Progress in Oceanography 56 (2003) 137–186
www.elsevier.com/locate/pocean
On the total geostrophic circulation of the Indian Ocean:
flow patterns, tracers, and transports
Joseph L. Reid ∗
Marine Life Research Group, Scripps Institution of Oceanography, 9500 GilmanDrive, La Jolla, CA 92093-0230, USA
Abstract
The large-scale circulation of the Indian Ocean has several major components. There is a cyclonic gyre in the far
southwest with its axis along about 60°S. It extends to the bottom. North of this the Circumpolar Current flows eastward
south of 40°S to more than 3000 m. The axis of the great anticyclonic gyre lies along 35°S to 40°S down to about
2000 m. Below there the western end shifts northward and the axis lies along the central and southeast Indian ridges,
with southward flow west of the ridges and northward flow on the east side.
There is a westward flow along 10°S to 15°S, which includes water from the Pacific, through the Banda Sea. The
flow near the equator is eastward down to the depth of the ridge near 73°E. Flow within both the Arabian Sea and
Bay of Bengal is cyclonic down to great depth.
There is a southward flow along the coast of Africa in the upper 2000 m joining the Circumpolar Current, and a
southward flow along the coast of Australia that does not reach the Circumpolar Current.
Below 2500 m there is a northward flow from the Circumpolar Current along the east coast of Madagascar and on
into the Somali and Arabian basins.
 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Indian Ocean; Circulation; Deep circulation; Geostrophic flow
Contents
1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
2.
Data presentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
3.
The near-surface waters [Fig. 3(a–d)] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
4.
The principal layers [Fig. 4(a–f)] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
5.
Surface circulation [Fig. 5(a)] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
∗
Tel.: +1-858-534-2055; fax: +1-858-822-4036.
E-mail address: [email protected] (J.L. Reid).
0079-6611/03/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0079-6611(02)00141-6
138
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
6.
Flow beneath the surface [Fig. 5(b–k)] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
7.
Total transport (Fig. 6) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
8. The pattern of tracers on isopycnals (Figs. 7–17) . . . . . . . . . .
8.1. The isopycnal where σ0 is 26.00 [Fig. 7(a–d)] . . . . . . . . .
8.2. The isopycnal where σ0 is 26.95 [Fig. 8(a–d)] . . . . . . . . .
8.3. The isopycnal where σ1 is 31.87 (27.25 in σ0) [Fig. 9(a–d)] .
8.4. The isopycnal where σ1 is 32.00 (27.362 in σ0) [Fig. 10(a–d)]
8.5. The isopycnal where σ2 is 36.805 [Fig. 11(a–d)] . . . . . . . .
8.6. The isopycnal where σ2 is 36.92 [Fig. 12(a–d)] . . . . . . . . .
8.7. The isopycnal where σ2 is 37.00 [Fig. 13(a–d)] . . . . . . . . .
8.8. The isopycnal where σ3 is 41.495 [Fig. 14(a–d)] . . . . . . . .
8.9. The isopycnal where σ4 is 45.89[Fig. 15(a–d)] . . . . . . . . .
8.10. The isopycnal where σ4 is 45.96[Fig. 16(a–c)] . . . . . . . . .
8.11. The isopycnal where σ4 is 45.98 [Fig. 17(a–d)] . . . . . . . . .
9.
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145
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148
148
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
1. Introduction
This is the fifth of a series of studies of the large-scale circulation of the world oceans. The first and
second of these (Reid, 1986, 1989) dealt with the South Pacific and South Atlantic, and third (Reid, 1994)
with the North Atlantic, and the fourth (Reid, 1997) with the Pacific.
The purpose of the present study is to estimate the general circulation of the entire Indian Ocean in a
manner that defines the flow at all depths and balances the total top-to-bottom geostrophic transport. The
estimation is made through a new examination of the characteristics and the geostrophic shear. The method
is the same as used in the earlier studies.
The emphasis will be on the deeper waters, below the high variability of the near-surface layer.
The two major assumptions used herein are that the flow is geostrophic and that both flow and mixing
take place approximately along isopycnal surfaces. Characteristics acquired where the isopycnals outcrop,
or in the case of the non-conservative characteristics by respiration or dissolution, are modified along the
flow by both lateral and vertical diffusion. Some tracers show both lateral and vertical extrema in concentration and their patterns can be used to estimate the sense of flow.
The baroclinic flow is given by the density field, that is, the geostrophic flow relative to the bottom
flow, which is estimated from examination of the various characteristics and taken as the reference speed.
The density field is defined fairly well over much of the Indian Ocean by the present data set, which
extends to the bottom, but is not synoptic. While the flow is known to vary with time, the large-scale flow
below the upper layer appears to be steady enough to allow data sets from different periods to be combined
and the general circulation to be examined usefully.
The characteristics used as tracers have various sources and lie in various ranges of depth and density,
and are spread throughout the ocean by both flow and mixing. Their patterns are examined along vertical
sections and along isopycnal surfaces. In some density ranges the patterns are sharply defined and show
features that appear to be the result of advection rather than horizontal diffusion alone. These and other
patterns, both shallower and deeper, can in some places indicate flow components at different depths that
are in opposite senses, and with the measured baroclinic component, constrain the value of the reference
velocity to a narrow range.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
139
The field of flow is presented by maps of adjusted steric height on isobaric surfaces. The characteristics
are mapped on isopycnals, each of which varies in depth.
From about 40°S to 50°S the isopycnals rise sharply to the south across the Circumpolar Current. North
of there their depths do not vary so much. The range of variation north of about 45°S along each isopycnal
is only about 200–300 m, except in the deep water beyond the constricted entrances to the Somali and
West Australia basins.
Each map of adjusted steric height represents the flow along one pressure-surface (depth). While it may
lie fairly close to the depth of an isopycnal north of 45°S, it cuts across many isopycnals south of there
and cannot represent the flow on any one isopycnal everywhere.
To compare the patterns of tracers along an isopycnal with the geostrophic flow one must look at the
flow along different isobaric surfaces as the isopycnal sinks or rises across the Circumpolar Current and
the great anticyclonic gyre.
