through Drake Passage on the northern side of the
Subantarctic Front. Some modification of properties
occurs in this region. When the thick layer reaches
the confluence of the Brazil and Malvinas Currents,
it plunges down and spreads out into the South
Atlantic/Indian subtropical gyre as a salinity minimum (McCartney, 1977; Talley, 1996a). An oxygen maximum is associated with the AAIW through
much of the South Atlantic. A pycnostad is also
associated with this new AAIW in the southwestern
South Atlantic, but is much weaker and more geographically restricted than in the South Pacific, presumably because of vigorous mixing at the western
boundary as it enters the Atlantic. There are no
sources of AAIW elsewhere in the Atlantic and
Indian Oceans, as is apparent from the changes in
salinity, oxygen and potential vorticity away from
the Brazil/Malvinas confluence. Major modification
of AAIW occurs through mixing, and the various
AAIWs referred to in the literature reflect these
regional mixings rather than new ventilation.
5.4.4 Temporal variability of mode water
properties and distribution
A marked characteristic of mode waters is their
apparent stability in properties and location
(Schroeder et al., 1959; Warren, 1972). Thus mode
water distributions, such as shown in Figure 5.4.3a
(see Plate 5.4.3a, p. 428), and approximate core
properties can be mapped using data sets from all
decades. The stability of properties and, indeed, the
interbasin similarity in temperature (although not
density because of interbasin salinity differences)
are remarkable and clearly associated with the
largest scale, longest time-scale wind and buoyancy
forcing. Of course, however, there is some variation in properties of the mode waters, which has
been studied in areas where time series are adequate. These variations in these near-surface water
masses, in temperature, salinity, density and thickness, are linked to surface forcing changes, although
in some cases the connection is not yet obvious. In
this section, we briefly review the temporal variability of mode waters where it has been documented.
5.4.4.1 Seasonal variation
After the formation of mode water in late winter, it
is capped by the seasonal thermocline, due to incident solar radiation. As the seasons progress, mode
water is advected away from the formation area
and sometimes becomes permanently capped. In
order to trace the seasonal evolution of North
Pacific STMW, Suga and Hanawa (1995b) mapped
potential vorticity and deduced that the main body
of NPSTMW formed in the western part of the formation area is advected westward by the Kuroshio
recirculation. During this movement, the NPSTMW
is subject to substantial diapycnal mixing.
The seasonality of NASTMW has not been
investigated to the same degree as NPSTMW. At
Bermuda, which is well downstream of the formation area, seasonality of the mixed layer depths is
clear, with a sudden shoaling of near-surface
isopycnals during late winter followed by a gradual deepening of the isopycnals through the rest of
the year (Talley and Raymer, 1982). However, a
seasonal cycle in mode water properties at this
site, which is some distance from the mode water
outcrop, is not easily apparent.
In the South Pacific STMW, Roemmich and
Cornuelle (1992) showed a large seasonal change
in mode water inventory and properties using seasonal XBT sampling.
The eastern Subtropical Mode Waters of the
North Atlantic and North Pacific have both been
studied at the seasonal cycle. Siedler et al. (1987)
and Hautala and Roemmich (1998) both show
the seasonal outcropping of the Madeira Mode
Water (MMW) and eastern North Pacific STMW
(NPESTMW). Siedler et al. show the advection of
the MMW downstream after capping in late winter and the near disappearance of the mode in late
fall after heating from above and possibly diapycnal mixing. Likewise Hautala and Roemmich
show the strength of the eastern North Pacific
mode just after it is capped at the end of winter,
and its decline in strength as the surface layer
restratifies further through the summer and fall.
The seasonality of other mode waters has not
been documented to our knowledge.
5.4.4.2 Interannual variations
Interannual variability is roughly defined as variability on time scales of 2 to about 7 years, encompassing the El Niño time scale, but not extending
to the decadal time scale.
