286
Chapter 10: The Indian Ocean
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
Major areas of flat shallow topography, <200 m deep, lie along the Mauritius-Seychelles
Ridge; these are the Nazareth, Saya do Malha, and Seychelles Banks.
Defining Characteristics of Regional Oceanography
We have very little organized knowledge about the subtropical gyre of the southern Indian
Ocean; some of the IIOE investigations did survey this province, especially by means of a
few long meridional sections by the then-Soviet ships, which generated scattered data on
primary and secondary production. In the southeastern part, the well-planned Australian
IIOE sections produced a serious body of organized ecological data, seasonally, although
this is really more relevant to the AUSW province. South African interest in the SW
Indian Ocean has illuminated some dark corners of ISSG in recent years, and the recent
review of Schott and McCreary (2000) is a most useful tool.
The zonal thermocline ridge at about 10
S is the limit of westward flow of the
low-salinity SEC on its southern slope and of the eastward flow of the SECC along
its northern slope. Below the southern slope of the ridge, and thus below the salinity minimum, Wyrtki’s hydrochemical front slopes down southward and separates the
high-nutrient/low-oxygen water of the monsoon gyre from the low-nutrient/high-oxygen
water of the subtropical gyre. The location of the ridge changes seasonally; as the SEC
strengthens during the Southwest Monsoon, the thermal ridge moves progressively northward (to 5
S) and shoals (Z m = <30 m) in the Southwest Monsoon, whereas during the
Northeast Monsoon it is at 10
S and deeper (Z m = <40 m).
During boreal summer, the southeast trades cross the equator northward together
with the ITCZ, which comes to lie along the southern flank of the Himalaya in August;
the wind-driven anticyclonic circulation south of the equator is then fully established.
Anticyclonic wind stress curl extends from about 10–15
S to the southern limb of the
gyre, associated with deepening of the mixed layer centrally in the gyre; this effect is
strongest in austral summer and autumn, reaching maximum effect at 30
S. Increasing
wind stress and decreasing insolation during austral winter impose negative heat flux
(−100 to −140 W m
−2 in August) that deepens the SW Indian Ocean mixed layer to
about 75–100 m. In austral summer, this shoals to 50–75 m. The ISSG province is yet
another case of a subtropical anticyclonic gyre that has nitrate isopleths deepest in
the central regions and shallowest around the periphery. As elsewhere (see SATL and
SPSG provinces), this peripheral shoaling of nitrate is clearly reflected in the surface
chlorophyll field.
Circulation within this subtropical gyre differs from those of the Atlantic and Pacific
Oceans because the eastward flow of the South Indian Ocean Current, along the South
Subtropical Convergence zone, does not generate a boundary current on encountering
the Australian continent (Tomczak and Godfrey, 1994). Instead, much of the flow around
the southern limb of the gyre progressively recirculates northward into the center of the
gyre as two main streams (Stramma and Lutjeharms, 1997). Even prior to reaching 70
E,
as much as 40 Sv of the original 60 Sv is recirculated into the SW Indian Ocean subgyre
that is both smaller and more dynamic than the equivalent feature in other oceans. The
remainder of the flow is drawn into the main subtropical gyre in a stream centered
on about 90
E, perhaps associated with the meridional ridge that occurs there: this is
prior to reaching the Australian continent, in any case. Between this recirculation stream
and the poleward Leeuwin Current at the Australian coast there forms an eddy field
within which Rossby waves are formed and from which they are strongly propagated
westward across the ocean (Andrews, 1977). Another eddy field is generated to the east of
Madagascar, comprising anticyclonic eddies with a periodicity of about 50 days. TOPEXPOSEIDON images reveal that a complex band of mesoscale eddying arcs across the
Chapter 10: The Indian Ocean
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
Major areas of flat shallow topography, <200 m deep, lie along the Mauritius-Seychelles
Ridge; these are the Nazareth, Saya do Malha, and Seychelles Banks.
Defining Characteristics of Regional Oceanography
We have very little organized knowledge about the subtropical gyre of the southern Indian
Ocean; some of the IIOE investigations did survey this province, especially by means of a
few long meridional sections by the then-Soviet ships, which generated scattered data on
primary and secondary production. In the southeastern part, the well-planned Australian
IIOE sections produced a serious body of organized ecological data, seasonally, although
this is really more relevant to the AUSW province. South African interest in the SW
Indian Ocean has illuminated some dark corners of ISSG in recent years, and the recent
review of Schott and McCreary (2000) is a most useful tool.
The zonal thermocline ridge at about 10
S is the limit of westward flow of the
low-salinity SEC on its southern slope and of the eastward flow of the SECC along
its northern slope. Below the southern slope of the ridge, and thus below the salinity minimum, Wyrtki’s hydrochemical front slopes down southward and separates the
high-nutrient/low-oxygen water of the monsoon gyre from the low-nutrient/high-oxygen
water of the subtropical gyre. The location of the ridge changes seasonally; as the SEC
strengthens during the Southwest Monsoon, the thermal ridge moves progressively northward (to 5
S) and shoals (Z m = <30 m) in the Southwest Monsoon, whereas during the
Northeast Monsoon it is at 10
S and deeper (Z m = <40 m).
During boreal summer, the southeast trades cross the equator northward together
with the ITCZ, which comes to lie along the southern flank of the Himalaya in August;
the wind-driven anticyclonic circulation south of the equator is then fully established.
Anticyclonic wind stress curl extends from about 10–15
S to the southern limb of the
gyre, associated with deepening of the mixed layer centrally in the gyre; this effect is
strongest in austral summer and autumn, reaching maximum effect at 30
S. Increasing
wind stress and decreasing insolation during austral winter impose negative heat flux
(−100 to −140 W m
−2 in August) that deepens the SW Indian Ocean mixed layer to
about 75–100 m. In austral summer, this shoals to 50–75 m. The ISSG province is yet
another case of a subtropical anticyclonic gyre that has nitrate isopleths deepest in
the central regions and shallowest around the periphery. As elsewhere (see SATL and
SPSG provinces), this peripheral shoaling of nitrate is clearly reflected in the surface
chlorophyll field.
Circulation within this subtropical gyre differs from those of the Atlantic and Pacific
Oceans because the eastward flow of the South Indian Ocean Current, along the South
Subtropical Convergence zone, does not generate a boundary current on encountering
the Australian continent (Tomczak and Godfrey, 1994). Instead, much of the flow around
the southern limb of the gyre progressively recirculates northward into the center of the
gyre as two main streams (Stramma and Lutjeharms, 1997). Even prior to reaching 70
E,
as much as 40 Sv of the original 60 Sv is recirculated into the SW Indian Ocean subgyre
that is both smaller and more dynamic than the equivalent feature in other oceans. The
remainder of the flow is drawn into the main subtropical gyre in a stream centered
on about 90
E, perhaps associated with the meridional ridge that occurs there: this is
prior to reaching the Australian continent, in any case. Between this recirculation stream
and the poleward Leeuwin Current at the Australian coast there forms an eddy field
within which Rossby waves are formed and from which they are strongly propagated
westward across the ocean (Andrews, 1977). Another eddy field is generated to the east of
Madagascar, comprising anticyclonic eddies with a periodicity of about 50 days. TOPEXPOSEIDON images reveal that a complex band of mesoscale eddying arcs across the