36
K. WYRTKI: Physical Oceanography of the Indian Ocean
Above the intermediate water the main oceanic thermocline is found, in which the
temperature increases from about 8° C to 15° C. The water of this layer is formed in the
transition region between the polar front and the subtropical convergence, as discussed
earlier, and is identical with the central water, as defined by SVERDRUP, JOHNSON and
FLEMING (1942). The most pronounced feature in this main thermocline is the oxygen
maximum, which clearly points to the origin of this entire water mass at the sea surface.
The oxygen maximum is found in the entire subtropical gyre at depths between 400 and
500 m and at temperatures between 10° and 12° C, and the O2 content decreases only
very slightly to the north. Between 15° Sand 10° S, where the strong hydrochemical
front of the Indian Ocean is situated, the O2 suddenly decreases from 5 ml 1-1 to less
than 3 mil-I, but the oxygen maximum can be followed much farther to about 8° N.
In the western part of the ocean, the oxygen distribution deviates from this general
pattern, and water of relatively high oxygen content spreads north, especially under the
influence of the Somali Current. The oxygen maximum layer is situated between the
shallow oxygen minimum near 200 m depth and the deep oxygen minimum (Fig. 7).
North of about 8° N these 2 layers melt into one huge layer of extremely low oxygen
content, and an organized oxygen maximum is no longer present, although at some
station a weak maximum can still be found.
Comparison with the Other Oceans
After this gross discussion of the physical oceanography of the Indian Ocean it seems
worthwhile to compare it with the other 2 oceans and to emphasize those features, which
are unique to the Indian Ocean. Morphologically, the Indian Ocean is landlocked in the
north, and does not extend into the cold climatic regions of the northern hemisphere.
This causes an asymmetrical development of its structure and circulation, which is most
obvious in the development of the huge layers of extremely low oxygen content in the
Arabian Sea and the Bay of Bengal. The land mass of Asia also affects the ocean climatologically by causing the seasonally changing monsoons, which in turn reverse the
oceanic -circulation over its northern part. Connected with this seasonally changing
circulation are various upwelling areas, which operate only during one season, in contrast
to all the other major upwelling areas in the world. Another outgrowth of the geographical
and climatological situation is the formation of high salinity waters in the Arabian Sea,
and even more extreme in the Red Sea and the Persian Gulf. These water masses form a
huge high-salinity layer in the Arabian Sea and affect the circulation in intermediate
depth rather drastically by preventing water of the southern hemisphere from penetrating
efficiently into the northern Indian Ocean.
K. WYRTKI: Physical Oceanography of the Indian Ocean
Above the intermediate water the main oceanic thermocline is found, in which the
temperature increases from about 8° C to 15° C. The water of this layer is formed in the
transition region between the polar front and the subtropical convergence, as discussed
earlier, and is identical with the central water, as defined by SVERDRUP, JOHNSON and
FLEMING (1942). The most pronounced feature in this main thermocline is the oxygen
maximum, which clearly points to the origin of this entire water mass at the sea surface.
The oxygen maximum is found in the entire subtropical gyre at depths between 400 and
500 m and at temperatures between 10° and 12° C, and the O2 content decreases only
very slightly to the north. Between 15° Sand 10° S, where the strong hydrochemical
front of the Indian Ocean is situated, the O2 suddenly decreases from 5 ml 1-1 to less
than 3 mil-I, but the oxygen maximum can be followed much farther to about 8° N.
In the western part of the ocean, the oxygen distribution deviates from this general
pattern, and water of relatively high oxygen content spreads north, especially under the
influence of the Somali Current. The oxygen maximum layer is situated between the
shallow oxygen minimum near 200 m depth and the deep oxygen minimum (Fig. 7).
North of about 8° N these 2 layers melt into one huge layer of extremely low oxygen
content, and an organized oxygen maximum is no longer present, although at some
station a weak maximum can still be found.
Comparison with the Other Oceans
After this gross discussion of the physical oceanography of the Indian Ocean it seems
worthwhile to compare it with the other 2 oceans and to emphasize those features, which
are unique to the Indian Ocean. Morphologically, the Indian Ocean is landlocked in the
north, and does not extend into the cold climatic regions of the northern hemisphere.
This causes an asymmetrical development of its structure and circulation, which is most
obvious in the development of the huge layers of extremely low oxygen content in the
Arabian Sea and the Bay of Bengal. The land mass of Asia also affects the ocean climatologically by causing the seasonally changing monsoons, which in turn reverse the
oceanic -circulation over its northern part. Connected with this seasonally changing
circulation are various upwelling areas, which operate only during one season, in contrast
to all the other major upwelling areas in the world. Another outgrowth of the geographical
and climatological situation is the formation of high salinity waters in the Arabian Sea,
and even more extreme in the Red Sea and the Persian Gulf. These water masses form a
huge high-salinity layer in the Arabian Sea and affect the circulation in intermediate
depth rather drastically by preventing water of the southern hemisphere from penetrating
efficiently into the northern Indian Ocean.