into the central basin and new descriptions of
Lagrangian pattern in this region are expected.
With a similar focus on gathering a Lagrangian
description of an important transport process, the
pathway of the North Atlantic’s deep western
boundary current and its interaction with the Gulf
Stream were examined by Bower and Hunt
(2000a,b) using RAFOS floats. They found only a
small amount of intermediate water from the
Labrador Sea to cross under the Gulf Stream into
the subtropical circulation while Nordic Seas overflow waters bifurcate, one branch following the
Gulf Stream offshore while the other remains
attached to the western boundary. The clarity of
these results suggests a similar acoustic-float study
to resolve the question, raised in the subsection
above, of how Labrador Sea Water enters the subtropical gyre.
A number of regional float experiments in the
North Atlantic have examined different elements
of the complex circulation there, both elucidating
specific processes and collectively forming the
backbone of that part of the global array. For
example, Fig. 3.2.11 (see Plate 3.2.11, p. 172),
adopted from Zenk (2000), shows daily-resolved
trajectories of RAFOS floats just east of the
Mid-Atlantic Ridge. These isobaric floats were
deployed after June 1997 at the depth of the
Labrador Sea Water, i.e. roughly at 1500 m. They
demonstrate the transport of this water mass eastward through Charlie-Gibbs Fracture Zone at
about 53°N in more detail. After passing this gap
the pathways bifurcate, one approaching the western extent of the European continental rise while
the other feeds the Iceland Basin. The Icelandic
branch meets Iceland–Scotland Overflow Water at
the northern tip of the Iceland Basin and then
advects the floats southwestward along the flanks
of the Reykjanes Ridge. Particularly along the
Icelandic branch, these continuously tracked trajectories give a quasi-Lagrangian view of the mean
flow patterns in Fig. 3.2.8 (see Plate 3.2.8, p. 172)
that are based on an entirely independent data set.
In another example of regional experiments giving a closer view of patterns smoothed over in
basin-scale coverage, Rossby et al. (2000) present
case studies of selected trajectories from the
Irminger and the Iceland Basins. Despite the
apparent eddy motion they see a rather robust
pattern of float pathways emerging. Rossby’s
isopycnal-following instruments provide explicit
information on advection and mixing along
their pathways. The authors expect particularly
challenging results that will include estimates of
cross-frontal fluxes of heat and oxygen between
the subpolar and the subtropical gyres.
Finally, Lavender et al. (2000b) present results
from a regional deployment of autonomous Vertical Current Meter floats to explore the vertical
motion associated with deep convection. One interesting question addressed was whether a region
of convection is one of general downwelling.
Although the floats (cf. Davis et al., 2000) can
measure vertical motion down to O(3 m day
91
), the
way they are entrained into, and detrained from,
convecting plumes causes them to give a biased
measure of the mean vertical motion, so the results
are equivocal. A perplexing paradox is that the
vertical temperature flux :wЈTЈ9 apparently
carried by the plume scale (quasi-Lagrangian time
scales of less than 88 h) is insufficient to explain the
observed cooling below 400 m depth.
3.2.5 The future
It is clear that the Swallow float has become an
observational tool capable of charting the world
ocean’s subsurface general circulation and providing numerous and economical profiles of ocean
properties. Even though the analysis of WOCE-era
floats has only begun, it is clear that they will have
significant impacts on the field. Floats have provided visualization of basin-scale flows including
intergyre and interbasin transport. In many regions
enough observations have been accumulated to
produce statistically reliable descriptions of absolute
flow from which hydrographic shears can be referenced. While many float data sets have yet to be
combined and the integration of mapped mean
flow and hydrography has so far been accomplished only on one hydrographic section, bringing
all these data together to produce consistent analysis will produce a significant increase in what is
known about the general circulation.
While this analysis is going on, oceanography will be undergoing a significant change made
possible, in part, by a new global array of 3000
profiling floats reporting profiles of temperature
and salinity over the upper 2000 m as well as
absolute velocity observations. This international
programme Argo, began implementation in 2000
with a goal of providing real-time observations
3.2 Subsurface Lagrangian Observations during the 1990s
137
Davis and Zenk
Lagrangian pattern in this region are expected.
