THE DEEP INDIAN OCEAN FLOOR
223
the Arabian Sea are not as high as those of coastal
upwelling systems in the Pacific and the Atlantic
Oceans. Upwelling also occurs off the southeast
margin of Oman, associated with enhanced biological
productivity (Hermelin and Shimmield, 1990). The
sediments of the Oman Margin and northwest Arabian
Sea vary laterally between facies rich in organic carbon,
in biogenic silica, or in carbonate, deposited under this
highly productive upwelling regime (Kennett, 1982;
Tomczak and Godfrey, 1994).
Upwelling along the eastern boundary of the Indian
Ocean is uncommon, because winds favorable for
upwelling are weak during the Northeast Monsoon
and absent during the Southwest Monsoon. This is in
contrast to the eastern boundary of the Pacific, where
there is significant upwelling in both the northern and
southern hemispheres (see Chapter 6). A small amount
of upwelling occurs along the coast of Java during the
Southwest Monsoon and weak upwelling also occurs
off southwest India (Wyrtki, 1973; Burkill et al., 1993).
Unlike typical eastern boundary currents, the Leeuwin
Current (Fig. 7.2) off the west coast of Australia flows
poleward against the wind, and the undercurrent is
equatorward, instead of moving toward the continent, as
in the Peru–Chile Margin. For these reasons, upwelling
does not occur on the Western Australian shelf, and
thus overall biological productivity in this region is
relatively low.
Deep water masses
The hydrology of the deep Indian Ocean is much
less affected by the seasonal monsoon cycle than
near-surface currents. Monsoonal influence is restricted
to the surface mixed layer and western boundary
currents. Three mediterranean-type seas influence the
hydrographic properties of the Indian Ocean: the
Persian Gulf, the Red Sea, and the Australasian
Mediterranean Sea. In the Persian Gulf and Red Sea,
evaporation exceeds precipitation and warm, dense
water flows out of the Persian Gulf and Red Sea into
the northern Indian Ocean forming North Indian Ocean
Intermediate Water. This intermediate water does not
reach the deep sea bed. In the southern Indian Ocean,
south of 10ºS, between the depths of ~500 and 1000 m,
the northward flow of Antarctic Intermediate Water is
blocked by the equatorial current system (Tomczak and
Godfrey, 1994).
Abyssal flow in the Indian Ocean is divided
into three components associated with three western
boundary currents. Western basins are penetrated by
bottom waters derived from the Indian and Atlantic
Basins (Warren, 1981). Like the Atlantic and Pacific
Oceans, the water masses in the Indian Ocean below
3500 m consist mostly of cold Antarctic Bottom Water (AABW), with a potential temperature T of 0.3ºC.
Antarctic Bottom Water leaves the circumpolar current
to enter and fill the Indian Ocean below a depth
of 3800 m at two locations: the Madagascar Basin,
and gaps in the Southwest Indian Ridge. Antarctic
Bottom Water then flows across the Madagascar
continental slope and forms a deep western boundary
current (Swallow and Pollard, 1988). Recirculation of
Antarctic Bottom Water in the Madagascar Basin is
fast, bottom currents flowing northward at a speed of
approximately 0.2 m s
−1 . The bottom water continues
along the western pathway in the Somali Basin, where
it eventually enters the Arabian Basin and disappears
through gradual mixing into overlying Deep Water. In
the eastern Indian Ocean, Antarctic Bottom Water, also
called Circumpolar Water, enters the South Australian
Basin via the Australian-Antarctic Discordance. It fills
the Great Australian Bight and moves west then north,
forming a western boundary current along the NinetyEast Ridge. The oxygen concentration follows the flow
pattern, with the concentration in the bottom water
decreasing towards the north from 5 ml °
−1 at 60ºS
to 0.2 ml °
−1 in the Arabian Sea and Bay of Bengal,
as the water mass increases in age and isolation.
This situation is in contrast to that in the Atlantic
and Pacific, where the most notable regions of low
oxygen are on the east side around the equator. From
a depth of 1500 to 3800 m, the Indian Ocean is filled
with Indian Deep Water formed from North Atlantic
Deep Water (NADW) carried into the Indian Ocean
with the upper Circumpolar Current. It spreads north
in the western boundary current to the Arabian Sea
and the Bay of Bengal. The water properties consist
of a moderate oxygen concentration (4.7 ml °
−1 ), low
temperature (2ºC), and high salinity (35.85), similar to
the North Atlantic Deep Water. The northern Arabian
Sea is closed by land mass in the north, and is semienclosed altogether (Fig. 7.1). The Arabian Sea is
deeper than 3000 m, and the basin is closed in the south
by the Central Indian Ridge, the Carlsberg Ridge, and
the Chagos–Laccadive (or Maldive) Ridge (Fig. 7.1).
Therefore, bottom water must enter in the west, through
the Owen Fracture Zone, rather than from the south
(Tomczak and Godfrey, 1994).
