Earth and Planetary Scwnce Letters, 113 (1992) 287-292 287 Elsevier Science Publishers B.V., Amsterdam
[MK]
First resolution of flow through the Hunter Channel in the South Atlantic
Kevin Speer a, W a l t e r Z e n k
aGerold Siedler
aJiJrgen P~itzold b a n d C l e m e n s H e i d l a n d c
a Instltut fiir Meereskunde, Ditsternbrooker Weg 20, D-2300 Kiel 1, Germany
b Unwersttat Bremen, Fachberelch 5, Geowtssenschaften 1, Postfach 330440, D-2800 Bremen 33, Germany CAlfred Wegener Instttut, Postfaeh 12 O1 61, D-2850 Bremerhaven 12, Germany
Received June 18, 1992; revision accepted July 6, 1992
ABSTRACT
Dense Antarctic Bottom Water formed around the continent of Antarctica spreads northward In the Atlantic underneath North Atlantic Deep Water, gradually mixing and upwelling into it. This Antarctic Water forms a significant element of the meridional circulation in both directions: northward as bottom water and southward as deep water. It is important to determine the strength of each component to assess ~ts role in ocean circulation. Such measurements are useful when made in constricted pathways because any flow is more clearly defined. A new set of fine-resolution hydrograhic measurements m the Hunter Channel of the South Atlantic Ocean has been obtained, which allow the geostrophic bottom flow there to be estimated for the first time The northward flow through the Hunter Channel of water cooler than 2°C is thus estimated to be 0.7 × 10 6 m 3 s -1.
T h e s o u r c e o f t h e d e n s e s t w a t e r for m o s t o f t h e A t l a n t i c O c e a n is t h e W e d d e l l S e a , s o u t h o f S o u t h A m e r i c a . T h i s is t h e l a r g e s t e m b a y m e n t o f t h e A n t a r c t i c c o n t i n e n t , a n d t h e o c e a n is c o o l e d t h e r e to s u c h a d e g r e e t h a t w a t e r d e n s e e n o u g h to s i n k t o t h e b o t t o m is p r o d u c e d [1]. T h i s A n t a r c t i c B o t t o m W a t e r flows n o r t h a n d e n t e r s t h e A t l a n t i c at its s o u t h w e s t e r n c o r n e r , w h e r e it fills t h e A r g e n t i n e B a s i n u p t o t h e level o f t h e s u r r o u n d i n g sills. I n i t i a l l y c l o s e to t h e f r e e z i n g p o i n t , t h e b o t t o m w a t e r m i x e s a l o n g its r o u t e w i t h o v e r l y i n g w a t e r a n d is d i l u t e d , b e c o m i n g w a r m e r a n d g e n e r a l l y l o o s i n g t h e t r a c e r c h a r a c t e r i s t i c s it a c q u i r e d in t h e W e d d e l l Sea. T h i s m i x t u r e f o r m s a l a y e r r o u g h l y 1 k m t h i c k c o v e r i n g v i r t u a l l y t h e e n t i r e A t l a n t i c O c e a n , b u t in o r d e r t o p e n e t r a t e n o r t h a w a y f r o m its s o u r c e it m u s t c r o s s a n u m b e r o f r i d g e s t h a t b l o c k its p a t h .
T h e m o s t i m p o r t a n t b a r r i e r to t h e n o r t h w a r d p r o g r e s s o f b o t t o m w a t e r in t h e A t l a n t i c is t h e s y s t e m c o n s i s t i n g o f t h e R i o G r a n d e R i s e a n d
Correspondence to: K. Speer, Laboratolre de Physique des Oc6ans, IFREMER, B.P. 70, 29280 Plouzan6, France.
W a l v i s R i d g e n e a r 30°S; t h e s e two r i d g e s b l o c k flow o n t h e w e s t e r n a n d e a s t e r n s i d e o f t h e M i d - A t l a n t i c R i d g e r e s p e c t i v e l y . W h i l e t h e W a l v i s R i d g e p r e v e n t s a s i g n i f i c a n t flow o f A n t a r c t i c B o t t o m W a t e r f r o m c o n t i n u i n g n o r t h in t h e e a s t - e r n S o u t h A t l a n t i c O c e a n [2], a s t r o n g flow o f b o t t o m w a t e r d o e s c r o s s t h e R i o G r a n d e R i s e . T h e p r i m a r y p a t h w a y a c r o s s this b a r r i e r h a s l o n g b e e n k n o w n to b e t h e V e m a C h a n n e l [3] (Fig. 1).
