Deep-Sea Research II 50 (2003) 1893–1931
A near-synoptic survey of the Southwest Indian Ocean
Kathleen A. Donohuea,*, John M. Tooleb
a
Graduate School of Oceanography, University of Rhode Island, BAY Campus 215 South Ferry Road, Narragansett, RI 02882, USA
b
Woods Hole Oceanographic Institution, USA
Abstract
This study focused on the southwest region of the Indian Ocean, where the poleward-directed Agulhas Current is
born, and where dense waters filter through fractures in the Southwest Indian Ridge to form an equatorward-directed
deep boundary current east of Madagascar. Both represent major circulation features of the Indian Ocean: the Agulhas
in its role as a western-boundary current closes the wind-driven subtropical gyre; the deep western-boundary current
renews the bottom waters of the Madagascar, Mascarene, and Somali basins to the north.
A regional, quasi-synoptic survey of the Southwest Indian Ocean carried out as part of the World Ocean Circulation
Experiment Hydrographic Program from May through July of 1995 occupied a cruise track that comprised a closed
‘‘box’’ in the Southwest Indian Ocean. Full-depth water properties and direct-velocity measurements were used to
diagnose the circulation patterns as a function of depth and to estimate the transports of the major currents. The
synoptic circulation was quantified by the construction of a referenced geostrophic velocity field. Water-mass
distributions, direct-velocity measurements, and mass conservation within bounded regions guided the placement of the
level of no motion. Errors in the reference scheme and from synoptic-scale circulation features such as eddies and
internal waves that are aliased by the hydrographic sampling led to uncertainties in the transport estimates.
The upper-ocean Agulhas transport (neutral density, gn o27.96 kg m3, depthst2000 m) was estimated to be
76 106 m3 s1. Contributions to the Agulhas consisted of 29 106 m3 s1 from the westward limb of the subtropical
gyre south of 25 S, 20 106 m3 s1 from the poleward flow east of Madagascar, which subsequently turns to move west
at about 25 S, and an additional poleward flow of 18 106 m3 s1 through the Mozambique Channel. Bathymetry
strongly controls the deep and bottom circulation: the African Coast, the Mozambique Plateau, and the Madagascar
Ridge support deep western-boundary currents along their eastern margins; however, the Natal Valley and the
Mozambique Basin are blocked to meridional flow beneath 2800 and 3000 m, respectively. East of the Madagascar
Ridge, net northward deep transport is small. In the Madagascar and Mascarene Basins, a deep cyclonic circulation is
present: high-latitude deep waters move northward along the western boundary, while North Indian Deep Water flows
southward in the basin interior. The deep western-boundary current along the east coast of Madagascar carries about
3 106 m3 s1 of bottom water (gn > 28:11 kg m3) northward.
r 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
*Corresponding author. Tel.: +1-401-874-6615; fax: +1401-874-6728.
E-mail address: [email protected] (K.A. Donohue).
The wind-stress curl over the subtropical
South Indian Ocean drives an anticyclonic
circulation characterized by broad northward
flow in the interior and a poleward westernboundary jet (e.g., Hellerman and Rosenstein,
0967-0645/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0967-0645(03)00039-0
1894
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1983; Grundlingh
.
et al., 1991; Stramma and
Lutjeharms, 1997). The Agulhas Current is the
strongest western-boundary current in the southern hemisphere, with estimates of its transport
based on data collected south of the African
continent as high as around 100 106 m3 s1, well
in excess of the Sverdrup flow (e.g., Jacobs and
Georgi, 1977; Gordon et al., 1987; Bennett, 1988).
Akin to the North Atlantic, the wind-driven
boundary current in the South Indian Ocean is
augmented by poleward flow associated with the
thermohaline circulation. In this case, the poleward thermohaline flow is supplied by deep waters
that have been warmed and upwelled at low
latitudes in the Indian Ocean (Warren, 1981b;
Toole and Warren, 1993; Robbins and Toole,
1997; Macdonald, 1998; Sloyan and Rintoul, 2001;
Ganachaud et al., 2001; Bryden and Beal, 2001).
The Indonesian Throughflow also appears to
contribute to the Agulhas transport in the form
of Pacific waters that pass through the archipelago
and then west across the Indian Ocean at tropical
latitudes to the western boundary (Wyrtki, 1971;
Fine, 1985; Gordon et al., 1997). Local recirculation is likely to be responsible for the remainder of
the transport in excess of the Sverdrup forcing.
Topography, particularly the island of Madagascar, complicates the upper-ocean circulation
pattern in the west. Westward interior transport
impacting the eastern shore of Madagascar potentially supplies poleward flow both along the
eastern coast of the island (East Madagascar
Current) and through the Mozambique Channel
(e.g., Lutjeharms et al., 1981; Sætre and daSilva,
1984; Schott et al., 1988). Westward interior
transport also contributes to northward flow along
the Horn of Africa in the East African Coastal and
Somali Currents (e.g., Duing
.
and Schott, 1978;
Schott and McCreary, 2001). Island Rule calculations (Godfrey, 1989; Pedlosky et al., 1997)
provide estimates of the wind-driven transport
between Madagascar and the basin boundary.
The tortuous bathymetry of the Indian Ocean
also greatly constrains the deep and abyssal
circulations. As detailed by Toole and Warren
(1993), North Atlantic Deep Water is largely
confined by the Madagascar and Daves ridges to
the extreme southwest of the region (Natal Valley
and Mozambique Basin) (Fig. 1). Circumpolar
Deep Water appears to move north through gaps
in the Southwest Indian Ridge (Warren, 1978),
and as boundary currents along the flanks of the
Southeast Indian Ridge and Broken Plateau
(Toole and Warren, 1993). In turn, water property
distributions suggest that Indian Deep Water
returns poleward in the interior regions of the
many South Indian Ocean basins, and in a deep
western-boundary current east of Madagascar
(Warren, 1974, 1981b).
The regional circulation in the Southwest Indian
Ocean has been examined previously based on
historical hydrographic observations and local
surveys (most of which only sampled to thermocline depth). From these, the basic structure of the
upper-ocean circulation has developed. (Notable
here are the works by Grundlingh
.
et al. (1991) and
Stramma and Lutjeharms (1997). Quantitative
questions remain, however, such as the relative
contributions to the Agulhas transport from the
East Madagascar Current, interior westward motion south of Madagascar, and flow through the
Mozambique Channel. Description of the intermediate and deep water circulation patterns is less
complete. Motivated by these questions, a regional, quasi-synoptic survey of the Southwest Indian
Ocean was carried out as part of the World Ocean
Circulation Experiment Hydrographic Program
(WHP) in 1995. Full-depth water properties and
direct-velocity measurements were acquired along
a series of sections that bracket the ocean region,
where the Agulhas Current forms into an intense
western-boundary current. Those data are used
here to diagnose the circulation patterns as a
function of depth and estimate the transports of
the major currents.
2. Data
From late May to mid July 1995 aboard the R/V
Knorr, a series of full-depth closely spaced hydrographic casts were occupied along a track that
comprised a closed ‘‘box’’ in the Southwest Indian
Ocean (Fig. 1). Station sampling along the northernmost section of this box, the western end of the
20 S trans-Indian ocean section (I3W), began on
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1895
4 oS
8 oS
Amirante
Passage
o
12 S
Mascarene
Basin
Daves Ridge
agasc
I5W
I7C
I3W
I4East
I4
Mauritius
Mad
o
20 S
ar
16 oS
o
24 S
Madagascar
Basin
Mozambique
Basin
o
28 S
Durban
ge
Atlantis II
Fracture
Zone
Indomed
Fracture Zone
Crozet
Basin
hw
at
So
ut
N
o
36 S
es
al
tI
V
al
nd
le
ia
y
n
R
32 S
Madagascar
Ridge
id
Mozambique
Plateau
o
Melville
Fracture
Zone
Agulhas Basin
o
40 S
44 oS
Prince Edward
Fracture Zone
48 oS o
26 E
o
30 E
o
34 E
o
38 E
o
42 E
o
46 E
o
50 E
o
54 E
o
58 E
o
62 E
Fig. 1. Map of hydrographic station locations for the World Ocean Circulation Experiment Hydrographic Program sections (I3W,
I4East, I4, I5W, and I7C) in the Southwest Indian Ocean conducted aboard the R/V Knorr in 1995. Bathymetry from Smith and
Sandwell (1997) is shaded every 1000 m from 3000 to 5000 m depth. A thin (thick) solid line represents the 2000 m (0 m) isobath.
28 May and reached the Madagascar coast on 1
June. After a brief port stop in Mauritius, station
work resumed on 13 June along 25 S off the
southeast tip of Madagascar with a short segment
of stations we named I4East. Sampling continued
westward across the Mozambique Channel along
1896
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
25 S as the I4 section, effectively extending the I3
section to the African Coast. After a 1-day stop in
Durban, South Africa, station work recommenced
along approximately 32 S with samples across the
Agulhas Current. This quasi-zonal section, I5W,
re-occupied the western end of a 1987 pre-World
Ocean Circulation Experiment (WOCE) transoceanic section (Toole and Warren, 1993). Finally,
a meridional segment, I7C, along 54.5 E from
33.5 S to 20 S was occupied, completing the box.
Sampling ended on 7 July. In total, the hydrographic survey required 40 days to complete.
Greatest station spacing for the full-depth
hydrographic casts was 55 km, with closer spacing
near topographic features and western boundaries
(Fig. 1). Hydrographic casts performed with a 36bottle rosette/CTD to within 10 m of the bottom
yielded full-water-column conductivity and temperature profiles as well as discrete salinity,
oxygen, and a suite of nutrient measurements at
all stations and bottles (Figs. 2–7). While technically silicic acid was measured, we refer to this
nutrient as silicate for simplicity. The resulting
data set meets WHP specifications outlined in
World Ocean Circulation Experiment Hydrographic Programme Office Report WHPO 91-1,
WOCE Report NO. 68/91 (1994). We also report
the proxy for density, neutral density, gn (Jackett
and McDougall, 1997) (Fig. 8).
Two independent acoustic Doppler current
profiling (ADCP) systems sampled ocean velocity.
A shipboard ADCP (SADCP) continuously monitored upper-ocean currents (Fig. 9). Additionally,
a lowered ADCP (LADCP) mounted on the
underwater hydrographic frame provided fulldepth profiles of horizontal currents at each
station (Figs. 10 and 11). SADCP data were
collected using an RD Instruments 150-kHz
narrowband system interfaced with the vessel’s
gyroscope. Real-time processing yielded 5-min
vector-averaged relative velocity estimates with 8m vertical resolution. Ship gyroscope errors were
quantified with an Ashtech 3DF GPS attitude
sensor (King and Cooper, 1993), and relative
velocities were corrected accordingly. Ship navigation data were an irregular mix of P-code precision
GPS fixes and dithered fixes when the former was
unavailable. Standard water track calibration
methods (Joyce, 1989; Pollard and Read, 1989)
provided a velocity-scale factor and constant
angular offset between the transducer and the
Ashtech antenna array. The inherent accuracy of
the 3DF together with the consistency of successive calibrations on the R/V Knorr indicate that
heading accuracy better than 0.1 was achieved,
and the corresponding cross-track velocity errors
were under 1 cm s1; however, episodes of bad
weather and/or periods of poor navigational fixes
degraded the accuracy of the velocities.
The LADCP measurements were acquired with
a 150-kHz broadband sonar from RD Instruments. Details of LADCP instrumentation and
processing may be found in Hacker et al. (1996)
and Fischer and Visbeck (1993). Uncertainty in the
barotropic component of LADCP profiles is
typically about 1 cm s1; uncertainty in velocity
at any given depth is larger and near 2–4 cm s1.
Waters east of the Madagascar Ridge tended to
have the weakest deep-scattering amplitudes and
thus greatest depth-dependent velocity uncertainty.
We are interested in the currents associated with
the regional synoptic geostrophic circulation. The
directly measured velocities include tidal (barotropic and baroclinic) flows and other ageostrophic currents such as internal waves and
near-inertial motion. We estimated barotropic
tidal currents using the OSU TOPEX/Poseidon
Cross-over Global Inverse Solution, Version 3.1
(Egbert et al., 1994) and removed these from the
SADCP and LADCP profiles. The predicted
tide contributes less than 2.5 cm s1 except in the
Mozambique Channel, near the Madagascar
Ridge, and along the western boundary of the
Madagascar Basin (Fig. 10).
