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Hydrographyandwatermassesinthesouth-easternIndianOcean

TECHNICALREPORT·JANUARY2007

DOI:10.13140/2.1.3119.2001

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3AUTHORS:

MunWoo

UniversityofWesternAustralia

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CharithaPattiaratchi

UniversityofWesternAustralia

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MingFeng

TheCommonwealthScientificandIndustrial…

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Availablefrom:CharithaPattiaratchi

Retrievedon:04February2016

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DRAFT

Hydrography and water masses in the

south-eastern Indian Ocean

SFRME collaborative research project

Mun Woo, Charitha Pattiaratchi School of Environmental Systems Engineering (SESE)

The University of Western Australia

Ming Feng CSIRO Marine and Atmospheric Research

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Summary Analysis of field data from 26 voyages in the eastern Indian revealed the presence of 8 major

masses in the eastern Indian ocean region in decreasing density (i.e decreasing depth) they can

be defined as:

1. Antarctic Bottom Water (AABW) – relatively higher oxygen, higher salinity

(depth range: 3700-5300m)

2. Lower Circumpolar Deep Water (LCDW) - higher salinity lower oxygen (depth:

2600-3100m)

3. Upper Circumpolar Deep Water (UCDW) – lower oxygen (depth: 1550m)

4. North West Indian Intermediate water (NWII) - lower oxygen, higher salinity

(depth 900-1000m)

5. Antarctic Intermediate Water (AAIW) - lower salinity (depth: 1000-2000m)

6. Subantarctic Mode Water (SAMW) - higher oxygen (depth 400-500m)

7. South Indian Central Water (SICW) - higher salinity (depth 0-300m)

8. Australasian Mediterranean Water (AAMW)- lower salinity (depth 0-1000m)

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Contents Summary .......................................................................................................................3 1 Introduction ..........................................................................................................5

2 Data Sources .........................................................................................................7 3 Results and Discussion .........................................................................................9

3.1 Antarctic Bottom Water (AABW) ...Error! Bookmark not defined. 3.2 Upper andf lower Circumpolar Deep Water (CDW)Error! Bookmark not

defined. 3.3 Northwest Indian Intermediate (NWII) water ..................................21 3.4 Antarctic Intermediate Water (AAIW) ............................................21 3.5 Australasian Mediterranean Water (AAMW)..................................22 3.6 Subantarctic Mode Water (SAMW).................................................27 3.7 South Indian Central Water (SICW) Error! Bookmark not defined.

5 Conclusions .........................................................................................................33 6 References ...........................................................................................................34

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1. Introduction This report collates two decades of historical shipboard data to produce a large-scale

hydrographic description of the oceanic environment around Western Australia (WA). The

major water masses in the Indian Ocean around Western Australia were found to consist of:

Antarctic Bottom Water, Lower Circumpolar Deep Water, Upper Circumpolar Deep Water,

North West Indian Intermediate Water, Antarctic Intermediate Water, Australasian

Mediterranean Water, Subantarctic Mode Water, and South Indian Central Water.

Bottom topography beneath these water masses is dominated by the presence of a chain of 3

basins, namely: South Australia Basin, Perth Basin and West Australian (or Wharton) Basin

(Fig. 1), whose maximum depths all exceed 5000m. A significant feature south of the South

Australia Basin is the Southeast Indian Ridge which forms a barrier between the South

Australia Basin and Australian-Antarctic Basin (further south). However, there is a multiple

fracture zone in the Southeast Indian Ridge near 50oS, 124oE, which allows the exchange of

deep waters between those two basins.

There are a number of large-scale ocean currents that influence this region of Indian Ocean.

In the north, along 10oS, the South Equatorial Current (SEC) flows east to west. It is caused

by the eastward rotation of the earth which causes a relative motion of water with respect to

the earth, and thus, a westerly current. In the Indian Ocean, SEC transports Indonesian

throughflow from the Pacific Ocean, as well as recirculated water from the southern

hemisphere subtropical gyre (Quadfasel, 1996).