T h i s s i t u a t i o n h a s a t t r a c t e d s e v e r a l r e s e a r c h p r o g r a m m e s d e d i c a t e d t o t h e V e m a C h a n n e l [ 4 - 6], a n d d i r e c t c u r r e n t m e a s u r e m e n t s [6] i n d i c a t e a m e a n t r a n s p o r t o v e r o n e y e a r o f 4 + 0.4 × 10 6 m 3 s - 1 o f b o t t o m w a t e r t h r o u g h this c h a n n e l . T h e t r a n s p o r t d e p e n d s , o f c o u r s e , o n h o w o n e d e f i n e s b o t t o m w a t e r , b u t it is o b s e r v e d t h a t t h e v e l o c i t y o f t h e flow d e c r e a s e s to z e r o ( o n a v e r a g e ) at a level a b o v e t h e b o t t o m w h e r e t h e p o t e n t i a l t e m - p e r a t u r e 1 is a b o u t 2°C, a n d it is c o m m o n to
i The temperature that a parcel of seawater has if raised to the surface adiabatically. The m-sire temperature is higher, at about 2.3°C.
0012-821X/92/$05.00 © 1992 - Elsevier Science Pubhshers B.V. All rights reserved
t-J ~o O~ u i u BRAZIL BASIN
i u u { LOWER SANTOS ~3 PLATEAU t~ °° j~ HUNTER CHANNEL ARGENTINE BASIN HUNTER GAP 50 ° 45 ° 40 ° 35 ° 30 ° 25 ° W 20 = Fig. 1. Stmphfied bathymetry of the Rio Grande Rise, which divides the Brazd Basra from the Argentine Basin near 30°S m the South Atlantic [13]. Station positions (dots) from a part of Meteor 15, leg 2 devoted to the Hunter Channel.
rn rn >
FIRST RESOLUTION OF FLOW THROUGH THE HUNTER CHANNEL. SOUTH ATLANTIC 289
m a k e transport estimates taking this isotherm as the top of the b o t t o m layer. T h e typical speed e n c o u n t e r e d at the b o t t o m of the V e m a Channel is 10 cm s-1, with occasional values higher than 30 cm s-1; speeds decrease to zero roughly 1 km above the bottom.
On the o t h e r hand, higher b o t t o m t e m p e r a - tures on the northern, Brazil Basin side of the H u n t e r Channel have led Wrist [3] and subse- quent investigators to the conclusion that the passage is shallow and flow is inhibited. Past evidence for flow of b o t t o m w a t e r through the H u n t e r Channel has come essentially f r o m geo- logical studies. T h e discovery of diatoms which survive in Antarctic Bottom W a t e r in the H u n t e r Channel gave a geological impetus to studies there, suggesting that this w a t e r did cross the rise in the past and may be doing so presently [7].
T h e first hydrographic m e a s u r e m e n t s with suf- ficient horizontal resolution to constitute a true section across the H u n t e r Channel were m a d e in 1959, for a total of seven stations along 32°S, between the Rio G r a n d e Ridge and the Mid- Atlantic Ridge [8]. No features suggesting deep.
flow stood out, but the t o p o g r a p h y is rough in the region and the horizontal and vertical resolution (discrete water samples with 200-400 m vertical spacing) were still rather coarse for measuring weak flow n e a r the bottom. T h e horizontal reso- lution n e a r the western b o u n d a r y of the H u n t e r Channel, the Rio G r a n d e Ridge, was later im- proved s o m e w h a t [9], but still no identifying structure b e c a m e apparent. A recent estimate of d e e p transport through the H u n t e r Channel is 1.8 × 106 m 3 s - l , based on these older data but assuming a weak northward b o t t o m flow, partly for the p u r p o s e of achieving an overall mass balance in the region [10].
