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On the Origins of Deep and Bottom Waters of the Indian Ocean

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On the Origins of Deep and Bottom Waters of the Indian Ocean 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, Scripps Institution of Oceanography,La Jolla,California Abstract. The characteristicsof the deepand bottomwaters of the Indian Ocean,when illustratedon potential-densityanomaly surfaces, indicate that the watersenter from both the Atlanticand Pacific Oceans. The paths of spreadingare constrained by thecomplex topography, andcharacteristics are seen to be alteredby exchangewith theoverlying and underlying water and with the sediments,especially in the northernIndian Ocean. The WeddellSea contributes to the densest waters found in the western basins and the Ross Sea and Adelie coast to the densest watersfound in theeastern basins. Both dense water varieties are altered by andincorporated in the lessdense water above;initially, water carried by thecircumpolar current, then water from thenorth Atlantic, and finally by deepwater whose characteristics are derivedin the northern Indian Ocean. Contact with the sedimentsincreases the silica contentof the bottom water in the SouthernOcean. In the northernIndian Ocean the sedimentsalter the silica of the water at the bottomand, together with enhanced salinity from diffusion of salineoverflows from the marginal seasabove, imprint unique markers to thedeep water that flows back to thesouth. At middepths theseries of ridgesbetween Madagascar and Australia confine the flow to a seriesof gyresthat carrycharacteristics from thecircumpolar current equatorward and the northernIndian Ocean characteristicssouthward. Within the circumpolar current, low-oxygen deep water from the Pacific is carried acrossthe Atlantic and into the Indian Ocean southof Africa. Part flows around the cyclonicWeddell Sea Gyre, andpart extendsacross the SouthernOcean. Waterfrom another Pacificsource can be seennear 2000 m extendingwestward from the Tasman Sea, south of Aus- traliaand across the Indian Ocean, and perhaps to theAghulas Current region southeast 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 qualitativesense to infer pathwaysof spreading progressiverefinement in detail as more observationsbe- of 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 severelyrestrict possible pathways of deepand bottomwa- Gordon, 1982; Mantyla and Reid, 1983; Johnsonet al., ter flow. In the following we will beginby lookingat char- 199la, b]. With the availability of newer data from recent acteristicsat the bottom of the ocean to reveal the path- decadesfor the presentstudy it was possibleto clarify and ways of spreadingof the densestbottom waters from basin add detail to the previous maps of bottom characteristics to basin. Then a series of six potential-densityanomaly surfacesat progressivelyshallower depths from about4000 shownby Mantyla and Reid [ 1983]. Nearly half of the sta- m up to about2000 m will be shownto seewhat character- tions used were taken during the 1960s' InternationalIndi- an OceanExpeditions (IIOE) or earlier. This early data set, isticsinfluence the deep watersand how pathwaysof flow while useful, suffersfrom somedeficiencies in comparison are constrainedby the bathymetryof the Indian Ocean. to the later data sets. For example, not all of the stations The choice of the specific density levels (Table 1) was reachedclose to the bottom, and some only attemptedto basedprimarily on variousproperty extrema shown by the samplethe upperhalf of the water column. Many salinities 18øSAtlantis II Cruise93 coloredsections that accompany were either titrationsalinities or run on early salinometers Warren's [1981] detailed discussionof the major features that occasionallyhad large systematicerrors. No attempt of the centralIndian Oceanbetween Madagascar and Aus- 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 deepdata set [Mantyla, 1994]. The IIOE oxygenanalyses well positionedto show featuresoriginating from both the north and the south,as well as somebrought into the Indi- were done before Carpenter's[1965] improvementsto the an Ocean by the AntarcticCircumpolar Current. The full- Winkler titration methodwere published,and they suffer oceanarea isopycnalmaps provide essentialevidence on from inaccuraciescommon to the older techniques.The the origins and fates of featuresseen in the sectionsand, nutrient data, analyzedmanually, show cruise-to-cruisebi- asesand were usuallynoisy. Wyrtki [1971] tabulatescruis- Copyright1995 by the AmericanGeophysical Union es that had large nutrient anomalies. Some could be used by makingallowances for offsetsin contouringthe data, as Papernumber 94JC02564. was done in the atlas of Wyrtki[1971, Tablesc-g]; others 0148-0227/95/94JC-02564505.00 wereomitted. Somelater cruises suffer by focusingon just 2417 2418 MANTYLA AND REID: ORIGINS OF INDIAN OCEAN DEEP AND BOTTOM WATERS 0 ø 20 ø 40 ø 60 ø 80ø 100ø 120ø 140ø 400 •' CrozetB. 