absolute velocity close to the bottom and resolving
the small scales that bathymetry introduces (Hogg
et al., 1996) and led to using RAFOS and MARVOR floats. The necessary acoustic network was
maintained by institutions in the US, Germany and
France covering the tropical and subtropical South
Atlantic. Similarly, toward the end of WOCE
observations, scientists from Germany, France, the
US and UK, observing the complex flow in and
around the eastern boundary currents and convoluted topography of the eastern North Atlantic,
chose to employ RAFOS and MARVOR floats as
part of the European initiative EUROFLOAT
(Speer et al., 1999) and of ACCE (Atlantic Circulation and Climate Experiment in the US and
Atlantic Climate Change Experiment in Europe).
3.2.4.2 Basin-wide studies
The float observations needed to carry out the
ambitious WOCE plan to accurately map flow
globally would have been prohibitively expensive
using relatively short-lived acoustically tracked
floats needing an array of sound sources. Thus
over much of the global ocean WOCE sampling
fell to autonomous floats with the consequent loss
of continuous tracking. Figure 3.2.5 (see Plate
3.2.5, p. 172) is an example of raw ALACE data
from near 1000 m depth in the southwestern
Indian Ocean. The very rapid flow in the Agulhas
is an extreme example of a region where the
higher spatial resolution of acoustic tracking
would be desirable. At the same time, the vigorous
chaotic motion where the Agulhas loses its coherence is a clear example of the need for the massive
sampling that is made possible by autonomous
operation.
The first large-scale WOCE deployment was
in the South Pacific, the largest of ocean basins,
where between 1991 and 1995 over 300 ALACEs,
some reporting temperature profiles, were deployed,
mainly from WOCE hydrographic cruises, to map
mean flow near 1000 m depth. Over half of these
floats were still reporting at the end of 1999, but a
preliminary analysis (Davis, 1998b) hints at what
can be expected when WOCE Float Programme
sampling is complete. Design of the WOCE array
assumed a Lagrangian velocity time scale of O(10
days) so that successive ALACE velocities, most of
which were averages over 25 days, should be serially uncorrelated. This was confirmed for all South
Pacific velocities except zonal flow within O(10°)
of the equator where variability on a seasonal time
scale dominates and confirms the design estimate
that more than 5 years of data are required to
achieve the desired accuracy for mean flows.
Remarkably, the spatial structure and evolution of
the low-frequency equatorial variability was well
predicted by the data-assimilating NCEP (National
Center for Environmental Prediction) model
(Behringer et al., 1998) and could, therefore, be
coherently subtracted from the observations before
incoherent averaging was carried out.
Perhaps the most visually striking feature of the
raw South Pacific data is how within about 15° of
the equator velocities are dominantly zonal with
remarkably short meridional scales manifested in
zonal trajectories that reverse and return in the
opposite direction within a few degrees of latitude.
Outside this equatorial band raw data is chaotic,
presumably because of mesoscale variability.
Simple time-area averages of the float data
show the main features of the South Pacific circulation but the most accurate results are obtained
from objective maps that take account of the horizontal continuity relation implied by geostrophy.
Figure 3.2.6 portrays the mean flow at 900 m
obtained from the 840 float-years of data available
in 1997 (1200 float-years were available at the
end of 1999). This map of geostrophic pressure
roughly corresponds with the scheme devised by
Reid (1997) from a combination of geostrophic
shear and tracer distributions. The dominant feature is the South Equatorial Current cutting northwestward across the gyre connecting the South
Pacific’s southeast corner off Chile to the lowlatitude western boundary currents along the northeastern coast of Australia and along New Guinea.
The ends of this current, evident in the floats and
Reid’s analysis, are likely spots for transgyre transport between the Southern Ocean and subtropics
in the southeast and between the subtropical and
tropical circulation in the northwest.
