using data near Bermuda dating back to the
Challenger voyage in 1873 and including the
then 4-year time series at the Panulirus station
(1954–58). Ebbesmeyer and Lindstrom (1986)
showed that newly formed NASTMW may persist
for several years within the Gulf Stream recirculation. On the other hand, within the relative
stability of Eighteen Degree Water existence
and properties, variations are clear (Talley and
Rayner, 1982; Talley, 1996) – thickness changes
reflect variations in formation rate while temperature and salinity changes also reflect changes in
surface forcing and possibly exchange with other
regions, as reviewed below (Section 5.4.4).
The Madeira Mode Water (MMW; Siedler et
al., 1987; earlier observations by Käse et al., 1985)
is the archetype of the relatively low-density subtropical mode waters of the eastern subtropical
gyres. Using hydrographic and historical XBT
(eXpendable BathyThermograph) data, Siedler et al.
documented the existence of this mode water,
which is clearly distinct from the Eighteen Degree
Water of the western subtropical gyre. It is associated with the warm side of the Azores Front and is
offshore of the coastal upwelling area. The MMW
has temperature and potential density ranges of
16–18°C and 26.5–26.8 . Winter mixed layers in
its formation region are about 200 m thick (Käse
et al., 1985). Although the MMW almost disappears as a thick mode by the end of summer
(Siedler et al., 1987), it is advected southwestward
from its formation and joins the thermocline as
part of the North Atlantic Central Water. This
layer has been the focus of intensive investigations
into the subduction process, in which the water
was observed from winter outcrop to restratification, deepening and potential vorticity homogenization (Joyce et al., 1998).
Mode waters of higher density in the northern
subtropical gyre and the subpolar gyre of the
North Atlantic were documented by McCartney
and Talley (1982), and called Subpolar Mode
Water (SPMW), with a density range of 26.9
east of Newfoundland to 27.75, in the Labrador
Sea. The very smooth, broad-scale description of
the SPMW in that first paper suggested that
SPMW originates as thick layers at 14–15°C in the
North Atlantic Current loop. The concept was
that these layers are advected eastward south of
the North Atlantic Current to the eastern Atlantic,
becoming cooler and denser along the path. These
11–12°C SPMWs were then thought to split to the
south and north, with the southward flow entering
the subtropical gyre thermocline (McCartney,
1982). These southward flowing SPMWs are often
called Eastern North Atlantic Water (e.g. Harvey,
1982; Pollard et al., 1996) The northward flow
becomes the inflowing warm surface water of the
subpolar gyre. These northward-flowing SPMWs
continue to cool and increase in density, with 8°C
water found east of Iceland. This SPMW then
splits, with a portion entering the Norwegian Sea
as the main part of the warm Atlantic inflow to the
Arctic, and hence the precursor to North Atlantic
Deep Water formation. The remainder was thought
to circulate westward past Iceland into the
Irminger Sea and then on into the Labrador Sea,
eventually cooling enough to become Labrador Sea
Water (Talley and McCartney, 1982).
There is a reason however to consider major
modifications to this picture of the SPMW formation, transformation and circulation. The portion
of the SPMW that is south of the North Atlantic
Current and the 11–12°C water of the northeastern subtropical gyre are really mode waters of the
northern subtropical gyre, located south of the
wind-stress curl gyre boundary. In this sense, these
waters are similar to the North Pacific Central
Mode Water described by Nakamura (1996a) and
Suga et al. (1997). The relative amount of the
SPMW that turns northward to continue transformation to higher density has not yet been quantified well. Other modifications to the large-scale
picture of SPMW transformation are emerging as
well (Talley, 1999b): the Subarctic Front, which is
the extension of the North Atlantic current, turns
northeastward between the Rekyjanes Ridge and
Rockall Plateau. It separates SPMWs of the eastern and western subpolar gyre. A connection
between the eastern and western SPMWs is not
clear since strong, permanent westward surface
flow just south of Iceland is not apparent in the
climatological circulation. The warmer SPMWs of
the eastern subpolar gyre feed the Norwegian Sea.
The origin of the colder SPMWs of the western
subpolar gyre, which feed the Labrador Sea convection, are not as clear, but may come from a different part of the North Atlantic Current/Subarctic
Front. This region is under intensive study as one
of the last WOCE process studies, leading into a
CLIVAR (Climate Variability and Predictability)
study of decadal and centennial change.