3. Water-property distributions
3.1. Upper ocean
Tropical surface water, TSW, characterizes the
upper ocean in the northern reaches of our study
region. TSW owes its relatively low salinity to
excess precipitation over evaporation in the
tropics, particularly in the Bay of Bengal, and to
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1897
Masc. Basin
I3W
20oS
I4
Moz. Plateau
Moz. Basin
I5W
I7C
Mad. Basin
Va
lle
y
o
30 S
I4East
Mad. Ridge
o
25 S
N
at
al
o
35 S
o
o
35 E
o
40 E
o
45 E
o
o
50 E
55 E
611
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
548
562
557
555
553
551
549
584
578
576
610
604
600
598
596
594
592
590
588
30 E
0
depth [m]
18
500
16
14
14
10
10
6
8
8
1000
0
12
10
8
12
12
12
8
6
16
6
1000
6
4
4
4
4
3
3
depth [m]
2000
3
2.5
2.5
2.5
4
2.5
2
2
2
2
2
3000
1.5
1.5
4000
1
Natal
Valley
0.8
0.8
40˚E
Mozambique
Channel
Mascarene
Basin
Madagascar Basin
I5W
35˚E
1
0.6
Mozambique Basin
6000
1.5
1
0.8
0.6
0.4
0.2
5000
1.5
1.5
I7C
45˚E
potential temperature
°
[ C]
0
33˚S
I3W
500
1000
distance [km]
29˚S
25˚S
1500
50˚E
I4East
54˚E 49˚E
I4
38˚E
42˚E
21˚S
Fig. 2. Upper panel: map of hydrographic stations. Lower panels: profiles of potential temperature in C for the Southwest Indian
Ocean WHP lines (left to right: I5W, I7C, I3W, I4East, and I4.) Station numbers and positions are indicated at the top of the 0–1000 m
panel.
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1898
Masc. Basin
I3W
o
20 S
I4
Moz. Plateau
o
N
at
al
Va
lle
y
30 S
I4East
Mad. Ridge
o
25 S
Moz. Basin
I5W
I7C
Mad. Basin
35oS
o
o
35 E
o
40 E
o
45 E
o
o
50 E
55 E
611
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
548
562
557
555
553
551
549
584
578
576
610
604
600
598
596
594
592
590
588
30 E
0
35.2
35
35
34.7
34
1000
0
34 5
1000
depth [m]
2000
34
34.7
34.76
34.
8
34.5
34.6
34.7
34.7 34.74
6
3000
34.7
34.7
34.5
34.6
.5
500
35.4
35.4
35.4
34
depth [m]
35.6
34.5
2
34.7
.4
34.5
34.6
34.7
34.7
34.72
34.76
34.74
34.8
34.76
4000
34.76
34.74
Natal
Valley
34.7
2
Mozambique
Channel
34.7
5000
Mozambique Basin
Madagascar Basin
I5W
6000
35˚E
40˚E
Mascarene
Basin
I7C
45˚E
0
I3W
500
1000
distance [km]
1500
50˚E
I4East
54˚E 49˚E
I4
38˚E
42˚E
salinity
33˚S
29˚S
25˚S
21˚S
Fig. 3. Distribution of salinity on the practical salinity scale along the sections.
the influence of low-salinity Indonesian throughflow waters carried westward in the South
Equatorial Current (Wyrtki, 1971; Gordon et al.,
1997). The freshest and warmest TSW is found
along the northernmost section, I3W (20 S),
where surface salinity and temperature values
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1899
Masc. Basin
I3W
o
20 S
I4
Moz. Plateau
N
at
al
Va
lle
y
30oS
I4East
Moz. Basin
I5W
Mad. Ridge
o
25 S
I7C
Mad. Basin
35oS
o
o
o
35 E
o
40 E
45 E
o
o
50 E
55 E
611
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
548
562
557
555
553
551
549
584
578
576
610
604
600
598
596
594
592
590
588
30 E
200
230
230
230
depth [m]
0
500
220
210
220
210
200
1000
0
220
200
1000
200
180
140
160
0
180
200
210
190
depth [m]
190
200
21
170
190
160
2000
140
170
3000
180
20
0
Natal
4000
Valley
180
Mozambique
Channel
200
220
5000
Mozambique Basin
Madagascar Basin
I5W
6000
35˚E
40˚E
oxygen
-1
[µmol kg ]
Mascarene
Basin
I7C
45˚E
0
I3W
500
1000
distance [km]
33˚S
29˚S
25˚S
1500
50˚E
I4East
54˚E 49˚E
I4
38˚E
42˚E
21˚S
Fig. 4. Distribution of dissolved oxygen in mmol kg1.
range from 34.94 to 35.31 and 24.7 C to 26.3 C,
respectively (Fig. 12). In the tropical Indian
Ocean, surface salinities are higher in the west
due to the influence of Arabian Sea Waters
(Wyrtki, 1971). Higher surface salinity in the
Mozambique Channel as compared to east of
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1900
Masc. Basin
I3W
20oS
I4
Moz. Plateau
I4East
Moz. Basin
35oE
40oE
o
I5W
Mad. Basin
30oE
45oE
50oE
55oE
611
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
548
562
557
555
553
551
549
584
578
576
610
604
600
598
596
594
592
590
588
N
at
al
o
35 S
Va
lle
y
30 S
I7C
Mad. Ridge
o
25 S
0
5
5
5
5
1000
10
10
5
30
40
60
50
20
30
40
30
20
1000
0
10
20
50
50
50
70
70
2000
80
100
110
3000
70
80
90
70
depth [m]
3020
10
10
10
10
20
depth [m]
5
5
5
5
500
0
12
5
12
90
Natal
Valley
125
120
4000
90
Mozambique
Channel
110
125
120
5000
Mozambique Basin
Madagascar Basin
I5W
6000
35˚E
40˚E
silicate
[µmol kg-1 ]
Mascarene
Basin
I7C
45˚E
0
500
1000
distance [km]
I3W
1500
50˚E
I4East
54˚E 49˚E
I4
38˚E
42˚E
33˚S
29˚S
25˚S
21˚S
Fig. 5. Distribution of dissolved silicate in mmol kg1.
Madagascar is likely a manifestation of the poleward spread of Arabian Sea Water properties
along the African coast.
TSW enters the subtropics via the poleward
arms of the South Equatorial Current bifurcations
at the African and Madagascar coasts and
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1901
Masc. Basin
I3W
o
20 S
I4
Moz. Plateau
N
at
al
Va
lle
y
30oS
I4East
Moz. Basin
I5W
I7C
Mad. Ridge
o
25 S
Mad. Basin
35oS
o
o
35 E
o
40 E
o
45 E
o
o
50 E
55 E
611
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
548
562
557
555
553
551
549
584
578
576
610
604
600
598
596
594
592
590
588
30 E
500
20
1000
0
1000
25
5
15
0
25 2
30 32
25
30
30
32
32
30
30
30
3000
34
32
depth [m]
20
10
32
30
2000
25
30
32
15
25
5
10
1
20 5
25
302
3
5
10
15
20
5
20
25
15
20
15
5
10
10
10
10
5
10
5
5
5
15
depth [m]
0
Natal
Valley
30
Mozambique
Channel
32
4000
5000
Mozambique Basin
Madagascar Basin
I5W
6000
35˚E
40˚E
nitrate
-1
[µmol kg ]
Mascarene
Basin
I7C
45˚E
0
I3W
500
1000
distance [km]
33˚S
29˚S
25˚S
1500
50˚E
I4East
54˚E 49˚E
I4
38˚E
42˚E
21˚S
Fig. 6. Distribution of dissolved nitrate in mmol kg1.
continues southward either in the Mozambique
Channel or along the eastern Madagascar Coast in
the East Madagascar Current (e.g., Swallow et al.,
1988; Schott et al., 1988). Along I3W (20 S),
surface currents do not reveal a southward
boundary current as the section is close to the
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1902
Masc. Basin
I3W
o
20 S
I4
Moz. Plateau
N
at
al
Va
lle
y
30oS
I4East
Moz. Basin
I5W
Mad. Ridge
o
25 S
I7C
Mad. Basin
35oS
o
o
o
35 E
o
40 E
45 E
o
o
50 E
55 E
611
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
548
562
557
555
553
551
549
584
578
576
610
604
600
598
596
594
592
590
588
30 E
0.5
0.5
500
1
1.5
1
1
1.5
1.5
1.5
1
5
0.5
1.
depth [m]
0
2.1
1000
0
1
1
1
1000
2
2
2.3
2.1 2
2.1
4
2.
2
3
2.
3000
4000
Natal
Valley
2.1
Mozambique
Channel
2.3
depth [m]
3
2.
2.3
2000
5000
Mozambique Basin
Madagascar Basin
I5W
6000
35˚E
40˚E
phosphate
-1
[µmol kg ]
Mascarene
Basin
I7C
45˚E
0
I3W
500
1000
distance [km]
33˚S
29˚S
25˚S
1500
50˚E
I4East
54˚E 49˚E
I4
38˚E
42˚E
21˚S
Fig. 7. Distribution of dissolved phosphate in mmol kg1.
South Equatorial Current bifurcation latitude
(Swallow et al., 1988; Rao et al., 1989) (Fig. 9).
Surface velocities here are mainly southward, but
they are broad and relatively weak (o15 cm s1).
A maximum (minimum) in surface salinity (temperature) along I3W is co-located with a weak
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1903
Masc. Basin
I3W
o
20 S
I4
Moz. Plateau
o
Moz. Basin
I5W
I7C
Mad. Basin
Va
lle
y
30 S
I4East
Mad. Ridge
o
25 S
N
at
al
o
35 S
o
o
35 E
o
o
40 E
45 E
o
o
50 E
55 E
611
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
548
562
557
555
553
551
549
584
578
576
610
604
600
598
596
594
592
590
588
30 E
0
depth [m]
24.65
26.5
500
26.5
26.7
26.9
26.9
26.9
26.5
26.7
26.7
27.13
6
3
27.
1000
0
26.7
26.9
1000
26.9
27.13
27.36
27.36
27.53
27.7
27.7
depth [m]
27.96
27.96
27.96
28.04
28.04
28.04
3000
4000
27.83
27.83
2000
26.5
26.9
27.36
27.7
27.83
26.5
26.5
28.
11
28.11
28.11
28.17
Natal
Valley
28.17
Mozambique
Channel
28.23
28.
27
5000
Mozambique Basin
I5W
6000
35˚E
40˚E
Mascarene
Basin
Madagascar Basin
I7C
45˚E
0
n
neutral density, γ
-3
[kg m ]
33˚S
I3W
500
1000
distance [km]
29˚S
25˚S
1500
50˚E
I4East
54˚E 49˚E
I4
38˚E
42˚E
21˚S
Fig. 8. Upper panel: map of hydrographic stations. Lower panels: profiles of neutral density, gn ; in kg m3. The level of no motion
used to reference geostrophic velocities is shown by a white dot for each station pair, except where the level of no motion is the deepest
common level.
