depth-averaged CFC-11 concentrations in the
dense (neutral density 928.27 kg m
93 ) layer of
AABW around Antarctica, based on CFC data
collected along a number of WHP sections, with
additional data from pre-WOCE sections, especially for the Atlantic sector, are shown in Figure
4.6.13 (Rintoul et al., Chapter 4.6). Now that the
One-Time Survey has been completed, significantly better coverage is available. However, the
more limited CFC coverage presented in Figure
4.6.13 clearly highlights the major regions where
new AABW is being formed, the major pathways
of transport away from the source regions, and the
importance of topography in limiting the areal
extent of these dense waters around Antarctica.
Of particular interest are the very high
(92 pmol kg
91
) concentrations of CFC-11 found
in the southern Weddell Sea (Weddell Sea Bottom
Water), in the Ross Sea (Ross Sea Bottom Water),
and along Adélie Land in Antarctica (south of
Tasmania; Adélie Land Bottom Water). The intensity of the CFC-11 signal extending from the
southern Weddell Sea northward toward the tip of
the Antarctic Peninsula, and then branching northward (toward the Argentine Basin) and eastward,
confirm that this region is a major source of new
AABW. The presence of strong CFC signals in
RSBW and in bottom waters off Adélie Land
demonstrate that vigorous formation of bottom
waters also occurs in these regions. The high CFC
signal found in the Australian–Antarctic Basin can
be observed extending eastward towards the
Princess Elizabeth Trough (Ϸ80°E) and northward
along the Kerguelan Plateau. These combined CFC
observations confirm the presence of multiple
regions of bottom water formation around the
Antarctic continent. The distributions also outline
the major pathways for the flow of these waters
away from the source regions.
The observed CFC concentrations in this
region, when utilized in models of deep water formation, can also help to provide quantitative
information on the rates of bottom water formation and outflow from this region. Haine et al.
(1998) used CFC and carbon tetrachloride (CCl 4 )
data collected in 1993 and 1994 (not shown
in Figure 4.6.13) to map the flow path of AABW
from the Weddell Sea to the Crozet–Kerguelen
Gap. They applied a streamtube advection–mixing
model to estimate a transit time of 23 <5 years
which corresponds to an apparent current speed
of 1.2 <0.3 cm s
91
. They also estimated that
the water was diluted by a factor of 8 to 15
since it departed the near-surface region of the
Weddell continental shelves, and combining this
with current meter measurements, estimated a
volume flux of 0.8 to 1.6 Sv from the Weddell Sea
shelves.
As discussed above, CFC inventories provide
quantitative information on formation rates of
subsurface water masses. Orsi et al. (1999) determined the total CFC inventories for the Atlantic
sector and the combined Indian–Pacific sectors
shown in Figure 4.6.13. The dissolved CFC content in newly formed AABW as a function of time
was modelled using the CFC atmospheric input
histories (Walker et al., 2000), observations of the
CFC content of newly formed AABW, and the
assumption thatd the CFC saturation level of
newly formed AABW (relative to the overlying
atmosphere) remained constant over time. Observations indicate that the dissolved CFC content of
newly forming AABW is not in equilibrium with
the overlying atmosphere. Measurements of dissolved CFCs in dense waters formed on the shelves
in the southern Weddell Sea in the region around
the Filchner Ice Shelf are substantially undersaturated with respect to the overlying atmosphere
(Schlosser et al., 1991b; Mensch et al., 1996).
Measurements of CFCs in these dense waters sinking along the slope in this region show that they
undergo further mixing with low-CFC Weddell
Deep Water as they descend to form new WSBW.
Orsi et al. (1999) examined CFC saturations in
newly formed AABW, based on observations near
several source regions around Antarctica (Weddell
Sea, Ross Sea and Adelie region). They found
roughly similar CFC saturation ratios of 35% (relative to the atmosphere at the time of sampling)
for newly formed Weddell, Ross and Adélie Bottom Waters descending across the 2500 db isobath. Using this CFC saturation, Orsi et al. (1999)
derived average production rates for new AABW
of 4.9 Sv for the Atlantic sector, and 3.2 Sv for the
Indian and Pacific sectors (8.1 Sv total). These production rate estimates are for newly formed
AABW, of density 928.27, which has reached a
depth of 92500 m along the slope around Antarctica. Such water represents the product of extensive, rapid mixing of high-CFC shelf waters
(Ϸ70% saturated relative to the overlying atmosphere) with very low-CFC Circumpolar Deep
5.8 Transformation and Age of Water Masses
447
Schlosser et al.
dense (neutral density 928.27 kg m
93 ) layer of
AABW around Antarctica, based on CFC data
collected along a number of WHP sections, with
additional data from pre-WOCE sections, especially for the Atlantic sector, are shown in Figure
4.6.13 (Rintoul et al., Chapter 4.6). Now that the
One-Time Survey has been completed, significantly better coverage is available. However, the
more limited CFC coverage presented in Figure
4.6.13 clearly highlights the major regions where
new AABW is being formed, the major pathways
of transport away from the source regions, and the
importance of topography in limiting the areal
extent of these dense waters around Antarctica.
