10
V. Barale
volume of 292,131,000 km
3 (including all its adjacent seas). Its deepest point, more
than 8,000 m below the surface, is in the Diamantina Trench. The basin extends
in latitude from the Tropic of Cancer to the Antarctic (where the boundary with
the Southern Ocean has been set at 60
◦ S), and reaches a longitudinal width of about
10,000 km, between the southern parts ofAfrica andAustralia. Surface salinity ranges
between 32 (in areas of monsoon rains, at high latitudes and along coasts impacted
by large rivers) to 37 psu (in the Arabian Sea and in a belt between southern Africa
and south-western Australia). Surface water temperatures vary between 22
◦ C and
28
◦ C, northward of the Tropic of Capricorn, but drop rather quickly southward.
In a schematic representation of the Indian Ocean water column, based on combined temperature and salinity characteristics, Upper Water at the surface is followed
by Central Water, formed by subduction at the tropics, as well as Indonesian, Bengal
Bay and Arabian Sea Water (surface to 500 m). Below this, Antarctic Intermediate
Water from the south, formed by deep convection east and west of the southern tip
of the American continent and spread into all oceans with the Circumpolar Current,
and by Indonesian, Red Sea and Persian Gulf Intermediate Water in the north (500–
1500 m). Still deeper in the water column, Circumpolar Deep Water, a mixture of
North Atlantic Deep Water and Antarctic Bottom Water, formed by deep convection
in the Weddell Sea and Ross Sea, occupy the deepest basins (1500 m to bottom).
The general thermohaline circulation of the Indian Ocean, as part of the global
Conveyor Belt system, is composed by a prevalent northeastward flow in lower
layers (primarily in the western basins), and by a prevalent southwestward flow
in the upper layers. At the surface, the wind-driven circulation pattern consists of
variable currents, dominated by the monsoon wind reversals, north of the equator,
and a main basin-wide gyre, circulating in an anticyclonic direction, i.e. counterclockwise, south of the equator (see Fig. 1.3 for a sketch of these systems’ African
components). During the summer monsoon, the northern component of the South
Equatorial Current (the East African Coastal Current) feeds a northward Somali
Current (recirculating also in an anticyclonic gyre off east Africa). This continues
northward and eastward into the Southwest Monsoon Current. During the winter
monsoon, the North Equatorial Current and the Northeast Monsoon Current, flowing
westward and southwestward, feed a southward Somali Current off east Africa. This
joins with the northern component of the South Equatorial Current (the East African
Coastal Current) into the Equatorial Countercurrent flowing eastward, away from
the African continent. In the southern gyre, the southward component of the South
Equatorial Current splits west of Madagascar, towards the Mozambique Channel,
and east of Madagascar, as the East Madagascar Current, only to re-join southward
to form the complex Agulhas Current system.
The Monsoon Currents (or Drifts) indicate the seasonally varying current regime
found in the tropical regions of the northern Indian Ocean. During summer, the
flow of the upper ocean is directed eastward, from the African area, around the
Indian sub-continent and towards the Indonesian region. During winter, the direction
reverses, with westward flow from the Indonesian region towards the African area.
The shift occurs over a time period of several weeks and is due to changes in the wind
stress associated with the Indian monsoon: winds blowing from the southwest result
V. Barale
volume of 292,131,000 km
3 (including all its adjacent seas). Its deepest point, more
than 8,000 m below the surface, is in the Diamantina Trench. The basin extends
in latitude from the Tropic of Cancer to the Antarctic (where the boundary with
the Southern Ocean has been set at 60
◦ S), and reaches a longitudinal width of about
10,000 km, between the southern parts ofAfrica andAustralia. Surface salinity ranges
between 32 (in areas of monsoon rains, at high latitudes and along coasts impacted
by large rivers) to 37 psu (in the Arabian Sea and in a belt between southern Africa
and south-western Australia). Surface water temperatures vary between 22
◦ C and
28
◦ C, northward of the Tropic of Capricorn, but drop rather quickly southward.
In a schematic representation of the Indian Ocean water column, based on combined temperature and salinity characteristics, Upper Water at the surface is followed
by Central Water, formed by subduction at the tropics, as well as Indonesian, Bengal
Bay and Arabian Sea Water (surface to 500 m). Below this, Antarctic Intermediate
Water from the south, formed by deep convection east and west of the southern tip
of the American continent and spread into all oceans with the Circumpolar Current,
and by Indonesian, Red Sea and Persian Gulf Intermediate Water in the north (500–
1500 m). Still deeper in the water column, Circumpolar Deep Water, a mixture of
North Atlantic Deep Water and Antarctic Bottom Water, formed by deep convection
in the Weddell Sea and Ross Sea, occupy the deepest basins (1500 m to bottom).
The general thermohaline circulation of the Indian Ocean, as part of the global
Conveyor Belt system, is composed by a prevalent northeastward flow in lower
layers (primarily in the western basins), and by a prevalent southwestward flow
in the upper layers. At the surface, the wind-driven circulation pattern consists of
variable currents, dominated by the monsoon wind reversals, north of the equator,
and a main basin-wide gyre, circulating in an anticyclonic direction, i.e. counterclockwise, south of the equator (see Fig. 1.3 for a sketch of these systems’ African
components). During the summer monsoon, the northern component of the South
Equatorial Current (the East African Coastal Current) feeds a northward Somali
Current (recirculating also in an anticyclonic gyre off east Africa). This continues
northward and eastward into the Southwest Monsoon Current. During the winter
monsoon, the North Equatorial Current and the Northeast Monsoon Current, flowing
westward and southwestward, feed a southward Somali Current off east Africa. This
joins with the northern component of the South Equatorial Current (the East African
Coastal Current) into the Equatorial Countercurrent flowing eastward, away from
the African continent. In the southern gyre, the southward component of the South
Equatorial Current splits west of Madagascar, towards the Mozambique Channel,
and east of Madagascar, as the East Madagascar Current, only to re-join southward
to form the complex Agulhas Current system.
The Monsoon Currents (or Drifts) indicate the seasonally varying current regime
found in the tropical regions of the northern Indian Ocean. During summer, the
flow of the upper ocean is directed eastward, from the African area, around the
Indian sub-continent and towards the Indonesian region. During winter, the direction
reverses, with westward flow from the Indonesian region towards the African area.
The shift occurs over a time period of several weeks and is due to changes in the wind
stress associated with the Indian monsoon: winds blowing from the southwest result