Indian Ocean Trade Wind Biome
279
Ocean. The climatology of the chlorophyll field also demonstrates that strong betweenyear variability characterizes the surface circulation of this ocean.
The southward coastal current that develops along East Africa during the Northeast
Monsoon (see later discussion) returns eastward across the ocean as the South Equatorial
Countercurrent (SECC) along the northern slope of the thermal ridge at 5–10
S (Reverdin
and Fieux, 1987). Convergence thus occurs at about 2–5
S between the NEC and the
SECC (Wyrtki, 1973a).
In boreal summer, the establishment of the Southwest Monsoon collapses the winter
circulation and, by April or May, forces the reversal of the NEC eastward as the Southwest
Monsoon Current (SMC) from about 5
S up to 10
N. At the same season, an anticyclonic
gyre is spun up in the Arabian Sea, and flow from the now-northward Somali Jet passes
around the eastern limb of the gyre and then eastward as the SMC along the same
latitudes that the NEC occupies in winter. The equatorial undercurrent of the Indian
Ocean is singular in that it is not an oceanwide, permanent feature of the circulation:
there is no persistent westward wind stress and thus no persistent westward wind drift
to be compensated by subsurface return flow. We need concern ourselves only with the
shallow Equatorial Jet (600 km wide) that passes eastward along the equator briefly at
each transition between monsoon seasons: in April–May and in September–October. To
judge from the evidence of the surface chlorophyll field, this is a highly variable feature.
It appears to have been exceptionally strong in October–November 1998; at this time,
there is evidence of the development in the eastern part of the ocean of Rossby waves, as
occurs in equatorial features in the Pacific and Atlantic Oceans.
In fact, the circulation of the entire northern Indian Ocean is extremely variable
between years, being closely linked to ENSO events. Sea-level anomalies (SLA) from
TOPEX-POSEIDON altimetry have been used to demonstrate variability at all appropriate
time scales in the Bay of Bengal (Somayajulu et al., 2003). Circulation was found to
respond to all phases of ENSO events, so that both SLA and SST anomalies are negatively
correlated with western Pacific values. The ENSO signal in the oceanic, equatorial region
was found to be transmitted to the Bay of Bengal by coastally trapped Kelvin and radiated
Rossby waves along the Sumatran and adjacent coasts.
The entire province has a permanent thermocline, usually at 30–50 m, except from
Sumatra to the south of Sri Lanka where it deepens in the Southwest Monsoon to 100 m
(Colborn, 1975). In the area of the oceanwide zonal monsoon currents, the thermocline
lies shallower (also thinner and steeper) in the west and deeper (also thicker and weaker)
in the east (Hastenrath, 1989).
An important low-oxygen layer underlies the thermocline at intermediate depths
through much of the eastern Arabian Sea, weakening progressively southward toward
the hydrochemical front discussed previously. This deficiency is maintained by the slow
passage through these depths of southern water, already oxygen deficient, within which
there are very high rates of oxygen consumption resulting from progressive bacterial
decomposition of large masses of organic material sinking from the superjacent algal
blooms (Olson et al., 1993; Kamykowski and Zentara, 1990; Vinogradov and Voronina,
1961). It is one of the three major denitrifying regions in the oceans, and the source
of significant flux of N 2 to the atmosphere. There is some evidence, from apparently
increasing abundance of Pleuromamma indica, a vertical migrant copepod characteristic
of such conditions, that the oxygen minimum layer has increased in importance during
the last 30 years or so (Smith and Madhupratap, 2005).
Because of their unique characteristics, the two northern embayments require individual attention.
Arabian Sea Gyre This account should be read alongside the discussion of seasonal
upwelling and other coastal processes forced by the reversing monsoon regime in the
279
Ocean. The climatology of the chlorophyll field also demonstrates that strong betweenyear variability characterizes the surface circulation of this ocean.
The southward coastal current that develops along East Africa during the Northeast
Monsoon (see later discussion) returns eastward across the ocean as the South Equatorial
Countercurrent (SECC) along the northern slope of the thermal ridge at 5–10
S (Reverdin
and Fieux, 1987). Convergence thus occurs at about 2–5
S between the NEC and the
SECC (Wyrtki, 1973a).
In boreal summer, the establishment of the Southwest Monsoon collapses the winter
circulation and, by April or May, forces the reversal of the NEC eastward as the Southwest
Monsoon Current (SMC) from about 5
S up to 10
N. At the same season, an anticyclonic
gyre is spun up in the Arabian Sea, and flow from the now-northward Somali Jet passes
around the eastern limb of the gyre and then eastward as the SMC along the same
latitudes that the NEC occupies in winter. The equatorial undercurrent of the Indian
Ocean is singular in that it is not an oceanwide, permanent feature of the circulation:
there is no persistent westward wind stress and thus no persistent westward wind drift
to be compensated by subsurface return flow. We need concern ourselves only with the
shallow Equatorial Jet (600 km wide) that passes eastward along the equator briefly at
each transition between monsoon seasons: in April–May and in September–October. To
judge from the evidence of the surface chlorophyll field, this is a highly variable feature.
It appears to have been exceptionally strong in October–November 1998; at this time,
there is evidence of the development in the eastern part of the ocean of Rossby waves, as
occurs in equatorial features in the Pacific and Atlantic Oceans.
In fact, the circulation of the entire northern Indian Ocean is extremely variable
between years, being closely linked to ENSO events. Sea-level anomalies (SLA) from
TOPEX-POSEIDON altimetry have been used to demonstrate variability at all appropriate
time scales in the Bay of Bengal (Somayajulu et al., 2003). Circulation was found to
respond to all phases of ENSO events, so that both SLA and SST anomalies are negatively
correlated with western Pacific values. The ENSO signal in the oceanic, equatorial region
was found to be transmitted to the Bay of Bengal by coastally trapped Kelvin and radiated
Rossby waves along the Sumatran and adjacent coasts.
The entire province has a permanent thermocline, usually at 30–50 m, except from
Sumatra to the south of Sri Lanka where it deepens in the Southwest Monsoon to 100 m
(Colborn, 1975). In the area of the oceanwide zonal monsoon currents, the thermocline
lies shallower (also thinner and steeper) in the west and deeper (also thicker and weaker)
in the east (Hastenrath, 1989).
An important low-oxygen layer underlies the thermocline at intermediate depths
through much of the eastern Arabian Sea, weakening progressively southward toward
the hydrochemical front discussed previously. This deficiency is maintained by the slow
passage through these depths of southern water, already oxygen deficient, within which
there are very high rates of oxygen consumption resulting from progressive bacterial
decomposition of large masses of organic material sinking from the superjacent algal
blooms (Olson et al., 1993; Kamykowski and Zentara, 1990; Vinogradov and Voronina,
1961). It is one of the three major denitrifying regions in the oceans, and the source
of significant flux of N 2 to the atmosphere. There is some evidence, from apparently
increasing abundance of Pleuromamma indica, a vertical migrant copepod characteristic
of such conditions, that the oxygen minimum layer has increased in importance during
the last 30 years or so (Smith and Madhupratap, 2005).
Because of their unique characteristics, the two northern embayments require individual attention.
Arabian Sea Gyre This account should be read alongside the discussion of seasonal
upwelling and other coastal processes forced by the reversing monsoon regime in the