400 • South B. 60 ø 60 ø Figure 1. Bathymetricfeatures and place names of the IndianOcean (B is basin;P, plateau;and R, rise). The 3000 m depthcontour is shown. conductivity-temperature-depth(CTD) observationsand in filling in the gaps with high-qualitycomplete observa- neglectingwater sample analyses for oxygenand nutrients. tions;the nonconservativechemicals are particularlyuseful It is to be hopedthe World OceanCirculation Experiment for indicatingorigins of water massesand relative degree (WOCE) cruisesplanned for thenext few yearswill result of isolationfrom formationregions. The contouringof thesemaps was highly subjectivebe- Table 1. Specificationsof the PotentialDensity Anomaly causeof the variability of the data quality. The Geochemi- Surfaces cal Ocean SectionsStudy (GEOSECS) stations,while few in number, sampled all of the importantbasins and were used as a guide to decide which data setsto acceptand > 3500 m 2500-3500 m 1500-2500 m 500-1500 m 0-500 m which to ignore in contouringthe fields. The GEOSECS (46.120) 41.711 37.198 data were assumedto be a consistentdata set,as the analyt- ical work was performedby a single group of personnel 37.104 32.521 27.824 (45.960) 41.584 [Weisset al., 1983]. (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 Waters (36.920) 32.375 27.712 In the Pacificand Atlantic Oceans, dense abyssal waters from Antarcticaspread far northward,primarily on the The potentialdensity is expressedas a0 from 0-500 dbar, westernsides of thoseoceans, while in the Indian Ocean, as a• from 500-1500 dbar,as a2 from 1500-2500 dbar,as abyssalwaters spread northward both in the westand in the a3 from 2500-3500 dbar,and as a4 from 3500 dbar east. The western and eastem sides of the Indian Ocean to the bottom. The numbersin parenthesesare thoseused have different initial sources of bottom water at their initial in thetext and figures to identifyeach isopycnal. entrypoints through the discordancezones in the Southeast MANTYLA AND REID:ORIGINS OF INDIAN OCEANDEEP AND BOTTOMWATERS 2419 IndianRidge at about50øS, 125øE [Rodman and Gordon, of the overlyingwater columnand are a resultof vertical 1982] and in the SouthwestIndian Ridge at about30øS, mixing. Silica doesnot changemonotonically from south 60øE[Warren, 1978], as can be seenin Figures2a-2e. The to north. It is high in the Enderby-CrozetBasins and in the bottom flow into the western Indian Ocean basins enters Arabian Basin, with a relative minimum in the Mascerene from the Weddell-EnderbyBasin, and the bottom flow into Basin at 10ø to 20 ø south, but this patternmay also be a the eastern Indian Ocean basins enters from the Australian- consequenceof mixing with waterabove. Verticalsections AntarcticBasin. Both basinshave primary and secondary alongthe westernbasins are shownby Mantylaand Reid sources to their dense bottom waters. The characteristics [1983,Figure 5] andin the GEOSECSIndian Ocean Atlas of the Weddell-EnderbyBasin are derived primarily from [Spenceret al., 1982,Plates 3-21]. From the verticalsec- the Weddell Sea and are alteredby mixing with deep cir- tion of Mantyla and Reid [1983, Figures5a and 5b] one cumpolarwater above as the densewater flows away from can seethe middepthsignature of waterfrom the northAt- the western Weddell shelf [Foster and Carmack, 1976]. A lantic by a salinitymaximum and low silica that extend small input from Enderby Land coast just west of the northward from the circumpolarcurrent until the silica Amery Ice shelf (70øE) can be seenin the bottomcharac- minimum intersects the bottom near 20øS. Up to this teristics,especially in dissolvedoxygen [Jacobs and Geor- point, the abyssalwaters from the Antarctichave been gi, 1977]. The bottom waters of the Australian-Antarctic mixingwith deepbut lessdense circumpolar water and wa- Basinare derivedprimarily from the RossSea, as indicated ter originatingprimarily from the Atlantic. To the northof by the higher salinity there and, secondarily,from the thispoint the mixtureof
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