north atlantic deep water in the south-western indian ocean

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Deep-Sea Research I 51 (2004) 755–776

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doi:10.1016/j.ds

North Atlantic deep water in the south-western Indian Ocean

Hendrik M. van Akena,*, Herman Ridderinkhofa, Wilhelmus P.M. de Ruijterb

aRoyal Netherlands Institute for Sea Research, P.O. Box 59, 1960 AB Den Burg/Texel, Netherlandsb Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, Netherlands

Received 11 September 2002; received in revised form 21 October 2003; accepted 13 January 2004

Abstract

The circulation of deep water in the south-western Indian Ocean has been studied from hydrographic observations

and current measurements, obtained during the Dutch–South African Agulhas Current Sources Experiment

programme, and from similar public data from the World Ocean Circulation Experiment. The three major water

masses involved are the saline North Atlantic deep water (NADW), its derivative in the Antarctic circumpolar current,

lower circumpolar deep water (LCDW), and the aged variety of deep water, North Indian deep water (NIDW).

Although bound by the shallow topography near Madagascar, about 2� 106m3/s from the upper half of the NADW

core appears to flow across the sill in the Mozambique Channel into the Somali Basin, while the remaining NADW

flows east at about 45�S and is transformed to LCDW by lateral and diapycnal mixing. East of Madagascar the deep

circulation is dominated by the southward flow of NIDW. Northward inflow of LCDW into the Indian Ocean therefore

can take place only in the eastern half of the Indian Ocean, along the Southeast Indian Ridge and the Ninetyeast Ridge.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Deep water; Indian Ocean; Inter-ocean exchange

1. Introduction

Cold and saline water, formed by convection inthe North Atlantic Ocean, spreads at depth via theSouthern Ocean towards the Indian and PacificOceans in the global thermohaline circulation(THC). In the Southern Ocean this North Atlanticdeep water (NADW) is transformed to the fresherlower circumpolar deep water (LCDW) by upwel-ling and mixing with other water masses likeWeddell Sea deep water (WDW; Callahan, 1972;

ing author. Royal Netherland Institute for Sea

sdiep 4, Horntje, Texel 1797 S2, Netherlands.

69-416; fax: +31-222-319-674.

ess: [email protected] (H.M. van Aken).

e front matter r 2004 Elsevier Ltd. All rights reserve

r.2004.01.008

Park et al., 1993). In the Indian and PacificOceans, these water masses are assumed to riseto shallower levels by large-scale upwelling (Stom-mel, 1958). During this process, they are trans-formed and heated by diapycnal mixing with theoverlying less dense water (Munk, 1966). Thetransformed NADW is assumed to flow back tothe North Atlantic, partly as warm thermoclinewater (Gordon, 1986), bringing heat gained in theIndian and Pacific Oceans into the North Atlantic(de Ruijter et al., 1999), and partly as colderintermediate water (Rintoul, 1991). Additional tothe large-scale upwelling an aged form of NADW,generally named North Indian deep water(NIDW), is known to leave the northern andequatorial Indian Ocean basins by a southward

d.

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H.M. van Aken et al. / Deep-Sea Research I 51 (2004) 755–776756

transport towards the Antarctic circumpolarcurrent (ACC; Park et al., 1993). The subtropicalfront at about 42�S forms the northern boundaryof the ACC in the Indian Ocean (Orsi et al., 1995).Although the particular hydrographic propertiesof NIDW are caused mainly by mineralization oforganic matter, unlike most major water masseswhich obtain their properties in the surface layersdue to air–sea interaction, NIDW is usuallytreated as a separate water mass (Toole andWarren, 1993; Mantyla and Reid, 1995; You,1999, 2000). Because of the long isolation from itsNADW source in the Southern Ocean, NIDW ischaracterized by a relatively low oxygen and highnutrient content (Read and Pollard, 1993; Parket al., 1993; You, 2000). In the south-westernIndian Ocean the North Atlantic origin of NADWis reflected by a deep salinity maximum, coincidingwith an oxygen maximum and nutrient minima ata depth of about 2500m (Reid and Lynn, 1971;Mantyla and Reid, 1995). Both the underlyingAntarctic bottom water (AABW) and the over-lying upper circumpolar deep water (UCDW) havelower salinities and larger nutrient concentrations(Park et al., 1993). Deep water in the IndianOcean, derived from NADW, is generally ob-served in the depth interval of 2000–3500m (Tooleand Warren, 1993; Mantyla and Reid, 1995; You,1999).Several contrasting schemes have been devised

for the northward inflow of NADW or itsderivative LCDW, into the Indian Ocean. Thepronounced topography of the area with its manyridges (Fig. 1a) certainly will influence the deepflow of NADW. A basic assumption for theinterpretation of tracer data by many authors,based on ideas about large-scale upwelling and theresulting deep circulation brought forward byStommel (1958), is that the northward flow ofNADW in the southern Indian Ocean is organizedin fast deep western boundary currents (Le Pichon,1960; Callahan, 1972; Warren, 1974, 1981; John-son et al., 1991; You, 2000). Such deep under-

Fig. 1. (a) Topography of the research area. The thick line gives the

than 5000m are lightly shaded. (b) The hydrographic stations used in

with an additional solid line and symbol a–d.

currents have recently been observed directly fromcurrent measurements (Beal and Bryden, 1997,1999; Ridderinkhof and de Ruijter, 2003; deRuijter et al., 2002). Several authors assume thatthe sill in the Mozambique Channel (slightly over2500m) prevents the NADW core near the SouthAfrican continental slope from entering the SomaliBasin between Africa and Madagascar (Le Pichon,1960; Warren, 1974; Toole and Warren, 1993;Read and Pollard, 1993; Mantyla and Reid, 1995;You, 2000). However, they ignore the finitevertical extension of the NADW layer with raisedsalinities extending upwards to a depth ofB2000m (Toole and Warren, 1993; Mantyla andReid, 1995; You, 1999). The lateral distributionsof salinity, oxygen, phosphate and silicate at2500m by Wyrtki (1971) indicate that someinfluence of the high salinity, high oxygen, lownutrient NADW may extend across Davie Ridgeto at least 10�S. From the large-scale Indian Oceancirculation scheme, derived from a global inversemodel, Ganachaud et al. (2000) have derived anorthward flow through the Mozambique Channelbelow 2000m of 271.5 Sv (1 Sv=106m3/s). Thisvalue is similar to the northward transport of deepwater through the Natal Valley near 32�S, basedon lowered acoustic doppler current profiler(LADCP) observations (Donohue et al., 2000).The Madagascar Ridge and Southwest Indian

