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, 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
Revised for Deep-Sea Research, September 2006
*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 Southern Ocean southeast Indian sector is higher in CFC
concentrations than in the southwest, due to the shorter distance of the southeast water to
the Adelie Land coast and Ross Sea sources. However, by 40ºS, CFC concentrations in
the southwest Bottom Water from the Weddell Sea are similar to those in the southeast.
Bottom Water undergoes turbulent mixing between entering the Southern Ocean
southeast Indian sector and before entering the subtropics. High CFC dilutions (3-14
fold) and a substantial concentration decrease (factor of 5) south to north of the Southeast
Indian Ridge, are consistent with independent observations of turbulent mixing. 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 Mascerene Basin. Comparing
2
subtropical Bottom Water CFC concentrations with those of the Pacific and Atlantic
oceans, at comparable latitudes, Indian Ocean Bottom Water CFC concentrations are
lower, consistent with its high dissipation rates as shown elsewhere. Thus, the high
dilutions and low CFC concentrations in Bottom Water of the southeast and southwest
subtropical Indian Ocean are due to the relative effectiveness of mixing on route and
distance to the water mass source regions. 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 its
boundaries with local sources from its marginal seas (Red Sea and Persian Gulf), by the
input of 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). Several
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 throughflow region (Gordon and Fine, 1996)
and at the location where this water enters the Indian Ocean (Fieux et al., 1996) do not
reveal a distinct signature in CFCs. 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 CFCs 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. Here we compare the contributions of these water masses
in ventilating the Indian Ocean at thermocline, intermediate, and bottom levels.
4
The World Ocean Circulation Experiment (WOCE) CFC and hydrographic data
are used to estimate decadal ventilation. The CFCs are transient tracers that have been
entering the ocean at the surface via gas exchange with the atmosphere. Their delivery to
the surface ocean is constrained by well-known atmospheric boundary conditions
(Walker et al., 2000) and solubilities as functions of temperature and salinity (Warner and
Weiss, 1985), allowing the use of these tracers quantitatively. Their distributions are
analogous to that of a signal imprinted upon the ocean surface (e.g., a climate anomaly)
with the additional advantage that they contain time scale information.
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 (pCFC11/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 of
water that has left the surface over a several-year period. The average age of this 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
5
downstream of the formation region this variability is averaged out by isopycnal mixing
and the pCFC ages represent a mean age as discussed above. For older ages, and in
waters that are mixtures from sources with different temperatures, non-linearities in the
atmospheric source function and solubility cause pCFC and ideal ages to diverge. This is
the case for intermediate and deep waters where we use ratio ages. Ratio ages represent
the age of the youngest component of a water parcel.
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) and Plähn et al. (1999) reported
CFC-12 concentration anomalies 8-40 times higher than expected (relative to CFC-11) at
depths above 400 m. The anomalies are coincident with Persian Gulf Water (PGW), and
were traced southwestward to 12ºN in the Arabian Sea. Since CFC-11 concentrations and
6
other hydrographic properties were at expected levels, they concluded that the sources of
the CFC-12 anomalies were probably solvents and fire extinguishers related to the first
Gulf War. These anomalous data are not included in figures presented here.
3. Discussion of Ventilation
During the WOCE Indian Ocean expedition, observed CFC concentrations
poleward of 40S were 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 of thermocline through intermediate layers
in the subtropics and tropics (e.g., Figs. 2c and 2d). A common pattern for thermocline,
intermediate, and Bottom Waters is a decrease in CFC concentrations from the south to
north along density layers.
3.1. Thermocline Water
Within the thermocline of the Indian Ocean, Indian Central Water (ICW) (15-8C,
35.5-34.6 salinity) (Sverdrup et al., 1942) and North ICW (15.7-7.8C, 34.8-35.1 salinity)
(You and Tomczak, 1993) are the dominant water types (~26-27 ). Several water
masses contribute to ICW and North ICW. Low salinity SAMW is found in the lower
thermocline (~26.7-27.0 ) of the subtropical gyre (McCartney, 1982) (I8S, I8N, I7N,
I5W). Low salinity Indonesian throughflow water is found from the surface though
intermediate depths (e.g., Fine, 1985; Gordon, 1986; Fieux et al., 1996; Talley and
Sprintall, 2005), and its signal is most pronounced in the eastern tropics.
