Decadal Ventilation and Mixing of Indian Ocean Waters rana a. Fine

3.1. Thermocline Water

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 (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 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 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 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.

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., Ivanenkov and Gubin, 1959; Mantyla and Reid, 1995) after undergoing substantial dilution (Haine et al., 1998). The second significant source is water primarily from the Adelie Land coast (~143ºE), (Gordon and Tchernia, 1972; Rintoul, 1998) with a contribution from the Ross Sea (Mantyla and Reid, 1995) that enters the Australia-Antarctic Basin, then the South Australia Basin, and the Perth Basin. There is an additional source of Bottom Water from the Amery Ice Shelf (near 70ºE), (Jacobs and Georgi, 1977), which is not presently a significant source of CFCs directly to the Indian Ocean. The Australia-Antarctic Basin is nearer to its source of Bottom Water than the Enderby Basin is to its source and therefore has higher CFC concentrations (Fig. 5) (Orsi et al., 1999; Rintoul and Bullister, 1999).
3.3.1. Dilutions between the Australia-Antarctic and South Australia Basins

While Bottom Water entering in the Australia-Antarctic Basin has higher CFC concentrations than that entering in the Enderby Basin (Fig. 5), concentrations in the Crozet Basin are similar to those in the South Australia Basin at 40ºS. Along I8S and I9S, CFC concentrations are highest at the poleward extreme of both lines (Fig. 2b). Oxygen and CFC concentrations are higher (and water colder and fresher) along the poleward extreme of I9S than I8S due to the proximity of I9S to the Adelie Land and Ross Sea sources. This source water flows westward at the base of the Antarctic continental slope and turns northward to follow the bathymetry at Princess Elizabeth Trough (~90ºE). On the southern flank of the Southeast Indian Ridge, the distance traveled from the source to line I8S is shorter than to line I9S. Consequently in a small latitude range from 56º-57ºS equatorward to ~50ºS, Bottom Water has higher CFC and oxygen concentrations (and is colder and fresher) along line I8S (95ºE) than I9S (115ºE).

As the Bottom Water with relatively high CFC concentrations continues eastward to the south of the Southeast Indian Ridge, it is diverted northward into the South Australia Basin at gaps in the Ridge near 85ºE and 120ºE (McCartney and Donohue, 2007). Some Bottom Water then follows the Southeast Indian Ridge northwestward in the South Australia Basin. Also, some water takes a more northward path that directly feeds the Perth Basin via the gap between the Naturaliste and Broken Plateaus (~33º-35ºS) (Mantyla and Reid, 1995; Reid, 2003; McCartney and Donohue, 2007).

Poleward of 51ºS along both meridians (lines I8S and I9S), Bottom Water CFC ratio ages are less than 20 years. Equatorward of the Southeast Indian Ridge on both meridians, ratio ages increase to about 32 years. There is a corresponding decrease in CFC concentrations by a factor of 5. The ratio ages are used to calculate CFC dilutions, which are a representation of the extent of mixing.

In this context, dilutions are defined as the ratio of the expected CFC-11 (or CFC-12) concentration of a sample to the measured CFC-11 (CFC-12) concentration of the sample (cf., Weiss et al., 1985). The expected CFC concentration is derived from the pCFC ratio of the sample as follows. From the observed dissolved pCFC ratio and the atmospheric CFC source functions (Walker et al., 2000), the year in which surface water with this pCFC ratio would be in equilibrium with the atmosphere is determined. The expected CFC-11 concentration is the atmospheric CFC-11 concentration for that date, multiplied by the CFC-11 solubility coefficient (Warner and Weiss, 1985) in the source 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 depth-averaged velocities from the Circumpolar Current as it passes over rough topography and the subsequent generation of internal lee-waves, to enhanced mixing in the southeast Indian Ocean. Recently, McCartney and Donohue (2007) using hydrography and LADCP data from WOCE (I8S and I9S) provided evidence for and quantified the circulation of a strong cyclonic gyre with elevated mixing in the Australia-Antarctic Basin, which they 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 Bottom Water entering from the Crozet Basin to the Madagascar Basin more than it dilutes water entering from the Agulhas Basin to the Mozambique Basin.

In the Madagascar Basin, CFC concentrations decrease northward along 55ºE, reaching blank levels at 17ºS. In the western boundary of the Mascarene Basin along 20ºS west of 54ºE, CFC-11 concentrations are two times blank level in four Bottom Water samples, and may represent the first influx of CFC-bearing Bottom Water (Min, 1999). While CFC-bearing Bottom Water reaches 20ºS in the Madagascar Basin, it reaches only about 30ºS in the Perth Basin (Figs. 2b and 5). These southwest-to-southeast differences seem not to be related to transport rates. Bottom Water transports into the Crozet and Perth Basins across 32ºS are similar within uncertainty (Robbins and Toole, 1997). Still it is not known how representative the Robbins and Toole (1997) transport estimates are, as a numerical simulation suggests that the DWBC in the southwest along 32ºS can be highly variable (Palmer, 2005). Thus, based on available observations, local mixing and dilution (see section 3.3.1) appear to be the reason for lower CFC concentrations in the Perth Basin than in the Madagascar Basin.

Bottom Water in the western boundary of the Madagascar Basin flows equatorward to feed the narrow opening between the Mascarene Basin and the Somali Basin at Amirante Passage (9ºS, 53ºE, line I2, see Fig. 1). Johnson et al. (1998) questioned why there are no measurable CFCs in the Amirante Passage during WOCE. Note that at this latitude in the South Pacific, there were low levels of CFC-11 (0.007 pmol kg^-1) in the Samoan Passage in 1996 (Orsi and Bullister, 1996). Furthermore, CFC-11 concentrations measured in 1991 were an order of magnitude higher (0.05 pmol kg^-1) at 19°S in the South Atlantic (Wallace et al., 1994). Among other possibilities, Johnson et 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.

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 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 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 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. 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.

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.

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