With a similar focus on gathering a Lagrangian
description of an important transport process, the
pathway of the North Atlantic’s deep western
boundary current and its interaction with the Gulf
Stream were examined by Bower and Hunt
(2000a,b) using RAFOS floats. They found only a
small amount of intermediate water from the
Labrador Sea to cross under the Gulf Stream into
the subtropical circulation while Nordic Seas overflow waters bifurcate, one branch following the
Gulf Stream offshore while the other remains
attached to the western boundary. The clarity of
these results suggests a similar acoustic-float study
to resolve the question, raised in the subsection
above, of how Labrador Sea Water enters the subtropical gyre.
A number of regional float experiments in the
North Atlantic have examined different elements
of the complex circulation there, both elucidating
specific processes and collectively forming the
backbone of that part of the global array. For
example, Fig. 3.2.11 (see Plate 3.2.11, p. 172),
adopted from Zenk (2000), shows daily-resolved
trajectories of RAFOS floats just east of the
Mid-Atlantic Ridge. These isobaric floats were
deployed after June 1997 at the depth of the
Labrador Sea Water, i.e. roughly at 1500 m. They
demonstrate the transport of this water mass eastward through Charlie-Gibbs Fracture Zone at
about 53°N in more detail. After passing this gap
the pathways bifurcate, one approaching the western extent of the European continental rise while
the other feeds the Iceland Basin. The Icelandic
branch meets Iceland–Scotland Overflow Water at
the northern tip of the Iceland Basin and then
advects the floats southwestward along the flanks
of the Reykjanes Ridge. Particularly along the
Icelandic branch, these continuously tracked trajectories give a quasi-Lagrangian view of the mean
flow patterns in Fig. 3.2.8 (see Plate 3.2.8, p. 172)
that are based on an entirely independent data set.
In another example of regional experiments giving a closer view of patterns smoothed over in
basin-scale coverage, Rossby et al. (2000) present
case studies of selected trajectories from the
Irminger and the Iceland Basins. Despite the
apparent eddy motion they see a rather robust
pattern of float pathways emerging. Rossby’s
isopycnal-following instruments provide explicit
information on advection and mixing along
their pathways. The authors expect particularly
challenging results that will include estimates of
cross-frontal fluxes of heat and oxygen between
the subpolar and the subtropical gyres.
Finally, Lavender et al. (2000b) present results
from a regional deployment of autonomous Vertical Current Meter floats to explore the vertical
motion associated with deep convection. One interesting question addressed was whether a region
of convection is one of general downwelling.
Although the floats (cf. Davis et al., 2000) can
measure vertical motion down to O(3 m day
91
), the
way they are entrained into, and detrained from,
convecting plumes causes them to give a biased
measure of the mean vertical motion, so the results
are equivocal. A perplexing paradox is that the
vertical temperature flux :wЈTЈ9 apparently
carried by the plume scale (quasi-Lagrangian time
scales of less than 88 h) is insufficient to explain the
observed cooling below 400 m depth.
3.2.5 The future
It is clear that the Swallow float has become an
observational tool capable of charting the world
ocean’s subsurface general circulation and providing numerous and economical profiles of ocean
properties. Even though the analysis of WOCE-era
floats has only begun, it is clear that they will have
significant impacts on the field. Floats have provided visualization of basin-scale flows including
intergyre and interbasin transport. In many regions
enough observations have been accumulated to
produce statistically reliable descriptions of absolute
flow from which hydrographic shears can be referenced. While many float data sets have yet to be
combined and the integration of mapped mean
flow and hydrography has so far been accomplished only on one hydrographic section, bringing
all these data together to produce consistent analysis will produce a significant increase in what is
known about the general circulation.
While this analysis is going on, oceanography will be undergoing a significant change made
possible, in part, by a new global array of 3000
profiling floats reporting profiles of temperature
and salinity over the upper 2000 m as well as
absolute velocity observations. This international
programme Argo, began implementation in 2000
with a goal of providing real-time observations
3.2 Subsurface Lagrangian Observations during the 1990s
137
Davis and Zenk