Also evident in both float data and Reid’s
analysis are an intermediate depth East Australia
Current, westward flow south of Australia, and
the confluence of the polar limb of the subtropical
circulation and the polar circulation east of New
Zealand. Differences between the float data and
Reid’s analysis are found in speeds of the Antarctic
Circumpolar Current and the South Equatorial
Current, in the double-gyre subtropical circulation
(with western boundary currents along the coasts
3.2 Subsurface Lagrangian Observations during the 1990s
131
Davis and Zenk
the small scales that bathymetry introduces (Hogg
et al., 1996) and led to using RAFOS and MARVOR floats. The necessary acoustic network was
maintained by institutions in the US, Germany and
France covering the tropical and subtropical South
Atlantic. Similarly, toward the end of WOCE
observations, scientists from Germany, France, the
US and UK, observing the complex flow in and
around the eastern boundary currents and convoluted topography of the eastern North Atlantic,
chose to employ RAFOS and MARVOR floats as
part of the European initiative EUROFLOAT
(Speer et al., 1999) and of ACCE (Atlantic Circulation and Climate Experiment in the US and
Atlantic Climate Change Experiment in Europe).
3.2.4.2 Basin-wide studies
The float observations needed to carry out the
ambitious WOCE plan to accurately map flow
globally would have been prohibitively expensive
using relatively short-lived acoustically tracked
floats needing an array of sound sources. Thus
over much of the global ocean WOCE sampling
fell to autonomous floats with the consequent loss
of continuous tracking. Figure 3.2.5 (see Plate
3.2.5, p. 172) is an example of raw ALACE data
from near 1000 m depth in the southwestern
Indian Ocean. The very rapid flow in the Agulhas
is an extreme example of a region where the
higher spatial resolution of acoustic tracking
would be desirable. At the same time, the vigorous
chaotic motion where the Agulhas loses its coherence is a clear example of the need for the massive
sampling that is made possible by autonomous
operation.
The first large-scale WOCE deployment was
in the South Pacific, the largest of ocean basins,
where between 1991 and 1995 over 300 ALACEs,
some reporting temperature profiles, were deployed,
mainly from WOCE hydrographic cruises, to map
mean flow near 1000 m depth. Over half of these
floats were still reporting at the end of 1999, but a
preliminary analysis (Davis, 1998b) hints at what
can be expected when WOCE Float Programme
sampling is complete. Design of the WOCE array
assumed a Lagrangian velocity time scale of O(10
days) so that successive ALACE velocities, most of
which were averages over 25 days, should be serially uncorrelated. This was confirmed for all South
Pacific velocities except zonal flow within O(10°)
of the equator where variability on a seasonal time
scale dominates and confirms the design estimate
that more than 5 years of data are required to
achieve the desired accuracy for mean flows.
Remarkably, the spatial structure and evolution of
the low-frequency equatorial variability was well
predicted by the data-assimilating NCEP (National
Center for Environmental Prediction) model
(Behringer et al., 1998) and could, therefore, be
coherently subtracted from the observations before
incoherent averaging was carried out.
Perhaps the most visually striking feature of the
raw South Pacific data is how within about 15° of
the equator velocities are dominantly zonal with
remarkably short meridional scales manifested in
zonal trajectories that reverse and return in the
opposite direction within a few degrees of latitude.
Outside this equatorial band raw data is chaotic,
presumably because of mesoscale variability.
Simple time-area averages of the float data
show the main features of the South Pacific circulation but the most accurate results are obtained
from objective maps that take account of the horizontal continuity relation implied by geostrophy.
Figure 3.2.6 portrays the mean flow at 900 m
obtained from the 840 float-years of data available
in 1997 (1200 float-years were available at the
end of 1999). This map of geostrophic pressure
roughly corresponds with the scheme devised by
Reid (1997) from a combination of geostrophic
shear and tracer distributions. The dominant feature is the South Equatorial Current cutting northwestward across the gyre connecting the South
Pacific’s southeast corner off Chile to the lowlatitude western boundary currents along the northeastern coast of Australia and along New Guinea.
The ends of this current, evident in the floats and
Reid’s analysis, are likely spots for transgyre transport between the Southern Ocean and subtropics
in the southeast and between the subtropical and
tropical circulation in the northwest.
Also evident in both float data and Reid’s
analysis are an intermediate depth East Australia
Current, westward flow south of Australia, and
the confluence of the polar limb of the subtropical
circulation and the polar circulation east of New
Zealand. Differences between the float data and
Reid’s analysis are found in speeds of the Antarctic
Circumpolar Current and the South Equatorial
Current, in the double-gyre subtropical circulation
(with western boundary currents along the coasts
3.2 Subsurface Lagrangian Observations during the 1990s
131
Davis and Zenk