The area studied is shown in Fig. 1 on a Mollweide projection, with the pertinent topographic features
labeled. The array of stations used in determining the fields of adjusted steric height and volume transport
(Table 1 and Fig. 2) is selected to include stations that reached near the bottom and, where it is possible,
along lines made by a single ship roughly normal to major flows. Some combinations of stations from
different expeditions are needed to complete lines. For the Indian Ocean 2187 stations are selected for
calculating the fields of flow, and they are identified in Table 1. A much larger set of stations (4287) is
used on the isopycnal maps.
The work was carried out in two stages. First, on selected lines of stations (Fig. 2), components of
geostrophic motion are calculated relative to the deepest common depth of each consecutive station pair
and compared with the tracer patterns. If necessary a reference speed is added to achieve the sense of flow
assumed from the tracer patterns for that pair of stations. The adjusted flows normal to the station pairs
along these lines define adjusted pressure gradients along the lines, and these are integrated horizontally
to obtain the adjusted steric height.
A second adjustment is necessary because no constraint of continuity is used in the first stage, and the
resulting transport across the line of stations may not be in balance. Transport into the Indian Ocean south
of Africa is taken to be 132 × 106 m3 s⫺1. The transport from the Indian Ocean into the South Pacific is
taken to be 135 × 106 m3 s⫺1. Transport from the Pacific into the Indonesian seas and the Indian Ocean
is taken to be 3 × 106 m3 s⫺1. Further adjustments to match these constraints and to balance the transport
at the intersections of the selected lines required very little change in the reference velocities and resulting
flow patterns.
Except for the specified net ocean-to-ocean transports the only constraint applied herein is quite simple:
that the field of flow should be qualitatively coherent with the tracer patterns. No constraint on heat or
salt transport is applied and no Ekman transport accommodated.
The patterns of characteristics along several isopycnals at and below 2000 m have been presented and
discussed by Mantyla and Reid (1995). Their detailed discussion is not repeated here. Herein some of
those isopycnal patterns are presented again, with some minor changes resulting from including recent
World Ocean Circulation Experiment data, and six shallower isopycnals are also shown. The fields of flow
are presented at standard pressures and as the total top-to-bottom flow.
2. Data presentation
All of the illustrations have been placed after the text. As some of them will be referred to in different
sections of the text it seems easier to have them grouped in order of surface maps, the vertical section,
maps of geostrophic flow, and maps of characteristics along isopycnals. The isopycnals are labeled at
different potential density values at the different pressures, as in Reid and Lynn (1971). Table 2 lists the
140
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Table 1
Expeditions from which stations were chosen to calculate the adjusted steric height
Expedition/ship
Dates
NODC #
09AR9404—1
316N145—5
316N145—6
316N145—7
316N145—8
316N145—9
316N145—10
316N145—11
316N145—12
316N145—13
316N145—14
316N145—15
74AB29—1
Africana II
Lacerda Almirante
AJAX Leg I, II
Atlantis II
Atlantis II
Conrad 17
Darwin
Darwin Cr. 29
Discovery
Discovery Cr. 164
Eltainin Cr. 41
Dec. 1994–Feb. 1995
Dec. 1994–Jan. 1995
Jan.–Feb. 1995
Mar.–Apr. 1995
Apr.–Jun. 1995
Jun.–Jul. 1995
Jul.–Aug. 1995
Aug.–Sep. 1995
Sep.–Oct. 1995
Nov. 1995
Dec. 1995
Dec. 1995–Jan. 1996
Nov.–Dec. 1987
Jun. 1963
Apr.–May 1964
Oct. 1983–Jan. 1984
Jul.–Dec. 1963
Sept. 1965
Jan.–Apr. 1974
Jan. 1987
Nov.–Dec. 1989
Feb.–Mar. 1935
Dec. 1986–Jan. 1987
Dec. 1969–Feb. 1970
Fuji Maru
Fuji Maru
Islas Orcadas
Islas Orcadas
JARE Cr. 22
Marion Dufresne MD43
Marion Dufresne
Feb. 1974
Feb. 1977
Dec. 1976
Jan. 1977
Feb.–Mar. 1981
Feb.–Mar. 1985
Feb.–Mar. 1992
490906
491002
311011
311012
Serrano
Feb.–Mar. 1963
310090
910007
680001
318628
310197
310247
749178
740039
311213
Source
WOCE S03/S041
WOCE I08S/I09S
WOCE I09N
WOCE I05E/I08N
WOCE I03
WOCE I04/I05W/I07C
WOCE I07N
WOCE I01W
WOCE I01E
WOCE I10
WOCE I02E
WOCE I02W
WOCE I05P
S. Africa
S. Africa, IIOE Cr. AM1/64
SIO, TAMU (1985)
WHOI, IIOE Cr. 8
WHOI
LDGO (1980)
United Kingdom
WHOI
United Kingdom
United Kingdom
SIO, Horace Lamb Center &
Johns Hopkins Univ. (1972)
Japan
Japan
LDGO (1981)
LDGO (1981)
Japan
France
French-Indonesian JADE Program
IIOE
range of numbers for each isopycnal. The figures are labeled with the densest (deepest) value for each
isopycnal. The reader may wish to look at the figures before reading the sections that follow.
3. The near-surface waters [Fig. 3(a–d)]
There are not enough WOCE data to provide a complete map for a single season. In order to have a
complete coverage the data from other expeditions and seasons must be added. Even north of the equator
the WOCE data are from January, March and October 1995 in the Bay of Bengal and August–September
1995 in the Arabian Sea.
