On the deep water circulation of the eastern South Atlantic Ocean

ARTICLE IN PRESS

Deep-Sea Research I 50 (2003) 889–916

On the deep water circulation of the eastern South Atlantic Ocean Michel Arhana,*, Herle! Merciera, Young-Hyang Parkb a

Laboratoire de Physique des Oc!eans, CNRS/IFREMER/UBO, IFREMER Centre de Brest, B.P. 70, 29280 Plouzan!e, France b D!epartement des Milieux et Peuplements Marins, Mus!eum National d’Histoire Naturelle, Paris, France Received 27 June 2002; received in revised form 13 March 2003; accepted 24 March 2003

Abstract The main flow patterns and transports of North Atlantic Deep Water (NADW) in the eastern South Atlantic and round southern Africa were inferred from two quasi-meridional WOCE hydrographic lines at 9
W and 5
E nominal longitudes, and from segments of other sections in the Atlantic-to-Indian Oceans transition region. An export of 1174 Sv of NADW from the Atlantic was estimated near southern Africa inshore of the Subtropical Front. Most of this flow can be traced back across the Cape Basin to passages south of 28
S in Walvis Ridge, and farther upstream to an entry in the eastern basin at 20–25
S. Other transfers of NADW from the western boundary current to the eastern boundary were confirmed near the equator and at 22
S near the Namib Col of Walvis Ridge, yet a cyclonic recirculation of the former in the Angola Basin, and the weakness of the latter (472 Sv), make their direct contributions to the export of NADW minor ones. As another consequence, the estimated transport of the deep southward boundary current against the western continental slope of Africa shows important latitudinal variations (1279 Sv at 7
300 S, 375 Sv at 22
300 S, 1174 Sv at 35
S), with even a suggestion of its reversal in the northern Cape Basin. To the southwest of Cape Town, the deep boundary current is influenced by the temporal variability of the Agulhas retroflection region, and most of it separates from the slope to enter the Indian Ocean in the deep return flow of the Agulhas Current. Only a 2–3 Sv remnant against the continental slope was estimated at 30
E to the east of the Agulhas Plateau. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: South Atlantic; Circulation; Current; Hydrography

1. Introduction Although the dominant role of the deep western boundary current of the Atlantic Ocean to convey the North Atlantic Deep Water (NADW) southward has long been known, Reid (1989) noted that a substantial part of this water mass does not *Corresponding author. Fax: +33-2-98-22-44-96. E-mail address: [email protected] (M. Arhan).

reach the Antarctic Circumpolar Current along the western side of the South Atlantic. Instead, he concluded from a geostrophic and tracer analysis that a fraction of the deep water turns back northward in the western basin of the South Atlantic, then eastward south of the equator and finally southward along the eastern boundary. The shift to the eastern ocean margin was confirmed by other studies, even though interior flow patterns at variance with the one suggested by Reid (1989)

0967-0637/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0967-0637(03)00072-4

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M. Arhan et al. / Deep-Sea Research I 50 (2003) 889–916

were proposed. Stramma and England (1999), in a schematics which summarizes the results from several studies, show the eastward transfer of NADW occurring along the equator (Weiss et al., . 1985; Boning and Schott, 1993; Andrie! , 1996) and in the so-called Namib Col Current near 22
S (Warren and Speer, 1991; Speer et al., 1995). Both zonal routes feed an eastern boundary current which contributes to the NADW export from the Atlantic, yet with an apparently lower contribution than the deep western boundary current. Transport computations across transatlantic sections exhibit the eastern boundary flow with transports around 6 Sv (1 Sv=106 m3 s1) at 11
S (Speer et al., 1996) and 12 Sv near 30
S (Saunders and King, 1995). The deep boundary flow to the south of Walvis Ridge is also present in model results presented by Stramma and England (1999) and de Miranda and Barnier (2000), with a transport of 12 Sv in the latter. Despite these results and the thorough analysis of Warren and Speer (1991) on the Angola Basin, the behavior and transports of deep waters in the eastern South Atlantic remain uncertain. Most previous studies rested on zonal hydrographic lines, with the one exception of the AJAX section realized in 1983 along the Greenwich meridian. This undersampling of the meridional direction certainly hindered the examination of the zonal flows now recognized as major circulation features (Wienders et al., 2000; Vanicek and Siedler, 2002). In this paper, we present an analysis of the hydrography and transports in the deep layers of the eastern equatorial and South Atlantic Ocean on the basis of two quasi-meridional sections (Fig. 1) that were part of the French contribution to the Hydrographic Program of the World Ocean Circulation Experiment (WOCE). The two lines were designed to be on either side of the former AJAX sampling, one at the nominal longitude 9
W above the eastern flank of the Mid-Atlantic Ridge (A14 under the WOCE designation), and the other one approximately parallel to the African continental slope at about 600 km from it (A13). The latter, which is complemented by a shorter transverse section between its southern end and the continental slope (hereafter the Cape Town line or CPT), was aimed at determining the

exchanges between the ocean interior and the system of eastern boundary currents. For specific issues we also use a few complementary data including parts of the WOCE sections A11, A12, A21 and I6 (Fig. 1), which provide information on the way the diluted NADW exported from the Atlantic skirts the southern tip of Africa. After a brief presentation of the data and bathymetric configuration in Section 2, we discuss the major hydrographic features of A13 and A14 in Section 3. The transports across these lines as determined from box inversions are presented in Section 4. In Section 5, finally, we illustrate the deep variability across the Cape Town line by a comparison of CPT with the A12 and A21 data along the same track, and we seek information on the extension of the deep eastern boundary flow in the I6 sampling of the 30
E meridian. This study is a deep water counterpart of another one by Mercier et al. (2003) which addresses the circulation of the upper and intermediate waters of the eastern South Atlantic from the same A13 and A14 data, also complemented by A11.

2. Data and bathymetric configuration The A13 and A14 measurements were carried out from the N/O L’ATALANTE between January 17th and March 28th, 1995, during cruise CITHER-3 of the WOCE-France project CITHER (circulation thermohaline). The data were acquired down to 15 m above the bottom, with water samples collected at 32 levels for tracer analysis and CTD-O2 calibration. A total of 243 stations were occupied at latitudes from 4
200 N at the northern border of the Guinea Basin to 40
S for A13 and 45
300 S for A14. The station spacing of 30 nm was decreased near the equator, across the Walvis Ridge, and above continental slopes. A thorough description of the data may be found in Le Groupe CITHER-3 (1996, 1998), and the vertical distributions of potential temperature (y), salinity (S) and dissolved oxygen (O2) along A14, A13 and the Cape Town line are shown in Figs. 2–4, respectively. Basic information on the additional data used in the paper is given in Table 1.

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891

Guinea Basin

G. R

.

Angola Basin

A14 A13 AJAX

W .R

.

R.J.F.Z

1 A1

W.P. A12

N.C.

Cape Basin

PT

C

T.B. A.P.

A

21

I6

Fig. 1. Tracks of the hydrographic lines used or referred to in the paper (see Table 1). Lines A14 and A11 form box I of the hydrographic inversions presented in Section 4. Lines A13 and CPT form box II. Additional lines A12, A21 and I6 are used to study the skirting of southern Africa by the deep waters (Section 5). Isobaths 200 m and multiples of 1000 m are shown, with the areas shallower than 4000 m shaded. The bathymetric features referred to in the text are indicated by their initials: G.R., Guinea Rise; R.J.F.Z., Rio de Janeiro Fracture Zone; W.R., Walvis Ridge; N.C., Namib Col; W.P. Walvis Passage; A.P., Agulhas Plateau; T.B., Transkei Basin.

