Decadal Ventilation and Mixing of Indian Ocean Waters

RANA A. FINEa*, WILLIAM M. SMETHIE, JRb, JOHN L. BULLISTERc, MONIKA

RHEINd, DONG-HA MINe, MARK J. WARNERf, ALAIN POISSONg, AND RAY F.

WEISSh

aRosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098, USA

bLamont Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

cNational Oceanic and Atmospheric Administration, Pacific Marine Environmental Laboratory, 7600 Sand Point Way, NE, Seattle, WA 98115, USA

dUniversity Bremen, Institute for Environmental Physics, Department of Oceanography, D28359 Bremen, Germany

eUniversity of Texas at Austin, Marine Science Institute, 750 Channel View Drive, Port Aransas, TX 78373-5015, USA

fUniversity of Washington, School of Oceanography, Seattle, WA 98195-7940, USA

gLaboratoire de Biogeochimie et Chimie Marines, IPSL-CNRS, Universite Pierre et Marie Curie, case 134, 4 place Jussieu, 75252, Paris cedex 05, France

hScripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0220, USA

Finally Revised for Deep-Sea Research, September 2007

*Corresponding Author: Tel: 305-421-4722; E-mail: rfine@rsmas.miami.edu

Abstract

Chlorofluorocarbon (CFC) and hydrographic data from the World Ocean

Circulation Experiment (WOCE) Indian Ocean expedition are used to evaluate

contributions to decadal ventilation of water masses. At a given density, CFC-derived

ages increase and concentrations decrease from the south to north, with lowest

concentrations and oldest ages in Bay of Bengal. Average ages for thermocline water are

zero to 40 years, and for intermediate water they are less than 10 years to more than 40

years. As compared with the Marginal Seas or throughflow, the most significant source

of CFCs for the Indian Ocean south of 12ºN is the Southern Hemisphere. A simple

calculation is used to show this is the case even at intermediate levels due to differences

in gas solubilities and mixing of Antarctic Intermediate Water and Red Sea

Water.Bottom Water in the Australia-Antarctic Basin is higher in CFC concentrations

than to the west in the Enderby Basin, due to the shorter distance of this water to the

Adelie Land coast and Ross Sea sources. However, by 40ºS, CFC concentrations in the

Bottom Water of the Crozet Basin originating from the Weddell Sea are similar to those

in the South Australia Basin. Independent observations, which show that Bottom Water

undergoes elevated mixing between the Australia-Antarctic Basin and before entering the

subtropics, are consistent with high CFC dilutions (3- to 14-fold) and a substantial

concentration decrease (factor of 5) south to north of the Southeast Indian Ridge. CFC-

bearing Bottom Water with ages of 30 years or more is transported into the subtropical

South Indian Ocean by three western boundary currents, and highest concentrations are

observed in the westernmost current. During WOCE, CFC-bearing Bottom Water reaches

to about 30ºS in the Perth Basin, and to 20ºS in the Mascarene Basin. Comparing

2

subtropical Bottom Water CFC concentrations with those of the South Pacific and

Atlantic oceans, at comparable latitudes, Indian Ocean Bottom Water CFC concentrations

are lower, consistent with its high dissipation rates from tidal mixing and current

fluctuations as shown elsewhere. Thus, the generally high dilutions and low CFC

concentrations in Bottom Water of the Indian Ocean are due to distance to the water mass

source regions and the relative effectiveness of mixing. While it is not surprising that at

thermocline, intermediate, and bottom levels, the significant ventilation sources on

decadal time scales are all from the south, the CFCs show how local sources and mixing

within the ocean affect the ventilation.

Keywords: ventilation, CFCs, water masses, Indian Ocean, circulation, tracers

3

1. Introduction

Properties of water masses in the Indian Ocean are affected by exchange at their

boundaries with local sources from Marginal Seas (Red Sea and Persian Gulf), by the

input of river runoff to the surface primarily into the Bay of Bengal, by exchange with

adjacent ocean basins via the Indonesian throughflow and the southern boundary, and by

modification due to mixing within the ocean (Schott and McCreary, 2001). Previous

studies have described chlorofluorocarbon (CFC) observations in these boundary regions.

Olson et al. (1993) found relatively low dissolved gas concentrations in the source

regions of Persian Gulf Water (PGW) due to the high temperatures and salinities and thus

lower solubilities. The distributions of dissolved gases in Red Sea Water (RSW) should

be similar, as it leaves the source region at relatively high temperatures and becomes

strongly diluted (Mecking and Warner, 1999; Bower et al., 2000). Observations within

the Indonesian Seas (Gordon and Fine, 1996) and in the region where throughflow water

enters the Indian Ocean (Fieux et al., 1996) do not reveal a distinct signature of CFCs in

the upper water column, and deep waters are CFC-free. In contrast, Fine (1993)

concluded, based on CFC distributions along 32°S, that Subantarctic Mode Water

(SAMW) from the southeast and Antarctic Intermediate Water (AAIW) from the

southwest transport high CFC concentrations into the subtropical gyre. Deeper in the

water column, cold, fresh Bottom Water carrying low-level CFC concentrations enters

the southern Indian Ocean from both the east and west (e.g., Orsi et al., 2002). Based on

earlier observations of dissolved oxygen (e.g., Wyrtki, 1971; Swallow, 1984), the

importance of the southern sources in ventilating the Indian Ocean is not surprising.

4

The World Ocean Circulation Experiment (WOCE) CFC and hydrographic data

are used to estimate decadal ventilation from tracer ages of thermocline, intermediate,

and Bottom Waters. Estimates of "age" can be calculated from the partial pressures of

dissolved CFCs 11 and 12 (pCFC-11 and pCFC-12) or from the ratio of the partial

pressures (pCFC-11/pCFC-12). In both cases, the observed partial pressures or partial

pressure ratios are compared to the atmospheric source function (Walker et al., 2000) to

determine the date at which the dissolved CFCs in the water sample would have been in

equilibrium with the atmosphere. The resulting age is the elapsed time from that date to

the time of sampling.

