North Atlantic: the upper NADW maximum at
about 1500 m depth and a deeper maximum in
lower NADW at about 3500 m depth. These features were strongest in the Deep Western Boundary Current (DWBC) (Fine and Molinari, 1988;
Smethie, 1993). Pickart (1992) showed that the
upper maxima occurred at a density too low to be
categorized as classical Labrador Sea Water and
referred to this water mass as upper Labrador Sea
Water. Using measurements collected in the early
1990s, Pickart et al. (1996) presented evidence that
the source of this water was winter convection on
the northern side of the North Atlantic Current
in the southern Labrador Sea. WOCE measurements in the subtropical North Atlantic in the early
1990s (Rhein et al., 1995) revealed the same two
maxima but at much lower concentrations. Both
the upper NADW maximum and lower NADW
maximum were observed to extend eastward along
the equator and to cross the equator along the
western boundary into the South Atlantic. The
upper maximum was observed as far as 10°S and
the deep maximum as far as 5°S.
In the western subtropical North Atlantic, relatively high CFC concentrations observed in the
interior basin revealed recirculation of the DWBC
water in the deep Gulf Stream recirculation gyre
(Hogg et al., 1986; Pickart and Smethie, 1993), the
Abaco gyre (Johns et al., 1997a) and the Guyana
gyre (Molinari et al., 1992). Water mass ages in the
DWBC derived from CFC-11/CFC-12 ratios indicated a spreading rate of 1–2 cms
91 (Weiss et al.,
1985; Smethie, 1993; Smethie et al., 2000). These
rates are considerably lower than those calculated
from current meter measurements in the DWBC.
This difference demonstrates the importance of
deep recirculation gyres in ventilating the interior
and slowing the overall equatorial spreading rate of
NADW from its northern source regions. Streamtube models of CFC concentrations in the DWBC
that include exchange with the interior (Pickart
et al., 1989; Rhein, 1994) yield current speeds of
5–10 cm s
91
. These values are in agreement with
results from long-term current meter measurements
in the DWBC. A detailed description of the evolution of the CFC signal in the North Atlantic is presented by Smethie et al. (2000).
In addition to providing information on spreading pathways and time scales, CFC inventories
provide quantitative information on rates of water
mass formation. The total amount of CFCs in a
particular water mass is a direct reflection of how
rapidly that water mass formed from near-surface
water and the formation rate can be calculated
from the CFC inventory of the water mass. Calculations based on pre-WOCE CFC-11 data reveal
a total NADW formation rate about 17 Sv, which
includes the first estimate of 2.2 Sv for upper
Labrador Sea Water (Smethie and Fine, 2001).
CFC concentrations in bottom water in the
South Atlantic in the late 1980s were much lower
than in the North Atlantic. The CFC distribution
in the Argentine Basin showed the entry of AABW
through gaps in the Falkland Ridge. They also
indicated a split of this flow with one branch flowing westward to the western boundary and the
other flowing to the northeast and then into the
interior (Warner et al., 1990; Coles et al., 1996).
The CFC signal had just begun to enter the Brazil
Basin with AABW, and by 1992 was barely
detectable at 5°S (Rhein et al., 1995). In NADW
the upper CFC maximum was barely detectable at
19°S in the DWBC and the lower NADW CFC
maximum extended only to 10°S. The CFC concentration was undetectable in the Angola Basin
at this time.
The WOCE programme was carried out at
different times in the North and South Atlantic
oceans. The North Atlantic programme was performed between 1996 and 1998 with 1997 being
the major field year. The South Atlantic programme, including the tropical Atlantic, was carried out primarily between 1993 and 1996, with a
portion of the field work being done as part of the
Deep Basin Experiment and as part of the German
WOCE programme between 1990 and 1996.