The Leeuwin Current (LC) flows southward along the Western Australia continental shelf. It

is a shallow (< 300 m), narrow band (< 100 km wide) of warm, lower salinity, nutrient

depleted water of tropical origin that flows poleward from Exmouth to Cape Leeuwin and into

the Great Australian Bight (Church et al., 1989; Smith et al., 1991; Ridgway and Condie,

2004). It is now accepted that the Leeuwin Current signature extends from North-West Cape

to Tasmania as the longest boundary current in the World (Ridgway and Condie, 2004).

Here, we follow the same definition as Cresswell and Petersen (1993) to define the LC as: a

warm water current of tropical origin which, during the summer months, is augmented by the

addition of (salty) water from the West Australian Current.

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The Flinders Current (FC) - the only northern boundary current in the Southern Hemisphere,

is the dominant feature along the southern coast of Australia extending from Tasmania to

Cape Leeuwin (Bye, 1972). The current extends through the water column to a maximum

depth of 800 m with maximum currents up to 0.5 ms-1 in the offshore region. The FC

interacts with the Leeuwin Current (LC) at the shelf break and slope. Here, the LC is

observable near the shelf break/slope as a surface current flowing eastward whilst the FC

located as a subsurface current flowing westward. Comparison of temperature and salinity

data collected along the south and west coasts indicate that the Flinders Current is the source

of the Sub Antarctic Mode Water which forms the core of the Leeuwin Undercurrent.

The Leeuwin Undercurrent (LU) is an equatorward undercurrent flowing beneath the Leeuwin

Current, driven by an equatorward geopotential gradient located at the depth of the

Undercurrent (Thompson, 1984, 1987; Woo et al., 2006). The Undercurrent transports 5 Sv of

higher salinity (> 35.8) oxygen-rich nutrient-depleted water at a rate of 0.32–0.40 ms-1

northward (Thompson, 1984). LU is closely associated with the Subantarctic Mode Water

(SAMW). A feature of this water mass, resulting from convection to the region south of

Australia, is high dissolved oxygen concentration and thus a cross-section of the LU core can

be identified from the dissolved oxygen distribution: the core of the current consists of

dissolved oxygen maximum (252 μM/L) centred at a depth of approximately 400 m.

The Leeuwin Undercurrent may be considered as an extension of the Flinders Current

northwards along the west coast. The Flinders Current has a subsurface maximum is located

at 400 m depth adjacent to the continental slope similar to that of the LU. Along the south

coast, the Flinders current interacts with the Leeuwin Current at the shelf break, where the

Flinders Current flows beneath the eastward flowing LC Leeuwin Current similar to the

Leeuwin Undercurrent observed on the west coast. This behaviour, together with T/S

characteristics indicates that the Flinders Current is the source of the Leeuwin Undercurrent

(Church et al., 1989; Woo et al., 2005).

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2. Data Sources The collected during 28 voyages by the RV Franklin and FRV Southern Surveyor National

Facility research vessels from 1987 to 2006 (Table 1) in the study region (Figure 1)

comprising of Conductivity-Temperature-Depth (CTD) data and Dissolved Oxygen (DOX)

concentrations, where available.

Table 1: Historical voyages from which dataset was acquired.