Ridge (Fig. 1a) form a barricade for the eastwardspreading of NADW towards the Crozet andMadagascar Basins. While, e.g. Read and Pollard(1993) and You (2000) assume that these ridgesconstrain the eastward flow of NADW from theMozambique Basin, others (Le Pichon, 1960;Warren, 1974; Toole and Warren, 1993; Donohueand Toole, 2003) report traces of NADW east ofthe Madagascar Ridge, which reflect at least alimited exchange across some deeper sills on theseridges. In a large-scale isopycnal analysis, Mantylaand Reid (1995) have observed a lateral maximumof salinity and oxygen and a minimum of silica inthe ACC, extending at a latitude of 45�S far

2500m isobath, the thin line the 5000m isobath. Basins deeper

this study (dots). The salinity sections given in Fig. 2 are marked

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H.M. van Aken et al. / Deep-Sea Research I 51 (2004) 755–776 757

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eastwards from the Mozambique Basin, along theDel Cano Rise and the Kerguelen towards 140�E,south of Australia. This result is similar to a moreadvanced analysis along neutral surfaces (You,2000) and can already be recognized from thelateral distributions of hydrographic properties ata depth of 2500m, presented by Wyrtki (1971).Apparently, a significant part of the NADW corenear southern Africa is transported around the DelCano Rise eastwards into the ACC, where it isconverted by mixing into fresher LCDW (Call-ahan, 1972).There is also no general agreement on the

meridional transport of NADW in the MascareneBasin, east of Madagascar. You (2000) proposed anorthward flow of NADW through the MascareneBasin east of Madagascar, while Warren (1974)proposed a northward flow east of the MascareneRidge. Robbins and Toole (1997) stated that thenorthward inflow of NADW at 32�S is limited tothe area west of 60�E, evenly distributed over theMozambique Basin and Natal Valley, west of theMadagascar Ridge, and the Madagascar Basin,east of that ridge. Other authors assumed that atthe NADW level NIDW flows southward throughthe Mascarene Basin (Le Pichon, 1960; Warren,1981; Park et al., 1993; Mantyla and Reid, 1995).With the injection of NIDW into the ACC, thisaged form of NADW may also contribute to theformation of LCDW (Park et al., 1993).Widely varying numbers on the strength of the

meridional overturning of the Indian Ocean,expressed as deep upwelling of the net northwardflowing NADW and AABW, can be found in theliterature. From geostrophic calculations, con-strained in inverse models of varied complexity,values of upwelling at 2000m, north of 32�S, havebeen reported to amount to 3.6 Sv (Fu, 1986),25 Sv (Toole and Warren, 1993), 17 Sv (Macdo-nald, 1995), 13 Sv (Robbins and Toole, 1997), and11 Sv (Ganachaud et al., 2000). The estimatedcontribution of NADW to this overflow variesbetween 1.5 Sv (Robbins and Toole, 1997) and10 Sv (Toole and Warren, 1993). The net NADWinput of 1.5 Sv, reported by Robbins and Toole(1997) resulted from a northward inflow ofB16.5 Sv between South Africa and 60�E and asouthward flow of B15 Sv between 60�E and

western Australia. You (1999) came to an evenlower estimate of the net northward flow below2000m at 32�S (0.4 Sv) of which only 0.2 Sv wascontributed by NADW, based on an estimate ofthe diapycnal mixing, with an ad hoc, low estimateof the turbulent diffusivity of 10�5m2/s. Gana-chaud et al. (2000) reported the net northwardflow between 2000 and 3500m (the NADW layer)with a value of 3, 5, and 0.5 Sv at, respectively,32�S, 20�S, and 8�S. None of these quantitativeestimates of the strength of the meridional over-turning in the Indian Ocean were based on directvelocity measurements. The two orders of magni-tude variation of the value of the net NADW input(0.2–10 Sv) as well the contrasting circulationschemes show that there are still large uncertaintiesin the importance of the flushing of the deepIndian Ocean for the global THC.Many analyses of the circulation of deep water

in the Indian Ocean have been performed on alarge scale (e.g. Warren, 1981; Toole and Warren,1993; Mantyla and Reid, 1995). In this paper, wedeal with the NADW as a possible direct source ofdeep water in the Indian Ocean. Therefore, theanalysis presented here focuses on the south-western Indian Ocean. We will present recenthydrographic data, obtained during the Dutch–South African Agulhas current sources experiment(ACSEX) near Madagascar, as well as hydro-graphic data from the World Ocean CirculationExperiment (WOCE) Hydrographic Program(WHP) and additional current measurements tostudy details of the circulation of NADW in thesouth-western Indian Ocean. To effectively use thehydrographic profiles a subjective regional stationgrouping into clusters is applied which is not thestandard statistical cluster analysis. By datareduction of hydrographic profiles to regionallymean cluster profiles we have attempted to useboth the lateral and vertical hydrographic struc-ture in our studies, in order to gain insight in thedirection of the flow of NADW in the south-western Indian Ocean. The vertical profiles andhorizontal distributions of hydrographic pro-perties of these a priori defined regional clustersare used to show that they are compatiblewith a circulation scheme with NADW enteringthe Indian Ocean northwards through the

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Mozambique Channel and eastwards into theLCDW core, while NIDW is exported southwardseast of Madagascar. The few available currentmeasurements are shown to agree with thiscirculation scheme, with highest meridional velo-cities near the continental slopes, and supply a firstestimate of the northward transport of NADWthrough the Mozambique Channel.

2. The data

In the Dutch–South African ACSEX pro-gramme three hydrographic surveys were carriedout with RV Pelagia, Pelagia cruise 156 in 2000and cruises 176 and 177 in 2001 (Lutjeharms et al.,2000; Ridderinkhof, 2000, 2001; van Aken, 2001).During these surveys, CTD casts were performedfrom the sea surface to within a few metres fromthe bottom, while at Pelagia cruise 176 part of thecasts south and east of Madagascar ended atshallower levels (B2500m) due to cable problems.During the up-casts, water samples were taken atstandard depths for the determination of nutrientand oxygen concentrations and salinity calibra-tions. The CTD/Rosette-system was supplementedwith an LADCP which was operational on allCTD stations. The CTD in use was a pumpedSeabird SBE9/11+ system. The CTD data wererecorded on board with 24Hz. After data proces-sing and calibration the data were reduced to1 dbar averages.The resulting accuracy of these calibrated and

processed CTD data (1� standard deviation) isestimated to be 1.2 dbar for pressure, 0.001�C fortemperature, and 0.002 for (practical) salinityrelative to the P137 standard water batch, meetingWOCE standards. This high accuracy of pressureand temperature is possible because of directaccess to the Dutch national calibration facility,NMI, and the use of SIS RPM-6000X and SBE35reference sensors (Ober, 1998). According to theIAPSO (International Association for the PhysicalSciences of the Ocean) recommendations (UN-ESCO, 1985) the density anomaly symbol g(gamma) and the dimensionless practical salinityare used throughout the paper. Note that thenumerical value of g in kg/m3 is systematically

