subtropical Atlantic. Earlier data (Molinari et al.,
1998) and CFC measurements from WOCE repeat
hydrography cruises in the mid-1990s clearly show
extensive spreading of this newly formed Labrador
Sea Water. Its transit time to the Irminger Sea, the
Newfoundland Basin, and the northeastern Atlantic
was determined to be 6 months, 1 year, and 5
years, respectively (Sy et al., 1997). On the basis of
the CFC-11 inventory calculated from the data
obtained during the North Atlantic One-Time Survey, the formation rate of classical Labrador Sea
Water during the high NAO index period was estimated to be 8–11 Sv (Rhein et al., 2001). The CFC
coverage obtained during the WOCE One-Time
Survey will allow us to determine the circulation
pathways of the various components of NADW in
greater detail than has been possible with data
collected in the past. Additionally, it will provide
the data needed to determine the CFC inventories
(and hence formation rates) for the various NADW
components more accurately than has been possible with earlier data.
In addition to the results described above, the
WOCE repeat hydrography programme allowed
us to establish a tracer time series along a section
across the Labrador Sea (AR7W; about 53 to
60°N; 48 to 56°W). This time series (Fig. 5.8.8,
see Plate 5.8.8, p. 428) will be used to put the
results of the One-Time Survey into the context of
the variability observed in the North Atlantic
Ocean, specifically that in the Labrador Sea. It
also will contribute to a better understanding of the
processes that determine the setting of the initial
tracer boundary conditions in areas where major
water masses are formed. Such time series provide
substantial insight into the initial boundary conditions for the individual tracers (e.g. saturation
levels of CFCs), as well as those for derived quantities (e.g. the tritium/
3
He age). Knowledge of the
temporal evolution of these boundary conditions is
important for proper use of tracers in qualitative,
as well as quantitative studies (e.g. integration into
ocean circulation models).
5.8.3.3 Antarctic Intermediate Water renewal
in the Atlantic Ocean
Antarctic Intermediate Water (AAIW) is apparent
as a salinity minimum over much of the Atlantic
(e.g. Emery and Meincke, 1986; Clarke et al.,
Chapter 1.2). It represents one of the principal
contributors to the upper, northward branch of
the global thermohaline circulation (THC) (e.g.
Rintoul, 1991), and is, thus, important for the
freshwater and salinity balance on a global scale.
The AAIW core depth is generally 800–1000 m,
but slopes toward shallower depths in the south
where it is formed by surface forcing at latitudes
close to the Antarctic Circumpolar Current. Details
of AAIW formation are still under investigation
(e.g. Gordon et al., 1992; Hanawa and Talley,
Chapter 5.4).
AAIW is renewed on a decadal time scale.
Therefore, its tracer signatures provide integrated
information on pathways of AAIW, as well as its
renewal rates. Warner and Weiss (1992), using
CFC data collected during TTO/TAS, AJAX and
SAVE, found high and rather uniform CFC concentrations in the AAIW core layer south of about
20°S, and far lower concentrations further north
with a concentration increase toward the western
boundary. They identified the high-CFC pool with
the subtropical anticyclonic gyre, and noted that
part of the gyre waters moved on northward as a
western boundary current. This is in contrast, for
example, to the circulation in the Indian Ocean,
where the most recently ventilated water enters
and circulates in a compact anticyclonic gyre in
the southwestern corner of the Indian Ocean (Fine,
1993).
The expansion of the tracer data base during
WOCE has prompted a more comprehensive tracer
study of AAIW (Rose, 1999). In this study, the
entire layer of AAIW was dealt with. The contributions of water admixed from the adjoining water
masses above and below it (South Atlantic Central
Water, SACW, and upper Circumpolar Deep
Water, uCDW) were separated by means of a multiparameter water mass analysis (Tomczak and
Large, 1989), utilizing the complete hydrographic
data set. Figure 5.8.9 shows the resulting fractions
of pure AAIW versus depth and versus density ( 0 )
along the WOCE WHP sections A8 (11°S), A9
(19°S) and A10 (30°S). A vertical integration of
this fraction in space yielded an effective vertical
extension (layer thickness) of pure AAIW (Fig.
5.8.10). By areal integration of the latter, the total
volume of AAIW was obtained as 14.110
6 km
3
.
The CFC data span about a decade. They were
converted into a quasisynoptic data set using temporal trends derived from the available observations repeated at (essentially) the same location.