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1904
o
40 cm s-1
20 S
24oS
28oS
32oS
36oS
28oE
32oE
36oE
40oE
44oE
48oE
52oE
56oE
40 cm s-1
20oS
50oE
52oE
54oE
56oE
Fig. 9. Detided velocity vectors from shipboard ADCP averaged from 100 to 300 m depth. The 1000 and 2000 m isobaths are
contoured. The coast is contoured with thick line. The lower panel gives an expanded view of the I3W area.
surface-trapped cyclonic eddy centered near
51.5 E (Figs. 9 and 12). To the south, along
I4East (25 S), a well-developed southward East
Madagascar Current was observed, along with a
strong offshore northward flow (Fig. 9). The
southward surface currents within the East Madagascar Current reach 80 cm s1. Surface temperatures increase and surface salinities decrease
towards the Madagascar coast as the East
Madagascar Current advects TSW southward
(Fig. 12). A retroflection in the East Madagascar
Current is often observed south of Madagascar
(Lutjeharms et al., 1981; Lutjeharms, 1988);
however, altimeter data suggest that a cyclonic
eddy east of the East Madagascar Current is
responsible for some if not all of the offshore
northward flow on I4East (not shown). Water
properties below the surface (discussed later)
further substantiate this claim. Within the Mozambique Channel (I4) no well-defined poleward
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
612
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
562
557
555
553
551
549
583
578
576
609
603
600
598
596
594
592
590
587
1905
depth [m]
0
500
1000
0
1000
depth [m]
2000
3000
4000
Natal
Valley
Mozambique
Channel
5000
Mozambique Basin
Madagascar Basin
I5W
6000
35˚E
40˚E
I7C
45˚E
0
I3W
500
1000
distance [km]
33˚S
velocity [cm s-1]
Mascarene
Basin
29˚S
25˚S
1500 50˚E
I4East
I4
54˚E 49˚E
38˚E
42˚E
54˚E 49˚E
38˚E
42˚E
21˚S
5
0
5
35˚E
40˚E
45˚E
0
500
1000
distance [km]
1500 50˚E
Fig. 10. Profiles of detided cross-track LADCP velocity in cm s1. The contour interval is 10 cm s1. Negative flow is shaded. For I3W,
I4, and I4East positive is to the north; for I5W positive is to the northeast and north; for I7C positive is to the northeast and east. The
predicted cross-track barotropic tide that has been removed from the LADCP profiles is shown in the bottom panels.
current is evident. Rather, the dominant features
are two cyclonic eddies (Fig. 9). This is consistent
with previous hydrographic and satellite altimetry
data which indicate that flow within the Channel is
frequently dominated by eddies (e.g., Sætre and
daSilva, 1984; DiMarco et al., 2002; de Ruijter
et al., 2002). The relatively low surface salinities
within the Agulhas Current (o35:6) compared to
values to the east evince a surface layer tropical-tosubtropical exchange (Fig. 3).
Excess evaporation over precipitation in the
subtropics creates the relatively high-salinity subtropical surface water (STSW). Away from the
Agulhas Current and south of about 28 S, STSW
surface salinities are quite uniform and in excess of
35.5. Away from the Agulhas, surface currents are
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1906
LADCP vertically averaged
surface to γ =27.96 kg m-3 (~2000 m)
n
o
18 S
-1
22oS
40 cm s
o
26 S
30oS
o
34 S
38oS
o
o
o
o
o
o
o
o
26 E 30 E 34 E 38 E 42 E 46 E 50 E 54 E
LADCP vertically averaged
γ =27.96 kg m-3 (~2000 m) to bottom
n
o
18 S
22 oS
20 cm s-1
o
26 S
30 oS
34 oS
38 oS
o
o
o
o
o
o
o
o
26 E 30 E 34 E 38 E 42 E 46 E 50 E 54 E
Fig. 11. Upper panel: detided velocity vectors from the
LADCP averaged from the surface to gn =27.96 kg m3. Lower
panel: detided velocity vectors from LADCP averaged from
gn =27.96 kg m3 to the bottom of each profile. Bathymetry
from Smith and Sandwell (1997) is shaded every 1000 m from
3000 to 5000 m depth.
weak and mostly oriented to the west and north
along I5W and I7C. At 28 S, a sharp gradient in
surface temperature and salinity marks the boundary between surface TSW and surface STSW.
Equatorward of the front, STSW subducts below
TWS producing a subsurface salinity maximum
near 200 m depth. A subsurface oxgyen minimum
near 200 m is present within the STSW layer and
most extreme on the northern sections. Warren
(1981a, b), in his analysis of data along 18 S,
suggested that in situ oxygen consumption creates
this shallow dissolved-oxygen minimum. A core of
low dissolved oxygen (o185 mmol kg1) near
150 m depth is found banked against the Madagascar Coast within the East Madagascar Current
(presumably carried south by this flow), but not
within the offshore northward flow. This supports
our inference that the northward flow is not an
East Madagascar Current retroflection. Each
cyclonic eddy in the Mozambique Channel contains a low-oxygen core o175 mmol kg1 at 200 m
depth, suggesting a northern source (Fig. 4). A
minimum in dissolved oxygen near 200 m depth
within the Agulhas also points to advection from
the tropics to the subtropics.
Two upper-ocean water-mass features noted in
the 1987 Darwin section are not evident in these
1995 sections. First, Toole and Warren (1993)
found a slight dissolved-oxygen maximum near
100 m depth, which they ascribed to the seasonal
capping of winter mixed layers. Our austral winter
1995 sections have no shallow (o200 m) dissolvedoxygen maximum, presumably because winter
mixing eroded the seasonal thermocline they
sampled. Second, Toole and Warren (1993) noted
a weak thermostad near 17 C, which could be
created in winter within the Agulhas Retroflection
zone and brought to the subtropics by the Agulhas
Return Current (Gordon et al., 1987). A pycnostad at temperatures in the high teens is not readily
apparent in the 1995 WOCE data.
Winter cooling and subsequent deep convection
poleward of the subantarctic front sets the properties of the subantarctic mode water (SAMW)
(McCartney, 1977, 1982; Warren, 1981b), which
subducts into the thermocline and spreads into the
subtropics by participating in the anticyclonic
wind-driven gyre (Toole and Warren, 1993;
McCarthy and Talley, 1999). The coincident
oxygen maximum and pycnostad (potential vorticity minimum) located near 500 m depth identifies
SAMW (Figs. 4, 8 and 14). The combination of
the west-to-east progression of increasingly
fresher, colder, and denser SAMW across the
southern Indian Ocean basin (McCartney, 1977,
1982) and the entrainment of these waters into the
gyre results in a subtropical distribution of
SAMW, which becomes colder and denser with
increasing distance from the gyre center. Within
our study region, the lowest SAMW potential
vorticity and highest oxygen values are found in
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
I7C
I5W
26
I3W I4East
1907
I4
temperature
[°C]
24
22
20
18
35.6
35.4
35.2
salinity
35
35˚E
40˚E
45˚E
0
500
1000
distance [km]
33˚S
29˚S
25˚S
1500
50˚E
54˚E 49˚E
38˚E
42˚E
21˚S
Fig. 12. Surface temperature (upper panels) and salinity (lower panels) for the I5W, I7C, I4East, I3W, and I4.
the southeast corner (50 E, 33.5 S to 51 E,
32.5 S). The warmest (gn range 12–13 C) and
lightest SAMW (gn range 26.6–26.75 kg m3) is
found east of the Agulhas Current extending
northward along I7C to 28 S (Fig. 14). Equatorward of 28 S, the potential temperature and
density of SAMW decrease from y ¼ 13 C
and gn ¼ 26:65 kg m3 at 28 S to y ¼ 11 C and
gn ¼ 26:8 kg m3 at 20 S (Fig. 14). These relatively
cold and dense modes originate near 80–110 E
and flow into our study region in the northward
and westward limbs of the subtropical gyre
(McCarthy and Talley, 1999). SAMW appears
along the Mozambique Channel section. The
westward limb of the wind-driven gyre brings
subantarctic water to the southern reaches of the
channel; our circulation scheme and that of
Ganachaud et al. (2001) and DiMarco et al.
(2002) show a mid-channel anticylonic circulation
feature within the upper ocean. SAMW within the
Agulhas Current has a relatively weak potential
vorticity minimum and low dissolved-oxygen
concentrations relative to values to the east
indicating that the SAMW pycnostad has undergone mixing during its journey through the
subtropical gyre (Fig. 14).
Along I3W, a subsurface cyclonic eddy centered
near 500 m depth and 50 E was sampled off the
Madagascar Coast (Fig. 10). Cross-track velocities
are greater than 30 cm s1. Compared to adjacent
waters along I3W, the eddy is colder, fresher, less
oxygenated, and higher in nutrients. These characteristics suggest a northern source.
3.2. Intermediate waters
Intermediate layers in the Southwest Indian
Ocean (gn ranging between 26.9 and 27.7 kg m3)
manifest both low-salinity contributions from
southern-origin Antarctic intermediate water
(AAIW) (e.g., Warren, 1981b; Fine, 1993) and
possibly Pacific-origin Banda Sea Water (e.g.,
1908
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
Warren, 1981b) and the influence of high-salinity
northwest Indian-origin Red Sea water (RSW)
(e.g., Beal et al., 2000).
McCartney (1977) hypothesized that the cold
fresh SAMW in the southeast Pacific is AAIW,
which subsequently flows eastward throughout the
Southern Ocean in the Antarctic Circumpolar
Current. The circumpolar evolution of AAIW
depends upon regional processes. For example,
Park et al. (1993) showed that cross-frontal
exchange north of the Kerguelen Plateau in the
Crozet Basin leads to the modification and
formation of mode and intermediate waters. This
low-salinity water mass enters the southern Indian
Ocean midbasin (50–65 E) with the most recently
ventilated AAIW injected in the western portion of
the basin. It subsequently participates in a
relatively tight anticyclonic gyre lying completely
west of 72 E (Fine, 1993). A salinity minimum
exists on all of our sections in the depth span 800–
1200 m (Fig. 3) between gn values 27.2 and
27.4 kg m3 (Fig. 14). The strongest AAIW signal
(low salinity B34:34; high dissolved oxygen
B200 mmol kg1) is found in the southeast corner
of our survey area. North and west of this region,
salinity and nutrient concentrations increase and
dissolved-oxygen concentrations decrease.
The high-salinity, low-oxygen core of RSW
spreads primarily along the western boundary of
the Indian Ocean through the Mozambique
Channel to be incorporated into the Agulhas
Current (Wyrtki, 1971; Beal et al., 2000). These
latter authors showed that the potential density
interval of RSW influence is between 27.0 and
27.7 kg m3 (equivalently gn from 27.1 to
27.8 kg m3). Southward spreading of RSW along
the western boundary of the Mozambique Channel is evident from the elevated salinity and
depressed dissolved-oxygen concentrations found
there (Figs. 13 and 14). Similar to the SAMW
layer, mid-channel intermediate depth water properties have relatively fresh, cold, and low dissolved-oxgyen concentrations indicative of AAIW
influence. Along the inshore edge of the Agulhas
Current, salinities greater than 34.6 and dissolvedoxygen concentrations less than 160 mmol kg1
between 27.2 and 27.7 kg m3 are characteristic
of RSW (Fig. 14). Donohue et al. (2000) showed
that while the Agulhas Undercurrent carries water
with RSW influence northward, the bulk of this
water mass is flowing southward, away from its
source, in the Agulhas Current. Grundlingh
.
(1985)
noted that filaments and lenses of RSW are
observed along 28 S as far east as the Mozambique Plateau. An apparent isolated chunk of RSW
was sampled within the Mozambique Basin and
appears along I5W near 37.5 E.
The dissolved-oxygen-minimum layer and middepth nutrient maxima are the product of in situ
consumptive nutrient regeneration (Warren,
1981a). In our study region, low dissolved-oxygen
and high nutrient concentrations mark waters of
northern influence (Wyrtki, 1971; Toole and
Warren, 1993). We see the oxygen minimum/
nutrient maximum centered near gn ¼ 27:6 kg m3
and depths between 1200 and 1800 m (Figs. 4 and
14). Lowest dissolved-oxygen concentration
(o140 mmol kg1) and highest nutrient concentrations (nitrate >34, phosphate >2.4 mmol kg1)
are found along the northern sections (I3W and
I4). Concentrations in the dissolved-oxygen-minimum layer increase southward along I7C with the
highest dissolved-oxygen concentrations observed
in the southeast corner of our survey. As
mentioned previously, there is a clear northern
influence within the Agulhas upper-ocean waters.
This is also true for the oxygen-minimum layer
where depressed dissolved-oxygen and elevated
nutrient concentrations within the Agulhas signal
northern contributions.
Within the oxygen-minimum layer, the lowest
dissolved-oxygen and highest nutrient and salinity
concentrations in our study region are found
along the eastern side of the Mozambique
Channel section (east of 42 E) where the crosstrack LADCP velocities are southward (Figs. 10
and 14).