Ages calculated using either the ratio or partial pressure method are appropriate

for different circumstances. For thermocline ventilation, water subducted in a given year

mixes with water subducted during prior years, so that a water parcel is a mixture that has

left the surface over a several-year period. The average age of a water parcel can be

approximated by the pCFC age. Studies using simple models (e.g., Doney et al., 1997;

Sonnerup, 2001) have shown that pCFC and ideal ages, which are mean ages, agree most

closely over periods when the atmospheric growth rates for the CFCs were roughly linear

(1965-1990). A recent ocean modeling study found considerable interannual variability

of pCFC ages in the North Pacific (Tsumune et al., submitted), but downstream of the

formation region this variability is smoothed out by isopycnal mixing and the pCFC ages

represent a mean age. For older waters, and in waters that are mixtures from sources with

different temperatures, non-linearities in the atmospheric source functions and solubilities

cause pCFC ages to diverge from ideal ages. In these cases, (e.g., intermediate waters),

we use ratio ages. Ratio ages represent the age of the CFC-bearing component in a

5

mixture of water parcels. Tracer ages are model-dependent, and the errors associated with

pCFC or ratio ages will vary for some of the reasons discussed. Errors in CFC ages range

from 10% in the most optimistic case, and may be off by factors of 2-3 in the most

pessimistic case. Thus, the ages are presented for relative comparisons rather than for

quantitative purposes.

2. Data

The quality of the one-time WOCE Indian Ocean CFC data are excellent and

generally meet the relaxed WOCE standards (defined as precisions better than 3% of the

concentrations or 0.015 pmol kg-1 whichever is greater). The station locations and dates

are given in Fig. 1 and its caption. The CFC data are reported on the SIO-98 calibration

scale (Prinn et al., 2000). Measurement groups used slightly-modified procedures of the

purge-and-trap technique of Bullister and Weiss (1988). The WOCE CFC Indian Ocean

data are available on DVDs (WOCE Data Products Committee, 2002). Vertical sections,

maps of ages and concentrations on isopycnal surfaces, surface saturation maps, and a

table of blank level corrections and precisions are available at gecko.rsmas.miami.edu.

Near the Persian Gulf, anomalously high dissolved CFC-12 concentrations were

measured, far above those expected from normal air-sea gas exchange with background

atmospheric CFC concentrations (Rhein et al., 1997; Plähn et al., 1999). The CFC-12

anomalies were probably from solvents and fire extinguishers related to the first Gulf

War. These anomalous data are not included in figures presented here.

3. Discussion of Ventilation

6

During the WOCE Indian Ocean expedition, observed CFC concentrations

poleward of 40°S are at least several times higher than the detection limit (roughly 0.005

pmol kg-1) throughout the water column (e.g., Figs. 2a and 2b). Equatorward of 40°S,

CFC-bearing waters are found at depths from the thermocline through intermediate layers

in the subtropics and tropics (e.g., Figs. 2c and 2d). A common pattern for thermocline,

intermediate, and Bottom Water is a decrease in CFC concentrations from the south to

north along isopycnal surfaces.

3.1. Thermocline Water

The CFC concentrations and ages vary considerably through Indian Central Water

(ICW) (~26-27 σθ). In the North Indian Ocean, CFC concentrations are relatively lower

and ages are older in the Bay of Bengal than in the Arabian Sea (Fig. 3a-f). There are

little (if any) sources of recently ventilated water in the Bay of Bengal, except for low

salinity runoff into the surface layers. Recently ventilated water enters the Arabian Sea

directly from the Marginal Seas, but the resulting CFC and oxygen concentrations of

PGW are relatively low (Olson et al., 1993) compared with water at comparable density

that is colder (and fresher) at its outcrop. Within the South Indian subtropical gyre, ICW

has pCFC-12 ages of 2-14 years (Figs. 3a-e). These ages are similar to those observed at

the same densities in the South Pacific Ocean (Fine et al., 2001).

In the thermocline of the subtropical gyre south of 15°S, there are zonal variations

in CFC concentrations and ages. Subantarctic Mode Water in the southeast Indian Ocean

has lower potential vorticity (McCartney, 1982), higher CFCs (Fine, 1993), and is denser

(26.7 σθ) (Karstensen and Tomczak, 1998) than the SAMW observed in the southwest

7

(26.5 σθ). Ages of the SAMW on 26.7 σθ vary from 2-4 years in the southeast to more

than 14 years in the southwest (Fig. 3e), and SAMW is the dominant water mass for

ventilation of the thermocline (Karstensen and Quadfasel, 2002). High CFC

concentrations observed in the southeast in the WOCE data (on 26.7 σθ Fig. 3e and 26.5

σθ not shown) lend support to earlier suggestions (e.g., Warren et al. 1966; Wyrtki, 1971;

Swallow, 1984; Prunier, 1992; Stramma and Lutjeharms, 1997; Sloyan and Rintoul,

2001) that the southeast Indian Ocean is the source for recently-ventilated thermocline

water transported into the North Indian Ocean primarily along the western boundary.

The contrast between low CFC concentrations of the thermocline and

intermediate waters of the North Indian Ocean and their high concentrations of the

subtropical gyre is dramatic (Figs. 2c and 2d). Largest meridional gradients of CFC-11

concentration are located between 10º-20ºS. These meridional CFC-11 gradients increase

with depth through the thermocline. Gradients are also observed in CFC ages (Figs. 3a-h)

and in other tracers (e.g., Wyrtki, 1971; Fine, 1985; Gordon, 1986; Gordon et al., 1997),

and reflect differences in the sources and the spreading rates of water masses. The

Indonesian throughflow contributes to the meridional gradients of some properties

including CFCs. Throughflow water has CFC concentrations that are lower than those of

the subtropical gyre and higher than those in the Bay of Bengal.

3.2. Intermediate Water

There are several sources of intermediate water to the Indian Ocean. There is

relatively high CFC AAIW (~27.1 σθ) entering in the southwest (Fine, 1993; also

observed in the 2002 transect along 32ºS), and low CFC AAIW from the Pacific entering

in the southeast (Fine, 1993). In addition, there are local sources of intermediate water in

8

the southeast Indian Ocean (Schodlok et al., 1997) and from the Marginal Seas. The CFC

ratio ages for intermediate water are less than 25 years (Fig. 3g) in the subtropical gyre.

Ratio ages are younger than the pCFC ages (Fig. 3f) because the dilution of CFC

concentration by mixing with very low or zero-CFC water has lowered the CFC

concentration resulting in an older age, but has not changed the pCFC-11/pCFC-12 ratio.

The oldest intermediate water is observed in the Bay of Bengal, where CFC ratio ages

exceed 40 years, while CFC ages in the Arabian Sea lie between those of the South

Indian and Bay of Bengal. At 27.3 σθ (Figs. 3g and 3h), there is an influence of relatively

younger RSW (Mecking and Warner, 1999) coincident with westward-intensified high

salinity. Ratio ages of RSW in the western Gulf of Aden are 18-27 years. There is a ratio

age difference of about 10 years between the western Arabian Sea and the northern Bay

of Bengal.

