r e s . P r e v i o u s d e s c r i p t i o n s [Gordon and Greengrove, 1986] w e r e mostly b a s e d o n s n a p s h o t s o b t a i n e d with h y d r o g r a p h i c d a t a . T h e first c o n t i n u o u s o b s e r v a t i o n s o f t h e a r e a w e r e o b t a i n e d d u r i n g 1 9 8 4 - 1 9 8 6 . Sea s u r f a c e t e m p e r a t u r e was m o n i t o r e d f r o m i n f r a r e d satellite s e n s o r s [Olson et ai, 1988], a n d dy n a m i c h e i g h t was o b s e r v e d h o u r l y for 16 m o n t h s u s i n g a pilot a r r a y of i n v e r t e d e c h o s o u n d e r s [Garzoli and Bianchi, 1987; Garzoli and Garraffo, 1989]. B o t h m e a s u r e m e n t s p r o v i d e d i n f o r m a t i o n o n t h e t i m e - s p a c e variabili ty of t h e C o n f l u e n c e a n d l a t i t u d e of its sepa r a t i o n f r o m t h e coast a n d d e m o n s t r a t e d t h e i m p o r t a n c e of a r e g i o n a l survey b a s e d o n long-term continuous observations. O n t h e basis of t h e s e results, a n i n t e r n a tional effort was initiated to p e r f o r m a h i g h r e s o l u t i o n s t u d y of t h e C o n f l u e n c e P r o g r a m . Objectives of t h e p r o g r a m follow. • T o m o n i t o r t h e variability of t h e d y n a m i c t o p o g r a p h y in t h e a r e a of t h e C o n f l u e n c e of t h e Brazil a n d Malvinas c u r r e n t s a n d to s t u d y t h e fluctuations in t i m e a n d s p a c e of t h e asso ciated t h e r m o h a l i n e f r o n t . • T o m o n i t o r t h e v a r i a t i o n s in t r a n s p o r t of t h e Brazil c u r r e n t in t h e vicinity of its s e p a r a tion f r o m t h e b o u n d a r y a n d to o b t a i n t h e first e s t i m a t e s of t h e b o u n d a r y c u r r e n t s in e a c h w a t e r m a s s s o u t h of t h e C o n f l u e n c e front. • T o o b t a i n d e t a i l e d w a t e r mass d e s c r i p t i o n s of t h e r e g i o n d u r i n g t h r e e d i f f e r e n t seasons, to d e t e c t c h a n g e s in w a t e r mass c o m p o s i t i o n a n d to e v a l u a t e t h e w a t e r mass modifications d u e to small-scale m i x i n g across t h e f r o n t a l region. • T o d e t e r m i n e t h e s t r e n g t h of t h e C 0 sink in t h e r e g i o n d u r i n g w i n t e r c o n d i t i o n s . 2

T o achieve t h e s e objectives, a n intensive field e x p e r i m e n t was o r g a n i z e d a n d t h r e e

m o o r i n g ( C M M ) o n d e c k of A r g e n t i n e R/V AVA Puerto Deseado. See " C o n f l u e n c e 1 9 8 8 - 1 9 9 0 : A n I n t e n s i v e S t u d y of t h e S o u t h w e s t e r n A t l a n t i c , " p r e p a r e d by t h e C o n f l u e n c e P r i n c i p a l I n v e s t i g a t o r s , this is sue.

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Variability of the Flows T h e C o n f l u e n c e f r o n t is c h a r a c t e r i z e d by s t r o n g baroclinicity a n d h i g h variability o v e r a wide r a n g e of spatial a n d t i m e scales. T h e C o n f l u e n c e s t a n d s o u t as a r e g i o n rich in e n e r g e t i c e d d i e s w h e n o b s e r v e d t h r o u g h satel lite i n f r a r e d i m a g e r y [Olson et ai, 1988], satel lite-tracked d r i f t i n g b u o y s [Patterson, 1 9 8 5 ; Piola et ai, 1987, Figueroa and Olson, 1984] a n d satellite a l t i m e t r y [Cheney et ai, 1983]. I n o r d e r t o s t u d y this variability, t i m e series of daily satellite i m a g e s w e r e collected a n d a n a r r a y of 10 i n v e r t e d e c h o s o u n d e r s was d e ployed. T h e IES are bottom-deployed instru m e n t s t h a t m e a s u r e a n d r e c o r d t h e t i m e it takes for a n acoustic signal to travel f r o m t h e b o t t o m , reflect at t h e s u r f a c e a n d r e t u r n . T h r o u g h a calibration p r o c e d u r e b a s e d o n C T D m e a s u r e m e n t s , this travel t i m e c a n b e transformed into dynamic heights. T h e I E S a r r a y was d e s i g n e d to m o n i t o r t h e d y n a m i c t o p o g r a p h y of t h e Brazil-Malvinas Confluence region and provide temporal continuity between the h y d r o g r a p h i c observa tions o b t a i n e d d u r i n g t h e t h r e e C o n f l u e n c e cruises. I E S estimates of sea s u r f a c e t o p o g r a p h y allow calculation of t h e g e o s t r o p h i c ve locities a n d t r a n s p o r t s of t h e associated flows a n d p r o v i d e a h i g h - r e s o l u t i o n , cost-effective m e t h o d to s t u d y t i m e - s p a c e variability of t h e t h e r m o h a l i n e f r o n t at t h e C o n f l u e n c e . T h e instruments were deployed along t h r e e z o n a l lines at a p p r o x i m a t e l y 35°, 36.5° a n d 38°S ( F i g u r e 1). A p r e l i m i n a r y calibration to d y n a m i c h e i g h t was o b t a i n e d b a s e d o n t h e d a t a collected d u r i n g t h e first two cruises. T h e series will b e a d j u s t e d , if necessary, with d a t a f r o m C T D casts o b t a i n e d d u r i n g t h e t h i r d c r u i s e . I n this way, 10 t i m e series of dy namic height were derived from the IES m e a s u r e d travel t i m e a n d g e o s t r o p h i c veloci ties calculated f r o m t h e d y n a m i c h e i g h t series ( F i g u r e 2).

