Weddell Sea several major programmes have contributed to this advance (see Fahrbach et al., 1998,
for a review). The new results are broadly consistent with the picture of circulation and water mass
formation in the Weddell developed by earlier
investigators. Relatively warm Circumpolar Deep
Water (sometimes called Warm Deep Water in
the Weddell) enters the basin from the east in the
Weddell gyre and is converted to Weddell Sea Deep
and Bottom Water through ice–ocean–atmosphere
interactions along the southern and western margin. Two mixing scenarios have been proposed
for the formation of Weddell Sea Bottom Water:
(1) mixing of Winter surface water, Warm Deep
Water, and Western Shelf Water, whose salinity
has been enriched by brine rejected during sea
ice formation (Foster and Carmack, 1976); and
(2) mixing of Ice Shelf Water (formed by cooling and freshening of Western Shelf Water beneath
the vast ice shelves in the southern Weddell Sea)
with Weddell Sea Deep Water and Warm Deep
Water (Carmack and Foster, 1975; Foldvik et al.,
1985a).
Fahrbach et al. (1994a) maintained a line of
current meter moorings and repeat hydrographic
sections across the central Weddell Sea between
1989 and 1993. They estimate a Weddell gyre
transport of 30 Sv, almost all of which is carried in
narrow currents along the continental slope.
Recent model results of Beckmann et al. (1999)
indicate that Fahrbach’s section does not cut
through the centre of the Weddell gyre, which has
a maximum transport exceeding 50 Sv, in agreement with observations along the Greenwich
meridian (Schröder and Fahrbach, 1999). By considering inflow and outflow in density layers
across this section, Fahrbach et al. (1991) infer a
net conversion of 3–4 Sv of Winter and Warm
Deep Water to Weddell Sea Deep and Bottom
Water. Direct measurements of the outflow of bottom water (potential temperature:90.7°C) from
moorings in the western boundary current show a
mean of 1.7 Sv, and a range from 0.8 to 3.9 Sv;
additional bottom water export occurs offshore of
the narrow boundary current (Fahrbach et al.,
1994). Dense shelf water also escapes from the
Weddell Sea to the Scotia Sea through gaps in the
island chain separating the basins (Whitworth
et al., 1994).
The 700-km-long drift of Ice Station Weddell in
1992 further refined our understanding of
ice–ocean–atmosphere interactions in the western
Weddell Sea (Gordon et al., 1993; Gordon, 1998).
Taken together, the results of recent programmes
identify a number of distinct AABW sources along
the southern and western rim of the Weddell Sea,
each with a characteristic temperature, salinity and
stable isotope signature. The relative importance
of the two mixing scenarios described above varies
along the rim of the Weddell. Entrainment of
warmer and saltier deep water found over the
slope largely determines the ultimate properties of
deep and bottom water leaving the Weddell Sea.
Gordon (1998) estimates the formation of Weddell
Sea Bottom Water (WSBW) (potential temperature
:90.7°C) during the period of the Ice Station
drift to be 4.0–4.8 Sv. Mensch et al. (1998) estimate a similar formation rate (about 5 Sv of deep
and bottom water) from tracer measurements
obtained during the Ice Station.
Satellite observations of a large polynya in the
Weddell Sea in the late 1970s, and the large changes
in temperature and salinity of Weddell Sea Deep
Water that resulted (Gordon, 1982), first sparked
interest in the variability of Weddell waters. A number of recent studies have documented variations of
deep and bottom water properties. Gordon (1998)
concludes that the average salinity of the WSBW
formed during the Ice Station is too low to provide
the end-member required to account for the deep
water of the Weddell gyre, and suggests that the
bottom water forming at the present time contains
more Ice Shelf Water. Nøst and Østerhus (1998)
describe the impact of several large grounded icebergs north of the Filchner depression, which have
caused the cessation of high-salinity shelf water formation there and led to a cooling anxd freshening in
the depression. Further afield, Coles et al. (1996)
and Hogg and Zenk (1997) have observed cooling
and freshening (on isopycnals) of AABW spreading
north in the South Atlantic, which they attribute to
changes in open ocean convective events in the
Weddell Sea.
