On the total geostrophic circulation of the Indian Ocean: flow patterns, tracers, and transports

Transcription

1 Progress in Oceanography 56 (2003) 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 , 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 to40 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 to15 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 Elsevier Science Ltd. All rights reserved. Keywords: Indian Ocean; Circulation; Deep circulation; Geostrophic flow Contents 1. Introduction Data presentation The near-surface waters [Fig. 3(a d)] The principal layers [Fig. 4(a f)] Surface circulation [Fig. 5(a)] Tel.: ; fax: address: jreid@ucsd.edu (J.L. Reid) /03/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. doi: /s (02)

2 138 J.L. Reid / Progress in Oceanography 56 (2003) Flow beneath the surface [Fig. 5(b k)] Total transport (Fig. 6) The pattern of tracers on isopycnals (Figs. 7 17) The isopycnal where σ 0 is [Fig. 7(a d)] The isopycnal where σ 0 is [Fig. 8(a d)] The isopycnal where σ 1 is (27.25 in σ 0 ) [Fig. 9(a d)] The isopycnal where σ 1 is ( in σ 0 ) [Fig. 10(a d)] The isopycnal where σ 2 is [Fig. 11(a d)] The isopycnal where σ 2 is [Fig. 12(a d)] The isopycnal where σ 2 is [Fig. 13(a d)] The isopycnal where σ 3 is [Fig. 14(a d)] The isopycnal where σ 4 is 45.89[Fig. 15(a d)] The isopycnal where σ 4 is 45.96[Fig. 16(a c)] The isopycnal where σ 4 is [Fig. 17(a d)] Conclusion 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.

3 J.L. Reid / Progress in Oceanography 56 (2003) 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 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 m 3 s 1. The transport from the Indian Ocean into the South Pacific is taken to be m 3 s 1. Transport from the Pacific into the Indonesian seas and the Indian Ocean is taken to be m 3 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

4 140 J.L. Reid / Progress in Oceanography 56 (2003) Table 1 Expeditions from which stations were chosen to calculate the adjusted steric height Expedition/ship Dates NODC # Source 09AR Dec Feb WOCE S03/S N145 5 Dec Jan WOCE I08S/I09S 316N145 6 Jan. Feb WOCE I09N 316N145 7 Mar. Apr WOCE I05E/I08N 316N145 8 Apr. Jun WOCE I03 316N145 9 Jun. Jul WOCE I04/I05W/I07C 316N Jul. Aug WOCE I07N 316N Aug. Sep WOCE I01W 316N Sep. Oct WOCE I01E 316N Nov WOCE I10 316N Dec WOCE I02E 316N Dec Jan WOCE I02W 74AB29 1 Nov. Dec WOCE I05P Africana II Jun S. Africa Lacerda Almirante Apr. May S. Africa, IIOE Cr. AM1/64 AJAX Leg I, II Oct Jan SIO, TAMU (1985) Atlantis II Jul. Dec WHOI, IIOE Cr. 8 Atlantis II Sept WHOI Conrad 17 Jan. Apr LDGO (1980) Darwin Jan United Kingdom Darwin Cr. 29 Nov. Dec WHOI Discovery Feb. Mar United Kingdom Discovery Cr. 164 Dec Jan United Kingdom Eltainin Cr. 41 Dec Feb SIO, Horace Lamb Center & Johns Hopkins Univ. (1972) Fuji Maru Feb Japan Fuji Maru Feb Japan Islas Orcadas Dec LDGO (1981) Islas Orcadas Jan LDGO (1981) JARE Cr. 22 Feb. Mar Japan Marion Dufresne MD43 Feb. Mar France Marion Dufresne Feb. Mar French-Indonesian JADE Program Serrano Feb. Mar 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

5 J.L. Reid / Progress in Oceanography 56 (2003) Table 2 Specifications of the isopycnal surfaces a Indian Ocean σ 0 σ 1 σ 2 σ 3 σ 4 σ a The potential density is expressed as σ 0: db, as σ 1: db, as σ 2: db, as σ 3: db, as σ 4: db, and as σ 5: 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

6 142 J.L. Reid / Progress in Oceanography 56 (2003) 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.

7 J.L. Reid / Progress in Oceanography 56 (2003) 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 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 m from about 10 N to20 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 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

8 144 J.L. Reid / Progress in Oceanography 56 (2003) 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 to35 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 to20 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 Weddell- Enderby Basin and the Australian-Antarctic Basin. At 4500 db (not shown) there are isolated cyclonic flows in the Cape and Agulhas basins.