7
The CFC concentrations and ages vary considerably through ICW and North
ICW. In the North Indian Ocean, CFC concentrations are relatively lower and ages 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, and contributes elevated concentrations of CFCs to the northwest as
discussed below. In the subtropical gyre, ICW has pCFC-12 ages of 2-14 years (Figs. 3ae). These ages are similar to those observed at the same densities in the South Pacific
Ocean (Fine et al., 2001).
At lower thermocline and intermediate depths, some of the youngest waters are
the low potential vorticity SAMW and AAIW entering the Indian Ocean in the
subtropical gyre across its southern boundary (e.g., McCartney 1982; Karstensen and
Tomczak, 1997). These waters then circulate within the subtropical gyre. Water
subducted in the southwest Indian Ocean remains in a westward-intensified recirculation
cell (Wyrtki, 1971; Fine, 1993; Stramma and Lutjeharms, 1997), and thus has little
potential to influence the North Indian Ocean. In the southeast, Sloyan and Rintoul
(2001) suggested that the SAMW entering the Perth Basin are transported equatorward
with the basin scale subtropical gyre circulation. The distributions of CFCs are consistent
with these suggested circulation schemes.
In the thermocline of the subtropical gyre, there are zonal variations in CFC
concentrations and ages south of 15S. Fine (1993) discussed the two South Indian
SAMW sources and the differences in their strengths based on a 1987 occupation of
32°S. SAMW in the southeast Indian Ocean has lower potential vorticity, higher CFCs,
8
and is denser (26.7 ) than the SAMW observed in the southwest. Karstensen and
Tomczak (1998) also found denser SAMW in the southeast. Ages of the SAMW on 26.7
 in this region vary from 2-4 years in the southeast to more than 14 years in the
southwest (Fig. 3e). Dissolved gases in SAMW are not likely to be in equilibrium with
the atmosphere due to lack of equilibration time during formation by wintertime
convection. Close to the outcrop, newly formed SAMW has an apparent age of about 3
years (Rintoul and Bullister, 1999). Still, high CFC concentrations observed in the
southeast in the WOCE data lend support to earlier suggestions (e.g., Wyrtki, 1971;
Stramma and Lutjeharms, 1997; Sloyan and Rintoul, 2001) that the southeast Indian
Ocean is the source for recently-ventilated ICW transported into the North Indian Ocean.
The ICW from the southeast circulates in the subtropical gyre, flowing westward
with the South Equatorial Current. Upon approaching the western boundary, the South
Equatorial Current bifurcates into southward and northward flows (e.g., Schott and
McCreary, 2001). The northward branch flows primarily along the western boundary,
rather than within the interior. Strong meridional property gradients (10º-20ºS) are an
indication that there is a barrier to exchange in the interior of the Indian Ocean. The
distributions of the CFCs support the presence of this barrier to exchange. The contrast
between low CFC concentrations of North Indian Ocean and high concentrations of the
thermocline and intermediate waters of the subtropical gyre is dramatic (Figs. 2c and 2d).
The sharp CFC concentration gradients equatorward of the subtropical gyre reflect
differences in the water mass sources and their spreading rates. There are closely-spaced
CFC-11 concentration isopleths between 10º-20ºS. The meridional CFC-11 gradients
increase dramatically with depth through the thermocline. These gradients are also
9
observed in CFC ages (Figs. 3a-h) and in other tracers (e.g., Wyrtki, 1971; Fine, 1985;
Gordon, 1986).
The Indonesian throughflow contributes to the meridional gradients of some
properties including CFCs at the equatorward boundary of the subtropical gyre.
