JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. C2, PAGES 2417-2439, FEBRUARY 15, 1995
On the origins of deep and bottom waters of the Indian Ocean
Arnold W. Mantyla and JosephL. Reid
MarineLife ResearchGroup,ScrippsInstitutionof Oceanography,
La Jolla,California
Abstract. The characteristics
of the deepandbottomwatersof the Indian Ocean,when
illustratedon potential-densityanomalysurfaces,indicatethatthe watersenterfrom both the
AtlanticandPacificOceans.Thepathsof spreading
areconstrained
by thecomplextopography,
andcharacteristics
areseento be alteredby exchange
withtheoverlyingandunderlyingwaterand
with the sediments,especiallyin the northernIndianOcean. The WeddellSeacontributesto the
densest waters found in the western basins and the Ross Sea and Adelie coast to the densest
watersfoundin theeasternbasins.Bothdensewatervarietiesarealteredby andincorporated
in thelessdensewaterabove;initially,watercarriedby thecircumpolarcurrent,thenwater
from thenorthAtlantic,andfinallyby deepwaterwhosecharacteristics
arederivedin thenorthern
Indian Ocean. Contact with the sedimentsincreasesthe silica contentof the bottom water in the
SouthernOcean. In the northernIndian Ocean the sedimentsalter the silica of the water at the
bottomand,together
withenhanced
salinityfromdiffusion
of salineoverflows
fromthemarginal
seasabove,imprintuniquemarkers
to thedeepwaterthatflowsbackto thesouth.At middepths
theseriesof ridgesbetweenMadagascar
andAustraliaconfinetheflowto a seriesof gyresthat
carrycharacteristics
from thecircumpolar
currentequatorward
andthenorthernIndianOcean
characteristics
southward.
Withinthecircumpolar
current,low-oxygen
deepwaterfromthe
Pacific is carried acrossthe Atlantic and into the Indian Ocean southof Africa. Part flows around
thecyclonicWeddellSeaGyre, andpartextendsacrossthe SouthernOcean. Waterfrom another
Pacificsourcecanbe seennear2000m extendingwestwardfromtheTasmanSea,southof Aus-
traliaandacrosstheIndianOcean,andperhaps
to theAghulasCurrentregionsoutheast
of Africa.
Introduction
for the most part, confirm Warren's conjectureson the
deeperfeatures.
In this study we show some maps of deep and bottom
The general pathwaysof bottom water spreadingin the
water characteristicsof the Indian Ocean, which may be Indian Ocean have been understood for some time, with
usedin a qualitativesenseto infer pathwaysof spreading progressiverefinementin detail as more observationsbeof water below a depth of about 2000 m. The Indian
come available [Wrist, 1939; Warren, 1974, 1978, 1982;
Ocean has numerousridges and basins(Figure 1) that
Kolla et al., 1976; Jacobsand Georgi, 1977; Rodmanand
severelyrestrictpossiblepathwaysof deepandbottomwaGordon, 1982; Mantyla and Reid, 1983; Johnsonet al.,
ter flow. In the followingwe will beginby lookingat characteristicsat the bottom of the ocean to reveal the pathwaysof spreadingof the densestbottomwatersfrom basin
to basin. Then a series of six potential-densityanomaly
surfacesat progressivelyshallowerdepthsfrom about4000
m up to about2000 m will be shownto seewhatcharacteristicsinfluencethe deep watersand how pathwaysof flow
are constrainedby the bathymetryof the Indian Ocean.
The choice of the specificdensity levels (Table 1) was
basedprimarily on variouspropertyextremashownby the
18øSAtlantisII Cruise93 coloredsectionsthataccompany
Warren's[1981] detailed discussionof the major features
199la, b]. With the availabilityof newerdatafrom recent
decadesfor the presentstudyit was possibleto clarify and
add detail to the previous maps of bottom characteristics
shownby Mantyla andReid [ 1983]. Nearlyhalf of the stations usedwere taken during the 1960s' InternationalIndian OceanExpeditions(IIOE) or earlier. This early dataset,
while useful, suffersfrom somedeficienciesin comparison
to the later data sets. For example, not all of the stations
reachedclose to the bottom, and someonly attemptedto
samplethe upperhalf of the watercolumn. Many salinities
were either titrationsalinitiesor run on early salinometers
that occasionallyhad large systematicerrors. No attempt
of the centralIndian OceanbetweenMadagascarandAus- has been made here to normalize the salinities to a modern
tralia. The sections, approximately midway between cruise standard, such as was done with an Atlantic Ocean
Antarctica and the northern Indian Ocean boundaries, are
deepdataset [Mantyla, 1994]. The IIOE oxygenanalyses
well positionedto show featuresoriginatingfrom both the
to the
north and the south,as well as somebroughtinto the Indi- weredonebeforeCarpenter's[1965] improvements
an Oceanby the AntarcticCircumpolarCurrent. The full- Winkler titration methodwere published,and they suffer
oceanarea isopycnalmaps provideessentialevidenceon from inaccuraciescommon to the older techniques.The
the origins and fates of featuresseen in the sectionsand, nutrient data, analyzedmanually,show cruise-to-cruisebiasesand were usuallynoisy. Wyrtki[1971] tabulatescruises that had large nutrient anomalies. Some could be used
Copyright1995 by the AmericanGeophysical
Union
by makingallowances
for offsetsin contouring
thedata,as
Papernumber94JC02564.
was donein the atlasof Wyrtki[1971, Tablesc-g]; others
0148-0227/95/94JC-02564505.00
wereomitted.Somelatercruisessufferby focusingonjust
2417
2418
MANTYLA
0ø
AND
20ø
400
REID:
ORIGINS
40 ø
OF INDIAN
60ø
•'
OCEAN
DEEP AND BOTTOM
80ø
100ø
Crozet
B.
•
120ø
South
60 ø
WATERS
140ø
B.
400
60 ø
Figure 1. Bathymetricfeaturesandplacenamesof theIndianOcean(B is basin;P,plateau;andR, rise).
The 3000 m depthcontouris shown.
conductivity-temperature-depth
(CTD) observationsand
neglectingwatersampleanalysesfor oxygenandnutrients.
It is to be hopedthe WorldOceanCirculationExperiment
(WOCE) cruisesplannedfor thenextfew yearswill result
Table 1. Specifications
of the PotentialDensityAnomaly
Surfaces
1500-2500 m
500-1500 m
0-500 m
in filling in the gaps with high-qualitycompleteobservations;the nonconservative
chemicalsare particularlyuseful
for indicatingorigins of water massesand relative degree
of isolationfrom formationregions.
The contouringof thesemapswas highly subjectivebecauseof the variability of the dataquality. The Geochemical Ocean SectionsStudy (GEOSECS) stations,while few
in number, sampledall of the importantbasinsand were
used as a guide to decide which data setsto acceptand
which to ignore in contouringthe fields. The GEOSECS
datawere assumedto be a consistentdataset,as theanalytical work was performedby a singlegroupof personnel
> 3500 m
2500-3500 m
(46.120)
41.711
37.198
(45.960)
41.584
37.104
32.521
27.824
(45.890)
41.526
37.063
32.493
27.806
(41.495)
37.039
32.475
27.794
(37.000)
32.446
27.773
The Bottom
(36.920)
32.375
27.712
In thePacificandAtlanticOceans,denseabyssalwaters
from Antarcticaspreadfar northward,primarilyon the
[Weisset al., 1983].