Water properties in the oxygen-minimum layer
and the LADCP further support our claim that the
northward flow adjacent to the East Madagascar
current is not a retroflection. Near 1000 m depth, a
core of low oxygen (o150 mmol kg1), high
nutrient (e.g., phosphate >2.4 mol kg1) and
salinity (>35.5) is found within the East Madagascar current, but not offshore. Additionally, the
LADCP shows that the southward flow of the East
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
n
1909
-3
γ =27.40 kg m
I7C
I3W I4East
I4
611
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
548
562
557
555
553
551
549
584
578
576
610
604
600
598
596
594
592
590
588
I5W
6
potential temperature
[°C]
5.5
5
salinity
34.6
34.5
34.4
-1
[µmol kg ]
200
180
160
oxygen
[µmol kg-1 ]
140
55
silicate
50
45
40
35
[µmol kg-1 ]
32
nitrate
31
30
35˚E
40˚E
45˚E
0
salinity anomaly 800 m contour
21oS
33˚S
500
1000
distance [km]
29˚S
25˚S
1500
50˚E
54˚E 49˚E
38˚E
42˚E
21˚S
o
24 S
27oS
o
30 S
o
33 S
30oE 35oE 40oE 45oE 50oE 55oE
-4.0 -3.0 -2.0 -1.0 -0.0 -1.0 -2.0 -3.0 -4.0
Fig. 13. Properties on gn surface 27.4 kg m3, which approximately corresponds to the AAIW salinity minimum. Salinity anomaly
(lower left corner) relative to the mean salinity on the 27.4 kg m3 surface has been normalized by its standard deviation.
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1910
I5W
I7C
I3W
I4East
I4
611
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
548
562
557
555
553
551
549
584
578
576
610
604
600
598
596
594
592
590
588
(a) salinity
26.9
34.8
34.8
.5
34
.6
34.6
34.7
34 76
(b) dissolved oxygen [µmol kg-1]
210
-3
3
200
27.5
0
17
190
180
4
0
27.7
140
27.3
layer
220
15
180
5
13.5
10
7.5
27.3
7.5
5
27.7
5
10
8.3
6.6
5
7.5
7.5
7.5
7.
10
7.5
10
7.5
7.5
10
4.6
5
3.7
5
5
12.0
10.2
7.5
10
27.1
10
7.5
10
26.9
7.5
7.5
7.5
-3
26.7
2.7
27.9
35˚E
40˚E
45˚E
0
33˚S
500
1000
distance [km]
29˚S
25˚S
1500
50˚E
54˚E 49˚E
38˚E
potential temperature [˚C]
(ms)-1 ]
16
0
7.5
(c) potential vorticity [-10
10
10
170
7.5
n
2
210
27.1
-11
n
4
5
34.7
0
22
27.9
γ neutral density [kg m ]
.6
34
34.7
26.9
27.5
3
34.5
34.6
26.7
26.5
34.8
34.76
34.7
34
34.5
6
26.5
2
34.6
34.5
27.3
27.7
.2
35
34.5
27.5
35
35.2
35.2
35
34.8
27.1
27.9
γ neutral density [kg m ]
35.4
34.6
34.
n
-3
γ neutral density [kg m ]
26.7
35.2
35
layer
5.4
26.5
42˚E
21˚S
Fig. 14. Upper-ocean properties contoured with neutral density, gn as the vertical ordinate. (a) Salinity, (b) dissolved oxygen, (c)
potential vorticity.
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
Madagascar Current extends deeper in the water
column than the northward flow.
3.3. Deep waters
Deep water (gn range 27.96–28.11 kg m3) enters
the three main basins of the southwestern Indian
Ocean basins (Natal Valley, Mozambique and
Madagascar Basins) via deep western-boundary
currents (Toole and Warren, 1993). Within each
basin, the general sense of circulation is cyclonic;
deep water with strong influence of North Atlantic
Deep Water (NADW) or Circumpolar Deep
Water (CDW) moves northward in concentrated
boundary currents, while broad southward return
flows carry North Indian Deep Water (NIDW)
and recirculating NADW and CDW (Toole and
Warren, 1993; Mantyla and Reid, 1995; McCarthy
and Talley, 1999). All of the deep water masses in
this region are characterized by a salinity maximum; NADW and CDW are best differentiated
from NIDW by their higher oxygen and lower
nutrient concentrations (Wyrtki, 1971; Warren,
1981b). The deep salinity maximum is found
between gn 28.0 and 28.1 kg m3 and depths 2200
and 3200 m (Figs. 3 and 16).
The South Atlantic Current and continentalmargin flows introduce NADW from the midlatitude South Atlantic first into the Natal Valley
and then to the Mozambique Basin (Toole and
Warren, 1993). Properties show pronounced differences east and west of the Madagascar Ridge
(Figs. 15 and 16). In contrast to the Madagascar
and Mascarne Basins, the Natal Valley and the
Mozambique Basin deep water have a deep silicate
minimum ascribable to NADW. The salinity
maximum and nutrient minimum are slightly more
pronounced in the Natal Valley as compared to
the Mozambique Basin, presumably because the
Natal Valley is closer to the NADW source.
Dissolved-oxygen concentrations decrease to the
north in the Mozambique Basin, possibly due to
the lower dissolved-oxygen concentrations in the
oxygen-minimum layer above the NADW in the
northern Mozambique Basin. Enhanced property
extrema in the western portions of the Natal
Valley and Mozambique Basin suggest northward
flow there.
1911
A deep anticyclonic eddy appears in the
Mozambique Basin near 41.4 E, station 645 near
3000 m depth (Fig. 8). A weaker NADW signal is
present at eddy center; salinity and dissolved
oxygen are reduced and silicate and nutrients are
enhanced relative to the surrounding stations in
the basin (Figs. 15 and 16), suggesting a northern
formation site.
The Madagascar Ridge with sill depth of
B3000 m north of the I5W section blocks deeper
water from entering the Madagascar Basin; however, small throughflows do occur at lesser depths
(Toole and Warren, 1993). Evidence of one
Madagascar Ridge throughflow appears at stations 661–663, which have high dissolved oxygen
(>200 mmol kg1) and salinity (>34.76) and low
nutrient (e.g., silicate o75 mmol kg1) for y > 2 C
(Fig. 16).
Circumpolar deep water, yo2:0 C (Toole and
Warren, 1993), gains entry to the Madagascar
Basin via several fracture zones within the Southwest Indian Ridge (Warren, 1978; Toole and
Warren, 1993) and eventually makes its way to
the western edge of the basin. Along I5W on the
eastern flank of the Madagascar Ridge, relatively
high dissolved-oxygen and low nutrient concentrations within the lower deep layer signal the
presence of CDW that has moved north from the
Crozet Basin (Toole and Warren, 1993) (Fig. 16).
The continuation of a deep water western-boundary current to 25 S is evident from the ribbon of
relatively high dissolved-oxygen and low nutrient
concentrations hugging the Madagascar coast
along I4East (Fig. 16). Offshore, the deep silicate
maxima (>125 mmol kg1) centered near neutral
density 28.1 kg m3 indicate the presence of lowlatitude-origin NIDW (Fig. 16). Further northward, penetration of deep waters appears to be
limited to the lightest portions; a sliver of relatively
high oxygen concentration (>160 mmol kg1) is
found adjacent to the Madagascar Coast on I3W
centered between gn 27.9 and 27.96 kg m3.
NIDW, indicated by its nutrient maxima, is
observed across I3W within the deep layers
(Figs. 5–7). The most extreme values of the NIDW
deep oxygen minimum are found just offshore
Madagascar (o160 mmol kg1) and can only have
arrived here from the north (Fig. 16). Rather than
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1912
n
-3
γ =28.05 kg m
I7C
I3W I4East
I4
611
621
624
626
628
631
636
640
641
643
645
647
650
655
658
660
662
664
666
668
669
671
673
675
677
679
681
684
687
690
693
696
699
701
703
548
562
557
555
553
551
549
584
578
576
610
604
600
598
596
594
592
590
588
I5W
2.1
[°C]
2
1.9
1.8
potential temperature
1.7
34.82
34.8
34.78
34.76
salinity
34.74
[µmol kg-1 ]
220
200
180
oxygen
[µmol kg-1 ]
160
120
100
80
[µmol kg ]
34
-1
silicate
nitrate
32
30
28
26
35˚E
40˚E
45˚E
0
oxygen anomaly 2500 m contour
21oS
33˚S
500
1000
distance [km]
29˚S
25˚S
1500
50˚E
54˚E 49˚E
38˚E
42˚E
21˚S
o
24 S
27oS
o
30 S
o
33 S
30oE 35oE 40oE 45oE 50oE 55oE
-4.0 -3.0 -2.0 -1.0 -0.0 -1.0 -2.0 -3.0 -4.0
Fig. 15. Same as Fig. 13, but for properties on gn surface 28.05 kg m3, roughly equivalent to the deep salinity maximum.
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
Fig. 16. Deep- and bottom-water properties contoured with neutral density, gn as the vertical ordinate.
1913
1914
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
a smooth northward progression from highlatitude origin CDW to low-latitude origin NIDW
along I7C, the transition is punctuated by several
eddies that appear as local anomalies with NIDW
core characteristics (relatively low dissolved-oxygen and high nutrient concentrations). They are
readily seen in the dissolved-oxygen field, notably
at stations 671, 682, 686, and 691 (Fig. 4).
3.4. Bottom waters
Bottom waters in the Southwest Indian Ocean
derive from the Weddell–Enderby Basin (Toole
and Warren, 1993). The Natal Valley and Mozambique Basin present dead ends to the northward passage of bottom water due to bathymetric
features that rise to about 3000 m depth, which
close the basins along all but their southern
reaches. Any net inflow of bottom water into
these cul de sacs must mix and upwell to escape.
Two routes appear to bring bottom water to the
Mozambique Basin: the primary path carries
water from the Weddell–Enderby Basin by way
of the Agulhas Basin before reaching the Mozambique Basin, while the direct path is via the Prince
Edward Fracture Zone (Read and Pollard, 1999).
The bulk of the bottom-water transport occurs
within the primary path. Although both routes
share a common source, strong mixing with
overlying NADW within the Prince Edward
Fracture Zone apparently elevates the salinity of
bottom water taking the fracture zone path (Read
and Pollard, 1999). These authors show that
salinity increases by 0.003 across the Mozambique
Basin on the s4 ¼ 46:00 kg m3 density surface for
a section stretching diagonally across the basin
from 35 E–36.5 S to 37.6 E–40.3 S (south of
I5W). Here we show salinity on the
gn ¼ 28:23 kg m3 surface (Bs4 ¼ 46:00 kg m3
and B4200 m depth) for the 1987 Darwin and
1995 I5W stations in the Mozambique Basin
(Fig. 18). As found by Read and Pollard (1999),
the salinity range for each section is quite small,
B0:002: Interestingly, the Darwin section has a
west-to-east increase in salinity seen by Read and
Pollard (1999) further south, but the I5W section
has maximum salinity mid-basin with minima at
the basin edges. Salinity on slightly lighter and
denser surfaces show the same pattern; so this
result is not the consequence of a calibration offset
feeding into the density calculation. Even more
interesting is the fact that the section described by
Read and Pollard (1999) was conducted during
January–February 1995—only months before the
I5W section. This may indicate variability in
bottom water-mass characteristics and circulation.
Perhaps deep-reaching eddies and meanders,
possibly those associated with the Agulhas Return
Current, are responsible for the variability. The
Mozambique Basin warrants further investigation.
Major inflow of bottom water into the Madagascar Basin from the Crozet Basin occurs through
the Atlantis II Fracture Zone and to a lesser extent
through the Melville and Indomed Fracture Zones
(Warren, 1978; Toole and Warren, 1993) (Fig. 1).
Two pathways appear plausible for bottom-water
entry into the Madagascar Basin and then towards
20 S: zonal flow across the basin and then north
along the western boundary (Warren, 1981b) or a
diagonal flow northwest across the basin (Swallow
and Pollard, 1988). The coldest bottom potential
temperature along I7C in the Madagascar Basin is
0.51 C at 27 S (station 687). Bottom water colder
than y ¼ 0:6 C with silicate concentrations in
excess of 125 mmol kg1 are found in two regions
along I7C; between 31.8 S and 30.2 S and
between 28.7 S and 25.3 S (Fig. 16). This bottom
water can be traced farther north: Along I4East
(25 S),
cold
(yo0:6 C)
high
silicate
(>125 mmol kg1) bottom water is found at the
two easternmost stations (east of 49 E); farther
north along I3W bottom temperatures are slightly
warmer, bottom water with yo0:61 C, silicate
>124 mmol kg1 exists between 50.6 E and
52.3 E. These two proposed flow pathways are
discussed further in Section 5.3.1.
