280
Chapter 10: The Indian Ocean
adjacent coastal provinces: ARAB and INDW. The reversing monsoons create a reversing
gyral circulation that occupies the entire Arabian Sea: cyclonic in boreal winter, anticyclonic in summer. The topographically trapped gyres that form adjacent to the Somali
coastline and the island of Socotra are here treated as being within the coastal provinces.
At the onset of the Southwest Monsoon winds (<15 m sec
−1 ) of boreal summer, the
pycnocline in the central Arabian Sea is shallow and the density profile is typical of
oligotrophic ocean regions. The mixed layer of the central part of the gyre deepens to
100–110 m as an anticyclonic gyre is spun up, with the center of the bowl lying rather
centrally in the basin at about 10
N 63
E; the seasonal response of the pycnocline of
the central Arabian Sea to physical forcing has been recently analyzed by Kumar and
Narvekar (2005), based on serial observations along a meridional transect from the coast
of Pakistan down to the Equator. The rate of deepening is greatest just after monsoon
onset and is associated with anticyclonic wind-stress curl. The central Arabian Sea is
cooled both by downward transfer of heat, and by evaporative heat flux despite the
relatively high atmospheric humidity at this time. In the southeastern Arabian Sea, the
mixed layer deepens by 25–35 m and cools by 2
C (Colborn, 1975; Rao, 1986; Hastenrath,
1989; Bauer et al., 1991). Thus, even beyond the effects of coastal upwelling generated by
the Southwest Monsoon, the entire Arabian Sea experiences some cooling and freshening.
During the cooler Northeast Monsoon winds of boreal winter, the wind maximum
forms a weaker (c. 6 m sec
−1 ), broader jet that forces relatively insignificant Ekman
dynamics, though some downwelling is predicted (Bauer et al., 1991) within the general
circulation that now proceeds in a cyclonic sense. The cool, dry winter monsoon forces
negative heat flux, and deep penetrative convection reaches the pycnocline at around
125 m. This results in a significant flux of nutrients into the mixed layer so that nitrate
concentrations >4 nM have been observed by the end of winter in the central Arabian
Sea at depths shoaler than 50 m (Wiggert et al., 2000). These observations have suggested
high-S conditions to some people, and this possibility will be discussed later.
Bay of Bengal Gyre Compared with the Arabian Sea, circulation in the Bay of Bengal
is weaker and less predictable, and its response to monsoon reversal more complex,
because it is partially open on its eastern margin where there is connection with the
western Pacific through the Indonesian archipelago. Circulation and mixed-layer depth
in the north of the Bay of Bengal is determined by winter cooling and by the very large
fluvial input of freshwater from the mouths of the Ganges and Irrawaddy rivers, as well
as by the reversing monsoon winds and the influence of the zonal currents lying across
the south of the embayment. The interaction between these diverse forcings is complex
and results in highly variable circulation. The northern area has very low surface salinities
year-round, so that the 33.5‰ isohaline lies zonally, and almost permanently, at the sea
surface across the Bay of Bengal at 10–15
N.
During the Northeast Monsoon, a cyclonic gyre forms in the Bay of Bengal (Wyrtki,
1973b; Rao and Sastry, 1981). With this is associated the East Indian Winter Jet of
Tomczak and Godfrey, a shallow current passing equatorward along the eastern coast of
India, rounding Sri Lanka and continuing into the Arabian Sea from October to March;
this jet can be traced in the surface thermal field as a warm feature. The Southwest
Monsoon spins up an anticyclonic gyre, though weaker than that of the Arabian Sea,
which transports warm water of southern origin (Shetye, 1993), observable in satellite
IR imagery, northward along the coasts of both India and Sri Lanka. Late in the season,
there is some flux from the west coast of India and around the east coast of Sri Lanka
that extends, while maintaining anticyclonic curl, into the cooler water of the northeast
sector of the Bay of Bengal (Legeckis, 1987).