Cruise Code Geographic Extent Data

FR04/87 15-32oS, 110-118oE CTD/DOX

FR06/87 29-37oS, 113-118oE CTD/DOX

FR07/87 22-36oS, 110-118oE CTD/DOX

FR08/87 22-32oS, 112.5-115oE CTD/DOX

FR09/87 10-22oS, 105.6-126oE CTD/DOX

FR03/94 32-38oS, 114-138oE CTD

FR08/94 5-29oS, 80-115oE CTD/DOX

FR09/94 7-32oS, 80-115oE CTD/DOX

FR10/94 34-48oS, 120-132oE CTD/DOX

FR11/94 32-38oS, 114-136oE CTD

FR02/95 10-22oS, 114-124oE CTD

FR03/95 9-25oS, 106-115oE CTD

FR04/95 19-20oS, 115.5-116.5oE CTD

FR06/95 30-34oS, 102-115.5oE CTD

FR07/95 31.5-35.5oS, 114-130oE CTD

FR08/95 9-25oS, 106-124oE CTD

FR10/95 13.5-30oS, 111-122oE CTD

FR01/96 23-32oS, 112-115.5oE CTD

FR02/96 12-32oS, 107-116oE CTD

FR04/96 20-32oS, 112-117oE CTD

FR05/96 20.5-34.5oS, 98.5-117oE CTD

FR06/96 21.5-22.1oS, 112.2-113.4oE CTD

FR08/96 31.5-34.5oS, 102-110.5oE CTD

FR09/2000 6-35oS, 94-116oE CTD/DOX

FR10/2000 21-32oS, 110-116oE CTD/DOX

SS08/2003 30-32.5oS, 109-116oE CTD

SS09/2003 27.46-35.26oS, 112.6-115.9oE CTD/DOX

SS04/2006 31-36oS, 112-128oE CTD

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The quantity of data available varied with location (Figure 1), with the majority of voyages

having been undertaken at the shelf and shelf break. In the open ocean, data was sparse, with

large gaps in the central regions of the West Australia and Perth Basins and no data in the

western end of the South Australia Basin. However, because CTD stations were strategically

positioned at margins and entrances of the basins, a detailed description of large-scale

hydrography of the region could be developed.

Figure 1: Map of the Indian Ocean identifying prominent basins and ridges. 4km-isobath is

shown. Crosses indicate positions of CTD stations that also measured dissolved oxygen, while dots indicate CTD stations that did not.

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3. Results and Discussion

Eight different large-scale water masses were identified in the ocean surrounding West

Australia coast and they correspond with accepted classical water masses of the Indian Ocean

(Wyrtki, 1971; Warren, 1981). These were observed in the vertical distribution of salinity

and dissolved oxygen as interleaving layers of salinity and dissolved oxygen (Fig. 2). In

order of decreasing density (i.e. decreasing depth from the sea bed) these water masses were:

(i) (relatively) higher oxygen, higher salinity Antarctic Bottom Water (AABW)

(ii) higher salinity Lower Circumpolar Deep Water (LCDW)

(iii) lower oxygen Upper Circumpolar Deep Water (UCDW)

(iv) lower oxygen, higher salinity North West Indian Intermediate water (NWII)

(v) lower salinity Antarctic Intermediate Water (AAIW)

(vi) higher oxygen Subantarctic Mode Water (SAMW)

(vii) higher salinity South Indian Central Water (SICW)

(viii) lower salinity Australasian Mediterranean Water (AAMW)

The location of the water masses, the neutral densities on which they flow and their relative

position relative to each other can be identified using both salinity and oxygen (Fig. 3). The

spread of these water masses were examined using spatial charts plotted on a neutral density

surface upon which the core of water masses flowed.

In the following sections, the characteristics of each of the water masses are discussed in

detail.

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Figure 2: Potential temperature-salinity and potential temperature-oxygen diagrams exhibit interleaving positions of water property extrema. Data taken at 31.4oS, 108.3oE.

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Figure. 3 Salinity (a) and dissolved oxygen concentration (b) isopleths plotted against

neutral density (kg/m3). South of 35oS, data taken from FR10/94 (along 120oE). North of 34oS, data taken from FR09/2000 (along 95oE).

