Indian Ocean Trade Wind Biome
281
characterized by abundant, strong, and variable mesoscale eddies of both signs. Thermal
stratification is strongest in April and May because of the northern excursion of the sun
and cloudless skies, though because salinity distribution largely determines mixed-layer
depth, the pycnocline remains rather shallow and abrupt. With the onset of the stronger
winds of the southwest summer monsoon, this density structure is broken down by
mixing, overturn, and cooling that are due to heavy seasonal cloud cover. During the
same period, the anticyclonic gyral circulation breaks down, and the Bay of Bengal then
contains two minor cyclonic eddies, which must include pycnocline uplift at their centers.
One is in the northern part, in July–September, centered at 20
N 90
E. The other appears
in October off Madras at 85
E, moves south, and dissipates off Sri Lanka in December
(Banse, 1990a).
The consequence of these processes is that mixed-layer depths in the north of the Bay
of Bengal (15–17
N) deepen by 50–75 m as the Northeast Monsoon season progresses
(Colborn, 1975) and a thermal ridge develops, lying southwest-northeast from Madras
to Burma, separating the Bay of Bengal circulation from an area of deeper mixing to
the northwest of Sumatra. Minimum mixed-layer depths along this ridge are 30–40 m
and south of the ridge, mixed-layer depths reach 90 m during the Northeast Monsoon.
Beyond the shelf regions, the mixed layer is permanently devoid of nitrate and silicate, the
main nutricline lying within with the principal thermocline at 50–100 m and responding
to shoaling of the density gradient in cold-core eddies (Madhupratap et al., 2003). The
thermocline is also coincident with a strong oxycline, leading down into a deeper water
mass having around 05 ml liter
−1 dissolved O 2 . This extends down to around 500 m.
Regional Response of the Pelagic Ecosystem
The serial chlorophyll images now available, more or less continuously since 1978, leave no
doubt that the phytoplankton responds strongly to the forcing of the reversing monsoon
wind systems. Although there are significant between-year differences, the main features
of this response are readily described in simple terms, as below; for a more detailed
account, based on 8 years of CZCS data, and partitioned within three open-ocean boxes
representing the entire Arabian Sea, see the masterly synthesis by Banse and English
(2000). It is to be noted that in that account, as in what follows, the division of the
early cycle into four periods is not always very clear: it is often stated that the two
intermonsoon periods both represent oligotrophic seasons, although it is clear that they
are far from equally so. Seasonality of chlorophyll biomass is dominated by the period of
clear, blue seawater that succeeds the winter monsoon during the period April–June: this
is the period of maximal solar irradiance at the sea surface, the minimum occurring at
the height of the Southwest Monsoon under very cloudy skies. The study by Kumar and
Narvekar (see earlier discussion) of the central Arabian Sea to physical forcing concluded
that seasonal changes in the productivity of the pelagic ecosystem were a direct response
to seasonal changes in incoming irradiance, wind stress at the sea surface, and the role
of external forcing in the seasonal variation of nutrient supply to the euphotic zone.
They also reported that westward propagation of Rossby waves, independent of the
seasonal reversal of monsoon winds, appears also to be involved in seasonal changes in
mixed-layer depth.
Simple examination of serial chlorophyll images shows that the Arabian Sea almost
invariably has a higher surface chlorophyll biomass than the Bay of Bengal and that
the strongest response of the phytoplankton is aligned approximately below the Findlater Jet—that is to say, rather in the northwest quadrant of the Arabian Sea than
toward its center. During the Southwest Monsoon, the region of higher chlorophyll
(∼10 mg chl m
−3 ) moves progressively toward the southeast across the Arabian Sea to
reach its maximal extent in the intermonsoon period; then, some reduction of biomass
281
characterized by abundant, strong, and variable mesoscale eddies of both signs. Thermal
stratification is strongest in April and May because of the northern excursion of the sun
and cloudless skies, though because salinity distribution largely determines mixed-layer
depth, the pycnocline remains rather shallow and abrupt. With the onset of the stronger
winds of the southwest summer monsoon, this density structure is broken down by
mixing, overturn, and cooling that are due to heavy seasonal cloud cover. During the
same period, the anticyclonic gyral circulation breaks down, and the Bay of Bengal then
contains two minor cyclonic eddies, which must include pycnocline uplift at their centers.
One is in the northern part, in July–September, centered at 20
N 90
E. The other appears
in October off Madras at 85
E, moves south, and dissipates off Sri Lanka in December
(Banse, 1990a).
The consequence of these processes is that mixed-layer depths in the north of the Bay
of Bengal (15–17
N) deepen by 50–75 m as the Northeast Monsoon season progresses
(Colborn, 1975) and a thermal ridge develops, lying southwest-northeast from Madras
to Burma, separating the Bay of Bengal circulation from an area of deeper mixing to
the northwest of Sumatra. Minimum mixed-layer depths along this ridge are 30–40 m
and south of the ridge, mixed-layer depths reach 90 m during the Northeast Monsoon.
Beyond the shelf regions, the mixed layer is permanently devoid of nitrate and silicate, the
main nutricline lying within with the principal thermocline at 50–100 m and responding
to shoaling of the density gradient in cold-core eddies (Madhupratap et al., 2003). The
thermocline is also coincident with a strong oxycline, leading down into a deeper water
mass having around 05 ml liter
−1 dissolved O 2 . This extends down to around 500 m.
Regional Response of the Pelagic Ecosystem
The serial chlorophyll images now available, more or less continuously since 1978, leave no
doubt that the phytoplankton responds strongly to the forcing of the reversing monsoon
wind systems. Although there are significant between-year differences, the main features
of this response are readily described in simple terms, as below; for a more detailed
account, based on 8 years of CZCS data, and partitioned within three open-ocean boxes
representing the entire Arabian Sea, see the masterly synthesis by Banse and English
(2000). It is to be noted that in that account, as in what follows, the division of the
early cycle into four periods is not always very clear: it is often stated that the two
intermonsoon periods both represent oligotrophic seasons, although it is clear that they
are far from equally so. Seasonality of chlorophyll biomass is dominated by the period of
clear, blue seawater that succeeds the winter monsoon during the period April–June: this
is the period of maximal solar irradiance at the sea surface, the minimum occurring at
the height of the Southwest Monsoon under very cloudy skies. The study by Kumar and
Narvekar (see earlier discussion) of the central Arabian Sea to physical forcing concluded
that seasonal changes in the productivity of the pelagic ecosystem were a direct response
to seasonal changes in incoming irradiance, wind stress at the sea surface, and the role
of external forcing in the seasonal variation of nutrient supply to the euphotic zone.
They also reported that westward propagation of Rossby waves, independent of the
seasonal reversal of monsoon winds, appears also to be involved in seasonal changes in
mixed-layer depth.
Simple examination of serial chlorophyll images shows that the Arabian Sea almost
invariably has a higher surface chlorophyll biomass than the Bay of Bengal and that
the strongest response of the phytoplankton is aligned approximately below the Findlater Jet—that is to say, rather in the northwest quadrant of the Arabian Sea than
toward its center. During the Southwest Monsoon, the region of higher chlorophyll
(∼10 mg chl m
−3 ) moves progressively toward the southeast across the Arabian Sea to
reach its maximal extent in the intermonsoon period; then, some reduction of biomass