310
Chapter 10: The Indian Ocean
Defining Characteristics of Regional Oceanography
Winter cooling occurs in the northern part of the Bay of Bengal, and because of its
semienclosed nature, coastal regions are strongly influenced both by the seasonal circulation of the whole bight (see MONS), and by regional freshwater inputs. The Brahmaputra
and Ganges release freshwater at a rate of about 10
12 m
3 y
−1 , making this the fourth
largest freshwater input, after the Amazon, Congo, and Orinoco. Even so, this is only a
small fraction of the total freshwater inputs, that include heavy rainfall at the sea surface.
The surface salinity of the entire northern Bay of Bengal strongly reflects the input of
freshwater.
This entire region experiences wind-forcing seasonality similar to that of the ARAB
and INDE provinces; perhaps the best modern guide to the coastal circulation of this
region is that of Shetye and Guveia (1998), on which I have relied heavily in what follows.
Coastal flows follow the gyral circulation of the entire Bay of Bengal, although the
mechanisms involved remain obscure. The best-observed flow is that of the seasonally
reversing East India Coastal Current (EICC) that lies above the narrow shelf regions from
Sri Lanka to Bangladesh. Forced by the Northeast Monsoon, a western boundary current
is initiated on the northern part of the coast in October so that southward flow occurs
along the east coast of India, observable as a meandering warm-water body in satellite
imagery; warm-core eddies are detached at the edge of the current, resembling those of
the Gulf Stream (Legeckis, 1987). As the Northeast Monsoon begins to fail in December,
the current remains strongest on the southern part of the coastline, and at this season
downwelling occurs the length of the coast and an inshore plume of low-salinity water
hugs the coastline, originating on the northern shelf.
In February, during the winter intermonsoon period, the EICC starts to flow northward
and by March–April, the period of strongest flow, the EICC is established the entire
length of the coastline of Sri Lanka and eastern India. This flow decays by the end of June
and is associated with an anticyclonic subtropical gyre that occupies the Bay of Bengal
and is unique in its seasonality. Along almost the coast south of 20
N, the contours for
density, temperature, and salinity slope upward toward the coast, and in a 40-km-wide
band the isolines appropriate to 70 m offshore break the surface. Thus, the southern
regime is upwelling, while the northern is downwelling. Modeling studies suggest that
upwelling-favorable winds during the Southwest Monsoon may force Ekman divergence
and upwelling south of Visakhatapam (175
N), but farther north, freshwater from the
Ganges mouths (especially of the Hoogli) is sufficiently strong as to inhibit upwelling
(Johns et al., 1992). However, observations suggest that deep water may be brought to
the surface south of Visakhatapam not only by upwelling but also perhaps by vertical
mixing in March and April and again in June–August off Waltair (Banse, 1990b; Shetye,
1993). Whatever the mechanism, the inshore flank of this warm current is marked by
cooler, high-salinity water brought to the surface by the eddying dynamics of the flow.
Eddies of scale 150-km diameter are observed in satellite SST images to become detached
from the flow of this western boundary current.
During this same season, on the eastern side of the Bay of Bengal the water column may
be rendered isothermal over the continental shelf by tidal mixing and thermal convection
(Sesama, 1989). Unfortunately, coastal dynamics have been much less well observed in
the eastern part of the province, and about all that can be said is that perpetual northward
flow occurs through the Straits of Macassar from the Indo-Pacific regions and that a
small reversing gyre occupies the shelf to the west of Burma.
Regional Response of the Pelagic Ecosystem
I have located only one recent regional-scale study using current techniques (JGOFS
protocols) of the ecology of the Bay of Bengal shelves; this is a single coastwise line
Chapter 10: The Indian Ocean
Defining Characteristics of Regional Oceanography
Winter cooling occurs in the northern part of the Bay of Bengal, and because of its
semienclosed nature, coastal regions are strongly influenced both by the seasonal circulation of the whole bight (see MONS), and by regional freshwater inputs. The Brahmaputra
and Ganges release freshwater at a rate of about 10
12 m
3 y
−1 , making this the fourth
largest freshwater input, after the Amazon, Congo, and Orinoco. Even so, this is only a
small fraction of the total freshwater inputs, that include heavy rainfall at the sea surface.
The surface salinity of the entire northern Bay of Bengal strongly reflects the input of
freshwater.
This entire region experiences wind-forcing seasonality similar to that of the ARAB
and INDE provinces; perhaps the best modern guide to the coastal circulation of this
region is that of Shetye and Guveia (1998), on which I have relied heavily in what follows.
Coastal flows follow the gyral circulation of the entire Bay of Bengal, although the
mechanisms involved remain obscure. The best-observed flow is that of the seasonally
reversing East India Coastal Current (EICC) that lies above the narrow shelf regions from
Sri Lanka to Bangladesh. Forced by the Northeast Monsoon, a western boundary current
is initiated on the northern part of the coast in October so that southward flow occurs
along the east coast of India, observable as a meandering warm-water body in satellite
imagery; warm-core eddies are detached at the edge of the current, resembling those of
the Gulf Stream (Legeckis, 1987). As the Northeast Monsoon begins to fail in December,
the current remains strongest on the southern part of the coastline, and at this season
downwelling occurs the length of the coast and an inshore plume of low-salinity water
hugs the coastline, originating on the northern shelf.
In February, during the winter intermonsoon period, the EICC starts to flow northward
and by March–April, the period of strongest flow, the EICC is established the entire
length of the coastline of Sri Lanka and eastern India. This flow decays by the end of June
and is associated with an anticyclonic subtropical gyre that occupies the Bay of Bengal
and is unique in its seasonality. Along almost the coast south of 20
N, the contours for
density, temperature, and salinity slope upward toward the coast, and in a 40-km-wide
band the isolines appropriate to 70 m offshore break the surface. Thus, the southern
regime is upwelling, while the northern is downwelling. Modeling studies suggest that
upwelling-favorable winds during the Southwest Monsoon may force Ekman divergence
and upwelling south of Visakhatapam (175
N), but farther north, freshwater from the
Ganges mouths (especially of the Hoogli) is sufficiently strong as to inhibit upwelling
(Johns et al., 1992). However, observations suggest that deep water may be brought to
the surface south of Visakhatapam not only by upwelling but also perhaps by vertical
mixing in March and April and again in June–August off Waltair (Banse, 1990b; Shetye,
1993). Whatever the mechanism, the inshore flank of this warm current is marked by
cooler, high-salinity water brought to the surface by the eddying dynamics of the flow.
Eddies of scale 150-km diameter are observed in satellite SST images to become detached
from the flow of this western boundary current.
During this same season, on the eastern side of the Bay of Bengal the water column may
be rendered isothermal over the continental shelf by tidal mixing and thermal convection
(Sesama, 1989). Unfortunately, coastal dynamics have been much less well observed in
the eastern part of the province, and about all that can be said is that perpetual northward
flow occurs through the Straits of Macassar from the Indo-Pacific regions and that a
small reversing gyre occupies the shelf to the west of Burma.
Regional Response of the Pelagic Ecosystem
I have located only one recent regional-scale study using current techniques (JGOFS
protocols) of the ecology of the Bay of Bengal shelves; this is a single coastwise line