near 30°S at the southern edge of the Brazil Basin,
strong (30 cm s
91 ) northward flow in the North
Brazil Undercurrent, and strong zonal flows
with little organized meridional motion in the
equatorial zone.
The last WOCE observations examined the
North Atlantic beginning in 1997. The float work
included extensive deployments of temperature
and salinity profiling autonomous floats from the
subpolar gyre to the tropics. These were supplemented by intermediate-depth RAFOS floats in the
North Atlantic Current and the northeastern
North Atlantic. This was an internationally coordinated effort by groups from France, Germany,
the UK and the US. In the subpolar gyre the scientific goal was to follow the warm-to-cold water
transformation from the North Atlantic Current
through the subpolar gyre, the process of intermediate-water formation and the subsequent spreading of Labrador Sea Water throughout the North
Atlantic. Other scientific foci were formation of
18°C water in the western subtropical gyre as well
as pathways of cross-equatorial flow and waterproperty variability in the tropics.
To date the densest gyre-scale float coverage
anywhere was in the subpolar gyre. Over 200
autonomous profiling floats were deployed,
mainly in the western two-thirds of the gyre. The
North Atlantic Current and eastern basin were
populated mainly with acoustic floats, as summarized by Bower et al. (2000). The results from the
various surveys in different regions have not been
fully integrated, but already a new, and in several
ways surprising, picture of the mean intermediatedepth flow has emerged.
Lavender et al. (2000a) used a very dense array
of 180 autonomous floats to construct the map
of mean flow near 700 m shown in Fig. 3.2.8 (see
Plate 3.2.8, p. 172) covering the Labrador and
Irminger Basins (see Lazier et al., Chapter 5.5;
Saunders, Chapter 5.6). In addition to the expected
peripheral current under the East and West
Greenland Currents and the Labrador Current, this
discloses some surprises. First, the plot and trajectories of individual floats indicate that most of the
flow in the deep Labrador Current turns back into
the subpolar gyre rather than joining the deep
western boundary current as it flows southwest.
Hydrographic tracers make it clear that Labrador
Sea Water does continue south of Flemish Cap and
the Grand Banks but the float observations raise
the question of how this pathway is maintained.
Equally surprising, offshore of the peripheral current there is a countercurrent that flows anticyclonically around both the Labrador and Irminger
Basins and provides a short, direct path for newly
formed Labrador Sea Water to reach the Irminger
Basin. These same observations show that 40%
of newly formed intermediate water leaves the
Labrador Basin within 1 year and that this water
can reach the eastern basin in 2–3 years by either
the pathway southward in the Labrador Current
and then east, or in the countercurrent and then
out of the Irminger Basin. By averaging float velocity along isobaths, Fischer and Schott (2000)
inferred, from 15 profiling floats injected into the
deep western boundary current, the structure and
the variability of the deep Labrador Current at
1500 m depth. They found this current to be about
100 km wide with an averaged core of 18 cm s
91
.
These floats also confirmed elements of the two
basic pathways by which freshly ventilated
Labrador Sea Water is advected away from its
formation zone.
WOCE activities in the North Atlantic also represented the first widespread use of profiling
autonomous floats to report repeated profiles of
temperature and salinity. Profiling floats were
deployed in the western subtropical gyre to examine formation and circulation of 18°C water and
in the tropical Atlantic to examine the hydrological cycle, pathways of cross-equatorial flow and
the generation of tropical sea surface temperature
anomalies. Autonomous CTD (ConductivityTemperature-Depth) profilers from these deployments and the subpolar array now regularly deliver
time series of temperature and salinity from most of
the North and tropical Atlantic. This is extending
regular profiling into regions not visited by shipping, like the Labrador Sea, and into severe winter
conditions when ocean processes are particularly
vigorous but there are few other observations.
An example of the profile data now routinely
available is the 2-year time series of salinity in
Fig. 3.2.9. This is from a float that spent most of
its life in the interior of the Labrador Sea, not far
from Weather Station B (57°N, 53°W), where
Labrador Sea Water is formed by deep convection.
The figure shows winter cooling overcoming salinity stratification to drive deep convection followed
by rapid spring restratification by fresh water.