Near the surface the flow between the tropic circles is very variable, as Schott and McCreary (2001)
have shown in their review of the monsoon circulation. Because of this variability the maps presented
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
141
Table 2
Specifications of the isopycnal surfacesa
Indian Ocean
σ0
26.000
26.950
27.250
27.362
27.615
27.712
27.773
27.794
27.806
27.824
27.831
σ1
31.870
32.000
32.270
32.375
32.446
32.475
32.493
32.521
32.527
σ2
σ3
36.805
36.920
37.000
37.039
37.063
37.104
37.116
41.495
41.526
41.584
41.600
σ4
45.890
45.960
45.980
σ5
50.236
50.257
a
The potential density is expressed as σ0: 0–500 db, as σ1: 500–1500 db, as σ2: 1500–2500 db, as σ3: 2500–3500 db, as σ4: 3500–
4500 db, and as σ5: 4500 db to the bottom. The potential density is given in units of σ, which is ρ ⫺ 1000, where ρ is in kg m-3.
This table lists the different numbers used for each isopycnal as it extends to the different pressure ranges. The numbers in bold are
those used in the text and figures to identify each isopycnal.
here, from the non-synoptic data set, cannot be expected to match the details of individual seasonal studies
of the flow near the surface in particular areas at different times.
Therefore there are some irregularities in the patterns in the upper layer, but the major features are still
clear. Temperature [Fig. 3(a)] is highest between 20°N and 20°S except along the western boundary, where
upwelling is indicated north of the equator. South of the equator relatively high values extend southward
along the western boundary.
Salinity [Fig. 3(b)] is highest, from excess evaporation, west of India, and lowest in the area of rainfall
along the eastern boundary north of the equator and high in the evaporation zone along about 30°S.
Oxygen [Fig. 3(c)] is close to 4.6 ml l⫺1 north of 25°S and rises to more than 8 ml l⫺1 in the colder
water near Antarctica.
Silica [Fig. 3(d)] is as high as 50 µm kg⫺1 at the coast of Antarctica. There appears to be a minimum
of less than 2 µm kg⫺1 along 40°S extending from the Atlantic to the Pacific.
4. The principal layers [Fig. 4(a–f)]
The characteristics of the Indian Ocean have been illustrated in maps and long top-to-bottom sections
by Wyrtki (1971). Warren (1981) and Toole and Warren (1993) have shown east–west sections along 32°S
and 18°S, and the GEOSECS vertical section have been published by Spencer, Broecker, Craig, and Weiss
(1982). Others, from the World Ocean Circulation Experiment are in preparation. Mantyla and Reid (1983,
1995) have discussed the various deep layers entering or formed in the Indian Ocean, and those discussions
are not repeated here.
Only one section, from Antarctica to the Gulf of Oman, is presented here [Fig. 4(a–f)]. It follows the
path of the coldest and densest bottom water, flowing northward from Antarctica through the Enderby,
Crozet, Madagascar, Mascarene, and Somali basins into the Arabian Basin and the Gulf of Oman. South
of 60°S the abyssal values are the coldest, densest, highest in oxygen, and lowest in silica along the section.
These values may include an input from the Enderby Land coast (Jacobs & Georgi, 1977). Other sections
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J.L. Reid / Progress in Oceanography 56 (2003) 137–186
in the South Australia Basin would have shown input from the Ross Sea and from the Adélie Coast, near
140°E (Gordon & Tchernia, 1972; Bindoff, Rosenberg, & Warner, 2000; Rintoul, 1998). They appear on
the maps of bottom characteristics prepared by Mantyla and Reid (1995).
The warm and saline waters from the North Atlantic appear as a vertical maximum in salinity south of
about 35°S on the section. North of there, there is a shallower vertical maximum in salinity extending
southward from the northern Indian Ocean to about 15°S. It results from the high net evaporation rate
there and the outflow from the Red Sea and Persian Gulf (Wyrtki, 1971; Premchand, Sastry & Murty,
1986a,b; Beal, Ffield, & Gordon, 2000).
A subsurface salinity minimum extends from the surface south of 50°S northward to near the equator.
Oxygen is everywhere lower north of the equator and from there a great oxygen minimum extends
southward across the Circumpolar Current. There is some interleaving of higher oxygen near 15°S to 30°S
near 500 and 1500 m.
Silica is highest in the north, and as in the Pacific, extends southward as a subsurface maximum from
the very highest values at the bottom in the Arabian Sea and the Bay of Bengal, but also high at the bottom
in the Enderby Basin.
Phosphate and nitrate are not shown, as the data available were not adequate.
You (1998) has mapped the characteristics above 1500 m and north of 45°S on neutral surfaces (Jackett &
McDougall, 1997) in both the summer and winter monsoons. He has mapped the contributions of the
various sources to each of four neutral surfaces and prepared schematic circulation patterns for summer
and for winter.
5. Surface circulation [Fig. 5(a)]
The Indian Ocean is quite variable, especially near the surface, and it is hardly to be expected that a
combination of data from different months and years would provide an entirely satisfactory pattern near
the surface. Not only the monsoons but also various other wind changes alter the patterns. Several of the
studies cited herein have investigated the temporal variations at shallow depths in some areas. But this
work requires stations that reach near the bottom, and there are not enough of these to compare the seasonal
changes in flow near the surface.
Circulation within the Arabian Sea and Bay of Bengal are shown to be cyclonic in December–February
in the atlases by Hastenrath and Greischar (1979) and Richardson and McKee (1989). They also show a
westward flow south of Sri Lanka connecting the two cyclonic gyres. The data used for the surface map
herein are from September to March in the Bay of Bengal, and indicate a cyclonic pattern. The Arabian
Sea data are from August and September. They do not show a cyclonic pattern at the surface, but they do
show it from 200 to 3000 db in the eastern part of the Sea. The maps by Cutler and Swallow (1984) show
the two cyclonic features in November–December but not clearly in January and February.
This pattern is seen in the Arabian Sea in November–January by Shetye, Gouveia, and Shenoi (1994)
and as an undercurrent along the east coast of India by Muraleedharan, Kumar, and Rao (1995), and
Stramma, Fischer, and Schott (1996). It is seen in the Bay of Bengal in November–January by Shetye and
Gouveia (1998); Murty, Sarma, Rao, and Murty (1992) and Murty, Suryanarayana, and Rao (1993).
