Antarctic Westerly Winds Biome
449
subtropical surface water. The STF of many previous authors represents a concatenation
of the two fronts of the STZ, but was most frequently identified by the conjunction
here of cold, low-salinity subantarctic surface water with water having some residual
subtropical characteristics.
The entire frontal zone is sufficiently dynamic to have a major eddy field associated
with it, so that several surface discontinuity fronts may be encountered in the same
transect (Deacon, 1984; Colborn, 1975; Lutjeharms, 1985). The STF on the cold side
of the STCZ is most frequently observed because of the transition across it to subantarctic conditions. Because of seasonal changes that occur in surface temperatures,
the associated surface salinity gradient is the best practical indicator of the presence of
the STF.
A fast zonal jet in the upper 1000 m of the water column passes eastward across each
ocean basin along the warm side of the STF (Stramma and Petersen, 1990; Stramma,
1992). Other names have been applied to it, but we may simply call this the STCZ flow,
because its lateral extent corresponds closely to the region of the entire STCZ as otherwise
determined. The STCZ flow is bounded to the north by the termination of the thermal
and salinity gradients at the surface, a change that is sufficiently sharply marked in surface
properties as to be recognized by some authors as a Northern Subtropical Front. Velocity
in the near-surface core of the STCZ flow is of order 10–30 cm sec
−1 , and transport is
two to three times faster than the flow to the north and south.
Since the STCZ lies between nitrate-depleted subtropical and nitrate-replete subantarctic water, there is a major gradient in dissolved nitrogen across the feature: NO 3 + NH 4
may be undetectable in subtropical water to the north, but take values of ∼2 M in the
STCZ itself, and 8–10 M in the subantarctic water to the south. Silicate values remain
undetectable across the STCZ and remain low (04 M) to the south, not increasing
(to >5 M) until the PF is crossed at about 50
S. Available illumination at the surface
(12–52 E m
−2 d
−1 ) and mean mixed-layer depth (30–125 m) each show a simple seasonal
pattern having maxima and minima in December–January and June–July, as appropriate.
Euphotic zone depth varies only rather slightly with the seasons (∼85 ± 15 m). Surface
temperature maxima and minima (12–15
C) lag by about 1 month.
The strength of the flow in the STCZ explains what is observed so clearly in satellite
imagery. TOPEX-POSEIDON sea-surface elevation data reveal that across each ocean a
zonal band of strong mesoscale meanders and eddies occupies a zone of approximately 5
latitude in extent. This is, at once, the flow of the STC current and an operational means
of defining the SSTC province, although the concept of a simple linear STF lying zonally
around the Southern Ocean is very far from reality. This had already been observed in
section data; a meridional section down the southern Indian Ocean at 70
E cut twice
across the STCZ, with a 100 km section of subantarctic water between, indicating very
strong meandering (Stramma, 1992). Although, as we shall see, the STCZ is clearly
represented in sea surface chlorophyll fields, these are somewhat misleading because, to
eliminate cloud masking, they are usually examined as 30-day composites. There is so
much motion within the meanders of the STCZ that it then appears in these images wider
than instantaneous images would reveal. Only from cloud-piercing TOPEX-Poseidon
radar maps of sea surface elevation can we obtain true dimensions and location of the
STCZ. Both 7-day chlorophyll images and sea-level anomaly maps show that where the
STCZ is most strongly developed, meanders take rather uniform scale: about 5–7
of
latitude seems in the SW Indian Ocean, for example. Meander formation is strongest
immediately downstream of where the zonal flow is deformed by topography to the east
of Patagonia, of South Africa, and of Tasmania and New Zealand.
South of Africa, the location of the Subtropical Convergence is determined by the
position of the retroflection zone of the Agulhas Current where the warm Agulhas water
thrusts south of the Cape of Good Hope (Lutjeharms et al., 1985; Lutjeharms and van
449
subtropical surface water. The STF of many previous authors represents a concatenation
of the two fronts of the STZ, but was most frequently identified by the conjunction
here of cold, low-salinity subantarctic surface water with water having some residual
subtropical characteristics.
The entire frontal zone is sufficiently dynamic to have a major eddy field associated
with it, so that several surface discontinuity fronts may be encountered in the same
transect (Deacon, 1984; Colborn, 1975; Lutjeharms, 1985). The STF on the cold side
of the STCZ is most frequently observed because of the transition across it to subantarctic conditions. Because of seasonal changes that occur in surface temperatures,
the associated surface salinity gradient is the best practical indicator of the presence of
the STF.
A fast zonal jet in the upper 1000 m of the water column passes eastward across each
ocean basin along the warm side of the STF (Stramma and Petersen, 1990; Stramma,
1992). Other names have been applied to it, but we may simply call this the STCZ flow,
because its lateral extent corresponds closely to the region of the entire STCZ as otherwise
determined. The STCZ flow is bounded to the north by the termination of the thermal
and salinity gradients at the surface, a change that is sufficiently sharply marked in surface
properties as to be recognized by some authors as a Northern Subtropical Front. Velocity
in the near-surface core of the STCZ flow is of order 10–30 cm sec
−1 , and transport is
two to three times faster than the flow to the north and south.
Since the STCZ lies between nitrate-depleted subtropical and nitrate-replete subantarctic water, there is a major gradient in dissolved nitrogen across the feature: NO 3 + NH 4
may be undetectable in subtropical water to the north, but take values of ∼2 M in the
STCZ itself, and 8–10 M in the subantarctic water to the south. Silicate values remain
undetectable across the STCZ and remain low (04 M) to the south, not increasing
(to >5 M) until the PF is crossed at about 50
S. Available illumination at the surface
(12–52 E m
−2 d
−1 ) and mean mixed-layer depth (30–125 m) each show a simple seasonal
pattern having maxima and minima in December–January and June–July, as appropriate.
Euphotic zone depth varies only rather slightly with the seasons (∼85 ± 15 m). Surface
temperature maxima and minima (12–15
C) lag by about 1 month.
The strength of the flow in the STCZ explains what is observed so clearly in satellite
imagery. TOPEX-POSEIDON sea-surface elevation data reveal that across each ocean a
zonal band of strong mesoscale meanders and eddies occupies a zone of approximately 5
latitude in extent. This is, at once, the flow of the STC current and an operational means
of defining the SSTC province, although the concept of a simple linear STF lying zonally
around the Southern Ocean is very far from reality. This had already been observed in
section data; a meridional section down the southern Indian Ocean at 70
E cut twice
across the STCZ, with a 100 km section of subantarctic water between, indicating very
strong meandering (Stramma, 1992). Although, as we shall see, the STCZ is clearly
represented in sea surface chlorophyll fields, these are somewhat misleading because, to
eliminate cloud masking, they are usually examined as 30-day composites. There is so
much motion within the meanders of the STCZ that it then appears in these images wider
than instantaneous images would reveal. Only from cloud-piercing TOPEX-Poseidon
radar maps of sea surface elevation can we obtain true dimensions and location of the
STCZ. Both 7-day chlorophyll images and sea-level anomaly maps show that where the
STCZ is most strongly developed, meanders take rather uniform scale: about 5–7
of
latitude seems in the SW Indian Ocean, for example. Meander formation is strongest
immediately downstream of where the zonal flow is deformed by topography to the east
of Patagonia, of South Africa, and of Tasmania and New Zealand.
South of Africa, the location of the Subtropical Convergence is determined by the
position of the retroflection zone of the Agulhas Current where the warm Agulhas water
thrusts south of the Cape of Good Hope (Lutjeharms et al., 1985; Lutjeharms and van