Eos, V o l . 7 1 , N o . 4 1 , October 9, 1990 A l o n g 35°S ( F i g u r e 2), t h e flow is mostly associated with t h e Brazil C u r r e n t . O n t h e w e s t e r n side of t h e I E S section, t h e d y n a m i c h e i g h t s l o p e a n d t h e n e g a t i v e velocities i n d i cate t h a t for m o s t of t h e o b s e r v e d p e r i o d t h e flow was s o u t h w a r d (Brazil C u r r e n t ) . D u r i n g only two s h o r t p e r i o d s o f t i m e , a r o u n d d a y s 4 3 0 a n d 7 0 0 , t h e flow r e v e r s e s , i n d i c a t i n g n o r t h w a r d i n c u r s i o n s of t h e Malvinas C u r r e n t . I n t h e c e n t e r o f t h e I E S section t h e di rection of g e o s t r o p h i c flow was m a i n l y n o r t h w a r d a n d was associated with t h e r e t u r n of t h e Brazil C u r r e n t . T h e flow b e c a m e s o u t h w a r d a g a i n n e a r t h e e a s t e r n side of t h e sec tion, r e p r e s e n t i n g a r e c i r c u l a t i o n of t h e Brazil C u r r e n t . T h e s h e d d i n g of e d d i e s is c h a r a c t e r istic of this r e g i o n a n d is reflected by d e e p d e p r e s s i o n s of t h e d y n a m i c t o p o g r a p h y . T h e IES array does not provide informa tion o n t h e vertical s t r u c t u r e o f t h e w a t e r col u m n , but a good approximation of the geo s t r o p h i c t r a n s p o r t s c a n b e o b t a i n e d by a s s u m i n g a l i n e a r d e c r e a s e of velocity with d e p t h [Garzoli and Garraffo, 1989]. O n this basis, m e a n t r a n s p o r t values of 8 x 1 0 m / s a n d 11 x 1 0 m / s w e r e o b t a i n e d across 35°S for t h e Brazil C u r r e n t a n d t h e Brazil r e t u r n , r e s p e c tively. A similar analysis was p e r f o r m e d with t h e IES series o b t a i n e d f a r t h e r s o u t h (not s h o w n ) . A t 36.5°S t h e flow was mostly s o u t h w a r d a n d t h e m e a n e s t i m a t e d t r a n s p o r t was 9 x 1 0 m / s . F o r t h e s o u t h e r n m o s t line ( n e a r 38°S) t h e series s h o w e d a l a r g e zonal d y n a m i c h e i g h t variability, associated with z o n a l dis p l a c e m e n t of t h e f r o n t . A l o n g t h e e d g e o f t h e c o n t i n e n t a l shelf, t h e o b s e r v e d flow c h a n g e s f r o m s o u t h w a r d to n o r t h w a r d d e p e n d i n g o n t h e p o s i t i o n of t h e t h e r m o h a l i n e f r o n t . T h e s e d a t a will b e a n a l y z e d o n t h e basis of a s i m p l e m o d e l to s t u d y t h e t i m e - s p a c e variability of the front. 6

6

3

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3

Circulation

Fig. 1. ( T o p ) I n f r a r e d satellite sea s u r f a c e t e m p e r a t u r e . R e d is cold; b l u e is w a r m . S u p e r i m p o s e d crosses i n d i c a t e t h e location of t h e m o o r i n g s (IES a n d c u r r e n t m e t e r s ) . [See t h e back of this v o l u m e for o r i g i n a l c o l o r figure.] ( B o t t o m ) S c h e m a t i c of g e o s t r o p h i c t i m e a v e r a g e d m a s s t r a n s p o r t of t h e u p p e r 1000 m in t h e w e s t e r n A r g e n t i n a Basin. T h e t r a n s p o r t s given a r e in S v e r d r u p s (1 Sv = 1 x 1 0 m / s ) . 6

3


W h i l e variability of t h e C o n f l u e n c e c a n b e m o n i t o r e d with IESs d u e to t h e baroclinic characteristics of t h e associated flow, o b s e r v a tion of t h e b a r o t r o p i c Malvinas C u r r e n t r e q u i r e d d i r e c t m e a s u r e m e n t s u s i n g a n a r r a y of c u r r e n t m e t e r m o o r i n g s to e s t i m a t e t r a n s ports a n d their variations. T h r e e moorings with a total of 14 c u r r e n t m e t e r s w e r e d e p l o y e d in a t r i a n g u l a r a r r a y n e a r 42°S ( F i g u r e 1). T h e w a t e r c o l u m n d e p t h at e a c h location was 1500, 3 0 0 0 a n d 4 5 0 0 m , respectively. C u r r e n t m e t e r s w e r e d i s t r i b u t e d vertically in o r d e r to s a m p l e e a c h w a t e r m a s s at least o n c e . T h e shallowest m o o r i n g utilized t h r e e c u r r e n t m e t e r s at d e p t h s o f 5 0 0 , 1000 a n d 1500 m . T h e 3 0 0 0 - m m o o r i n g h a d five c u r r e n t m e t e r s a n d t h e 4 5 0 0 m m o o r i n g h a d six. N i n e of t h e c u r r e n t m e t e r s w e r e e q u i p p e d with p r e s s u r e , t e m p e r a t u r e a n d c o n d u c t i v i t y s e n s o r s . T h e d a t a r e t u r n o n t h e velocity m e a s u r e m e n t s was a b o u t 7 0 % b e c a u s e a n u n k n o w n e v e n t c a u s e d p a r t o f t h e u p p e r flota tion o n t h e two d e e p e s t m o o r i n g s to i m p l o d e after 3 m o n t h s in t h e w a t e r . T h e 1 5 0 0 - m m o o r i n g was always in t h e Malvinas C u r r e n t a n d s h o w e d t h e s t r e n g t h a n d d i r e c t i o n of t h e flow to be r e m a r k a b l y s t e a d y ; tidal effects w e r e practically n o n e x i s t e n t . T h e d i r e c t i o n of t h e flow closely followed t h e b o t t o m slope a n d little variability was o b s e r v e d . T h e 3 0 0 0 a n d 4 5 0 0 - m m o o r i n g s w e r e subject to b o t h Malvinas C u r r e n t a n d r e t u r n flow.

Eos, V o l . 7 1 , N o . 4 1 , October 9 , 1990

DYNAMIC (dyn

HEIGHT m)

46°-

T"

48

1.60 1.50

UJ

49°-?

o z o

50°-

1.40 1.30 1.20 1.10

51H

1.00

I 52°-

322

422

522 JULIAN

i 722

622

DAY (1988-1990)

GEOSTROPHIC VELOCITY (cm/sec) 47°30 20 10 0 -10 -20 -30 -40 -50

JULIAN

DAY (1988-1990)

Fig. 2. ( T o p ) T i m e l a t i t u d e variability o f d y n a m i c h e i g h t a n d (bottom) g e o s t r o p h i c velocities a l o n g 35°S f r o m t h e i n v e r t e d e c h o s o u n d e r s . T h e u p p e r p a n e l shows t h e t i m e l a t i t u d e variability o f d y n a m i c h e i g h t for t h e n o r t h e r n I E S line, a l o n g 35°S, a n d t h e lower p a n e l s h o w s t h e g e o s t r o p h i c velocities o b t a i n e d f r o m p a i r s of stations, f r o m west to east. [See t h e b a c k o f this v o l u m e f o r original color figure.]

Water Masses T h e s t r o n g t h e r m o h a l i n e stratification o f t h e s o u t h w e s t e r n A t l a n t i c is a c o n s e q u e n c e of t h e location o f this basin at t h e c r o s s r o a d s o f several m a j o r w a t e r m a s s e s o f t h e W o r l d O c e a n . B a s e d o n e x t r e m e s f o u n d in t e m p e r a t u r e , salinity, dissolved o x y g e n a n d n u t r i e n t vertical d i s t r i b u t i o n s , Reid et al. [1977] i d e n t i fied seven distinct w a t e r m a s s e s below t h e t h e r m o c l i n e . T h e s e layers a r e a d v e c t e d i n t o t h e basin f r o m t h e N o r t h Atlantic, t h e S o u t h Pacific a n d t h e W e d d e l l Sea. Historical d a t a reveal t h a t t h e climatological sea s u r f a c e d y n a m i c h e i g h t relative t o 1 0 0 0 d b a r s c h a n g e s 0.5 d y n a m i c m e t e r s (5 J / K g ) . F r o n t a l s u r f a c e g e o s t r o p h i c velocities relative t o 1000 d b a r s r e a c h values of 7 5 c m / s with typical values o f a b o u t 5 5 cm/s. T h e Confluence Program has provided the h y d r o g r a p h i c data necessary to adequately