9 J.L. Reid / Progress in Oceanography 56 (2003) Total transport (Fig. 6) The top-to-bottom transport into the Indian Ocean from the Atlantic is set at m 3 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 m 3 s 1 (Gordon, Susanto, & Ffield, 1999), and m 3 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 m 3 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 m 3 s --1. The transport integration (Fig. 6) starts from zero at Antarctica and reaches m 3 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 m 3 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 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 s 0 is [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 to10 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 to10 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.

10 146 J.L. Reid / Progress in Oceanography 56 (2003) The isopycnal where s 0 is [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 σ 0 is 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 σ 0 ) to more than 1000 m ( in σ 2 ) The isopycnal where s 1 is (27.25 in s 0 ) [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 The isopycnal where s 1 is ( in s 0 ) [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 S is most obvious at this density and there is an eastward extension of high salinity just south of it, along 15 S to18 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 The isopycnal where s 2 is [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.

11 J.L. Reid / Progress in Oceanography 56 (2003) The new feature is the tongue of high salinity along 40 S to45 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 The isopycnal where s 2 is [Fig. 12(a d)] The high salinity from the Atlantic along 40 S to45 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 The isopycnal where s 2 is [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 The isopycnal where s 3 is [Fig. 14(a d)] As on the isopycnal where σ 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 The isopycnal where s 4 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.

12 148 J.L. Reid / Progress in Oceanography 56 (2003) The isopycnal where s 4 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 The isopycnal where s 4 is [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 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 Much of the Weddell Sea water turns back westward along the coast of Antarctica, and the salinity along the eastward flow drops from to less than 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 Sto15 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

13 J.L. Reid / Progress in Oceanography 56 (2003) 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 σ 4 is 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. References Beal, L. M., Ffield, A., & Gordon, A. L. (2000). Spreading of Red Sea overflow waters in the Indian Ocean. Journal of Geophysical Research, 105, Bindoff, N. L., Rosenberg, M. A., & Warner, M. J. (2000). On the circulation and water masses over the Antarctic continental slope and rise between 80 and 150 E. Deep-Sea Research, 1(47), Bye, J. A. (1972). Oceanic circulation south of Australia. In D. E. Hayes (Ed.), Antarctic oceanology II: the Australia New Zealand sector, Antarctic research series (pp ). Washington, DC: American Geophysical Union. Chong, J. C., Sprintall, J., Hautala, S., Morawitz, W. L., Bray, N. A., & Pandoe, W. (2000). Shallow throughflow variability in the outflow straits of Indonesia. Geophysical Research Letters, 27(125-12), 8. Cutler, A.N., & Swallow, J.C. (1984). Surface currents of the Indian Ocean. Institute of Oceanographic Sciences report 187. Fieux, M., Molcard, R., & Ilahude, A. G. (1996). Geostrophic transport of the Pacific Indian Oceans throughflow. Journal of Geophysical Research, 101(12), Fieux, M., & Swallow, J. C. (1988). Flow of deep water into the Somali Basin. Deep-Sea Research, 35, Gordon, A. L. (1986). Interocean exchange of thermocline water. Journal of Geophysical Research, 91, Gordon, A. L., Susanto, R. D., & Ffield, A. L. (1999). Throughflow within Makassar Strait. Geophysical Research Letters, 26, Gordon, A. L., & Tchernia, P. (1972). Waters of the continental margin off Adélie Coast, Antarctica. In D. E. Hayes (Ed.), Antarctic oceanology II: the Australia New Zealand Sector. Antarctic research series (pp ). Washington, DC: American Geophysical Union. Gründlingh, M. L. (1978). Drift of a satellite-tracked buoy in the southern Agulhas Return Current. Deep-Sea Research, 25, Harris, T. F. W. (1972). Sources of the Agulhas Current in the spring of Deep-Sea Research, 19, Hastenrath, S., & Greischar, L. (1979). Climatic atlas of the Indian Ocean, upper-ocean structure, 3. Madison, WI: University of Wisconsin Press. Jackett, D. R., & McDougall, T. J. (1997). A neutral density variable for the world s oceans. Journal of Physical Oceanography, 27, Jacobs, S. S., & Georgi, D. T. (1977). Observations on the southwest Indian/Antarctic Ocean. In M. Angel (Ed.), Voyage of discovery (pp ). New York: Pergamon. Johnson, D. A., & Damuth, J. E. (1979). Deep thermohaline flow and current-controlled sedimentation in the Amirante Passage: western Indian Ocean. Marine Geology, 33, 1 44.

