Observations of the Dynamics of the Antarctic Circumpolar Current
39
standard instruments [13]. Theirs is now a standard design; their water-following
characteristics minimizing any direct wind drag effects hâve been assessed [14,
15]. Besides the above reason, they were selected because we wanted to be able to
make these data available to the broadest potentially interested scientific community, and they are in fact pooled in the WOCE-TOGA Global Drifter Center at the
NOAA Atlantic Océanographie and Meteorological Laboratory in Miami. The
instruments hâve been put to sea on the ship route from New Zealand to the
Italian base on Terra Nova Bay or vice versa.
We hâve been trying to keep a relatively constant deployment schedule over the
years; variations of the number of deployed instruments dépend on year-to-year
funding and also on the fact that we hâve experienced a few instrument failures
just after launch. The data utilized in this work are relative to 11 successfully
deployed surface drifters, ail manufactured by Technocean; six were deployed in
March 1994, three in January 1995 and two in January 1996.
WOCE-TOGA standard drifters are localized by the Argos satellite System with
a précision of about 300 m, at uneven time intervals. Typically, we collected a
number of five or six fixes a day. The First operation to be done, after editing positions as well as surface températures in order to get rid of spurious data (this was
carried out with a routine based on a simplification of the technique proposed in
[16]), was a linear interpolation in time of the location data so as to hâve regular
time sériés at 3-hour intervals. Thereafter, locations and velocities hâve been
resampled every 24 hours by means of the application of a triangular weight filtering technique (M.S. Swenson, pers. comm.). Data presented in this work are relative to the period March 1994 through December 1996, even if the oceanography
program in the framework of PNRA, including the Lagrangian investigation, is
still ongoing and deployments hâve been carried out in January 1997 and 1998
and are planned for the austral summers of the next years up to 2001.
2.1 Trajectory Analysis
A visual analysis of the trajectories is sufficient to illustrate the main features of
the surface dynamics of the ACC in the PSSO (for a general view of the physical
oceanography of the région, see in addition to the above references [11]). A map
of ail the trajectories of the surface drifters examined in this work is shown in Fig.
1 (a sketch of the 3000- and 4000-m isobaths is shown in Fig. 2 along with the
Lagrangian velocities, which will be discussed below). The observed overall
Lagrangian flow pattern is in good agreement with the Eulerian one estimated
from hydrographie data [7-9].
Our drifters sample extensively the variability band of the ACC, whose méridional limits, as redefmed in [7] on the basis of the water mass characteristic zonation, approximately correspond to the southernmost and northernmost drifter
trajectories over the whole PSSO. As expected, the overall flow pattern in that portion of the océan is zonal, directed eastwards. As shown also by t h» resuit s of the
analysis of hydrographie data [7,8], the band of the ACC in the PSSO is obviously affected by the presence of the Pacific-Antarctic Ridge (besides being possibly
constrained by the presence of sea ice); its influence on the first portion of the tra-
39
standard instruments [13]. Theirs is now a standard design; their water-following
characteristics minimizing any direct wind drag effects hâve been assessed [14,
15]. Besides the above reason, they were selected because we wanted to be able to
make these data available to the broadest potentially interested scientific community, and they are in fact pooled in the WOCE-TOGA Global Drifter Center at the
NOAA Atlantic Océanographie and Meteorological Laboratory in Miami. The
instruments hâve been put to sea on the ship route from New Zealand to the
Italian base on Terra Nova Bay or vice versa.
We hâve been trying to keep a relatively constant deployment schedule over the
years; variations of the number of deployed instruments dépend on year-to-year
funding and also on the fact that we hâve experienced a few instrument failures
just after launch. The data utilized in this work are relative to 11 successfully
deployed surface drifters, ail manufactured by Technocean; six were deployed in
March 1994, three in January 1995 and two in January 1996.
WOCE-TOGA standard drifters are localized by the Argos satellite System with
a précision of about 300 m, at uneven time intervals. Typically, we collected a
number of five or six fixes a day. The First operation to be done, after editing positions as well as surface températures in order to get rid of spurious data (this was
carried out with a routine based on a simplification of the technique proposed in
[16]), was a linear interpolation in time of the location data so as to hâve regular
time sériés at 3-hour intervals. Thereafter, locations and velocities hâve been
resampled every 24 hours by means of the application of a triangular weight filtering technique (M.S. Swenson, pers. comm.). Data presented in this work are relative to the period March 1994 through December 1996, even if the oceanography
program in the framework of PNRA, including the Lagrangian investigation, is
still ongoing and deployments hâve been carried out in January 1997 and 1998
and are planned for the austral summers of the next years up to 2001.
2.1 Trajectory Analysis
A visual analysis of the trajectories is sufficient to illustrate the main features of
the surface dynamics of the ACC in the PSSO (for a general view of the physical
oceanography of the région, see in addition to the above references [11]). A map
of ail the trajectories of the surface drifters examined in this work is shown in Fig.
1 (a sketch of the 3000- and 4000-m isobaths is shown in Fig. 2 along with the
Lagrangian velocities, which will be discussed below). The observed overall
Lagrangian flow pattern is in good agreement with the Eulerian one estimated
from hydrographie data [7-9].
Our drifters sample extensively the variability band of the ACC, whose méridional limits, as redefmed in [7] on the basis of the water mass characteristic zonation, approximately correspond to the southernmost and northernmost drifter
trajectories over the whole PSSO. As expected, the overall flow pattern in that portion of the océan is zonal, directed eastwards. As shown also by t h» resuit s of the
analysis of hydrographie data [7,8], the band of the ACC in the PSSO is obviously affected by the presence of the Pacific-Antarctic Ridge (besides being possibly
constrained by the presence of sea ice); its influence on the first portion of the tra-