Chapter 10
The Indian Ocean
T
he Indian Ocean, north of the Subtropical Convergence zone, is the smallest
ocean basin (about 50 × 10
6 km
2 ) and has some special characteristics that must
be accommodated when it is partitioned into biogeochemical provinces. It has
become much better known in recent years, although the new research has been concentrated mostly in the Arabian Sea, adjacent to Madagascar and in the Agulhas Current.
The subtropical gyre of the southern hemisphere—like that of the other oceans—has
been relatively neglected. This uneven coverage is especially unfortunate because of the
unusual complexity of the circulation of this ocean that is forced by the characteristic
seasonal reversal of the monsoon wind systems.
Our organized knowledge of the Indian Ocean begins with results of the classical John
Murray Expedition (1933–34) and of the International Indian Ocean Expedition (IIOE)
of 1959–1965 that was an early version of the international multiship investigations that
have since come to dominate the progress of oceanography. Unfortunately, though much
biology was done during the IIOE, very little ecology emerged and, because the wide grid
of stations was sampled quite irregularly, no uniform field of values for any property
was obtained. In fact, this was expressly not the intention of the organizers; rather,
their intention was to permit complete freedom of action to the scientists aboard the
11 participating ships. A centrally planned grid of stations and agreed procedures to wring
answers from critical problems would have been “ideal but unrealistic,” according to the
principal coordinator. Nevertheless, the Indian Ocean Oceanographic Atlas of Wyrtki
(1971) remains the best source of maps of the physical oceanography of this ocean. The
results of the IIOE were discussed at a SCOR-IBP Symposium in 1971; the resultant
volume (Zeitzschel, 1973) is an important document, both historically and as a source
of information on subjects that have not been studied subsequently over the entire basin
of this ocean. A glance will demonstrate the extent to which the southern subtropical
gyre was relatively neglected, and the great difficulty of deriving any comprehensive
information for that region.
Fortunately, we have come a long way since the days of the IIOE, because of the intense
and disciplined exploration of the ecological consequences of monsoon reversal during
the Arabian Sea component of Joint Global Ocean Flux Study (JGOFS, 1994–96), and the
Netherlands Indian Ocean program (1992–93). The Arabian Sea JGOFS was one of the
most successful investigations in recent decades (see Smith et al., 1998a); five ships from
Europe and the United States, one from India, and several from Pakistan undertook about
20 voyages along standard meridional transects across the northern Arabian Sea that
were arranged to cover the critical periods of the monsoon cycle. Stations were specially
chosen to investigate the seasonal upwellings that are a consequence of the Southwest
Monsoon; of particular interest was the spatial extent of the biological consequences
of this process and the distribution at the surface of oxygen-depleted, upwelled water.
Special emphasis was given to the response of biological carbon flux to the reversing
275
The Indian Ocean
T
he Indian Ocean, north of the Subtropical Convergence zone, is the smallest
ocean basin (about 50 × 10
6 km
2 ) and has some special characteristics that must
be accommodated when it is partitioned into biogeochemical provinces. It has
become much better known in recent years, although the new research has been concentrated mostly in the Arabian Sea, adjacent to Madagascar and in the Agulhas Current.
The subtropical gyre of the southern hemisphere—like that of the other oceans—has
been relatively neglected. This uneven coverage is especially unfortunate because of the
unusual complexity of the circulation of this ocean that is forced by the characteristic
seasonal reversal of the monsoon wind systems.
Our organized knowledge of the Indian Ocean begins with results of the classical John
Murray Expedition (1933–34) and of the International Indian Ocean Expedition (IIOE)
of 1959–1965 that was an early version of the international multiship investigations that
have since come to dominate the progress of oceanography. Unfortunately, though much
biology was done during the IIOE, very little ecology emerged and, because the wide grid
of stations was sampled quite irregularly, no uniform field of values for any property
was obtained. In fact, this was expressly not the intention of the organizers; rather,
their intention was to permit complete freedom of action to the scientists aboard the
11 participating ships. A centrally planned grid of stations and agreed procedures to wring
answers from critical problems would have been “ideal but unrealistic,” according to the
principal coordinator. Nevertheless, the Indian Ocean Oceanographic Atlas of Wyrtki
(1971) remains the best source of maps of the physical oceanography of this ocean. The
results of the IIOE were discussed at a SCOR-IBP Symposium in 1971; the resultant
volume (Zeitzschel, 1973) is an important document, both historically and as a source
of information on subjects that have not been studied subsequently over the entire basin
of this ocean. A glance will demonstrate the extent to which the southern subtropical
gyre was relatively neglected, and the great difficulty of deriving any comprehensive
information for that region.
Fortunately, we have come a long way since the days of the IIOE, because of the intense
and disciplined exploration of the ecological consequences of monsoon reversal during
the Arabian Sea component of Joint Global Ocean Flux Study (JGOFS, 1994–96), and the
Netherlands Indian Ocean program (1992–93). The Arabian Sea JGOFS was one of the
most successful investigations in recent decades (see Smith et al., 1998a); five ships from
Europe and the United States, one from India, and several from Pakistan undertook about
20 voyages along standard meridional transects across the northern Arabian Sea that
were arranged to cover the critical periods of the monsoon cycle. Stations were specially
chosen to investigate the seasonal upwellings that are a consequence of the Southwest
Monsoon; of particular interest was the spatial extent of the biological consequences
of this process and the distribution at the surface of oxygen-depleted, upwelled water.
Special emphasis was given to the response of biological carbon flux to the reversing
275