Models suggest that convective plumes mainly mix
SECTION 3 NEW WAYS OF OBSERVING THE OCEAN
134
strong (30 cm s
91 ) northward flow in the North
Brazil Undercurrent, and strong zonal flows
with little organized meridional motion in the
equatorial zone.
The last WOCE observations examined the
North Atlantic beginning in 1997. The float work
included extensive deployments of temperature
and salinity profiling autonomous floats from the
subpolar gyre to the tropics. These were supplemented by intermediate-depth RAFOS floats in the
North Atlantic Current and the northeastern
North Atlantic. This was an internationally coordinated effort by groups from France, Germany,
the UK and the US. In the subpolar gyre the scientific goal was to follow the warm-to-cold water
transformation from the North Atlantic Current
through the subpolar gyre, the process of intermediate-water formation and the subsequent spreading of Labrador Sea Water throughout the North
Atlantic. Other scientific foci were formation of
18°C water in the western subtropical gyre as well
as pathways of cross-equatorial flow and waterproperty variability in the tropics.
To date the densest gyre-scale float coverage
anywhere was in the subpolar gyre. Over 200
autonomous profiling floats were deployed,
mainly in the western two-thirds of the gyre. The
North Atlantic Current and eastern basin were
populated mainly with acoustic floats, as summarized by Bower et al. (2000). The results from the
various surveys in different regions have not been
fully integrated, but already a new, and in several
ways surprising, picture of the mean intermediatedepth flow has emerged.
Lavender et al. (2000a) used a very dense array
of 180 autonomous floats to construct the map
of mean flow near 700 m shown in Fig. 3.2.8 (see
Plate 3.2.8, p. 172) covering the Labrador and
Irminger Basins (see Lazier et al., Chapter 5.5;
Saunders, Chapter 5.6). In addition to the expected
peripheral current under the East and West
Greenland Currents and the Labrador Current, this
discloses some surprises. First, the plot and trajectories of individual floats indicate that most of the
flow in the deep Labrador Current turns back into
the subpolar gyre rather than joining the deep
western boundary current as it flows southwest.
Hydrographic tracers make it clear that Labrador
Sea Water does continue south of Flemish Cap and
the Grand Banks but the float observations raise
the question of how this pathway is maintained.
Equally surprising, offshore of the peripheral current there is a countercurrent that flows anticyclonically around both the Labrador and Irminger
Basins and provides a short, direct path for newly
formed Labrador Sea Water to reach the Irminger
Basin. These same observations show that 40%
of newly formed intermediate water leaves the
Labrador Basin within 1 year and that this water
can reach the eastern basin in 2–3 years by either
the pathway southward in the Labrador Current
and then east, or in the countercurrent and then
out of the Irminger Basin. By averaging float velocity along isobaths, Fischer and Schott (2000)
inferred, from 15 profiling floats injected into the
deep western boundary current, the structure and
the variability of the deep Labrador Current at
1500 m depth. They found this current to be about
100 km wide with an averaged core of 18 cm s
91
.
These floats also confirmed elements of the two
basic pathways by which freshly ventilated
Labrador Sea Water is advected away from its
formation zone.
WOCE activities in the North Atlantic also represented the first widespread use of profiling
autonomous floats to report repeated profiles of
temperature and salinity. Profiling floats were
deployed in the western subtropical gyre to examine formation and circulation of 18°C water and
in the tropical Atlantic to examine the hydrological cycle, pathways of cross-equatorial flow and
the generation of tropical sea surface temperature
anomalies. Autonomous CTD (ConductivityTemperature-Depth) profilers from these deployments and the subpolar array now regularly deliver
time series of temperature and salinity from most of
the North and tropical Atlantic. This is extending
regular profiling into regions not visited by shipping, like the Labrador Sea, and into severe winter
conditions when ocean processes are particularly
vigorous but there are few other observations.
An example of the profile data now routinely
available is the 2-year time series of salinity in
Fig. 3.2.9. This is from a float that spent most of
its life in the interior of the Labrador Sea, not far
from Weather Station B (57°N, 53°W), where
Labrador Sea Water is formed by deep convection.
The figure shows winter cooling overcoming salinity stratification to drive deep convection followed
by rapid spring restratification by fresh water.
Models suggest that convective plumes mainly mix
SECTION 3 NEW WAYS OF OBSERVING THE OCEAN
134