5.4 Mode Waters
379
Hanawa and Talley
Challenger voyage in 1873 and including the
then 4-year time series at the Panulirus station
(1954–58). Ebbesmeyer and Lindstrom (1986)
showed that newly formed NASTMW may persist
for several years within the Gulf Stream recirculation. On the other hand, within the relative
stability of Eighteen Degree Water existence
and properties, variations are clear (Talley and
Rayner, 1982; Talley, 1996) – thickness changes
reflect variations in formation rate while temperature and salinity changes also reflect changes in
surface forcing and possibly exchange with other
regions, as reviewed below (Section 5.4.4).
The Madeira Mode Water (MMW; Siedler et
al., 1987; earlier observations by Käse et al., 1985)
is the archetype of the relatively low-density subtropical mode waters of the eastern subtropical
gyres. Using hydrographic and historical XBT
(eXpendable BathyThermograph) data, Siedler et al.
documented the existence of this mode water,
which is clearly distinct from the Eighteen Degree
Water of the western subtropical gyre. It is associated with the warm side of the Azores Front and is
offshore of the coastal upwelling area. The MMW
has temperature and potential density ranges of
16–18°C and 26.5–26.8 . Winter mixed layers in
its formation region are about 200 m thick (Käse
et al., 1985). Although the MMW almost disappears as a thick mode by the end of summer
(Siedler et al., 1987), it is advected southwestward
from its formation and joins the thermocline as
part of the North Atlantic Central Water. This
layer has been the focus of intensive investigations
into the subduction process, in which the water
was observed from winter outcrop to restratification, deepening and potential vorticity homogenization (Joyce et al., 1998).
Mode waters of higher density in the northern
subtropical gyre and the subpolar gyre of the
North Atlantic were documented by McCartney
and Talley (1982), and called Subpolar Mode
Water (SPMW), with a density range of 26.9
east of Newfoundland to 27.75, in the Labrador
Sea. The very smooth, broad-scale description of
the SPMW in that first paper suggested that
SPMW originates as thick layers at 14–15°C in the
North Atlantic Current loop. The concept was
that these layers are advected eastward south of
the North Atlantic Current to the eastern Atlantic,
becoming cooler and denser along the path. These
11–12°C SPMWs were then thought to split to the
south and north, with the southward flow entering
the subtropical gyre thermocline (McCartney,
1982). These southward flowing SPMWs are often
called Eastern North Atlantic Water (e.g. Harvey,
1982; Pollard et al., 1996) The northward flow
becomes the inflowing warm surface water of the
subpolar gyre. These northward-flowing SPMWs
continue to cool and increase in density, with 8°C
water found east of Iceland. This SPMW then
splits, with a portion entering the Norwegian Sea
as the main part of the warm Atlantic inflow to the
Arctic, and hence the precursor to North Atlantic
Deep Water formation. The remainder was thought
to circulate westward past Iceland into the
Irminger Sea and then on into the Labrador Sea,
eventually cooling enough to become Labrador Sea
Water (Talley and McCartney, 1982).
There is a reason however to consider major
modifications to this picture of the SPMW formation, transformation and circulation. The portion
of the SPMW that is south of the North Atlantic
Current and the 11–12°C water of the northeastern subtropical gyre are really mode waters of the
northern subtropical gyre, located south of the
wind-stress curl gyre boundary. In this sense, these
waters are similar to the North Pacific Central
Mode Water described by Nakamura (1996a) and
Suga et al. (1997). The relative amount of the
SPMW that turns northward to continue transformation to higher density has not yet been quantified well. Other modifications to the large-scale
picture of SPMW transformation are emerging as
well (Talley, 1999b): the Subarctic Front, which is
the extension of the North Atlantic current, turns
northeastward between the Rekyjanes Ridge and
Rockall Plateau. It separates SPMWs of the eastern and western subpolar gyre. A connection
between the eastern and western SPMWs is not
clear since strong, permanent westward surface
flow just south of Iceland is not apparent in the
climatological circulation. The warmer SPMWs of
the eastern subpolar gyre feed the Norwegian Sea.
The origin of the colder SPMWs of the western
subpolar gyre, which feed the Labrador Sea convection, are not as clear, but may come from a different part of the North Atlantic Current/Subarctic
Front. This region is under intensive study as one
of the last WOCE process studies, leading into a
CLIVAR (Climate Variability and Predictability)
study of decadal and centennial change.
5.4 Mode Waters
379
Hanawa and Talley