4. Geostrophic transport calculations
We quantify the synoptic circulation in the
Southwest Indian Ocean by constructing a referenced geostrophic velocity field. We partition
geostrophic transports in the vertical using the
neutral-density-based layer definitions found in
Robbins and Toole (1997) and reproduced in
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
Table 1
Neutral density, gn surfaces (Jackett and McDougall, 1997)
taken to define layers used in the transport analysis (after
Robbins and Toole, 1997)
Layer number and name
1 Surface water
2 Subantarctic mode
water
3 Upper Antarctic
intermediate water
4 Lower Antarctic
intermediate water
5 Upper deep water
6 Lower deep water
7 Upper bottom water
8 Lower bottom water
gn (kg m3)
s (kg m3)
Ocean surface
26.50
26.90
Ocean surface
s0 =26.45
s0 =26.80
18 oS
22 oS
Layers 1 through 5
γn < 27.96 kg m-3
22
26 oS
s0 =27.20
27.70
s0 =36.65
27.96
28.11
28.23
Sea floor
s0 =36.92
s0 =45.89
s0 =46.01
Sea floor
Approximate potential density values, s (referenced to the
surface, 2000 and 4000 dbar), corresponding to layer boundaries are also given.
2
17
18
20
30 oS
18
2
o
34 S
27.36
1915
8
o
38 S 76
25oE 31oE 37oE 43oE 49oE 55oE 61oE
o
18 S
o
Layer 6
-3
n
27.96 < γ < 28.11 kg m
5
2
22 S
0
2
26 oS
0
30 oS
1
1
1
1
o
34 S
Table 1. Bottom triangles are filled by uniformly
extrapolating the velocity at the deepest common
level. Fig. 17 shows integrated volume transport
schematics for selected groups of layers.
Water-mass distributions and mass conservation
within bounded regions guide the placement of the
level of no motion. The water-property distributions along our sections have been presented in the
previous section. In general, a deep level of no
motion is preferred over a shallow one. Placing the
level of no motion below the pycnocline in a region
of weak vertical geostrophic shear minimizes
transport sensitivity to variations in the level of
no motion. As these sections are nearly synoptic,
mass should be conserved in closed boxes within
some reasonable error. In addition to the main
box, I5W-I7C-I3W-I4, described in Section 2, two
other sub-boxes are available: I5W-I7C-I4East
and I3W-I4East-I7C. (Note, however, the large
gap of 453 km between the eastern edge of I4East
and I7C.) Based on our referencing scheme, the
derived imbalance of net geostrophic full-depth
transport for each box is less than 6 106 m3 s1
(Table 2). The Ekman flux convergence into the
subtropical region, determined using the Hellerman and Rosenstein (1983) wind-stress climatology for June/July, is small, 2.5 106 m3 s1 for the
o
38 S
25oE 31oE 37oE 43oE 49oE 55oE 61oE
o
18 S
22 oS
Layers 7 through 8
o
34 S
2
1
o
26 S
30 oS
3
-3
γn > 28.11 kg m
0
0
2
1
2
1
38 oS
o
o
o
o
o
o
o
25 E 31 E 37 E 43 E 49 E 55 E 61 E
Fig. 17. Integrated volume transport schematics in sverdrups
(1 106 m3 s1) for selected groups of layers. The shading
indicates water shallower than 2000 m (middle panel) and
3000 m (bottom panel).
large box (Table 2). Bathymetric constraints define
additional regions where mass should be conserved. Following Ganachaud et al. (2001), net
mass transport should be small beneath
gn ¼ 28:07 kg m3 (2900 m) in the Mozambique
Channel (Fig. 18), and beneath gn ¼ 28:11 kg m3
(3400 m) in the Natal Valley, Mozambique, and
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1916
Table 2
Net volume transports in sverdrups (1 106 m3 s1) for each
circulation layer for the three available boxes in our study
region
Layer
Water mass
I3W, I4,
I5W, I7C
I4,
I4East,
I5W, I7C
1
1
2
Ekman
Surface water
Subantarctic
mode water
Upper Antarctic
intermediate
water
Lower Antarctic
intermediate
water
Upper deep
water
Lower deep
water
Upper bottom
water
Lower bottom
water
Total
2.5
8.2
3.2
2.1
8.8
3.4
0.4
0.7
0.2
0.5
1.4
1.9
3.2
1.8
1.4
1.9
1.8
0.2
0.9
1.3
0.4
0.9
0.9
0.0
0.8
0.8
0.0
0.6
5.1
4.4
3
4
5
6
7
8
I3W,
I4East,
I7C
Positive transports are directed into the boxes.
γn=28.23 kg m-3
Darwin
I5W
34.719
salinity
34.718
34.717
34.716
34.715
38˚E 40˚E 42˚E
38˚E 40˚E 42˚E
Fig. 18. Salinity on the g surface 28.23 kg m3 for the 1995
I5W (left) and 1987 Darwin (right) sections in the Mozambique
Basin.
n
Madagascar Basins. Based on our referencing
scheme, net transports in these bathymetrically
constrained areas are less than 1 106 m3 s1
except for deep flow within the Mozambique
Basin where net transport below gn ¼ 28:11 kg m3
m3 is 1.6 106 m3 s1 (0.6 106 m3 s1 of which is
contained within the bottom triangles).
What is a reasonable error for the net transports
within the boxes? Non-zero net transports could
result from errors in the reference scheme and
from synoptic-scale circulation features such as
eddies and internal waves that are aliased by the
hydrographic sampling. Using the output from an
ocean general circulation model (Semtner and
Chervin, 1992; Stammer et al., 1996), Ganachaud
(1999) investigated the difference between transAtlantic transports determined using the
circulation field measured during a simulated
hydrographic cruise compared to those calculated
from the instantaneous circulation field. He concluded that the combination of synoptic-scale
variability and finite sampling time leads to errors
in full-depth trans-ocean transports of around
4 106 m3 s1 rms. Ganachaud (1999) also estimated uncertainties due to internal wave heaving
from the Garrett and Munk (1972) spectrum and
showed that the rms noise in total transport
between any two stations in a water depth of
4000 m is 2 106 m3 s1 at 30 latitude. We
estimate the variability in transport due to internal
wave activity in a manner similar to Fieux et al.
(1996), who calculated transport differences between repeat casts taken a few hours apart in
the Indonesian Throughflow south of Java.
They found transport fluctuations of about
9 106 m3 s1, which is similar to the 10 106 m3 s1 predicted by Ganachaud et al. (2001)
using the Garrett and Munk (1972) spectrum for
the appropriate latitude (10 ) and ocean depth.
Here we evaluate the differences between the up
and down hydrographic cast traces (Firing et al.,
1998). Specifically, for each station pair we
calculated the bottom-referenced geostrophic
transport using the down/down, up/up, up/down,
and down/up traces. Note that the time separation
between up and down casts was always less than
6 h; so we fail to sample the full range of the
variability since a 6 h time separation can sample
the minimum or maximum difference in isopycnal
displacements due to the semi-diurnal internal
tide but only quarter-period diurnal tide displacements.
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
602
607
597
592
587
560
555
550
702
697
692
687
682
using only stations with a duration exceeding 2 h,
with one standard deviation is 4.2 (2.9) cm s1 for
velocity and 1.2 (1.9) 106 m3 s1 for transport. In
addition to the effect of internal wave activity,
677
672
667
662
647
652
657
642
631
636
626
611
616
621
The maximum full-depth transport and velocity
(0–1000 dbar average) differences of the four trace
combinations for each of our station pairs are
shown in Fig. 19. The overall mean, calculated
1917
-1
velocity [cm s ] velocity [cm s-1 ]
transport [sv]
6
(a)
4
2
0
20
(b)
10
0
10
(c)
0
distance [km]
10
u
v
6
(d)
4
2
(e)
2
4000
I5W
I7C
I3W
I4
Madagascar
2000
(f)
Mad. Ridge
depth [m]
0
0
Moz. Plateau
time [hours]
0
4
Mozambique
Channel
Natal Valley
Mozambique Basin
6000
500
34˚E
1000
38˚E
42˚E
1500
46˚E
Madagascar Basin
2000
50˚E
2500 3000
distance [km]
29˚S
25˚S
Mascarene Basin
3500
21˚S 53˚E
4000
49˚E
4500
5000
43˚E
39˚E
Fig. 19. (a) Maximum difference between the full-depth, bottom-referenced transports in sverdrups (1 10 m3 s1) using up/up,
down/down, up/down, and down/up hydrographic traces, (b) same as (a) but for velocity averaged between the surface and 1000 dbar,
(c) predicted barotropic tide from the OSU TOPEX/Poseidon Cross-over Global Inverse Solution, Version 3.1 (Egbert et al., 1994)
zonal (meridional) velocity is shown with thin (thick) lines, (d) horizontal distance between the start of the down trace and the end of
the up trace for each hydrographic station, (e) duration of each hydrographic station, and (f) bathymetry.
6
1918
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
traces might be different if the ship moved while
on station in such a manner that the down- and
up-casts were taken in different physical environments, for example near the boundary of a sharp
front. This may account for the larger velocity and
transport differences within the Agulhas Current.
Relatively large velocity and transport differences
along I3W and I4 cannot be accounted for by large
on-station ship displacements. The amplitude of
the barotropic tide is quite large, near 4 cm s1;
within the Mozambique Channel, large transport
and velocity differences along I4 are presumably
due to elevated baroclinic tidal activity.
Our net box transports balance to just under
6 106 m3 s1. Given the errors introduced by
synoptic variability (B4 106 m3 s1), internal
tides (B2 106 m3 s1), and the subjective nature
of the level of no motion-referencing strategy, we
conclude that our circulation field balances mass
within the estimated uncertainty.
Previous studies have referenced the geostrophic
shears with the LADCP and SADCP velocities
(e.g., Donohue et al., 1999; Beal and Bryden,
1999), but full-depth transport estimates require
excellent velocity accuracy, especially for long
sections. A bias of only 1 cm s1 along the I5WI7C-I3W-I4 sections, for example, produces a
90 106 m3 s1 error in total transport. Despite
this sensitivity, we performed the exercise of
referencing geostrophic velocities within the I5WI7C-I3W-I4 box to the ADCP data (Fig. 20). The
geostrophic velocity is referenced to the ADCP
data by applying a depth-independent adjustment
to the integrated geostrophic shear to match the
depth-averaged geostrophic and ADCP-derived
cross-track velocities. For the SADCP reference,
in the horizontal we average between stations, and
in the vertical we average over the thickest layer
for which the SADCP velocities are consistently
available, but avoiding the surface layer where
near-inertial energy is usually highest. In general,
we average from 150 to 400 m, except in a few
cases where the SADCP range did not extend
this deep. For the LADCP reference, we
average the two adjacent stations together; in
the vertical we average from 75 m to the deepest
common level between the two stations. The
SADCP reference produced a net mass conver-
gence of 88 106 m3 s1 into the box, while the
LADCP reference resulted in a divergence of
40 106 m3 s1 out of the box.
To achieve a circulation that more closely
conserved mass, we adjusted these by subtracting
0.99 cm s1 from each SADCP reference velocity
and adding 0.45 cm s1 to each LADCP reference
value so that the total flow into our box was
conserved to within 3 106 m3 s1 for both ADCP
references. These adjustments are within our
uncertainty for the directly measured flow. Errors
due to ageostrophic horizontal currents such as
internal tides and near-inertial oscillations have
not been assessed nor have errors in the LADCP
reference that arise from inadequate station
sampling (i.e., when the average velocity from
two stations in a pair is not the same as the true
horizontal average between the two stations.) The
results are encouraging. Our three circulation
estimates (referenced with SADCP, LADCP, and
a deep level of no motion) resemble each other,
suggesting that our choice of deep reference levels
is appropriate throughout much of the study area
(Figs. 20 and 21).
