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
a
Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600
Rickenbacker Causeway, Miami, FL 33149-1098, USA
b
Lamont Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
c
National Oceanic and Atmospheric Administration, Pacific Marine Environmental
Laboratory, 7600 Sand Point Way, NE, Seattle, WA 98115, USA
d
University Bremen, Institute for Environmental Physics, Department of Oceanography,
D28359 Bremen, Germany
University of Texas at Austin, Marine Science Institute, 750 Channel View Drive, Port
Aransas, TX 78373-5015, USA
e
f
University of Washington, School of Oceanography, Seattle, WA 98195-7940, USA
g
Laboratoire de Biogeochimie et Chimie Marines, IPSL-CNRS, Universite Pierre et
Marie Curie, case 134, 4 place Jussieu, 75252, Paris cedex 05, France
h
Scripps 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. CFCbearing 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 ). 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 ) (Karstensen and Tomczak, 1998) than the SAMW observed in the southwest
7
(26.5 ). Ages of the SAMW on 26.7  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  Fig. 3e and 26.5
 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 ) 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  (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  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  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  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 , 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 AustraliaAntarctic 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 (~33º35ºS) (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 CFC12) 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  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 -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 depthaveraged 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, CFC11 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. DeepSea 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, 25732576.
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 BottomWater 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 lowoxygen 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, 1403914056.
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 19582000 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 chlorofluorocarbons 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 finescale 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 MarchApril 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