Throughflow water (Gordon and Fine, 1996) does not have a distinct signature in CFCs
as it enters the Indian Ocean (Fieux et al., 1996), although it has a low salinity signature
(e.g., Gordon, 1986; Gordon et al., 1997). Throughflow water has CFC concentrations –
that are lower than those of the subtropical gyre and higher than those in the Bay of
Bengal. In addition to spreading westward, You and Tomczak (1993) found that some
throughflow water also spreads northward in the thermocline along the eastern boundary
into the Bay of Bengal. However, based on CFC concentrations and ages within the Bay
of Bengal, it appears that the most recently ventilated water enters the Bay from the
Arabian Sea to the south of Sri Lanka. The WOCE CFC data support previous studies
(Swallow et al., 1991; Haines et al., 1999) that show the main meridional exchange of
throughflow water and South ICW occurs along the western boundary.
Thermocline water that originates in and south of the South Indian subtropical
gyre (e.g., SAMW) has higher CFC and oxygen concentrations than throughflow water
and is the major ventilation source for the North Indian Ocean, including the Bay of
Bengal, in agreement with earlier studies (Warren et al. 1966; Wyrtki, 1971; Swallow,
1984; Prunier, 1992). Through the thermocline to 26.7 , You and Tomczak (1993)
calculated a 30-40% contribution of Southern Hemisphere water in the Arabian Sea south
of 12ºN (the latitude of zero wind stress curl).
10
The ICW also contributes to ventilation of water north of 12ºN, although PGW
dominates the ventilation of this region. Rhein et al. (1997) used a CFC budget to
estimate that the inflow of ICW north of 12ºN to be 1-6 Sv, depending on the strength of
the RSW input to the Arabian Sea. In the upper thermocline, there are minima in pCFC12 ages on 24.0  (Fig. 3a) and maxima in CFC and oxygen concentrations centered at
20ºN associated with the high salinity Persian Gulf outflow. At 24.0-24.7  surfaces,
CFC age and concentration (not presented) isopleths are oriented zonally in the western
Arabian Sea (Figs. 3a and 4b) while isohalines are oriented meridionally on each surface
throughout the lower thermocline. The orientation of CFC age and concentration
isopleths suggests a ventilation source from the north (i.e., the Persian Gulf). Yet, due to
its higher temperatures and lower gas solubilities, the resulting CFC and oxygen
concentrations of PGW are relatively low (Olson et al., 1993) compared with water at
comparable density which is colder (and fresher) at its outcrop. PGW has also been
diluted by a factor of four when it reaches 12ºN (Plähn et al., 1999; Bower et al., 2000).
Thus, PGW is not a strong source of ventilated water to the Indian Ocean, particularly
south of 12ºN. Instead, water that originates in and south of the South Indian subtropical
gyre has a relatively greater effect on thermocline ventilation throughout the Indian
Ocean.
3.2. Intermediate Water
There are several sources of intermediate water to the Indian Ocean. The AAIW is
found throughout most of the Indian Ocean at ~27.1 , and has some of the lowest
subsurface salinities (I8S and I9S). There is relatively high CFC AAIW entering in the
southwest (Fine, 1993; also observed in the 2002 transect at 32ºS), and low CFC AAIW
11
from the Pacific entering in the southeast (Fine, 1993). In addition, there is locally
formed intermediate water in the southeast Indian Ocean (Schodlok et al., 1997) and in
Marginal Seas. The RSW has a multi-layered structure (Bower et al., 2000) (>27 ) in
the western Indian Ocean, and can be traced into the Agulhas Current (e.g., Grundlingh,
1985; Beal et al., 2000). The RSW has the highest salinity (exceeding 37) in the WOCE
Indian Ocean data set (I1). The RSW CFC concentrations are highly diluted (Mecking
and Warner, 1999). Banda Intermediate Water enters in the Indonesian throughflow (e.g.,
Rochford, 1966; Talley and Sprintall, 2005) and flows westward carrying very low CFC
concentrations (Waworuntu et al., 2000). The CFC ratio ages for intermediate water (27.1
) 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. As mentioned earlier, the ratio age is that
of the youngest component of a water parcel. The oldest intermediate water is observed
in the Bay of Bengal, where CFC ratio ages exceed 30 years, while CFC ages in the
Arabian Sea lie between those of the South Indian and Bay of Bengal.