14 150 J.L. Reid / Progress in Oceanography 56 (2003) Johnson, G. C., Musgrave, D. L., Warren, B. A., Ffield, A., & Olson, D. B. (1998). Flow of bottom and deep water in the Amirante Passage and Mascarene Basin. Journal of Geophysical Research, 103(30), Lutjeharms, J. R. E., Bang, N. D., & Duncan, C. P. (1981). Characteristics of the currents east and south of Madagascar. Deep-Sea Research, 28, Mantyla, A. W., & Reid, J. L. (1983). Abyssal characteristics of the world ocean waters. Deep-Sea Research, 30, Mantyla, A. W., & Reid, J. L. (1995). On the origins of deep and bottom waters of the Indian Ocean. Journal of Geophysical Research, 100, McCarthy, M. S., & Talley, L. D. (1999). Three dimensional isoneutral potential vorticity structure in the Indian Ocean. Journal of Geophysical Research, 104, Muraleedharan, P. M., Kumar, M. R. R., & Rao, G. V. L. (1995). A note on poleward undercurrent along the southwest coast of India. Continental Shelf Research, 15, Murty, V. S. N., Sarma, Y. V. B., Rao, D. P., & Murty, C. S. (1992). Water characteristics, mixing and circulation in the Bay of Bengal during southwest monsoon. Journal of Marine Research, 50, Murty, V. S. N., Suryanarayana, A., & Rao, D. P. (1993). Current structure and volume transport across 12 N in the Bay of Bengal. Indian Journal of Marine Science, 22, Orsi, A. H., Johnson, G. C., & Bullister, J. L. (1999). Circulation, mixing, and production of Antarctic bottom water. Progress in Oceanography, 43, Premchand, K., Sastry, J. S., & Murty, C. S. (1986a). Watermass structure in the western Indian Ocean part ii: the spreading and transformation of the Persian Gulf water. Mausam, 37, Premchand, K., Sastry, J. S., & Murty, C. S. (1986b). Watermass structure in the western Indian Ocean part II: the spreading and transformation of the Red Sea watermass. Mausam, 37, Quadfasel, D. R., & Schott, F. (1983). Southward subsurface flow below the Somali Current. Journal of Geophysical Research, 88, Quadfasel, D., Fischer, J., Schott, F., & Stramma, L. (1997). Deep water exchange through the Owen Fracture Zone in the Arabian Sea. Geophysical Research Letters, 24, Reid, J. L. (1981). On the mid-depth circulation of the world ocean. In B. A. Warren, & C. Wunsch (Eds.), Evolution of physical oceanography (pp ). Cambridge, MA: MIT Press. Reid, J. L. (1986). On the total geostrophic circulation of the south Pacific Ocean: flow patterns, tracers, and transports. Progress in Oceanography, 16, Reid, J. L. (1989). On the total geostrophic circulation of the south Atlantic Ocean: flow patterns, tracers, and transports. Progress in Oceanography, 23, Reid, J. L. (1994). On the total geostrophic circulation of the north Atlantic Ocean: flow patterns, tracers, and transports. Progress in Oceanography, 33, Reid, J. L. (1997). On the total geostrophic circulation of the Pacific Ocean: flow patterns, tracers, and transports. Progress in Oceanography, 39, Reid, J. L., & Lynn, R. J. (1971). On the influence of the Norwegian Greenland and Weddell seas upon the bottom waters of the Indian and Pacific oceans. Deep-Sea Research, 18, Richardson, P.L. & McKee, T.K. (1989). Surface velocity in the equatorial oceans (20 N 20 S) calculated from historical ship drifts. Woods Hole Oceanographic Institution technical report, 89-9 Rintoul, S. R. (1998). On the origin and influence of Adélie Land bottom water. In S. S. Jacobs, & R. F. Weiss (Eds.), Ocean, ice, and atmosphere: interactions at the Antarctic Continental Margin. Antarctic research series (pp ). American Geophysical Union. Rochford, D. J. (1966). Distribution of Banda intermediate water in the Indian Ocean. Australian Journal of Marine Freshwater Research, 17, Schott, F., Fischer, J., Garternicht, U., & Quadfasel, D. (1997). Summer monsoon response of the northern Somali Current, Geophysical Research Letters, 24, Schott, F. A., & McCreary, J. P. Jr. (2001). The monsoon circulation of the Indian Ocean. Progress in Oceanography, 51, Schott, F., Reppin, J., Fischer, J., & Quadfasel, D. (1994). urrents and transports of the Monsoon current south of Sri Lanka. Journal of Geophysical Research, 99(25), Schott, F., Swallow, J. C., & Fieux, M. (1990). The Somali Current at the equator: annual cycle of currents and transports in the upper 1000 m and connection to neighbouring latitudes. Deep-Sea Research, 37, Schott, G. (1935). Geographie des Indischen und Stillen Ozeans. Hamburg: C. Boysen. Sharma, G. S. (1971). Water characteristics at 200 cl/t in the intertropical Indian Ocean during the southwest monsoon. Journal of Marine Research, 30, Shetye, S. R., & Gouveia, A. D. (1998). Coastal circulation in the north Indian Ocean. In K. H. Brink, & R. Robinson (Eds.), The sea, 11. New York: John Wiley and Sons.