The flow at the sea surface includes a cyclonic gyre near 60°S in the southwest, the Circumpolar Current
south of 40°S, and an anticyclonic gyre with its axis along about 35°S to 45°S, extending westward south
of Africa to about 15°E and eastward south of Australia almost to Tasmania (Wyrtki, 1971; Reid, 1981;
McCarthy & Talley, 1999).
In the west a satellite-tracked drogue was deployed near 36°S 28°E and drifted for 268 days and passed
part-way around the gyre (Gründlingh, 1978). It was released off the east coast of Africa, drifted westward
to about 13°E and then eastward with the anticyclonic gyre to 38°E before it was lost.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
143
At the eastward end of the gyre flow is westward along the south coast of Australia (Sverdrup, Johnson, &
Fleming, 1942; Wyrtki, 1971; Bye, 1972).
The flow is eastward between 8°S and 5°N. Although the flow at the surface does not show the same
patterns that are seen in the various atlases (Schott, 1935), especially in the north, it is not inconsistent
with the patterns of characteristics at the surface. The flows southward across 5°N and northward across
8°S along the coast of Africa, and southward west of Madagascar to the tip of Africa fit fairly well with
the temperature field.
The westward flow near 10°S and eastward flow along the equator are supported by the salinity pattern
(Fig. 3b). Schott, Reppin, Fischer, and Quadfasel (1994) found westward flow near the surface just south
of Sri Lanka in winter.
Swallow and Bruce (1966) found the flow across 5°N near Somali to be northward down to 100 m in
August 1964.
Some evidence of the Great Whirl is seen near 9°N 54°E at the sea surface and 200 db (Schott, Fischer,
Garternicht, & Quadfasel, 1997).
Along the west coast of Australia the map of adjusted steric height indicates southward flow south of
20°S, in agreement with Smith, Huyer, Godfrey, and Church (1991).
6. Flow beneath the surface [Fig. 5(b–k)]
Warren, Stommel, and Swallow (1966) used data taken in August–September 1964 from 5°S to 12°N
and west of 56°E to identify the major temperature, salinity and oxygen extrema in the Somali Basin. In
the upper layer the temperature and salinity defined the Somali Current and showed warm saline inflows
from the Arabian Sea.
Near the equator Schott, Swallow, and Fieux (1990) found the flow along the western boundary in the
upper 100 m to be southward from December through April and northward the rest of the year.
With direct current measurements Quadfasel and Schott (1983) found southward flow across 5°N at the
coast at depths from 150 to 600 m, though the flow at the surface was northward from April to October.
Swallow, Fieux, and Schott (1988), with measurements along the east coast of Madagascar, found strong
westward flow across the north tip and southward flow across 23°S.
In the upper 500 m Lutjeharms, Bang, and Duncan (1981) found a southward flow along the east coast
of Madagascar, a part of which turns westward around Madagascar into the Mozambique Basin, and another
part forming eddies near 25°S.
The fields of flow on various standard pressures are presented in Fig. 5.
In this set of data the flow appears to be cyclonic in both the Arabian Sea and the Bay of Bengal [Fig.
5(b–k)]. In the Arabian Sea, however, there is a narrow northward flow along the western boundary at
depths of 800–1200 m from about 10°N to 20°N, which carries some of the Red Sea water northward.
The flow across 5°N at the western boundary is northward in the upper 200 m, mostly southward from
800 to 3000 db, and at 3500–4000 db the flow is northward.
Harris (1972) examined the area from 10°S to 32°S east of Madagascar and proposed that above 2000
m the Agulhas Current at 32°S contains mostly water from the South Equatorial Current both north and
south of Madagascar, and from the southward limb of the great gyre.
The deep flow along the east side of Madagascar was shown to be northward by Wyrtki (1971), and
by Warren (1974,1981) who showed, with a vertical section along 18°S, that there the deep flow below
2000 m derives from the Circumpolar Current.
North of the Circumpolar Current the dominant feature of the South Indian Ocean is the great anticyclonic
gyre, which extends from the sea surface to more than 3000 m.
Between 200 and 2000 db the westward limb of the anticyclone feeds a southward flow along the coast
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J.L. Reid / Progress in Oceanography 56 (2003) 137–186
of Africa. At 2000 db part of the northward flow along the east coast of Madagascar returns along the
coast of Africa. Part of it joins the eastward flow along the equator. Part of it turns back southeastward
from the Mascarene Basin and joins the eastward limb of the great gyre [Fig. 5(g)]. This part of the gyre
extends to more than 3000 db, and its axis shifts and lies roughly along the Central Indian Ridge, with
southeastward flow on the western side and northwestward flow in the east.
The pattern is consonant with the northwestward flow below 2000 db along the eastern side of the
Central and Southeast Indian ridges from about 40°S to 10°S shown by Wyrtki (1971). It has also been
shown by Warren (1981) across 18°S and by Toole and Warren (1993) across 32°S.
Below 2000 db a westward flow develops along Antarctica, first in the east near 140°E, extending to
the Kerguelen Plateau. At 3500 db it divides, part passing westward south of the Plateau and part turning
northward and eastward along the ridges (Speer & Forbes, 1994).
From 200 db down to 3500 db there is a southward flow along the coast of Australia, from 10°S to
about 32°S. However, unlike the flow in the Pacific, it does not continue southward and eastward around
the continent, but turns offshore and westward near 30° to 35°S.
At 800 db and deeper there is a substantial difference from the overlying flow. Some of the westward
flow between 10°S and 15°S turns back eastward near 80°E and then southward along the boundary from
about 15°S to 35°S and then westward again with the anticyclonic gyre [Fig. 5(d)]. At 800 db this is a
small feature in the east but at greater depths the turn-back takes place farther west, at the Mascarene
Plateau at 1000 db.
From 1000 to 3000 db there is some resemblance between this feature in the Indian Ocean, with eastward
flow along 15°S to 20°S, and the Pacific Ocean, which at 1000 db and below has such an eastward flow
between 20°S and 30°S (Reid, 1997).
