Physical Oceanography of the Indian Ocean
31
at these locations. In any case, over most of the central Indian Ocean and the Great
Australian Bight, the subtropical convergence is a zonal strip of water along the boundary
between warm subtropical water of high salinity and cooler, temperate water of lower
salinity, along which under favorable wind conditions sinking may occur. The mean
position of this strip is 40 0 to 41 0 S, and before reaching Tasmania the feature has
practically vanished.
Subtropical surface water of high salinity forms in the subtropical anticyclonic gyre
under the influence of an excess of evaporation over precipitation. The highest surface
salinities are found in a belt between 25 0 and 35 0 S, where water movements are generally
to the east. The absolute maximum of surface salinity occurs rather close to Australia and
coincides with the maximum of excess evaporation. Winter convection and downward
fluxes of salt and heat cause the subtropical water to extend with salinities above 35'1'00 to
more than 500 m depth. From the high salinity core a subsurface salinity maximum
extends throughout the entire subtropical anticyclonic gyre. This maximum spreads
chiefly equatorwards within the thermocline and is carried west by the South Equatorial
Current, where it lies between 250 and 150 m depth beneath fresher surface water (Fig. 7).
Its southward penetration is rather limited and its structure is often destroyed by sinking
along the subtropical convergence. A weak oxygen minimum is found near a depth of
180 m in the central and northern parts of the subtropical gyre. The subtropical gyre is
of very low nutrient content down to a considerable depth. At 500 m depth most of the
gyre shows P0 4 3 --P of less than 1.0 ,ug-at 1-\ N03--Nless than 15 ,ug-at I-I, and
Si04 4 --Si less than 10 ,ug-at 1-1 (Fig. 3). At 100 m depth only the coastal regions off
Australia, South Africa, and Madagascar Channel show somewhat higher nutrient concentrations (Fig. 2).
Below the subtropical water a layer of high oxygen content is found at temperatures
between 10 0 and 12 0 C in depths of 400 to 500 m. This layer originates in the transition
area south of the subtropical convergence by vertical convection and spreads north as an
oxygen maximum. It participates in the general anticyclonic circulation of the subtropical
gyre. Also the Antarctic intermediate water characterized by a salinity minimum between
800 and 1200 m depth follows an anticyclonic path. The movements of these 2 water
masses clearly demonstrate the great vertical penetration of the subtropical anticyclonic
gyre, which is considerably deeper than that of the monsoon gyre.
III. The Antarctic Waters
The hydrographic structure in Antarctic waters in the Indian Ocean is very similar to
that in the other 2 oceans. It is governed by the surfacing of the main oceanic thermocline
and by the strong, deep-reaching Circumpolar Current. Between about 40 0 Sand 50 0 S the
temperature and salinity decrease rapidly from more than 15 0 C and 35.0%0 to less than
50 C and 34.0'1'00, indicating the surfacing of the main oceanic thermocline, which divides
the warm-water sphere form the cold-water sphere. The strong inclination of this
boundary, rising from almost 1000 m depth to the sea surface, causes a powerful geostrophic current, the Antarctic Circumpolar Current, to flow east. Strong west winds
over the whole area keep the front in position by preventing the light water of the warmwater sphere from flowing over the heavier water of the cold-water sphere.
31
at these locations. In any case, over most of the central Indian Ocean and the Great
Australian Bight, the subtropical convergence is a zonal strip of water along the boundary
between warm subtropical water of high salinity and cooler, temperate water of lower
salinity, along which under favorable wind conditions sinking may occur. The mean
position of this strip is 40 0 to 41 0 S, and before reaching Tasmania the feature has
practically vanished.
Subtropical surface water of high salinity forms in the subtropical anticyclonic gyre
under the influence of an excess of evaporation over precipitation. The highest surface
salinities are found in a belt between 25 0 and 35 0 S, where water movements are generally
to the east. The absolute maximum of surface salinity occurs rather close to Australia and
coincides with the maximum of excess evaporation. Winter convection and downward
fluxes of salt and heat cause the subtropical water to extend with salinities above 35'1'00 to
more than 500 m depth. From the high salinity core a subsurface salinity maximum
extends throughout the entire subtropical anticyclonic gyre. This maximum spreads
chiefly equatorwards within the thermocline and is carried west by the South Equatorial
Current, where it lies between 250 and 150 m depth beneath fresher surface water (Fig. 7).
Its southward penetration is rather limited and its structure is often destroyed by sinking
along the subtropical convergence. A weak oxygen minimum is found near a depth of
180 m in the central and northern parts of the subtropical gyre. The subtropical gyre is
of very low nutrient content down to a considerable depth. At 500 m depth most of the
gyre shows P0 4 3 --P of less than 1.0 ,ug-at 1-\ N03--Nless than 15 ,ug-at I-I, and
Si04 4 --Si less than 10 ,ug-at 1-1 (Fig. 3). At 100 m depth only the coastal regions off
Australia, South Africa, and Madagascar Channel show somewhat higher nutrient concentrations (Fig. 2).
Below the subtropical water a layer of high oxygen content is found at temperatures
between 10 0 and 12 0 C in depths of 400 to 500 m. This layer originates in the transition
area south of the subtropical convergence by vertical convection and spreads north as an
oxygen maximum. It participates in the general anticyclonic circulation of the subtropical
gyre. Also the Antarctic intermediate water characterized by a salinity minimum between
800 and 1200 m depth follows an anticyclonic path. The movements of these 2 water
masses clearly demonstrate the great vertical penetration of the subtropical anticyclonic
gyre, which is considerably deeper than that of the monsoon gyre.
III. The Antarctic Waters
The hydrographic structure in Antarctic waters in the Indian Ocean is very similar to
that in the other 2 oceans. It is governed by the surfacing of the main oceanic thermocline
and by the strong, deep-reaching Circumpolar Current. Between about 40 0 Sand 50 0 S the
temperature and salinity decrease rapidly from more than 15 0 C and 35.0%0 to less than
50 C and 34.0'1'00, indicating the surfacing of the main oceanic thermocline, which divides
the warm-water sphere form the cold-water sphere. The strong inclination of this
boundary, rising from almost 1000 m depth to the sea surface, causes a powerful geostrophic current, the Antarctic Circumpolar Current, to flow east. Strong west winds
over the whole area keep the front in position by preventing the light water of the warmwater sphere from flowing over the heavier water of the cold-water sphere.