CFCs increase strongly above the AAIW layer
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
438
1998) and CFC measurements from WOCE repeat
hydrography cruises in the mid-1990s clearly show
extensive spreading of this newly formed Labrador
Sea Water. Its transit time to the Irminger Sea, the
Newfoundland Basin, and the northeastern Atlantic
was determined to be 6 months, 1 year, and 5
years, respectively (Sy et al., 1997). On the basis of
the CFC-11 inventory calculated from the data
obtained during the North Atlantic One-Time Survey, the formation rate of classical Labrador Sea
Water during the high NAO index period was estimated to be 8–11 Sv (Rhein et al., 2001). The CFC
coverage obtained during the WOCE One-Time
Survey will allow us to determine the circulation
pathways of the various components of NADW in
greater detail than has been possible with data
collected in the past. Additionally, it will provide
the data needed to determine the CFC inventories
(and hence formation rates) for the various NADW
components more accurately than has been possible with earlier data.
In addition to the results described above, the
WOCE repeat hydrography programme allowed
us to establish a tracer time series along a section
across the Labrador Sea (AR7W; about 53 to
60°N; 48 to 56°W). This time series (Fig. 5.8.8,
see Plate 5.8.8, p. 428) will be used to put the
results of the One-Time Survey into the context of
the variability observed in the North Atlantic
Ocean, specifically that in the Labrador Sea. It
also will contribute to a better understanding of the
processes that determine the setting of the initial
tracer boundary conditions in areas where major
water masses are formed. Such time series provide
substantial insight into the initial boundary conditions for the individual tracers (e.g. saturation
levels of CFCs), as well as those for derived quantities (e.g. the tritium/
3
He age). Knowledge of the
temporal evolution of these boundary conditions is
important for proper use of tracers in qualitative,
as well as quantitative studies (e.g. integration into
ocean circulation models).
5.8.3.3 Antarctic Intermediate Water renewal
in the Atlantic Ocean
Antarctic Intermediate Water (AAIW) is apparent
as a salinity minimum over much of the Atlantic
(e.g. Emery and Meincke, 1986; Clarke et al.,
Chapter 1.2). It represents one of the principal
contributors to the upper, northward branch of
the global thermohaline circulation (THC) (e.g.
Rintoul, 1991), and is, thus, important for the
freshwater and salinity balance on a global scale.
The AAIW core depth is generally 800–1000 m,
but slopes toward shallower depths in the south
where it is formed by surface forcing at latitudes
close to the Antarctic Circumpolar Current. Details
of AAIW formation are still under investigation
(e.g. Gordon et al., 1992; Hanawa and Talley,
Chapter 5.4).
AAIW is renewed on a decadal time scale.
Therefore, its tracer signatures provide integrated
information on pathways of AAIW, as well as its
renewal rates. Warner and Weiss (1992), using
CFC data collected during TTO/TAS, AJAX and
SAVE, found high and rather uniform CFC concentrations in the AAIW core layer south of about
20°S, and far lower concentrations further north
with a concentration increase toward the western
boundary. They identified the high-CFC pool with
the subtropical anticyclonic gyre, and noted that
part of the gyre waters moved on northward as a
western boundary current. This is in contrast, for
example, to the circulation in the Indian Ocean,
where the most recently ventilated water enters
and circulates in a compact anticyclonic gyre in
the southwestern corner of the Indian Ocean (Fine,
1993).
The expansion of the tracer data base during
WOCE has prompted a more comprehensive tracer
study of AAIW (Rose, 1999). In this study, the
entire layer of AAIW was dealt with. The contributions of water admixed from the adjoining water
masses above and below it (South Atlantic Central
Water, SACW, and upper Circumpolar Deep
Water, uCDW) were separated by means of a multiparameter water mass analysis (Tomczak and
Large, 1989), utilizing the complete hydrographic
data set. Figure 5.8.9 shows the resulting fractions
of pure AAIW versus depth and versus density ( 0 )
along the WOCE WHP sections A8 (11°S), A9
(19°S) and A10 (30°S). A vertical integration of
this fraction in space yielded an effective vertical
extension (layer thickness) of pure AAIW (Fig.
5.8.10). By areal integration of the latter, the total
volume of AAIW was obtained as 14.110
6 km
3
.
The CFC data span about a decade. They were
converted into a quasisynoptic data set using temporal trends derived from the available observations repeated at (essentially) the same location.
CFCs increase strongly above the AAIW layer
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
438