resolve t h e relatively n a r r o w flows o f t h e B r a zil a n d Malvinas c u r r e n t s u p s t r e a m f r o m a n d after t h e i r c o n f l u e n c e . N u t r i e n t d a t a collected d u r i n g C o n f l u e n c e - 3 cruises will aid in i n t e r p r e t i n g t h e c o m p l e x flows a n d m i x i n g of d e e p - w a t e r m a s s e s p r e s e n t in t h e r e g i o n . P r e l i m i n a r y r e s u l t s b a s e d o n d a t a collected d u r ing the Confluence P r o g r a m show that the s o u t h w a r d m a s s t r a n s p o r t across 40°S relative to 1000 d b a r s a v e r a g e s 2 2 S v e r d r u p s (1 Sv = 1 0 m / s ) . T h i s t r a n s p o r t is d u e t o a p p r o x i mately e q u a l c o n t r i b u t i o n s of t h e Brazil C u r r e n t a n d t h e Malvinas C u r r e n t . A s c h e m a t i c of u p p e r w a t e r t r a n s p o r t s in t h e a r e a is s h o w n in F i g u r e lb. T h e s c h e m a t i c is b a s e d o n t h e climatic g e o s t r o p h i c m a s s t r a n s p o r t function a n d is a qualitative p i c t u r e o f t h e av e r a g e d u p p e r o c e a n circulation b a s e d o n t h e geostrophic balance. 6

3

N u t r i e n t s ( p h o s p h a t e , silicate, n i t r a t e , ni trite) a n d 0 w e r e m e a s u r e d at e a c h C T D 2


station d u r i n g t h e t h r e e cruises. S u c h t r a c e r s a r e very useful in m u l t i p l e t r a c e r analysis o f w a t e r mass c o m p o s i t i o n a n d m i x i n g . T h e y also h e l p to identify w a t e r masses f o r m e d by w i n t e r c o n v e c t i o n , a n d to follow t h e i r p a t h way within t h e b a s i n i n t e r i o r . A p r e l i m i n a r y q u a n t i t a t i v e analysis of w a t e r masses in t h e C o n f l u e n c e h a s b e e n p e r f o r m e d with a n i n v e r s e m e t h o d u s i n g d a t a f r o m t h e s e c o n d c r u i s e . T h e p u r p o s e was t o find t h e m i x t u r e o f s o u r c e w a t e r types t h a t best d e s c r i b e d t h e c o m p o s i t i o n of a given wa t e r s a m p l e [Mackas et al, 1987]. T h e solution v e c t o r t h e r e f o r e p r o v i d e s t h e best fitting frac tional c o n t r i b u t i o n (%) o f each s o u r c e t y p e t o t h e s a m p l e . T h i s m e t h o d is valuable in r e g i o n s involving s t r o n g m i x i n g a m o n g a rela tively l a r g e n u m b e r o f s o u r c e w a t e r types, as is t h e Brazil-Malvinas C o n f l u e n c e z o n e . S e v e n s o u r c e w a t e r t y p e s w e r e r e t a i n e d : S o u t h At lantic C e n t r a l W a t e r ( S A C W ) , S u b a n t a r c t i c S u r f a c e W a t e r (SASW), A n t a r c t i c I n t e r m e d i ate W a t e r ( A A I W ) , U p p e r C i r c u m p o l a r D e e p W a t e r ( U C D W ) , N o r t h Atlantic D e e p W a t e r (NADW), Lower Circumpolar Deep Water, ( L C D W ) , a n d W e d d e l l Sea D e e p W a t e r ( W S D W ) [Peterson and Whitworth, 1989]. T r a c e r s selected w e r e t e m p e r a t u r e , salinity, dis solved n u t r i e n t s a n d o x y g e n . T h e m a i n results ( s h o w n only for N A D W a n d S A C W , F i g u r e 3) w e r e in a c c o r d a n c e with t h e p r e v i o u s d e s c r i p t i o n of circulation in t h e Brazil-Malvinas C o n f l u e n c e . I n t h e u p p e r layer t h e c o n t r i b u t i o n o f S A C W ( > 9 6 % ) to t h e w a t e r s a m p l e s was l a r g e r at t h e n o r t h e r n section (35°S) t h a n at t h e s o u t h e r n section ( < 7 5 % at 39°S). T h e S A C W flows s o u t h w a r d close t o t h e c o n t i n e n t a l p l a t e a u . A t 39°S, its c o n t r i b u t i o n r e m a i n s below 10%. T h e S A S W m o v i n g n o r t h w a r d is c o m p r e s s e d a l o n g t h e coast v a n i s h i n g at 35°S (less t h a n 10%). T h e c o r e layer of A A I W is d e t e c t e d b e t w e e n d e p t h s of 5 0 0 - 1 5 0 0 m . I t c o n t r i b u t e s m o r e t h a n 8 0 % at all sections. T h e N A D W , at about 2500 m depth, moves southward along t h e c o n t i n e n t a l slope ( 7 0 % ) . Its c o n t r i b u t i o n b e c o m e s w e a k e r at t h e s o u t h e r n section (low e r t h a n 2 5 % ) with a s t r o n g d i l u t i o n b e t w e e n 37°S a n d 39°S. T h e c i r c u m p o l a r d e e p w a t e r is s e p a r a t e d by t h e N A D W in two layers: t h e u p p e r b r a n c h (UCDW) a n d the lower b r a n c h ( L C D W ) . T h e U C D W is well r e p r e s e n t e d at all stations. Its c o n t r i b u t i o n is h i g h e r t h a n 70%. T h e L C D W contribution decreases to w a r d t h e n o r t h e r n stations. Below 3 5 0 0 m , t h e W S D W is well m a r k e d ( c o n t r i b u t i o n h i g h er than 90%). Despite the nonconservativeness o f n u t r i e n t s a n d dissolved 0 , this sim ple m e t h o d s e e m s efficient. W e f o u n d h i g h e r r e s i d u a l s in t h e s u r f a c e s a m p l e s a l o n g t h e coast. T h e y p r o b a b l y reflect m o r e i n t e n s e bio logical activity a n d / o r m i x t u r e of r i v e r p l u m e w a t e r f r o m t h e Rio d e la Plata n o t yet consid e r e d in o u r d e s c r i p t i o n . 2

Mixing C r o s s - f r o n t h e a t flux e s t i m a t e s in d e e p wa t e r s [Georgi, 1981] a n d i n t e r m e d i a t e w a t e r s [Piola and Georgi, 1982] s u g g e s t t h a t lateral m i x i n g m a y play a key r o l e in w a t e r m a s s modification at t h o s e levels. I n t h e f r o n t a l r e g i o n t h e t h e r m o h a l i n e s t r u c t u r e shows i n t e r l e a v i n g o f t h e distinct w a t e r masses f o u n d at e i t h e r side of t h e f r o n t at v a r i o u s d e p t h s . T h e layering e n h a n c e s cross-front heat a n d salt t r a n s f e r by small-scale m i x i n g p r o c e s s e s .

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l a t i o n s h i p b e t w e e n t h e fine-scale variability of vertical scales in t h e 10- t o 1 0 0 - m r a n g e a n d t h e cross-frontal p r o p e r t y fluxes. T h e m o d e l fluxes a r e given in t e r m s o f t h e vertical e d d y diffusivity, t h e vertical p r o p e r t y v a r i a n c e a n d t h e large-scale cross-front p r o p e r t y g r a d i e n t . A s s u m i n g a diffusivity o f 1 x 10 " m / s a n d calculating t h e typical large-scale cross-front p r o p e r t y g r a d i e n t , t h e m o d e l l e a d s t o a crossf r o n t t e m p e r a t u r e flux o f 1 x 10" °C m/s a n d salinity flux o f 1 x 10" m / s . S u c h m e c h a n i s m s a r e also c a n d i d a t e s f o r dissipation o f l a r g e scale e d d i e s a n d m e a n d e r s t h a t d e t a c h f r o m the Confluence downstream of t h e separation point from t h e continental slope.