The eastern South Atlantic is constituted of three subbasins, namely, the Guinea Basin, the Angola Basin and the Cape Basin, that all have maximum depths in excess of 5000 m. They are separated by the southwest–northeast oriented Guinea Rise and Walvis Ridge. From the bathymetry of Smith and Sandwell (1997), the deepest passage in the Guinea Rise slightly exceeds 4500 m near 5
S–0
W. The Walvis Ridge is well known as a nearly impassable barrier for bottom waters, yet two passages in excess of 4000 m exist in the southern part of the ridge near 36
S–7
W (the Walvis Passage), and near 32
400 S–2
200 W (Fig. 1), that allow some bottom water transfer from the Cape Basin to the Angola Basin (Connary and Ewing, 1974). Farther northeast, the general shallowing of the Walvis Ridge gives way to flows deeper than 3000 m in only two

regions, one between 28
S and 30
S (with a 3600 m-deep saddle at 30
S), and the other one near 22
S (the Namib Col). Although A14 was positioned along the nominal isobath 4000 m above the eastern flank of the Mid-Atlantic Ridge, its intersections with the Guinea Rise and Walvis Ridge are apparent around 16
S and 39
S in Fig. 2. Both ridges stand out unequivocally on the vertical distributions of A13, which intersected the Guinea Rise at a short distance to the east of its deepest passage and Walvis Ridge close to the Namib Col to its west (Fig. 3).

3. Meridional hydrographic structure The NADW flowing southward in the western equatorial Atlantic is structured vertically in three

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892

SAF

STF

UDW Z (m)

MDW LDW

Z (m)

(a)

>34.88

Z (m)

(b)

µ

(c)

(°S)

Fig. 2. Vertical distributions of potential temperature (a), salinity (b), and dissolved oxygen (c) along A14. The isopycnals limiting the upper, middle, and lower-deep water varieties (s1 ¼ 32:1; s2 ¼ 36:95; s3 ¼ 41:49; s4 ¼ 45:90; Table 2) are superimposed. STF and SAF stand for Subtropical Front and Subantarctic Front, respectively, and the vertical dash-dotted line marks the intersection with A11.

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893

STF

UDW

z (m)

MDW

LDW

z (m)

(a)

z (m)

(b)

µ

(c)

(°S)

Fig. 3. Same as Fig. 2 but for A13.

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894 STF

LDW

z (m)

MDW

z (m)

z (m)

UDW

µ

(°E)

(a)

(°E)

(b)

(°E)

(c)

Fig. 4. Same as Fig. 2 but for CPT. Table 1 Summary of the hydrographic lines other than those of the CITHER-3 cruise used in this paper Section

A11

A12

A21

I6

Date Ship Institution and chief scientist Source

Dec. 1992–Feb. 1993 RRS Discovery IOSDL P. Saunders

May–August 1992 R/V Polarstern A.W.I. P. Lemke

Jan.–March 1990 R/V Meteor Univ. Bremen W. Roether

Feb.–March 1996 N/O Marion Dufresne LBCM, Paris A. Poisson

Saunders and King (1995)

http://whpo.ucsd.edu

Roether et al. (1990)

Park et al. (2001)

The acronyms have the following meanings: IOSDL, Institute of Oceanographic Sciences Deacon Laboratory; A.W.I., Alfred Wegener Institute; LBCM, Laboratoire de Biog!eochimie et Chimie Marine.

components that Wust . (1935) named upper-, middle-, and lower-NADW, the first characterized by a salinity maximum around 1700 m depth, and the two others by oxygen maxima around 2200 and 3800 m depth (e.g. McCartney, 1993). Although the three classes progressively lose their identity in the South Atlantic, it is useful, for the purpose of examining the evolution of the water mass vertical structure, to superimpose the three layers on the vertical property distributions along A14, A13 and CPT (Figs. 2–4). The following hydrographic analysis is also helpful with a view to estimating the volume transports (Section 4). As an upper limit for the deep water we chose the isopycnal s1 ¼ 32:1 which was retained as the lower limit of intermediate water by Mercier et al.

(2003). The isopycnic separations between upper-, middle-, and lower-deep water (UDW, MDW, LDW) and that with the bottom water below (s2 ¼ 36:95; s3 ¼ 41:49; s4 ¼ 45:90; Table 2) are taken from analyses of another WOCE section near the western boundary by Me! mery et al. (2000) and Wienders et al. (2000). 3.1. The upper-deep water In the eastern equatorial Atlantic, the salinity maximum of the upper NADW is found within the UDW layer (Figs. 2b and 3b), at a depth of B1700 m similar to the one it has against the South American continental margin (McCartney, 1993). However, the northern deep water is not the

ARTICLE IN PRESS M. Arhan et al. / Deep-Sea Research I 50 (2003) 889–916 Table 2 isopycnal limits chosen to subdivide the deep water layer, and central isopycnals of each layer used for the property mapping in Fig. 5 Water mass layer

Upper limit

UDW MDW LDW Bottom water

s1 s2 s3 s4

¼ 32:1 ¼ 36:95 ¼ 41:49 ¼ 45:90

Lower limit

Central isopycnal

s2 ¼ 36:95 s3 ¼ 41:49 s4 ¼ 45:90 Bottom

s2 ¼ 36:85 s3 ¼ 41:45 s4 ¼ 45:855

only water mass occupying the UDW layer in the South Atlantic. Reid (1989) observed that circumpolar water enters the South Atlantic at densities comparable to those of the NADW and Me! mery et al. (2000) pointed out the important density overlap between upper NADW and upper circumpolar water. This stands out in Figs. 2b and c, where the UDW layer along A14 is occupied by the salinity maximum of the northern water near the equator, and by the oxygen minimum characteristic of upper circumpolar water in the southern part of the section. As a consequence, significant meridional changes of property occur along A14 in the UDW. The equatorial salinity maximum first weakens southward, then disappears from the UDW layer. Although the potential temperature and dissolved oxygen also both decrease southward, with a sharper transition near 24
S, the NADW influence (e.g. O2>200 mmol kg1) remains detectable in the lower part of the UDW layer as far south as 42
S. Still considering the UDW, the tracer patterns along A13 (Fig. 3) are qualitatively similar to those along A14, with the main transition between the northern and southern waters detected at 27
S from the potential temperature distribution, instead of 24
S. Along CPT (Fig. 4), most of the UDW layer is occupied by circumpolar water, but NADW is also still present in its lower part, particularly near the continental slope. For a better understanding of the vertical property distributions we show, in Fig. 5, the basin-wide lateral distributions of salinity on three isopycnals representative of each deep water class (Table 2), which were constructed with data from WOCE, from the South Atlantic Ventilation

895

Experiment, and from a few additional cruises including AJAX and the 11
S and 24
S sections used by Warren and Speer (1991). These distributions are generally comparable to those drawn by Reid (1989) on neighboring isopycnal surfaces, yet with more details provided by a denser sampling in the ocean interior. In the UDW layer (Fig. 5a), the well-known branching of the upper NADW, eastward along the equator and southward along the western boundary, stands out, and an anticyclonic flow of upper circumpolar water at the base of the subtropical gyre is visible south of B25
S. The anticyclonic flow, already diagnosed by Reid (1989) from a map of steric height at 1500 m, is here seen to rejoin the western boundary near 30
S, as in the overlying intermediate water (Boebel et al., 1999). Along A14 (Fig. 2), the anticyclonic gyre in the UDW is signaled by a trough of the isopycnal s2 ¼ 36:95 from 45
S to 25
S with the deepest point near 40
S. From this we expect an eastward flow of UDW to the south of 40
S and a westward flow from 40
S to 25
S. More to the north, the tracer patterns suggest a dominant eastward transport near the equator. The moderately saline UDW observed from B5
S to 25
S along A14 could either flow eastward from the western boundary, or be recirculated water from the east-equatorial region. Low oxygen concentrations in the underlying water at 5–15
S, and flat isopycnals suggestive of a barotropic behavior, support the second hypothesis. Similar inferences may be drawn from the tracer observations in the UDW layer along A13 (Fig. 3), although no trough of the isopycnal s1 ¼ 36:95 is visible because of the shorter latitudinal extent of this line. Along CPT, oxygen values (O2o180 mmol kg1) lower than along the two other lines in the circumpolar water suggest an influence from the Agulhas Current system (see Section 5) and a northwestward flow of this water mass. The flow might be reversed, however, in the diluted NADW (O2>200 mmol kg1) present in the lower part of the UDW layer. 3.2. The middle-deep water On both A14 and A13 (Figs. 2 and 3) the oxygen signature of the middle NADW near the equator is

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M. Arhan et al. / Deep-Sea Research I 50 (2003) 889–916

Fig. 5. Hand-drawn salinity distributions (S-34) on three isopycnal surfaces representative of the UDW (a), MDW (b) and LDW (c) layers. In (a), note the change of contour interval at S ¼ 34:900; and the additional contours 34.715 and 34.725 (dashed and dotdashed) that show the anticyclonic pattern of the deep subtropical gyre. In (c), note the contour discontinuities across the Mid-Atlantic Ridge and parts of Walvis Ridge, and the uncertain isohalines (dashed) around 35
S–20
W. Strong mesoscale signals south of 40
S in the western basin prevented the drawing of reliable isohalines in this region.