The topography of the mixed layer of the central Bay of Bengal is less predictable than
in the Arabian Sea, perhaps because of undersampling, but also because this region is
Chapter 10: The Indian Ocean
adjacent coastal provinces: ARAB and INDW. The reversing monsoons create a reversing
gyral circulation that occupies the entire Arabian Sea: cyclonic in boreal winter, anticyclonic in summer. The topographically trapped gyres that form adjacent to the Somali
coastline and the island of Socotra are here treated as being within the coastal provinces.
At the onset of the Southwest Monsoon winds (<15 m sec
−1 ) of boreal summer, the
pycnocline in the central Arabian Sea is shallow and the density profile is typical of
oligotrophic ocean regions. The mixed layer of the central part of the gyre deepens to
100–110 m as an anticyclonic gyre is spun up, with the center of the bowl lying rather
centrally in the basin at about 10
N 63
E; the seasonal response of the pycnocline of
the central Arabian Sea to physical forcing has been recently analyzed by Kumar and
Narvekar (2005), based on serial observations along a meridional transect from the coast
of Pakistan down to the Equator. The rate of deepening is greatest just after monsoon
onset and is associated with anticyclonic wind-stress curl. The central Arabian Sea is
cooled both by downward transfer of heat, and by evaporative heat flux despite the
relatively high atmospheric humidity at this time. In the southeastern Arabian Sea, the
mixed layer deepens by 25–35 m and cools by 2
C (Colborn, 1975; Rao, 1986; Hastenrath,
1989; Bauer et al., 1991). Thus, even beyond the effects of coastal upwelling generated by
the Southwest Monsoon, the entire Arabian Sea experiences some cooling and freshening.
During the cooler Northeast Monsoon winds of boreal winter, the wind maximum
forms a weaker (c. 6 m sec
−1 ), broader jet that forces relatively insignificant Ekman
dynamics, though some downwelling is predicted (Bauer et al., 1991) within the general
circulation that now proceeds in a cyclonic sense. The cool, dry winter monsoon forces
negative heat flux, and deep penetrative convection reaches the pycnocline at around
125 m. This results in a significant flux of nutrients into the mixed layer so that nitrate
concentrations >4 nM have been observed by the end of winter in the central Arabian
Sea at depths shoaler than 50 m (Wiggert et al., 2000). These observations have suggested
high-S conditions to some people, and this possibility will be discussed later.
Bay of Bengal Gyre Compared with the Arabian Sea, circulation in the Bay of Bengal
is weaker and less predictable, and its response to monsoon reversal more complex,
because it is partially open on its eastern margin where there is connection with the
western Pacific through the Indonesian archipelago. Circulation and mixed-layer depth
in the north of the Bay of Bengal is determined by winter cooling and by the very large
fluvial input of freshwater from the mouths of the Ganges and Irrawaddy rivers, as well
as by the reversing monsoon winds and the influence of the zonal currents lying across
the south of the embayment. The interaction between these diverse forcings is complex
and results in highly variable circulation. The northern area has very low surface salinities
year-round, so that the 33.5‰ isohaline lies zonally, and almost permanently, at the sea
surface across the Bay of Bengal at 10–15
N.
During the Northeast Monsoon, a cyclonic gyre forms in the Bay of Bengal (Wyrtki,
1973b; Rao and Sastry, 1981). With this is associated the East Indian Winter Jet of
Tomczak and Godfrey, a shallow current passing equatorward along the eastern coast of
India, rounding Sri Lanka and continuing into the Arabian Sea from October to March;
this jet can be traced in the surface thermal field as a warm feature. The Southwest
Monsoon spins up an anticyclonic gyre, though weaker than that of the Arabian Sea,
which transports warm water of southern origin (Shetye, 1993), observable in satellite
IR imagery, northward along the coasts of both India and Sri Lanka. Late in the season,
there is some flux from the west coast of India and around the east coast of Sri Lanka
that extends, while maintaining anticyclonic curl, into the cooler water of the northeast
sector of the Bay of Bengal (Legeckis, 1987).
The topography of the mixed layer of the central Bay of Bengal is less predictable than
in the Arabian Sea, perhaps because of undersampling, but also because this region is