A rough estimate can be made of the contribution of RSW, AAIW, and

Indonesian throughflow to the measured CFC concentration at 27.1 σθ and 5ºN along the

western boundary by combining the fractional water mass contributions from the source

regions as estimated by You (1998, his Figs. 9 and 19) with WOCE data (Fig. 4) for three

components. At 27.1 σθ and 32ºS, 55ºE water is about 80% AAIW (You, 1998) with

CFC-11 concentrations of ~1.2 pmol kg-1. (They imply 1.5 pmol kg-1 at 100% if the

mixing occurs with CFC-free water). At 15ºN, 55ºE there is 100% RSW (You, 1998)

with concentrations of approximately 0.18 pmol kg-1. [At RSW outcrop, CFC saturations

are 20-50% (Mecking and Warner, 1999) with dilution factors of ~2.5 (Bower et al.,

2000).] At 10ºS, 120ºE there is 100% Indonesian throughflow water with concentrations

of 0.06 pmol kg-1. Along the western boundary at 5ºN, You (1998) estimated the water at

9

27.1 σθ to be a mixture of 20% AAIW, 70% RSW, and 10% Indonesian throughflow.

Because of lack of information, we assume that these three end-member waters take the

same amount of time to get to the boundary at 5ºN, without their concentrations

changing. Then we can sum the product of these percentages times their CFC-11

concentrations at the sources (0.2*1.5 + 0.7*0.18 + 0.1*0.06) to get a concentration of

0.43 pmol kg-1. It is nearly double the 0.25 pmol kg-1 on the map (Fig. 4), which suggests

that the high concentration AAIW portion is probably even smaller than 20%. Still this

simple calculation shows that even though there is a lower percentage of AAIW than

RSW along the western boundary, AAIW contributes most of the CFCs to the mixture

because of the differences in gas solubilities and dilution at the sources of these two

water masses. At 27.3 σθ, below the core of AAIW, RSW is also a relatively weak and

localized CFC source. Thus, when the effect on the large scale is considered, although

RSW is a significant source of salt to the thermohaline circulation (e.g., Beal et al.,

2000), it is not a significant influence as a source of water recently ventilated with CFCs

(and oxygen) relative to AAIW.

Schodlok et al. (1997) discussed the possibility of another localized source of

intermediate water formed in the Indian Ocean near Australia. This water was observed at

depths of 400-500 m just south of the subantarctic front at 47º-48ºS, 115ºE, and was

characterized by oxygen concentrations greater than 280 umol kg-1 at potential

temperatures of 6ºC, and salinities less than 34.3. Several months later, WOCE stations

were occupied on the I9S line along 115ºE. Dissolved oxygen concentrations greater than

280 umol kg-1 at 6ºC are found at 48ºS-50ºS, but at depths of only 80-150 m. At one of

these stations, CFC-11 concentration is greater than 4.8 pmol kg-1. At a few stations from

10

46º-48ºS along section I8S (93ºE, Fig. 2b), there are oxygen concentrations greater than

280 umol kg-1 and CFC-11 concentrations greater than 4.4 pmol kg-1 at depths near 150

m. Yet stations further south have lower CFC and oxygen concentrations at the same

temperature of 6ºC. Thus, the WOCE data support the 1994 observations of Schodlok et

al. (1997) of unusually high tracer concentrations at intermediate densities for several

stations south of the subantarctic front near Australia. The spatial extent and temporal

persistence of this high-tracer water cannot be determined from the available data.

However, low CFC concentrations at intermediate levels in the WOCE data from the

southeast Indian Ocean suggest that, the local source has at most a small-scale influence

on ventilating the Indian Ocean. In summary, the major ventilation source of CFCs in

intermediate water of the Indian Ocean is AAIW entering in the south, as opposed to

local sources (southeast AAIW and RSW) and throughflow.

3.3. Bottom Water

Antarctic Bottom Water with low salinity is prominent in data from lines

extending into the Southern Ocean (e.g., Fig. 2a and 2b). The WOCE data provide an

opportunity for a basin scale examination of how CFC concentrations in Bottom Water

entering the Southern Ocean Indian sector and subsequently the subtropical Indian Ocean

are influenced by proximity to source regions and mixing.

There are two significant sources for Bottom Water entering the Indian Ocean.

Bottom Water from the most productive source, the Weddell Sea (e.g., Worthington,

1981; Rintoul, 1998; Orsi et al., 1999), enters the Enderby Basin (Fig. 1), then the

Agulhas and Crozet Basins, and the Mozambique and Madagascar Basins (e.g.,

11

Ivanenkov and Gubin, 1959; Mantyla and Reid, 1995) after undergoing substantial

dilution (Haine et al., 1998). The second significant source is water primarily from the

Adelie Land coast (~143ºE), (Gordon and Tchernia, 1972; Rintoul, 1998) with a

contribution from the Ross Sea (Mantyla and Reid, 1995) that enters the Australia-

Antarctic Basin, then the South Australia Basin, and the Perth Basin. There is an

additional source of Bottom Water from the Amery Ice Shelf (near 70ºE), (Jacobs and

Georgi, 1977), which is not presently a significant source of CFCs directly to the Indian

Ocean. The Australia-Antarctic Basin is nearer to its source of Bottom Water than the

Enderby Basin is to its source and therefore has higher CFC concentrations (Fig. 5) (Orsi

et al., 1999; Rintoul and Bullister, 1999).

3.3.1. Dilutions between the Australia-Antarctic and South Australia Basins

While Bottom Water entering in the Australia-Antarctic Basin has higher CFC

concentrations than that entering in the Enderby Basin (Fig. 5), concentrations in the

Crozet Basin are similar to those in the South Australia Basin at 40ºS. Along I8S and I9S,

CFC concentrations are highest at the poleward extreme of both lines (Fig. 2b). Oxygen

and CFC concentrations are higher (and water colder and fresher) along the poleward

extreme of I9S than I8S due to the proximity of I9S to the Adelie Land and Ross Sea

sources. This source water flows westward at the base of the Antarctic continental slope

and turns northward to follow the bathymetry at Princess Elizabeth Trough (~90ºE). On

the southern flank of the Southeast Indian Ridge, the distance traveled from the source to

line I8S is shorter than to line I9S. Consequently in a small latitude range from 56º-57ºS

12

equatorward to ~50ºS, Bottom Water has higher CFC and oxygen concentrations (and is

colder and fresher) along line I8S (95ºE) than I9S (115ºE).