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897

Fig. 5 (continued).

approximately centred on the isopycnal s2 ¼ 36:95 which separates the UDW and MDW layers. As this surface was determined from western basin characteristics, this reveals an eastward shallowing of the middle NADW signature along the equator, which is confirmed in Figs. 6b–d showing the zonal evolution of the oxygen maximum (3
S to 3
N averages) and associated depth and densities from a series of cross-equatorial sections. The same parameters for the salinity maximum of the upper NADW are also shown for comparison. While the salinity maximum hardly deepens by 100 m from 35
W to 2
E, the oxygen signal shallows by more than 300 m, so that both features are found within 100 m of each other near the eastern boundary. The generally smooth trends in Fig. 6 rule out the non-synopticity of the measurements as a cause of zonal variations. There are several potential reasons for the more shallow oxygen maximum in the eastern basin. One could be an influence of southern deep water shown in Fig. 5b to extend northward in the eastern basin, then westward into

the western basin between the equator and about 17
S. Another reason could be a low oxygen source emanating at about 4000 m in the region of the Congo River cone near 6
S (van Bennekom and Berger, 1984). Warren and Speer (1991) showed that this feature rises to about 3000 mdepth in the ocean interior, and, provided that it can reach the equator, the low oxygen influence could be further advected westward by equatorial deep jets such as those described by Gouriou et al. (2001) from lowered Acoustic Doppler Current Profiler (L-ADCP) measurements, which reached down to 3500 m at 10
W. During the CITHER-3 cruise, L-ADCP measurements were only realized around 2
E to the north of 2
200 S along A13. Fig. 7 showing the zonal velocities along that line (uncertain by B5 cm s1) also suggests a dominant westward flow, yet not extending beyond 3000 m at the equator. Finally, the shallowing of the oxygen maximum in Fig. 6 could be related to a less pronounced equatorial branching of middle NADW, relative to upper NADW. This is

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Smax

898

(a) -1

O2max (µmol kg )

(°W)

z (m)

(b)

(°W)

z(Smax ) z(O2max)

σ2 (kg m-3 )

(c)

(°W)

σ 2 (Smax)

(d)

σ 2 (O2max )

(°W)

Fig. 6. Longitudinal variations of the NADW salinity maximum (a), oxygen maximum (b), and of the depth (c) and density (d) of these maxima, along the equator. The reported maxima are averages between 3
S and 3
N. The line at s2 ¼ 36:95 in (d) shows the boundary between the UDW and MDW layers.

suggested by the tracer patterns in Fig. 5a and b, and would match transport computations near the northeastern tip of Brazil by Rhein et al. (1995). At non-equatorial latitudes, the oxygen signal of the middle NADW is accentuated from 14
S to 24
S along A14 (Fig. 2) and around 20
S to the immediate north of Walvis Ridge on A13 (Fig. 3), in patterns similar to the one observed along AJAX by Warren and Speer (1991). Along A14 the high oxygen values mark an entry of deep northern water into the eastern basin. A part of this water proceeds eastward as described by Speer et al. (1995) to give the high oxygen core (>235 mmol kg1) visible on A13 at the entry of the Namib Col. Another part certainly proceeds

southeastward to the Cape Basin through the gaps present from 28
S to 32
400 S in the ridge (Fig. 1), to give the values in excess of 230 mmol kg1 between the Walvis Ridge and 35
S on A13. Contrary to the situation at the equator, the oxygen signals around 20
S on both lines fit almost exactly in the original density range of the middle NADW, an indication that the process causing the rising of the maximum at the equator does not operate here. Because of the deep oxygen sink at the African continental slope, the lower oxygen values present at 10–15
S in the MDW layer of both sections certainly reveal a westward flow. To the south of 24
S on A14 and to the south of Walvis Ridge on A13, a deepening of the

ARTICLE IN PRESS M. Arhan et al. / Deep-Sea Research I 50 (2003) 889–916 2°S

1°S

EQ

1°N

2°N

899

3°N

4°N

0

10

0 -10 0

0

Z (m)

-10 0 0

A13

-1

U(cm s )

Fig. 7. Zonal velocity component between 2
200 S and the African continental slope along the northern part of A13, as measured with a L-ADCP. Westward velocities are shaded.

NADW oxygen maximum reflects a more pronounced mixing of the northern water with overlying circumpolar water. For a more precise examination of the deep water properties along the eastern boundary we show in Fig. 8 the meridional distributions of the deep water salinity maximum (Smax ) and oxygen maximum (O2max) along A13 (Fig. 8a and b), and the corresponding depths and densities (Fig. 8c and d). Also displayed are the same values right at the continental slope at discrete latitudes, taken from zonal transects. The continental slope values of Smax exceed the A13 values at the same latitudes, except at 4
300 S and 21
S where they equal them. This corroborates the idea of a deep southward slope current fed by sources near the equator and the Namib Col. A jump at 27
S in the A13 Smax curve marks the southernmost limit of the water from the Namib Col on this section. South of this latitude, the pronounced differences between the continental slope and A13 values (Figs. 8a, c and d) reveal a still significant Namib Col component in the most inshore water of the boundary flow. This does not mean that the routes through the other Walvis Ridge passages are less significant, however. Their signatures stand out south of 27
S along A13 (Figs. 3b and c), and the

next section will reveal their dominance in terms of transport. In keeping with a map of Smax in Park et al. (2001), the NADW highest salinities exiting the Atlantic (Figs. 4 and 8) are about 34.86, at depths around 2500 m and densities s3 ¼ 41:45: These parameters are subject to some variability, however, as will be illustrated in Section 5. Because of the deep oxygen sink at the eastern boundary, a comparison of the oxygen values against the continental slope and along A13 (Fig. 8b) is less informative, in terms of alongshore flow direction, than the comparison using salinity. However, the oxygen sink provides the O2max distribution along A13 with more structure than that of Smax : Pronounced gradients mark the lateral limits of the regions of eastward transfer near the equator and the Namib Col. Boundaries of the former at 2
300 S and 3
N are suggested, which are compatible with the eastward flow patterns at B2
S and 2–3
N in Fig. 7, and the northern bound of the latter stands out at 18
450 S. From Fig. 8b the domain from 3
S to 18
S would then be dominated by westward flows. As a tentative explanation for the additional sharp gradient near 7
S, we observe that this latitude approximately corresponds to that of the sedimentary cone of the Congo River, so that water

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Smax

900

(a) -1 O2max (µmol kg )

(°S)

(b)

(°S)

z (m)

z(Smax )

z(O2max)

σ3 (kg m-3)

(c)

(°S)

σ3 (Smax)

σ3 (O2max )

(d)

(°S)

Fig. 8. Latitudinal variations of the NADW salinity maximum (a), oxygen maximum (b) and of the depth (c) and density (d) of these maxima, along A13. Values of the same parameters against the continental slope, estimated from transverse sections, are shown for comparison.

flowing westward to the north of it would have left the boundary upstream of the region of maximum oxygen depletion, contrary to the water sampled farther south. To the south of Walvis Ridge, the poleward oxygen decrease is more regular, with no gradient associated with the 27
S jump in salinity. The oxygen maximum in the NADW exiting the Atlantic is about 230 mmol kg1 at a depth near 3000 m and a density s3 ¼ 41:49 which corresponds to the MDW/LDW interface. 3.3. Lower-deep water and bottom water The water present in the LDW layer in the Guinea and Angola Basins is a mixture of lower