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T h e i n t e n s e survey p e r f o r m e d d u r i n g t h e Confluence-3 cruise provides a description of t h e s u m m e r h y d r o g r a p h i c characteristics o f t h e r e g i o n ( F i g u r e s 5a, 5b a n d 5c). F o u r m a i n features can be distinguished. Surface waters a r e s e p a r a t e d f r o m u n d e r l y i n g w a t e r s by a s h a r p seasonal t h e r m o c l i n e . T h e s u r f a c e sa linity was i n f l u e n c e d by a l a r g e runoff, result i n g t o s o m e values less t h a n 3 2 p s u a n d a n in t e n s e e v a p o r a t i o n l e a d i n g t o a salinity close t o 37 p s u . S u r f a c e w a t e r s w e r e also often largely o v e r s a t u r a t e d in o x y g e n . At t h e surface t h e Brazil-Malvinas t e m p e r a t u r e front is w e a k with a n east-west d i r e c t i o n , w h e r e a s at 5 0 m it is m u c h s t r o n g e r a n d with a north-south direction. T h e shape a n d d e p t h of this seasonal t h e r m o c l i n e varies with t h e u n d e r l y i n g w a t e r masses. T h e seasonal m i x e d layer is shallower in t h e s o u t h ( 5 - 3 0 m ) a n d d e e p e n s in t h e n o r t h t o r e a c h 6 0 m at 35°S ( F i g u r e 5a). T h e g r a d i e n t s in salinity r e a c h u p t o 1 p s u / 1 0 k m ( F i g u r e 5b). T h i s is p a r t l y d u e t o very low values (less t h a n 3 2 p s u ) e n c o u n t e r e d o n t h e shelf at t h e l a t i t u d e of t h e Rio d e la Plata. T h e Rio d e L a Plata dis c h a r g e into t h e o c e a n is g e n e r a l l y e n h a n c e d in s u m m e r d u e t o s u b t r o p i c a l r a i n s . T h e very l a r g e salinity values f o u n d in t h e s u r f a c e wa ters c o r r e s p o n d i n g t o t h e Brazil C u r r e n t (at 35°S) a r e p r o b a b l y d u e t o intensified local s u m m e r e v a p o r a t i o n . O x y g e n is s u p e r s a t u r a t ed at t h e s u r f a c e in t h e Malvinas C u r r e n t ( F i g u r e 5c). S u p e r s a t u r a t i o n is r o u g h l y in versely c o r r e l a t e d with s u r f a c e t e m p e r a t u r e , with cold w a t e r b e i n g w a r m e d u p b e c o m i n g s u p e r s a t u r a t e d with o x y g e n . T e m p e r a t u r e is n o t t h e only factor. Biological p r o c e s s e s also play a n i m p o r t a n t r o l e as e v i d e n c e d by t h e chlorophyll a n d nutrient data.

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Fig. 3. S o u r c e c o m p o s i t i o n solution s h o w i n g c o n c e n t r a t i o n ( 0 - 1 . 0 , o r 0 - 1 0 0 % ) o f t h e s o u r c e w a t e r t y p e N A D W a n d S A C W v e r s u s h o r i z o n t a l position a n d d e p t h for t h e f o u r C o n f l u e n c e - 2 sections. T h e n o r t h e r n section is at t h e t o p of t h e figure, t h e s o u t h e r n at t h e b o t t o m . S a m p l e locations a r e i n d i c a t e d by d o t s . C o n t o u r s a n d s h a d i n g i n d i c a t e d t h e e s t i m a t e d 0 . 1 , 0 . 2 5 , 0 . 5 , 0 . 7 5 a n d 0 . 9 0 s o u r c e c o n c e n t r a t i o n isopleths.

CO2 Program During Confluence-2 Expedition (September 1989-Austral Winter) T h e C o n f l u e n c e r e p r e s e n t s a significant oceanic sink a r e a for a t m o s p h e r i c C 0 . T h e partial p r e s s u r e o f C 0 ( p C 0 ) in t h e w a r m Brazil C u r r e n t w a t e r in t h e s t u d y a r e a ( 3 4 ° S 40°S) is r e d u c e d m a i n l y d u e t o c o o l i n g o c c u r r i n g d u r i n g its p o l e w a r d flow. T h e p C 0 in t h e cold, n u t r i e n t - r i c h Malvinas C u r r e n t wa t e r is also r e d u c e d d u r i n g its n o r t h w a r d flow by p h o t o s y n t h e t i c utilization o f C 0 . As t h e s e two effects a r e s u p e r p o s e d in t h e C o n f l u e n c e , t h e surface w a t e r s b e c o m e highly u n d e r s a t u r a t e d with r e s p e c t t o a t m o s p h e r i c C 0 [Takahashi and Chipman, 1985; T . P e n g a n d T . T a 2

T h e t i m e - a v e r a g e d t e m p e r a t u r e a n d salini ty sections o b s e r v e d across t h e Brazil/Malvinas C o n f l u e n c e a t 38°S d u r i n g t h e Conflu e n c e P r o g r a m a r e s h o w n in F i g u r e s 4 a a n d 46. T h e s u b t r o p i c a l (Brazil C u r r e n t ) w a t e r s a r e c h a r a c t e r i z e d by t e m p e r a t u r e h i g h e r t h a n 10°C a n d salinity h i g h e r t h a n 3 5 . 0 practical salinity u n i t s (psu), a n d t h e s u b a n t a r c t i c (Mal vinas C u r r e n t ) w a t e r s in t h e u p p e r 5 0 0 m show t e m p e r a t u r e s lower t h a n 10°C a n d sa linities lower t h a n 3 4 . 3 p s u . T h e lower p a n e l s

in F i g u r e s 4c a n d 4d s h o w t h e vertical t e m p e r a t u r e a n d salinity g r a d i e n t v a r i a n c e s esti m a t e d for t h e 0 . 0 1 - t o 0 . 1 0 - c p m w a v e n u m b e r b a n d . V a r i a n c e i n c r e a s e d by a n o r d e r o f m a g n i t u d e t o w a r d t h e front. T h i s i n c r e a s e is consistent with t h e i n t e r l e a v i n g a n d m i x i n g e x p e c t e d a t t h e front. T h e v a r i a n c e m a g n i t u d e is a n o r d e r of m a g n i t u d e h i g h e r t h a n t h a t o b s e r v e d in t h e G u l f S t r e a m [Joyce, 1976]. T h e m o d e l of Joyce [1977] established a r e This page may be freely copied.