As the Bottom Water with relatively high CFC concentrations continues eastward

to the south of the Southeast Indian Ridge, it is diverted northward into the South

Australia Basin at gaps in the Ridge near 85ºE and 120ºE (McCartney and Donohue,

2007). Some Bottom Water then follows the Southeast Indian Ridge northwestward in

the South Australia Basin. Also, some water takes a more northward path that directly

feeds the Perth Basin via the gap between the Naturaliste and Broken Plateaus (~33º-

35ºS) (Mantyla and Reid, 1995; Reid, 2003; McCartney and Donohue, 2007).

Poleward of 51ºS along both meridians (lines I8S and I9S), Bottom Water CFC

ratio ages are less than 20 years. Equatorward of the Southeast Indian Ridge on both

meridians, ratio ages increase to about 32 years. There is a corresponding decrease in

CFC concentrations by a factor of 5. The ratio ages are used to calculate CFC dilutions,

which are a representation of the extent of mixing.

In this context, dilutions are defined as the ratio of the expected CFC-11 (or CFC-

12) concentration of a sample to the measured CFC-11 (CFC-12) concentration of the

sample (cf., Weiss et al., 1985). The expected CFC concentration is derived from the

pCFC ratio of the sample as follows. From the observed dissolved pCFC ratio and the

atmospheric CFC source functions (Walker et al., 2000), the year in which surface water

with this pCFC ratio would be in equilibrium with the atmosphere is determined. The

expected CFC-11 concentration is the atmospheric CFC-11 concentration for that date,

multiplied by the CFC-11 solubility coefficient (Warner and Weiss, 1985) in the source

13

region (a function of θ and salinity of the sample), and then multiplied by the observed

disequilibrium for the CFC in the source region during winter.

The CFC dilutions are estimated for the South Australia Basin over a range of θ-S

values (-0.8º to -0.3ºC, 34.65 to 34.72) that correspond to a range of Ross Sea and Adelie

Land Bottom Water, and surface shelf water CFC equilibration with the present

atmosphere of 50% in winter (Orsi et al., 2002). To the south of the Southeast Indian

Ridge along lines I8S and I9S, water is too young to calculate ratio ages and thus

dilutions. North of the Ridge along line I8S, Bottom Water dilutions range from 14- to

30-fold. However, since the calculation is dependent on ratio ages and the measured

concentrations are very close to blank levels, these estimates are not robust in this region.

North of the Southeast Indian Ridge along line I9S, dilutions are 3- to 21-fold. The

findings of a substantial concentration decrease (factor of 5) on either side of the Ridge,

together with the high CFC dilutions on both meridians north of the Ridge, suggest that

Bottom Water undergoes substantial mixing between entering the Australia-Antarctic

Basin and the South Australia Basin. (CFC concentrations are too low in the WOCE data

of the southwest Indian Ocean to calculate dilutions there.)

Polzin and Firing (1997) and Polzin (1999) used data from I8S near 55ºS to

suggest turbulent mixing of Bottom Water in this region. They related large depth-

averaged velocities from the Circumpolar Current as it passes over rough topography and

the subsequent generation of internal lee-waves, to enhanced mixing in the southeast

Indian Ocean. Recently, McCartney and Donohue (2007) using hydrography and LADCP

data from WOCE (I8S and I9S) provided evidence for and quantified the circulation of a

strong cyclonic gyre with elevated mixing in the Australia-Antarctic Basin, which they

14

conclude increases the Bottom Water isolation with the subtropics. Furthermore, they

find considerable diapycnal mixing associated with the equatorward transport of Bottom

Waters across the fracture zones into the South Australia Basin. These independent

observations are consistent with changes in CFC concentrations and high dilutions

between the Australia-Antarctic and South Australia Basins.

3.3.2. Subtropics

Along the 1987 32ºS transect, the westernmost of the three boundary currents

(Warren, 1981) transported Bottom Water into the subtropical Indian Ocean carrying the

highest CFC (Fine, 1993) and oxygen concentrations (Toole and Warren, 1993; Mantyla

and Reid, 1995). Using GEOSECS data, Srinivasan et al. (2000) describe younger

radiocarbon in Bottom Water of the western subtropical Indian Ocean. During WOCE,

Bottom Water entering the subtropical Indian Ocean carries measurable CFC

concentrations at the Mozambique, Madagascar, and Perth Basins (Fig. 5). Only seven

samples contain CFC concentrations exceeding blank levels (≥0.006-0.011 pmol kg-1) in

the Bottom Water entering the Perth Basin along 32ºS (I5E). At the same latitude along

line I9, similar concentrations are observed at several stations.

In the Mozambique Basin, the highest CFC-11 concentrations are observed in the

western boundary with lower concentrations in the eastern part of the Basin. This pattern

is consistent with a cyclonic circulation pathway (e.g., LePichon, 1960; Kolla et al.,

1976; Read and Pollard, 1999; Reid, 2003). Bottom Water of the western Mozambique

Basin is lower in salinity and higher in CFCs than that in the western Madagascar Basin.

This is likely due to mixing over the fracture zones that dilutes CFC concentrations in

15

Bottom Water entering from the Crozet Basin to the Madagascar Basin more than it

dilutes water entering from the Agulhas Basin to the Mozambique Basin.

In the Madagascar Basin, CFC concentrations decrease northward along 55ºE,

reaching blank levels at 17ºS. In the western boundary of the Mascarene Basin along

20ºS west of 54ºE, CFC-11 concentrations are two times blank level in four Bottom

Water samples, and may represent the first influx of CFC-bearing Bottom Water (Min,

1999). While CFC-bearing Bottom Water reaches 20ºS in the Madagascar Basin, it

reaches only about 30ºS in the Perth Basin (Figs. 2b and 5). These southwest-to-southeast

differences seem not to be related to transport rates. Bottom Water transports into the

Crozet and Perth Basins across 32ºS are similar within uncertainty (Robbins and Toole,

1997). Still it is not known how representative the Robbins and Toole (1997) transport

estimates are, as a numerical simulation suggests that the DWBC in the southwest along

32ºS can be highly variable (Palmer, 2005). Thus, based on available observations, local

mixing and dilution (see section 3.3.1) appear to be the reason for lower CFC

concentrations in the Perth Basin than in the Madagascar Basin.