0.025 higher than the traditional dimensionlessKnudsen parameter s which was based on thespecific gravity, not the density (Gill, 1982). Thepotential density of seawater derived from theCTD observations is expressed as the potentialdensity anomaly gN, where N is the referencepressure in N� 1000 dbar (JPOTS Editorial Panel,1991). Since the NADW core was observed in anarrow density interval near a pressure level ofB2500 dbar the potential density anomaly relativeto that pressure, g2.5, has been used as vertical co-ordinate in this paper. This removes effects ofvertical displacements of isopycnals by geostrophicshear, eddies and internal waves. Water sampleswere taken with 5 and 10 dm3 samplers, mountedin the CTD rack. The water samples were analysedfor dissolved phosphate, nitrate, silica and oxygen.The resulting precision (root mean square of thedifference between duplicates) of the chemicalanalyses is estimated to be 0.6, 0.02, 0.2, and0.5 mmol/kg for, respectively, oxygen, phosphate,nitrate, and silica.The LADCP system consisted of two synchro-

nized self-contained 300 kHz RDI Workhorsebroadband ADCP’s. A vertical bin size of 8mwas applied. The LADCP data were processedwith a modified MATLAB master script fileoriginally developed by Visbeck (2002). Theaccuracy of velocities measured with an LADCPis still difficult to determine, but, referring toVisbeck (2002), the accuracy of bottom trackedprofiles is estimated to be of the order of 1–2 cm/s.Seven current meter moorings were deployed

during Pelagia cruise 156 in April 2000, roughly atB17�S along a line between Mozambique andMadagascar in the Mozambique Channel. Thesemoorings were serviced and re-deployed in April2001 and finally recovered in November 2001 byRV Charles Darwin. One mooring was located inthe deep channel along Davie Ridge. The deepestcurrent meter from this mooring was at the levelof NADW (2500m). Reliable records fromthis current meter are available for the periodApril–November 2001 (Ridderinkhof and deRuijter, 2003).WOCE hydrographic data (CTD, oxygen and

nutrients) from sections in the South AtlanticOcean, Southern Ocean and Indian Ocean

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(WOCE Data Products Committee, 2000) havealso been used, additional to the ACSEXdata. Additional hydrographic data from theAgulhas Passage and the area just east of DavieRidge were downloaded from the World DataCenter for Oceanography, operated by the USNational Oceanographic Data Center. Fig. 1bshows the hydrographic stations used for thisstudy. Use has been made of current measure-ments from literature (Schott et al., 1988), andpublic current meter data from WOCE mooringarrays ICM1 (Agulhas current) and ICM3 (EastMadagascar current,), additional to the ACSEXcurrent observations.Since the oxygen saturation concentration de-

pends strongly on temperature, the sub-surfaceoxygen concentrations show the effects not only ofageing (decrease in oxygen concentration andincrease in nutrient concentrations due to miner-alization of organic matter), but also of tempera-ture. In order to separate the temperature effectsfrom the ageing effects on the oxygen concentra-tion, the parameter apparent oxygen utilization(AOU), often used in oceanography, was analysed

Fig. 2. The salinity distribution along (a) a hydrographic section perp

nominal zonal section at B34�S in the south-western Indian Ocean, (

Indian Ocean, and (d) a nominal zonal section across the narrow secto

potential density anomaly relative to a pressure of 2500dbar, g2.5hydrographic sections is shown in Fig. 1b.

instead. AOU is defined as

AOU ¼ ½O2sat� � ½O2� ð1Þ

with [O2sat] the saturation value of the oxygenconcentration [O2], in balance with the atmo-sphere.

3. Results

3.1. Sections

The deep salinity maximum, connected with theNADW core, was found at pressures close to2500 dbar around southern Africa. This salinitymaximum could be followed in a narrow band(50–100 km) along the African continental slopefrom the Atlantic WHP A10 section at 30�S nearwestern South Africa (Fig. 2a) to the MozambiqueChannel east of Africa at at least the same latitude(Fig. 2b). Further south, off the African con-tinental slope, the salinity decreased to valuestypical of LCDW. Along the zonal WHP sectionIO5 (Toole and Warren, 1993) at a nominal

endicular to the coast line at B30�S in the Atlantic Ocean, (b) a

c) a nominal meridional section at B40�E in the south-western

r of the Mozambique Channel at B17�S surveyed in 2000. The

, has been used as vertical co-ordinate. The location of the

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latitude of B32�S, the NADW salinity maximumwas found at approximately g2.5=39.25 kg/m3

(Fig. 2b). Both in the Natal Valley and in theMadagascar Basin, the high salinity NADW corewas confined in a relatively narrow band, along theAfrican continental slope and along the slope ofthe Madagascar Ridge respectively. In the Mo-zambique Basin, the lateral salinity gradients inthe NADW core along this section were lesspronounced, although also here the salinitydecreased to the east. The existence of the narrowsalinity maximum along the Madagascar Ridge isascribed to a small leakage of NADW throughgaps in this ridge (Donohue and Toole, 2003).During the Southwest Indian Ocean Experiment(SWINDEX) programme RV Discovery surveyeda nearly meridional section along a longitude ofabout 40�E (Read and Pollard, 1999; Pollard andRead, 2001). This section reached from theConrad Rise, over the Del Cano Rise via theDiscovery Fracture Zone, across the southernreaches of the Madagascar Ridge towards theMozambique Basin. Along this section, the salinitydistribution (Fig. 2c) showed narrow high salinityNADW cores over the southern slope of the DelCano Rise and over the south-eastern slope of theMadagascar Ridge centred around theg2.5=39.25 kg/m3 isopycnal. At this density levelthe salinity difference across the MadagascarRidge was less than 0.01. In the MozambiqueChannel at a latitude of approximately 17�S,nearly zonal hydrographic sections were surveyedby RV Pelagia in 2000 and 2001 (Ridderinkhof,2000, 2001; de Ruijter et al., 2002). In both years,potential density maxima over g2.5=39.25 kg/m3

were observed in the narrow deep channel(B2600m) west of the Davie Ridge. The salinitydistribution along this section (Fig. 2d) showedthat there the maximum salinity was in the nearbottom layer at the western side of the channel. Inthe shallower basin east of the Davie Ridge muchlower densities were observed in the near bottomlayer (g2.5o39.17 kg/m3). The low salinity, over-lying the NADW in the western part of theMozambique Channel, was connected with afresher low oxygen core of UCDW between 1000and 1500dbar, flowing northward along the Afri-can continental margin (de Ruijter et al., 2002).