(b)

(a)

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3.1 Antarctic Bottom Water (AABW)

Antarctic Bottom Water (AABW) is the densest water mass found in the World Ocean

(Tomczak & Liefrink, 2005). Formed close to the Antarctic continent by deep convection

(principally in the Weddell Sea, Ross Sea and along Adélie Coast) (Sverdrup et al., 1942;

Carmack, 1977; Gordon & Tchernia, 1972; Mantyla & Reid, 1995; Baines & Condie, 1998,

Park et al., 1998; Rintoul, 1998; Orsi et al., 1999; Whitworth, 2002), it flows northward

beneath all the other water masses, hugging the ocean bed. In the Indian Ocean, AABW is

also known as Antarctic Circumpolar Water because on its passage into the Indian Ocean,

AABW passes through and mixes with the water of the Antarctic Circumpolar Current (ACC)

so that at its exit from the ACC, its water properties correspond to those of Antarctic

Circumpolar Water, i.e. 0.3oC potential temperature and 34.7 salinity (Tomczak & Godfrey,

1994).

AABW flowed into the South Australia Basin through the Australian Antarctic Discordance,

a multiple fracture zone in the Southeast Indian Ridge. This entry was clearly seen in Fig.

4(d), where AAIW flowed on a neutral density surface of 28.2 kg/m3 through the Discordance

near 50oS 124oE into the depths of the Great Australian Bight. AABW then move into the

Perth Basin. There was little spatial variation in the signature of water properties of the

AABW (Fig. 4), with attributes remaining constantly at 34.71 salinity, 0.62oC potential

temperature and an oxygen content ranged at 218-210μM/L (Fig. 5). Using a year-long

current mooring array between the Broken and Naturaliste plateaus Sloyan (2006) has

estimated a 2 – 2.5Sv flow of ABBW from the South Australia Basin into the Perth Basin.

ABBW carried on into the West Australian Basin, forming a western boundary current along

the Ninety East Ridge (Tomczak & Godfrey, 1994). As the area of the 28.2 kg/m3 neutral

density surface is limited to the southern part of the Perth Basin, AABW must upwell

(Sloyan, 2006). Indeed, the continuity of AABW into the West Australian Basin was clearly

detected in deep CTD records (>4000m) taken in the West Australian Basin. Across 8oS

where data was sampled down to 5000m (28.18 kg/m3 neutral density), the bottom water

exhibited a salinity of 34.71, potential temperature of 0.6oC and dissolved oxygen

concentration of 210μM/L, typical of AABW in these basins. Data was also sorted into

10ox10o squares throughout our study area (Fig. 6). Temperature-Salinity (T/S) diagrams

plotted in every square showed that the lower end of all the T/S charts terminate at the same

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point, which was the T/S signature of AABW. Thus, AABW had spread across the ocean bed

of this region.

Figure. 4: Spread of AABW on 28.2kg/m3 neutral density surface.

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0.55 0.6 0.65 0.7 0.7534.7

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Figure 5 Relationships between salinity, oxygen content, potential temperature, depth and

latitude of AABW on 28.2kg/m3 γn. (Different markers used within each 10o latitude interval.)

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Figure 6: Potential temperature – salinity diagrams drawn for data separated into a 10ox10o

grid.

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3.2 Circumpolar Deep Water (CDW)

A large northward flowing Circumpolar Deep Water (CDW) overlaid AABW. CDW may be

further separated into two distinct water masses, namely: Lower Circumpolar Deep Water

(LCDW) and Upper Circumpolar Deep Water (UCDW).

Lower Circumpolar Deep Water (LCDW) directly overlaid AABW. It was characterised by a

salinity maximum, reaching 34.75 in the south (Fig. 7) and 34.72 in the north (Fig. 8). This

water mass has also been referred to as Indian Deep Water (IDW) and upper deep water

(Tomczak & Godfrey, 1994). The high salinity content in LCDW is formed by North Atlantic

Deep Water (NADW) being injected into the Antarctic Circumpolar Current (ACC)

(Whitworth 3rd et al., 1998; Fieux et al., 2005), and then gradually getting modified along its

course through constant mixing with deep waters during its eastward journey in the ACC

(Tomczak & Liefrink, 2005). LCDW moved on the 28.07kg/m3 neutral density surface. Its

oxygen content decreased from >200μM/L in the south to 170μM/L in the north, thus

indicating northward motion. LCDW maintained potential temperature at around 1.5oC within

a depth range of 2600-3100m (Fig. 8).