Bottom Water in the western boundary of the Madagascar Basin flows

equatorward to feed the narrow opening between the Mascarene Basin and the Somali

Basin at Amirante Passage (9ºS, 53ºE, line I2, see Fig. 1). Johnson et al. (1998)

questioned why there are no measurable CFCs in the Amirante Passage during WOCE.

Note that at this latitude in the South Pacific, there were low levels of CFC-11 (0.007

pmol kg-1) in the Samoan Passage in 1996 (Orsi and Bullister, 1996). Furthermore, CFC-

11 concentrations measured in 1991 were an order of magnitude higher (0.05 pmol kg-1)

at 19°S in the South Atlantic (Wallace et al., 1994). Among other possibilities, Johnson et

16

al. (1998) suggested that this could be due to transport differences [1-1.7 Sv in the Indian

Ocean, (Johnson et al., 1998) versus 7.8 Sv in the Pacific, (Roemmich et al., 1996)].

However, the volume transport of Bottom Water in the western tropical Atlantic falls

between the other two oceans.

Johnson et al. (1998) suggested that Indian Ocean Bottom Water may be affected

by enhanced topographic mixing (e.g., Southwest and Southeast Indian Ridges). Mixing

may be diluting CFC concentrations in the subtropical/tropical Indian more than those of

the Pacific and Atlantic. Egbert and Ray (2000) used altimeter data to estimate

dissipation rates due to tidal forcing. They found considerable dissipation associated with

the rough topography of the ridge system in the subtropical/tropical western Indian

Ocean. In addition, Warren et al. (2002) and Dengler et al. (2002) observed large

fluctuations in deep currents in the western Mascarene and northern Somali Basins,

respectively, which could result in topographic mixing. The combination of effects of

high dissipation rates from tidal mixing and current fluctuations are consistent with

observations presented here of dilution of CFC concentrations in the western subtropical

Indian Ocean. In addition, the distance of the western Indian Ocean Bottom Water from

the Weddell Sea as discussed above, is probably also contributing to their lower CFC

concentrations as compared with the western subtropical South Atlantic and Pacific

oceans.

4. Conclusions

In the Indian Ocean poleward of 40ºS, there are measurable CFCs observed

throughout the water column. In the subtropics and tropics, there are CFC-bearing

17

thermocline through intermediate waters. The general northward decrease at a given

density in CFC concentrations highlights the dominance of southern sources in

ventilating the Indian Ocean at thermocline, intermediate, and bottom levels on decadal

time scales. While this is not surprising based on earlier observations, we show how local

water mass contributions and mixing within the ocean affect the ventilation.

As compared with the sources of SAMW and AAIW, Marginal Seas and

Indonesian throughflow are considerably less important and more localized ventilation

sources for thermocline and intermediate layers. Even though they do not have a

significant influence on the large-scale ventilation of the Indian Ocean, the Marginal Seas

and throughflow are sources of high and low salinity water, respectively, which in turn

influence the properties and circulation.

For Indian Ocean thermocline water, there is a large contrast between the high

CFC concentrations and relatively young ages of the South Indian Ocean, low

concentration and relatively old ages of the North Indian Ocean, and intermediate

concentrations and ages of the Indonesian throughflow. The throughflow contributes to

some of the property gradients at the equatorward boundary of the subtropical gyre. High

tracer concentrations in the southeast Indian Ocean are the most likely ventilation source

for the Arabian Sea south of 12°N, where average ages are 20-30 years. The PGW

contributes to the ventilation for the Arabian Sea north of 12°N, but it does not affect

ventilation on a larger scale.

For Indian Ocean intermediate water, including the Arabian Sea, the most

significant ventilation source is AAIW, which enters the Indian Ocean in the southwest

(Fine, 1993). There is a lower percentage of AAIW than RSW in the tropics along the

18

western boundary (You, 1998). However, a simple calculation suggests that due to

considerably higher concentrations of CFCs in AAIW and dilution (e.g., Mecking and

Warner, 1999) and gas solubility of RSW at the source, AAIW contributes most of the

CFCs. The AAIW formed in the southeast (Schodlok et al., 1997), has at most an

influence of small scale on ventilating intermediate levels of the Indian Ocean.

The CFC concentrations in Bottom Water present some seeming contradictions.

The Weddell Sea is the strongest source of Bottom Water (e.g., Worthington, 1981;

Rintoul, 1998; Orsi et al., 1999). However, higher CFC concentrations are observed in

the Australia-Antarctic Basin (south of 60°S), and these waters are from the Adelie Land

coast and Ross Sea. Water from the Weddell Sea in the Enderby Basin is considerably

farther from its source, and it has more time to undergo dilution and mixing on route than

Bottom Water in the Australia-Antarctic Basin. However, by 40°S, CFC concentrations

in the Crozet Basin are similar to those in the South Australia Basin. There is a

substantial concentration decrease (factor of 5) from the southern to the northern side of

the Southeast Indian Ridge. This together with high CFC dilutions (3- to 21-fold) are

consistent with independent observations (Polzin and Firing, 1997; Polzin, 1999;

McCartney and Donohue, 2007) suggesting that Bottom Water undergoes substantial

mixing after entering the Australia-Antarctic Basin and before entering the subtropics.

Although highest CFC concentrations are in the subpolar southeast Indian Ocean,

in the subtropics CFCs are highest in the southwest. The three western boundary currents

carry CFC-bearing Bottom Water into the subtropics, and highest concentrations are in

the westernmost current. There are lower concentrations at comparable latitudes (30ºS) in

the Perth Basin than in the west. The location of the detection limit of equatorward

19

spreading of CFC-bearing Bottom Water was at 20ºS in the Mascarene Basin of the

western Indian Ocean, as compared with at least 10ºS in the Pacific and Atlantic based on

observations a few years earlier in WOCE. This difference between oceans may be

related to a combination of effects including high dissipation rates from tidal mixing

(Egbert and Ray, 2000) and large fluctuations observed in deep currents (Warren et al.,

2002; Dengler et al., 2002) that could result in topographic mixing. Distance away from

the Weddell Sea and mixing result in the Bottom Waters of the Madagascar and

Mascarene Basins of the Indian Ocean being more diluted than in the other oceans.

Furthermore, Sloyan (2006) finds that for a given energy dissipation, weak stratification

and numerous topographic features may support a larger Indian Ocean meridional

overturning circulation (Sloyan and Rintoul, 2001).