The deep salinity distribution, as illustrated inFig. 2, shows the presence of high salinity coresover the steeply sloping topography extendingfrom the south-western African slope to theMadagascar Ridge, Del Cano Rise and theMozambique Channel in the Indian Ocean. Thissuggests that the inflow of NADW into the south-western Indian Ocean is organized as an intensifiedslope current, more or less following the slopecontours around the complex topography. Thepresence of high salinity cores near theg2.5=39.25 kg/m3 isopycnal in the DiscoveryFracture Zone, over the southern slope of theDel Cano Rise, and over the eastern slope of theMadagascar Ridge may indicate that part of theNADW core reached the Madagascar Basin in thisslope current. The decrease of the maximumsalinity from the Natal Valley to the MadagascarBasin suggests that continuing mixing, eitherdiapycnal or isopycnal, modifies the NADW corein the slope current. This modification will bediscussed below in more detail.The high density and high salinity, observed at

about 2600m in the Mozambique Channel,indicates that the upper part of the NADW coremay cross the sill on Davie Ridge. This overflow isapparently fed by a deep current bound to theAfrican continental slope. Such a ‘‘Mozambiqueundercurrent’’ has been observed (with LADCP)during the first ACSEX cruise by RV Pelagia (deRuijter et al., 2002).In the following sections, we will describe the

development and modification of the NADW coreby tracking the mean properties of regional groupsor clusters of stations, representing the NADWcore. This is an alternative to the analysis of thelateral distribution of hydrographic properties inisobaric or isopycnal surfaces (e.g. Callahan, 1972;Mantyla and Reid, 1995; You, 1999, 2000).

3.2. Hydrographic properties of water type clusters

A total of 48 regional station clusters werechosen a priori, based on similarity of water massstructure in the NADW range and geographicvicinity, in order to follow the circulation andmodification of the NADW core when it enters theIndian Ocean. The clusters consisted of a number

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Fig. 3. Mean positions of the regional clusters of CTD stations used in this study. The range of the salinity within each cluster near the

NADW level is generally less that 0.005. At the clusters indicated with an open crossed circle an AOU minimum is found at the level of

the NADW core. At the clusters indicated with a black dot an AOU maximum is present, while at the clusters indicated with a cross,

the presence of the NADW core can be discerned from an inflection point in the AOU profile. The position of the Subtropical Front,

which forms the northern boundary of the Antarctic Circumpolar Current, derived from Orsi et al. (1995), is sketched with a thick

dashed line.


of neighbouring hydrographic stations (1–8) on ahydrographic section with a very similar hydro-graphic structure (salinity in the isopycnal, char-acteristic for the NADW core, varied less than0.005). Most clusters were located at geographicpositions where a hydrographic section crossed asteep slope or other topographic feature (num-bered symbols in Fig. 3) and generally representedthe maximum salinity cores, shown in Fig. 2.Clusters 1–13 did connect the NADW in theBenguela region with the Somali Basin. Clusters14–44 formed the connection from the Mozambi-que Basin over and around the Madagascar Ridgeand the Southwest Indian Ridge with the equator-ial Central Indian Basin, where cluster 27 pre-sented an eastward extension over the northernslope of the Kerguelen Islands Plateau. Finally,

clusters 45–48 connected the Central Indian Basin,east of the Mascarene Ridge, with the CrozetBasin. A number of additional clusters weredefined in the eastern basins. For each cluster ofstations the characteristic mean profiles of hydro-graphic properties were determined.The Y–S diagram for these clusters (Fig. 4)

shows that the deep water mass generally ischaracterized by a deep salinity maximum inbetween fresher UCDW above (So34.70) andAABW below (Yo1�C). The salinity maxima areobserved at a mean pressure of 2650 dbar, with apotential density anomaly of approximatelyg2.5=39.25 kg/m3. At potential temperatures be-low 1�C, the Y–S curves converge to the homo-geneous characteristics of AABW. At the largedepths where that water mass is found (>3500m)

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Fig. 4. Potential Temperature-Salinity plot for the station clusters shown in Fig. 3. Cluster 10 near the mooring line in the

Mozambique Channel is indicated with crosses, cluster 11, just across the Davie Ridge, with black dots, and cluster 13 in the Somali

Basin near 4�S by open circles. The arrow indicates the direction of transition from the Benguela area in the Atlantic Ocean (clusters 1

and 2) towards the clusters in the Madagascar Basin and Crozet Basin (clusters 47 and 48). The dashed isopycnal lines give the

potential density anomaly g2.5 in kg/m3.


sills in the topography very strongly determine theflushing of AABW in the deep basins of the IndianOcean (Read and Pollard, 1999). The transitionbetween the NADW core and the overlying watermass is defined tentatively at a density level ofg2.5=39.15 kg/m3 observed at a mean depth ofabout 2000m (Toole and Warren, 1993; Mantylaand Reid, 1995; You, 1999). The succession ofcurves suggests a continuing modification andfreshening of the NADW core from the inflowpoint in the Benguela area (clusters 1 and 2)towards the Somali Basin and Central IndianBasin (the arrow direction in Fig. 4). Also thethickness of the salinity maximum layer decreasedfrom the Benguela region towards the tropical

Indian Ocean (not shown). The modification of thesaline NADW core may be achieved by diapycnalmixing with overlying and underlying fresherwater types as well as by isopycnal mixing withthe LCDW which also has a lower salinity thanNADW. At a limited number of clusters close tothe equator, the NADW core was overlain by ahigh salinity water mass reflecting the influence ofRed Sea outflow water (RSW) from the ArabianSea (Clowes and Deacon, 1935; Beal et al., 2000).The presence of this saline intermediate water willsuppress the expression of a deep salinity max-imum. However, the presence of remnants of asaline water mass at NADW densities still can bederived from an inflection point in the Y–S curve

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near the g2.5E39.21 kg/m3 isopycnal. This curva-ture of the Y–S curve implies that progressivediapycnal mixing will lead to a decrease of salinityin the NADW core, even in the near equatorialregion, when diapycnal mixing is dominant (Reidand Lynn, 1971). This indicates that the generalflow direction of NADW in our research area isfrom high to low salinities.A near bottom salinity maximum was observed

in the Mozambique Channel, just south of the sillon Davie Ridge at a potential density anomaly g2.5just over 39.25 kg/m3 (crosses in Fig. 4). Just northof Davie Ridge a similar, but weaker, salinitymaximum was encountered at a slightly lowerdensity (black dots in Fig. 4) while over theAfrican slope at 4�S an inflection point in the Y–S

curves was found (open circles in Fig. 4). Thissuccession of Y–S curves is compatible with a

Fig. 5. Property–potential density plots for the stations clusters shown

(d) dissolved phosphate. Cluster 10 near the mooring line in the Moza

across the Davie Ridge, is indicated with black dots in (a) and (b). The

(clusters 1 and 2) to NIDW north and east of the Mascarene Ridge (

northward transport of NADW from the Mozam-bique Channel along the east African slope toequatorial latitudes.