Upper Circumpolar Deep Water (UCDW), which occurred just beneath Antarctic

Intermediate Water (AAIW), was characterised by an oxygen minimum (Tomczak &

Liefrink, 2005). UCDW was observed flowing at depths of around 1550m on a neutral density

surface of 27.8kg/m-3, with its oxygen concentration decreasing from 180μM/L in the south to

120μM/L in the north. (Fig. 9). Potential temperatures and salinities both increased

northward, from 2.6oC/ 34.57 to 3.6oC/34.75 respectively.

Poleward slanting isopleths were found at the eastern ocean margin (Fig. 9). This

corroborated with suggestions by Warren (1981, 1982) and Tally & Baringer (1997) that

southward flow of water from the northern ocean occurs on the eastern side of the basin,

while northward flow of CDW occurs on the western side of the basin.

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Figure 7: Spread of LCDW on 28.07kg/m3 neutral density surface.

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1.4 1.45 1.5 1.55 1.6 1.6534.7

34.72

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Figure 8: Relationships between salinity, oxygen content, potential temperature, depth and

latitude of LCDW on 28.07kg/m3 γn. (Different markers used within each 10o latitude interval.)

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Figure 9: Spread of UCDW on 27.8kg/m3 neutral density surface.

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2.5 3 3.5 434.5

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Figure 10: Relationships between salinity, oxygen content, potential temperature, depth and

latitude of UCDW on 27.8kg/m3 γn. (Different markers used within each 10o latitude interval.)

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3.3 Northwest Indian Intermediate (NWII) water

Occupying depths of 900–1000m on the neutral density level of 27.5kg/m3, a signature of

minimum oxygen (90-110 µM/L) north of 24oS (Fig 11) and clearly poleward slanting

isopleths at the eastern ocean margin revealed the presence of Northwest Indian Intermediate

(NWII) water pushing southward near Western Australia (Fig. 12). The low oxygen values

are the result of in-situ consumption of dissolved oxygen in water that has not been in contact

with the atmosphere for long, presumably due to much slower overall horizontal flow at such

depths (Warren, 1981). NWII is of Red Sea origin (Rochford, 1961, 1964). Thus, it carried

water of relatively high salt content. With a higher salinity range of 34.55-34.65 at a potential

temperature of 5oC, it could be most clearly seen in the T/S chart for data close to Australia in

the 20o-30oS, 110o-120oE grid (Fig. 6). Some indication of NWII water was also seen in the

two grid boxes to the west at this latitude. The core of NWII water had likely flowed into this

region from the northeast, similar to Rochford’s (1961) Fig. 9. However, because of the gap

in our dataset, the core of the tongue of NWII leading eastward towards the Australia was not

revealed.

As is seen in Fig.3, NWII has a similar oxygen content as CDW. This, coupled with the lack

of data in mid basins made the meeting of these two water masses unclear. However, by the

spatial orientation of these water masses, it can be known that NWII flows most strongly

southward closer to the Western Australia, while CDW flows more strongly further offshore.

3.4 Antarctic Intermediate Water (AAIW)

Above the oxygen minimum layer (UCDW or NWII), a salinity minimum was observed,

revealing the presence of Antarctic Intermediate Water. AAIW is formed in the Antarctic

Polar Zone (Molinelli, 1981; Fine, 1993; Toole & Warren, 1993; Sloyan & Rintoul, 2001). In

the South Australian Basin, AAIW was observed at depths of 1000-1200m, with fairly

constantly low potential temperatures and salinities of about 4.5oC and 34.4 respectively, and

dissolved oxygen concentrations of ~200μM/L (Fig. 13). After moving northward of 30oS in

the Perth Basin, the thickness of AAIW decreased equatorward as it is eroded by higher

salinity water masses above and below (Fieux et al., 2005).

