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Chapter 10: The Indian Ocean
this as a separate province (Western India Coastal Province, INDW). From Somalia to
the Gulf of Oman, the Southwest Monsoon forces a wide zone of coastal divergence;
this feature is continuous with an offshore region of Ekman suction forced by the same
wind system. Both are also areas of strong biological enhancement. It is unreasonable to
separate these two related processes, so it is best to group part of the deep northwest
Arabian Sea together with the Somalia-Arabian coastal boundary in a single province
(Northwest Arabian Sea Upwelling Province, ARAB).
The two adjacent evaporative basins, the Red Sea and the Arabian Gulf, are taken
together simply as a matter of convenience and because together they make an interesting
and instructive comparison (as the Red Sea, Arabian Gulf Province, REDS).
Indian Ocean Trade Wind Biome
Indian Monsoon Gyres Province (MONS)
Extent of the Province
The MONS province extends from the hydrochemical front at 10
S (see later discussion)
north to the offshore limits of the coastal provinces ARAB, INDE, and INDW. The
province also includes the central Bay of Bengal and the southern part of the Arabian
Sea. Thus, it is similar to Colborn areas 2 and 12–21.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
Though continental shelf topography is not relevant to this province, the shallow water of
the Maldive Islands aligned along 73
E, from the equator north to the Indian continental
shelf, has consequences for both circulation and biological response.
Defining Characteristics of Regional Oceanography
This province is synonymous with the reversing circulations of the two monsoon gyres
of the northern Indian Ocean, with relatively low salinity and high surface temperatures, as befits an extension westward of the warm-water pool of the Pacific Ocean (see
Chapter 11, WARM).
The monsoon gyral circulation occupies the whole northern Indian Ocean south to
the hydrochemical front of Wyrtki (1973a) that lies above a zonal thermocline ridge at
10–15
S separating high-oxygen, high-salinity regions to the south from low-oxygen, lowsalinity regions to the north. This is a convergent front and lies on the equatorward flank
of the perpetual westward flow of the South Equatorial Current (SEC). In the eastern
part of the province it is less well marked and the tropical-subtropical distinction (based
on subsurface nitrate) occurs at 20–22
S along the 110
E IIOE section of Tranter (1973).
When the Northeast Monsoon becomes established in boreal winter, westward flow
across the ocean as the Northeast Monsoon Drift or the North Equatorial Current (NEC)
is established between about 2
N and the Indian subcontinent. This flow originates in the
Bay of Bengal and in water that has been transported westward through the Indonesian
archipelago; accordingly, surface salinities are low in the eastern part of the province
during this season. Because of the different characteristic latitude here of the intertropical
convergence zone than in Pacific and Atlantic Oceans, divergence of the surface water
mass at the equator is very weak, or does not occur. Surface chlorophyll is a very good
tracer of this process and a glance at the SeaWiFS serial chlorophyll images will convince
you that the linear equatorial feature so prominent elsewhere is absent in the Indian
Chapter 10: The Indian Ocean
this as a separate province (Western India Coastal Province, INDW). From Somalia to
the Gulf of Oman, the Southwest Monsoon forces a wide zone of coastal divergence;
this feature is continuous with an offshore region of Ekman suction forced by the same
wind system. Both are also areas of strong biological enhancement. It is unreasonable to
separate these two related processes, so it is best to group part of the deep northwest
Arabian Sea together with the Somalia-Arabian coastal boundary in a single province
(Northwest Arabian Sea Upwelling Province, ARAB).
The two adjacent evaporative basins, the Red Sea and the Arabian Gulf, are taken
together simply as a matter of convenience and because together they make an interesting
and instructive comparison (as the Red Sea, Arabian Gulf Province, REDS).
Indian Ocean Trade Wind Biome
Indian Monsoon Gyres Province (MONS)
Extent of the Province
The MONS province extends from the hydrochemical front at 10
S (see later discussion)
north to the offshore limits of the coastal provinces ARAB, INDE, and INDW. The
province also includes the central Bay of Bengal and the southern part of the Arabian
Sea. Thus, it is similar to Colborn areas 2 and 12–21.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
Though continental shelf topography is not relevant to this province, the shallow water of
the Maldive Islands aligned along 73
E, from the equator north to the Indian continental
shelf, has consequences for both circulation and biological response.
Defining Characteristics of Regional Oceanography
This province is synonymous with the reversing circulations of the two monsoon gyres
of the northern Indian Ocean, with relatively low salinity and high surface temperatures, as befits an extension westward of the warm-water pool of the Pacific Ocean (see
Chapter 11, WARM).
The monsoon gyral circulation occupies the whole northern Indian Ocean south to
the hydrochemical front of Wyrtki (1973a) that lies above a zonal thermocline ridge at
10–15
S separating high-oxygen, high-salinity regions to the south from low-oxygen, lowsalinity regions to the north. This is a convergent front and lies on the equatorward flank
of the perpetual westward flow of the South Equatorial Current (SEC). In the eastern
part of the province it is less well marked and the tropical-subtropical distinction (based
on subsurface nitrate) occurs at 20–22
S along the 110
E IIOE section of Tranter (1973).
When the Northeast Monsoon becomes established in boreal winter, westward flow
across the ocean as the Northeast Monsoon Drift or the North Equatorial Current (NEC)
is established between about 2
N and the Indian subcontinent. This flow originates in the
Bay of Bengal and in water that has been transported westward through the Indonesian
archipelago; accordingly, surface salinities are low in the eastern part of the province
during this season. Because of the different characteristic latitude here of the intertropical
convergence zone than in Pacific and Atlantic Oceans, divergence of the surface water
mass at the equator is very weak, or does not occur. Surface chlorophyll is a very good
tracer of this process and a glance at the SeaWiFS serial chlorophyll images will convince
you that the linear equatorial feature so prominent elsewhere is absent in the Indian