223
the Arabian Sea are not as high as those of coastal
upwelling systems in the Pacific and the Atlantic
Oceans. Upwelling also occurs off the southeast
margin of Oman, associated with enhanced biological
productivity (Hermelin and Shimmield, 1990). The
sediments of the Oman Margin and northwest Arabian
Sea vary laterally between facies rich in organic carbon,
in biogenic silica, or in carbonate, deposited under this
highly productive upwelling regime (Kennett, 1982;
Tomczak and Godfrey, 1994).
Upwelling along the eastern boundary of the Indian
Ocean is uncommon, because winds favorable for
upwelling are weak during the Northeast Monsoon
and absent during the Southwest Monsoon. This is in
contrast to the eastern boundary of the Pacific, where
there is significant upwelling in both the northern and
southern hemispheres (see Chapter 6). A small amount
of upwelling occurs along the coast of Java during the
Southwest Monsoon and weak upwelling also occurs
off southwest India (Wyrtki, 1973; Burkill et al., 1993).
Unlike typical eastern boundary currents, the Leeuwin
Current (Fig. 7.2) off the west coast of Australia flows
poleward against the wind, and the undercurrent is
equatorward, instead of moving toward the continent, as
in the Peru–Chile Margin. For these reasons, upwelling
does not occur on the Western Australian shelf, and
thus overall biological productivity in this region is
relatively low.
Deep water masses
The hydrology of the deep Indian Ocean is much
less affected by the seasonal monsoon cycle than
near-surface currents. Monsoonal influence is restricted
to the surface mixed layer and western boundary
currents. Three mediterranean-type seas influence the
hydrographic properties of the Indian Ocean: the
Persian Gulf, the Red Sea, and the Australasian
Mediterranean Sea. In the Persian Gulf and Red Sea,
evaporation exceeds precipitation and warm, dense
water flows out of the Persian Gulf and Red Sea into
the northern Indian Ocean forming North Indian Ocean
Intermediate Water. This intermediate water does not
reach the deep sea bed. In the southern Indian Ocean,
south of 10ºS, between the depths of ~500 and 1000 m,
the northward flow of Antarctic Intermediate Water is
blocked by the equatorial current system (Tomczak and
Godfrey, 1994).
Abyssal flow in the Indian Ocean is divided
into three components associated with three western
boundary currents. Western basins are penetrated by
bottom waters derived from the Indian and Atlantic
Basins (Warren, 1981). Like the Atlantic and Pacific
Oceans, the water masses in the Indian Ocean below
3500 m consist mostly of cold Antarctic Bottom Water (AABW), with a potential temperature T of 0.3ºC.
Antarctic Bottom Water leaves the circumpolar current
to enter and fill the Indian Ocean below a depth
of 3800 m at two locations: the Madagascar Basin,
and gaps in the Southwest Indian Ridge. Antarctic
Bottom Water then flows across the Madagascar
continental slope and forms a deep western boundary
current (Swallow and Pollard, 1988). Recirculation of
Antarctic Bottom Water in the Madagascar Basin is
fast, bottom currents flowing northward at a speed of
approximately 0.2 m s
−1 . The bottom water continues
along the western pathway in the Somali Basin, where
it eventually enters the Arabian Basin and disappears
through gradual mixing into overlying Deep Water. In
the eastern Indian Ocean, Antarctic Bottom Water, also
called Circumpolar Water, enters the South Australian
Basin via the Australian-Antarctic Discordance. It fills
the Great Australian Bight and moves west then north,
forming a western boundary current along the NinetyEast Ridge. The oxygen concentration follows the flow
pattern, with the concentration in the bottom water
decreasing towards the north from 5 ml °
−1 at 60ºS
to 0.2 ml °
−1 in the Arabian Sea and Bay of Bengal,
as the water mass increases in age and isolation.
This situation is in contrast to that in the Atlantic
and Pacific, where the most notable regions of low
oxygen are on the east side around the equator. From
a depth of 1500 to 3800 m, the Indian Ocean is filled
with Indian Deep Water formed from North Atlantic
Deep Water (NADW) carried into the Indian Ocean
with the upper Circumpolar Current. It spreads north
in the western boundary current to the Arabian Sea
and the Bay of Bengal. The water properties consist
of a moderate oxygen concentration (4.7 ml °
−1 ), low
temperature (2ºC), and high salinity (35.85), similar to
the North Atlantic Deep Water. The northern Arabian
Sea is closed by land mass in the north, and is semienclosed altogether (Fig. 7.1). The Arabian Sea is
deeper than 3000 m, and the basin is closed in the south
by the Central Indian Ridge, the Carlsberg Ridge, and
the Chagos–Laccadive (or Maldive) Ridge (Fig. 7.1).
Therefore, bottom water must enter in the west, through
the Owen Fracture Zone, rather than from the south
(Tomczak and Godfrey, 1994).