In conclusion, there are important questions that repeated observations with CFCs

can address. In particular there is the issue of the fate of Bottom Water in the subtropical

and tropical Indian Ocean. As the transient evolves, re-occupation of key sections in the

7-10 year time frame will provide an opportunity to observe the equatorward spreading of

the CFCs in the eastern and western Indian Ocean. Together with other observations,

processes contributing to dilution of the tracer can be addressed. In addition, recent

observations have shown warming and increased salinity of upper waters in low latitudes

(e.g., Howe, 2000; Bryden et al., 2003), and freshening of intermediate and Bottom

Water of high latitude origins (Whitworth, 2002; Jacobs et al., 2002). If these changes

that are consistent with anthropogenic warming continue, they may have an impact on

ventilation patterns. These are key issues in understanding and modeling the conversion

of cold to warm water, and the ocean's ability to absorb atmospheric gases such as CO2.

20

Absence of a subpolar and subtropical North Indian Ocean has a direct effect on decadal

time scale water mass ventilation. It also is a reason the North Indian Ocean is a strong

source for CO2 as compared with the North Atlantic and Pacific, which are overall sinks.

Acknowledgements

The authors thank the lead analysts Kevin Sullivan, Ricky VanWoy, Dave

Wisegarver, Steve Covey, Tina Elbraechter for the excellent quality CFC data, and

particularly to Debbie Willey for quality control, data processing, and preparing figures.

Thank you also to the Chief Scientists and analysts from the Scripps Ocean Data Facility

and Woods Hole CTD group for the hydrographic data. R.A. Fine acknowledges support

of National Science Foundation grants OCE-9811535, OCE-0136973, OCE-0424744;

W.M. Smethie acknowledges National Oceanic and Atmospheric Administration for

support of line S4; J. L. Bullister acknowledges support from NOAA’s Office of Global

Programs; M. Rhein acknowledges support from the German Bundesministerium für

Bildung und Forschung BMBF. We also thank two anonymous reviewers for their

helpful comments, particularly on adding focus to this paper.

21

References

Beal, L.M., Ffield, A., Gordon, A.L., 2000. Spreading of Red Sea overflow waters in the

Indian Ocean. Journal of Geophysical Research 105, 8549-8564.

Bower, A.S., Hunt, H.D., Price, J.F., 2000. Character and dynamics of the Red Sea and

Persian Gulf outflows. Journal of Geophysical Research 105, 6387-6414.

Bryden, H.L., McDonagh, E.L., King, B.A., 2003. Changes in ocean water mass

properties: Oscillations or trends? Science 300, 2086-2088.

Bullister J.L., Weiss, R.F., 1988. Determination of CCl3F and CCl2F2 in seawater and air.

Deep-Sea Research 35 (5A), 839-853.

Dengler, M., D. Quadfasel, F. Schott, and J. Fischer, 2002. Abyssal circulation in the

Somali Basin. Deep-Sea Research II: Topical Studies in Oceanography 49 (7-8),

1297-1322.

Doney, S.C., Jenkins, W.J., Bullister, J.L., 1997. A comparison of ocean tracer dating

techniques on a meridional section in the eastern North Atlantic, Deep-Sea Research I

44 (4), 603-626.

Egbert, G.D., Ray, R.D., 2000. Significant dissipation of tidal energy in the deep ocean

inferred from satellite altimeter data. Nature 405, 775-778.

Fieux, M., Andrie, C., Harriaud, E., Ilahude, A.G, Metzl, M., Molcard, R., Swallow, J.C.,

1996. Hydrological and chlorofluoromethane measurements of the Indonesian

throughflow entering the Indian Ocean. Journal of Geophysical Research 101,

12,433-12,454.

Fine, R.A., 1985. Direct evidence using tritium data for the throughflow from the Pacific

to the Indian Ocean. Nature 315, 478-480.

22

Fine, R.A., 1993. Circulation of Antarctic Intermediate Water in the Indian Ocean. Deep-

Sea Research I, 40, 2021-2042.

Fine, R.A., Maillet, K.A., Sullivan, K.F., Willey, D., 2001. Circulation and ventilation

flux of the Pacific Ocean. Journal of Geophysical Research, 106(10), 22,159-22,178.

Gordon, A.L., 1986. Interocean exchange of thermocline water. Journal of Geophysical

Research 91, 5037-5046.

Gordon, A.L., Tchernia, P., 1972. Waters of the continental margin off Adelie coast,

Antarctica. In: Hayes D.E. (Ed.), Antarctic Oceanology II: The Australian-New

Zealand Sector, Antarc. Res. Ser., vol. 19, AGU, Washington, D.C., pp. 59-69.

Gordon, A.L., Fine, R.A., 1996. Pathways of water between the Pacific and Indian

Oceans in the Indonesian Seas. Nature 379, 146-149.

Gordon, A.L., Ma, S., Olson, D.B., Hacker, P., Ffield, A., Talley, L.D., Wilson, D.,

Baringer, M., 1997. Advection and diffusion of Indonesian Throughflow water within

the Indian Ocean South Equatorial Current. Geophysical Research Letters 24, 2573-

2576.

Haine, T.W.N., Watson, A.J., Liddicoat, M.I., Dickson, R.R., 1998. The flow of Antarctic

bottom water to the southwest Indian Ocean estimated using CFCs. Journal of

Geophysical Research 103, 27,637-27,653.

Howe, S.F., 2000. Decadal changes in the Indian Ocean WOCE I3 section: Water mass

changes from 1960s to 1995. International WOCE Newsletter 40, 6-8.

Ivanenkov, V.N., Gubin, F.A., 1959. Water masses and hydrochemistry of the western

and southern parts of the Indian Ocean. Transactions of the Marine Hydrophysical

23

Institute, Academy of Sciences of the U.S.S.R., Physics of the Sea, Hydrology, 22,

27-99.

Jacobs, S.S., Georgi, D.T., 1977. Observations on the southwest Indian/Antarctic Ocean.

In: Angel, M. (Ed.), A Voyage of Discovery. Deep-Sea Research 24 (Supplement),

43-84.

Jacobs, S.S., Giulivi, C.F., Mele, P.A., 2002. Freshening of the Ross Sea during the late

20th Century. Science 297, 386-389.

Johnson, G.C, Musgrave, D.L., Warren, B.A., Ffield, A., Olson, D.B., 1998. Flow of

Bottom and Deep Water in the Amirante Passage and Mascarene Basin. Journal of

Geophysical Research 103, 30,973-30,984.

Karstensen, J., Tomczak, M., 1998. Age determination of mixed water masses using CFC

and oxygen data. Journal of Geophysical Research 103, 18,599-18,609.