NADW and Antarctic Bottom Water from the western basin that was formed through strong vertical mixing downstream of the sills of the Romanche and Chain Fracture Zones (Mercier and Morin, 1997). The vertical homogenization resulting from mixing is reflected along A14 and A13 by the LDW layer reaching the bottom in the two basins, except for a reduced area in the Guinea Abyssal Plain (Figs. 2 and 3). Oxygen values exceeding 240 mmol kg1 about the equator in both sections signal an eastward extent of the Fracture Zones outflows to the approaches of the African continental slope. Along A13 in the Angola Basin, lower salinities and higher oxygen near the bottom signal a southeastward spreading of water that

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spilled over the Guinea Rise. This pattern and the bottom potential temperatures along A14 and A13 match a map of bottom temperature in this region by van Bennekom (1996), which suggests an abyssal cyclonic flow in the Angola Basin, first southward along the African continental rise, then southwestward against the northern side of Walvis Ridge. Warren and Speer (1991) and Stephens and Marshall (2000) reproduced a similar flow pattern in models. In the upper part of the LDW layer, the oxygen distributions of Figs. 2c and 3c (high values near the equator, lower values south of B4
S) also suggest a cyclonic pattern, yet shifted northward relative to the deeper one. In the southwestern corner of the Angola Basin, lower temperature, salinity, and oxygen values at 32– 38
S in the LDW layer along A14 (Fig. 2) mark Antarctic Bottom Water from the Cape Basin that has flowed through the southern passages of Walvis Ridge, and is visible in Fig. 5c as a southwest–northeast elongated fresh anomaly. At 22
S above the eastern flank of the Mid-Atlantic Ridge (Figs. 2 and 5), a throughflow from the Rio de Janeiro Fracture Zone, already discussed by Mercier et al. (2000), is apparent. The LDW and BW layers are both present in the Cape Basin portions of A14, A13, and CPT (Figs. 2–4), the former being occupied by NADW and the latter by Antarctic Bottom Water (AABW). We suggested above that the NADW marked by O2max values higher than B230 mmol kg1 along A13 in the Cape Basin crossed the Mid-Atlantic Ridge at latitudes around 20
S where a high oxygen pattern is visible in Fig. 2. This is supported by maps of adjusted steric heights at 2500 and 3000 dbar in Reid (1989), which trace back the flow farther to latitudes 30– 35
S near the western boundary. Me! mery et al. (2000) noted a significant escape of middle and lower NADW from the deep western boundary current at these latitudes, and the MDW and LDW salinity distributions in Figs. 5b and c also show a sharp front there, extending northeastward to about 20
S above the Mid-Atlantic Ridge. These observations match the idea that the NADW observed north of the Subtropical Front (at 39
S) on A13 and (at 36
300 S) on A14 also left the western boundary in the subtropical region. It

901

is mostly this water that feeds the deep eastern boundary current visible on CPT (Fig. 4). From Fig. 8 (and the corresponding figure for A14, not shown) it may be characterized by Smax > 34:85; and O2max>230 mmol kg1. The NADW observed south of the Subtropical Front (mostly sampled on A14), on the contrary, probably left the western boundary in the southern Argentine Basin, where some NADW is known to be trapped in the Falkland Return Current associated with the Subantarctic Front (Peterson and Whitworth, 1989; Whitworth and Nowlin, 1987). This southernmost NADW component mixes with (and feeds) the lower circumpolar deep water (LCDW) of the Southern Ocean, itself characterized by a maximum salinity of B34.73 at s2 ¼ 37:04 in the Drake Passage (Sievers and Nowlin, 1984). We observe in Fig. 8 that the density of the salinity maximum at the southern end of A13 is s3 ¼ 41:49 (corresponding to s2 ¼ 37:04). The same density, associated with a value Smax ¼ 34:80; was observed at the southern end of A14. Comparing these values, it is remarkable that the injection of NADW in the LCDW does not alter the density of the salinity maximum. The AABW present in the BW layer of the Cape Basin was found to have a density lower than s4 ¼ 46:0 in all three sections, as in A11 (Fig. 1). From this we recognize it as LCDW, the shallowest constituent of AABW, for which Sievers and Nowlin (1984) placed the lower bound at s4 ¼ 46:0 in the Drake Passage. Reid (1989) described a cyclonic circulation for this water in the Cape Basin. On the Cape Town line (Fig. 4), the outflow of AABW from the Cape Basin appears as a core of colder (yo0:6
C) and fresher (So34:72) water near 15
W against the South African continental rise.

4. Deep water transports across A14, A13 and CPT In order to estimate absolute transports in the upper and intermediate layers, Mercier et al. (2003) applied an inverse model based on geostrophy and property conservation to the two boxes defined by A14 and the eastern part of A11 (box I; Fig. 1) and by A13 and CPT (box II). Here we

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902

follow the same procedure for the deep water transports, using a different subdivision of the water column more suited to the deep circulation. 4.1. Model constraints and unknowns Conservation constraints for volume, heat and salt were written for the whole water column and the four isopycnal layers defined in Table 2. In the additional ‘‘upper layer’’ bounded by the surface and s1 ¼ 32:1; we imposed transport estimates of some equatorial currents deduced from vesselmounted ADCP measurements. The Ekman transports and, when relevant, the air–sea heat fluxes averaged over the three months of the cruise, were included in the surface-to-bottom and upper layer constraints (data from the European Centre for Medium range Weather Forecasting). The transports in the bottom triangles were estimated using the velocity at the deepest common level of each station pair. Specific constraints for the deep waters blocked by the Guinea Rise and Walvis Ridge were also applied. The imposed values and their associated uncertainties for boxes I and II are given in Tables 3 and 4, and we refer the reader to Mercier et al. (2003) for justifications of the uncertainties. The solutions presented below led to constraint residuals only occasionally exceeding the prescribed uncertainties, and always within twice these uncertainties. The model unknowns are *

The reference surface velocities for all station pairs located at off-equatorial latitudes (|lat.|>3
).

*

*

The transports in the five layers and for each station pair in the equatorial band (|lat.|o3
). The vertical diffusivities at the layer interfaces.

The inversion provides best estimates of the unknowns that minimize a weighted sum of their squared departures from a priori values and the squared residuals of the constraints. It also provides the error covariance matrix of the unknowns, from which the transport uncertainties are deduced (Tarantola and Valette, 1982). The weights are diagonal error covariance matrices. At off-equatorial latitudes, the reference surface velocities and thermal wind equation provide the absolute velocity profiles. In the equatorial band, the water mass transports are directly estimated by the model. The vertical diffusivities are used in the tracer constraints written as a balance between 3D-advection and vertical diffusion. The vertical advection is deduced from volume conservation. 4.2. Reference surfaces In Figs. 9a and b we present the cumulative transports of our preferred solutions in the four deep layers and the upper layer, along with the a priori solutions and error bars, for boxes I and II. A schematic NADW circulation inferred from this inversion and the hydrographic analysis of Section 3 is proposed in Fig. 10. The transports in the upper and intermediate layers were shown by Mercier et al. (2003) to be only moderately sensitive to reasonable changes in the model

Table 3 Constraints used for the inversion of box I (A14+A11) Constraint

Imposed value

Surface-to-bottom volume conservation Surface-to-bottom heat conservation Surface-to-bottom salt conservation Volume conservation in the five layers Heat and salt conservation in layers UDW, MDW, LDW, BW Upper layer transport at latitudes of the Equatorial Undercurrent (1
300 N–1
300 S) Upper layer transport at latitudes of the equatorial branch of the South Equatorial Current (1
300 S–3
S)

072.5 Sv (0.470.4)  1015 W 0780  109 kg s1 072 Sv qF 07Kv0 A qz 2075 Sv 674 Sv

Transports are positive eastward. Kv0 is the a priori value of the vertical diffusivity. A is the area of the given layer interface and qF=qz the vertical tracer gradient. The five layers used for volume conservation are the four layers of Table 2 and the upper layer from the surface to s1 ¼ 32:1:

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Table 4 Constraints used for the inversion of box II (A13+CPT) Constraint

Imposed value

Surface-to-bottom volume conservation Surface-to-bottom heat conservation Surface-to-bottom salt conservation Volume conservation in the five layers Heat and salt conservation in layers UDW, MDW, LDW, BW Upper layer transport at latitudes of the northern branch of the South Equatorial Current (1
300 N–3
N) Upper layer transport at latitudes of the Equatorial Undercurrent (1
300 N–1
300 S) Conservation of water denser than s4 ¼ 45:87 between the Guinea Rise and Walvis Ridge Conservation of water denser than s3 ¼ 41:49 south of the Walvis Ridge

