Observations of the Dynamics of the Antarctic Circumpolar Current
45
A thorough analysis of the importance of bottom topography in the dynamics
of the ACC was carried out in [27]. From this investigation bathymétrie features
resuit particularly felt where they lie perpendicular to the flow, as in our case of
the Ridge. In particular, we expect a Lagrangian analysis like ours to show
stronger evidence of areas characterized by eddy downstream propagation, i.e. in
the lee of topographie reliefs. The strong influence of a bottom topography whose
main features in the PSSO are deeper than 2500 m on the surface velocity structures obviously suggests a strong degree of vertical cohérence in the large-scale
flow. This is also suggested by the results of the dynamic analyses carried out over
the years on conventional and XBT hydrological data over the whole area and
more specifically in the Drake Passage (see Sect. 3). Confirmation of this character of the flow régime is being sought by looking at the potential vorticity balance
along the drifter trajectories, and preliminary results [28] suggest the tendency of
the water column sampled by our drifters to change its relative vorticity in order
to keep the ratio Ç/H constant in correspondence with the Crossing of the PacificAntarctic Ridge, where the flow is essentially zonal and therefore changes in the
planetary vorticity are negligible.
3 Lagrangian Observations at Intermediate Depth
In order to explore the vertical structure of the velocity field, five Autonomous
LAgrangian Circulation Explorer (ALACE) floats were also deployed along with
the first surface drifters release undertaken in 1994. Out of the five instruments
released in March 1994, three of them started transmitting regularly after launch,
whereas another one, for no understood reason, started working only 10 months
after release. ALACEs [29] are Lagrangian subsurface instruments which float at
a predetermined depth, where they are neutrally buoyant in virtue of the ballasting provided by the manufacturer. Periodically, they corne to the surface where
they are located by, and transmit data to, the Argos satellite System.
In our experiment the nominal depth was set at 900 m and the surfacing cycle
at 30 days, in order to approximately comply with the standards used in other
Lagrangian studies of the subsurface circulation in the Southern Océan ([30] R.E.
Davis and D.C. Webb, pers. comm.). The actual floating depths were within the
range 950-1200 m. Figure 5a shows the trajectories followed by our ALACEs in the
PSSO as long as ail four floats were transmitting, i.e. until August 1996. There is
an évident resemblance between the surface and the subsurface field, even though
speeds are obviously quite different: the overall flow is zonal, even if a recirculation zone can be seen downstream of the Campbell Plateau, approximately in correspondence of the recirculation displayed by the surface drifters and discussed
in Section 2. Recirculations are also évident in correspondence of the Crossing of
the ridge in the trajectories of the two ALACEs which get there.
In Fig. 5b we présent the Lagrangian velocities computed from the float observations. Obviously, the calculated velocities only refer to the submerged portion
of the trajectories. The resulting speeds are of the order of 1/4 of those at the surface: (u,v)=(4±5,0.6±3 cm/s). Given the sparsity of our data, pseudo-Eulerian statistics are not meaningful in this case. A compact way to compare surface and
45
A thorough analysis of the importance of bottom topography in the dynamics
of the ACC was carried out in [27]. From this investigation bathymétrie features
resuit particularly felt where they lie perpendicular to the flow, as in our case of
the Ridge. In particular, we expect a Lagrangian analysis like ours to show
stronger evidence of areas characterized by eddy downstream propagation, i.e. in
the lee of topographie reliefs. The strong influence of a bottom topography whose
main features in the PSSO are deeper than 2500 m on the surface velocity structures obviously suggests a strong degree of vertical cohérence in the large-scale
flow. This is also suggested by the results of the dynamic analyses carried out over
the years on conventional and XBT hydrological data over the whole area and
more specifically in the Drake Passage (see Sect. 3). Confirmation of this character of the flow régime is being sought by looking at the potential vorticity balance
along the drifter trajectories, and preliminary results [28] suggest the tendency of
the water column sampled by our drifters to change its relative vorticity in order
to keep the ratio Ç/H constant in correspondence with the Crossing of the PacificAntarctic Ridge, where the flow is essentially zonal and therefore changes in the
planetary vorticity are negligible.
3 Lagrangian Observations at Intermediate Depth
In order to explore the vertical structure of the velocity field, five Autonomous
LAgrangian Circulation Explorer (ALACE) floats were also deployed along with
the first surface drifters release undertaken in 1994. Out of the five instruments
released in March 1994, three of them started transmitting regularly after launch,
whereas another one, for no understood reason, started working only 10 months
after release. ALACEs [29] are Lagrangian subsurface instruments which float at
a predetermined depth, where they are neutrally buoyant in virtue of the ballasting provided by the manufacturer. Periodically, they corne to the surface where
they are located by, and transmit data to, the Argos satellite System.
In our experiment the nominal depth was set at 900 m and the surfacing cycle
at 30 days, in order to approximately comply with the standards used in other
Lagrangian studies of the subsurface circulation in the Southern Océan ([30] R.E.
Davis and D.C. Webb, pers. comm.). The actual floating depths were within the
range 950-1200 m. Figure 5a shows the trajectories followed by our ALACEs in the
PSSO as long as ail four floats were transmitting, i.e. until August 1996. There is
an évident resemblance between the surface and the subsurface field, even though
speeds are obviously quite different: the overall flow is zonal, even if a recirculation zone can be seen downstream of the Campbell Plateau, approximately in correspondence of the recirculation displayed by the surface drifters and discussed
in Section 2. Recirculations are also évident in correspondence of the Crossing of
the ridge in the trajectories of the two ALACEs which get there.
In Fig. 5b we présent the Lagrangian velocities computed from the float observations. Obviously, the calculated velocities only refer to the submerged portion
of the trajectories. The resulting speeds are of the order of 1/4 of those at the surface: (u,v)=(4±5,0.6±3 cm/s). Given the sparsity of our data, pseudo-Eulerian statistics are not meaningful in this case. A compact way to compare surface and