In the Pacific the anticyclonic gyre does not extend all the way to the eastern boundary at depths below
500 db, and water can flow southward east of the gyre and leave the Pacific through the Drake Passage.
In the Indian Ocean the anticyclonic gyre extends eastward south of Australia and there is no free passage
eastward along the south coast of Australia.
At 3500 m the western Indian Ocean is separated from the eastern basin by the Chagos-Laccadive,
Central, and Southeast Indian ridges and there is no exchange between the two at this depth. The flow is
eastward just south of Africa, looping northward into the Mozambique Basin and then southward to pass
eastward south of the Crozet Ridge. From there the flow is northward past Madagascar and through the
Amirante Trench (Johnson & Damuth, 1979; Fieux & Swallow, 1988; Johnson, Musgrave, Warren,
Ffield, & Olson, 1998). It continues onward through the Carlsberg Ridge near 56°E and into the Arabian
Basin. Part of the northward flow turns southward near 10°S along the eastern boundary of the basin to
about 60°S where it joins the westward limb of the Weddell-Enderby gyre.
East of the dividing ridge there is westward flow along Antarctica that turns northward and eastward at
the Kerguelen Plateau as proposed by Orsi, Johnson, and Bullister (1999). It then turns northward across
50°S through the Australian–Antarctic Discordance near 50°S 120° to 125°E (Fig. 5j). From there it circulates within the South Australian Basin and northward along the Southeast Indian and Central Indian ridges
and to the equator.
The cyclonic flow in the Arabian Sea and Bay of Bengal appears to hold down through 3000 db. At
3500 db the flow is northward through the Carlsberg Ridge near 56°E and possibly cyclonic east of there.
At 4000 db the flow is much like that at 3500 db. The Circumpolar Current is interrupted at this depth
by the various ridges, particularly the Kerguelen Ridge. The principal flows are northward along the deep
channels toward the Arabian Sea and the Bay of Bengal and westward along Antarctica in the WeddellEnderby Basin and the Australian-Antarctic Basin.
At 4500 db (not shown) there are isolated cyclonic flows in the Cape and Agulhas basins.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
145
7. Total transport (Fig. 6)
The top-to-bottom transport into the Indian Ocean from the Atlantic is set at 132 × 106 m3 s⫺1 (Reid,
1994). Pacific water enters through the Indonesian seas, but estimates of the throughflow vary widely.
Recent reports of the average flow within the Makassar Strait are about 9 × 106 m3 s⫺1 (Gordon, Susanto, &
Ffield, 1999), and 10–15 × 106 m3 s⫺1 through the southern passages (Chong et al., 2000). The data of the
French-Indonesian JADE Program (Fieux, Molcard, & Ilahude, 1996) along about 115°E were taken in
February 1992, a season of minimum or reversed throughflow (Schott, 1935; Wyrtki, 1958; Fieux, Molcard, & Ilahude, 1996). A value of 3 × 106 m3 s⫺1 has been used here to be consonant with the earlier
studies (Reid, 1986, 1989, 1994, 1997), so the eastward transport to the Pacific south of Australia becomes
135 × 106 m3 s--1.
The transport integration (Fig. 6) starts from zero at Antarctica and reaches 132 × 106 m3 s⫺1 along the
western and northern boundaries and southward to the throughflow near 10°S. South of the throughflow
the value along the coast of Australia is 135 × 106 m3 s⫺1.
The transport shows the Antarctic Circumpolar Current near 40°S south of Africa and 55°S south of
Australia. The axis of the cyclonic Weddell Sea gyre extends along about 60°S as far as 30°E.
The field of total transport shows the axis of the great anticyclonic gyre along 35°S south of Africa
extending to about 50°S south of Tasmania.
The transport near the equator is eastward between about 5°N and 10°S.
There does not appear to be any significant net transport across the equator near the boundaries. In the
west the flow across 10°N is northward above about 400–500 db and below about 3000 db, with southward
flow in between. This is consonant with the tracer fields.
In the east the flow across 6°N and 10°N is northward, but the flow does not appear to come only from
south of the equator, but also from the west, as part of a cyclonic gyre that extends all across the Arabian
Sea and Bay of Bengal down to about 2000 db. Below there the gyre is divided by the ridge along 73°E,
becoming one cyclonic gyre in the Arabian Sea and one in the Bay of Bengal.
8. The pattern of tracers on isopycnals (Figs. 7–17)
8.1. The isopycnal where s0 is 26.00 [Fig. 7(a–d)]
This isopycnal lies between about 100 and 250m in the south and outcrops near 40°S.
The salinity pattern reflects some of the near-surface features. Values are highest near the Persian Gulf
and Red Sea (Premchand, Sastry, & Murty, 1986a, 1986b) and near 30°S in the east, beneath the great
evaporation zone.
The westward flow at 200 db along about 8°S to 10°S carries a tongue of low salinity from the areas
of excess precipitation in the east. The eastern Bay of Bengal is low in salinity because it lies under the
zone of heavy rainfall.
The flow near the equator carries high salinity from the western boundary eastward between the two
zones of low salinity. The cyclonic flow at 200 db within the Bay of Bengal carries some of the low
salinity along the boundary around the Bay and part turns westward at the southern tip of India.
Higher values of oxygen extend northward across the equator in the west and then eastward along the
equator. The Arabian Sea and Bay of Bengal are very low in oxygen, and account for the low values that
extend westward along about 8°S to 10°S. South of the equator the pattern is much like that in the South
Pacific, where the flow carries high oxygen from the west along the equator and low oxygen from the
eastern boundary extends westward south of the equatorial flow.
North of the outcrop silica has much the same pattern as oxygen, but with the high and lows reversed.
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J.L. Reid / Progress in Oceanography 56 (2003) 137–186
8.2. The isopycnal where s0 is 26.95 [Fig. 8(a–d)]
This isopycnal outcrops near 50°S, lies near 450 m in the north, and is deepest, more than 900 m, in
the western part of the anticyclonic gyre.
The highest values of salinity on this isopycnal are seen just outside the Red Sea and the Persian Gulf,
and the lowest in the far south.