In contrast to shallower surfaces in the Arabian Sea, the CFC age isopleths at 27.1
and 27.3  (Figs. 3g and 4h) are oriented meridionally due to the input of relatively
younger RSW. Age isopleths are also oriented meridionally along the western boundary,
as northward flowing AAIW is relatively younger than RSW (e.g., Fine et al., 1988, Fine,
1993). At 27.3 , 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. RSW undergoes considerable horizontal mixing
12
through the Gulf of Aden (Mecking and Warner, 1999), and properties are diluted rapidly
(Beal et al., 2000; Bower et al., 2000). 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 CFC concentration at 27.1  and 5ºN along the western
boundary. This estimate uses the fractional water mass contribution maps of You (1998,
his Figs. 9 and 19) and WOCE data (Fig. 4) as sources 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 27.1  to be a
mixture of 20% AAIW, 70% RSW, and 10% Indonesian throughflow. Due to lack of
information, we assume that these three end member waters take the same amount of
time to get to the boundary at 5ºN. 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 due to the differences in gas solubilities and dilution at the sources of these
13
two water masses. At 27.3 , below the core of AAIW, RSW is also a relatively weak
and localized CFC source. Thus, when considering the effect on the large scale, although
RSW is a significant source for 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) as compared with AAIW.
Schodlok et al. (1997) discussed the possibility of another localized source of
intermediate water. From data collected in 1994, they designated a locally formed AAIW
in the southeast Indian Ocean. 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 salinity
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 46º-48ºS along section
I8S (93ºE, Fig. 2b), there are oxygen concentrations greater than 280 umol kg-1 and CFC11 concentrations greater than 4.4 pmol kg-1 at depths near 150 m, while stations further
south have lower CFC and oxygen concentrations at the same temperature of 6ºC. Thus,
the WOCE data support the observations of Schodlok et al. (1997) of unusually high
tracer concentrations at intermediate densities of 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 an influence of small-scale on ventilating the Indian Ocean.
14
Thus, the major ventilation source of intermediate water to the Indian Ocean is AAIW
entering in the south relative to local sources (southeast AAIW and RSW) and
throughflow.
3.3. Bottom Water
The deep water, which enters the Indian Ocean from the south, has, at most, low
level CFCs within the subtropics. Additionally, there is an Indonesian Seas source of
some deep water that is CFC-free and can be traced by its helium-3 anomaly (JeanBaptiste et al., 1997). Below the deep water, Antarctic Bottom Water (AABW) with low
salinity was previously observed along the southern part of the Indian Ocean (e.g.,
Warren, 1981; Toole and Warren, 1993). Since lines I8S, I9S, I6S, and S4I extended
furthest into the Southern Ocean, AABW is most prominent in data from those lines.
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,
circulation, and mixing. There are two significant sources for Bottom Water entering the
Indian Ocean. Water from the Weddell Sea enters the Mozambique and Madagascar
Basins from the Agulhas and Crozet Basins, respectively (e.g., Ivanenkov and Gubin,
1959; Mantyla and Reid, 1995; 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 from
the South Australia Basin. There is additional input from the Amery Ice Shelf (~70ºE),
(Jacobs and Georgi, 1977)
15
3.3.1. Southern Ocean Indian Sector
Cold Bottom Water formed at various locations around the Antarctic continent
during the past few decades carries high concentrations of CFCs away from the source
regions (Orsi et al., 1999; Rintoul and Bullister, 1999). In the Southern Ocean Indian
sector, Bottom Water (bottom bottle at depths below 3500 m) has measurable CFCs (Fig.
