470
Chapter 12: The Southern Ocean
maximum (19–20 × 10
6 km
−2 ) in August–October when it extends to within a few degrees
of the PF. It lies farthest from the continent in the central Atlantic and Pacific sectors.
Especially in the western Pacific/Drake Passage sector the permanently open surface water
is very narrow, and seasonal pack ice closely approaches the PF.
Water of the East Wind Drift is very cold: shoaler than 500 m it is always <2
C even
in summer and is still colder in winter; it is also thermally unstratified. Where slight
surface warming occurs, this results in a mixed-layer depth of no more than about 25 m.
In austral winter, a combination of low sun angle, heat loss to the atmosphere, and deep
convection cools the entire surface water mass so that mixing extends to halocline depth
at 200–400 m (Trull et al., 2001).
The convergent Continental Water Boundary (CB) that circles the Antarctic continent
poleward of the AD (see Fig. 12.2) lies between shelf water and the warmer circumpolar
deep water upwelled at the divergence that otherwise occupies the surface of this province.
At the CB, deep convection occurs of the very dense (cold and salty) shelf water down
into the interior of the ocean. It is only in places such as this, where surface evaporation
from cold dry winds occurs simultaneously with ice formation and brine rejection,
that sufficiently dense water is evolved to sink and form the deep water masses of the
oceans. The Antarctic bottom water formed in this way along the coast of Antarctica
is one of the two principal roots of the global thermohaline circulation of the oceans
(Lutjeharms, 1985).
Because wind-stress curl is at its regional minimum here, Ekman transport diverges
on either side of the front and density surface slopes downward both to the north and to
the south; a convenient way of locating the axis of upwelling is the salinity maximum on
the 200-m horizon. LCDW is upwelled at the AD and this—compared with UCDW (see
earlier discussion)—has a relatively low content of macronutrients and iron: this fact has
significance for the ecology of the province. It is only closer to the Antarctic continent, in
the coastal region, that higher nutrient levels are encountered. To the south of the CWB
(and consequently south of the Si max ) the surface water mass is permanently silica-replete.
Although winter mixing of course occurs here, Central Deep Water is not reached and
so the potentially strong input of dissolved Fe from this water mass is therefore not
realized.
There are two major gyres in the westward flow around Antarctica, occupying the
Weddell and Ross Seas, and each is confluent with the flow of the ACC to the north. The
Weddell Sea gyre comprises very cold coastal water (−18
C in the center of the gyre
even in summer), occupies the area enclosed by the Antarctic Peninsula, and extends
eastward to about 10–15
W before rejoining the westward flow along the coast into the
southern Weddell Sea. The eastward flow in this gyre from the tip of the peninsula forms
a confluent front (at the AD) with the ACC. The smaller Ross Sea gyre is generated by
flow past the coastal prominences on either side of this embayment.
Because so much of the research performed in Antarctic seas has concentrated on only
a few places, it will be useful to discuss these in more detail. The Larsen ice shelf on the east
coast of the Antarctic Peninsula and the Filchner-Rønne ice shelf at the head of the bight
occupy a significant part of the surface of the Weddell Sea. The coastal current of the East
Wind Drift enters the Weddell Sea at 20
W and contributes to its cyclonic circulation
(clockwise in the Southern Hemisphere), whereas near Halley Bay (25
E, on the southeast
coastline) the incoming coastal current passes through a divergence zone that somewhat
separates the ecology of the sea into a northeastern and southern component. The gyral
circulation passes toward the east at the tip of the Antarctic Peninsula and can be traced to
about 15
W where it returns south toward the coast. Weddell Sea gyre water has anomalously high silicate and nitrate concentrations compared with the water beyond the gyre.
Although the Weddell Sea is within the zone usually mapped as “permanently icecovered,” much open water does occur seasonally within the confines of its gyral
Chapter 12: The Southern Ocean
maximum (19–20 × 10
6 km
−2 ) in August–October when it extends to within a few degrees
of the PF. It lies farthest from the continent in the central Atlantic and Pacific sectors.
Especially in the western Pacific/Drake Passage sector the permanently open surface water
is very narrow, and seasonal pack ice closely approaches the PF.
Water of the East Wind Drift is very cold: shoaler than 500 m it is always <2
C even
in summer and is still colder in winter; it is also thermally unstratified. Where slight
surface warming occurs, this results in a mixed-layer depth of no more than about 25 m.
In austral winter, a combination of low sun angle, heat loss to the atmosphere, and deep
convection cools the entire surface water mass so that mixing extends to halocline depth
at 200–400 m (Trull et al., 2001).
The convergent Continental Water Boundary (CB) that circles the Antarctic continent
poleward of the AD (see Fig. 12.2) lies between shelf water and the warmer circumpolar
deep water upwelled at the divergence that otherwise occupies the surface of this province.
At the CB, deep convection occurs of the very dense (cold and salty) shelf water down
into the interior of the ocean. It is only in places such as this, where surface evaporation
from cold dry winds occurs simultaneously with ice formation and brine rejection,
that sufficiently dense water is evolved to sink and form the deep water masses of the
oceans. The Antarctic bottom water formed in this way along the coast of Antarctica
is one of the two principal roots of the global thermohaline circulation of the oceans
(Lutjeharms, 1985).
Because wind-stress curl is at its regional minimum here, Ekman transport diverges
on either side of the front and density surface slopes downward both to the north and to
the south; a convenient way of locating the axis of upwelling is the salinity maximum on
the 200-m horizon. LCDW is upwelled at the AD and this—compared with UCDW (see
earlier discussion)—has a relatively low content of macronutrients and iron: this fact has
significance for the ecology of the province. It is only closer to the Antarctic continent, in
the coastal region, that higher nutrient levels are encountered. To the south of the CWB
(and consequently south of the Si max ) the surface water mass is permanently silica-replete.
Although winter mixing of course occurs here, Central Deep Water is not reached and
so the potentially strong input of dissolved Fe from this water mass is therefore not
realized.
There are two major gyres in the westward flow around Antarctica, occupying the
Weddell and Ross Seas, and each is confluent with the flow of the ACC to the north. The
Weddell Sea gyre comprises very cold coastal water (−18
C in the center of the gyre
even in summer), occupies the area enclosed by the Antarctic Peninsula, and extends
eastward to about 10–15
W before rejoining the westward flow along the coast into the
southern Weddell Sea. The eastward flow in this gyre from the tip of the peninsula forms
a confluent front (at the AD) with the ACC. The smaller Ross Sea gyre is generated by
flow past the coastal prominences on either side of this embayment.
Because so much of the research performed in Antarctic seas has concentrated on only
a few places, it will be useful to discuss these in more detail. The Larsen ice shelf on the east
coast of the Antarctic Peninsula and the Filchner-Rønne ice shelf at the head of the bight
occupy a significant part of the surface of the Weddell Sea. The coastal current of the East
Wind Drift enters the Weddell Sea at 20
W and contributes to its cyclonic circulation
(clockwise in the Southern Hemisphere), whereas near Halley Bay (25
E, on the southeast
coastline) the incoming coastal current passes through a divergence zone that somewhat
separates the ecology of the sea into a northeastern and southern component. The gyral
circulation passes toward the east at the tip of the Antarctic Peninsula and can be traced to
about 15
W where it returns south toward the coast. Weddell Sea gyre water has anomalously high silicate and nitrate concentrations compared with the water beyond the gyre.
Although the Weddell Sea is within the zone usually mapped as “permanently icecovered,” much open water does occur seasonally within the confines of its gyral