5. Derived circulation
5.1. Natal Valley
In the Natal Valley, the strong Agulhas Current
flows poleward with surface speeds in excess of
100 cm s1 (Figs. 9 and 10). Beneath the core of the
Agulhas Current and banked against the continental slope, the Agulhas Undercurrent flows
equatorward with speeds near 30–40 cm s1 (e.g.,
Fig. 2 in Donohue et al., 2000). Using the I5W
LADCP data, Donohue et al. (2000) determined
the net, full-depth southward transport of the
combined Agulhas Current and Undercurrent
system to be 7672 106 m3 s1. In Donohue
et al. (2000) and this study, we take the first
maximum in the barotropic transport streamfunction, 31.5 E, as defining the eastern edge of the
Agulhas Current. Here we apply a level of no
motion that achieves comparable Agulhas transport (Fig. 8). A level of no motion set at the
deepest common level for the first two stations
602
607
597
592
587
560
555
550
1919
LNM
SADCP
LADCP
Layers 1-4, γn < 27.70 kg m-3
-50
transport [sv]
5
0
-5
Layer 5, 27.70 < γn < 27.96 kg m-3
10
5
0
-5
n
-3
Layer 6 27.96 < γ < 28.11 kg m
10
5
0
-5
2000
4000
6000
I5W
Mad. Ridge
0
Layers 7-8 γn > 28.11 kg m-3
Moz. Plateau
transport [sv]
transport [sv]
-10
Natal Valley
Mozambique Basin
500
34˚E
1000
38˚E
42˚E
1500
46˚E
I7C
Madagascar Basin
2000
50˚E
2500 3000
distance [km]
29˚S
25˚S
6
I3W
Madagascar
transport [sv]
0
-100
depth [m]
702
697
692
687
682
677
672
667
662
647
652
657
642
631
636
626
611
616
621
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
I4
Mozambique
Mascarene Basin Channel
3500
21˚S 53˚E
4000
49˚E
4500
43˚E
5000
39˚E
3 1
Fig. 20. Cumulative geostrophic volume transport in sverdrups (1 10 m s ) integrated counterclockwise around the periphery of
the I5W-I7C-I3W-I4 box. Positive transport is into the box and to the left of track. Uppermost panel: LADCP- and SADCPreferenced transports for the upper four layers are shown with grey and dashed lines, respectively. Bottom panel: bottom depth in
meters around the box.
allows the inshore Agulhas to extend to the
bottom. A variable level of no motion that
descends from 500 dbar for station pair 613–614
through 1600 dbar for station pair 616–617 results
in an equatorward undercurrent. LADCP velo-
cities indicate that the the Agulhas extends to
about 2500 m depth; thus a 2500 dbar level of no
motion is applied across the remainder of this
basin. The resulting net full-depth southward
Agulhas transport is 74.7 106 m3 s1 (Figs. 20
561
556
551
703
693
584
579
40
20
0
-20
-40
-60 Layers 1-4, γn < 27.70 kg m-3
transport [sv]
transport [sv]
698
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1920
5
0
-5
transport [sv]
-10
Layer 5, 27.70 < γn < 27.96 kg m-3
5
0
-5
n
-3
-10 Layer 6 27.96 < γ < 28.11 kg m
transport [sv]
10
5
0
-5
Layers 7-8 γn > 28.11 kg m-3
0
depth [m]
I4East
I7C
I3W
2000
4000
500
1000 1500
distance [km]
51˚E
21˚S 53˚E
49˚E
Fig. 21. Same as Fig. 20 but for the I4East-I7-I3 box. Positive
is into the box and to the left of the track.
and 22). This reference scheme produces a small
net transport of 0.1 106 m3 s1 southward beneath gn ¼ 28:11 kg m3 between the African coast
and the Mozambique Plateau, thus satisfying our
bathymetric constraint (Fig. 20, Table 3).
Offshore of the Agulhas, there is a weak surfaceintensified eddy (Fig. 10). Farther east, there is
general tendency for upper-ocean isoneutral surfaces to rise eastward indicating a weak northward
baroclinic flow. By the longitude of the Mozambique Plateau (35.4 E), this interior recirculation
decreases the full-depth poleward transport on the
I5W section to 68.4 106 m3 s1 (Figs. 20 and 22).
Net deep-water transport over this longitude range
beneath gn > 27:96 kg m3 is 1.2 106 m3 s1 equatorward (Fig. 20), slightly less than estimates
from the Darwin 1987 section (Toole and Warren,
1993: 1.9 106 m3 s1; Robbins and Toole, 1997:
2.8 106 m3 s1; Ganachaud et al., 2001: 3.073 106 m3 s1).
Our poleward full-depth Agulhas transport
estimate of 75 106 m3 s1 is in agreement with
previous calculations near 33 S. The magnitude of
the Agulhas transport, however, is sensitive to the
reference scheme, as is reflected in reported
estimates of the top-to-bottom poleward Agulhas
Current export based on the 1987 Darwin section
which ranges between 48 and 93 106 m3 s1
(Toole and Warren, 1993: 85 106 m3 s1; Robbins and Toole, 1997: 90 106 m3 s1; Macdonald,
1998: 93 106 m3 s1; Sloyan and Rintoul, 2001:
48 106 m3 s1; and Ganachaud et al., 2001:
71 106 m3 s1). Inspection of the Darwin and
I5W basin-averaged velocity profiles referenced to
the bottom show remarkable similarity (Fig. 23)
and full-depth bottom-referenced transports between 30.54 E and 34.17 E differ from one
another by only 1 106 m3 s1. Donohue et al.
(2000) or more recently Bryden and Beal (2001)
give a more complete discussion of the impact of
the reference scheme on the Agulhas Current and
Undercurrent transport calculation. More importantly, our estimate falls within one standard
deviation of the mean Agulhas transport determined from a current-meter array; its 267 day time
series yielded a mean transport of 69.7 106 m3 s1
with a standard deviation of 21.4 106 m3 s1
(Bryden and Beal, 2001).
5.2. Mozambique Basin (I5W)
Except for possible spillovers from the Natal
Valley, no northern sources exist for the NADW
602
607
1921
597
592
587
560
555
550
702
697
692
687
682
677
672
667
662
647
652
657
642
631
636
626
611
616
621
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
20
0
n
-3
0
1000
-20
2000
-40
3000
-60
4000
-80
5000
-100
Layers 1-5, γn < 27.96 kg m-3
500
34˚E
1000
38˚E
42˚E
1500
46˚E
depth [m]
transport [sv]
Layers 6-8, γ > 27.96 kg m
6000
2000
2500 3000
distance [km]
50˚E
29˚S
25˚S
3500
21˚S 53˚E
4000
4500
49˚E 43˚E
5000
39˚E
3
Fig. 22. Cumulative geostrophic volume transport for upper ocean (layers 1–5, gn o27.96 kg m , thick) and deep ocean (layers 6–8,
gn >27.96 kg m3, thin) in sverdrups (1 106 m3 s1).
Table 3
Absolute value of the derived net volume transports for regions where bathymetry indicates that these transports should be small
Region
Upper gn
(kg m3)
Upper depth
(m)
Full transport
(1 106 m3 s1)
Triangle transport
(1 106 m3 s1)
Mozambique Channel (I4)
Natal Valley (I5W)
Mozambique Basin (I5W)
Madagascar, Mascarene Basins (I5W, I7C, I3W)
Madagascar Basin (I5W, I7C, I4East)
28.07
28.11
28.11
28.11
28.11
2900
3400
3400
3400
3300
0.8
0.1
1.6
0.1
0.1
0.1
0.0
0.6
0.1
0.3
Transports within bottom triangles are included and reported separately. Approximate depths corresponding to the gn interfaces are
provided.
found along the flank of the Mozambique Plateau,
and bottom waters in the Mozambique Basin are
topographically blocked to the north. LADCP
cross-track velocities support the inference that
lower deep and bottom waters flow north along
the eastern flank of the Plateau (Fig. 10). We
therefore placed the level of no motion as
detailed below to both achieve a consistent flow
pattern and minimize net bottom transport
(gn >28.11 kg m3). For station pairs over the
Mozambique Plateau and eastward to and
including station pair 638–639, the level of no
motion was set at the transition between the
oxygen minimum and the lower deep water,
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
1922
20
Layers 6 to 8
transport [sv]
0
Toole and Warren 1993 (Darwin 1987)
Robbins and Toole 1997 (Darwin 1987)
Ganachaud et al. 2001 (Darwin 1987)
this study (I5 1995)
-20
-40
-60
-80
depth [m]
0
2000
34˚E
Darwin 1987
I5 1995
4000
6000
Natal Valley
32˚E
34˚E
36˚E
38˚E
36˚E
40˚E
42˚E
Mozambique Basin
38˚E
40˚E
42˚E
44˚E
Madagascar
Ridge
Layers 1 to 5
32˚E
Mozambique
Plateau
-100
44˚E
basin geostrophic velocity referenced to deepest common level
0
1000
depth [m]
2000
3000
4000
Natal Valley
5000
-20
Mozambique Basin
basin transport (sv)
Darwin 1987 24
I5 1995
-5
basin transport (sv)
Darwin 1987 -68
I5 1995
-69
-15
-10
-5
velocity [cm s-1]
0 -5
0
5
10
velocity [cm s-1]
15
Fig. 23. Top panel: cumulative geostrophic volume transport for upper ocean (layers 1–5, gn o27.96 kg m3) and deep ocean (layers 6–
8, gn >27.96 kg m3) along 33 S in sverdrups (1 106 m3 s1). Positive is to the north (Toole and Warren, 1993: blue, Robbins and
Toole, 1997: green, Ganachaud et al., 2001: magenta, and I5W). Middle panel: bathymetry in meters. Blue (red) dots correspond to
Darwin 1987 (Knorr I5W 1995) stations used to construct the velocities shown in the bottom panels. Bottom panel: basin averaged
bottom-referenced geostrophic velocities for the Natal Valley (left) and Mozambique Basin (right) using the station pairs shown in the
middle panel. Darwin 1987 is blue; Knorr I5W 1995 is red.
gn =27.96 kg m3. Between the deep westernboundary current and the Madagascar Ridge,
except for an interruption near 41 E (station
646) due to a deep eddy, water properties are
relatively uniform. Therefore, we opted for the
simplest reference scheme—a single level of no
motion. The deepest common level minimizes net
flow in the bottom water (gn >28.11 kg m3),
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
resulting in a small net northward transport of
1.6 106 m3 s1 in layers 7 and 8, which are totally
blocked to the north (Fig. 20). This is the largest
deep-water-mass convergence derived for our
collection of bathymetrically constrained areas
(Table 3). Almost 40% of this transport is in the
bottom triangle beneath station pairs 636–638 and
638–639. Dividing this net deep inflow by the basin
area north of the section below 3500 m
(6 1011 m2) produces a vertical velocity estimate
of 3 104 cm s1—an order of magnitude larger
than the 4.5 105 cm s1 basin-averaged upwelling values for Indian Ocean of Robbins and Toole
(1997), but comparable to the 7–13 105 cm s1
determined by Read and Pollard (1999) for the
Mozambique Basin. Given the uncertainties in our
transport estimates and the size of this basin, we
refrain from assigning physical meaning to this
inferred upwelling rate.
The reference scheme sends lower deep and
bottom water northward in a narrow region along
the flank of the Mozambique Plateau and creates
an adjacent recirculation region. Along the western boundary, 4.8 106 m3 s1 of lower deep and
bottom water flows north (Figs. 20 and 22).
The LADCP data show a reversal in the crosstrack velocity from northward to southward flow
between stations 639 and 640, suggesting that the
deep western-boundary current is narrowly confined along the plateau’s western flank (Fig. 10).
Coincident with this measured velocity transition,
the slopes of deep isoneutral surfaces also change
sign (Fig. 8). Between stations 640 and 644, deep
isoneutral surfaces (gn > 28:11 kg m3) descend to
the east and geostrophic shears decrease monotonically with depth; the bottom reference results
in southward flow here. This interior recirculation
brings 6.5 106 m3 s1 of lower deep and bottom
water southward (Figs. 20 and 22).
The deep eddy indicated by the large dip in
isoneutral surfaces at station 645 has very little
impact on the net deep transport; yet it is not a
trivial feature of the circulation (Fig. 8). Beneath
neutral density 27.96 kg m3, 6.3 106 m3 s1 flows
south at station pair 645–646 and 5.4 106 m3 s1
returns north in the next station pair (Figs. 20 and
22). We infer that the LADCP fails to reveal this
anticyclonic eddy because it is a point measure-
1923
ment and the station sampling did not resolve the
velocity structure (Fig. 10).