5). Orsi et al. (1999) attributed relatively high CFC concentrations in the southeast Indian
sector to a combination of inflow of Ross Sea Bottom Water, and local formation of
Bottom Water along the southern margins of the basin (Adelie Land). In contrast, the
Weddell Sea is the most productive source of Bottom Water around the Antarctic
continent (e.g., Worthington, 1981; Rintoul, 1998; Orsi et al., 1999). Yet, there is a
considerably greater distance from the Weddell to the southwest Indian Ocean relative to
that for the southeastern sources, resulting in longer travel time and greater opportunity
for dilution of tracer concentrations by mixing. Haine et al. (1998) found that Bottom
Water in the Crozet-Kerguelen Gap (~46ºS, 56ºE) had undergone an 8-15-fold dilution
from its Weddell source due to topographic mixing (e.g., Polzin et al., 1997; Quadfasel et
al., 1997). Increased travel time and dilution by mixing (Haine et al., 1998; Boswell and
Smythe-Wright, 2002) lower the CFC concentrations. Furthermore, based on Reid’s
(2003) transports, Bottom Water which appears to originate near the Amery Ice Shelf
flows westward along the Antarctic continental rise and must traverse the entire Weddell
Sea before entering the South Indian Ocean Thus, the Amery Ice Shelf is not presently a
significant source of CFCs for the Indian Ocean. The southeast Indian sector is nearer to
the Bottom Water sources than the southwest, and as a result its Bottom Water undergoes
less dilution and mixing on route. In the WOCE data, there are higher CFC inventories
16
(Willey et al., 2004) and concentrations over a considerably larger area in the southeast
than the southwest Southern Ocean Indian sector (south of 60ºS).
3.3.2. Dilutions in the Southeast
While Bottom Water entering in the southeast has higher CFC concentrations than
that entering in the southwest (Fig. 5), concentrations in the southwest are similar to those
in the southeast at 40ºS. In the southeast Indian Ocean, CFC concentrations are highest at
the poleward extreme of both lines I8S (Fig. 2b) and I9S. Oxygen and CFC
concentrations are higher (and water colder and fresher) along the poleward extreme of
I9S than I8S due to I9S proximity to the Adelie Land and Ross Sea sources. This source
water flows westward south of the Antarctic Circumpolar Current along the continental
rise to the Kerguelen Plateau (e.g., Gordon and Tchernia, 1972; Kolla et al., 1976). Some
of this water continues westward south of the Kerguelen Plateau (e.g., Mantisi et al.,
1991; Rintoul, 1998), while most of the circulation is diverted to the northeast (Speer and
Forbes, 1994; Donohue et al., 1999). Just south of the Southeast Indian Ridge, the
distance from the source to line I8S is shorter than to line I9S. Consequently in a small
latitude band from 56º-57ºS equatorward to the Southeast Indian Ridge, Bottom Water
has higher CFC and oxygen concentrations (and is colder and fresher) along line I8S
(95ºE) than I9S (115ºE).
Bottom Water with relatively high CFC concentrations then flows eastward along
the southern flank of the Southeast Indian Ridge. It is diverted northward into the South
Australia Basin at the gap in the Ridge (about 120ºE). Some Bottom Water then follows
the Southeast Indian Ridge northwestward into the South Australia Basin. Also, some
17
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).
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 in the source region (a function of  and
salinity) of the sample, and multiplied by the observed disequilibrium for the CFC in the
source region at present.
The CFC dilutions are estimated for the southeastern region over a range of -S
values (-0.8º to -0.3ºC, 34.65-34.72) 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% (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 14-30-fold. However, since
18
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 3-21-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 southeast sector Bottom Water undergoes
substantial mixing between entering the southeast Indian sector of the Southern Ocean
and before entering the subtropics. (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. These observations are consistent with changes in CFC concentrations and
high dilutions within the Australia-Antarctic Basin.
3.3.3. Subtropics
Along the 1987 32ºS transect, CFC-11 concentrations exceeding blank levels
were confined to the Mozambique Basin and the fracture zones of the Southwest Indian
Ridge (Fine, 1993). 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.
19
The westernmost of the three boundary currents (Warren, 1981) transports the highest
concentration CFC and oxygen (Mantyla and Reid, 1995) Bottom Water into the
subtropical Indian Ocean.