3.3. Chemical tracers

The vertical structure of chemical tracers(Fig. 5) reflects a similar continuous modificationof the vertical structure of the NADW core. In theBenguela region NADW is characterized by aminimum in AOU and nutrient concentrations.The horizontal distribution of clusters with AOUminima at the level of the NADW core (opensymbols in Fig. 3) reveals that AOU minima arefound along the African slope to the MozambiqueChannel, in the Mozambique Basin, and overthe Madagascar Ridge and Del Cano Rise,extending from there eastwards to the Kerguelen.

in Fig. 3: (a) AOU, (b) dissolved silica, (c) dissolved nitrate, and

mbique Channel is indicated with crosses, while cluster 11, just

arrow indicates the transition of NADW in the Atlantic Ocean

clusters 43–48).

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Everywhere over the deep Crozet Basin, theMadagascar Basin, the Central Indian Basin andthe Mascarene Basin, AOU minima connectedwith an NADW core were absent. The chemicaltracers in the g2.5=39.25 kg/m3 isopycnal showeda clear bimodal distribution with a bundle ofprofiles characteristic of the original NADWstratification (minima in AOU and nutrients) anda bundle of profiles characteristic for the northernbasins (Fig. 5). Only a few profiles were found inbetween these bundles. In the northern CrozetBasin, the Madagascar Basin and the southernMascarene Basin the AOU minima did change tomaxima (black dots in Fig. 3). At the northern-most clusters the NADW core was overlain bywater with very high AOU, nitrate and phosphateconcentrations, which changed the NADW core inthe diagrams of Fig. 5 from local maxima in theMadagascar Basin to inflection points furthernorth (crosses in Fig. 3). However, in dissolvedsilica (Fig. 5b) the local maximum in the Mada-gascar Basin was maintained further north becauseof relatively low silicate values in the overlyingwaters. Diapycnal mixing with overlying andunderlying waters only can diminish the magni-tude of an AOU or nutrient minimum, but cannotchange a minimum connected with the NADWcore into a maximum. The observed change fromlocal minima in AOU and nutrients in the NADWcore to local maxima in the Madagascar Basin alsocannot be achieved by isopycnal mixing withLCDW, since the waters south of our researcharea are also characterized by an AOU minimumat a potential density anomaly 39.25og2.5o39.30 kg/m3. Apparently, a northern sourceof aged water (high AOU and nutrients) isrequired at NADW densities to explain theobserved AOU and nutrient maxima. NIDW, theaged variety of NADW/LCDW, is a likely sourcefor this water.The profiles of chemical tracers show a near-

bottom minimum of AOU and nutrients on theMozambique Channel just south of Davie Ridge(cluster 10, crosses in Fig. 5), reminiscent of theNADW core. Just north of the sill on Davie Ridge,cluster 11 with a local AOU minimum wasobserved, but at a potential density anomalyg2.5E39.21 kg/m3, above the density level of the

NADW core south of the Ridge (the dots inFig. 5a). The dissolved silica stratification alsoshowed a minimum near the g2.5=39.21 kg/m3

isopycnal (dots in Fig. 5b). These minima coin-cided with the salinity maximum for this cluster(black dots in Fig. 4). Similar to the succession ofY–S diagrams along the east African slope thistracer structure is compatible with a northwardtransport of the upper part of the NADW corethrough the Mozambique Channel and acrossDavie Ridge into the Somali Basin. In the deepchannel west of Davie Ridge (cluster 10) waterwith potential density anomaly over 39.25 kg/m3

was observed in the 100m thick near bottom layerduring the ACSEX surveys of RV Pelagia in 2000and 2001, at depths over 200m shallower thannorth-east of the ridge (cluster 11). Apparently, adensity gradient across the sill was present to drivean overflow of this water across the Davie Ridge.It may be expected that entrainment of overlyingwater will lower the density of the overflow waterto a level where the above-mentioned minima inAOU and silica were found.

3.4. Trends along sections of station clusters

The development of the salinity maximum alongthe line of clusters (Fig. 6a) shows a continuingdecrease of the salinity from the Benguela regiontowards the Somali Basin. From cluster 3 onwardsthe maximum salinity was observed around theg2.5=39.25 kg/m3 isopycnal, suddenly rising to the39.21 kg/m3 isopycnal in cluster 11 across theDavie Ridge (Fig. 6b). Also on the western side ofthe Madagascar Ridge, around the Del Cano Riseand near the Kerguelen slope the salinity max-imum was found near the g2.5=39.25 kg/m3

isopycnal. From the Indomed Fracture Zone(clusters 31 and 32) to the Mascarene and CentralIndian Basins this maximum shifted to shallowerisopycnals (39.20o g2.5o39.23 kg/m3). The lowestsalinity maxima were found in the Crozet Basinand southern Central Indian Basin (clusters 47 and48) at the highest potential densities(g2.5E39.27 kg/m3), suggesting a relatively strongLCDW influence. The isopycnal evolution of thesalinity along the line of clusters showed a similardevelopment (Fig. 6c). Apparently, there are direct

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Fig. 6. The development of the salinity of the NADW core for successive station clusters: (a) the salinity of the deep salinity maximum,

(b) the potential density anomaly at the deep salinity maximum, (c) the salinity in g2.5=39.20 (triangles), 39.25 (black dots), and

39.30 kg/m3 isopycnals (inverted triangles). The arrows indicate shortcuts, explained in the text.


flow connections across the Mozambique Basin(arrows I and II) and the Mozambique Channel(arrow III) indicative of a cyclonic re-circulationof NADW south-east of Africa. This contrasts theanti-cyclonic circulation in the Mozambique Ba-sin, proposed by You (2000), since such acirculation would bring less modified, more salineNADW to the eastern side of the basin, while themore modified, fresher NADW would be found in

the western Mozambique Basin and Natal Valley.The high salinity in the northern DiscoveryFracture Zone (cluster 30, see also Fig. 2b)suggests a direct connection at NADW level(arrow IV) with the Mozambique Basin (clusters19 and 20). A similar salinity maximum just east ofthe Madagascar Ridge (cluster 34) confirms theprobable eastward flow of NADW (arrow V)south of the plateau in the Madagascar Ridge