Recent studies have also identified or confirmed
a number of important sources of AABW outside
the Weddell Sea. Rintoul (1998) shows that the
Adélie coast (140–150°E) is likely to be a more significant source of bottom water than previously
appreciated. The Adélie Land Bottom Water is evident in Fig. 4.6.3 as a thin layer of cold, fresh,
dense, high-oxygen water found over the continental slope and rise of Antarctica. He argues on the
4.6 The Antarctic Circumpolar Current System
295
Rintoul, Hughes and Olbers
for a review). The new results are broadly consistent with the picture of circulation and water mass
formation in the Weddell developed by earlier
investigators. Relatively warm Circumpolar Deep
Water (sometimes called Warm Deep Water in
the Weddell) enters the basin from the east in the
Weddell gyre and is converted to Weddell Sea Deep
and Bottom Water through ice–ocean–atmosphere
interactions along the southern and western margin. Two mixing scenarios have been proposed
for the formation of Weddell Sea Bottom Water:
(1) mixing of Winter surface water, Warm Deep
Water, and Western Shelf Water, whose salinity
has been enriched by brine rejected during sea
ice formation (Foster and Carmack, 1976); and
(2) mixing of Ice Shelf Water (formed by cooling and freshening of Western Shelf Water beneath
the vast ice shelves in the southern Weddell Sea)
with Weddell Sea Deep Water and Warm Deep
Water (Carmack and Foster, 1975; Foldvik et al.,
1985a).
Fahrbach et al. (1994a) maintained a line of
current meter moorings and repeat hydrographic
sections across the central Weddell Sea between
1989 and 1993. They estimate a Weddell gyre
transport of 30 Sv, almost all of which is carried in
narrow currents along the continental slope.
Recent model results of Beckmann et al. (1999)
indicate that Fahrbach’s section does not cut
through the centre of the Weddell gyre, which has
a maximum transport exceeding 50 Sv, in agreement with observations along the Greenwich
meridian (Schröder and Fahrbach, 1999). By considering inflow and outflow in density layers
across this section, Fahrbach et al. (1991) infer a
net conversion of 3–4 Sv of Winter and Warm
Deep Water to Weddell Sea Deep and Bottom
Water. Direct measurements of the outflow of bottom water (potential temperature:90.7°C) from
moorings in the western boundary current show a
mean of 1.7 Sv, and a range from 0.8 to 3.9 Sv;
additional bottom water export occurs offshore of
the narrow boundary current (Fahrbach et al.,
1994). Dense shelf water also escapes from the
Weddell Sea to the Scotia Sea through gaps in the
island chain separating the basins (Whitworth
et al., 1994).
The 700-km-long drift of Ice Station Weddell in
1992 further refined our understanding of
ice–ocean–atmosphere interactions in the western
Weddell Sea (Gordon et al., 1993; Gordon, 1998).
Taken together, the results of recent programmes
identify a number of distinct AABW sources along
the southern and western rim of the Weddell Sea,
each with a characteristic temperature, salinity and
stable isotope signature. The relative importance
of the two mixing scenarios described above varies
along the rim of the Weddell. Entrainment of
warmer and saltier deep water found over the
slope largely determines the ultimate properties of
deep and bottom water leaving the Weddell Sea.
Gordon (1998) estimates the formation of Weddell
Sea Bottom Water (WSBW) (potential temperature
:90.7°C) during the period of the Ice Station
drift to be 4.0–4.8 Sv. Mensch et al. (1998) estimate a similar formation rate (about 5 Sv of deep
and bottom water) from tracer measurements
obtained during the Ice Station.
Satellite observations of a large polynya in the
Weddell Sea in the late 1970s, and the large changes
in temperature and salinity of Weddell Sea Deep
Water that resulted (Gordon, 1982), first sparked
interest in the variability of Weddell waters. A number of recent studies have documented variations of
deep and bottom water properties. Gordon (1998)
concludes that the average salinity of the WSBW
formed during the Ice Station is too low to provide
the end-member required to account for the deep
water of the Weddell gyre, and suggests that the
bottom water forming at the present time contains
more Ice Shelf Water. Nøst and Østerhus (1998)
describe the impact of several large grounded icebergs north of the Filchner depression, which have
caused the cessation of high-salinity shelf water formation there and led to a cooling anxd freshening in
the depression. Further afield, Coles et al. (1996)
and Hogg and Zenk (1997) have observed cooling
and freshening (on isopycnals) of AABW spreading
north in the South Atlantic, which they attribute to
changes in open ocean convective events in the
Weddell Sea.
Recent studies have also identified or confirmed
a number of important sources of AABW outside
the Weddell Sea. Rintoul (1998) shows that the
Adélie coast (140–150°E) is likely to be a more significant source of bottom water than previously
appreciated. The Adélie Land Bottom Water is evident in Fig. 4.6.3 as a thin layer of cold, fresh,
dense, high-oxygen water found over the continental slope and rise of Antarctica. He argues on the
4.6 The Antarctic Circumpolar Current System
295
Rintoul, Hughes and Olbers