072.5 Sv (0.170.3)  1015 W 0787  109 kg s1 072 Sv qF 07Kv0 A qz 371.5 Sv 6.372.4 Sv 071 Sv 071 Sv

Transports are positive eastward. Kv0 is the a priori value of the vertical diffusivity. A is the area of the given layer interface and qF=qz the vertical tracer gradient. The five layers used for volume conservation are the four layers of Table 2 and the upper layer from the surface to s1 ¼ 32:1:

setting. Sensitivity runs for the deep water transports led to the same conclusion, with the difference of a greater variability of the deep equatorial transport estimates which, unlike their upper layer counterparts, were not constrained by direct measurements. As an illustration of the transport sensitivity, we show in the Appendix (Fig. 14) curves similar to those of Fig. 9, that were obtained by changing some model parameters, primarily the reference surfaces, and we compare them with those of the solution that we finally retained. This solution rests on the following a priori values of the unknowns: 4.2.1. Box I (A14–A11) The equatorial transport unknowns were all set to 0 Sv with an uncertainty of 3 Sv. The a priori vertical diffusivities were assumed to be 171.104 m2 s1 at the upper layer interface and 575.104 m2 s1 at the three other interfaces, to account for expected higher diffusivities at depth (Polzin et al., 1997). The reference surfaces, associated with a priori cross-track velocities 072 cm s1, were inferred from the analysis of the tracer distributions and isopycnal slopes presented in Section 3. North of 3
N along A14, it was chosen at the MDW/LDW interface (s3 ¼ 41:49) to account for a likely westward flow of oxygen-poor MDW against the African continental slope. From 3
S to latitude 38
S on the poleward side of the Subtropical Front, a reference surface at s2 ¼ 36:86; in the middle of the UDW,

was found to best separate the mostly westward flow of upper circumpolar water, in the upper part of this layer, from the mostly eastward flow of NADW below. The same surface was retained east of 9
W along A11 on the basis of a similar reasoning. South of 38
S along A14, the barotropic character of the Antarctic Circumpolar Current was reproduced by bottom-referencing the velocities. The solution thus obtained for box I is referred to below and in the Appendix as S0-I. The relatively high uncertainty (2 cm s1) of the a priori reference velocities was retained because of local high geostrophic shears in the deep Angola Basin, although a lower value might have been more appropriate elsewhere. This probably makes the errors bars of Fig. 9 somewhat conservative. Experiences with uncertainties of 1 cm s1 (solutions S3-I and S3-II in Appendix) had transport errors diminished by a factor of B1.7. In Fig. 14, the scatter of the solutions in the sensitivity study (all found within the error bars of Fig. 9) provides another measure of the transport uncertainties. 4.2.2. Box II (A13—CPT) The equatorial transport and diffusivity unknowns were given the same values as for S0-I. The same reference surfaces were also chosen north of 3
N (s3 ¼ 41:49) and south of Walvis Ridge (s2 ¼ 36:86) where the upper circumpolar water and NADW are thought to flow in opposite directions. Between 3
S and the Walvis Ridge, a generally cyclonic flow of abyssal waters near the

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UW

UDW

UDW

MDW

MDW T (Sv)

UW

T (Sv)

904

LDW

LDW

WR

WR BW

BW

(a)

(°S)

(°E)

(b)

(°S)

(°E)

Fig. 9. Cumulative cross-track transports of the preferred inversion solutions along the perimeters of box I (a) and box II (b). The transports are shown for the upper layer (surface to s1 ¼ 32:1) and the four other layers defined in Table 2. They are positive entering the boxes and integrated first southward (along A14 and A13), then northeastward (along A11 and CPT). The transport uncertainties (shaded) and initial guess solution (dash-dotted) are also shown. WR stands for Walvis Ridge.

eastern boundary of the Angola Basin (Warren and Speer, 1991) was reproduced by a reference surface at s3 ¼ 41:51 (or B3500 m-depth) after a few trials. The solution thus obtained for box II is referred to as S0-II. 4.3. The preferred solution (box I) The transports shown in Fig. 9 were accumulated southward from the northern ends of A14 and A13, then northeastward along A11 (Fig. 9a)

and CPT (Fig. 9b). Their vanishing (within 1 Sv) at the eastern ends of the two latter sections in all layers is indicative of a good satisfaction of the layer conservation constraints. The integrated transports are positive entering the boxes and, owing to volume conservation, their value at a given latitude of A14 or A13 may also be regarded as the net southward transport between the hydrographic line and the African continental slope. The curves for the upper layer, not discussed here, are shown to point out their similarity with

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905

15.9 ±10

12.1±9

4 ±2

2.7 ±5

10.7 ±7

4.8 ±3

4 ±3

? 9.7

6.5 ±3

±5

?

7.5 ±2 3 11.1 ±4

7

Fig. 10. Schematic circulation of NADW (green) and UCDW (red) in the eastern South Atlantic (with the 3000 m isobath), as suggested by the transport computations across A14, A13, CPT, the eastern part of A11, and the northern part of I6. The inversion transport estimates (Sv) of major circulation features are reported. The thick arrow at the equator shows the net transport of the system of alternate deep jets in the approximate band 4
N–5
S. The dashed cyclonic pattern in the southern Angola Basin is mostly contributed to by LDW. The circulations of Southern Ocean waters other than UCDW are not shown. Question marks near the cyclonic flow in the northern Cape Basin and the UCDW bifurcation in the Indian Ocean refer to uncertain flow patterns.

those presented by Mercier et al. (2003) with, for instance, transitions between the cyclonic tropical and anticyclonic subtropical circulations detected at 20
S and 25
S on A14 and A13, respectively, from the curves’ zero-crossings. Along A14 in the three deep layers, the transport curves show a general doming between 4
N and about 20
S, which is comparable to that of the upper layer in the same latitude range, and suggests a similar cyclonic flow pattern. The

eastward limb of this circulation occurs from 4
N to about 5
S. Although flow reversals associated with the equatorial current system occur at these latitudes, we estimate a total eastward flow of 15.9710 Sv of deep water to the north of 5
S. Such values are reminiscent of comparable ones obtained from 4
N to 4
300 S at 4
W by Lux et al. (2001) from an independent calculation on other WOCE lines. To the south of B5
S, the decrease of the UDW transport curve

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over the rest of A14 indicates a westward flow (Figs. 9 and 10). This suggests that the high salinity water present in this layer from 5
S to B24
S (Fig. 2b) is upper NADW recirculated from the eastern equatorial region. The zerocrossing of the transport curve near 24
S corroborates this view. A westward circulation of MDW and LDW from 5
S to 20
S is also suggested by Fig. 9a with, to the north of B15
S, a decreased oxygen signature (Fig. 2c) caused by the transit in the eastern oxygen sink region. South of the westward recirculation of NADW, the basic circulation patterns across A14 are different for the UDW, on the one hand, and the MDW and LDW, on the other hand. The curve of the former layer exhibits a trough in the southern part of A14 and along A11. This signals a northwestward flow of upper circumpolar water (473 Sv) across A11 in the Benguela Current, and the westward flow of this water south of 24
S across A14, in the northern limb of the subtropical gyre. The reversed pattern in the MDW and LDW panels is indicative of cyclonic flows in these layers. A net eastward flow of 10.777 Sv is diagnosed in these layers between 20
S and 31
S, yet with local reversals probably caused by the proximity of the Mid-Atlantic Ridge. This latitude band is shifted poleward by about 5
relative to the high oxygen pattern present at 15–25
S in Fig. 2c. On the northern side, the shift is indicative of a westward recirculation of some of the NADW entering the eastern basin at 20–25
S. On the southern side, it suggests an eastward entrainment of less oxygenated water from the southern Angola Basin. As a westward flow of MDW and LDW is found between 31
S and the Walvis Ridge in Fig. 9a, a cyclonic abyssal circulation of 6.573 Sv similar to the one proposed by Warren and Speer (1991; their Fig. 4) in the southern Angola Basin is hinted from the A14 transports. Across A11, finally, the flow in the MDW and LDW is weak except to the east of 10
E where a southeastward deep boundary current of 7.172.5 Sv is observed. This value, which is lower than the 12 Sv diagnosed by Saunders and King (1995) over a thicker layer (36:80os2 ; s4 o45:95), might be an underestimate, as some NADW, not counted here, is present at the base of the UDW (Fig. 4). These

