34
K. WYRTKI:
the deep sea basins of the Indian Ocean with water of potential temperatures as cold as
1.2° C in the Arabian Basin, 1.0° C in the Central Indian Basin and o.r C in the NW
Australian Basin. The spreading of the bottom water will be affected by some active flow,
at least when it crosses the sills in the various ridges dividing the basins.
The deep water of the Indian Ocean, characterized by a weak salinity maximum,
originates from the North Atlantic deep water. It enters the Indian Ocean to the south
of Africa at a depth between 2500 and 3200 m with salinities of about 34.84%0, a potential
temperature of 2.20 C and a rather high O2 content of 5 mIl-I. It spreads east with the
Circumpolar Current and its core layer can be followed into the Pacific Ocean. Below
the polar front, the core layer rises sharply and reaches to less than 800 m depth below the
Antarctic divergence (Fig. 7). Together with the bottom water it forms a rather uniform
water mass of great volume, the circumpolar water, which flows east in the lower parts
of the Circumpolar Current. The deep water also spreads north into the central Indian
Ocean, where its core layer can be followed as a salinity maximum to the north of
Madagascar and into the NW Australian Basin. Its northward spreading seems to be
diffusive rather than caused by a real flow. The O2 content in both the bottom water
and the deep water is high, but decreases in the direction of spreading. Nutrient concentrations, which are also high, generally increase to the north.
The Arabian Sea is filled with high-salinity water. It is formed at the sea surface and
intensified by the outflow of high salinity water from the Persian Gulf near 300 m depth
and from the Red Sea near 800 m depth. Vertical mixing in this rather isolated sea forms
a thick layer of high salinity which may be called the North Indian intermediate water
and downward fluxes of salt extend it to great depth. At 2000 m depth salinity in the
Arabian Sea is still as high a 34.8%0. Horizontal mixing and probably some advection
spread this layer throughout the northern Indian Ocean, even into the Bay of Bengal,
and salinity at 1500 m depth exceeds 34.80/00 everywhere to the north of 5° S. The isolation
and stagnation of the North Indian intermediate water and the lack of substantial
horizontal advection together with the high productivity of the northern Indian Ocean
cause the development of a large layer of extremely low oxygen concentration. This layer
extends from above 200 m to more than 1200 m depth in the Arabian Sea, and has O2
content of less than 1.0 mll- I everywhere to the north of 3 ° N.
Directly connected with this huge layer of very low oxygen content is the deep oxygen
minimum, which is present over the entire Indian Ocean (Fig. 8). Near the Equator it lies
at about 800 m depth, deepens to more than 1700 m near 40° S, and rises again to
about 400 m depth below the Antarctic divergence. In the South Indian Ocean it lies
above the deep water and below the Antarctic intermediate water. The O2 content within
the oxygen minimum increases slowly southward and reaches 4 ml I-I at 40° S. Without
being able to substantiate the conclusion, it appears that most of the water ascending
from the bottom and the deep water returns south in the layer near the oxygen minimum,
especially along the western side of the ocean.
The lowest oxygen concentrations in the oxygen minimum of the Indian Ocean are
in an entirely different location compared to the other 2 oceans. There the strongest
oxygen minimum is found at the eastern side of the ocean on both sides of the Equator.
In the Indian Ocean it is found in the 2 northern bays, the Arabian Sea and the Bay of
Bengal. This difference may be due not only to the fact that the Indian Ocean is landlocked
in the north, but more so to the advection of water of moderate oxygen concentration
from the Pacific Ocean through the Indonesian waters.
K. WYRTKI:
the deep sea basins of the Indian Ocean with water of potential temperatures as cold as
1.2° C in the Arabian Basin, 1.0° C in the Central Indian Basin and o.r C in the NW
Australian Basin. The spreading of the bottom water will be affected by some active flow,
at least when it crosses the sills in the various ridges dividing the basins.
The deep water of the Indian Ocean, characterized by a weak salinity maximum,
originates from the North Atlantic deep water. It enters the Indian Ocean to the south
of Africa at a depth between 2500 and 3200 m with salinities of about 34.84%0, a potential
temperature of 2.20 C and a rather high O2 content of 5 mIl-I. It spreads east with the
Circumpolar Current and its core layer can be followed into the Pacific Ocean. Below
the polar front, the core layer rises sharply and reaches to less than 800 m depth below the
Antarctic divergence (Fig. 7). Together with the bottom water it forms a rather uniform
water mass of great volume, the circumpolar water, which flows east in the lower parts
of the Circumpolar Current. The deep water also spreads north into the central Indian
Ocean, where its core layer can be followed as a salinity maximum to the north of
Madagascar and into the NW Australian Basin. Its northward spreading seems to be
diffusive rather than caused by a real flow. The O2 content in both the bottom water
and the deep water is high, but decreases in the direction of spreading. Nutrient concentrations, which are also high, generally increase to the north.
The Arabian Sea is filled with high-salinity water. It is formed at the sea surface and
intensified by the outflow of high salinity water from the Persian Gulf near 300 m depth
and from the Red Sea near 800 m depth. Vertical mixing in this rather isolated sea forms
a thick layer of high salinity which may be called the North Indian intermediate water
and downward fluxes of salt extend it to great depth. At 2000 m depth salinity in the
Arabian Sea is still as high a 34.8%0. Horizontal mixing and probably some advection
spread this layer throughout the northern Indian Ocean, even into the Bay of Bengal,
and salinity at 1500 m depth exceeds 34.80/00 everywhere to the north of 5° S. The isolation
and stagnation of the North Indian intermediate water and the lack of substantial
horizontal advection together with the high productivity of the northern Indian Ocean
cause the development of a large layer of extremely low oxygen concentration. This layer
extends from above 200 m to more than 1200 m depth in the Arabian Sea, and has O2
content of less than 1.0 mll- I everywhere to the north of 3 ° N.
Directly connected with this huge layer of very low oxygen content is the deep oxygen
minimum, which is present over the entire Indian Ocean (Fig. 8). Near the Equator it lies
at about 800 m depth, deepens to more than 1700 m near 40° S, and rises again to
about 400 m depth below the Antarctic divergence. In the South Indian Ocean it lies
above the deep water and below the Antarctic intermediate water. The O2 content within
the oxygen minimum increases slowly southward and reaches 4 ml I-I at 40° S. Without
being able to substantiate the conclusion, it appears that most of the water ascending
from the bottom and the deep water returns south in the layer near the oxygen minimum,
especially along the western side of the ocean.
The lowest oxygen concentrations in the oxygen minimum of the Indian Ocean are
in an entirely different location compared to the other 2 oceans. There the strongest
oxygen minimum is found at the eastern side of the ocean on both sides of the Equator.
In the Indian Ocean it is found in the 2 northern bays, the Arabian Sea and the Bay of
Bengal. This difference may be due not only to the fact that the Indian Ocean is landlocked
in the north, but more so to the advection of water of moderate oxygen concentration
from the Pacific Ocean through the Indonesian waters.