Waters
The potentialdensityis expressedas a0 from 0-500 dbar, westernsidesof thoseoceans,while in the Indian Ocean,
as a• from 500-1500 dbar,as a2 from 1500-2500dbar,as
abyssalwatersspreadnorthwardbothin the westandin the
east. The western and eastem sides of the Indian Ocean
a3 from 2500-3500 dbar,and as a4 from 3500 dbar
have different initial sourcesof bottom water at their initial
to the bottom. The numbersin parentheses
are thoseused
in thetextandfiguresto identifyeachisopycnal.
entrypointsthroughthe discordancezonesin the Southeast
MANTYLA AND REID:ORIGINSOF INDIAN OCEANDEEPANDBOTTOMWATERS
2419
IndianRidgeat about50øS,125øE[Rodman
andGordon, of the overlyingwater columnand are a resultof vertical
from south
1982]and in the Southwest
IndianRidgeat about30øS, mixing. Silicadoesnot changemonotonically
Basinsandin the
60øE[Warren,1978],ascanbe seenin Figures2a-2e. The to north. It is highin theEnderby-Crozet
bottom flow into the western Indian Ocean basins enters
Arabian Basin, with a relative minimum in the Mascerene
from theWeddell-Enderby
Basin,andthebottomflow into Basin at 10ø to 20ø south,but this patternmay also be a
of mixingwith waterabove. Verticalsections
the easternIndian Ocean basins enters from the Australian- consequence
AntarcticBasin. Both basinshaveprimaryand secondary alongthe westernbasinsare shownby MantylaandReid
sources to their dense bottom waters. The characteristics
[1983,Figure5] andin theGEOSECSIndianOceanAtlas
of the Weddell-EnderbyBasin are derived primarily from [Spenceret al., 1982,Plates3-21]. From the verticalsecthe Weddell Sea and are alteredby mixing with deep cir- tion of Mantyla and Reid [1983, Figures5a and 5b] one
cumpolarwater aboveas the densewater flows away from can seethemiddepthsignatureof waterfrom thenorthAtthe western Weddell shelf [Foster and Carmack, 1976]. A lantic by a salinitymaximumand low silicathat extend
small input from Enderby Land coast just west of the northwardfrom the circumpolarcurrent until the silica
Amery Ice shelf (70øE) can be seenin the bottomcharac- minimum intersectsthe bottom near 20øS. Up to this
teristics,especiallyin dissolvedoxygen[Jacobsand Geor- point, the abyssalwatersfrom the Antarctichave been
waterandwagi, 1977]. The bottom waters of the Australian-Antarctic mixingwith deepbutlessdensecircumpolar
Basinare derivedprimarilyfrom the RossSea,asindicated ter originatingprimarilyfrom the Atlantic. To thenorthof
by the higher salinity there and, secondarily,from the thispoint the mixtureof circumpolardeepwatersandbotAdelie coast (140øE), seenbest by the dissolvedoxygen tom waters mix with deep waters whose characteristics
data [Gordon and Tchernia, 1972] in Figure 2d. Alter- have been obtainedin the Arabian Sea. Interactionof deep
ations of the bottom
water
in the Australian-Antarctic
Basinalsotake placeby mixing with the deepcircumpolar
water
above.
Dissolved
silica levels in both basins are
made high by dissolutionof opaline silica depositson the
bottom [Edmondet al., 1979].
The densest waters that flow into each basin enter the
basinsover sills that are typically of the orderof a kilometer shallowerthan the deepestpart of the basin. They are
confined to those basins by topographyuntil they have
been altered to a lower density. This occursprimarily by
mixing with lessdensewater aboveand, to a lesserextent,
by geothermal heat flow from the bottom. The lowerdensitywater risesabovedenser,infiowing,freshetbottom
water and escapesthroughdeep sills and fracturezonesto
other deep basins. This alterationis thoughtto occurduring a spiralflow within the basin,as evidencedby the silica
maximum
above the bottom in the western Weddell
Sea
[Carmack, 1977]. The abyssalwaters escapefrom the
Weddell Basin in the following threemajor locations:(1)
in the west throughthe SouthSandwichTrenchto begin
the long pathway northward in the westernAtlantic (not
seenon presentmaps, see Mantyla and Reid [1983]; (2)
througha gapin the SouthwestIndianRidgeat about50øS,
30øE to fill the dead-end Agulhas, Cape, Natal, and
MozambiqueBasins(Figures.2a-2e); and (3) throughthe
discordance
zone in the SouthwestIndian Ridge at about
30øS, 60øE by way of the relatively unobstructed
Crozet
Basin. The abyssalwater from the third passageflows
northwardin the westernIndian Oceanto the Madagascar
Basin, into the shallowerMascereneBasin, throughthe
Amirante Trench to the Somali Basin, and throughthe
Owen FractureZone to the Arabian Basin. The abyssal
waters become less dense in each downstream basin until
the water at the bottom of the Arabian Basin has reached a
water with bottom sediments there increases the silica and
decreases
the oxygen[Edmondet al., 1979]. Althoughextremely saline overflowsfrom the Red Sea and Persian
Gulf do not directly penetrateto great depths,the high
salinitiesat intermediatedepthscausean increasein the
salinityof deeperwatersbelow by verticalmixing, analogous to the high-salinityinfluenceof the Mediterranean
outflow on deep watersof the easternAtlantic below the
actualdepthof theoverflow[Reid,1994]. Verticalmixing
also causesan additional reduction in deepwateroxygen
concentrations.Spenceret al. [1982, Plate 5] show two
high-salinitydeep sources>34.74 southof about 30øS
broughtin by thecircumpolarcurrentand>34.74 salinities
northof about 10øS,increasinginto the ArabianSea,with
slightlylowerdeepwater
salinities
in betweenfrom10øSto
30øS.
The primaryinflow of densebottomwaterfromtheAustralian-AntarcticBasin can be seen in Figures 2a-2e at
about 50øS, 125øE throughthe Australian-Antarctic
DiscordanceZone. A smallerpassageexistseastof 150øE,as
shownby Rodmanand Gordon[ 1982],thoughmostof the
water there entersthe TasmanSea. Althoughinitially the
bottomsalinity in the Australian-Antarctic
Basinis higher
than the Weddell-EnderbyBasin, the differencesbecome
less as the bottom waters spreadnorthwardin the eastern
Indian Oceanbasins,reflectingthe fact thatthe major componentof abyssalwatersto the northof the near-Antarctic
basinsis a mixture of water from the deepcircumpolarcurrent [Mantyla and Reid, 1983]. However,the abyssalwatersto the north near the equatorare as cold as they are because they include bottom water from the AustralianAntarctic Basin. At this latitude the eastern basin is colder
and denser than the western basin at the bottom. The
densitythat is aboutthe sameas the deepwaterdensity2
spreadingof abyssalwatersin the easternIndian Oceanis
by way of the South Australia Basin througha gap near
km above the bottom of the Crozet Basin, where the west-
33øS, 105øE between the Broken and Naturaliste Plateaus
em Indian Oceanabyssalflow began. Along this western
abyssalspreadingpath,potential-density
anomalydecreases from tr4 = 46.18 in the Weddell-Enderby
Basinto tr4 =
45.85 in theArabianBasin;potentialtemperature
increases
from-0.8 ø to 1.5ø;salinityincreasesfrom 34.66 to 34.75;
and dissolvedoxygen decreasesfrom more than 5.8 to less
into the Perth Basin, then to the deeper West Australian,
North Australian,and CocosBasins,and finally to the Central Indian Basin througha gap in the NinetyeastRidge at
about10øS[Warren,1982]. Minor passages
into the southern end of the Central Indian Basin can be seenby the the
45.92 tr4 contours,revealinga deepgap in the Ninetyeast
than3.0mLL-•. These
areallchanges
toward
thevalues Ridgeat about28øS,87øE[Tooleand Warren,1993]and
2420
MANTYLA
0ø
AND REID: ORIGINS
20 ø
40 ø
OF INDIAN
60 ø
80 ø
OCEAN DEEP AND BOTTOM
1O0ø
120 ø
WATERS
140 ø
40 ø
40 ø
.5.94
60 ø
60 ø
Figure 2a. Bottompotential-density
anomalyor4referredto 4000 dbarin the IndianOcean. Shaded
area is less than 3500 m.