15 J.L. Reid / Progress in Oceanography 56 (2003) Shetye, S. R., Gouveia, A. D., & Shenoi, S. S. C. (1994). Circulation and water masses of the Arabian Sea. In D. Lal (Ed.), Biogeochemistry of the Arabian Sea (pp. 9 26). Bangalore: Indian Academy of Sciences. Smith, R. L., Huyer, A., Godfrey, J. S., & Church, J. A. (1991). The Leeuwin current off Western Australia, Journal of Physical Oceanography, 21, Speer, K. G., & Forbes, A. (1994). A deep western boundary current in the south Indian Basin. Deep-Sea Research I, 41, Spencer, D., Broecker, W. S., Craig, H., & Weiss, R. (1982). GEOSECS Indian Ocean expedition, sections and profiles, 6. Washington, DC: National Science Foundation. Stramma, L., Fischer, J., & Schott, F. (1996). The flow field off southwest India at 8 N during the southwest monsoon of August Journal of Marine Research, 54, Sverdrup, H. U., Johnson, M. W., & Fleming, R. H. (1942). The oceans: their physics, chemistry and general biology. Englewood Cliffs, NJ: Prentice-Hall. Swallow, J. C., & Bruce, J. G. (1966). Current measurements off the Somali coast during the southwest monsoon of Deep- Sea Research, 13, Swallow, J. C., Fieux, M., & Schott, F. (1988). The boundary currents east and north of Madagascar 1. Geostrophic currents and transports. Journal of Geophysical Research, 93, Toole, J. M., & Warren, B. A. (1993). A hydrographic section across the subtropical south Indian Ocean. Deep-Sea Research I, 40, Warren, B. A. (1974). Deep flow in the Madagascar and Mascarene basins. Deep-Sea Research, 21, Warren, B. A. (1981). Transindian hydrographic section at latitude 18 S: property distributions and circulation in the south Indian Ocean. Deep-Sea Research, 28, Warren, B. A., Stommel, H., & Swallow, J. C. (1966). Water masses and patterns of flow in the Somali Basin during the southwest monsoon of Deep-Sea Research, 13, Wyrtki, K. (1958). The water exchange between the Pacific and Indian oceans in relation to upwelling processes. In Proceedings of the Ninth Pacific Science Congress of the Pacific Science Association, Chulalongkorn University, Bangkok, Thailand, November 19 December 9, Bangkok: Secretariat, Ninth Pacific Science Congress, Department of Science. Wyrtki, K. (1961). Physical oceanography of the Southeast Asian waters. Scientific results of marine investigations of the South China Sea and the gulf of Thailand In: NAGA Report, vol. 2, (pp ). Scripps Institution of Oceanography, University of California, San Diego. Wyrtki, K. (1971). Oceanographic atlas of the international Indian Ocean expedition. Washington, DC: National Science Foundation. You, Y. (1998). Intermediate water circulation and ventilation of the Indian Ocean derived from water mass contributions. Journal of Marine Research, 56,

16 152 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 1. Principal topographic features shown on a Mollweide projection. The 3000 m depth contour is indicated.

17 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 2. Lines of stations used in the calculation of the geostrophic flow.

18 154 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 3.ab (a) Temperature ( C) at the sea surface. (b) Salinity at the sea surface.

19 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 3.cd (c) Oxygen (ml l 1 ) at the sea surface. (d) Silica (m kg 1 ) at the sea surface.

20 156 J.L. Reid / Progress in Oceanography 56 (2003) 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.

21 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 4.cd (c) Salinity on the north south section. (d) Potential density (σ 0 σ 5 ) on the north south section.

22 158 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 4.ef (e) Oxygen (ml l 1 ) on the north south section. (f) Silica (µm kg 1 ) on the north south section.

23 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 5.ab (a) Adjusted steric height at 0 db (10 m 2 s 2 or 10 J kg 1 ). (b) Adjusted steric height at 200 db (10 m 2 s 2 or 10 J kg 1 ).