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(Toole and Warren, 1993; Donohue and Toole,2003). Further south along the Madagascar Ridge(cluster 33) as well further north near the south-eastern Madagascar slope (cluster 35), the salinityof the deep water over the slope was definitelylower, suggesting that the high salinity deep waterof cluster 34 is not part of a large-scale northwardslope current. In the Mascarene Basin (clusters 35–42), the salinity at NADW levels hardly changed,increasing to slightly higher values in the equator-ial Central Indian Basin (cluster 44). A similarsalinity increase was observed from the CrozetBasin to the Central Indian Basin (clusters 48–44).Since we expect a decrease of salinity in theNADW isopycnal due to diapycnal mixing thislatter change in salinity gradient suggests a south-ward flow at the NADW level towards theMadagascar and Crozet Basins. The isopycnaldistribution of chemical tracers along the clusterline (Fig. 7) showed a similar trend, but withslowly increasing AOU values and nutrient con-centrations from the Benguela region along theAfrican slope, via the Mozambique Basin, andaround the Del Cano Rise. This increase may beattributed to mixing as well as to ageing of thewater mass. Across the Davie Ridge a suddenincrease in AOU values and nutrient concentra-tions could be observed, partly due to the shift ofthe NADW core to lower densities. Between thewestern Madagascar Basin and the MascareneBasin also a sudden increase in AOU anddissolved nutrients is observed. Their values stayedhigh throughout the Mascarene and CentralIndian Basins, coincident with the disappearanceof the AOU minima (compare with Fig. 3). Thisprobably reflects the presence of a deep thermoha-line frontal zone between two different watermasses, not a sudden increase in mineralizationof organic matter nor entrainment of water poor inoxygen and with high nutrients. This front wasmost pronounced between clusters 34 and 35 alongthe eastern flank of the Madagascar Ridge(DAOU=39 mmol/kg, DSi=42 mmol/kg) andcould also be recognized in the salinity trend inFig. 6c (DS=�0.05). Cluster 36 had hydrographicproperties similar to cluster 35 and can beconsidered to be on the same side of the front.The geostrophic shear between clusters 34 and 36

agrees with a maximum south-eastward geos-trophic transport at about 2400m, the depthwhere the deep salinity maximum was found inthe neighbouring clusters.The pressure of the g2.5=39.25 kg/m3 isopycnal

did not show any large-scale trend north of 37�S(27007200 dbar), while south of that latitude itrose to a level of approximately 700 dbar at 60�S,where LCDW with a salinity maximum of B34.73and an AOU of B140 mmol/kg was the dominantwater mass. The relation between salinity andAOU in the g2.5=39.25 kg/m3 isopycnal showedtwo different groups of clusters (Fig. 8a). A lowAOU group with a negative correlation (A)represents all clusters with a local AOU minimumnear the g2.5=39.25 kg/m3 isopycnal (clusters 1–10and 14–34). The other group (B), with higherAOU values and representing those clusters wherethe AOU minimum was not found (clusters 12 and13 and 35–48) plus cluster 11 north of DavieRidge, had increasing AOU values with highersalinities. The apparently increasing age (=AOU)of the NADW core initially coincided in group Awith a further dilution of the salinity maximum.This may be due to diapycnal mixing with theoverlying and underlying fresher water with higherAOU values, or due to the progressive mineraliza-tion of organic matter. But between the SomaliBasin and the northern Crozet Basin (group B) theapparent ageing of the core was connected with anincrease of the salinity. The significant linearregression lines in Fig. 8 indicate that locally ingroups A and B the change in the non-conserva-tive AOU co-occurred linearly with changes in theconservative salinity. This suggests that locally inour research area mixing was dominant overageing for the observed AOU distribution. Thejump in AOU from group A to group B co-occurred with a similar jump in nutrient concen-trations, e.g. dissolved silica (Fig. 8b), reminiscentof a two endpoint member situation in the NADWisopycnal, suggested by Minster and Boulahdid(1987). Similar graphs could be drawn for phos-phate and nitrate. In the clusters with high AOUand nutrient concentrations, diapycnal mixing willlead to progressively lower values given thecurvature of the profiles (see Fig. 5) as well aslower salinities. If diapycnal mixing is locally

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Fig. 7. The development of (a) AOU, (b) dissolved silica, (c) dissolved phosphate, and (d) dissolved nitrate in the g2.5=39.25 kg/m3

isopycnal for successive clusters. The triangles in (a) show the g2.5=39.20 kg/m3, the inverted triangles the 39.30 kg/m3 isopycnal.


dominant over oxygen consumption due to miner-alization, the mean flow is expected to be directedfrom the equatorial region to the Mascarene Basin

and the Crozet Basin. The high linear correlationof dissolved silica and AOU (Fig. 8b), also agreeswith the dominance of mixing over ageing in our

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Fig. 8. Plots of AOU in the g2.5=39.25 kg/m3 isopycnal versus (a) salinity and (b) dissolved silica, with regression lines. The data

points from group A with a negative correlation R are from all clusters where a deep AOU minimum can be observed, except cluster

11, which is added to group B, the clusters where no AOU maximum is observed.


area, since the dissolution of silica frustules occurscompletely independent of the mineralization oforganic matter.

3.5. Current measurements

LADCP measurements present instantaneoussamples of the velocity structure which maycontain considerable contributions of the eddyfield, and internal (tidal) waves. One has to keepthis in mind when such measurements are inter-preted. Generalization of LADCP results isallowed only when certain current structures arerepeatedly encountered. Beal and Bryden (1997,1999) reported a northward undercurrent over thecontinental slope below the Agulhas Current,while Donohue et al. (2000) showed that thiscurrent structure was encountered during threedifferent LADCP surveys.Similar to the LADCP observations in the Natal

Valley near 32�S (Beal and Bryden, 1997, 1999;Donohue et al., 2000) a deep northward flow wasencountered regularly during the Pelagia ACSEXsurveys along the south-eastern African slope inthe depth and density interval of the NADW core(g2.5>39.15 kg/m3) from 30�S to the Somali Basin(Fig. 9). At 24�S the northward transport of

NADW was estimated roughly to be 2 Sv (deRuijter et al., 2002). During both ACSEX surveysof the 17�S section in 2000 and 2001, a net deepequatorward flow has been observed in the deepchannel west of the Davie Ridge below 2000m.Even at a latitude of 12�S between the Africancontinental slope and the Comores a net equator-ward flow in the NADW density range wasobserved. Despite the presence of occasionalsouthward flow in the NADW range, the presenceof a net equatorward undercurrent over theAfrican continental slope in all LADCP surveysappears to be a recurring feature, although theLADCP measurements do not resolve the deepflow over the slope in detail because of theirinstantaneous nature. At shallower levels(o2000m) the currents observed with the LADCPwere dominated by the presence of southwardmoving anti-cyclonic eddies (de Ruijter et al.,2000).A mooring with a current meter at 2500m was

deployed west of Davie Ridge at B17�S in theMozambique Channel, at the section of stationcluster 10, from April to November 2001 (Ridder-inkhof and de Ruijter, 2003). During 71% of thetotal record (214 days), the flow at a depth of2500m was directed towards the equator, with a

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Fig. 9. Sections of the northward velocity component (cm/s) measured with the LADCP at near zonal hydrographic sections near the