0ø
20 ø
40 ø
60 ø
80 ø
1O0ø
120ø
140ø
40 ø
40 ø
60 ø
60 ø
Figure 2b. Bottompotentialtemperaturein the IndianOcean.
MANTYLA
AND REID: ORIGINS
0ø
20 ø
40 ø
OF INDIAN
60 ø
80 ø
OCEAN DEEP AND BOTrOM
100 ø
120 ø
WATERS
140 ø
40 ø
40 ø
34. ÷
60 ø
60 ø
Figure 2c. Bottomsalinityin theIndianOcean.
0ø
20ø
40ø
60ø
80ø
100ø
120ø
ß.
40 ø
140ø
.
ß
40 ø
60 ø
60 ø
5.8
Figure2d. Bottom
dissolved
oxygen
(mLL-1)intheIndian
Ocean.
2421
2422
MANTYLA
0ø
AND
20 ø
REID:
40 ø
ORIGINS
OF INDIAN
60 ø
OCEAN
80 ø
DEEP AND BOTFOM
100 ø
120ø
WATERS
140 ø
40 ø
40 ø
11o
60 ø
60 ø
11o
Figure2e. Bottom
silicate
(/•molL-1)intheIndian
Ocean.
anothergap betweenthe BrokenPlateauandthe Southeast bottomin turbulentnepheloidlayers,which may locally
IndianRidgenear 34øS,83øE.
enhancesediment-water
interfacechemicalchanges.The
The bottom water characteristics north of the Central
IndusandGangesRiversform verylargedeep-sea
fansor
Basin in the Bay of Bengal are similar to thoseseenin the conesin the ArabianSeaandBay of Bengal,respectively,
Arabian Sea, temperaturegreater than 1.5øC, relatively and are possiblecandidatesto explainsomeof the effects
saline, low in oxygen, and high in silica. Edmondet al. seenon the chemistryof the bottomwatersin both north[1979] noted a distinction between the northern Indian em regions. A more thoroughsamplingof stationsnear
Oceanhigh bottom silica sourceand the SouthernOcean the submarinecanyonsand deep-seafans of both rivers is
high silica source. In the latter region the bottom sedi- neededto documentpossibleeffectsof the river plumeson
mentsshowno effect on the near-bottomoxygenor nutri- enhancedsurfaceproductivityandon effectsfromthe sedislopes.
ents (excludingsilica), while in the northernIndian Ocean mentsat thebaseof thecontinental
The bottomwaterspreading
illustratedin Figures2a-2e
both deep embaymentsshow strongeffectson the bottom
water oxygen, phosphate,and nitrate. They took that to appearsto be purely thermohaline,with the densestwater
mean considerableorganicmaterial reachesthe oceanbot- seekingthe deepestpassagesand basins.Lessdensewater
for the graditom throughhigh primary productivityin the surfacewa- floatsabovethe bottomwaterandaccounts
tersabove. The SouthernOceanalsohashighproductivi- entsat theedgesof thebasinswherethebottomslopesupnorthward,it is altered
ty, yet little organicmaterial appearsto reach the bottom. ward. As the bottomwaterspreads
of
Productivityappearsto be several times greaterin the by interactionwith thebottomandby thecharacteristics
northernIndian Oceancomparedwith the SouthernOcean the deep water aboveit, alteringthe deepwater itself as
[Koblentz-Mishke et al., 1970], and an additional sourceof
well, until at the northern end of the western and eastern
organicmaterialmay be from river discharges
in the north,
analogousto possibleeffectsof the Zaire River Plumeand
deep-seafan illustratedby van Bennekomand Berger
[1984] andby Warrenand Speer[1991]on thedeepnutrient patternsin the Angola Basin. An excessof silica, nitrate, and phosphateand a deficiencyin oxygencan be
seenspreadingout at abouta depthof 4000 m away from
the base of the continentalslopeand the Zaire deep-sea
fan. Suspended
river material(includingorganicmaterial)
is believedto be transporteddown the Zaire canyonto the
pathwaysthe bottomwater has beenloweredin density
andchanged
in characteristics
untilit hasthesameproperties as deepwater in the north. The oxygenandnutrient
signatures
imprintedon the deepandabyssalwatersin the
northern Indian Ocean can be seen as advective cores that
extendsouthwardabovethe generallynorthwardflowing
bottomwaters [Edmondet al., 1979, Figures21 and 22;
Spenceret al., 1982, Plates 11-21 and 31-41]. Since the
forcingof deepand bottomwatersis primarilyfrom the
bottomup, the next seriesof mapswill be examinedfrom
MANTYLA
0ø
20 ø
AND
REID:
40 ø
ORIGINS
OF INDIAN
60 ø
OCEAN
80 ø
DEEP AND BOTTOM
1O0ø
WATERS
120 ø
2423
140 ø
40 ø
40 ø
60 ø
60 ø
20
Figure 3a. Depth (hectometers)
of the 46.12-tr4 potential-density
anomalysurface.Area is shaded
wherethe bottomis shallowerthanthis isopycnaldepth.
the densest(deepest)isopycnalto the shallowest
isopycnal shelf salinitiesappearto be too low to contributewater of
illustrated.
this density.The majority of water on this surfaceis from
either the west end (Weddell Sea) or the east end (Ross
Sea). The limited dataquality and the smallrangeof variation may lead to someuncertaintyin detail, but the general
This isopycnalsurface(Figure3) lies well belowthe trend and patternsshouldbe all right. The two basinsare
AntarcticCircumpolarCurrent(ACC) salinitymaximum similar in oxygen and silicate(not shown)concentrations,
andis continuous
throughthepassage
betweentheKergue- with no discerniblepattern. Oxygenis mostlybetween5.4
100to 150gmolL-• .
len Plateauand Antarctica(PrincessElizabethTrough). It and5.7mLL-• andsilicate
doesnot extendnorthwardbeyondmid-oceanridges. It is
The 46.12-a4 Surface
generallydeeperthan 3000 m, intersecting
the bottom The 45.96-a4 Surface
alongthenorthern
edgeof theWeddell-End•rby
Basinand
the Australian-AntarcticBasin. A large eddy centeredat
This isopycnal(Figure4) is alsobelow the ACC salinity
about57øS, 10øEis indicatedby the domingof the isopyc- maximum; it reaches the bottom in the northem Mascarene
hal to depthsof less than2300 m. This is partof theeast- Basin and northernWest AustraliaBasin. It is not present
em end of the cyclonicWeddellSeaGyre. Thereis some in the Central Indian, Somali, or Arabian Basins. The
questionas to whethertheWeddellGyre is composed
of a isopycnalis at its shallowestpoint in the easternWeddell
single,elongatedgyre or of two or more separategyres,as Sea Gyre, less than 200 m near 60øS, 10øE. Along the
suggested
by Mosby[1934] andDeacon[1937]. The re- Antarcticshelf the isopycnalis typicallyfoundat depthsof
centgeostrophic
shearmapsby Orsi et al. [1993] suggest 600 m to 1000 m, exceptalong the Adelie coast,where it
the near-surfaceflow consistsof a single gyre, while the occurs somewhat shallower.