In the North Pacific STMW, large year-to-year
variabilities in temperature and its distribution
have been described, especially in relation to the
strength of interannual Wintertime East Asian
Monsoon. Monsoon strength is well represented
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
384
Subantarctic Front. Some modification of properties
occurs in this region. When the thick layer reaches
the confluence of the Brazil and Malvinas Currents,
it plunges down and spreads out into the South
Atlantic/Indian subtropical gyre as a salinity minimum (McCartney, 1977; Talley, 1996a). An oxygen maximum is associated with the AAIW through
much of the South Atlantic. A pycnostad is also
associated with this new AAIW in the southwestern
South Atlantic, but is much weaker and more geographically restricted than in the South Pacific, presumably because of vigorous mixing at the western
boundary as it enters the Atlantic. There are no
sources of AAIW elsewhere in the Atlantic and
Indian Oceans, as is apparent from the changes in
salinity, oxygen and potential vorticity away from
the Brazil/Malvinas confluence. Major modification
of AAIW occurs through mixing, and the various
AAIWs referred to in the literature reflect these
regional mixings rather than new ventilation.
5.4.4 Temporal variability of mode water
properties and distribution
A marked characteristic of mode waters is their
apparent stability in properties and location
(Schroeder et al., 1959; Warren, 1972). Thus mode
water distributions, such as shown in Figure 5.4.3a
(see Plate 5.4.3a, p. 428), and approximate core
properties can be mapped using data sets from all
decades. The stability of properties and, indeed, the
interbasin similarity in temperature (although not
density because of interbasin salinity differences)
are remarkable and clearly associated with the
largest scale, longest time-scale wind and buoyancy
forcing. Of course, however, there is some variation in properties of the mode waters, which has
been studied in areas where time series are adequate. These variations in these near-surface water
masses, in temperature, salinity, density and thickness, are linked to surface forcing changes, although
in some cases the connection is not yet obvious. In
this section, we briefly review the temporal variability of mode waters where it has been documented.
5.4.4.1 Seasonal variation
After the formation of mode water in late winter, it
is capped by the seasonal thermocline, due to incident solar radiation. As the seasons progress, mode
water is advected away from the formation area
and sometimes becomes permanently capped. In
order to trace the seasonal evolution of North
Pacific STMW, Suga and Hanawa (1995b) mapped
potential vorticity and deduced that the main body
of NPSTMW formed in the western part of the formation area is advected westward by the Kuroshio
recirculation. During this movement, the NPSTMW
is subject to substantial diapycnal mixing.
The seasonality of NASTMW has not been
investigated to the same degree as NPSTMW. At
Bermuda, which is well downstream of the formation area, seasonality of the mixed layer depths is
clear, with a sudden shoaling of near-surface
isopycnals during late winter followed by a gradual deepening of the isopycnals through the rest of
the year (Talley and Raymer, 1982). However, a
seasonal cycle in mode water properties at this
site, which is some distance from the mode water
outcrop, is not easily apparent.
In the South Pacific STMW, Roemmich and
Cornuelle (1992) showed a large seasonal change
in mode water inventory and properties using seasonal XBT sampling.
The eastern Subtropical Mode Waters of the
North Atlantic and North Pacific have both been
studied at the seasonal cycle. Siedler et al. (1987)
and Hautala and Roemmich (1998) both show
the seasonal outcropping of the Madeira Mode
Water (MMW) and eastern North Pacific STMW
(NPESTMW). Siedler et al. show the advection of
the MMW downstream after capping in late winter and the near disappearance of the mode in late
fall after heating from above and possibly diapycnal mixing. Likewise Hautala and Roemmich
show the strength of the eastern North Pacific
mode just after it is capped at the end of winter,
and its decline in strength as the surface layer
restratifies further through the summer and fall.
The seasonality of other mode waters has not
been documented to our knowledge.
5.4.4.2 Interannual variations
Interannual variability is roughly defined as variability on time scales of 2 to about 7 years, encompassing the El Niño time scale, but not extending
to the decadal time scale.
In the North Pacific STMW, large year-to-year
variabilities in temperature and its distribution
have been described, especially in relation to the
strength of interannual Wintertime East Asian
Monsoon. Monsoon strength is well represented
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
384