results agree with nearby direct current measurements at 3000 m depth presented by Nelson (1989), which revealed the existence of a deep poleward boundary flow near 30
S. 4.4. The preferred solution (box II) Although remnants of the doming of the A14 deep transport curves north of B20
S may be recognized about the equator and near 7
300 S, the circulation of UDW and MDW across A13 north of Walvis Ridge is less well-defined than across A14. Right against the northern side of the Guinea Basin, an eastward flow existed at the time of A13 which matches a similar one in Fig. 7. Our results, despite the unresolved details of the equatorial band, suggest that this flow recirculates westward to the north of 5
S, a pattern that might be related to the nearby eastern end of the Guinea Basin. Another doming between 6
S and 10
S suggests a deep southward boundary current of 12.179 Sv at these latitudes, which was observed by recent LADCP measurements along a zonal line at 6
S (Andrie! and Bourle" s, 2001). Adjacent to Walvis Ridge at its northern side, narrow eastward flows of 2.871 and 1.271 Sv at 20
400 –22
300 S in the UDW and MDW layers, respectively, might be related to the Namib Col Current just upstream of the passage from which it gets its name. The deep cyclonic circulation to the north of the Walvis Ridge is better defined in the LDW layer (Fig. 9b). A 4.474 Sv eastward equatorial transport is diagnosed for this water mass between 3
N and 3
S, which bifurcates northward and southward near the African continental slope into apparently equal branches. The transport curve between 4
S and 7
S suggests an outflow of 4.973 Sv of LDW from the 5
S–0
W passage in the Guinea Rise into the Angola Basin. South of 7
S, the LDW recirculates westward in the Angola Basin with a slightly enhanced intensity against the northern side of Walvis Ridge. In accordance with Speer et al. (1995), the southward transport of deep water inshore of A13 south of the Namib Col is 2.775 Sv. A more important NADW arrival (9.775 Sv) toward the eastern boundary is detected at 30–40
S across A13, particularly pronounced in the MDW layer, as suggested by the

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S A12 (a)

(B2 Sv) anticyclonic flow in the northern corner of the basin.

5. Deep water around the southern tip of Africa 5.1. Tracer patterns

z (m)

The region around southern Africa is known for its high variability related to the retroflection of the Agulhas Current and associated shedding of Agulhas eddies. Fig. 11, which displays the salinity distributions along the nearly coincident Sections A12, CPT and A21 (Fig. 1, Table 1), shows that the upper layer variability in this region has an imprint extending into the deep water. Along A12 (Fig. 11a), a regular eastward increase of the NADW high salinities signals the deep eastern boundary current. Along CPT (Fig. 11b), the high salinity layer has a similar signature, yet with a pinching at 15
E. Such a pinching is also observed at 16
E along A21 (Fig. 11c) in a more pronounced form, and apparently associated with an Agulhas ring present above. In Fig. 11c, the separation of the S > 34:84 domain

z (m)

z (m)

tracer patterns (Fig. 3). Our results suggest a reinforcement of this eastward flow by a 4.873 Sv anticyclonic circulation equatorward of 30
S in the northern corner of the Cape Basin, visible through negative cumulative transports of MDW and LDW in this region (Fig. 9b). As this anticyclone may appear contradictory to the observation of a detectable Namib Col water signature at the northeastern end of CPT (Section 3), we show it with a question mark in Fig. 10, and we note that the equatorward transport inshore of A13 near 25
S might be the net result of opposing flows. The anticyclone was a robust feature in several inversions run with different reference surfaces. A contribution of Namib Col water to it could explain the poleward decrease of oxygen concentrations south of Walvis Ridge (Fig. 3c). Farther south, an eastern boundary flow of deep water amounting to 11.174 Sv, with a dominant contribution of MDW (5 Sv), is estimated across CPT. The general cyclonic circulation of BW in the Cape Basin, finally, is reproduced with magnitudes of 873 and 7.173 Sv in boxes-I and II, respectively, and in this layer also, an apparent weak

S A21

S CPT (°E)

(b)

907

(°E)

(c)

(°E)

Fig. 11. Vertical salinity distributions along CPT (b) and the northeastern parts of the nearly coincident lines A12 (a) and A21 (c).

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908

into two parts suggests that the anticyclonic flow of the eddy extended downward to the deep layer and entrained a fraction of the deep boundary current offshore. Given the longitude of the breach in the high salinity pattern (16
E), and the longitude range of the Agulhas retroflection (16–20
E from Lutjeharms and Van Ballegooyen, 1988), the edge of thermocline water present above the breach is more likely an eddy than the tip of the retroflection itself. However, we may expect a similar effect of the Agulhas Current retroflection, as indeed depicted in a vertical section along 20
E shown by Reid (1989; his Fig. 14) where a pronounced pinching of the NADW signature is also present and associated with the surface-intensified retroflection signature. In maps of steric height at 2500 and 3000 m depth, Reid suggests that the deep boundary current separates from the continental slope near 20
E to continue eastward in the Agulhas return flow. Toole and Warren (1993) propose a similar flow configuration. To the east of the Agulhas Plateau, along I6, Park et al. (2001) suggested that the most saline

SAF

STF

z (m)

z (m)

PF

deep waters (S > 34:82) observed north of the Subtropical Front come from the Cape Basin, rather than from the southern Argentine Basin. This distinction between the deep water origins corroborates the one drawn above from A13 and A14. The vertical distributions of salinity and dissolved oxygen along I6 (Fig. 12) further confirm that the Agulhas Return Current is the main eastward route for the NADW from the boundary current. The return flow of the Agulhas Current was intersected twice along that line, first at 36– 38
S to the northeast of the Agulhas Plateau, then at 40–41
S. In the deep layer, the highest salinity and oxygen values are not observed against the continental slope as would have been the case in an extension of the deep eastern boundary flow, but below the Agulhas Return Current intersections. Because of the isopycnal slope reversals between 36
S and 41
S, we think it unlikely that the double intersection reveals a branching of the flow, and rather favor a current meander frequently observed to the east of the Agulhas Plateau (Lutjeharms, 1996; his Fig. 1b), or an eddy-like feature (Park et al., 2001).

A.P. µ

(a)

(°S)

(b)

(°S)

Fig. 12. Vertical distributions of salinity (a) and dissolved oxygen (b) along the northern part of I6. The isopycnic layers defined in Table 2 are also shown. PF, SAF and STF stand for Polar Front, Subantarctic Front and Subtropical Front, respectively. AP shows where I6 intersected the rim of the Agulhas Plateau.

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5.2. Transports up- and downstream of the Agulhas Plateau

909

These values apply to the layer 36:86os2 ; s4 o45:90 which encompasses the LDW, MDW, and the lower part of the UDW formed of NADW. The estimate obtained for CPT (6.5 Sv) is lower than the sum of the UDW, MDW and LDW transports provided by the inversion (1174 Sv; Fig. 9b) because of the barotropic component in the latter, and the different density range. Despite some sensitivity of the geostrophic transports to the reference surface choice (e.g., the CPT value changed to 10 and 4.5 Sv for surfaces

For a comparison of the deep water flows upand downstream of Cape Agulhas, we first estimated the geostrophic transports across CPT, A12, and A21 using as a surface of no motion that of the a priori solution of the inversion of Section 4 (s2 ¼ 36:86). The results in Fig. 13a show comparable net transports ranging from 6.5 to 9.5 Sv for the three lines inshore of the Subtropical Front.

A21

T (Sv)

A12

STF

CPT

(a) STF

T (Sv)

Reference surface in Agulhas Current: Bottom σ2=36.86 σ1=32.1 σ1=31.8

AC

(b)

(km)

Fig. 13. (a) Cross-track geostrophic transports of NADW accumulated southwestward along CPT, A12 and A21, using s2 ¼ 36:86 as a zero-velocity isopycnal. (b) Cross-track geostrophic transports of NADW accumulated southward along I6, using the indicated zerovelocity surfaces north of 34
S in the Agulhas Current (AC) region. South of 34
S the zero-velocity surfaces common to all computations are s4 ¼ 45:90 (34–40
S) and the bottom (south of 40
S).