As p a r t of the World O c e a n Circulation Ex- p e r i m e n t a p r o g r a m m e was designed to m e a s u r e the total source of d e e p water to the Brazil Basin, including inflow across the Rio G r a n d e Rise and outflow at the e q u a t o r [11]. In the f r a m e w o r k of this p r o g r a m m e a hydrographic section was occu- pied at the H u n t e r Channel by the F.S. Meteor (cruise 15, leg 2) (Fig. 1). A total of ten C T D stations with nearly continuous vertical coverage were placed across a narrow p a r t of the channel between two ridges. T h e 200 km width of the channel is roughly ten times that of the V e m a
Stn.no
84 86
O ~ 86 87 9493 ~ 91 90 89
5O0
1000
1500
5 ~
3 ~
21000 ~,~
"E 2.8
. . . ~ ~ S ~ - . . . ~ . . . ! .
3000 ~,"
1
350O
" '[ ~~ ~Jr- ",I ~ ,w-'l" 0 ~ I ' " I
- 1 . 5
~. 1.0
400O
45OO
IO0 20O
Dtstance (kin)
Fig. 2. P r o f i l e o f p o t e n t i a l t e m p e r a t u r e (°C) across the H u n t e r
Channel obtained during Meteor cruise 15 (February, 1991).
The Hunter Gap is the broad valley in the 150-220 km distance range.
Channel to the w e s t - - a n important point be- cause spreading a mass source over a b r o a d area weakens the flow and tends to mask any dynamic signal such as sloping isotherms. T h e H u n t e r Channel section is just north of a d e e p fracture (depths g r e a t e r than 5000 m) which a p p e a r s to be an isolated piece of a larger scale fracture zone emanating f r o m the Mid-Atlantic Ridge.
T h e coldest b o t t o m water was found in what is evidently the northward extension of the H u n t e r G a p [12] (Fig. 2, between distances of 150 and 220 km). Values below 0.2°C occurred on the eastern side of the gap, which was reminiscent of a similar, but m o r e p r o n o u n c e d p a t t e r n of east- ern intensification of tracer characteristics in the V e m a Channel [5,6]. W a t e r of this t e m p e r a t u r e
290 K SPEER ETAI can only have c o m e from south of the section, as
all o t h e r observations show higher b o t t o m tem- p e r a t u r e to the north. At s o m e w h a t shallower depths and for t e m p e r a t u r e s of b e t w e e n about 0.5°C and 2°C, the coolest w a t e r is at the western b o u n d a r y of the section and isotherms slope downward to the east, indicating the northward flow of b o t t o m w a t e r relative to the w a r m e r sur- face. Taking the flow to be zero at the 2°C surface, the b o t t o m (geostrophic) velocities were typically 1 - 2 cm s -1, and the total t r a n s p o r t below 2°C was 0.7 × 10 6 m 3 s-1. This figure in- cludes transport contributions below the d e e p e s t c o m m o n level of a given station pair, obtained by multiplying the d e e p e s t velocity by the area be- tween the stations. D e s p i t e the relatively high horizontal resolution of the new section, this con- tribution a m o u n t e d to about 20%, suggesting that some b o t t o m intensified flow m a y have b e e n missed. With even higher resolution p e r h a p s the transport estimate would be raised closer to 1 × 106 m 3 s-1.
Although the d o m i n a n t source of e r r o r is probably still related to horizontal resolution, and t h e r e f o r e difficult to estimate, the e r r o r related to the choice of reference level m a y be shown by
choosing nearby levels and performing the calcu- lation again. T h r e e constant density surfaces were chosen, both to verify that the same result obtains for this choice, and because at higher t e m p e r a - tures there is an extreme, making t e m p e r a t u r e unsuitable as a reference surface. T h e shallow surface was on the potential density anomaly surface tr 4 = 45.80 kg m -3, n e a r 2.4°C, which gave 0.8 × 106m3s -1. T h e choice tr 4 = 45.85 kg m -3 is close to 2.0°C, and p r o d u c e d the figure of 0.7 x 106 m 3 s-1. Finally, a d e e p e r surface at tr 4 = 45.90 kg m - 3, n e a r 1.5°C, is clearly within the region of flow and thus p r o d u c e d the low result 0.3 × 106 m 3 s-1. Based on this m o d e r a t e reference level d e p e n d e n c e and on a standard deviation of 0.1 × 106 m 3 s -1 for the set of transports at each station pair, the above transport estimate at 2°C is thought to be accurate to b e t t e r than 50%.
It is unlikely that significant b o t t o m w a t e r flow was missed east of the section, because a b r o a d ridge (or eastern continuation of the rise) shal- lower than 3600 m joins the Mid-Atlantic Ridge.
Thus the connection b e t w e e n the Brazil Basin and Argentine Basin is closed off at this level, which is close to the 2°C zero velocity reference surface.