East African continental slope at successive latitude from 30�S (lowest panel) to 12�S (upper left panel). The dashed line shows the

approximate upper boundary of the NADW layer found near 2000m.


northward velocity component over 10 cm/s for19% of the record, and with extremes well over25 cm/s (0.7% of the record). The mean equator-ward velocity amounted to 4.3 cm/s. The twoLADCP sections along the current meter mooringin 2000 and 2001 suggest that the deep flow wasdecoupled from the overlying flow of large anti-cyclonic eddies at a depth of about 2000m. If weassume that the observed mean velocity is typicalfor the flow below 2000m we arrive at a meanequatorward transport of NADW of 2.8 Sv, whilewith a linear velocity profile decreasing to zerovelocity at 2000m a transport of 1.7 Sv is found.This transport of deep water through the Mozam-bique Channel below 2000m agrees with theinverse estimate by Ganachaud et al. (2000), but

is 3–4 times the large-scale upwelling at that levelin the Indian Ocean, derived by You (1999), and 4times less than the inversely determined northwardinflow of NADW west of the Madagascar Ridge at32�S, reported by Robbins and Toole (1997).More long-term current meter records from the

south-western Indian Ocean data are availablefrom WOCE mooring arrays ICM1 and ICM3(Beal and Bryden, 1999; WOCE Data ProductsCommittee, 2000) and from literature (Schott et al.,1988). All three mooring arrays were designed tomonitor western boundary currents. When we plotthe long term mean velocity vectors from thearrays in the 1900–3100 depth interval from theavailable records (Fig. 10), we can discern thatnear 32�S (array ICM1) at about 500m above the

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Fig. 10. Mean velocity vectors from long-term moored current meters between 1900 and 3100m.


African continental slope a narrow northwardundercurrent was found with a velocity of over9 cm/s, in agreement with the LADCP measure-ments also reported by Beal and Bryden (1999)and Donohue et al. (2000). Further offshore asouthward velocity of 5.5 cm/s was observed,probably connected with the lower part of thedeep reaching Agulhas Current (Donohue et al.,2000). The two other moorings of the ICM1 arrayshow velocities in the NADW depth range of theorder of 1 cm/s which do not differ significantlyfrom zero.In the Mascarene Basin, the long-term mean

flow along the eastern Madagascar slope in the1900–3100m interval (average current meter depthis 2530m) is directed to the south with meanvelocities up to 6.5 cm/s at 2876m. At a latitude of20�S, east of Madagascar (ICM3 array, seeWOCE Data Products Committee, 2000) the mean

southward velocity amounts to 4.2 cm/s within50 km from the continental slope. The mooringsbetween 50 and 250 km from the slope give amean southward velocity an order of magnitudesmaller, 0.3 cm/s, which does not differ signifi-cantly from zero. Over the eastern continentalslope off Madagascar at 23�180S a long-termmean southward velocity of 2.5 cm/s has beenreported (Schott et al., 1988). Apparently, in theMascarene Basin at NADW levels a narrow deepwestern boundary current transports NIDWsouthwards. Apparently, the high salinity core ofcluster 34 does not reach that far north as a deepboundary current. The instantaneous LADCPobservations at 25�S from the Pelagia 2001ACSEX survey, south-east of Madagascar at adepth of 2500m, gave a mean southward velocityof 4.2 cm/s in the first 120 km from the continentalslope.

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Near Cape Amber, at the northern point ofMadagascar, a long-term mean velocity from theMascarene Basin into the Somali Basin of 3.3 cm/shas been observed at a relatively shallow depth of2070m, nearly 1000m above the sea floor (Schottet al., 1988). This is close to the depth of the upperboundary of the NADW layer. However, noobservations at deeper levels, where the centre ofthe NADW density interval is found, are availablefor that position.

4. Discussion and conclusions

The circulation scheme for NADW, discussed inthis chapter, is given in Fig. 11, with the lateralsalinity distribution on the g2.5>39.25 kg/m3 iso-pycnal surface as background. The hydrographicdata, presented above, confirm that NADW in the

Fig. 11. Tentative circulation scheme (arrows) for the deep water (20

background, the lateral salinity distribution in the g2.5=39.25 kg/m3

line). The 34.77 isohaline (thick line) shows the approximate boundary

and NIDW.

Indian Ocean is characterized by a deep salinitymaximum near a potential density of aboutg2.5=39.25 kg/m3. If we tentatively take a max-imum salinity of 34.77 (thick isohaline in Fig. 11)as the boundary between NADW and the fresherLCDW, following You (2000), NADW is foundjust over the eastern slope the Madagascar Ridgeand Southwest Indian Ridge, extending over theDel Cano Rise and Crozet slopes eastwards toabout 55�E. NADW reaches the positions ofclusters 33 and 34 probably from cluster 17 (arrowV in Fig. 6c) across the saddle in the MadagascarRidge near 35�S as proposed by Gr .undling et al.(1991), Toole and Warren (1993) and Donohueand Toole (2003). Apparently, an additionalconnection across the ridges exists via the Dis-covery Fracture Zone (see Fig. 2b and cluster 30 inFig. 6). When we follow the NADW core from theAtlantic Ocean along the African continental slope

00–3500m) based on the analysis presented in this paper. In the

isopycnal surface is drawn, bound by the 2500m isobath (thin

between the high salinity NADW and the lower salinity LCDW

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(clusters 1–5), across the Mozambique Basin(arrow I in Fig. 6c) and around the SouthwestIndian Ridge and Del Cano Rise towards theregion of the Kerguelen (clusters 18–27), weobserve only a gradual change of water massproperties from NADW to LCDW. The salinitydecreases from over 34.86 in cluster 1 at 12�E to34.76 for cluster 27 at 68�E in the Crozet Basin.AOU and Silica increase between those stationsfrom 105 to 129 and from 50 to 80 mmol/kgrespectively (see Figs. 6 and 7), while the density ofthe deep salinity maximum hardly changes(g2.5=39.2570.01 kg/m3). These changes in sali-nity, AOU and nutrient concentrations may beascribed to the effects of diapycnal mixing with theoverlying intermediate and underlying bottomwater and possibly lateral mixing with moresouthern LCDW. The mineralization of organicmatter and dissolution of silica frustules may havesome additional effects on the chemical tracers.Apparently, the NADW, transformed by mixingto LCDW, moves eastwards near 45�S. Thisconfirms results presented by Wyrtki (1971),Mantyla and Reid (1995) and You (2000).The gradual change from NADW towards

LCDW contrasts the abrupt transition betweenthese water masses and the deep water massobserved in the northern Kerguelen, Madagascar,and Mascarene Basins (clusters 35–46), wherewater with NIDW characteristics is encountered.This region with strong gradients in AOU andsilicate was already shown in the analyses pre-sented by Mantyla and Reid (1995). Whereas thedeep salinity maxima in the Mascarene Basin areonly 0.012 fresher than the salinity maximum ofcluster 27, they are found in shallower isopycnals(g2.5=39.2270.01). This may be ascribed to thediapycnal advection and mixing during the large-scale upwelling of fresher AABW in the northernIndian Ocean (Fu, 1986; Toole and Warren, 1993;Robbins and Toole, 1997; Ganachaud et al., 2000)which will shift the deep water core from the in-flowing NADW isopycnal to the out-flowingNIDW isopycnal. The minima in AOU andnutrient concentrations in the NADW core havedisappeared, to be replaced by deep maxima insome station clusters, but in general by muchhigher values in all NIDW cores (Figs. 5 and 7).

