276
Chapter 10: The Indian Ocean
monsoon winds, and a highly instrumented mooring at 15
S was maintained to monitor
this process. Shipborne studies of all components of the pelagic ecosystem—including
nutrient dynamics, productivity, the organization of the microbial food web, and the
dynamics of herbivore consumption—were given prominence in the scientific program
aboard the participating ships.
The results of many of these investigations were published during the period 1998–2002
in a series of issues of Deep-Sea Research Part II, and in 2005 in a special edition of Progress
in Oceanography; these present an excellent description of the dynamic response of the
pelagic ecosystem of this region to the seasonal uplifting and advection of nutrient-rich
water. The changing community composition of producers and consumers is now as well
understood here, far from the great oceanographic institutions, as anywhere else in the
oceans. Obviously, the organization and execution of the JGOFS Arabian Sea Expedition
was a far cry from that of the IIOE, and this shows in the results obtained. However,
nothing is perfect and an ideal synthesis remains difficult to achieve, principally because
so many key studies and observations were not performed in each critical environment,
in each season. Unfortunately for the possibility of taking another look at these problems,
the world has changed since 1994–1996: after the U.S.S. Cole incident, the Mogadishu
affair, and the invasion of Iraq, it is unlikely that further research can be undertaken in
these waters aboard Western ships—or, at least, not for a very long time.
The anomalous circulation and ecology of the Indian Ocean are principally forced
by two factors: its basin form, and the consequent wind regime. The Indian Ocean
extends only marginally into the northern hemisphere, adjacent to the high terrain of the
Asiatic landmass, and this is the key to the singularity of this ocean because its northern
region is dominated by the most striking example of a seasonally reversing wind regime
that occurs anywhere. The reversing wind regime here has given the Arabic name of
monsoon to such wind systems elsewhere—as in western Africa and southern Argentina.
The monsoons carried the dhows from Arabia to Zanzibar and then home again “laden
with sandalwood, apes and ivory” and human merchandise that we would rather forget.
Monsoon winds peak in February and July; the winter high-pressure belt over central
Asia feeds the cool, dry Northeast Monsoon that sweeps down across the Arabian Sea and
forces seasonal circulation patterns across the equator and into the northern part of the
southern subtropical gyre. At this season, the atmospheric intertropical convergence zone
(ITCZ) is located well to the south of the equator, a fact having important consequences
for the development of the equatorial current system. The Southwest Monsoon is induced
by the low atmospheric pressure that forms over Arabia and Pakistan during the heat
of summer and forms a strong jetlike stream that is constrained to the west by the
highlands of eastern Africa. Although the Southwest Monsoon has important effects along
the coastline, wind speed is maximal in the Findlater Jet that passes SW-NE across the
northern part of the Arabian Sea, generally aligned from Somalia to Bombay. Because
the meridional pressure gradient is significantly greater across the relevant latitudes in
July than it is in February, the Southwest Monsoon winds are stronger and steadier than
those of the Northeast Monsoon. The effects on the pelagic ecosystem induced by the
Southwest Monsoon were a principal interest of the 1994–96 JGOFS ecologists.
Any partition of the Indian Ocean must accommodate two seasonal circulation states,
because the seasonal monsoon wind reversal rapidly induces reversal in both shallow
and deep currents because (as noted in Chapter 3) wind stress in low latitudes induces
momentum rather than mixing. Indeed, the reversal of the Somali Current and the rapid
spin-up of a deep boundary jet in response to the onset of the Southwest Monsoon is
the classical model for this phenomenon in low-latitude oceans.
The Northeast Monsoon forces weak westward flow across the whole ocean between
India and the equator so that some southerly flow must occur at the coast of Africa,
while the Arabian Sea and Bay of Bengal are occupied by cyclonic flow. The strong,
Chapter 10: The Indian Ocean
monsoon winds, and a highly instrumented mooring at 15
S was maintained to monitor
this process. Shipborne studies of all components of the pelagic ecosystem—including
nutrient dynamics, productivity, the organization of the microbial food web, and the
dynamics of herbivore consumption—were given prominence in the scientific program
aboard the participating ships.
The results of many of these investigations were published during the period 1998–2002
in a series of issues of Deep-Sea Research Part II, and in 2005 in a special edition of Progress
in Oceanography; these present an excellent description of the dynamic response of the
pelagic ecosystem of this region to the seasonal uplifting and advection of nutrient-rich
water. The changing community composition of producers and consumers is now as well
understood here, far from the great oceanographic institutions, as anywhere else in the
oceans. Obviously, the organization and execution of the JGOFS Arabian Sea Expedition
was a far cry from that of the IIOE, and this shows in the results obtained. However,
nothing is perfect and an ideal synthesis remains difficult to achieve, principally because
so many key studies and observations were not performed in each critical environment,
in each season. Unfortunately for the possibility of taking another look at these problems,
the world has changed since 1994–1996: after the U.S.S. Cole incident, the Mogadishu
affair, and the invasion of Iraq, it is unlikely that further research can be undertaken in
these waters aboard Western ships—or, at least, not for a very long time.
The anomalous circulation and ecology of the Indian Ocean are principally forced
by two factors: its basin form, and the consequent wind regime. The Indian Ocean
extends only marginally into the northern hemisphere, adjacent to the high terrain of the
Asiatic landmass, and this is the key to the singularity of this ocean because its northern
region is dominated by the most striking example of a seasonally reversing wind regime
that occurs anywhere. The reversing wind regime here has given the Arabic name of
monsoon to such wind systems elsewhere—as in western Africa and southern Argentina.
The monsoons carried the dhows from Arabia to Zanzibar and then home again “laden
with sandalwood, apes and ivory” and human merchandise that we would rather forget.
Monsoon winds peak in February and July; the winter high-pressure belt over central
Asia feeds the cool, dry Northeast Monsoon that sweeps down across the Arabian Sea and
forces seasonal circulation patterns across the equator and into the northern part of the
southern subtropical gyre. At this season, the atmospheric intertropical convergence zone
(ITCZ) is located well to the south of the equator, a fact having important consequences
for the development of the equatorial current system. The Southwest Monsoon is induced
by the low atmospheric pressure that forms over Arabia and Pakistan during the heat
of summer and forms a strong jetlike stream that is constrained to the west by the
highlands of eastern Africa. Although the Southwest Monsoon has important effects along
the coastline, wind speed is maximal in the Findlater Jet that passes SW-NE across the
northern part of the Arabian Sea, generally aligned from Somalia to Bombay. Because
the meridional pressure gradient is significantly greater across the relevant latitudes in
July than it is in February, the Southwest Monsoon winds are stronger and steadier than
those of the Northeast Monsoon. The effects on the pelagic ecosystem induced by the
Southwest Monsoon were a principal interest of the 1994–96 JGOFS ecologists.
Any partition of the Indian Ocean must accommodate two seasonal circulation states,
because the seasonal monsoon wind reversal rapidly induces reversal in both shallow
and deep currents because (as noted in Chapter 3) wind stress in low latitudes induces
momentum rather than mixing. Indeed, the reversal of the Somali Current and the rapid
spin-up of a deep boundary jet in response to the onset of the Southwest Monsoon is
the classical model for this phenomenon in low-latitude oceans.
The Northeast Monsoon forces weak westward flow across the whole ocean between
India and the equator so that some southerly flow must occur at the coast of Africa,
while the Arabian Sea and Bay of Bengal are occupied by cyclonic flow. The strong,