The pattern of salinity shows the eastward and westward limbs of the great anticyclonic gyre, and the
southward flow along the coast of Africa from about 10°N to the Cape.
Near 10°S in the east the low salinity may represent a contribution from the Banda Sea.
The vertical salinity minimum (Fig. 4c) lies below the isopycnal where σ0 is 26.95 in the south, but
north of the equator, where the minimum over-rides the shallow salinity maximum from the Red Sea and
Persian Gulf, the minimum is found at successively lower densities.
The oxygen shows high values extending westward north of Madagascar and continues along the equator,
and low values in the Arabian Sea and Bay of Bengal, extending south along the eastern boundary.
Silica shows high values in the northern seas extending, like the low oxygen values, southward along
the eastern boundary and westward along 10°S. The high values along 10°S to 12°S in the east indicate
an input from the Banda Sea. The westward extension of low-salinity water from the Banda Sea was first
shown by Wyrtki (1961) to about 90°E by Rochford (1966) and Sharma (1971) to Madagascar, and to the
coast of Africa by Wyrtki (1971) and Gordon (1986). It is seen here in the patterns of tracers from about
500 m (26.95 in σ0) to more than 1000 m (36.805 in σ2).
8.3. The isopycnal where s1 is 31.87 (27.25 in s0) [Fig. 9(a–d)]
South of about 20°S the salinity minimum lies close to the depth of this isopycnal [Fig. 9(a–d) and
4(c)]. The salinity pattern still shows the highest values to be in the western Arabian Sea, extending
eastward along about 5°N and southward along the coast of Africa to the Cape. The values are lower near
10°S in the east, from the Banda Sea, and in the far south.
Like salinity, both oxygen and silica indicate a northwestward flow east of Madagascar and a southward
flow on the western side. Silica also shows low values extending eastward from the Arabian Sea near the
equator. A tongue of high silica extending westward along 10°S to 12°S indicates a contribution from the
Banda Sea.
8.4. The isopycnal where s1 is 32.00 (27.362 in s0) [Fig. 10(a–d)]
Along this isopycnal there is an extension of high salinity from the Arabian Sea eastward along about
5°N. The westward extension of low salinity from the Banda Sea along 10°–12°S is most obvious at this
density and there is an eastward extension of high salinity just south of it, along 15°S to 18°S.
The high salinity from the Arabian Sea extends southward along the coast of Africa to the Cape, and
lower salinity from the east extends northward east of Madagascar. Part turns back southward with the
flow along Africa and part turns eastward with the flow along the equator.
On this isopycnal silica, like salinity, indicates a substantial input of Banda Sea water into the westward
flow along about 12°S. Oxygen and salinity indicate an eastward flow just to the south along about 15°S
to 18°S. Silica indicates eastward flow along the equator, and low values entering from the Atlantic south
of Africa and from the Red Sea.
8.5. The isopycnal where s2 is 36.805 [Fig. 11(a–d)]
The low-salinity signal from the Banda Sea is very small at this depth. Otherwise the pattern north of
20°S is much like that in the overlying layer.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
147
The new feature is the tongue of high salinity along 40°S to 45°S in the west. This is from the moresaline Atlantic water and the southward flow along Africa. North of it, along 30°S to 35°S, lower-salinity
water from the circumpolar flow extends westward to about 55°E and then northward east of Madagascar.
The oxygen pattern is much like the salinity pattern, and only the silica shows any possible effect of
the Banda Sea.
8.6. The isopycnal where s2 is 36.92 [Fig. 12(a–d)]
The high salinity from the Atlantic along 40°S to 45°S is part of a tongue that extends from a maximum
value in the North Atlantic all across the Atlantic, Indian, and Pacific oceans to the Drake Passage (Reid &
Lynn, 1971).
Along the tongue the salinity decreases eastward. Part of it turns back westward south of Australia [Fig.
5(g)]. There is an isolated low centered about 30°S, 70°E between the higher salinity from the Atlantic
and the higher salinity from the Arabian Sea. This is a result of vertical exchange with the overlying less
saline Intermediate Water. A similar feature is seen in the Pacific (Reid, 1997).
There is a northward extension of lower-salinity water into the Bay of Bengal.
Along this isopycnal the tongues of higher salinity, high oxygen, and lower silica from the Atlantic
extend eastward all across the Indian Ocean.
8.7. The isopycnal where s2 is 37.00 [Fig. 13(a–d)]
Water from the Atlantic extends eastward just south of Africa and on to the Pacific as tongues of higher
oxygen and salinity and lower silica. Some of it turns northward into the Mozambique Basin and some
turns northward east of the about 80°E and across the equator.
At this density the strong vertical maximum in salinity extends southward from the Arabian Sea and
the high-salinity layer from the Atlantic spreads northward leaving an isolated low near 20°S [Fig. 13(b)].
The axis of the great anticyclonic gyre has shifted at 2500 db to lie roughly along the Central and
Southeast Indian ridges [Fig. 5(h)]. At this density the gyre carries high oxygen and low silica northwestward east of the ridges and low oxygen and high silica southeastward west of the ridges.
8.8. The isopycnal where s3 is 41.495 [Fig. 14(a–d)]
As on the isopycnal where σ2 ⫽ 37.0 the salinity is high in the south from the saline Atlantic water
and high in the north from the Arabian Sea. Away from these lateral sources vertical diffusion results in
a lateral minimum in between. The more saline waters from the south extend northward east of the Central
and Southeast Indian ridges, and lower salinities from the minimum extend southward on the western side.
Also higher oxygen and lower silica values from the circumpolar flow extend northwestward east of the
Central and Southeast Indian ridges and lower oxygen and higher silica extend southeastward west of the
ridges. This is consonant with the axis of the anticyclonic gyre, which lies along the ridges between about
15°S and 35°S. There is equatorward flow along the western boundary.