Additional deep- and bottom-water inflow
appears to occur along the eastern boundary of
the basin-counter to the traditional notion of deep
cyclonic circulation where narrow deep westernboundary currents are to some extent balanced by
weak flow across the interior of the basin. East of
the eddy, deep isoneutral surfaces mainly rise
(Fig. 8); the bottom reference results in northward
transport of 5.1 106 m3 s1 along the eastern
boundary in the lower deep and bottom layers
(Figs. 20 and 22). A similar density structure was
found in the 1987 Darwin section and a section
traversing the Mozambique Basin south of I5W
(Read and Pollard, 1999). Toole and Warren
(1993) and Read and Pollard (1999) suggested
that this deep northward flow along the eastern
boundary of the Madagascar Ridge occurs when
eastward flow associated with either the Agulhas
Return Current or the Antarctic Circumpolar
Current meet the Southwest Indian Ridge; these
deep-reaching currents may turn northward in
order to conserve potential vorticity.
In contrast to the Natal Valley, basin-averaged
relative velocity profiles from the Darwin and I5W
sections are quite different in the Mozambique
Basin (Fig. 23). As noted by Toole and Warren
(1993), the Darwin basin-averaged shear is monotonic, which indicates that no reference will yield
zero net deep mass transports. The I5W shear,
however, is weak and the slope of the shear does
change sign near 3000 m depth; the inability of our
reference scheme to perfectly conserve flow
beneath the bathymetric constraint (3000 m) is
due to northward flow in the bottom triangles at
the basin boundaries, particularly the western
boundary. Note that differences between the
shears sampled on the two cruises extend throughout the water column. The full-depth basin-wide
bottom-referenced transports differ by nearly
30 106 m3 s1.
Integrating the deep and bottom transports
(gn > 27:96 kg m3) from the African coast across
both the Natal Valley and Mozambique Basin
yields 2.2 106 m3 s1 northward. Again, our
estimates are slightly less than those determined
from the Darwin 1987 section (Fig. 23, Toole and
1924
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
Warren, 1993: 8.2 106 m3 s1; Robbins and
Toole, 1997: 9.4 106 m3 s1; Ganachaud et al.,
2001: 5.0 106 m3 s1).
In the upper ocean (gn o27:96 kg m3), a large
cyclonic eddy with magnitude of about
25 106 m3 s1 sits over the basin and is apparent
in an altimetry-derived sea-surface height anomaly
field (not shown). Cumulative transport in the
upper five layers (gn o27:96 kg m3) from the
African coast to the apex of the Madagascar
Ridge (43.8 E) is 84.4 106 m3 s1 southward
(Fig. 22).
5.3. Madagascar and Mascarene Basins (I5W,
I7C, I3W, I4EAST)
Our reference strategy for the Madagascar and
Mascarene Basins hinges on the assumption that
northern-origin NIDW and southern-origin bottom water move in opposite directions throughout
much of the basin. This translates into a level of no
motion between the NIDW and bottom water:
gn ¼ 28:14 kg m3. Our reference approximately
corresponds to a potential temperature of 1.06 C
and falls nearly in the middle of the range of
reference levels (1.0–1.1 C) used by Johnson et al.
(1998) for sections to the north in the Mascarene
Basin and Amirante Passage. As was the case on
I5W, at the western end of I4East (25 S) line, the
referencing was adjusted so that high-latitudeorigin deep and bottom water move northward;
Section 5.3.1 discusses the details of the level of no
motion shown in Fig. 8. Net transports beneath
gn ¼ 28:11 kg m3 within the bathymetrically constrained regions are less than 0.3 106 m3 s1
(Table 3).
5.3.1. Deep- and bottom-water circulation
The Madagascar Ridge effectively separates
deep waters in the Natal Valley and Mozambique
Basin from the Madagascar Basin; however, a
small amount of NADW appears to flow east
through cols in the Ridge and then north along its
eastern flank (Toole and Warren, 1993). Below
1000 m and west of station 645 on I5W, isoneutral
surfaces descend towards the ridge and shears are
monotonic with depth (Fig. 8). A bottom reference
for station pairs 657–658 through 664–665
maximizes the northward flow of lower deep water
and bottom water.
Farther east along I5W, the relatively high
dissolved-oxygen and low nutrient concentrations
within the lower deep layer indicate a southern
source (Fig. 16). Between stations 665 and 667,
deep isoneutral surfaces are basically parallel and
descend to the east (Fig. 8). The level of no motion
placed between the deep oxygen-minimum and the
lower deep-water layer (gn ¼ 27:96 kg m3) for
station pairs 665–666 and 666–667 sends the lower
deep and bottom waters northward.
Integrating from the Madagascar Ridge crest to
station pair 666–667 (48.8 E) below gn ¼ 27:96;
the deep western-boundary current carries
3.4 106 m3 s1 northward nearly equally partitioned between 1.8 106 m3 s1 of lower deep
water (layer 6, 27.96 kg m3 >gn 28.11 kg m3)
and 1.6 106 m3 s1 of bottom water (layer 7;
gn >28.11 kg m3) (Fig. 20). East of the deep
western-boundary current along I5W and south
of 30 S along I7C, deep isopleths of neutral
density are parallel and nearly horizontal. Integrating the lower deep-water layer velocities from
east of the deep western-boundary current along
I5W and north along I7C to 30.4 S (677–678)
yields an additional input of 0.7 106 m3 s1 of
deep and bottom waters (gn >27.96 kg m3) into
the box.
Water properties along I7C indicate a northward transition from CDW to NIDW (Fig. 16),
and accordingly, the circulation field shows a
transition from the north and northwest flow in
the CDW regime to eastward flow within the
NIDW regime. North of 30.4 S, the baroclinic
structure on I7C changes: deep isoneutral density
surfaces are no longer parallel but instead, the
lower deep water layer thickens as upper isoneutral isopleths rise (o3000 m) and lower isopleths
(> 3000 m) descend to the north. Hence, the
average shear structure on the northern part of
I7C has a relative minimum near 3000 m, with
increasing westward flow above and below 3000 m.
Placing the level of no motion at 28.14 kg m3,
below the shear minimum, sends NIDW eastward,
albeit with undulations in the geostrophic streamfunction due to eddies. We estimate a net eastward
transport of lower deep water (layer 6) between
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
30 S and 20 S of 6.0 106 m3 s1 (Fig. 20). The
bulk of this eastward transport, 4.8 106 m3 s1,
occurs north of 25 S (the latitude of I4East).
Weak deep shears coupled with our deep
reference yield small bottom-water layer
(gn >28.11 kg m3) transports north of 30.4 S
along I7C (Fig. 20). Between 28.7 S and 25.3 S,
the bottom water circulates cyclonically with a
magnitude near 1 106 m3 s1; the net flow across
this circulation feature is even smaller. Net
bottom-water transport integrated from station
667 (east of the deep western-boundary current)
along the cruise track to 20 S (station pair 704–
548) is 0.9 106 m3 s1 into the box. This is
partitioned into 0.2 106 m3 s1 entering the box
south of 25 S and 0.7 106 m3 s1 entering north
of 25 S.
As discussed in the property section, a deep
western-boundary current is evident along I4East
(25 S) extending from the lower deep water to the
bottom; so we placed the level of no motion at
2000 dbar from the Madagascar coast eastward
through station pair 578–579. For the remainder
of the stations, the level of no motion is
gn ¼ 28:14 kg m3. Integrated from the Madagascar Coast to I7C (54 E including the very wide
station pair), we accumulate 2.0 106 m3 s1
northward in layer 6 (27.96 kg m3ogn o
28:11 kg m3) and 2.0 106 m3 s1 northward in
layer 7 (gn >28.11) (Fig. 21).
Note that the northward flow of bottom water is
confined west of 49.8 E in a well-defined deep
western-boundary current. Thus, it appears that
bottom water, after entering the Madagascar
Basin from the Crozet Basin (30 S, 56 E–59 E),
flows directly west to the Madagascar Ridge and
then north in a deep western-boundary current as
suggested by Warren (1978), rather than traversing
across the basin from 30 S to 23 S in a northwest
direction as suggested by Swallow and Pollard
(1988).
Along 20 S, the bottom waters continue north
in the deep western-boundary current, while
deep waters move south across I3W. Deep
isoneutral density surfaces descend to the east
(Fig. 8); placing the level of no motion at
28.14 kg m3 sends 2.6 106 m3 s1 of lower deep
(27.96 kg m3ogn o28:11 kg m3) waters south
1925
and 2.5 106 m3 s1 of bottom (gn >28.11 kg m3)
waters north (Figs. 20 and 22).
Our southward deep and northward bottom
fluxes are in qualitative agreement with the mean
field determined from a current meter array off the
coast of Madagascar along the western edge of the
I3W section (Warren et al., 2002). Deep currents
were dominated by a nearly barotropic 60-day
signal with amplitudes upward of 10 cm s1.
Warren et al. (2002) ascribed these fluctuations
to wind-forced resonant Rossby-wave response in
the Mascarene Basin.
5.3.2. Upper-ocean circulation
East of the Madagascar Ridge crest to 46.5 E
(station 662), upper-ocean (gn o27:96 kg m3) isoneutral surfaces are relatively flat; our bottom
reference results in negligible transports in the
upper ocean (layers 1–5, gn o27:96 kg m3,
Fig. 22).
Two eddies appear in the southeast corner of
our study region: a cyclone and anticyclone
centered near 48.5 E, 33.5 S (station 666) and
50.4 E, 33.15 S (station 670), respectively. Surface
speeds are near 25 cm s1 (Fig. 9), and flow in
excess of 10 cm s1 extends to about 1000 m with
weaker velocities to the ocean floor (Figs. 10 and
11). The density field similarly reflects the deepreaching nature of the eddies; the dome and
depression associated with the cyclone and anticyclone, respectively, penetrate through the upper
five layers. Consequently, our deep reference yields
a large transport amplitude within the upper ocean
(gn o27:96 kg m3) of around 20 106 m3 s1 for
each eddy (Fig. 22).
Along I7C the overall sense of the dissolvedoxygen isoneutral surfaces (o27:96 kg m3) is for
a northward rise (Fig. 8); the deep reference results
in westward flow in all the five dissolved-oxygen
layers (Fig. 22). Net upper-ocean transport integrated from the Madagascar Ridge crest (station
pair 657–658) to 22 S (station pair 704–548) is
39.4 106 m3 s1 into the box. The bulk of this
transport accumulates south of 25 S; only
2.1 106 m3 s1 flows west along I7C between
the 25 S and 20 S sections. In this more
northern latitude range, only SAMW and upper
AAIW contribute net westward transport, the
1926
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
upper-ocean flow is variable in direction. Nearsurface neutral density isopleths (o26:5 kg m3)
descend to the north while mid-depth
(o27:96 kg m3) neutral density surfaces ascend
to the north. Geostrophic shears have a relative
maxima near 500 m depth, with the relative
velocity increasingly westward above and below
this level. The deep reference sends 4.9 106 m3 s1
of SAMW (26.50 kg m3ogn o26:90 kg m3) and
2.4 106 m3 s1 of upper AAIW (26.90 kg m3
ogn o27:36 kg m3) layers westward; 0.4 106 m3 s1 of surface (ogn o26:50 kg m3), 1.3 106 m3 s1 of lower AAIW (27.36 kg m3ogn o
27:70 kg m3) and 3.4 106 m3 s1 of upper
deep water (27.70 kg m3ogn o27:96 kg m3)
eastward.
As noted in Section 3.1, the I3W section does
not show a well-defined poleward East Madagascar Current; nevertheless, net transports on
I4East and I3W are similar (Fig. 21). For
gn o27:96 kg m3, we estimate net southward
upper-ocean transport across the I4East and
I3W sections of 19.3 and 21.6 106 m3 s1,
respectively. Our East Madagascar Current
estimate is within the range of previous estimates (Warren, 1981b: 20 106 m3 s1; Lutjeharms et al., 1981: 40 106 m3 s1; Swallow et al.,
1988: 20 106 m3 s1; Schott et al., 1988: 20 106 m3 s1; and Sloyan and Rintoul, 2001: 28 106 m3 s1).
