Observations of the Dynamics of the Antarctic Circumpolar Current
41
The eastward general flow also présents some smaller-scale recirculation zones,
whose connection with the bottom topography will be explored in more detail
later. The First encountered and most energetic of these areas is the one downstream of the Campbell Plateau, sampled by two surface drifters as well as by one
subsurface float (see also Sec. 3). In this area the flow sampled by the drifters bends
equatorward, weakens and recirculates downstream of the continental shelf. This
complexity has been extensively discussed in [17], as being due to the possibility
of ring formation at the Macquarie Ridge. In our case, however, the recirculations
displayed in the trajectories show up well downstream with respect to the ridge. In
the région west of 140°W, sea ice and bottom topography strongly confine the area
spanned by the drifters. This displays a shrinking around 150°W, followed by a
wide spreading of the Lagrangian paths thereafter.
The Crossing of the Pacific-Antarctic Ridge takes place essentially in correspondence of the two major gaps of that topographie feature, the Eltanin and the
Udintsev Fracture Zones, as happened with some of the FGGE drifters (First
GARP (Global Atmospheric Research Pogram) Global Experiment) [il], with just
two exceptions. In fact, east of 150°W our surface drifter set can be divided into
two subsets, représentative of very different flow régimes. While typically the
drifters proceed following the main path of the ACC, two of them are caught in
the southernmost branch of the Pacific subtropical gyre, the South Pacific
Current [18], eventually entering the Peru-Chile current. The trajectories of these
two instruments cross the relief farther north than the others, in the région of the
East Pacific Rise, and their trajectories do not appear strongly affected by the
Rise. This suggests a less strong barotropic character of the current in correspondence of the subtropical gyre [ 18]. A similar behaviour just occasionally occurred
with FGGE drifters [6,19], whereas hydrographie analyses do not seem to be able
to résolve it [8]. It is worth noticing, however, that this is consistent with the localization of the Subtropical Front determined in [20] and refined in [7]: the front,
typically located south of 40°S, is displaced equatorward beyond 30°S just in the
eastern half of the South Pacific Océan.
Finally, as to the flow in the eastern portion of the PSSO, the four drifters left
in the basin show a spreading of the trajectories which is, again, consistent with
the analysis in [7, 8]. The ACC is most likely constituted by individual jets coinciding with frontal positions [6], but in the eastern PSSO, i.e. in correspondence
of and downstream of the Pacific-Antarctic Ridge, the continuity of the jet patterns is not demonstrated [ 11 ]. As will be seen in the discussion of the Lagrangian
and pseudo-Eulerian estimated velocities, the four trajectories in that région
indeed show a sensible méridional shear suggesting the presence of separated
jets, but the data density is too little to draw any conclusion as to that.
Out of the four drifters approaching Drake Passage, possibly the most studied
zone in the Southern Océan (for a review see [4]), just one, in the period of observation, reaches and crosses it, and is eventually caught in the northward branch of
the ACC entering the Southern Atlantic and following the Malvinas Current; its
path is characterized by the presence of recirculations and meanders, in particular in the Brazil/Malvinas confluence région (a general description of the flow in
this area is given, e.g., in [21]).
41
The eastward general flow also présents some smaller-scale recirculation zones,
whose connection with the bottom topography will be explored in more detail
later. The First encountered and most energetic of these areas is the one downstream of the Campbell Plateau, sampled by two surface drifters as well as by one
subsurface float (see also Sec. 3). In this area the flow sampled by the drifters bends
equatorward, weakens and recirculates downstream of the continental shelf. This
complexity has been extensively discussed in [17], as being due to the possibility
of ring formation at the Macquarie Ridge. In our case, however, the recirculations
displayed in the trajectories show up well downstream with respect to the ridge. In
the région west of 140°W, sea ice and bottom topography strongly confine the area
spanned by the drifters. This displays a shrinking around 150°W, followed by a
wide spreading of the Lagrangian paths thereafter.
The Crossing of the Pacific-Antarctic Ridge takes place essentially in correspondence of the two major gaps of that topographie feature, the Eltanin and the
Udintsev Fracture Zones, as happened with some of the FGGE drifters (First
GARP (Global Atmospheric Research Pogram) Global Experiment) [il], with just
two exceptions. In fact, east of 150°W our surface drifter set can be divided into
two subsets, représentative of very different flow régimes. While typically the
drifters proceed following the main path of the ACC, two of them are caught in
the southernmost branch of the Pacific subtropical gyre, the South Pacific
Current [18], eventually entering the Peru-Chile current. The trajectories of these
two instruments cross the relief farther north than the others, in the région of the
East Pacific Rise, and their trajectories do not appear strongly affected by the
Rise. This suggests a less strong barotropic character of the current in correspondence of the subtropical gyre [ 18]. A similar behaviour just occasionally occurred
with FGGE drifters [6,19], whereas hydrographie analyses do not seem to be able
to résolve it [8]. It is worth noticing, however, that this is consistent with the localization of the Subtropical Front determined in [20] and refined in [7]: the front,
typically located south of 40°S, is displaced equatorward beyond 30°S just in the
eastern half of the South Pacific Océan.
Finally, as to the flow in the eastern portion of the PSSO, the four drifters left
in the basin show a spreading of the trajectories which is, again, consistent with
the analysis in [7, 8]. The ACC is most likely constituted by individual jets coinciding with frontal positions [6], but in the eastern PSSO, i.e. in correspondence
of and downstream of the Pacific-Antarctic Ridge, the continuity of the jet patterns is not demonstrated [ 11 ]. As will be seen in the discussion of the Lagrangian
and pseudo-Eulerian estimated velocities, the four trajectories in that région
indeed show a sensible méridional shear suggesting the presence of separated
jets, but the data density is too little to draw any conclusion as to that.
Out of the four drifters approaching Drake Passage, possibly the most studied
zone in the Southern Océan (for a review see [4]), just one, in the period of observation, reaches and crosses it, and is eventually caught in the northward branch of
the ACC entering the Southern Atlantic and following the Malvinas Current; its
path is characterized by the presence of recirculations and meanders, in particular in the Brazil/Malvinas confluence région (a general description of the flow in
this area is given, e.g., in [21]).