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Northward of 25oC, dissolved oxygen concentration rapidly dropped toward 100μM/L, and

salinity and potential temperature rose to >34.6 and 5.75oC respectively (Fig. 14). This may

be the result of mixing with NWII, whose southern extent (24oS) and water properties

correspond with these changes observed in AAIW at this location.

Following a neutral density of 27.4kg/m3, AAIW shoaled equatorward, reaching up to a depth

of 800m at 18oS (Fig. 13). At this depth, AAIW comes into range of the South Equatorial

Current (SEC), which sweeps zonally along 10oS through the top 1000m of water (Tomczak

& Godfrey, 1994). Therefore, AAIW was restricted to the region south of 10-15oS, and this is

clearly seen in its water property verses latitude charts (Fig. 14).

3.5 Australasian Mediterranean Water (AAMW)

A jet of Australasian Mediterranean Water (AAMW) moved westward at 10o-15oS from the

Indonesian Archipelago, carrying relatively fresh (34.6 salinity), lower oxygen (~90μM/L)

water. The signature of AAMW was seen in T/S diagrams at latitudes north of 20oS, and most

prominently so at locations nearest to the Indonesian Archipelago (Fig. 6). In waters with

AAMW content, the T/S curve extends almost vertically upward, indicating uniformly fresher

salinity throughout its temperature range.

Because the AAMW jet extended from the surface to depths of 1000m (seen at 1000m depth

in Fig. 12), at these latitudes it creates a hindrance to the northward flow of NWII, AAIW,

SICW as well as SAMW (Figs. 3, 12, 13, 15 and 18). AAMW is a tropical water mass formed

by the transit of Pacific Ocean Central Water through the Australasian Mediterranean Sea

(Tomczak & Godfrey, 1994). In observing a similar jet, Fieux et al. (1994) concluded that it

comprised a mixture of low oxygen North Indian Deep Water and Banda Sea Intermediate

Water. This jet-like inflow of AAMW is said to produce one of the strongest frontal systems

of the world ocean’s thermocline (Tomczak & Godfrey, 1994). As there is no fixed

nomenclature for this water, it has been known as Banda Sea Water and Indonesian

Throughflow jet (Tally, 1995), among others.

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3 4 5 634.2

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Figure 11: Relationships between salinity, oxygen content, potential temperature, depth and

latitude of NWII on 27.5kg/m3 γn. (Different markers used within each 10o latitude interval.)

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Figure 12: Spread of NWII on 27.5kg/m3 neutral density surface.

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Figure 13: Spread of AAIW on 27.4kg/m3 neutral density surface.

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3 4 5 6 734.2

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a o C

-50 -40 -30 -20 -10 03

4

5

6

7

latitude oN

thet

a o C

400 600 800 1000 120050

100

150

200

250

pressure db

oxyg

en m

icro

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-50 -40 -30 -20 -10 050

100

150

200

250

latitude oN

oxyg

en m

icro

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-50 -40 -30 -20 -10 0400

600

800

1000

1200

latitude oN

pres

sure

db

Figure 14: Relationships between salinity, oxygen content, potential temperature, depth and

latitude of AAIW on 27.4kg/m3 γn. (Different markers used within each 10o latitude interval.)

- 27 - DRAFT

3.6 Subantarctic Mode Water (SAMW)

Overlying AAIW, a layer of high dissolved oxygen concentration (240–260µM/L) can be

identified as Subantarctic Mode Water (SAMW). The middle of this layer occurred around

400–500 m. Data revealed that SAMW consisted of water with very uniform water properties.