This is ascribed to the mineralization of organicmatter and the dissolution of silica frustules duringthe ageing of the deep water in the northern IndianOcean. The long-term current meter observationsand LADCP observations, presented above, in-dicate that probably the southward flow of NIDWthrough the Mascarene Basin is organised in adeep Western Boundary Current, extending to atleast 23�180S. The lateral homogeneity of thehydrography in the Mascarene Basin suggestssome lateral mixing in this basin, possibly by adeep anti-cyclonic re-circulation within the basin.The presence of a deep maximum in AOU anddissolved nutrients in the Mascarene Basin reflectsthe contrast between the aged, south flowingNIDW and the relatively young north flowingintermediate and bottom water (Warren, 1974;Swallow and Pollard, 1988; Mantyla and Reid,1995). The above-mentioned geostrophic shearbetween clusters 34 and 36 which agrees with asouth-eastward transport maximum near the frontat the level of the deep salinity maximum fits intothis scheme. This circulation scheme, with southflowing NIDW in the Mascarene Basin, confirmsearlier publications (Le Pichon, 1960; Warren,1981; Park et al., 1993; Mantyla and Reid, 1995),but contradicts the recent publications on the deepcirculation by Robbins and Toole (1997) and You(2000). The similarity of the properties of the deepwater east of the Mascarene Ridge with the NIDWwest of it also appears to contradict the proposi-tion by Warren (1974) of an inflow of LCDWalong the eastern flank of the Mascarene Ridge.The southward flow of NIDW through the

Mascarene Basin forms an injection into the ACCwhich has its northern boundary at about 42�S(Fig. 3), as suggested by Park et al. (1993). Thiswill lead to a transformation of the LCDW bylateral mixing with NIDW further east. Any inflowof deep water from the ACC into the Indian Oceanfurther east of our research area therefore willcontain parts of NIDW which re-circulates north-wards. The apparent deep front between the areasdominated by NADW/LCDW and NIDW isclearly visible in the south-east running 34.74–34.76 isohalines in Fig. 11, and in the chemicaltracers (e.g. between the open symbols and blackdots in Fig. 3). This front also separates cluster

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groups A and B in Fig. 8a. Group A is dominatedby relatively young NADW while group Bpresents the aged NIDW flowing south-wards. The front probably is formed by theconvergence of the flows of these water types.There is no indication for a considerable cross-frontal transport of NADW. That implies thatthe NADW which manages to cross theMadagascar Ridge ultimately will flow south-eastwards in the direction of the lateral salinitymaximum near 45�S.The circulation sketched above leaves only one

source of relatively saline deep water in the south-western Indian Ocean: the Mozambique Channel.Although most authors on this subject (e.g. LePichon, 1960; Warren, 1974; Toole and Warren,1993; Mantyla and Reid, 1995) assume thatNADW cannot pass the sill over Davie Ridge inthe Mozambique Channel, about half of thevolume of NADW in the Mozambique Channelis found above the level of the sill (the layer from2000 to 2600m, crosses in Figs. 4 and 5). At leastthis upper half can flow across the ridge. Thehydrographic properties of cluster 11 furthersupport such a scheme with a deep maximum insalinity (black dots in Fig. 4) and a minimum inAOU and nutrients (black dots in Fig. 5a and b).The fact that the extremes are found at slightlyshallower isopycnals that further south can beexplained by entrainment of the fresher overlyingwater mass, and the absence of water withdensities above g2.5=39.25 kg/m3 over the sill.LADCP observations along the south-easternAfrican slope (Fig. 9) agree with the existence ofa slope bound deep boundary current flowing tothe north, while deep long-term current measure-ments in the Mozambique Channel show a meanequatorward flow at 2500m of over 4 cm/s atB17�S (Ridderinkhof and de Ruijter, 2003). Thevolume transport derived from these currentmeasurements is estimated to be of the order of2 Sv, a value in agreement with an inverse estimateby Ganachaud et al. (2000) and a direct estimate at24�S by de Ruijter et al. (2002). It appears to bepossible to follow the inflow of the saline NADWinto the Somali Basin further north to at least theequatorial region at 4�S (open symbols in Fig. 4).The magnitude of the northward flow of NADW

through the Mozambique Channel is of the orderof the net northward NADW transport (Fu, 1986;Toole and Warren, 1993; Macdonald, 1995;Robbins and Toole, 1997; Ganachaud et al.,2000). It forms a small but possibly importantpart of a complicated circulation scheme withupwelling and diapycnal mixing in the northernIndian Ocean as well as lateral circulation feedinga southward outflow of NIDW.The estimate of the northward transport rate of

NADW is based on a single current meter mooringwith limited vertical resolution. Plans have beenfunded to monitor the transport through theMozambique Channel for at least 5 years with abetter horizontal and vertical resolution of thedeep flow.The circulation scheme, as sketched in Fig. 11, is

mainly based on water mass analysis, withsupporting current meter and LADCP observa-tions. It only leaves space for additional north-ward inflow of LCDW further east, modified in thesouth-eastern Indian Ocean by admixture ofsouthward flowing NIDW. Such additional inflowprobably takes place east of the Southeast IndianRidge and the Ninetyeast Ridge (e.g. Mantyla andReid, 1995). This definitely contradicts the lateralcirculation scheme of Robbins and Toole (1997),based on a weakly constrained geostrophic inversemodel. Future inverse models of the deep circula-tion in the Indian Ocean will require a combina-tion of dynamics (e.g. geostrophic models) andconstraints based not only on conservation ofwater, heat and dissolved substances, but alsoconstraints derived from current observations andwater mass distributions.

Acknowledgements

The ACSEX programme has been funded by thefoundation of Earth and Life Sciences (ALW), asubsidiary of the Netherlands Foundation forScientific Research (NWO) as part of the CLI-VARNET theme. The WOCE mooring arraysICM1 (PI: H.L. Bryden) and ICM3 (PIs W.D.Nowlin, R.D. Pillsbury, B. Warren, and T. Whit-worth) are used in this study. This is NIOZpublication 3723.

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