Of particular interest are the very high
(92 pmol kg
91
) concentrations of CFC-11 found
in the southern Weddell Sea (Weddell Sea Bottom
Water), in the Ross Sea (Ross Sea Bottom Water),
and along Adélie Land in Antarctica (south of
Tasmania; Adélie Land Bottom Water). The intensity of the CFC-11 signal extending from the
southern Weddell Sea northward toward the tip of
the Antarctic Peninsula, and then branching northward (toward the Argentine Basin) and eastward,
confirm that this region is a major source of new
AABW. The presence of strong CFC signals in
RSBW and in bottom waters off Adélie Land
demonstrate that vigorous formation of bottom
waters also occurs in these regions. The high CFC
signal found in the Australian–Antarctic Basin can
be observed extending eastward towards the
Princess Elizabeth Trough (Ϸ80°E) and northward
along the Kerguelan Plateau. These combined CFC
observations confirm the presence of multiple
regions of bottom water formation around the
Antarctic continent. The distributions also outline
the major pathways for the flow of these waters
away from the source regions.
The observed CFC concentrations in this
region, when utilized in models of deep water formation, can also help to provide quantitative
information on the rates of bottom water formation and outflow from this region. Haine et al.
(1998) used CFC and carbon tetrachloride (CCl 4 )
data collected in 1993 and 1994 (not shown
in Figure 4.6.13) to map the flow path of AABW
from the Weddell Sea to the Crozet–Kerguelen
Gap. They applied a streamtube advection–mixing
model to estimate a transit time of 23 <5 years
which corresponds to an apparent current speed
of 1.2 <0.3 cm s
91
. They also estimated that
the water was diluted by a factor of 8 to 15
since it departed the near-surface region of the
Weddell continental shelves, and combining this
with current meter measurements, estimated a
volume flux of 0.8 to 1.6 Sv from the Weddell Sea
shelves.
As discussed above, CFC inventories provide
quantitative information on formation rates of
subsurface water masses. Orsi et al. (1999) determined the total CFC inventories for the Atlantic
sector and the combined Indian–Pacific sectors
shown in Figure 4.6.13. The dissolved CFC content in newly formed AABW as a function of time
was modelled using the CFC atmospheric input
histories (Walker et al., 2000), observations of the
CFC content of newly formed AABW, and the
assumption thatd the CFC saturation level of
newly formed AABW (relative to the overlying
atmosphere) remained constant over time. Observations indicate that the dissolved CFC content of
newly forming AABW is not in equilibrium with
the overlying atmosphere. Measurements of dissolved CFCs in dense waters formed on the shelves
in the southern Weddell Sea in the region around
the Filchner Ice Shelf are substantially undersaturated with respect to the overlying atmosphere
(Schlosser et al., 1991b; Mensch et al., 1996).
Measurements of CFCs in these dense waters sinking along the slope in this region show that they
undergo further mixing with low-CFC Weddell
Deep Water as they descend to form new WSBW.
Orsi et al. (1999) examined CFC saturations in
newly formed AABW, based on observations near
several source regions around Antarctica (Weddell
Sea, Ross Sea and Adelie region). They found
roughly similar CFC saturation ratios of 35% (relative to the atmosphere at the time of sampling)
for newly formed Weddell, Ross and Adélie Bottom Waters descending across the 2500 db isobath. Using this CFC saturation, Orsi et al. (1999)
derived average production rates for new AABW
of 4.9 Sv for the Atlantic sector, and 3.2 Sv for the
Indian and Pacific sectors (8.1 Sv total). These production rate estimates are for newly formed
AABW, of density 928.27, which has reached a
depth of 92500 m along the slope around Antarctica. Such water represents the product of extensive, rapid mixing of high-CFC shelf waters
(Ϸ70% saturated relative to the overlying atmosphere) with very low-CFC Circumpolar Deep
5.8 Transformation and Age of Water Masses
447
Schlosser et al.