not changed, the buoyancy is not changed and the
depth of convection, for a given heat loss, will not
be altered.
The foregoing one-dimensional view of convection is expanded to two dimensions in the contour
plot of salinity (Fig. 5.5.4, see Plate 5.5.4, p. 428)
obtained in the Labrador Sea in July following the
exceptionally cold winter of 1992–93. The water
mass resulting from convection is the large volume
of homogeneous water lying between 360 and
800 km on the horizontal scale and 500 and
2300 m in the vertical. Because of its large volume
and unique temperature and salinity properties that
are renewed in the Labrador Sea, the water mass
has its own name; the Labrador Sea Water (LSW).
The newly formed Labrador Sea Water is a
mixture of all the water down to 2300 m including
the water in contact with the atmosphere at the
surface. In these uppermost layers the concentration of gases such as oxygen, carbon dioxide,
tritium and chlorofluorocarbons (CFCs) are at or
near equilibrium with the atmosphere. By transporting these gases down from the upper layers of
the ocean to intermediate depths, convection provides a mechanism to ventilate the deeper layers,
which is one of the most important consequences
of deep convection. Dissolved oxygen, for example,
is slowly used up in the deep ocean by biological
processes and would eventually vanish without the
renewal via convection. Also most of the carbon
dioxide ever put in the atmosphere by volcanoes
since the formation of the earth became dissolved in
the ocean and is now contained in sediments in the
bottom of the ocean. As combustion of fossil fuels
over the earth raises the carbon dioxide content of
the atmosphere it is important to understand the
rate this gas is entering the deeper layers of the
ocean through processes such as deep convection.
The upper layer (0–500 m) in the central part of
the salinity section in Figure 5.5.4 (see Plate 5.5.4,
p. 428) is clearly not as well mixed as the layer
between 500 and 2300 m. This is because the
observations were obtained in July about 3 months
after deep convection ceased at the end of the
cooling season about 1 April. Since that time the
surface layer has been flooded with fresh water
derived from melting ice and river runoff. As well,
the layer below this low-salinity surface layer, to
about 500 m, has been invaded by higher-salinity
water from the right, that is, the northeast. This
more saline water is the water found under the
convection layer in Figure 5.5.3 and is known as
the Irminger Water (IW) because it is transported
into the Labrador Sea from the Irminger Sea in the
East and West Greenland Currents that lie over
the continental shelf and slope (Fig. 5.5.1). On the
left or southwest part of the section there is again
a salinity maximum at about 300 m over the
Labrador continental slope, which also tends to
invade the central region. This is again Irminger
Water, which has been transported around the
Labrador Sea in the West Greenland and Labrador
Currents. Beneath the LSW, between 2300 and
3300 m, lies a water mass identified by the salinity
maximum at about 2800 m. This is the North East
Atlantic Deep Water (NEADW). It originates in
the eastern basin of the North Atlantic and flows
into the western basin through gaps in the MidAtlantic Ridge. At the bottom of the section is the
Denmark Strait Overflow Water (DSOW) with a
slightly lower salinity than in the NEADW above.
DSOW is the densest water in the northern North
Atlantic; it originates in the seas north of Iceland
and comes to the Labrador Sea after flowing over
the sill in Denmark Strait between Greenland and
Iceland (see Saunders, Chapter 5.6). The last water
mass of note in the section is the low-salinity
water over the Labrador continental shelf that
flows south out of Baffin Bay in the Baffin Island
Current and the Labrador Current. A similar band
of low-salinity water of Arctic origin lies over the
Greenland continental shelf but it was covered by
heavy ice in July 1993 and not sampled when the
rest of these data were collected. The rapid transition between the low-salinity waters over the
shelves and the higher-salinity waters of the sea’s
interior mark the baroclinic currents lying over
the upper part of the continental slopes, i.e. the
Labrador and West Greenland Currents.
In the Atlantic Ocean, ventilation of the intermediate layers by deep convection occurs only in
the Labrador Sea. The historical notion has been
that the overturning occurs primarily in the western interior of the basin. Indeed, this is where it
has been observed the most (Clarke and Gascard,
1983; Wallace and Lazier, 1988; Lilly et al., 1999).
Recent evidence has demonstrated, however, that
convection can (at least on occasion) occur closer
to the western boundary, directly into the rim current system (Pickart et al., 2000a). Not surprisingly, this boundary current water mass product
is less dense than the interior water mass, and
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
390
depth of convection, for a given heat loss, will not
be altered.