Taking the variationsin depthof the isopycnalsas qualideeperlevels consistof two or more gyres. The isolated
gyrein Figure3a is similarto Orsi et al.'s [1993]mapof tatively indicative of the vertical shear and assuminga
the geopotential
heightanomalyat 1500 dbarwith respect shallowerreferencepressure,northwardwesternboundary
to 4000 dbar.
currents and offshore return flows are seen east of Mada-
The two deep basinsare distinctlydifferentin salinity,
with the Australian-Antarctic
Basinbeingabout0.02 higher thanthe Weddell-EnderbyBasin. Thereis someindication of contributionfrom shelvesnear Prydz Bay (75øE)
andalongthe Adelie coastnear 138øE,but elsewhere,
the
gascarand east of the NinetyeastRidge. Weaker anticyclonic shearis seenin the Cape Basin and perhapsin the
SouthAustralianBasin. In the Cape Basinthe deeperflow
has been shown to be cyclonic [Tucholkeand Embley,
1984;Reid, 1989];thedeeper,strongflow makestheactual
o
20 ø
40 ø
60 ø
80 ø
1O0 ø
120 ø
140 ø
40 ø
60 ø
60 ø
Figure3b. Salinityonthe46.12-rr4surface.
2424
MANTYLA
0ø
AND REID: ORIGINS OF INDIAN
20 ø
40 ø
60 ø
80 ø
OCEAN DEEP AND BOTTOM WATERS
100 ø
120 ø
140 ø
40 ø
40 ø
60 ø
50 o
Figure 4a. Depth (hectometers)of the 45.96-a4 potential-density
anomalysurface. Area is shaded
where the bottomis shallowerthanthis isopycnaldepth.
0ø
20 ø
60 ø
40 ø
60 ø
80 ø
100ø
120ø
140ø
60 ø
Figure 4b. Salinityon the45.96-tr4potential-density
anomalysurface.
MANTYLA
0ø
AND
20ø
REID:
40ø
ORIGINS
OF INDIAN
60ø
80ø
OCEAN
DEEP AND BOTTOM
WATERS
1O0ø
140ø
120ø
ß
60 ø
o
50 ø
4.7
Figure4c.Oxygen
(mLL-•) onthe45.96-o'4
potential-density
anomaly
surface.
0ø
40 ø
20 ø
40 ø
60 ø
80 ø
100 ø
120 ø
140 ø
40 ø
Figure4d. Silicate
(#molL-1)onthe45.96-o'4
potential-density
anomaly
surface.
2425
2426
MANTYLA
AND REID: ORIGINS
OF INDIAN
OCEAN DEEP AND BOTTOM
WATERS
circulationcyclonic (a deep, strongnorthwardflow along cumpolarDeep Water" describedby Gordon [1967], that
the westernboundaryof the Cape Basinreturningin the has been raised to its shallowest location in the center of a
eas0. This maybe thecasein otherbasinsaswell. Farther strongcyclonicgym. Whitworthand Nowlin [1987] examsouth, the shear is strongly zonal where the isopycnal ined the featurein detail andpointedout thatwith its high
slopesby morethan2 km in depthgenerallysouthof 45øS, densityit was too deepto play a part in the Weddellshelfslopeprocessesof bottom water formationand was left to
an indicationof theAntarcticCircumpolarCurrent.
Highestsalinitiesare seenin the CapeBasin,exceeding spiralaroundin the WeddellGyre, losingoxygenfrom in
34.75. Lowestsalinitiesoccurwherethe isopycnalis shal- situ consumption.A low-salinitysurfacelayer providesa
lowest(andcoldes0in the WeddellSeaGym andalongthe low-densitycap above the featureand minimizesgas exAntarctic shelf. That will be the case for all of the less changefrom above. An even lower shallowoxygenminidense isopycnals examined in the following sections. mum is seen on the Eltanin Cruise 41 section at 132øE
There is little salinity variationeastof 70øE;most values southof Australia[Callahan,1972,Figure2], but it is at a
are within 0.01 of 34.71.
differentdensityanddoesnot appearon thismap.
North of the AntarcticCircumpolarCurrent,dissolved Silica is uniformlyhigh, rangingfrom about100 to 135
oxygengenerallydecreasesmonotonicallyfrom valuesex- $tmol
L-• onthisdensity
surface.
Thesmallrangeand
ceeding
5.1mL L-1 in theCapeBasintolessthan4.4mL variable quality of the data makes the contourssomewhat
L-1 in thenorthern
WestAustralia
Basin.Oxygen
values speculative.
exceed
5 mL L-• alongtheAntarctic
continental
shelf,but
surprisingly,oxygen levels are low in the shallow, nearsurfacecenter of the Weddell Sea Gym eddy near 60øS,
10øE,similarto thoseseenon thedeeperisopycnals
in the
South Atlantic [Reid, 1989]. Oxygen levels are higher
both shallowerand deeper than this level at this location.
Examinationof the Ajax Expeditionverticalsectionsat the
Greenwich Meridian shown by Whirworthand Nowlin
[ 1987,Figure2] andotherIndianOceansections
shownby
Wyrtki[1971],showtheoxygenminimumis alsoa vertical
temperaturemaximum,thusthe featureis part of the "Cir-
0ø
20 ø
40 ø
60 ø
The 45.89-a4 Surface
The water at the bottom of the northern Indian Ocean
canbe thoughtof as both the end of the northwardbottom
waterspreading
andas thestartof thecompensating
southward deepwaterreturnflow. High silicaoriginatesin the
northandis seennearlyeverywhere
on the45.89-tr4 surface (Figure5) exceptfor the part enteringfrom the Atlanticsouthwestof Africa. The highestsilicas,> 1503tmol
L-1,arein theArabian
SeaandBayof Bengal.
Theyare
80 ø
100ø
120ø
140ø
40 ø
40 ø
ß
60 ø
o
50ø
Figure 5a. Depth (hectometers)
of the 45.89-tr4potential-density
anomalysurface.Area is shaded
wherethebottomis shallowerthanthisisopycnaldepth.
MANTYLA
0ø
AND REID: ORIGINS OF INDIAN OCEAN DEEP AND BOTFOM WATERS
20ø
40ø
60ø
80ø
100ø
120ø
140ø
40 ø
40 ø
60 ø
60 o
34.68
Figure$b. Salinityon the45.89-a4potential-density
anomalysurface.
0o
40 ø
20 ø
400
60 ø
80 ø
100ø
120ø
140ø
40 ø
Figure5c. Oxygen
(mLL-1)onthe45.89-a
4potential-density
anomaly
surface.
2427
2428
MANTYLA
0ø
AND REID: ORIGINS OF INDIAN
20 ø
40 ø
60 ø
80 ø
OCEAN DEEP AND BOTrOM
100 ø
120 ø
WATERS
140 ø
40 ø
40 ø
•o
•o
Figure
5d.Silicate
(/•mol
L-•) onthe45.89-a4
potential-density
anomaly
surface.
clearly the origin of the verticalsilica maximaseenon the
silica sectionsshownby Edmondet al. [1979], on Warren's [1981] sectionalong 18øS,in the GEOSECSIndian
Ocean Atlas [Spenceret al., 1982], and by Mantyla and
Saline(and warm) water,originatingfrom the deepAt-
lantic, can be seen entering the Indian Ocean south of
Africa and leaving south of Australia. A warm, saline
branchcanbe seenturningbackwestwardaroundtheWeddell Sea Gyre near 65øS, 10øE. The freshest,coldestwater
Reid [1983].
