Physical Oceanography of the Indian Ocean
23
and northern Arabian Sea. This water spreads SW into the area off Somalia during the
NE monsoon and is drawn from there into the Equatorial Countercurrent, where it can be
followed as a distinct tongue of comparatively high salinity to 90 ° E. During the SW
monsoon it first spreads south and then turns east and penetrates with the monsoon
current into the region south of Ceylon. Some portions of this high-salinity water sink in
the Arabian Sea and form a subsurface-salinity maximum in the upper portions of the
thermocline at temperatures between 26 ° and 22 ° C, Fig. 7. This salinity maximum is
spread at the different seasons with the various branches of the surface circulation
throughout the entire monsoon gyre north of 10 ° S. It does not penetrate into the Bay of
Bengal. Two other sources of high-salinity water, the outflow from the Persian Gulf and
from the Red Sea, are affecting the intermediate layers of the Arabian Sea. Persian Gulf
water appears as a strong salinity maximum at 300 m depth only in the Gulf of Oman, and
Red Sea water in the Gulf of Aden at 800 m depth. In the central Arabian Sea the
subsurface high-salinity water masses, the Arabian Sea water, the Persian Gulf water and
the Red Sea water, form a thick layer which is vertically of almost uniform salinity,
although the individual layers may still be recognizable at most stations as weak salinity
maxima. This whole layer may be called the North Indian high-salinity intermediate water
and occupies a depth range from about 150 to 900 m in the Arabian Sea. A comprehensive
study of the vertical and horizontal fluxes of heat and salt which lead to this structure and
of the effect of contraction during mixing on the exchange processes has been made by
BENNE1l (1970). From this high-salinity layer water proceeds at various depths with the
different branches of the circulation throughout most of the monsoon gyre. Near 300 m
depth high-salinity water spreads east with the SW monsoon current and at that depth fills
the region to the west of Sumatra and also the entire Bay of Bengal. A weak salinity
maximum of Red Sea water can be recognized between 600 and 900 m depth in the entire
equatorial region. This Red Sea water is most pronounced in the western part of the
ocean, where it penetrates near 1 100 m through the Madagascar Channel to 25 ° S.
The Hydro-Chemical Front at 10° S
The monsoon gyre is separated from the subtropical gyre of the southern Indian
Ocean by a strong front in the hydrographic and chemical structure. This front is more
pronounced in subsurface layers than at the surface and also more in the structure than in
the circulation. This is because during the NE monsoon the South Equatorial Current does
not belong to the monsoon gyre, while during the SW monsoon, its northern parts form
part of the monsoon gyre. At the surface the boundary zone between the 2 gyres is marked
by a horizontal salinity minimum stretching from Sumatra to Africa. This band of low
salinity is rather wide in a north-south direction and its center is difficult to define.
However, in maps of salinity at all horizons between 100 and 500 m depth, there is a
distinct horizontal salinity minimum stretching along 10° S all the way from Timor to
the north of Madagascar. This minimum separates the high-salinity water masses of the
northern Indian Ocean from the subtropical high-salinity water of the subtropical gyre. It
is caused by advection of low-salinity water by the South Equatorial Current from the
Timor Sea and the waters between Australia and Indonesia.
The front is even more pronounced in the distribution of chemical properties (Fig. 3).
It separates the low-nutrient, high - O 2 content waters of the subtropical gyre from the
23
and northern Arabian Sea. This water spreads SW into the area off Somalia during the
NE monsoon and is drawn from there into the Equatorial Countercurrent, where it can be
followed as a distinct tongue of comparatively high salinity to 90 ° E. During the SW
monsoon it first spreads south and then turns east and penetrates with the monsoon
current into the region south of Ceylon. Some portions of this high-salinity water sink in
the Arabian Sea and form a subsurface-salinity maximum in the upper portions of the
thermocline at temperatures between 26 ° and 22 ° C, Fig. 7. This salinity maximum is
spread at the different seasons with the various branches of the surface circulation
throughout the entire monsoon gyre north of 10 ° S. It does not penetrate into the Bay of
Bengal. Two other sources of high-salinity water, the outflow from the Persian Gulf and
from the Red Sea, are affecting the intermediate layers of the Arabian Sea. Persian Gulf
water appears as a strong salinity maximum at 300 m depth only in the Gulf of Oman, and
Red Sea water in the Gulf of Aden at 800 m depth. In the central Arabian Sea the
subsurface high-salinity water masses, the Arabian Sea water, the Persian Gulf water and
the Red Sea water, form a thick layer which is vertically of almost uniform salinity,
although the individual layers may still be recognizable at most stations as weak salinity
maxima. This whole layer may be called the North Indian high-salinity intermediate water
and occupies a depth range from about 150 to 900 m in the Arabian Sea. A comprehensive
study of the vertical and horizontal fluxes of heat and salt which lead to this structure and
of the effect of contraction during mixing on the exchange processes has been made by
BENNE1l (1970). From this high-salinity layer water proceeds at various depths with the
different branches of the circulation throughout most of the monsoon gyre. Near 300 m
depth high-salinity water spreads east with the SW monsoon current and at that depth fills
the region to the west of Sumatra and also the entire Bay of Bengal. A weak salinity
maximum of Red Sea water can be recognized between 600 and 900 m depth in the entire
equatorial region. This Red Sea water is most pronounced in the western part of the
ocean, where it penetrates near 1 100 m through the Madagascar Channel to 25 ° S.
The Hydro-Chemical Front at 10° S
The monsoon gyre is separated from the subtropical gyre of the southern Indian
Ocean by a strong front in the hydrographic and chemical structure. This front is more
pronounced in subsurface layers than at the surface and also more in the structure than in
the circulation. This is because during the NE monsoon the South Equatorial Current does
not belong to the monsoon gyre, while during the SW monsoon, its northern parts form
part of the monsoon gyre. At the surface the boundary zone between the 2 gyres is marked
by a horizontal salinity minimum stretching from Sumatra to Africa. This band of low
salinity is rather wide in a north-south direction and its center is difficult to define.
However, in maps of salinity at all horizons between 100 and 500 m depth, there is a
distinct horizontal salinity minimum stretching along 10° S all the way from Timor to
the north of Madagascar. This minimum separates the high-salinity water masses of the
northern Indian Ocean from the subtropical high-salinity water of the subtropical gyre. It
is caused by advection of low-salinity water by the South Equatorial Current from the
Timor Sea and the waters between Australia and Indonesia.
The front is even more pronounced in the distribution of chemical properties (Fig. 3).
It separates the low-nutrient, high - O 2 content waters of the subtropical gyre from the