The foregoing one-dimensional view of convection is expanded to two dimensions in the contour
plot of salinity (Fig. 5.5.4, see Plate 5.5.4, p. 428)
obtained in the Labrador Sea in July following the
exceptionally cold winter of 1992–93. The water
mass resulting from convection is the large volume
of homogeneous water lying between 360 and
800 km on the horizontal scale and 500 and
2300 m in the vertical. Because of its large volume
and unique temperature and salinity properties that
are renewed in the Labrador Sea, the water mass
has its own name; the Labrador Sea Water (LSW).
The newly formed Labrador Sea Water is a
mixture of all the water down to 2300 m including
the water in contact with the atmosphere at the
surface. In these uppermost layers the concentration of gases such as oxygen, carbon dioxide,
tritium and chlorofluorocarbons (CFCs) are at or
near equilibrium with the atmosphere. By transporting these gases down from the upper layers of
the ocean to intermediate depths, convection provides a mechanism to ventilate the deeper layers,
which is one of the most important consequences
of deep convection. Dissolved oxygen, for example,
is slowly used up in the deep ocean by biological
processes and would eventually vanish without the
renewal via convection. Also most of the carbon
dioxide ever put in the atmosphere by volcanoes
since the formation of the earth became dissolved in
the ocean and is now contained in sediments in the
bottom of the ocean. As combustion of fossil fuels
over the earth raises the carbon dioxide content of
the atmosphere it is important to understand the
rate this gas is entering the deeper layers of the
ocean through processes such as deep convection.
The upper layer (0–500 m) in the central part of
the salinity section in Figure 5.5.4 (see Plate 5.5.4,
p. 428) is clearly not as well mixed as the layer
between 500 and 2300 m. This is because the
observations were obtained in July about 3 months
after deep convection ceased at the end of the
cooling season about 1 April. Since that time the
surface layer has been flooded with fresh water
derived from melting ice and river runoff. As well,
the layer below this low-salinity surface layer, to
about 500 m, has been invaded by higher-salinity
water from the right, that is, the northeast. This
more saline water is the water found under the
convection layer in Figure 5.5.3 and is known as
the Irminger Water (IW) because it is transported
into the Labrador Sea from the Irminger Sea in the
East and West Greenland Currents that lie over
the continental shelf and slope (Fig. 5.5.1). On the
left or southwest part of the section there is again
a salinity maximum at about 300 m over the
Labrador continental slope, which also tends to
invade the central region. This is again Irminger
Water, which has been transported around the
Labrador Sea in the West Greenland and Labrador
Currents. Beneath the LSW, between 2300 and
3300 m, lies a water mass identified by the salinity
maximum at about 2800 m. This is the North East
Atlantic Deep Water (NEADW). It originates in
the eastern basin of the North Atlantic and flows
into the western basin through gaps in the MidAtlantic Ridge. At the bottom of the section is the
Denmark Strait Overflow Water (DSOW) with a
slightly lower salinity than in the NEADW above.
DSOW is the densest water in the northern North
Atlantic; it originates in the seas north of Iceland
and comes to the Labrador Sea after flowing over
the sill in Denmark Strait between Greenland and
Iceland (see Saunders, Chapter 5.6). The last water
mass of note in the section is the low-salinity
water over the Labrador continental shelf that
flows south out of Baffin Bay in the Baffin Island
Current and the Labrador Current. A similar band
of low-salinity water of Arctic origin lies over the
Greenland continental shelf but it was covered by
heavy ice in July 1993 and not sampled when the
rest of these data were collected. The rapid transition between the low-salinity waters over the
shelves and the higher-salinity waters of the sea’s
interior mark the baroclinic currents lying over
the upper part of the continental slopes, i.e. the
Labrador and West Greenland Currents.
In the Atlantic Ocean, ventilation of the intermediate layers by deep convection occurs only in
the Labrador Sea. The historical notion has been
that the overturning occurs primarily in the western interior of the basin. Indeed, this is where it
has been observed the most (Clarke and Gascard,
1983; Wallace and Lazier, 1988; Lilly et al., 1999).
Recent evidence has demonstrated, however, that
convection can (at least on occasion) occur closer
to the western boundary, directly into the rim current system (Pickart et al., 2000a). Not surprisingly, this boundary current water mass product
is less dense than the interior water mass, and
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
390