In addition to its constant oxygen content (240μM/L in most parts), its potential temperature

and salinity remained at 9°–10°C and 34.6–34.8 respectively (Fig. 15). The core of this layer

may be observed at 26.9kg/m3 neutral density. SAMW is the product of deep winter

convection at 40°–50°S in the zone between the Subtropical Convergence and the Subantarctic

Front to the south of Australia (Wyrtki, 1971; McCartney, 1977; Toole and Warren, 1993;

Karstensen and Tomczak, 1997). As SAMW is formed by deep convection rather than

subduction, newly formed SAMW penetrates to a greater depth and is better ventilated than

the newly subducted South Indian Central Water (SICW) and then moves northward from its

formation region. Due to its high oxygen content, the SAMW plays an important role in

ventilating the lower thermocline of the southern hemisphere subtropical gyres (McCartney,

1982).

It is postulated that the SAMW formed to the south of Australia is transported westward by

the Flinders Current (Middleton and Cirano, 2002) and is the source waters for the Leeuwin

Undercurrent transporting water northward along the WA coast.

SAWM also corresponds to the Indian Ocean Central Water (ICW) defined by Sverdrup et al.

(1942). SAMW and ICW often have similar temperatures and salinities; consequently SAMW

has been thought to contribute to the depth range of ICW (Karstensen and Tomczak, 1997).

According to Karstensen and Tomczak (1997), the source characteristics of SAMW differ

from region to region depending on prevailing atmospheric conditions during its formation.

In the West Australia Basin, 18oS was found to be the northern extent of the SAMW (Fig. 3).

North of this latitude, a rapid decline in oxygen concentration was seen (Fig. 16). This

resulted from mixing with the low-oxygen jet of AAMW at 10-15oS.

- 28 - DRAFT

3.7 South Indian Central Water (SICW)

A salinity maximum layer of 35.2-36 identified the South Indian Central Water (SICW).

SICW could be located at the surface between latitudes of 30oS and 36oS (Fig. 17). The

observation of surface salinity maximum is in agreement with Wyrtki (1971) who found

higher salinity water was found across the breath of the surface Indian Ocean at latitude range

25°–35°S. At these latitudes, an excess of evaporation over precipitation forms the higher

salinity water at the sea surface (Baumgartner & Reichel, 1975). Extending northward from

its formation zone, SICW was subducted below the surface water to a depth of 200-300m

(Fig. 18). SICW had a temperature range of 16-18oC and an oxygen concentration of 180-

240μM/L. Data showed no southward spread of SICW from its formation latitudes. However,

north of its formation latitude, SICW has been observed moving poleward with the Leeuwin

Current at the West Australia shelf break (Woo et al., 2006). Along the 1000m bathymetric

contour, ADCP data has revealed the core of SICW moving northward at a maximum speed

of 0.3 ms-1. However, near the shelf break this same water mass is part of the Leeuwin

Current flowing southwards and extending to the total depth (300m) of the SICW (Woo et al.,

2006). Thus, the pattern of isohalines on the neutral density surface of 26kg/m3 (on which the

SICW core followed), was one which indicated SICW sweeping poleward near the Australian

continent, between North West Cape (21oS) and Perth (32oS) (Fig. 17).

Northward, SICW extended to about 16oS where it abutted the lower salinity jet of

Australasian Mediterranean Water (AAMW) flowing westward from the Indonesian

Archipelago in the South Equatorial Current (Fig. 3).

In addition to being known as ‘South Indian Central Water’ (Webster et al., 1979; Rochford,

1969) and ‘Indian Central Water’ (Karstensen and Tomczak, 1997), this high salinity band

has also been referred to as ‘southern subtropical surface water’ (Muromtsev, 1959), ‘tropical

surface waters’ (Ivanenkov and Gubin, 1960), ‘subtropical surface water’ (Wyrtki, 1973) and

‘subtropical water’ (Fieux et al., 2005).

Finally, a summary of the attributes of all the large-scale water masses is presented in Table 2.

- 29 - DRAFT

Figure 15: Spread of SAMW on 26.9kg/m3 neutral density surface.

