1.3
Physical Oceanography of the Indian Ocean
K. WYRTKI
During the preparation of the Atlas on the physical oceanography of the Indian Ocean
(WYRTKI, 1971) a comprehensive analysis of its oceanographic conditions had to be
undertaken to construct meaningful maps, sections and diagrams, and to interpret the
structure and circulation of this ocean, which is in so many ways different from the
other oceans. On a large scale 3 distinct circulation systems can be delineated.
These are:
I. the seasonally changing monsoon gyre
II. the south hemispheric subtropical anticyclonic gyre
III. the Antarctic waters with the Circumpolar Current
While the last 2 systems are essentially similar to the corresponding systems in the
other oceans, a reversing monsoon gyre is not found anywhere else in the oceans. This
gyre is separated from the southern subtropical gyre by a very pronounced front in the
hydro-chemical structure at about 10° S, a unique feature of the Indian Ocean. But also
the nutrient-poor southern subtropical gyre differs in many respects from those in other
oceans. It lacks a well developed eastern boundary current along Australia. The Agulhas
Current, its western boundary current, is the strongest in the southern hemisphere.
The division into these 3 circulation systems is not too obvious from an inspection
of maps of the surface circulation alone, Fig. 1. However, when structural features of the
ocean are included in the analysis, this division becomes quite apparent. In the northern
portions of the South Equatorial Current a tongue of low salinity water originating off
Sumatra stretches west near 10° S extending all the way to Africa. This boundary is even
more marked by a subsurface front in the chemical characteristics, which separates the
waters of low oxygen content and high nutrients in the northern Indian Ocean from those
of high oxygen content and low nutrient content in the subtropical gyre. The monsoon
gyre consists during the NE monsoon of the North Equatorial Current, a southward flow
off the coast of Somalia, and the countercurrent, which runs east between the Equator
and about 8 ° S, across the entire width of the ocean. During the SW monsoon, the gyre
consists of the northern portions of the South Equatorial Current, which now extends
almost to the Equator, the strong Somali Current flowing north as a western boundary
current and the monsoon current, into which the countercurrent has merged.
The anticyclonic subtropical gyre consists of the South Equatorial Current, the Agulhas
Current system and the eastward flow situated to the north of the subtropical convergence
and driven by the prevailing westerly winds south of the atmospheric high pressure ridge.
The circulation is closed by slow equatorward movements over most of the subtropical
gyre, but a pronounced eastern boundary current is lacking. To the south of the sub-
Physical Oceanography of the Indian Ocean
K. WYRTKI
During the preparation of the Atlas on the physical oceanography of the Indian Ocean
(WYRTKI, 1971) a comprehensive analysis of its oceanographic conditions had to be
undertaken to construct meaningful maps, sections and diagrams, and to interpret the
structure and circulation of this ocean, which is in so many ways different from the
other oceans. On a large scale 3 distinct circulation systems can be delineated.
These are:
I. the seasonally changing monsoon gyre
II. the south hemispheric subtropical anticyclonic gyre
III. the Antarctic waters with the Circumpolar Current
While the last 2 systems are essentially similar to the corresponding systems in the
other oceans, a reversing monsoon gyre is not found anywhere else in the oceans. This
gyre is separated from the southern subtropical gyre by a very pronounced front in the
hydro-chemical structure at about 10° S, a unique feature of the Indian Ocean. But also
the nutrient-poor southern subtropical gyre differs in many respects from those in other
oceans. It lacks a well developed eastern boundary current along Australia. The Agulhas
Current, its western boundary current, is the strongest in the southern hemisphere.
The division into these 3 circulation systems is not too obvious from an inspection
of maps of the surface circulation alone, Fig. 1. However, when structural features of the
ocean are included in the analysis, this division becomes quite apparent. In the northern
portions of the South Equatorial Current a tongue of low salinity water originating off
Sumatra stretches west near 10° S extending all the way to Africa. This boundary is even
more marked by a subsurface front in the chemical characteristics, which separates the
waters of low oxygen content and high nutrients in the northern Indian Ocean from those
of high oxygen content and low nutrient content in the subtropical gyre. The monsoon
gyre consists during the NE monsoon of the North Equatorial Current, a southward flow
off the coast of Somalia, and the countercurrent, which runs east between the Equator
and about 8 ° S, across the entire width of the ocean. During the SW monsoon, the gyre
consists of the northern portions of the South Equatorial Current, which now extends
almost to the Equator, the strong Somali Current flowing north as a western boundary
current and the monsoon current, into which the countercurrent has merged.
The anticyclonic subtropical gyre consists of the South Equatorial Current, the Agulhas
Current system and the eastward flow situated to the north of the subtropical convergence
and driven by the prevailing westerly winds south of the atmospheric high pressure ridge.
The circulation is closed by slow equatorward movements over most of the subtropical
gyre, but a pronounced eastern boundary current is lacking. To the south of the sub-