Stn.no.
84
Stn.no.
4*8
30*W 290W 2O*W 270W
Fig. 3. Perspective plot of along-track bathymetry showing a part of the Rio Grande Ridge near 30°W, another ridge near 26°30"W which continues to the east, and the Hunter Gap at 27'W. The ship's track included an excursion to the south, forming a triangular
route.
FIRST R E S O L U T I O N O F F L O W T H R O U G H T H E H U N T E R CHANNEL, S O U T H ATLANTIC 291
T h e western b o u n d a r y of the section is the Rio G r a n d e Ridge, which blocks flow d e e p e r t h a n about 3400 m all the ~vay to the Rio G r a n d e Rise.
H y d r o g r a p h i c m e a s u r e m e n t s were also m a d e in the V e m a Channel to the west of the rise as part of the s a m e p r o g r a m m e , but on a s e p a r a t e expe- dition. T h e s e m e a s u r e m e n t s verified that the ear- lier transport estimates through the V e m a Chan- nel still held at the time of the H u n t e r Channel observations.
T h e salinity field varies mainly in the vertical direction, and is correlated with t e m p e r a t u r e such that below about 3000 m isopycnals are parallel to isotherms. A t the westernmost part of the section there is a t e m p e r a t u r e m i n i m u m n e a r 1500 m and a t e m p e r a t u r e m a x i m u m n e a r 2200 m. This is a weaker, m u c h r e d u c e d signal of the strong inversion at the western boundary, which is a well-known characteristic of the South At- lantic t h e r m o h a l i n e structure [1]. It is p r o d u c e d by the p r e s e n c e of a cool, Antarctic I n t e r m e d i a t e W a t e r layer, fresh enough to be of lower density than the relatively salty N o r t h Atlantic D e e p W a t e r below it, despite the lower t e m p e r a t u r e .
T h e continuity of the H u n t e r G a p , at least as far north as o u r section, is clear f r o m a profile of b a t h y m e t r y along the cruise track at 27°W (Fig.
3). T h e n a r r o w e r valleys just to the west are deeper, but do not s e e m to be an open, important pathway since t e m p e r a t u r e s are higher there (Fig.
2). M o r e observations are n e e d e d to d e t e r m i n e where these valleys begin and end, and their role (if any) in guiding the flow of b o t t o m water.
T h e d e p t h of the H u n t e r G a p is close to 4300 m, which is only about 300 m shallower than the V e m a Channel. Nevertheless this difference, to- gether with the geostrophic d e p t h increase of isotherms f r o m west to east across the V e m a Channel, eliminates water colder than 0°C f r o m the H u n t e r G a p . T h u s the average t e m p e r a t u r e of the b o t t o m current is higher there, about I°C, versus a value closer to 0.5°C in the V e m a Chan- nel. T h e w a r m e r H u n t e r Channel flow supplies w a t e r to the accordingly w a r m e r southeastern corner of the Brazil Basin.
O u r new, closely spaced C T D section across the H u n t e r Channel resolves the hydrographic structure of Antarctic B o t t o m W a t e r there, and confirms the p r e s e n c e of northward flow. T h e geostrophic t r a n s p o r t of this current, obtained
from the new hydrographic data, is 0.7 × 10 6 m 3 s -1, or roughly one-fifth of that entering the Brazil Basin through the V e m a Channel. Al- though the contribution of this flow to the flux of heat across this latitude of the South Atlantic is small, it nevertheless plays a significant role in the mass balance of the b o t t o m layer of the South Atlantic Ocean. T h e importance of this is that observations such as these usually provide the starting point for constructing dynamic models of the larger scale circulation in an ocean basin. T h e key physical processes in these models d e p e n d crucially on the vertical m o v e m e n t of water driven, in part, by the sources of mass. T h e H u n t e r Channel flow is an important c o m p o n e n t of the mass balance of the b o t t o m water in the Brazil Basin (and, obviously, the Argentine Basin), and so it will be a necessary part of such dynamic studies.
Acknowledgements
We wish to acknowledge the help of Captain H. Bruns and the crew of the F.S. Meteor for a successful cruise and a winch that was in good working order, This work was supported by the Bundesmimster fiir Forschung und Technologie (contract 03F0535 A) and the Deutsche For- schungsgemeinschaft (contract 111/37-1).
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