42
E. Zambianchi et al.
2.2 The Velocity Field
The Lagrangian velocities obtained from interpolated drifter positions are shown
in Fig. 2, averaged over intervals of 20 days, i.e. approximately 5 times the
Lagrangian decorrelation time scale [22, 23]. The average surface Lagrangian
velocities over the whole PSSO and the correspondent standard déviations
derived by our data resuit (u,v)=(14±18,1 ± 17 cm/s). The limited number of available drifters does not allow for the identification of preferential pathways, as was
possible with the FGGE data [6]; yet, the velocity field obviously shows that the
ACC in the PSSO is characterized by strong méridional shear patterns and by the
possible presence of multiple jets. This complexity of the flow field shows up particularly clearly in the western and central portion of the PSSO, whereas after
Crossing the Ridge a well defined jet is left, directed straight eastwards, in agreement with the geostrophic picture drawn in [8]. The question arises as to whether
this can be an artifact due to the non-simultaneity of the Lagrangian observations, and if some degree of interannual variability is présent. However, an analysis of the velocities measured by two selected pairs of drifters deployed respectively in March 1994 and in January 1995, namely the four buoys which hâve
extensively sampled the band of the ACC up to the Drake Passage, does not show
significant interannual variability in the overall field: namely, the average
Lagrangian velocities computed over the trajectory pairs spanning the whole
zonal extension of the PSSO are for the first drifter pair (u,v)=(14±17,1 ± 18 cm/s),
and for the second (u,v)=(17±19,1± 17 cm/s).
Another way to look at the overall velocity field is shown in Fig. 3, where we
présent partial drifter trajectories corresponding to speeds, respectively, lower
than 10 cm/s (Fig. 3a), between 10 and 30 cm/s (Fig. 3b), and higher than 30 cm/s
(Fig. 3c). A comparison between the intermediate and larger speed pattern shows
again both characteristics, namely the complexity in the west and how the ACC is
clearly stronger than the South Pacific Current and the Peru-Chile Current in its
southernmost branch.
The surface drifter data hâve been processed in order to obtain pseudoEulerian statistics over the whole région. The PSSO has been divided into bins
measuring 5 degrees of longitude by 2.5 degrees of latitude, i.e. approximately
square at these latitudes (for a critical analysis of this technique see [24] and référencés therein). Figure 4 shows the pseudo-Eulerian statistics as to the kinetic
energy: the kinetic energy of the mean flow (MKE) is presented in Fig. 4a, the
eddy kinetic energy (EKE) in Fig. 4b.
As suggested by the results of the analysis of FGGE data [25] and more recently of altimeter data [26], the PSSO is characterized by a relatively high ratio of
MKE/EKE. The reason for this is probably partly to be found in the absence of a
proper western boundary current in the basin, as is the case for the two other
basins of the Southern Océan: the Brazil/Malvinas Current in the Atlantic and the
Agulhas Current in the Indian Océan [18],
Our data confirm the steady character of the large-scale flow in the région sampled by our surface drifters: the MKE is typically larger than the EKE, with the
exception of the areas in the lee of strong topographie features. This is particularly well verified at the Crossing of the Pacific-Antarctic Ridge. There, the effect
E. Zambianchi et al.
2.2 The Velocity Field
The Lagrangian velocities obtained from interpolated drifter positions are shown
in Fig. 2, averaged over intervals of 20 days, i.e. approximately 5 times the
Lagrangian decorrelation time scale [22, 23]. The average surface Lagrangian
velocities over the whole PSSO and the correspondent standard déviations
derived by our data resuit (u,v)=(14±18,1 ± 17 cm/s). The limited number of available drifters does not allow for the identification of preferential pathways, as was
possible with the FGGE data [6]; yet, the velocity field obviously shows that the
ACC in the PSSO is characterized by strong méridional shear patterns and by the
possible presence of multiple jets. This complexity of the flow field shows up particularly clearly in the western and central portion of the PSSO, whereas after
Crossing the Ridge a well defined jet is left, directed straight eastwards, in agreement with the geostrophic picture drawn in [8]. The question arises as to whether
this can be an artifact due to the non-simultaneity of the Lagrangian observations, and if some degree of interannual variability is présent. However, an analysis of the velocities measured by two selected pairs of drifters deployed respectively in March 1994 and in January 1995, namely the four buoys which hâve
extensively sampled the band of the ACC up to the Drake Passage, does not show
significant interannual variability in the overall field: namely, the average
Lagrangian velocities computed over the trajectory pairs spanning the whole
zonal extension of the PSSO are for the first drifter pair (u,v)=(14±17,1 ± 18 cm/s),
and for the second (u,v)=(17±19,1± 17 cm/s).
Another way to look at the overall velocity field is shown in Fig. 3, where we
présent partial drifter trajectories corresponding to speeds, respectively, lower
than 10 cm/s (Fig. 3a), between 10 and 30 cm/s (Fig. 3b), and higher than 30 cm/s
(Fig. 3c). A comparison between the intermediate and larger speed pattern shows
again both characteristics, namely the complexity in the west and how the ACC is
clearly stronger than the South Pacific Current and the Peru-Chile Current in its
southernmost branch.
The surface drifter data hâve been processed in order to obtain pseudoEulerian statistics over the whole région. The PSSO has been divided into bins
measuring 5 degrees of longitude by 2.5 degrees of latitude, i.e. approximately
square at these latitudes (for a critical analysis of this technique see [24] and référencés therein). Figure 4 shows the pseudo-Eulerian statistics as to the kinetic
energy: the kinetic energy of the mean flow (MKE) is presented in Fig. 4a, the
eddy kinetic energy (EKE) in Fig. 4b.
As suggested by the results of the analysis of FGGE data [25] and more recently of altimeter data [26], the PSSO is characterized by a relatively high ratio of
MKE/EKE. The reason for this is probably partly to be found in the absence of a
proper western boundary current in the basin, as is the case for the two other
basins of the Southern Océan: the Brazil/Malvinas Current in the Atlantic and the
Agulhas Current in the Indian Océan [18],
Our data confirm the steady character of the large-scale flow in the région sampled by our surface drifters: the MKE is typically larger than the EKE, with the
exception of the areas in the lee of strong topographie features. This is particularly well verified at the Crossing of the Pacific-Antarctic Ridge. There, the effect