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Eos, V o l . 7 1 , N o . 4 1 , October 9, 1990 f o r m e d by c o m b i n i n g t h e v a r i o u s d a t a sets in a n o p t i m a l f a s h i o n . W e will s t u d y t h e ex c h a n g e s of e n e r g y b e t w e e n t h e m e a n flow a n d t h e p e r t u r b a t i o n s a n d try to u n d e r s t a n d t h e s o u r c e s of e n e r g y in t h e r e g i o n , a n d es tablish links b e t w e e n a t m o s p h e r i c forcings (local o r r e m o t e ) a n d local circulation. T h e C T D a r r a y s p r o v i d e us with a u n i q u e d o c u m e n t a t i o n t h a t will e n a b l e u s t o follow seasonal c h a n g e s of t h e r m o d y n a m i c p r o c e s s e s in t h e r e g i o n . M u l t i p a r a m e t e r analysis of wa t e r masses ( T , S, n u t r i e n t s , C 0 ) will d e t e r mine water mass origins, proportions a n d m i x i n g of w a t e r s o u r c e s . S t u d i e s of w i n t e r c o n v e c t i o n a n d d o u b l e diffusion processes will b e p e r f o r m e d . T h e m a s s t r a n s p o r t in e a c h w a t e r m a s s a n d its v a r i a t i o n s will b e esti mated from the CMM, IES, C T D , and X B T . T h i s will allow a m a j o r i m p r o v e m e n t o n t h e e s t i m a t e s in F i g u r e lb. 2

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DISTANCE FROM FRONT (km)

Fig. 4. ( T o p ) (a) T i m e - a v e r a g e d t e m p e r a t u r e a n d (b) salinity sections across t h e B r a zil/Malvinas C o n f l u e n c e at 38°S b a s e d o n C T D a n d X B T d a t a collected d u r i n g t h e C o n fluence Project. ( B o t t o m ) (c) Vertical t e m p e r a t u r e g r a d i e n t a n d (d) salinity g r a d i e n t vari a n c e in t h e 0 . 0 1 - to 0 . 1 - c p m w a v e n u m b e r b a n d p l o t t e d as a f u n c t i o n of t h e d i s t a n c e f r o m t h e f r o n t . N o t e t h e v a r i a n c e i n c r e a s e as d i s t a n c e to t h e f r o n t d e c r e a s e s .

k a h a s h i , u n p u b l i s h e d m a n u s c r i p t , 1990]. I n a d d i t i o n , t h e s u b p o l a r M o d e W a t e r , w h i c h is k n o w n to b e f o r m e d in t h e s t u d y a r e a via d e e p convective m i x i n g p r o c e s s e s d u r i n g win t e r [McCartney, 1982; Gordon, 1981], p r o v i d e s a vehicle for t r a n s p o r t of a t m o s p h e r i c C 0 into the thermocline regime. T h u s the Con fluence a r e a plays a n i m p o r t a n t r o l e in oce anic s e q u e s t r a t i o n of a t m o s p h e r i c C 0 . C 0 d a t a a r e n e e d e d for e s t i m a t i n g t h e air-sea C 0 flux a n d o c e a n i c C 0 t r a n s p o r t flux. T h e m a j o r p u r p o s e of t h e C 0 p r o g r a m in t h e C o n f l u e n c e e x p e d i t i o n was t o d e t e r m i n e t h e C 0 p a r t i a l p r e s s u r e in s u r f a c e w a t e r s d u r i n g t h e a u s t r a l w i n t e r ( A u g u s t ) of 1989, a n d to d e t e r m i n e t h e total C 0 c o n c e n t r a t i o n ( T C 0 ) in t h e t h e r m o c l i n e a n d d e e p w a t e r s a l o n g with s t a n d a r d h y d r o g r a p h i c m e a s u r e m e n t s i n c l u d i n g t h e c o n c e n t r a t i o n of o x y g e n a n d n u t r i e n t salts. T h e n e g a t i v e sea-air p C 0 differences ( A p C 0 = ( p C 0 ) s w - ( p C 0 ) a i r ) in t h e s t u d y a r e a s h o w e d t h a t s u r f a c e w a t e r s in the study area were strongly u n d e r s a t u r a t e d . T h e c o l d e r Malvinas C u r r e n t w a t e r a p p e a r e d to b e less u n d e r s a t u r a t e d ( A p C 0 = 1 0 - 1 5 jxatm) t h a n t h e w a r m e r w a t e r s o f t h e Brazil C u r r e n t ( A p C 0 = - 4 0 to - 4 5 |xatm). A n a r r o w z o n e of e x c e p t i o n a l l y low P C 0 values ( A p C 0 as low as - 2 2 4 p-atm; a b o u t 3 5 % satu r a t i o n ) was o b s e r v e d off Rio d e la Plata. T h e s e low p C 0 values a r e associated with low salinity values (as low as 3 3 p s u ) , r e d u c e d c o n c e n t r a t i o n o f total C 0 a n d z e r o n i t r a t e , a n d h e n c e a p p e a r to r e p r e s e n t t h e effect of 2

2

Rio d e la Plata w a t e r . A similar low p C 0 z o n e h a s b e e n o b s e r v e d off t h e A m a z o n River m o u t h . T h e total C 0 c o n c e n t r a t i o n s in t h e N A D W in t h e s t u d y a r e a h a v e b e e n f o u n d to r a n g e b e t w e e n 2 1 7 0 a n d 2 1 9 0 u-M/kg, w h e r e as t h o s e in t h e A A B W r a n g e b e t w e e n 2 2 6 0 a n d 2 2 7 0 n-M/kg. 2

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As t h e initial p r o c e s s i n g of t h e field d a t a m o v e s t o w a r d c o m p l e t i o n , it is t i m e to b e g i n a full analysis a n d synthesis of t h e v a r i o u s d a t a sets to p r o v i d e a n e w p i c t u r e of t h e d y n a m i c s of t h e C o n f l u e n c e r e g i o n . T h e o b servations obtained are c o m p l e m e n t a r y : C T D a r r a y s p r o v i d e d i s c r e t e m e a s u r e m e n t s in t i m e ( t h r e e d i f f e r e n t seasons), c o n t i n u o u s vertical profiles a n d h i g h h o r i z o n t a l r e s o l u t i o n , I E S f u r n i s h c o n t i n u o u s i n f o r m a t i o n of t h e posi tion of t h e f r o n t a n d e s t i m a t e s of t h e Brazil C u r r e n t t r a n s p o r t n o r t h of t h e front, C M M give c o n t i n u o u s e s t i m a t e s of velocity in e a c h w a t e r m a s s s o u t h o f t h e f r o n t , a n d satellite SST provides a surface coverage quasi-contin u o u s in t i m e . G e o s a t a l t i m e t r y d a t a will also b e a n a l y z e d . E a c h g r o u p is p r e s e n t l y analyz i n g its o w n d a t a set a n d p r e l i m i n a r y d a t a analysis ( d e s c r i p t i o n a n d statistics) is b e i n g achieved. C o m p a r i s o n of the different data types will p r o v i d e v a l i d a t i o n of t h e d i f f e r e n t d a t a sets. A f t e r t h e s e p r e l i m i n a r y s t u d i e s a n objective analysis o f t h e fields will b e p e r


T h e m e r i d i o n a l m o t i o n s of t h e Brazil C u r r e n t will b e o b t a i n e d t h r o u g h S S T i m a g e s a n d I E S , a n d f r o n t a l m o t i o n s will b e r e l a t e d t o v a r i a t i o n s o f t r a n s p o r t in t h e Malvinas a n d Brazil c u r r e n t s . Initial c o n s i d e r a t i o n of m e t h o d s for c o m b i n i n g I E S , satellite S S T a n d altim e t r i c d a t a s u g g e s t t h a t t h e a r r a y is c a p a b l e of p r o d u c i n g s y n o p t i c d e p i c t i o n . T h e vertical e x t e n t of s u r f a c e m o t i o n s will b e o b t a i n e d t h r o u g h correlations between surface a n d d e e p m o t i o n s ( C M M ) , e n a b l i n g u s to s t u d y t h e vertical p r o p a g a t i o n of signals ( C M M , S S T , IES).