The dominantfeatureof the depthof theisopycnalis the on this density surface is found on the Antarctic shelves
steeprise southof about45øSassociated
with the ACC andalongthe axisof the WeddellSeaGyre. Bothregions
high-speed
core. Flow is particularlystrongnorthof Ker- are shallowandsubjectto eithersurfacecoolingor mixing
guelenIslandat about72øE, wherethe isopycnalrisesby with cold, low-salinity surfacewinter water. Thesecharacabout 1100 m over a lateral distance of 2 ø latitude. The
teristicsare foundabovethe warm deepwatercoreof the
part of the ACC flow between KerguelenPlateau and ACC and are different from the characteristics found on
Antarcticaappearsto be weaker, but the stationspacing this densitysurfacenorth of the ACC in the Indian Ocean;
(from variouscruises)makesthis uncertain.The isopycnal the ACC is an effective barrier between these waters that
risesby morethan 1000 m therealso. The easternedgeof are of the same density but have little contactwith each
thecyclonicWeddellSeaGyreis evidentsouthof Africaat other.
55ø-70øS and to about 30øE. Aside from some shelf sta-
tions, this is the shallowestdepth of this isopycnal.The
isopycnaldoesnot appearto outcropanywherein thispredominatelynonwinterdataset. Suggestions
of theAgulhas
Current and its return flow are seenjust southof Africa.
Anticyclonic shear patternsare inferred in the Cape,
Mozambique,and South AustraliaBasins,thoughin the
CapeBasinthedeeperflow hasbeenshownto be cyclonic.
A northwardshearis seenjust eastof Madagascarbut not
off the Central Indian Ridge or NinetyeastRidge at this
Thelowestoxygens,
<4 mL L-•, arefoundin theArabi-
an Sea and Bay of Bengal;the highestare in the nearsurfacewatersand thoseenteringfrom the Atlanticjust
southwest of Africa. The low at about 45øS to 55øS can be
traced back all across the South Atlantic to the Drake Pas-
sage[Reid,1989]. The totaloxygenrangeon thisdensity
surfaceis aboutone half the rangeon the bottommaps
(Figure2d) or shallowerdensitysurfaces.
The 41.495-tr3 Surface
depth. The isopycnalreachesthe bottomnear 13øNand
showslittle variationin depthnorthof about20øS.
This isopycnalsurface(Figure 6) lies mostly between
The majorityof the Indian Oceannorthof about40øS 3000 m and 3400 m northof the circumpolarcurrentand
and east of Madagascarhas a potentialtemperaturerange showsinfluencesfrom the Arabian Sea, from the Atlantic,
from 1.15ø to 1.3ø and a salinityrangeof 34.72 to 34.75 and from the Pacific. The majority of the Indian Ocean
(but much less north of 20øS) on this density surface. northof 30øSandeastof Madagascar
hasa potentialtern-
MANTYLA
AND REID: ORIGINS
0ø
20 ø
40 ø
OF INDIAN
60 ø
OCEAN DEEP AND BOTrOM
80 ø
100 ø
120 ø
WATERS
140 ø
40 ø
40 ø
60 ø
50ø
Figure 6a. Depth(laectometers)
of the 41.495-tr3 potential-density
anomalysurface.Area is shaded
wherethebottomis shallowerthanthisisopycnaldepth.
0ø
20 ø
40 ø
60 ø
80 ø
100o
120ø
140ø
40 ø
40 ø
60 ø
60 ø
34.65
34.65
Figure 6b. Salinityon the41.495-tr3potential-density
anomalysurface.
2429
2430
MANTYLA
0ø
AND
20 ø
REID:
40 ø
ORIGINS
OF INDIAN
60 ø
80 ø
OCEAN
DEEP AND BOTFOM
100 ø
120 ø
WATERS
140 ø
40 ø
40 ø
60 ø
fi0 ø
Figure
6c.Oxygen
(mLL-1)onthe41.495-a3
potential-density
anomaly
surface.
0ø
20ø
40ø
60ø
80ø
100o
120ø
140ø
40 ø
40 ø
60ø
50ø
110
Figure6d.Silicate
(/•molL-1)onthe41.495-a3
potential-density
anomaly
surface.
MANTYLA AND REID: ORIGINS OF INDIAN OCEAN DEEP AND BOTI'OM WATERS
2431
perature
rangeof 1.4ø to 1.6ø O anda salinityrangeof staysnorthof Kerguelen.It appearsthatthe rapidmodifi-
34.73to 34.76on thisisopycnal
surface.Warmandsaline cation of NADW as it enters the Indian Ocean is accomNorth AtlanticDeep Water (NADW) entersthe Indian plishedby mixing with water from the northernIndian
Oceanjust southof Africa. The salinityextremaof this Ocean, with its high silica and low oxygen,but not very
warmdeeplayerliejustabovethisdensitysurface,
butthe differenttemperatureand salinity. Thus the NADW low
North Atlantic signal can be clearly seen extendingto silicaandhigh oxygenare erodedon the northernedgeof
northof Madagascar
in the westernboundaryand across the AntarcticCircumpolarCurrent,while temperatureand
the entiresouthernIndian Oceanalongthe AntarcticCir- salinityarenot changedsomuch.
highin
cumpolar
Current.Mostof the temperature
andsalinity The axisof theWeddellSeaGyre is surprisingly
the closeness
to the surface.Apparentchanges
takeplacebeforereaching
50øE.Anotherregion silica,considering
of highsalinitycanbe seenin theArabianSea. As dis- ly, it is deeperwaterdomingup in the middleof the cyin oxygenby its proximityto
cussedearlier in the bottomwater section,the high-salinity clonicgyrebut is freshened
influencesfrom the saline marginal seas are detected to
great depthsin the northernArabian Sea. Betweenthe
high-salinitysourcesfrom the north and the southlies a
weak lateral salinityminimum(<0.005) at about 10øSto
20øS. Since this densitylevel is below the vertical salinity
maximum layers, the lower salinity must be a result of
mixing with lower-salinitywaterfrom below.
The NADW signal also appearsas a relative high in
oxygenthat can be seenbeyondthe KerguelenPlateau.
The oxygenon the 41.495 a3 surfacevariesmonotonically
from high NADW valuesnear SouthAfrica to low values
in the Arabian Sea and the Bay of Bengal. The nearsurface(-• 200 m) high at 60øS, 10øEoriginatesfrom the
surfacelayer of the Weddell Sea along the axis of the cyclonic gyre. Althoughhigh in oxygen,it is below 100%
saturation,indicating some deeper water inclusion. The
the surfacelayer.
The channelbetweenMadagascarand Mauritius Island
is dominatedby high silica from the north,althoughit is
slightly lower in the westernboundarycurrentnext to
Madagascar.Somewhatlower silicasare seenalongthe
CentralIndianRidge,as notedby Warren[1981]. As with
the high oxygen,this is evidentlyfrom the south,with a
tinctureof NADW added. Very high silicasare found in
the Arabian Sea and in the Bay of Bengal. This isopycnal
surface is close to the bottom there, where a high silica
sourceis found [Edmond et al., 1979]. Southeastwardextensionsof high silica are seen in the same locationsas
with the low-oxygenextensions.