- 30 - DRAFT

8.5 9 9.5 10 10.534.5

34.6

34.7

34.8

34.9

theta oC

salin

ity

50 100 150 200 250 30034.5

34.6

34.7

34.8

34.9

oxygen microM/L

salin

ity

0 200 400 600 80034.5

34.6

34.7

34.8

34.9

pressure db

salin

ity

-50 -40 -30 -20 -10 034.5

34.6

34.7

34.8

34.9

latitude oN

salin

ity

50 100 150 200 250 3008.5

9

9.5

10

10.5

oxygen microM/L

thet

a o C

0 200 400 600 8008.5

9

9.5

10

10.5

pressure db

thet

a o C

-50 -40 -30 -20 -10 08.5

9

9.5

10

10.5

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a o C

0 200 400 600 8000

100

200

300

pressure db

oxyg

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-50 -40 -30 -20 -10 00

100

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300

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oxyg

en m

icro

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-50 -40 -30 -20 -10 00

200

400

600

800

latitude oN

pres

sure

db

Figure 16: Relationships between salinity, oxygen content, potential temperature, depth and

latitude of SAMW on 26.9kg/m3 γn. (Different markers used within each 10o latitude interval.)

- 31 - DRAFT

12 14 16 18 2034.5

35

35.5

36

36.5

theta oC

salin

ity

50 100 150 200 250 30034.5

35

35.5

36

36.5

oxygen microM/L

salin

ity

0 100 200 300 40034.5

35

35.5

36

36.5

pressure db

salin

ity

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35

35.5

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ity

50 100 150 200 250 30012

14

16

18

20

oxygen microM/L

thet

a o C

0 100 200 300 40012

14

16

18

20

pressure db

thet

a o C

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14

16

18

20

latitude oN

thet

a o C

0 100 200 300 4000

100

200

300

pressure db

oxyg

en m

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-50 -40 -30 -20 -10 00

100

200

300

latitude oN

oxyg

en m

icro

M/L

-50 -40 -30 -20 -10 00

100

200

300

400

latitude oN

pres

sure

db

Figure 17: Relationships between salinity, oxygen content, potential temperature, depth and

latitude of SICW on 26kg/m3 γn. (Different markers used within each 10o latitude interval.)

- 32 - DRAFT

Figure 18: Spread of SICW on 26kg/m3 neutral density surface.

- 33 - DRAFT

4. Conclusions Analysis of field data from 26 voyages in the eastern Indian revealed the presence of 8 major

masses in the region. Their core- water mass characteristics are listed on Table 2 and in terms

of decreasing density (i.e decreasing depth) they can be defined as:

• Antarctic Bottom Water (AABW) – relatively higher oxygen, higher salinity

• Lower Circumpolar Deep Water (LCDW) - higher salinity lower oxygen

• Upper Circumpolar Deep Water (UCDW) – lower oxygen

• North West Indian Intermediate water (NWII) - lower oxygen, higher salinity

• Antarctic Intermediate Water (AAIW) - lower salinity

• Subantarctic Mode Water (SAMW) - higher oxygen

• South Indian Central Water (SICW) - higher salinity

• Australasian Mediterranean Water (AAMW)- lower salinity

Table 2: Characteristics of the large-scale water masses found in the ocean around Western Australia.

Water mass Core depth (m)

Neutral density (kg/m3)

Salinity Temperature (oC)

Oxygen (μM/L)

AABW 3700-5300 28.2 34.71 0.6 210-218

LCDW 2600-3100 28.07 34.72-34.75 1.5 170-200

UCDW 1550 27.8 34.57-34.75 2.6-3.6 120-189

NWII 900-1000 27.5 34.55-34.65 5.0 90-110

AAIW 1000-2000 27.4 34.40-34.60 4.5-5.8 100-200

SAMW 400-500 26.9 34.60-34.80 9.0-10.0 240-260

SICW 0-300 26.0 35.20-36.00 16.0-18.0 180-240

AAMW 0-1000 22-27.4 34.6 5.0-30.0 90

- 34 - DRAFT

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