Confluence Principal Investigators Scientific p a r t i c i p a n t s , t h e i r affiliations a n d m a i n responsibilities follow. U . S . : Silvia L. Garzoli, L a m o n t - D o h e r t y Geological O b s e r v a tory of C o l u m b i a University, I n v e r t e d E c h o S o u n d e r s ; Otis B r o w n , R o b e r t E v a n s , D o n a l d O l s o n , a n d G u i l l e r m o P o d e s t a , Rosenstiel School of M a r i n e a n d A t m o s p h e r i c Science, University of M i a m i , Satellite I m a g e s , Sea Surface T e m p e r a t u r e . France: Christine Pro vost, C e n t r e N a t i o n a l d e la R e c h e r c h e Scientifique/Laboratoire d'Oceanographie Dynamiq u e et C l i m a t o l o g i e , University d e Paris S u d , C u r r e n t Meter Moorings/Hydrography; Veronique Garcon CNRS/UM39, Nutrients; C a t h e r i n e M a i l l a r d , I n s t i t u t F r a n c a i s d e Re c h e r c h e p o u r l'Exploitation d e la M e r , Hy drography; Laurent Memery, CNRS/LODYC, University d e Paris S u d , H y d r o g r a p h y ; T a r o T a k a h a s h i , L - D G O of C o l u m b i a University, C 0 . A r g e n t i n a : A l b e r t o Piola, Servicio d e H i d r o g r a f i a Naval, B u e n o s A i r e s , H y d r o g r a p h y ; A l e j a n d r o B i a n c h i , Servicio d e H i d r o grafia Naval, B u e n o s Aires, H y d r o g r a p h y . 2

Acknowledgments C r u i s e s u p p o r t was p r o v i d e d by t h e follow i n g o r g a n i z a t i o n s : C O N F L U E N C E 1, R/V ARA Puerto Deseado, f u n d e d by t h e N a t i o n a l Science F o u n d a t i o n ; C O N F L U E N C E 2, R/V BIP Oca Balda, f u n d e d by t h e Servicio d e H i drografia Naval, Argentina; C O N F L U E N C E 3, R/V Le Suroit, f u n d e d by I n s t i t u t F r a n c a i s d e R e c h e r c h e p o u r l'Exploitation d e la M e r (IFREMER), France. T h e hydrographic work in cruises 1 a n d 2 was c a r r i e d o u t by t h e D e p a r t a m e n t o d e O c e a n o g r a f i a , Servicio d e H i d r o g r a f i a N a v a l , while o n cruise 3 , it was c o n d u c t e d by L a b o r a t o i r e d ' O c e a n o g r a p h i e D y n a m i q u e et C l i m a t o l o g i e , U n i v e r s i t e d e Paris, France.

Eos, V o l . 7 1 , N o . 4 1 , October 9 , 1990 T h e U . S . p r o g r a m h a s b e e n fully f u n d e d by a g r a n t O C E 8 7 - 1 1 5 2 9 of t h e Division of O c e a n Sciences o f N S F . T h e e n g i n e e r in c h a r g e of t h e I E S was M i g u e l Maccio. S u p p o r t for scientific e x c h a n g e b e t w e e n scientists from U.S. a n d Argentina a n d U.S. a n d F r a n c e h a s b e e n p r o v i d e d by N S F g r a n t s I N T 8 9 - 1 4 5 2 5 a n d I N T 8 9 - 1 4 5 3 6 . D o n Ol son's p a r t i c i p a t i o n is s u p p o r t e d by a g r a n t of t h e Office of Naval R e s e a r c h O N R N 0 0 0 1 4 89-51137. We acknowledge the Tinker Foun dation that t h r o u g h grant C U 00327801 par tially s u p p o r t e d t h e p a r t i c i p a t i o n o f A r g e n tine scientific a n d technical p e r s o n n e l in t h e cruises. T h e satellite i m a g e r y was r e c o r d e d at t h e Estacion H R P T e n Alta R e s o l u c i o n o p e r a t e d by t h e A r g e n t i n e M e t e o r o l o g i c a l Service (Servicio M e t e o r o l o g i c o Nacional) as p a r t of a c o o p e r a t i v e a g r e e m e n t with t h e U n i v e r s i t y of Miami.

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T h e financial s u p p o r t o f t h e F r e n c h p r o gram comes from three sources: T h e current m e t e r w o r k was f u n d e d by I F R E M E R . T h e C T D w o r k a n d p a r t i c i p a t i o n (scientists a n d s h i p p i n g of e q u i p m e n t ) by I n s t i t u t e N a t i o n a l e d e Sciences d e l ' U n i v e r s e , a n d C e n t r e N a t i o n al d ' E t u d e s Espatiales. I F R E M E R s u p p o r t e d the third Confluence cruise. Takahashi's par ticipation in t h e p r o g r a m is b e i n g f u n d e d by the CNRS-Relations Internationales, Support for scientific e x c h a n g e s b e t w e e n scientists from France a n d the U.S., France a n d Ar g e n t i n a is b e i n g p r o v i d e d by C N R S - R e l a t i o n s I n t e r n a t i o n a l e s . W e a c k n o w l e d g e t h e partici p a t i o n in t h e d a t a collection of: A. Billant, C. H e s l o u i n , a n d R. P e r c h o c f r o m I F R E M E R ; S. G a n a a n d J . Lanoisell f r o m L O D Y C ; B . B o u r l a n d C. Colin f r o m t h e Office d e la R e c h e r c h e Scientifique et T e c h n i q u e O t r e - M e r ; B. Boudjellal a n d M. B o u l a h d i d f r o m I n s t i t u t d e s Sciences d e la M e r e t d e l ' A m e n a g e m e n t d u Littoral, A l g e r i a ; J . C. E l g u e f r o m t h e I n stitute N a c i o n a l d e Pesca, U r u g u a y ; H . Freije from the Instituto Argentino d e Oceanografia, A r g e n t i n a ; a n d L. M e d i n a f r o m C e n t r o d e Tecnologia Pesquera, Spain.

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T h e Naval H y d r o g r a p h i c Service o f A r g e n tina (Servicio d e H i d r o g r a f i a Naval ( S H N ) ) fully s u p p o r t e d t h e p a r t i c i p a t i o n o f t h e A r g e n t i n e scientists in t h e p r o g r a m . T h e Insti t u t o A n t a r c t i c o A r g e n t i n o kindly m a d e avail able scientific e q u i p m e n t for t h e first c r u i s e . W e a c k n o w l e d g e t h e d a t a collection a n d anal ysis h e l p o f M a r c e l a C h a r o , C l a u d i a Giulivi, A n a P a u l a Osiroff a n d C a r l o s Ballestrini, all from the S H N . W e a c k n o w l e d g e t h e g o v e r n m e n t s of A r g e n t i n a a n d U r u g u a y for g r a n t i n g t o t h e p r o g r a m p e r m i s s i o n to w o r k in t h e i r t e r r i t o r i a l waters. W e also wish t o t h a n k t h e Servicio d e H i d r o g r a f i a Naval a n d in p a r t i c u l a r , C a p d e Navio G u i l l e r m o V i d e l a a n d C a p s E d u a r d o R o d r i g u e z a n d J a v i e r V a l l a d a r e s for t h e logis tical s u p p o r t p r o v i d e d to t h e p r o g r a m . T h e crews f r o m t h e r e s e a r c h vessels Puerto Deseado, Oca Balda a n d Le Suroit a r e kindly ac k n o w l e d g e d for t h e i r c o o p e r a t i o n .

