Antarctic Westerly Winds Biome
455
water has perhaps some consequences for the micronutrient concentration of surface
water in the current jet along the PF (see also Chapter 7).
Defining Characteristics of Regional Oceanography
The general characteristics of the northern flow of the ACC were noted briefly earlier, but
it will be useful to reiterate that the two deep fronts with which we shall be concerned,
the SAF and the PF, are each associated with frontal jet currents (Nowlin and Klinck,
1986). The PF jet is the stronger, but these two together comprise the main eastward
transport within the northern part of the ACC. Flow in these jet currents is especially
fast above the southwest Pacific midocean ridge, in the Drake passage, and south of the
Agulhas Retroflection loop current. With each velocity maximum is associated a zone of
intense baroclinicity (Carmack, 1990).
The Polar Frontal Zone (PFZ) occupies almost half of the latitudinal extent of the
Subantarctic Water Ring (SWR), and is a complex feature bounded by two fronts, the SAF
and the PF. At the PF, Antarctic Surface Water slips below and mixes with warmer water
to the north, to form Atlantic Intermediate Water (AAIW). The strongest subsurface
expression of the PF is usually 50 km north of the surface feature (Lutjeharms et al.,
1985). South of the PF, down to the southern boundary of the ACC, upsloping UCDW
lies sufficiently close to the surface that deep winter mixing replenishes surface water
with nutrients.
To the east of South America, the SAF and PF separate more widely than elsewhere
(Tréguer and Jacques, 1992). Here, the SAF passes to the north close around the Falkland
Islands, before looping around the Argentine Basin (see SSTC) to meet the southerly flow
of the Brazil Current at about 40
N. Meanwhile, the PF passes directly north over deep
water between the Ewing Bank and the Falkland Plateau to approach the SAF again, after
which both pass eastward across the ocean, defining anew the PFZ lying between them
(see Fig 12.2). This alignment will be referred to again later in relation to a claim that
iron enrichment of the PF occurs by entrainment from shallow sediments during this
passage.
These are the windiest seas in all oceans; only in some parts of the North Atlantic in
winter does comparable wind stress occur at the sea surface. Even in December (austral
midsummer) wind speed at 50–60
S is similar to almost anywhere in the winter northern
hemisphere at similar latitudes, and the passage of depressions associated with force 6–7
winds must be expected even at that season. Winds are strongest in the region from the
longitude of South Africa to the mid-Pacific, where mean wind speed over large regions
exceeds 14 m sec
−1 . Wind direction is consistently from the west or northwest, and the
zonal component of velocity varies by a factor of about 2 between summer and winter.
Wind-stress curl south of the PF imparts buoyancy gain through Ekman divergence while,
north of the PF, downwelling and buoyancy loss is imparted.
The pattern of solar irradiance over the Southern Ocean involves winter darkness
poleward of 66
S; the opposite side of this coin is that day length there is longer at
midsummer than at lower latitudes. In some parts of this province, therefore, the total
solar radiation received at the surface each summer day is greater than in the central subtropical gyres. Consequently, near-surface phytoplankton may become light-saturated at
the surface so that the standardized photosynthetic rate may be maximal at 5–10 m depth.
The SAZ is characterized by deep winter mixing (April–October) and a shallow summer
thermocline that has an unusually weak thermal gradient. Winter mixed-layer depths are
greater here than anywhere else in the oceans; heat loss and deep convection may carry
mixing sufficiently far down as to entrain deep high-salinity, high-nutrient water into a
surface homogenous layer. Deepest winter mixing (>400 m) occurs at about 50
S and
may prevent the formation of a permanent thermocline in an annulus around the entire
455
water has perhaps some consequences for the micronutrient concentration of surface
water in the current jet along the PF (see also Chapter 7).
Defining Characteristics of Regional Oceanography
The general characteristics of the northern flow of the ACC were noted briefly earlier, but
it will be useful to reiterate that the two deep fronts with which we shall be concerned,
the SAF and the PF, are each associated with frontal jet currents (Nowlin and Klinck,
1986). The PF jet is the stronger, but these two together comprise the main eastward
transport within the northern part of the ACC. Flow in these jet currents is especially
fast above the southwest Pacific midocean ridge, in the Drake passage, and south of the
Agulhas Retroflection loop current. With each velocity maximum is associated a zone of
intense baroclinicity (Carmack, 1990).
The Polar Frontal Zone (PFZ) occupies almost half of the latitudinal extent of the
Subantarctic Water Ring (SWR), and is a complex feature bounded by two fronts, the SAF
and the PF. At the PF, Antarctic Surface Water slips below and mixes with warmer water
to the north, to form Atlantic Intermediate Water (AAIW). The strongest subsurface
expression of the PF is usually 50 km north of the surface feature (Lutjeharms et al.,
1985). South of the PF, down to the southern boundary of the ACC, upsloping UCDW
lies sufficiently close to the surface that deep winter mixing replenishes surface water
with nutrients.
To the east of South America, the SAF and PF separate more widely than elsewhere
(Tréguer and Jacques, 1992). Here, the SAF passes to the north close around the Falkland
Islands, before looping around the Argentine Basin (see SSTC) to meet the southerly flow
of the Brazil Current at about 40
N. Meanwhile, the PF passes directly north over deep
water between the Ewing Bank and the Falkland Plateau to approach the SAF again, after
which both pass eastward across the ocean, defining anew the PFZ lying between them
(see Fig 12.2). This alignment will be referred to again later in relation to a claim that
iron enrichment of the PF occurs by entrainment from shallow sediments during this
passage.
These are the windiest seas in all oceans; only in some parts of the North Atlantic in
winter does comparable wind stress occur at the sea surface. Even in December (austral
midsummer) wind speed at 50–60
S is similar to almost anywhere in the winter northern
hemisphere at similar latitudes, and the passage of depressions associated with force 6–7
winds must be expected even at that season. Winds are strongest in the region from the
longitude of South Africa to the mid-Pacific, where mean wind speed over large regions
exceeds 14 m sec
−1 . Wind direction is consistently from the west or northwest, and the
zonal component of velocity varies by a factor of about 2 between summer and winter.
Wind-stress curl south of the PF imparts buoyancy gain through Ekman divergence while,
north of the PF, downwelling and buoyancy loss is imparted.
The pattern of solar irradiance over the Southern Ocean involves winter darkness
poleward of 66
S; the opposite side of this coin is that day length there is longer at
midsummer than at lower latitudes. In some parts of this province, therefore, the total
solar radiation received at the surface each summer day is greater than in the central subtropical gyres. Consequently, near-surface phytoplankton may become light-saturated at
the surface so that the standardized photosynthetic rate may be maximal at 5–10 m depth.
The SAZ is characterized by deep winter mixing (April–October) and a shallow summer
thermocline that has an unusually weak thermal gradient. Winter mixed-layer depths are
greater here than anywhere else in the oceans; heat loss and deep convection may carry
mixing sufficiently far down as to entrain deep high-salinity, high-nutrient water into a
surface homogenous layer. Deepest winter mixing (>400 m) occurs at about 50
S and
may prevent the formation of a permanent thermocline in an annulus around the entire