Consistentwith the tracerpatterns,equatorwardwestern
boundarycurrentsare suggested
by the isopycnalslope
eastof Madagascarand eastof the CentralIndian Ridge,
low between 45 ø and 55øS, 0 ø and 20øE, can be traced but are not as clearly seeneastof the NinetyeastRidge at
back across the entire South Atlantic Ocean to the Drake
this depth. The bathymetryfrom 10øSto 40øSand from
Passage[Reid, 1989]. Its origin is the deepsoutheast
Pa- Madagascarto Australiaappearsto confinethe flow into a
cific [Callahan, 1972]. The lateral oxygen minimum seriesof gyres between the north-southtrendingridges,
wrapsaroundthe Weddell Sea Gyre, alongwith the rela- onebranchof eachgyre carryingsouthernpropertiesnorthtive warm and salinejet, and part of it also extendseast- ward and the otherlimb of the gyre carryingnortherncharacteristicssouthward. In the Arabian Sea thereappearsto
ward to the gap southof the KerguelenPlateau.
High-oxygentonguescan be seenprojectingnorthward be an anticyclonicshearbetween5øN and 20øN,just east
along Madagascarand along the Central Indian Ridge, in- of the Gulf of Aden, and a cyclonicpatternalongthe west
dicating that the oxygen maximum seen on Warren's India continentalslope. Over the West AustralianBasin,
[1981] 18øSsectionoriginatesfrom the south,as he pro- northernCentral Indian Basin, and the Bay of Bengal the
posed. Tooleand Warren [1993] also point out that this depthvariationof the 41.495-•3 surfaceis too smallto rewater mustbe carriedin by the circumpolarcurrent,how- solvethe shearwith any confidence.
ever,Figures6a-6e reveal that the water takesa muchmore
directrouteto the CentralIndianRidgethantheirpostulatThe 37.00-•: Surface
ed pathwayoriginmingfrom the Australian-Antarctic
Discordanceat 120-125øE. Interestingly,the oxygenon this
This isopycnallevel (Figure7) is closeto the densityof
isopycnalis higheralongthe CentralIndianRidgethanit is the NADW salinitymaximumcorejust southof Africa. It
off either Madagascaror the NinetyeastRidge, although is numericallythe sameas thatusedby Reid [1981]in his
the NADW potentialtemperature/salinity
(E}/ S) signalis
strongereast of Madagascar.The oxygen is lessthan 2.2
mL L-• in the northernmostArabian Sea and lessthan 3.2
global middepth circulationstudy,however,the newer
equationof state(EOS80)usedherefindsthisvalueabout
300 m deeper.
The strongslopein the depthof the isopycnalsouthof
mL L-• in theBayof Bengal.Lowoxygen
levelscanbe
seenextendingsoutheastwardfrom the Mascereneinto the about45øSreflectsthepathof the ACC. Waterof thisdenCrozetBasin. The patternon this densitysurfaceis similar sity breaksinto threestreamsas it approaches
Kerguelen
to the 3000-m oxygendistributionshownby Park et al. Plateau,the deeperpartpassingnorthof KerguelenIsland,
[ 1993]. They took the low-oxygenfeatureas an indication the intermediatedepthspassingthrougha gap in the Kerof North Indian Deep Water extendinginto the areanorth- guelen Plateau (the Fawn Trough 56øS, 77øE) southof
eastof KerguelenIsland.
Heard Island, and the shallowestportionspassingthrough
The lowest silica is associatedwith the NADW signal the PrincessElizabeth Trough south of the Kerguelen
south of Africa but is quickly modified by the time it Plateau.The isopycnalis lessthan200 m deepin theWedreachesthe Crozet Plateau, as are all of the NADW charac- dell SeaGyre andon partsof the Antarcticshelf. An inditeristics. The silica minimum can be traced south of Kercation of the AgulhasCurrent and return flow is apparent
guelen and perhapsacrossthe rest of the southernIndian south and southeastof Africa, and anticyclonic shear is
Ocean. The patternsof both silicaand oxygendiffer from seen in the South Australia Basin.
The northward shear along the Central Indian Ridge
the NADW temperatureand salinitymaximum core that
2432
MANTYLA
0ø
AND
20ø
REID:
40ø
ORIGINS
OF INDIAN
60ø
80ø
OCEAN
DEEP AND
1O0ø
BOTFOM
120ø
WATERS
140ø
ß
Figure7a. Depth(hectometers)
of the 37.00-ty:potential-density
anomaly
surface.Areais shaded
wherethebottomis shallower
thanthisisopycnal
depth.
0ø
20 ø
40 ø
60 ø
80 ø
100ø
120ø
140ø
ß
40 ø
40 ø
34.60
Figure 7b. Salinityon the37.00-•r2potential-density
anomalysurface.
MANTYLA
0ø
AND REID: ORIGINS
20 ø
40 ø
OF INDIAN
60 ø
80 ø
OCEAN DEEP AND BOTI'OM
1O0ø
120 ø
WATERS
140 ø
40 ø
40 ø
60 ø
60 ø
Figure7c.Oxygen
(mLL-1)onthe37.00-a2
potential-density
anomaly
surface.
0ø
20 ø
40 ø
120
60 ø
80 ø
1O0ø
120ø
140ø
ß
40 ø
40 ø
60 ø
60 ø
Figure7d. Silicate
(/tmolL-1)onthe37.00-a:
potential-density
anomaly
surface.
2433
2434
MANTYLA
AND REID: ORIGINS OF INDIAN
OCEAN DEEP AND BOTTOM WATERS
seenin the 41.495-a3 depthmaphasshiftedslightlynorth- The 36.92-a: Surface
westwardto the easternedge of the MascarenePlateau,as
Thisisopycnalsurface(Figure8) wasselected
to reprethe deeperridgesof the CentralIndianRidgebecomeless
of an obstacleto shearflow at the depthof the 37.00-a9. sentthe deeplayer of high salinitythatextendssouthward
feasurface.Much of the shearin thenortheast
partof themap fromtheArabianSeato at least15øS.It is a separate
ture
from
the
salinity
maximum
to
the
south,
which
is
from
is quite weak, as the depthof the surfaceis mostlywithin
100 m of 2600
m.
There
seems to be some southward
shearalong the west coastof India. A large,anticyclonic
shearpatternextendsovermostof the Crozet,Madagascar,
and MascereneBasinsand possiblyeastwardto includethe
gyre in the SouthAustraliaBasin. There is a cyclonicpattern over the Central Indian Ridge and its extensionto the
SoutheastIndianRidge.
the Atlantic. The high-salinitysourceseenin the circumpolar currentsouthof 40øS is separatedfrom the Arabian
Seahigh salinityby a broadbandof lower-salinity
water
between20ø and 40øS. The two highest(by about0.02)
salinitymaximaseenon Warren's[1981] 18øSsectionappear as isolated contours at 80ø and 86øE of salinifies
greaterthan34.75, valuesthatexceedany otherobservaThe high-salinity
signalof theNADWcanbe seen tionswithin 10ø of latitudeon this isopycnal.If the data
wrapping around southernAfrica close to the continent. are correct,they may represent"meddylike"featuresthat
Salinity greaterthan 34.90 reachesthe WalvisRidgefrom originatedin marginal sea overflowsto the Arabian Sea,
to isolatedsalinitylensesfoundin the Atlantic
the North Atlantic and is reducedto 34.80 by the time it analogous
Ocean
originating
from the Mediterranean
Sea [Armiand
reachesthe Madagascarand SouthwestIndianRidge. The
salinity maximum extendsacrossthe entire southIndian
Ocean, taking the deeperpath north of Kerguelen. The
salinity decreasesnorthward to about 15øS, presumably
from the effect of the lower-salinitywater above and below, and then increasesnorthward,reachingvaluesgreater
than34.80 in the northernArabianSea,dueto thepresence
of high-salinity waters above which originate in the
marginal seasthere.