References

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Longitude (°W) Fig. 5. S u r f a c e c o n d i t i o n s in t h e C o n f l u e n c e r e g i o n in a u s t r a l s u m m e r 1 9 9 0 : (top) surface t e m p e r a t u r e , ( m i d d l e ) s u r f a c e salinity, ( b o t t o m ) s u r f a c e o x y g e n s a t u r a t i o n .


C h e n e y , R. E., J . G. M a r s h , a n d B . D . Beckley, Global mesoscale variability f r o m collinear tracks of Seasat a l t i m e t r y d a t a , J. Geophys. Res., 88, 4 3 4 3 , 1 9 8 3 . F i g u e r o a , H . A., a n d D. B . O l s o n , L a g r a n g i a n statistics in t h e S o u t h Atlantic as d e r i v e d f r o m S O S a n d F G G E d r i f t e r s , J. Mar. Res., 47, 5 2 5 , 1984.

Eos, V o l . 7 1 , N o . 4 1 , October 9, 1 9 9 0 Garzoli, S. L., a n d A. B i a n c h i , T i m e - s p a c e availability of t h e local d y n a m i c s o f t h e Brazil-Malvinas C o n f l u e n c e as r e v e a l e d by i n v e r t e d e c h o s o u n d e r s , / . Geophys. Res., 92, 1.914, 1987. Garzoli, S. L., a n d Z. G a r r a f f o , T r a n s p o r t s , frontal m o t i o n s a n d e d d i e s at t h e BrazilMalvinas C u r r e n t s C o n f l u e n c e , Deep Sea Res., 36, 6 8 1 , 1989. G e o r g i , D. T . , O n t h e r e l a t i o n s h i p b e t w e e n t h e large-scale p r o p e r t y v a r i a t i o n s a n d t h e fine s t r u c t u r e in t h e C i r c u m p o l a r D e e p W a t e r , J. Geophys. Res., 86, 6 5 5 6 , 1 9 8 1 . G o r d o n , A. L., S o u t h A t l a n t i c t h e r m o c l i n e ventilation, Deep Sea Res., 28, 1239, 1 9 8 1 . G o r d o n , A. L., a n d C. L. G r e e n g r o v e , G e o s t r o p h i c circulation o f t h e B r a z i l / F a l k l a n d C o n f l u e n c e , Deep Sea Res., 3 3 , 5 7 3 , 1986. J o y c e , T . M., L a r g e - s c a l e v a r i a t i o n s in smallscale t e m p e r a t u r e / s a l i n i t y fine s t r u c t u r e in t h e m a i n t h e r m o c l i n e of t h e n o r t h w e s t At lantic, Deep Sea Res., 23, 1 1 7 5 , 1976.

J o y c e , T . M., A n o t e o n lateral m i x i n g o f wa t e r m a s s e s , / . Phys. Oceanogr., 7, 6 2 6 , 1977. Legeckis, R., a n d A. L. G o r d o n , Satellite o b s e r v a t i o n s of t h e Brazil a n d F a l k l a n d C u r r e n t s 1975 to 1976 a n d 1978, Deep Sea Res., 29, 3 7 5 , 1982. M a c k a s , D . L., K. L. D e n m a n , a n d A. F. B e n n e t t , Least s q u a r e s m u l t i p l e t r a c e r analysis of w a t e r m a s s c o m p o s i t i o n , / . Geophys. Res., 92, 2 9 0 7 , 1987. M c C a r t n e y , M . S., T h e s u b t r o p i c a l r e c i r c u l a tion of m o d e w a t e r s , / . Mar. Res., 40, (suppi), 4 2 7 , 1982. O l s o n , D., G. P o d e s t a , R. E v a n s , a n d O . B r o w n , T e m p o r a l v a r i a t i o n s in t h e s e p a r a tion of Brazil a n d Malvinas C u r r e n t s , Deep Sea Res., 35, 1 9 7 1 , 1 9 8 8 . P a t t e r s o n , S. L., S u r f a c e circulation a n d ki netic e n e r g y d i s t r i b u t i o n in t h e s o u t h e r n hemisphere oceans from FGGE drifting b u o y s , / . Phys. Oceanogr., 15, 8 6 5 , 1 9 8 5 . P e t e r s o n , R. G., a n d T . W h i t w o r t h , T h e s u b -

a n t a r c t i c a n d p o l a r f r o n t s in r e l a t i o n to d e e p water masses t h r o u g h the Southwest e r n A t l a n t i c , / . Geophys. Res., 94, 10,817, 1989. Piola, A. R., H . A . F i g u e r o a , a n d A. A. Bian chi, S o m e a s p e c t s of t h e s u r f a c e circulation s o u t h of 20°S r e v e a l e d by First G A R P Glob al E x p e r i m e n t d r i f t e r s , / . Geophys. Res., 92, 5 1 0 1 , 1987. Piola, A. R., a n d D . T . G e o r g i , C i r c u m p o l a r p r o p e r t i e s of A n t a r c t i c I n t e r m e d i a t e W a t e r a n d S u b a n t a r c t i c M o d e W a t e r , Deep Sea Res., 29, 7 8 6 , 1982. Reid, J . L., J r . , W . D. N o w l i n , j r . , a n d W . C. Patzert, O n t h e characteristics a n d circula tion of t h e s o u t h w e s t e r n Atlantic O c e a n , / . Phys. Oceanogr., 7, 6 2 , 1977. T a k a h a s h i , T . , a n d D . W. C h i p m a n , C i r c u m p o l a r o c e a n i c C 0 sink z o n e (abstract), IAM A P / I A P S O J o i n t Assembly P r o g r a m . , A u g . 5 - 1 6 , H o n o l u l u , H a w a i i , p . 34, 1985. 2

Meetinns AGU's Pacific Northwest Regional Meeting PAGES 1143-1145 T h e 1990 Pacific N o r t h w e s t r e g i o n a l A G U m e e t i n g h e l d S e p t e m b e r 1 3 - 1 4 at t h e U n i versity of W a s h i n g t o n , Seattle, was a t t e n d e d by m o r e t h a n 8 0 p e o p l e ; 24 p a p e r s a n d 17 posters were presented. About 25 people par ticipated in a field t r i p t o look at d e b r i s flow processes, d e p o s i t s a n d h a z a r d s at M o u n t R a n i e r , o r g a n i z e d by Patrick P r i n g l e of t h e W a s h i n g t o n D e p a r t m e n t of N a t u r a l R e s o u r c e s , Division of G e o l o g y . P u b l i s h e d copies o f t h e field t r i p g u i d e will b e available in 1991 f r o m t h e W a s h i n g t o n S t a t e D N R . Also f e a t u r e d at t h e m e e t i n g was a n i n t e r disciplinary s t u d y by t h e G e o l o g y , F o r e s t Re sources a n d Engineering d e p a r t m e n t s of the University of W a s h i n g t o n , " C h a r a c t e r i z a t i o n a n d M o d e l i n g o f a H o l l o w in t h e C a s c a d e s W i t h R e s p e c t to S l o p e Stability." S l o p e stabil ity is a n i m p o r t a n t topic in t h e Pacific N o r t h west b e c a u s e of its r u g g e d t o p o g r a p h y a n d t h e e c o n o m i c i m p o r t a n c e of b o t h f o r e s t r y and fisheries. Results of last y e a r ' s election o f r e g i o n a l section officers w e r e a n n o u n c e d at t h e m e e t ing. T h e Pacific N o r t h w e s t R e g i o n a l C o m m i t tee for J u l y 1, 1 9 9 0 - J u n e 3 0 , 1 9 9 1 , consists of: O c e a n o g r a p h y , Roy W. K o c h , P o r t l a n d State University; G e o m a g n e t i s m / P a l e o m a g n e tism, S h a u l Levi, O r e g o n State U n i v e r s i t y ; O c e a n Science, C u r t D . P e t e r s o n , P o r t l a n d State University; Seismology, E u g e n e H u m p h r e y s , U n i v e r s i t y of O r e g o n ; T e c t o n o p h y sics, A n n e T r e h u , O r e g o n State U n i v e r s i t y ; a n d V o l c a n o l o g y , R i c h a r d W a i t t e , D a v i d A. Johnston Cascade Volcano Observatory. A b s t r a c t s of t h e m e e t i n g follow. This report was prepared by Ruth S. Ludwin, Secretary-Treasure of the Pacific Northwest Meet ing.