Karstensen, J., and D. Quadfasel, 2002. Water subducted into the Indian Ocean

subtropical gyre. Deep-Sea Research II 49, 1441-1457.

Kolla, V., Sullivan, L., Streeter, S.S., Langseth, M.G., 1976. Spreading of Antarctic

Bottom Water and its effects on the floor of the Indian Ocean inferred from Bottom-

Water potential temperature, turbidity and sea-floor photography. Marine Geology

21, 171-189.

LePichon, X., 1960. The deep water circulation in the southwest Indian Ocean. Journal of

Geophysical Research 65(12), 4061-4074.

Mantyla, A.W., Reid, J.L., 1995. On the origins of deep and bottom waters of the Indian

Ocean. Journal of Geophysical Research 100, 2417-2439.

24

McCartney, M., 1982. The subtropical recirculation of mode waters. Journal of Marine

Research 40 (Supplement), 427-464.

McCartney, M.S., Donohue, K.A., 2007. A deep cyclonic gyre in the Australia-Antarctic

Basin. Progress in Oceanography, in press, doi: 10.1016/j.pocean.2007.02.008.

Mecking, S., Warner, M.J., 1999. Ventilation of Red Sea Water with respect to

chlorofluorocarbons. Journal of Geophysical Research, 104, 11,087-11,098.

Min, D.-H., 1999. Studies of large-scale intermediate and deep water circulation and

ventilation in the North Atlantic, South Indian and Northeast Pacific Oceans, and in

the East Sea (Sea of Japan), using chlorofluorocarbons as tracers. Ph.D. dissertation,

University of California, San Diego, CA, 171 pp.

Olson, D.B., Hitchcock, G.L., Fine, R.A., Warren, B.A., 1993. Maintenance of the low-

oxygen layer in the central Arabian Sea. Deep-Sea Research II 40 (3), 673-685.

Orsi, A.H., Bullister, J.L., 1996. Synthesis of WOCE Chlorofluorocarbon data in the

Pacific Ocean. U. S. WOCE Implementation Report 8, 11-13.

Orsi, A.H., Johnson, G.C., Bullister, J.L., 1999. Circulation, mixing, and production of

Antarctic Bottom Water. Progress in Oceanography 43, 55-109.

Orsi, A.H., Smethie Jr, W.M., Bullister, J.L., 2002. On the total input of Antarctic waters

to the deep ocean: A preliminary estimate from chlorofluorocarbon measurements.

Journal of Geophysical Research 107, doi:10.1029/2001/C000976.

Palmer, M.D., 2005. Decadal variability of the subtropical gyre and deep meridional

overturning circulation of the Indian Ocean. Ph.D. dissertation, University of

Southampton, 118 pp.

25

Plähn, O., Rhein, M., Fine, R.A., Sullivan, K.F., 1999. Pollutants from the Gulf War

serve as water mass tracer in the Arabian Sea. Geophysical Research Letters 26,

71-74.

Polzin, K.L., 1999, A rough recipe for the energy balance of quasi-steady internal lee

waves. Proceedings of the 11th 'AHA Huliko'a Hawaiian Winter Workshop,

Honolulu, Hawaii, pp. 117-128.

Polzin, K.L., Firing, E., 1997. Estimates of diapycnal mixing using LADCP and CTD

data from I8S. International WOCE Newsletter 29, 39-42.

Prinn, R. G., Weiss, R.F., Fraser, P.J., Simmonds, P.G., Cunnold, D.M., Alyea, F.N.,

O'Doherty, S., Salameh, P., Miller, B.R., Huang, J., Wang, R.H.J., Hartley, D.E.,

Harth, C., Steele, L.P., Sturrock, G., Midgley P.M., McCulloch, A., 2000. A history

of chemically and radiatively important gases in air deduced from

ALE/GAGE/AGAGE. Journal of Geophysical Research 105, 17751-17792.

Prunier, K., 1992. The spreading, mixing and ventilation of thermocline and intermediate

waters in the Arabian Sea. MS Thesis, University of Miami, Miami, FL, 45 pp.

Read, J.F, Pollard, R.T., 1999. Deep inflow into the Mozambique Basin. Journal of

Geophysical Research, 104, 3075-3090.

Reid, J.L., 2003. On the total geostrophic circulation of the Indian Ocean: flow patterns,

tracers, and transports. Progress in Oceanography, 56, 137-186.

Rhein, M., Stramma, L., Plähn, O., 1997. Tracer signals of the intermediate layer of the

Arabian Sea. Geophysical Research Letters 24, 2561-2564.

Rintoul, S.R., 1998. On the origin and influence of Adelie Land Bottom Water. In:

Jacobs, S.R., Weiss, R.F. (Eds.), Ocean, Ice, and Atmosphere: Interactions at the

26

Antarctic Continental Margins, Antarctic Research Series, Vol. 75, American

Geophysical Union, Washington, D.C., pp. 151-171.

Rintoul, S.R., Bullister, J.L., 1999. A late winter hydrographic section from Tasmania to

Antarctica. Deep-Sea Research I 46 (8), 1417-1454.

Robbins, P.E., Toole, J.M. 1997. The dissolved silica budget as a constraint on the

meridional overturning circulation of the Indian Ocean. Deep-Sea Research I 44 (5),

879-906.

Roemmich, D., Hautala, S., Rudnick, D., 1996. Northward abyssal transport through the

Samoan Passage and adjacent regions. Journal of Geophysical Research 101, 14039-

14056.

Schodlok, M.P., Tomczak, M., and White, N., 1997. Deep sections through the South

Australian Basin and across the Australian-Antarctic Discordance. Geophysical

Research Letters 24, 2785-2788.

Schott, F., McCreary, Jr., J.P., 2001. The monsoon circulation of the Indian Ocean.

Progress in Oceanography 51 (1), 1-123.

Sloyan, B.M., 2006. Antarctic bottom and lower circumpolar deep water circulation in

the eastern Indian Ocean. Journal of Geophysical Research 111,

doi:10.1029/2005JC003011.

Sloyan, B.M., Rintoul, S.R., 2001. Circulation, Renewal, and Modification of Antarctic

Mode and Intermediate Water. Journal of Physical Oceanography 31, 1005-1030.

Sonnerup, R.E., 2001. On relations among CFC derived water mass ages. Geophysical

Research Letters 28, 1739-1742.

27

Speer, K.G., Forbes, A., 1994. A deep western boundary current in the South Indian

Basin. Deep-Sea Research I, 41, 1289-1303.