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200 m shallower and deeper, respectively), these estimates and the inversion result all indicate a flow of diluted NADW inshore of the Subtropical Front to the southwest of Cape Town amounting to around 10 Sv. Considering the choice of zero-velocity surfaces along I6, we are led to distinguish three domains. In the region of the Agulhas Current recognized by a steep northward shallowing of the upper isohalines north of 34
S in Fig. 12a, reference surfaces around 2000 m (Toole and Warren, 1993) and 800 m (Beal and Bryden, 1997, 1999) have been proposed, both based on measurements near 31
S, some two degrees to the north of where I6 reaches the continental slope. Farther south, the region 34–40
S is where I6 intersects the Transkei Basin and the eastern spur of the Agulhas Plateau. There, the indication of a cyclonic flow of abyssal water in sediment patterns (Reid, 1989), in a direction opposed to that of the overlying NADW, led us to choose the LDW/BW interface (s4 ¼ 45:90) as a zero-velocity surface. South of 40
S, finally, the intersection by I6 of abyssal water flowing eastward from south of the Agulhas Plateau suggests the bottom as an appropriate zero-velocity surface. Fig. 13b displays the southward accumulated transports of NADW based on the above-indicated reference surfaces to the south of 34
S, and four trials in the Agulhas Current region: the bottom, the upper boundary of the NADW (s2 ¼ 36:86; or B1800 m-depth) comparable to the 2000 m choice of Toole and Warren (1993), the UW/UDW interface (s1 ¼ 32:1 or B1300 m-depth), and the surface s1 ¼ 31:8 (B900 m-depth) representative of the direct measurements of Beal and Bryden (1999). If we compare the cumulative transports estimated inshore of the Subtropical Front with those obtained across CPT, A12 and A21, the bottom and s2 ¼ 36:86 reference surfaces, which provide 8 and 10 Sv, respectively, appear as plausible choices. The much higher values based on the shallower surfaces (22 Sv, 36 Sv), on the other hand, suggest they be discarded. Park et al. (2001) noted that a 2000 dbreferenced solution on I6 (close to the s2 ¼ 36:86 solution of Fig. 13b) led to an Agulhas Current transport of 83 Sv, closer to the L-ADCP estimate

(75 Sv) of Beal and Bryden (1999) than the bottom-referenced solution. Here, focusing on the deep water transports, we observe that both plausible solutions in Fig. 13b reproduce the two intensified eastward flows of NADW likely associated with a meander of the Agulhas Return Current. The one with the bottom zero-velocity shows no trace of a continuation of the deep boundary current round southern Africa. The other one (s2 ¼ 36:86) shows a remnant of it (B2.5 Sv), yet clearly lower than the transport beneath the Agulhas Return Current. The LADCP measurements of Beal and Bryden (1999) at 31
S, and others by de Ruijter et al. (2002) at 24
S and 17
S, which all show a weak (B2 Sv) equatorward boundary flow near 2800 m depth, suggest the latter solution be given preference over the former. Beal and Bryden (1999), however, pointed out differences between their direct measurements, which revealed the presence of an equatorward Agulhas Undercurrent below B800 m, and geostrophic profiles referred to 2000 m which did not display this feature. Considering the curve with the reference surface s2 ¼ 36:86 (B1800 m) in Fig. 13b as our preferred solution, we are faced with a similar contradiction with the results of Beal and Bryden (1999), as our Agulhas Current extends downward to this density through the layer (800–1800 m) where those authors observe the highest values of the countercurrent. A southwestward flow in this layer across I6 is corroborated by the presence of a low oxygen core (o160 mmol kg1) in the upper circumpolar water (B1500 m-depth) against the continental slope in I6 (Fig. 12b), and its absence in CPT (Fig. 4c), suggesting either downstream erosion or retroflection of the low oxygen water with the Agulhas Current. Given the latitude difference between the inshore parts of their section (31
S) and I6 (33
S), the different flow directions could be indicative of a local deep recirculation at 31
S. Beal and Bryden (1999), however, did not adhere to this hypothesis. Using results of Mantyla and Reid (1995), we suggest another possibility to reconcile the observations. Those authors show maps of depth, salinity and dissolved oxygen in the Indian Ocean on the density surface s2 ¼ 36:92; which lies at 2000–2200 m depth near the southern tip of Africa.

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At these levels, they note ‘‘a westward shear near 30–40
S all the way from Tasmania to Madagascar and on to the Agulhas Current southeast of Africa’’, and their salinity map indeed reveals a westward protrusion of lower salinities at 30–35
S. This westward flow, which is the deep part of the northern limb of the South Indian subtropical gyre, may be regarded as the deep (and southernmost) imprint of the South Equatorial Current. The salinity map of Mantyla and Reid (1995) gives no indication on the behavior of the deep westward flow near the coast of Africa, but their oxygen map, through a westward separation of originally tightened isopleths, suggests a bifurcation at latitudes 30–32
S. Provided that the bifurcation occurs between 31
S and 33
S at the depths (800–1800 m) of Antarctic Intermediate Water and upper circumpolar water, it could explain the opposite boundary flows of these water masses as measured by Beal and Bryden (1999) and estimated across I6. Because this interpretation would need confirmation, we show it with a question mark in Fig. 10. As a side issue it is worth noting that, if correct, the suggested flow pattern would be an analogue to the bifurcation of the deep part of the South Equatorial Current in the Atlantic Ocean, which takes place at 28
S against the Brazilian continental slope (Boebel et al., 1999; Wienders et al., 2000), and gives birth, through its northbound branch, to the narrow North Brazil Undercurrent at the intermediate and upper-deep levels. In the Atlantic Ocean, the intermediate boundary current thickens and strengthens equatorward beneath the Brazil Current. In the Indian Ocean, de Ruijter et al. (2002) showed that the boundary flow at 1000–1500 m depth also extends equatorward to at least 17
S, yet weakens significantly in the Mozambique Channel.

6. Conclusion The deep South Atlantic is an important link of the global thermohaline cell, as a location of transfer of the NADW to other oceanic regions were the water mass experiences upwelling. Because of the complicated and as yet imperfectly known deep circulation in this basin, regional

911

analyses aiming at a better description and quantification of the various flow patterns are useful steps toward more synthetic approaches. In this context, the present study based on the new WOCE transects A13, A14, and I6 contributes to improved knowledge of the NADW behavior in the three subbasins of the eastern South Atlantic, and further downstream round southern Africa. The transport computations presented above obviously suffer some limitations. The first one is related to the variability of the flow, including the flow at depth. Deep variability at the annual and interannual periods is most important near the equator (e.g. Thierry, 2000). Because of it, the inversion results should be regarded as representative of the flow at the periods of A13 and A14, not of the time-averaged circulation. In this respect, combining A11 with A14, for lack of a more synoptic transect to close box I, should have only limited drawbacks, as the Subantarctic Front was present on both lines near their junction point (Mercier et al., 2003). The mesoscale variability, though intense and deep-reaching in the Agulhas retroflection region, should also have weak net effects on the large scale transports mostly considered in this study. We also checked that no eddy was intersected at the A14–A11 junction. Another concern with such box inversions is the non-uniqueness of their solutions. Although the formal error bars deduced from the inversion (Fig. 9) are pretty high, the sensitivity study performed with plausible initial guess solutions (Fig. 14) led to results generally exhibiting the same robust features. We have focussed our analysis on these robust, and generally large scale, features. Their compatibility with the tracer patterns described in Section 3 provides further evidence of a satisfactory representation of the real circulation. This study confirmed the two eastward routes of NADW, near the equator and at 20–25
S. The former, which should be regarded as the net result of alternating and time-variable jets, was observed with net magnitudes of 15.9710 Sv and 12.179 Sv across A14 and A13, respectively, and with contributions from the three sublayers (UDW, MDW, LDW). The second was embedded in a series of alternating zonal flows with a B10 Sv net

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UW

UDW

UDW

MDW

MDW T (Sv)

UW

T (Sv)

912

S3 -I

S1-I

S0 -I

S4 -I

S1-II

LDW

LDW

S0-II S2 -I

S3-II

BW

(a)

(°S)

BW

(°E)

(b)

(°S)

(°E)

Fig. 14. Cumulative cross-track transports of several tested inversion solutions for box I (a) and box II (b) described in the Appendix. The characteristics of the a priori solutions used for these sensitivity tests are given in Table 5. The transports are shown for the upper layer (surface to s2 ¼ 32:1) and the four layers defined in Table 2. They are positive entering the boxes and integrated first southward (along A14 and A13), then northeastward (along A11 and CPT).

transport between 20
S and 30
S across A14, in the MDW and LDW layers only. Across A13, the transport at these latitudes reduced to 472 Sv likely attributable to the Namib Col Current. Similar to a circulation pattern at 2500 m presented by Reid (1989), our results suggest that most of the NADW entering the eastern basin at 20–25
S subsequently turns southeastward and proceeds to the Cape Basin through passages to the south of 28
S in Walvis Ridge. A B10 Sv transport of this water was seen proceeding southeastward to the south of 30
S across A13.