FZ Terminations: Charleston, Lisbon Seismic Catastrophes R.C.Bostrom, U. of Washington AJ-20,

Seattle, WA 88195

Filtered gravimetric images indicate that Charleston (a) & Lisbon (b) are located at the continental termination of fracture zones. Using magnetic data Salisbury (1974) suggested that creep continues for geologic periods on intra-plate FZ's, accumulating displacements of tens of kilometers. Lubricatory serpentinous peridotites render the process aseismic. However, it would seem that this must result in efficient accumulation of major elastic strain at a landward termination in old, cold lithosphere. Prolonged silent accumulation of strain, punctuated by infrequent but catastrophic seismic events may be expected to characterize Charleston, Lisbon, and analogous settlements, located at the termination of FZ's acting as strain conduits.

d e f o r m a t i o n . The f a u l t - b o u n d e d QC T e r r a c e may c o n t a i n a downdropped b l o c k of c o n t i n e n t a l (not o c e a n i c ) c r u s t . The c o m b i n a t i o n of t h i c k c r u s t , l a c k of a s t r i p e d magnetic p a t t e r n , and l a c k of c l e a r e v i d e n c e for c o m p r e s s i o n a l s t r u c t u r e s s u g g e s t s t h a t a c c r e t i o n a r y p r o c e s s e s may not have p l a y e d a major r o l e i n t h e e v o l u t i o n of t h e t e r r a c e ; h i g h s e i s m i c v e l o c i t i e s a t depth may i n d i c a t e p r e s e n c e of v o l c a n i c s and Mesozoic or o l d e r s e d i m e n t s . S i m i l a r c o n c l u s i o n s can be drawn about e a s t e r n Winona B a s i n . On Vancouver and QC i s l a n d s , g r a v i t y h i g h s c o r r e s p o n d t o u p l i f t e d b l o c k s , and magnetic h i g h s t o p l u t o n s . In t h e QC B a s i n , g r a v i t y l o w s and s e i s m i c r e f l e c t i o n p a t t e r n s show T e r t i a r y d e p o c e n t e r s , and l i n e a r a n o m a l i e s i n p o t e n t i a l - f i e l d maps help d e l i n e a t e f a u l t networks; magnetic h i g h s l i k e l y r e f l e c t p l u t o n i s m . G e o l o g i c mapping on land s u g g e s t s t h a t some T e r t i a r y f a u l t s may be r e a c t i v a t e d Mesozoic or o l d e r f e a t u r e s , and block f a u l t i n g i s predominant. Many d e p o c e n t e r s i n t h e QC B a s i n o c c u r i n downdropped p o l y g o n a l b l o c k s whose b o u n d a r i e s were p o s s i b l y i n h e r i t e d from o l d e r f a u l t t r e n d s . V a r i a t i o n s i n c r u s t a l t h i c k n e s s seem t o be r e l a t i v e l y a b r u p t . L a r g e s c a l e b l o c k f a u l t i n g and i n v e r s i o n t e c t o n i c s a r e s u f f i c i e n t t o e x p l a i n t h e g e o p h y s i c a l and g e o l o g i c a l d a t a i n t h e QC B a s i n .

b Galactic Cosmic Rays (GCR) and Variations Upper Troposphere Ionization (VUTI)

in

J T A Ely, J J Lord, S W Smith, and F D Lind (all a t : G e o p h y s i c s P r o g r a m , S p a c e Sciences Div, Univ of Washington, Seattle, WA 98195)

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\ / A l t e r n a t i v e I n t e r p r e t a t i o n of G e o p h y s i c a l i n t h e Queen C h a r l o t t e (QC) B a s i n Area,

Data B.C.

H V LYATSKY. R L CHASE (UBC, D e p t . G e o l . S c i . , 6339 S t o r e s R d . , Vancouver, BC V6T 2B4) R I THOMPSON, 6 J WOODSWORTH ( G e o l . S u r v e y o f Canada, 100 W P e n d e r , Vancouver, BC V6B 1R8) An i n t e g r a t e d i n t e r p r e t a t i o n of Bouguer, f r e e a i r and i s o s t a t i c g r a v i t y maps, m a g n e t i c n a p s and s e i s m i c r e f r a c t i o n p r o f i l e s h a s been made t o l o c a t e downdropped b l o c k s and c h a n g e s i n c r u s t a l t h i c k n e s s and depth t o Mono; s e i s m i c r e f l e c t i o n p r o f i l e s have imaged s u p r a c r u s t a l


The d i s c o v e r y of local m o d u l a t i o n s of GCR (that c o r r e l a t e with v a r i o u s a s p e c t s of solar a c t i v i t y ) and t h e i r p o s s i b l e roles (in sunweather effects (such as d r o u g h t s , l i g h t n i n g incidence, v o r t i c i t y area index e v e n t s (VAI), etc) and in a crucial aspect of global warming) have been p r e v i o u s l y d e s c r i b e d (Ely JGR 1977; NASA CR 3812, 1984; Proc IEEE Conf Oceans '89; and Huang GRL 1 9 8 7 ; and Lord Bull APS 1990) . These e f f e c t s h a v e been e x p l a i n e d as mediated by GCR m o d s via their production of systematic VUTI and a n t a g o n i s m of cirrus (by a transport d e h y d r a t i o n m e c h a n i s m ) a c c o r d i n g to the M a g netic Coupling Model (MCM) theory (Ely ' 8 4 ) . We discuss here a finding from one of our ongoing studies of the local GCR m o d u l a t i o n s ; in this case, a test of the MCM prediction that the GCR mods called n o r t h - s o u t h asymmetries (NSA) p r o duce V A I : (1) Our satellite data from 1967 (0V186) showed the (local) NSA to be strong in the rising part of Sunspot Cycle 20, a result sup ported by neutron monitor analyses; however (2) in 1978, circa the same part of Sunspot Cycle 21, our GCR t e l e s c o p e on the S3-4 satellite showed no e v i d e n c e of strong NSA, also sup ported by the neutron record. Thus, if the MCM is correct that the NSA is a cause of VAI (via the cirrus antagonism m e c h a n i s m ) , one might ex pect a significant decline in the incidence or amplitude of VAI e v e n t s in Cycle 2 1 . This has, in fact, been reported (Wilcox & Scherrer, EOS 60 (18) : 272, 1979) .