Srinivasan, A., Rooth, C.G.H., Top, Z., and Olson, D.B., 2000. Abyssal upwelling in the

Indian Ocean: Radiocarbon diagnostics. Journal of Marine Research 58, 755-778.

Stramma, L., Lutjeharms, J.R.R., 1997. The flow field of the subtropical gyre of the

South Indian Ocean. Journal of Geophysical Research, 102, 5513-5530.

Swallow, J.C., 1984. Some aspects of the physical oceanography of the Indian Ocean.

Deep-Sea Research 31 (6-8A), 639-650.

Toole, J.M., and Warren, B.A., 1993. A hydrographic section across the subtropical

South Indian Ocean. Deep-Sea Research I 40 (10), 1973-2019.

Tsumune, D., Bryan, F.O., Doney, S.C., and Hecht, M.W., 2005. Interannual variability

of chlorofluorocarbons, pCFC ages and ideal ages in the North Pacific from 1958-

2000 as simulated by an ocean general circulation model. Journal of Geophysical

Research, submitted.

Walker, S.J., Weiss, R.F., Salameh, P.K., 2000. Reconstructed histories of the annual

mean atmospheric mole fractions for the halocarbons CFC-11, CFC-12, CFC-113 and

carbon tetrachloride. Journal of Geophysical Research, 105, 14,285-14,296.

Wallace, D.W.R., Beining, P., Putzka, A., 1994. Carbon tetrachloride and chloro-

fluorocarbons in the South Atlantic Ocean (19ºS). Journal of Geophysical Research,

99, 7803-7819.

Warner, M.J., and R.F. Weiss, 1985. Solubilities of chlorofluorocarbons 11 and 12 in

water and seawater. Deep-Sea Research., 32, 1485-1497.

28

Warren, B.A., 1981. Transindian hydrographic section at Lat. 18ºS: Property distributions

and circulation in the South Indian Ocean. Deep-Sea Research 28 (8A), 759-788.

Warren, B.A., Stommel, H., Swallow, J.C., 1966. Water masses and patterns of flow in

the Somali Basin during the southwest Monsoon of 1964. Deep-Sea Research 13,

825-860.

Warren, B.A., T. Whitworth, III, and J.H. LaCasce, 2002. Forced resonant undulation in

the deep Mascarene Basin. Deep-Sea Research II, 49 (7-8), 1513−1526.

Weiss, R.F., Bullister, J.L., Gammon, R.H., Warner, M.J., 1985. Atmospheric

chlorofluoromethanes in the deep equatorial Atlantic. Nature, 314, 608-610.

Whitworth, T., 2002. Two modes of bottom water in the Australia-Antarctic Basin.

Geophysical Research Letters, 29, 17(1-3).

WOCE Data Products Committee, 2002. WOCE Global Data, Version 3.0, WOCE

Report No. 180/02, WOCE International Project Office, Southampton, UK.

Worthington, L.V., 1981. The water masses of the world ocean: some results of a fine-

scale census. In: Warren, B.A., Wunsch, C. (Eds.), Evolution of Physical

Oceanography: Scientific Surveys in Honor of Henry Stommel, Chapter 2. MIT

Press, pp. 42-69.

Wyrtki K., 1971. Oceanographic Atlas of the International Indian Ocean Expedition.

National Science Foundation, Washington, D.C. 531 pp.

You, Y., 1998. Dianeutral mixing and transformation of Antarctic Intermediate Water in

the Indian Ocean. Journal of Geophysical Research 103, 30,941-30,971.

29

Figure Captions

Fig. 1. Map of WOCE Indian Ocean line locations, superimposed over 3500 m isobath,

names of major topographic features included. The map includes the following lines: I1

August-October 1995; I2 December 1995-January 1996; I3 April-June 1995; I4 and I5W

June-July 1995; I6S February-March 1996; I7 July-August 1995; I8N and I5E March-

April 1995; I9N January-March 1995; I8S and I9S December 1994-January 1995; I10

November 1995; S4I May-July 1996. The abbreviations are B. for Basin, and P. for

Plateau.

Fig. 2. Sections of CFC-11 versus pressure 0-2000 db are given for select one time

WOCE cruises in the Indian Ocean. All CFC concentrations are in picomoles per

kilogram seawater (pmol kg-1). Full water column sections are presented where there are

CFCs at concentrations exceeding 0.015 pmol kg-1 below 2000 db (I6S and I8S). Top

axis shows station numbers and locations, bottom axis shows distance along track and

latitude or longitude. The dots show location of the discrete samples. The sections

presented and their nominal latitudes and longitudes are: a) I6S (30ºE), b) I8S (90ºE), c)

I7C-I7N (65ºE), d) I9N (90ºE). Dashed curves show contours for 24.7, 25.7, 26.2, 26.7,

27.1, and 27.3 σθ.

Fig. 3. Maps of pCFC-12 ages (panels a-f for 24.0, 24.7, 25.7, 26.2, 26.7, 27.1 σθ) and

pCFC-11/pCFC-12 ratio ages (panels g-h for 27.1 and 27.3 σθ) in the Indian Ocean using

one time WOCE lines. In the North Indian Ocean, ratio ages on 27.3 σθ (panel h) could

30

not be calculated as the CFC concentrations there are too low. The dots only show

locations of stations for which data are used in mapping (e.g., CFC >0.02 pmol/kg),

dashed contours show the location of the surface austral winter outcrop. Unit of contour

is 2 years. In areas with gray tones there are not enough data to contour.

Fig. 4. Map of CFC-11 concentrations (pmol kg-1) on the 27.1 σθ isopycnal in the

subtropical and tropical Indian Ocean using one time WOCE cruises. The dots only show

locations of stations for which data are used in mapping. In areas with gray tones there

are not enough data to contour.

Fig. 5. Map of CFC-11 concentrations (pmol kg-1) using bottom bottles >3500 m. Unit of

contour is the same as for the sections in Fig. 2 with the addition of 0.01 pmol kg-1

contour. Bottom topography shallower than 3500 m is shaded in gray tones. The CFC-11

data are scarce in the area 50º-65ºS, 40º-70ºE. To compensate, we used the correlation

between oxygen >5.7 ml/l and CFC-11 >0.35 pmol/kg in Enderby Basin Bottom Water

along I6S. We then used the placement of oxygen contours on Mantyla and Reid’s (1995)

map of bottom oxygen (their Fig. 2d) as a guide for CFC-11 contours.

31