The zonal CFC distribution in the Argentine
Basin was measured in 1993 (WOCE line A11;
about 45 to 30°S; 60°W to 20°E) and was similar
to that measured during the SAVE programme, but
concentrations were higher. Highest concentrations
in AABW were found overlying the western flank of
the Mid-Atlantic Ridge. These data, together with
other evidence, revealed that the primary inflow of
AABW through gaps in the Falkland Escarpment
flowed to the northeast and then back to the
west in a deep anticyclonic gyre (Smythe-Wright
and Boswell, 1998), confirming the SAVE observations. A meridional section through the western
South Atlantic (WOCE line A17; about 10°N to
51°S; 60 to 30°W) parallel to the South American
coastline (Fig. 5.8.5, see Plate 5.8.5, p. 428)
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
436
about 1500 m depth and a deeper maximum in
lower NADW at about 3500 m depth. These features were strongest in the Deep Western Boundary Current (DWBC) (Fine and Molinari, 1988;
Smethie, 1993). Pickart (1992) showed that the
upper maxima occurred at a density too low to be
categorized as classical Labrador Sea Water and
referred to this water mass as upper Labrador Sea
Water. Using measurements collected in the early
1990s, Pickart et al. (1996) presented evidence that
the source of this water was winter convection on
the northern side of the North Atlantic Current
in the southern Labrador Sea. WOCE measurements in the subtropical North Atlantic in the early
1990s (Rhein et al., 1995) revealed the same two
maxima but at much lower concentrations. Both
the upper NADW maximum and lower NADW
maximum were observed to extend eastward along
the equator and to cross the equator along the
western boundary into the South Atlantic. The
upper maximum was observed as far as 10°S and
the deep maximum as far as 5°S.
In the western subtropical North Atlantic, relatively high CFC concentrations observed in the
interior basin revealed recirculation of the DWBC
water in the deep Gulf Stream recirculation gyre
(Hogg et al., 1986; Pickart and Smethie, 1993), the
Abaco gyre (Johns et al., 1997a) and the Guyana
gyre (Molinari et al., 1992). Water mass ages in the
DWBC derived from CFC-11/CFC-12 ratios indicated a spreading rate of 1–2 cms
91 (Weiss et al.,
1985; Smethie, 1993; Smethie et al., 2000). These
rates are considerably lower than those calculated
from current meter measurements in the DWBC.
This difference demonstrates the importance of
deep recirculation gyres in ventilating the interior
and slowing the overall equatorial spreading rate of
NADW from its northern source regions. Streamtube models of CFC concentrations in the DWBC
that include exchange with the interior (Pickart
et al., 1989; Rhein, 1994) yield current speeds of
5–10 cm s
91
. These values are in agreement with
results from long-term current meter measurements
in the DWBC. A detailed description of the evolution of the CFC signal in the North Atlantic is presented by Smethie et al. (2000).
In addition to providing information on spreading pathways and time scales, CFC inventories
provide quantitative information on rates of water
mass formation. The total amount of CFCs in a
particular water mass is a direct reflection of how
rapidly that water mass formed from near-surface
water and the formation rate can be calculated
from the CFC inventory of the water mass. Calculations based on pre-WOCE CFC-11 data reveal
a total NADW formation rate about 17 Sv, which
includes the first estimate of 2.2 Sv for upper
Labrador Sea Water (Smethie and Fine, 2001).
CFC concentrations in bottom water in the
South Atlantic in the late 1980s were much lower
than in the North Atlantic. The CFC distribution
in the Argentine Basin showed the entry of AABW
through gaps in the Falkland Ridge. They also
indicated a split of this flow with one branch flowing westward to the western boundary and the
other flowing to the northeast and then into the
interior (Warner et al., 1990; Coles et al., 1996).
The CFC signal had just begun to enter the Brazil
Basin with AABW, and by 1992 was barely
detectable at 5°S (Rhein et al., 1995). In NADW
the upper CFC maximum was barely detectable at
19°S in the DWBC and the lower NADW CFC
maximum extended only to 10°S. The CFC concentration was undetectable in the Angola Basin
at this time.
The WOCE programme was carried out at
different times in the North and South Atlantic
oceans. The North Atlantic programme was performed between 1996 and 1998 with 1997 being
the major field year. The South Atlantic programme, including the tropical Atlantic, was carried out primarily between 1993 and 1996, with a
portion of the field work being done as part of the
Deep Basin Experiment and as part of the German
WOCE programme between 1990 and 1996.
The zonal CFC distribution in the Argentine
Basin was measured in 1993 (WOCE line A11;
about 45 to 30°S; 60°W to 20°E) and was similar
to that measured during the SAVE programme, but
concentrations were higher. Highest concentrations
in AABW were found overlying the western flank of
the Mid-Atlantic Ridge. These data, together with
other evidence, revealed that the primary inflow of
AABW through gaps in the Falkland Escarpment
flowed to the northeast and then back to the
west in a deep anticyclonic gyre (Smythe-Wright
and Boswell, 1998), confirming the SAVE observations. A meridional section through the western
South Atlantic (WOCE line A17; about 10°N to
51°S; 60 to 30°W) parallel to the South American
coastline (Fig. 5.8.5, see Plate 5.8.5, p. 428)
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
436