8.9. The isopycnal where s4 is 45.89[Fig. 15(a–d)]
This isopycnal does not extend into the Arabian Basin, but incrops at the Carlsberg Ridge (Quadfasel,
Fischer, Schott, & Stramma, 1997; Mantyla & Reid, 1995). South of Africa and Australia the high salinity
from the Atlantic water extends all across the Indian Ocean and across the Pacific. Water at this density
also extends northward on the western boundaries of the basins east and west of the Central Indian Ridge.
Its salinity decreases northward by exchange with the less-saline underlying water from the far south.
Oxygen and silica show much the same pattern as salinity.
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J.L. Reid / Progress in Oceanography 56 (2003) 137–186
8.10. The isopycnal where s4 is 45.96[Fig. 16(a–c)]
Along this isopycnal the more saline and warmer and higher-oxygen waters from the North Atlantic still
provide lateral salinity maxima in the south. They extend eastward from the Cape Basin along about 50°S
and northward into the Mozambique and Madagascar Mascarene basins, and northward through the South
Australian Basin almost to the equator.
8.11. The isopycnal where s4 is 45.98 [Fig. 17(a–d)]
This isopycnal lies near 600 m along the axis of the Weddell-Enderby gyre in the east. It incrops near
10°S in the Mascarene Basin, and near 30°S in the east. The North Atlantic source is still recognized by
lateral maxima in salinity and oxygen at this density.
On this isopycnal waters from the Weddell Sea enter at depths as shallow as 600 m near 60°S, with
values as low as 34.68 in salinity, and waters from the warmer and more saline South Atlantic enter at
depths as great as 4400 m near 30°S, with salinity as high as 34.73.
Much of the Weddell Sea water turns back westward along the coast of Antarctica, and the salinity
along the eastward flow drops from 34.74 to less than 34.72 as the waters at this density enter the Pacific.
Oxygen and silica have much the same sort of pattern. In the case of oxygen the low beginning at the
Weddell gyre extends all across the Indian Ocean, with a high just south of it.
9. Conclusion
Water enters the Indian Ocean from the Atlantic and, through the Indonesian seas, from the Pacific. It
is caught up in the gyral patterns and gyre-to-gyre exchanges, and can extend and mix throughout the
entire Indian Ocean before departing. The paths along which it circulates before returning are revealed by
the patterns of the tracers and by the geostrophic balance of the density field. These patterns are displayed
here on isopycnal surfaces, but they indicate that there is significant cross-isopycnal exchange by diffusion.
Water enters the Indian Ocean from the Circumpolar Current with the northwestward flowing part of
the anticyclonic gyre, a northward-flowing western boundary current that passes east of Madagascar, and
a deep western boundary current along the eastern edge of the Central and South Indian ridges. Above
2000 db some of this northward flow turns back southward along the coast of Africa and another part
turns eastward near the equator. This eastward flow divides at the eastern boundary. Part turns northward
to join the cyclonic pattern north of the equator and part turns southward and westward south of 10°S.
Some water from the Banda Sea joins this westward flow, and its presence is shown by the silica pattern
down to 1000m or more. From 800 to 3000 db part of the westward flow along about10°S to 15°S turns back
and flows eastward near 15°S to the eastern boundary, and then westward with the great anticyclonic gyre.
Water leaves the Indian Ocean by rejoining the Circumpolar Current. Above 2000 db this comes from
the southward flow along the western boundary. Below 2000 db it comes from southward flow just west
of the Central and Southeast Indian ridges.
Above 2000 m the net transport across the section along 32°S is southward. This takes place mostly
along the western boundary, from the part of the westward flow between about 8°S and 20°S that turns
southward along the western boundary to about 40°S and then eastward.
There is northward flow east of Madagascar from 1500 db to the bottom. Along 1500 db it is an extension
of the westward flow of the great gyre. At greater depths the northward flow is not from the gyre, but
from part of the circumpolar flow turning north near 46°S, east of the Southwest Indian Ridge as a deep
western boundary current, and extending across the equator. At 2000 db some of this northward flow turns
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
149
back southward in the Mascarene Basin and along the western side of the Central and Southwest Indian
ridges. There is a northward flow along the eastern side of the ridge.
Below 2500 db there is a northward flow from the Australian–Antarctic Basin through the Discordance
Zone near 50°S 125°E into the South Australia Basin, and into the Central Indian and West Australia
basins. Below 3000 db there is no northern exit for the water that has flowed in from the south to the
West Australia and Central basins, and nothing in the tracer patterns indicates southward extensions along
the surface where σ4 is 45.89. Waters at this depth and density escape only after being mixed to lower density.
Acknowledgements
The work reported here represents one of the results of research supported by the National Science
Foundation and the Marine Life Research Program of the Scripps Institution of Oceanography. I wish to
acknowledge the assistance given by Arnold Mantyla in selecting the data for and preparing the vertical
sections and by David Newton for writing the various programs, and to Arnold Mantyla, Lynne Talley,
and Lisa Beal for their comments on the manuscript. I wish to acknowledge especially Sarilee Anderson
for the great skill in handling the various data formats, in arranging the data and calculating and plotting
the data points along the isopycnals and on the fields of steric height and for her patience in the long
succession of adjustments.
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J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 1.
Principal topographic features shown on a Mollweide projection. The 3000 m depth contour is indicated.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 2.
Lines of stations used in the calculation of the geostrophic flow.
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154
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Fig. 3.ab
(a) Temperature (°C) at the sea surface. (b) Salinity at the sea surface.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 3.cd
(c) Oxygen (ml lⴚ1) at the sea surface. (d) Silica (m kgⴚ1) at the sea surface.
155
156
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Fig. 4.ab (a) Positions of the stations used in the vertical section (b–f). (b) Potential temperature (°C) on a vertical section along
about 50° to 60°E from Antarctica to the Persian Gulf.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 4.cd
(c) Salinity on the north–south section. (d) Potential density (σ0 – σ5) on the north–south section.
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158
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Fig. 4.ef (e) Oxygen (ml lⴚ1) on the north–south section. (f) Silica (µm kgⴚ1) on the north–south section.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 5.ab
159
(a) Adjusted steric height at 0 db (10 m2 sⴚ2 or 10 J kgⴚ1). (b) Adjusted steric height at 200 db (10 m2 sⴚ2 or 10 J kgⴚ1).