The northernAtlantic sourceis relatively high in oxygen, but like the salinity,it decreasessubstantially
between
the Walvis Ridge and MadagascarRidge, from 5.5 to 4.8
Zenk, 1984]. Red Sea water salt lenseshave also been doc-
umentedin severallocationsnorth of the equatorby
Shapiroand Meschanov[1991].
The depthof the 36.92-a2surfaceis closeto 2000m everywherenorth of about 20øS, with lessverticalrelief than
wasseenin the deeperdensitysurfaces.That indicatesthat
the shearis weakerthanat the otherdepths.The depth
gradientwithin the circumpolarcurrentis equivalentto the
deeperdensitysurfaces,
about1 km within2ø latitudejust
northeast
of KerguelenIsland. This 36.92-a2depthmap,
however,differsmarkedlyfrom the deeperonesin showinga westwardshearnear30øS-40øS
all thewayfromTasmLL-1, thenremains
greater
than4.6mLL-• across
the mania
to Madagascar
andon to theAgulhasCurrentsouthentireIndian Oceandeepwaterpath. Southof thesehigh
eastof Africa. It represents
a potentialpathwayfor ex-
values,a lateraloxygenminimumcan be seenenteringthe changeof deep water from the Pacificto the Indian Ocean.
Indian Oceansectorjust southof 50øS. This is continuous
The featurewasfirstnoticedon the mapsof geostrophic
acrossthe Atlantic along about 55øS from the Pacific
transport
relativeto 3000dbarsprepared
by Sverdrup
etal.
throughthe Drake Passage.It tums southwardnear 35øE [1942, Figure 163]; extendingfrom the TasmanSea to
and westward along 65øS around the lateral maximum
100øE.It canalsobe seenin the middepth
geostrophic
foundalongthe axisof the WeddellSeaGyre. Partof the shearmapsof Wyrtki[ 1971] andReid [ 1981].
minimum oxygen also extends acrossthe Indian Ocean,
At thisdensitylevelthedeepsalinitymaximumfromthe
mostlyalongthemiddepthpath. Highestoxygenlevelsare Atlantic extendsmostly eastwardsouthof the zone of
found on the Antarctic shelves and in the center of the
westward
shearanddoesnot extendnorthward
as clearly
WeddellGyre wherethe isopycnalis shallowenoughto as on the deepersurfaces,exceptperhapsnearthe Madaoutcropin the winteror to be influenced
by high,mixed- gascar
Ridge.Thehighestsalinityseenonthisdensity
surlayer oxygenlevels. Relativelyhigh oxygencan be seen face,greaterthan34.86, appearsin theArabianSeaandin
extendingnorthwardfrom thecircumpolar
currentbetween the Gulf of Aden.
the CentralIndianRidgeandtheMascarenePlateau.RelaThe Mozambiquechannel(betweenMadagascar
and
tively low oxygencanbe seenextendingsoutheastward
in Africa)is no longerblockedat thisdensitylevelasit is at
the CrozetBasin,but theseslightlyreducedoxygenlevels greaterdepths. Influencesfrom the north can be seenexare not directlyconnectedto the very low levelsfoundin tending
southward
in theoxygenand(lessclearly)thesilithe ArabianSea. The oxygenpatternsare consistent
with ca patterns.The southwardpenetrationof northernIndian
the anticyclonicand cyclonicgyres suggested
by the OceandeepwaterthroughtheMozambique
Channel
was
37.00-•r
2 depthmapsdiscussed
aboveand with Wyrtki's notedby ClowesandDeacon[1935]withearlyDiscovery
[ 1971] map of 2500/3500dbar geopotential
topographyExpedition
data and has alsobeenillustrated
by Taft
maps.
[1963],Wyrtki[1971],andCallahan[1972],among
others.
The silicapatternhasmanysimilarities
with theoxygen The oxygenandsilicapatterns
seemto suggest
a northpatterns,althoughthe gradientsare weaker. The NADW wardflow alongAtica anda southward
flow alongthe
silicalevelssouthof Africa are only abouta thirdof those westcoastof Madagascar.
seenin the ArabianSea. The lack of connection
through Northof thecircumpolar
current,
thedissolved
oxygen
theMadagascar
Channelat thedepthof thisisopycnal
is decreases
monotonically
fromvaluesgreater
than4.6 mL
evident
bythe50-/•mol
L-• silica
concentration
difference
L-• forwaters
entering
fromtheAtlantic,
tolessthan2.0
north
andsouth
ofthesillbetween
Madagascar
andAfrica.mLL-• intheArabian
Seaandnearly
aslowintheBayof
MANTYLA
AND REID: ORIGINS
OF INDIAN
OCEAN DEEP AND BOTTOM
WATERS
2435
Bengal.A lateralminimum
inoxygen
ofabout
4.2mLL-1
This isopycnalmay be the shallowestone that can be exappearsjust southof the circumpolarcurrentjet, extending amined with a nonseasonal data set. The Indian Ocean has
to the KerguelenPlateau.
strongreversalsin currentsthat changeseasonallyin reA band of relatively low silica,reflectingdeepAtlantic sponseto the monsoonalvariationand vary from year to
influence, extends acrossthe entire southernIndian Ocean, year. Thoseseasonal
effectsmay extendto depthsas deep
essentiallycoincident with the lateral, high temperature, as 2000 m in the Somali Basin [Warren and Johnson,
dataset selectionmay be neededto
salinity,and oxygen maximums. Slightly higher values 1992], so a seasonal
look
at
shallow
to
intermediate
depthfeatures.
are carriedby the circumpolarcurrentthroughflowat about
50øS to 60øS. Farther south, the shallowest observations
arein therangeof 60 to 90/•molL-1, thevariation
duein
Discussion
part to cruiseto cruisebiasesandperhapsdueparfly to sea-
sonalvariations.
Silicasexceed140/•mol
L-1 in theArabian Seaandare nearlyashighin the Bay of Bengal.
The patterns of characteristicsand shear flow at this
isopycnaldepth are not as stronglyaffectedby the various
mid-ocean ridges as they were on the deeper surfaces.
The deepandbottomwatersof the IndianOceanare influencedby characteristics
derivedfrom many sources,includingsomefairly remotefrom the Indian Ocean. Most,
but not all, enter the Indian Ocean from the west with the
circumpolar current. Some come in from the east, and
some ,•e derivoa Inc'ally in
water from the circumpolarcurrent. Instead,the deepanti- The Antarctic sectors of the Indian Ocean do not contribute
cyclonic gyre, with its northern limb along about much to the abyssalwatersof the ocean,asidefrom slight
30øS-40øS,seemsto carry lower-salinitywater from the ventilationof watersthat originatein the Rossand Weddell
Pacific. The isopycnal lies between the high-salinity Seas.