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
Bottom Water entering from the Enderby 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). This water then flows equatorward to feed the narrow opening between the
Mascarene Basin and the Somali Basin at Amirante Passage (9ºS, 53ºE, line I2).
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 al. (1998) suggested that this could be due to
20
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 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. These
high dissipation rates are consistent with observations presented here of dilution of CFC
concentrations in the Indian as compared with Pacific and Atlantic oceans.
While CFC-bearing Bottom Water reaches 20ºS in the southwest, it reaches only
about 30ºS in the Perth Basin (Figs. 2b, 2d and 5). These southwest-to-southeast
differences are not related to transport rates. Bottom Water transports into the Crozet and
Perth Basins across 32ºS are similar within uncertainty (Robbins and Toole, 1997).
Bottom Water from the Australia-Antarctic Basin flows equatorward through the
South Australia Basin into the Perth Basin, on route it undergoes local mixing (see
section 3.3.2). Local mixing appears to be the reason for lower CFC concentrations in the
Perth Basin than in the subtropical southwest Indian Ocean. Thus, the relatively high
dilutions and low CFC concentrations in Bottom Water of the southeast and southwest
subtropical Indian Ocean are due to the relative effectiveness of mixing during transit and
distance to the water mass source regions.
21
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
thermocline through intermediate waters. The general northward decrease at a given
density in CFC concentrations for thermocline, intermediate, and Bottom Waters
highlights the dominance of southern sources in ventilating the Indian Ocean at all three
levels on decadal time scales. While this is not surprising based on earlier observations,
we show how local contributions and mixing within the ocean affect the ventilation.
As compared with the southern sources, Marginal Seas and Indonesian
throughflow are considerably less significant 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, 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 and for the Bay of Bengal, there 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.
22
For Indian Ocean intermediate water, including the Arabian Sea, the most
significant ventilation source is AAIW, which enters the Indian Ocean in the southwest.
There is a lower percentage of AAIW than RSW in the tropics along the 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 et al., 1998; Orsi et al., 1999). However, higher CFC concentrations are observed
in the southeast Indian sector of the Southern Ocean (south of 60°S), these waters are
from the Adelie Land coast and Ross Sea. Water from the Weddell Sea in the southwest
Indian sector is considerably further from its source, it has more time to undergo dilution
and mixing on route than Bottom Water in the southeast Indian sector. However, by
40°S, CFC concentrations in the southwest are similar to those in the southeast. There is a
substantial concentration decrease (factor of 5) on either side of the Southeast Indian
Ridge. This together with high CFC dilutions (3-21 fold) suggest that southeast sector
Bottom Water undergoes substantial mixing between entering the Southern Ocean
southeast Indian sector and before entering the subtropics. These changes in CFC
concentrations and dilutions are consistent with observations of mixing (Polzin and
Firing, 1997; Polzin, 1999).
23
Although highest CFC concentrations are in the subpolar southeast, 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 spreading
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 tidal
forced turbulent mixing in the subtropical southwest Indian Ocean (Egbert and Ray,
2000) being more effective in diluting the Bottom Water there than in the other oceans.
Furthermore, Sloyan (2006) finds that weak stratification and numerous topographic
features per energy dissipation 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
Waters 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
24
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.
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 Chief Scientists and analysts from the Scripps Ocean Data
Facility and Woods Hole CTD group for the hydrographic data. Thanks also to 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. W.M. Smethie acknowledges
National Oceanic and Atmospheric Administration for support of line S4; R.A. Fine
acknowledges support of National Science Foundation grants OCE-9811535 and OCE0136973. 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 manuscript.
25
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35
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 (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
CFC-11/CFC-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
36
not be calculated as the CFC concentrations there are too low. The dots show location of
the discrete samples, dashed contours show the location of the surface 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 on the 27.1 σθ isopycnal in the subtropical and
tropical Indian Ocean using one time WOCE cruises. The dots show location of the
discrete samples. 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.
37