160
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 5.cd (c) Adjusted steric height at 500 db (10 m2 sⴚ2 or 10 J kgⴚ1). Depths less than 500 m are shaded. (d) Adjusted steric
height at 800 db (10 m2 sⴚ2 or 10 J kgⴚ1). Depths less than 1000 m are shaded.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
161
Fig. 5.ef (e) Adjusted steric height at 1000 db (10 m2 sⴚ2 or 10 J kgⴚ1). Depths less than 1000 m are shaded. (f) Adjusted steric
height at 1500 db (10 m2 sⴚ2 or 10 J kgⴚ1). Depths less than 1500 m are shaded.
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Fig. 5.gh (g) Adjusted steric height at 2000 db (10 m2 sⴚ2 or 10 J kgⴚ1). Depths less than 2000 m are shaded. (h) Adjusted steric
height at 2500 db (10 m2 sⴚ2 or 10 J kgⴚ1). Depths less than 2500 m are shaded.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
163
Fig. 5.ij (i) Adjusted steric height at 3000 db (10 m2 sⴚ2 or 10 J kgⴚ1). Depths less than 3000 m are shaded. (j) Adjusted steric
height at 3500 db (10 m2 sⴚ2 or 10 J kgⴚ1). Depths less than 3500m are shaded.
164
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 5.k
(k) Adjusted steric height at 4000 db (10 m2 sⴚ2 or 10 kgⴚ1). Depths less than 4000 m are shaded.
Fig. 6.
Transport (106 m3 s⫺1). Depths less than 3500 m are shaded.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
165
Fig. 7.ab (a) Depth (hm) of the isopycnal defined by 26.00 in σ0. The dashed line indicates the outcrop. (b) Salinity on the isopycnal
defined by 26.00 in σ0.
166
Fig. 7.cd
σ0 .
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(c) Oxygen (ml lⴚ1) on the isopycnal defined by 26.00 in σ0. (d) Silica (µm kgⴚ1) on the isopycnal defined by 26.00 in
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 8.ab (a) Depth (hm) of the isopycnal defined by 26.95 in σ0. (b) Salinity on the isopycnal defined by 26.95 in σ0.
167
168
Fig. 8.cd
σ0 .
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
(c) Oxygen (ml lⴚ1) on the isopycnal defined by 26.95 in σ0. (d) Silica (µm kgⴚ1) on the isopycnal defined by 26.95 in
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 9.ab (a) Depth (hm) of the isopycnal defined by 31.87 in σ1. (b) Salinity on the isopycnal defined by 31.87 in σ1.
169
170
Fig. 9.cd
σ1 .
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(c) Oxygen (ml lⴚ1) on the isopycnal defined by 31.87 in σ1. (d) Silica (µm kgⴚ1) on the isopycnal defined by 31.87 in
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 10.ab
(a) Depth (hm) of the isopycnal defined by 32.00 in σ1. (b) Salinity on the isopycnal defined by 32.00 in σ1.
171
172
Fig. 10.cd
σ1 .
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(c) Oxygen (ml lⴚ1) on the isopycnal defined by 32.00 in σ1. (d) Silica (µm kgⴚ1) on the isopycnal defined by 32.00 in
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 11.ab
(a) Depth (hm) of the isopycnal defined by 36.805 in σ2. (b) Salinity on the isopycnal defined by 36.805 in σ2.
173
174
Fig. 11.cd
in σ2.
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(c) Oxygen (ml lⴚ1) on the isopycnal defined by 36.805 in σ2. (d) Silica (µm kgⴚ1) on the isopycnal defined by 36.805
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 12.ab
(a) Depth (hm) of the isopycnal defined by 36.92 in σ2. (b) Salinity on the isopycnal defined by 36.92 in σ2.
175
176
Fig. 12.cd
σ2 .
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
(c) Oxygen (ml lⴚ1) on the isopycnal defined by 36.92 in σ2. (d) Silica (µm kgⴚ1) on the isopycnal defined by 36.92 in
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 13.ab
(a) Depth (hm) of the isopycnal defined by 37.00 in σ2. (b) Salinity on the isopycnal defined by 37.00 in σ2.
177
178
Fig. 13.cd
σ2 .
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
(c) Oxygen (ml lⴚ1) on the isopycnal defined by 37.00 in σ2. (d) Silica (µm kgⴚ1) on the isopycnal defined by 37.00 in
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 14.ab
(a) Depth (hm) of the isopycnal defined by 41.495 in σ3. (b) Salinity on the isopycnal defined by 41.495 in σ3.
179
180
Fig. 14.cd
in σ3.
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(c) Oxygen (ml lⴚ1) on the isopycnal defined by 41.495 in σ3. (d) Silica (µm kgⴚ1) on the isopycnal defined by 41.495
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 15.ab
(a) Depth (hm) of the isopycnal defined by 45.89 in σ4. (b) Salinity on the isopycnal defined by 45.89 in σ4.
181
182
Fig. 15.cd
σ4 .
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
(c) Oxygen (ml lⴚ1) on the isopycnal defined by 45.89 in σ4. (d) Silica (µm kg-1) on the isopycnal defined by 45.89 in
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 16.ab
(a) Depth (hm) of the isopycnal defined by 45.96 in σ4. (b) Salinity on the isopycnal defined by 45.96 in σ4.
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184
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Fig. 16.c (c) Oxygen (ml lⴚ1) on the isopycnal defined by 45.96 in σ4.
J.L. Reid / Progress in Oceanography 56 (2003) 137–186
Fig. 17.ab
(a) Depth (hm) of the isopycnal defined by 45.98 in σ4. (b) Salinity on the isopycnal defined by 45.98 in σ4.
185
186
Fig. 17.cd
σ4 .
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(c) Oxygen (ml lⴚ1) on the isopycnal defined by 45.98 in σ4. (d) Silica (µm kgⴚ1) on the isopycnal defined by 45.98 in