The southeastPacific Ocean is the principal sourceof
NADW layer below and the low-salinityAntarcticIntermediate Water (AAIW) above. The freshest water in the low oxygen for the circumpolarcurrent,as can be seenin
patterns
onCallahan's
[1972]30cLt-• and50
gyre that appearsat 70øE may be a result of progressive theoxygen
mixing along the flow path with the low-salinityAAIW cL t-• isanosteric
surfaces.
Thelow-oxygen
waterpasses
above.
throughthe Drake Passageandacrossthe SouthAtlanticto
0ø
20 ø
40 ø
60 ø
80 ø
100 ø
120 ø
140 ø
ß
ß
40 ø
40 ø
60 ø
50 o
Figure 8a. Depth (hectometers)
of the 36.92-a2potential-density
anomalysurface.Area is shaded
wherethebottomis shallowerthanthisisopycnaldepth.
2436
MANTYLA
0ø
AND
20ø
REID: ORIGINS
40 ø
OF INDIAN
60 ø
80 ø
OCEAN
DEEP AND BOTI'OM
100ø
120ø
WATERS
140ø
34.71
40 ø
40 ø
ß 34.72 .
60 ø
60 ø
Figure 8b. Salinityon the 36.92-a2potential-density
anomalysurface.
0ø
20ø
40ø
60ø
80ø
100ø
120ø
140ø
40 ø
40 ø
60 ø
60 ø
7.0
Figure8c. Oxygen
(mLL-1)onthe36.92-a2
potential-density
anomaly
surface.
MANTYLA
0ø
AND REID: ORIGINS OF INDI•
20ø
40ø
60ø
OCEAN DEEP AND BOTI'OM
80ø
'120
100ø
120ø
WATERS
2437
140ø
.
40 ø
40 ø
60ø
60ø
9œ
Figure
8d. Silicate
(/•molL-1)onthe36.92-0'2
potential-density
anomaly
surface.
the southwestIndian Ocean, where it has risen to depths
shallowerthan 1000 m. As can be seenin Figures4c, 5c,
6c, 7c, and 8c, part of the low-oxygen water continues
acrossthe Indian Ocean southof the KerguelenPlateau,
and part goes aroundthe Weddell Sea Gyre. On the deep
isopycnalsurface45.96 a4 (or 27.82 a0) the oxygenis at a
minimum both vertically and laterally in the centerof the
gyre as well.
The Atlanticputsits imprinton the deepandbottomwaters of the Indian Ocean from two major sources.The
Weddell Sea providesdense,low-salinitybottomwater to
the Enderby,Mozambique,and Crozet Basins. This bottom water mixes with densewater from the circumpolar
currentand spreadsnorthwardinto the westernbasinsof
theIndianOcean. The signatures
from deepNorth Atlantic
are seenon most of the deepdensitysurfacesby relatively
high salinityand oxygenand low silicateenteringthe Indian Oceanimmediatelysouthof Africa. Part of this water
joins the circumpolarcurrentand crossesthe southernIndian Oceanto the Pacific Ocean,part extendsnorthwardto
contributeto the deep layers in the interior of the Indian
Ocean,someinteractswith deep water from the northern
throughflow[Wyrtki, 1961], occursin the upper thermocline and is too shallow to be considered here. The Ross
Sea and Adelie coast provide bottom water to the Australian-AntarcticBasin that is higher in salinity, although
not as cold as that found in the Weddell-EnderbyBasin.
This bottom water, mixed with dense water from the cir-
cumpolarcurrent, spreadsnorthwardinto the easternand
central basins of the Indian Ocean. The other Pacific influ-
encecan be seenat about2100 m, wherethe depthof the
36.92-0'2surfacesuggestsflow from at leastas far eastas
the Tasman Sea. It carries low-salinity water westward
acrossthe Indian Oceanto the AghulasCurrentsoutheast
of Africa.
Althoughnone of the water in the northernIndian Ocean
belowabout2000 m is directlyderivedfrom themarginal
seasthere, someof the deepwatercharacteristics
do originate there. Silica is clearly added from the sedimentsto
the deep watersthere. The influenceof the organic-rich
sedimentsat the bottomof the Bay of Bengaland the Arabian Seamay increasethe nutrientsanddecrease
the oxygen of the abyssal waters of the northernIndian Ocean
[Edmondet al., 1979; Broecker et al., 1980]. However,
Indian Ocean, and some mixes with waters above and be- verticalmixing with high-nutrientand low-oxygenwater
low. The effect of NADW on the bottom water below is
above could also explain the patternobservedin the prebest seenin the map of bottomsilica that showssilica de- sent,limited dataset. Also, the deepwatersof the northern
creasedin midlatitudesby admixturesof low-silica water Indian Oceanare too salineto have beenderivedentirely
from the north Atlantic.
from the south. The very saline overflowsfrom the Red
The Pacificprovidestwo othersourcesof deepor bot- Sea and PersianGulf, althoughinitially denserthanany at
tom water to the Indian Ocean from the east, in addition to the bottom of the Indian Ocean, only sink directly to a
thatpreviouslymentionedfrom thewestthroughtheDrake depthof 300 m to 600 m [Wyrtki, 1971], due to the small
Passage. A shallower Pacific source, the Indonesian outflow and mixing with the much lessdensewater at the
2438
MANTYLA
AND REID: ORIGINS
OF INDIAN
sills of the overflows. Nonetheless,the very high salinity
from the marginal seas clearly diffuses to much deeper
depths and results in higher deepwater salinities than
would be expectedfrom a purely southernsource.
Influencesfrom the deeperportionsof the water carried
by the circumpolarcurrenthave been seento alter the bottom waters as they spreadnorthwardbeneaththe ACC.
The circumpolarcurrentalso appearsto providea relatively high oxygen imprint to the east of Kerguelenand extending northward close to the Central Indian Ridge at
depthsof 2500 m to 3000 m. Although the sourceof this
deepwater is probablyoriginally from the northAtlantic,it
has been highly alteredby the time it hasbeencarriedby
the ACC to the longitudeeastof Kerguelen.
The complex topographyof the Indian Ocean,with its
shallowplateausand ridgesand numerousbasins,imposes
serious constraintson the deep circulation. Reid and
Arthur [1975] noted in their studyon the deep circulation
of the Pacific Ocean that the wind-driven
surface circula-
tion was reflected in deeper geopotentialanomaly maps
down to at least 3000 m. The westwardflowing limb of
the subtropicalanticyclonicgyreswere seento shift poleward in the deepermaps. In the Indian Oceanthe anticyclonic subtropicalgyre is seen to only about 2000 m, as
shownin the geostrophicshearmapsof Wyrtki[ 1971] and
in the depthof the 36.92-o'2isopycnal(Figure8a) by the
troughstartingat about 35øSjust southeast
of Africa extendingto about45øS southof Australia. Thus the westward shear from Tasmaniato the Agulhasrepresentsthe
northernlimb of an elongated,deep subtropicalgym. At
greaterdepthsthe bathymetrybreaksup the gyre into isolated circulation cells, as can be seen in the 2500 dbar rela-
OCEAN DEEP AND BOTTOM
WATERS
searchProgramof the ScrippsInstitutionof Oceanography.
We
wish to acknowledgethe assistance
given by SarileeAnderson
in arrangingthedataandcalculatingandplottingthedatapoints
alongisopycnalsand by David Newton for writing the various
programs. We thank Mizuki Tsuchiyaand Lynne Talley for
reading and commentingon the manuscript. Commentsfrom
two anonymousreviewerswere alsohelpfulin clarifyingdetails
in the text.
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(ReceivedApril 6, 1994; revisedSeptember27, 1994; accepted
September27, 1994.)