In the Guinea Basin and northern Angola Basin, our results fit in with a broad cyclonic flow present in a circulation schematics by Stramma and England (1999). Although the data analyzed in this study provide no information on a possible western closure of this cyclonic pattern, results of Warren and Speer (1991) suggest that the westward limb of the cyclonic flow at least partially feeds a northward boundary current above the eastern flank of the Mid-Atlantic Ridge. Another part of it, however, likely proceeds westward above the ridge and contributes to the tongue of

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diluted NADW present at 5–20
S in the western basin (Fig. 5). Though not inferred from this study, these two probable flow extensions are shown in Fig. 10. In the southern Angola basin, a westward flow component between B30
S and the Walvis Ridge in the MDW and LDW layers suggests a partial entrainment of the 20–30
S transport in another deep cyclonic pattern. The UDW layer has a different circulation in this region, as it is mostly occupied, south of about 24
S (along A14), by upper circumpolar deep water entrained in the deepest part of the subtropical gyre. From our analysis, the deep eastern boundary flow appears as constituted of the successive eastern boundary imprints of different interior flows, rather than as an individualized feature in itself. Magnitudes around 12 Sv and 7–11 Sv were diagnosed for it at B7
S and 30–35
S, respectively, but only B3 Sv at the Namib Col, and indications of a deep anticyclonic flow pattern in the northern corner of the Cape Basin even suggest a discontinuity of the boundary current there. The repeat WOCE samplings to the southwest of Cape Town suggest that Agulhas rings induce temporal variations of the deep boundary flow near 35
S. The analysis of I6 further confirms previous suggestions by Reid (1989) and Toole and Warren (1993) of an entrainment of most of the boundary current in the deep return flow of the Agulhas Current, leaving only a weak continuation (2–3 Sv) of it against the continental slope. Having observed that the corresponding solution for the I6 transports does not exhibit the northeastward Agulhas Undercurrent measured by Beal and Bryden (1999) at 800–1800 m some 250 km more to the northeast, we propose an explanation for this apparent contradiction in a possible bifurcation of the deepest part of the westward limb of the South Indian subtropical gyre, as it impinges on the African coast between 31
S and 33
S.

Acknowledgements Support for this study was provided by the Institut Fran@ais de Recherche pour l’Exploitation de la Mer (IFREMER) for M.A., by the Centre National de la Recherche Scientifique (CNRS) for

913

H.M., and by the Muse! um National d’Histoire Naturelle (MNHN) for Y.-H.P. We wish to thank G. Weatherly for useful discussions. P. Le Bot helped with the preparation of the figures.

Appendix The characteristics of the a priori solutions for S0-I, S0-II, and for other solutions shown in the sensitivity tests of Fig. 14, are summarized in Table 5. The transports in the upper layer are hardly sensitive to the variations of the a priori solution, as pointed out by Mercier et al. (2003). The transport differences are generally limited in the deep layers also, yet local features led us to retain S0-I and S0-II. In S1-I (Fig. 14a), the use of the UDW/MDW interface s2 ¼ 36:95 as a reference surface (instead of s2 ¼ 36:86) along A11 and a part of A14 led to an unrealistic vanishing of the deep eastern boundary current. In S2-I, the choice of a deeper reference surface (s3 ¼ 41:49 at the MDW/LDW interface) in the region of the deep oxygen maximum around 20
S removed any trace of the Namib Col Current from this solution. Solution S3-I, in which the uncertainties of the a priori reference velocities were divided by two, shows no basic difference with S0-I. Having observed that S0-I exhibits an unexpected westward flow (2 Sv) of LDW in the equatorial band, we ran an experiment with a transport of 472 Sv between 3
S and 3
N. Although the resulting solution S4-I is the most different from S0-I, this difference is limited to the LDW layer, at latitudes lower than 30
S. S4-I might be a better timeaveraged representation of the eastward flow from the equatorial fracture zones. Given the acknowledged deep variability of the equatorial flows, however, we retained S0-I as a representation of the flow at the time of A14. The same exercise on box II (Fig. 14b) also revealed a generally weak sensitivity to reasonable changes in the a priori choices. South of Walvis Ridge, we also placed the reference surface at the transition between upper circumpolar water and NADW. As for box I, the presence of some of the latter water mass in the UDW layer led us to choose a surface (s2 ¼ 36:86) within this layer

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Table 5 Characteristics of the a priori solutions used for the sensitivity tests shown in Fig. 14 Solution S0-I

S1-I

S2-I

S3-I

Characteristics of a priori solution

Hydrographic line 1

Reference surfaces with a priori velocities of 072 cm s : s3 ¼ 41:49 at 3
NoLatitude s2 ¼ 36:86 at 38
SoLatitudeo3
S Bottom at Latitudeo38
S s2 ¼ 36:86 at 9
WoLongitude

A14 A14 A14 A11

Identical to S0-I, except for: s2 ¼ 36:95 at 38
SoLatitudeo3
S s2 ¼ 36:95 at 9
WoLongitude

A14 A11

Identical to S0-I, except for: s3 ¼ 41:49 at 24
SoLatitudeo18
S s4 ¼ 45:90 at 9
WoLongitude Identical to S0-I, except for: A priori reference surface velocities of 071 cm s1

A14 A11

S4-I

Identical to S0-I, except for an imposed transport of 472 Sv in the LDW layer and the equatorial band.

S0-II

Reference surfaces with a priori velocities of 072 cm s1: s3 ¼ 41:49 at 3
NoLatitude s3 ¼ 41:51 at 22
300 SoLatitudeo3
S s2 ¼ 36:86 at 40
So Latitudeo22
300 S s2 ¼ 36:86 at 10
EoLongitude

A13 A13 A13 CPT

Identical to S0-II, except for: s3 ¼ 41:49 at 22
300 SoLatitudeo3
S

A13

Identical to S0-II, except for: s2 ¼ 36:95 at 40
So Latitudeo22
300 S s2 ¼ 36:95 at 10
EoLongitude

A13 CPT

S1-II

S2-II

S3-II

Identical to S0-II, except for: A priori reference surface velocities of 071 cm s1

(S0-II), rather than at its lower boundary (S2-II). Also to the south of Walvis Ridge, another trial using the LDW/BW interface (s4 ¼ 45:90) led to no significant difference with S0-II. In the Angola Basin, the reproduction of a cyclonic abyssal flow required a mid-depth reference surface. After observing that the MDW boundaries led to too high transport magnitudes (e.g.: S1-II in Fig. 14), we opted for an isopycnal inside the LDW (s3 ¼ 41:51; at about 3500 m-depth). Finally, solution S3-II in Fig. 14b was obtained, like S3-I, by dividing the uncertainties of the a priori preferred solution by two. At variance with box I, this change led to non-negligible differences for

box II. The reason for this lies in higher deep vertical velocity shears at places (B7
S) along A13, and an ensuing lower capacity of the barotropic velocity components to contribute to the constraints satisfaction. Observing some difficulty of S3-II to satisfy the conservation of bottom water (s4 > 45:87) between the Guinea Rise and Walvis Ridge, we retained S0-II.

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