5.4. Mozambique Channel
In the Mozambique Channel, the LADCP
cross-track velocities reveal a northward deep
western-boundary current with southward interior
recirculation. Placing the level of no motion at
1500 m depth west of 37.25 E (stations 610–601)
sends the deep waters northward, consistent with
the measured velocity field. In mid-basin, deep
measured velocities are weak and nearly unidirectional with depth; deep baroclinic shears are also
weak and mostly monotonic with depth. Setting
the level of no motion at the deepest common level
here appears reasonable. East of 42 E, LADCP
velocities show a deep-reaching southward flow.
Here the slightly diluted NADW property distributions signal a return arm of recirculating
NADW. Directly above this reduced NADWcore,
properties in the oxygen-minimum layer are at
their most extreme, also indicating southward
flow. A bottom reference achieves this inferred
southward flow in the oxygen-minimum layer and
NADW layer consistent with the LADCP observations. The reference strategy yields a small net
northward transport across I4 of 0.7 106 m3 s1
below the depth of the Daves Ridge (gn =28.07,
B2900 m), thus satisfying our bathymetric constraint (Table 3).
With this reference scheme, a vigorous cyclonic
recirculation occurs in the NADW layer: nearly
4 106 m3 s1 flows northward in the deep western-boundary current with a southward return
recirculation on the eastern side of the basin. The
interior deep flow is weak. Net equatorward
transport in this layer is 1.2 106 m3 s1. This
estimate is comparable to the figure of Ganachaud
et al. (2001) of 271.5 106 m3 s1, who used the
I4 section in their Indian Ocean inverse model.
The inverse model of Macdonald (1998) used an
older hydrographic section across the Mozambique Channel with the result of negligible net flow
through the channel. The inverse model run by
Sloyan and Rintoul (2001) assumed no flow
through the channel.
Integrating over our upper-ocean layers (1–5,
gn o27.96 kg m3), we estimate a southward transport through the Mozambique Channel of
17.7 106 m3 s1. Net transport is southward in
all five of these layers. Poleward transport along
the shelf break at the western boundary merges
with a mid-basin anticyclonic recirculation comprised of southern-origin surface, SAMW, and
AAIW waters. In the oxygen-minimum layer
(circulation layers 4 and 5, 27.36 kg m3
ogn o27.70 kg m3), large poleward transport
occurs along the eastern boundary. Given the
eddy-like nature of the flow (e.g., Sætre and
daSilva, 1984) and the large barotropic and
inferred baroclinic tidal variability within the
channel, we suspect that this eastern boundary
current is ephemeral. Our results are comparable
to the 1476 106 m3 s1 estimate by Ganachaud
et al. (2001), but larger in magnitude than the
5.9 106 m3 s1 estimate of DiMarco et al. (2002)
(all using the same data). Our transport values fall
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
within the range of past estimates in the literature.
(See DiMarco et al. (2002) and de Ruijter et al.
(2002) for comprehensive reviews of previous
transport estimates.)
5.5. Discussion of the regional circulation
5.5.1. Upper-ocean circulation
In our study area, upper-ocean contributions
(gn o27.96 kg m3, depths >2000 m) to the
76 106 m3 s1 Agulhas transport consist of
29 106 m3 s1 from the westward limb of the
subtropical gyre south of 25 S, 20 106 m3 s1
from poleward flow east of Madagascar that then
moves west along 25 S, and an additional poleward flow of 18 106 m3 s1 through the Mozambique Channel (Fig. 17). Note that our circulation
scheme does not produce zero net flow within any
of the boxes when transports are summed over the
upper five layers (Table 2); the I5W-I7C-I4East-I4
and I5W-I7C-I3-I4 boxes have 8 and 3 106 m3 s1 mass imbalances, respectively. While
clearly there are transport uncertainties, our
estimates of the proportional contributions to the
Agulhas are robust.
Our results indicate that nearly 14% of the
Agulhas is fed by flow through the Mozambique
Channel. Stramma and Lutjeharms (1997) provided a climatological view of the subtropical
Indian Ocean circulation field for depths less than
1000 m and determined that flow through the
Mozambique Channel contributed only a small
amount, 5 106 m3 s1, to their estimated Agulhas
transport of 65 106 m3 s1. One explanation for
the discrepancy between the climatology and our
results is that our sections may not be representative of the circulation field at longer time scales.
Modeling studies suggest that while the Madagascar Ridge might effectively shield the Agulhas
Current from incoming baroclinic Rossby waves
and thus reduce the amplitude of the seasonal
signal within the Agulhas (Matano et al., 1999),
flow both through the Mozambique Channel and
along the east coast of Madagascar appears to
exhibit a seasonal cycle (e.g., Matano et al., 1999;
Maltrud et al., 1998). Curiously, an 11-month
current meter mooring array off Madagascar at
23 S that monitored the East Madagascar Current
1927
showed no obvious annual signal (Schott et al.,
1988).
At 33 S, the Sverdrup transport using the
Hellerman and Rosenstein (1983) climatological
wind-stress curl ranges from 40 106 m3 s1 in
December to 65 106 m3 s1 in August. The
average of their June and July estimates is
50 106 m3 s1, 25 106 m3 s1 less than our
estimate for the Agulhas transport. The Agulhas
transport includes not only the wind-driven transport but also about 10 106 m3 s1 from the
Indonesian Throughflow (Gordon and Susanto,
1999) and possibly another 10 106 m3 s1 from
the meridional overturning cell (upwelled deep and
bottom waters that return south) (Robbins and
Toole, 1997). We can thus infer that additional
15 106 m3 s1 in the Agulhas is due to local
recirculation and/or the meridonal overturning
cell.
Here we apply Godfrey’s (1989) island rule in
order to estimate the wind-driven component of
the flow through the channel, and with our I4
transport calculation we deduce the partitioning of
the Indonesian Throughflow east and west of
Madagascar. Using the Hellerman and Rosenstein
(1983) wind-stress climatology, application of the
island rule yields Mozambique Channel transports
that range from 17 106 m3 s1 northward in
February to 16 106 m3 s1 southward in August.
The mean of the July and June estimates is
13 106 m3 s1 southward, 5 106 m3 s1 less than
we determine for our Mozambique Channel
section. This suggests that half of the Indonesian
Throughflow passes west of Madagascar and
south through the Mozambique Channel.
We note several caveats in interpreting our
Sverdrup calculations beyond providing a broader
context for our transports. First the Hellerman
and Rosenstein (1983) wind-stress values are likely
too large (Josey et al., 2002). Second, numerical
and laboratory studies suggest that the island rule
overestimates transport by 0–25% (Pedlosky et al.,
1997) and additional studies find that the island
rule overpredicts transport between the island and
the basin’s western boundary when the island is
close to the boundary (where ‘‘close’’ depends
upon the width of the channel relative to the
frictional boundary layer scale) (Pratt and
1928
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
Pedlosky, 1998). Finally, a more appropriate
island rule and Sverdrup calculation would include
the effects of time dependence (baroclinic Rossbywave adjustment) (Firing et al., 1999).
5.5.2. Deep and bottom circulation
Bathymetry strongly controls the deep and
bottom circulation in our study region: the African
Coast, the Mozambique Plateau, and the Madagascar Ridge support deep western-boundary
currents along their eastern margins; the Madagascar Ridge separates the deep waters of the
Natal Valley and Mozambique Basin from those
in the Madagascar Basin; and the Natal Valley
and the Mozambique Basin present dead ends to
meridional flow beneath 2800 and 3000 m, respectively. Very little net deep flow appears to flow
meridionally within the Mozambique Channel.
East of the Madagascar Ridge, net northward
deep transport (28.96 kg m3>gn >28.11 kg m3)
is small. There is, however, a deep cyclonic
circulation present; high-latitude deep waters
move northward along the western boundary,
while NIDW flows southward in the basin interior
(Fig. 17). We cannot unambiguously report the net
deep transports across the the Mascarene Basin,
because we only considered the portion of the I3W
section west of 54 S. In fact, if we carry out the
geostrophic transport estimate with a zero-velocity
surface at gn 28.14 kg m3 to the Mascarene
Ridge (57.5 E), a net transport in layer 6 of
0.13 106 m3 s1 southward is obtained.
We find that 2.56 106 m3 s1 of bottom water
(gn >28.11 kg m3) flows north across 20 S. The
transport calculation extended to 57.5 E yields
2.52 106 m3 s1 of equatorward bottom water
flow; the I3W section has captured most of the
bottom-water transport across the Mascarene
Basin. These waters presumably continue north
through the Amirante Passage to the North Indian
Ocean (Barton and Hill, 1989; Johnson et al.,
1998). There the bottom water upwells and some
portion is transformed to NIDW, which in turn
returns south as the upper limb of the deep
overturning cell.
Our bottom-water transports are consistent with
recent estimates of equatorward bottom-water
flow in the Mascarene Basin and Amirante
Passage: Johnson et al. (1998) determined that
2.5–3.8 106 m3 s1 flow north in the Mascarene
Basin and 1.0–1.7 106 m3 s1 flow north in
Amirante Passage. These transport bounds reflect
the range of zero-velocity surfaces (potential
temperature 1.0–1.1 C) utilized by their study.
Our referenced I3W section yields a northward
transport of 2.7 106 m3 s1 below the potential
temperature 1.1 C. (In fact, the agreement with
Johnson et al. (1998) is not too surprising,
considering that we use a similar level of no
motion.)
Our deep western-boundary current transports
are smaller than historical (pre-WOCE) estimates
in the Madagascar and Mascarene Basins. A
comprehensive list of published bottom-water
transport estimates in the Madagascar and Mascarene Basins and Amirante Passage can be found
in Table 1 of Johnson et al. (1998); here we
highlight the relevant subset. Swallow and Pollard
(1988) found 5.2 106 m3 s1 along a section
running from 35 S to 20 S using a 3500 m level
of no motion; Warren (1974) determined
5.7 106 m3 s1 of flow northward at 23 S using
a 3100 level of no motion; Warren (1981b)
calculated 5 106 m3 s1 of northward flow past
18 S using a variable levels of no motion; the
inverse model of Ganachaud (1999) produced
4.0 106 m3 s1 of equatorward flow across
the 20 S I3W line, and finally, Sloyan and
Rintoul (2001) calculated 3.5 106 m3 s1 below
gn =28.10 kg m3 passing north across 18 S.
6. Final thoughts
Based on near-synoptic observations, we have
derived an estimate of the Southwest Indian Ocean
circulation for late May through July 1995. Our
scheme balances total mass within closed boxes to
within 5 106 m3 s1 and within individual layers
to within 8 106 m3 s1. We have not attempted to
interpret layer transport imbalances as diapycnal
fluxes, as our transport uncertainties are comparable to these imbalances. A logical next analysis
step would be to utilize steady inverse methods
(e.g., Ganachaud et al., 2001; Sloyan and Rintoul,
2001) to rigorously constrain the three-dimen-
K.A. Donohue, J.M. Toole / Deep-Sea Research II 50 (2003) 1893–1931
sional circulation and derive more formal error
estimates. Our referenced circulation field would
serve as an excellent initial condition to such
effort. Given the scale of our survey area and
known temporal variability, it may be more
appropriate to explore time-dependent models.
Such models may help place our synoptic circulation scheme in the context of the general ocean
circulation.
Acknowledgements
Collection of the data from the WOCE Indian
Ocean Expedition was supported by the National
Science Foundation through grants OCE-9413164,
OCE-9401343, and OCE-9413172. Support for the
analysis of these observations derived from grants
OCE-9710102, OCE-9818947, and OCE-0079383.
We thank the officers and crew of the R/V Knorr
for their support. J. Hummon and E. Firing
provided the ADCP data for I5W, I7C, I4, and
I4East. M. Kosro provided ADCP data for the
I3W section. P. Robbins generously shared Matlab
code fundamental in the data analysis and the
geostrophic transports from the Toole and Warren
(1993) and Robbins and Toole (1997) analysis of
the 1987 Darwin section. We thank Bruce Warren
and T. Whitworth for helpful comments on the
manuscript and A. Ganachaud for providing
results from his Indian Ocean inverse model
(Ganachaud et al., 2001).
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