24 160 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 5.cd (c) Adjusted steric height at 500 db (10 m 2 s 2 or 10 J kg 1 ). Depths less than 500 m are shaded. (d) Adjusted steric height at 800 db (10 m 2 s 2 or 10 J kg 1 ). Depths less than 1000 m are shaded.

25 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 5.ef (e) Adjusted steric height at 1000 db (10 m 2 s 2 or 10 J kg 1 ). Depths less than 1000 m are shaded. (f) Adjusted steric height at 1500 db (10 m 2 s 2 or 10 J kg 1 ). Depths less than 1500 m are shaded.

26 162 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 5.gh (g) Adjusted steric height at 2000 db (10 m 2 s 2 or 10 J kg 1 ). Depths less than 2000 m are shaded. (h) Adjusted steric height at 2500 db (10 m 2 s 2 or 10 J kg 1 ). Depths less than 2500 m are shaded.

27 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 5.ij (i) Adjusted steric height at 3000 db (10 m 2 s 2 or 10 J kg 1 ). Depths less than 3000 m are shaded. (j) Adjusted steric height at 3500 db (10 m 2 s 2 or 10 J kg 1 ). Depths less than 3500m are shaded.

28 164 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 5.k (k) Adjusted steric height at 4000 db (10 m 2 s 2 or 10 kg 1 ). Depths less than 4000 m are shaded. Fig. 6. Transport (106 m3 s 1). Depths less than 3500 m are shaded.

29 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 7.ab (a) Depth (hm) of the isopycnal defined by in σ 0. The dashed line indicates the outcrop. (b) Salinity on the isopycnal defined by in σ 0.

30 166 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 7.cd σ 0. (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 0. (d) Silica (µm kg 1 ) on the isopycnal defined by in

31 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 8.ab (a) Depth (hm) of the isopycnal defined by in σ 0. (b) Salinity on the isopycnal defined by in σ 0.

32 168 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 8.cd σ 0. (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 0. (d) Silica (µm kg 1 ) on the isopycnal defined by in

33 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 9.ab (a) Depth (hm) of the isopycnal defined by in σ 1. (b) Salinity on the isopycnal defined by in σ 1.

34 170 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 9.cd σ 1. (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 1. (d) Silica (µm kg 1 ) on the isopycnal defined by in

35 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 10.ab (a) Depth (hm) of the isopycnal defined by in σ 1. (b) Salinity on the isopycnal defined by in σ 1.

36 172 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 10.cd σ 1. (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 1. (d) Silica (µm kg 1 ) on the isopycnal defined by in

37 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 11.ab (a) Depth (hm) of the isopycnal defined by in σ 2. (b) Salinity on the isopycnal defined by in σ 2.

38 174 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 11.cd (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 2. (d) Silica (µm kg 1 ) on the isopycnal defined by in σ 2.

39 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 12.ab (a) Depth (hm) of the isopycnal defined by in σ 2. (b) Salinity on the isopycnal defined by in σ 2.

40 176 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 12.cd σ 2. (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 2. (d) Silica (µm kg 1 ) on the isopycnal defined by in

41 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 13.ab (a) Depth (hm) of the isopycnal defined by in σ 2. (b) Salinity on the isopycnal defined by in σ 2.

42 178 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 13.cd σ 2. (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 2. (d) Silica (µm kg 1 ) on the isopycnal defined by in

43 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 14.ab (a) Depth (hm) of the isopycnal defined by in σ 3. (b) Salinity on the isopycnal defined by in σ 3.

44 180 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 14.cd (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 3. (d) Silica (µm kg 1 ) on the isopycnal defined by in σ 3.

45 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 15.ab (a) Depth (hm) of the isopycnal defined by in σ 4. (b) Salinity on the isopycnal defined by in σ 4.

46 182 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 15.cd σ 4. (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 4. (d) Silica (µm kg -1 ) on the isopycnal defined by in

47 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 16.ab (a) Depth (hm) of the isopycnal defined by in σ 4. (b) Salinity on the isopycnal defined by in σ 4.

48 184 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 16.c (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 4.

49 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 17.ab (a) Depth (hm) of the isopycnal defined by in σ 4. (b) Salinity on the isopycnal defined by in σ 4.

50 186 J.L. Reid / Progress in Oceanography 56 (2003) Fig. 17.cd σ 4. (c) Oxygen (ml l 1 ) on the isopycnal defined by in σ 4. (d) Silica (µm kg 1 ) on the isopycnal defined by in