38
E. Zambianchi et al.
use of expendable Eulerian instruments. Drifter and XBT deployments hâve been
regularly carried out on the ship route from New Zealand to Terra Nova Bay, or on
the way back, one (drifters) or more times (XBTs) a year. In addition, subsurface
floats were released during the expédition of the austral summer 1993-1994.
The Southern Océan is the only région in the world océan where the driving
action of the wind on the surface waters is essentially unconstrained by the présence of continental margins [2, 3]. The direct action of the westerlies, in combination with thermohaline effects not completely understood as yet [4, 5], gives
origin to the ACC, an extremely massive eastward zonal current limited in the
north by the Subtropical Convergence [2, 3], or Front [6], and whose Southern
limit has been recently identified and denominated as the ACC Front [7]. The classical description of the surface circulation around Antarctica is that given by
geostrophic analyses carried out on historical hydrographie data [8, 9]. More
recently this picture has been updated taking into considération additional
hydrographie stations available up to 1990 [7].
The eastward flow associated with the ACC is characterized by relatively slow
speeds at the surface (of the order of 15 cm/s, see e.g. [10]) but very strong transport (a typical value of 100 Sv above 3000 m is estimated in [7], in agreement with
previous calculations; for a review of zonal transport estimâtes proposed in the
past see [4,11] and for a discussion of their variability range [10]).
Gordon et al. [8] show that the choice of a reference level at 1000 db yields a
meaningful pattern for the surface geostrophic flow, but the eastward flow is seen
to reach depths of the order of 3000 m: apart from the above hydrographie data
analyses, at intermediate and deep levels the velocity field in the PSSO has been
extensively investigated essentially just in the area of the Drake Passage, e.g. in the
framework of the ISOS (International Southern Océan Studies) experiment (see
the reviews in [4, 11]), with particular emphasis on transport estimâtes. The jet
structure of the ACC (discussed in Sec. 2 below) is seen to extend almost to the
bottom [11]; a strong coupling in the mid-depth range has been observed in current meter data which show corrélation from the surface down to depths of 2000
m and more [12].
The first aspect of the dynamics of the PSSO studied in this work is the surface
circulation in correspondence of the ACC, investigated by means of surface drifter
data. This is done in Section 2. In Section 3 the flow at a depth of around 1000 m
is evaluated using subsurface float data. In Section 4, XBT data collected along a
section approximately coinciding with the PUS WOCE section are presented; an
estimate of the velocity field transverse to the section based on the geostrophic
method is first given using the maximum measurement depth (750 m) as the reference level. This is later tentatively refïned by forcing the flow to comply with the
velocity estimâtes derived from the surface Lagrangian data. Finally, summary
and conclusions can be found in Section 5.
2 The Lagrangian Picture of the Surface Flow
The surface drifters deployed over the years in the ACC by the Istituto
Universitario Navale in the framework of the PNRA are WOCE-TOGA holey-sock
E. Zambianchi et al.
use of expendable Eulerian instruments. Drifter and XBT deployments hâve been
regularly carried out on the ship route from New Zealand to Terra Nova Bay, or on
the way back, one (drifters) or more times (XBTs) a year. In addition, subsurface
floats were released during the expédition of the austral summer 1993-1994.
The Southern Océan is the only région in the world océan where the driving
action of the wind on the surface waters is essentially unconstrained by the présence of continental margins [2, 3]. The direct action of the westerlies, in combination with thermohaline effects not completely understood as yet [4, 5], gives
origin to the ACC, an extremely massive eastward zonal current limited in the
north by the Subtropical Convergence [2, 3], or Front [6], and whose Southern
limit has been recently identified and denominated as the ACC Front [7]. The classical description of the surface circulation around Antarctica is that given by
geostrophic analyses carried out on historical hydrographie data [8, 9]. More
recently this picture has been updated taking into considération additional
hydrographie stations available up to 1990 [7].
The eastward flow associated with the ACC is characterized by relatively slow
speeds at the surface (of the order of 15 cm/s, see e.g. [10]) but very strong transport (a typical value of 100 Sv above 3000 m is estimated in [7], in agreement with
previous calculations; for a review of zonal transport estimâtes proposed in the
past see [4,11] and for a discussion of their variability range [10]).
Gordon et al. [8] show that the choice of a reference level at 1000 db yields a
meaningful pattern for the surface geostrophic flow, but the eastward flow is seen
to reach depths of the order of 3000 m: apart from the above hydrographie data
analyses, at intermediate and deep levels the velocity field in the PSSO has been
extensively investigated essentially just in the area of the Drake Passage, e.g. in the
framework of the ISOS (International Southern Océan Studies) experiment (see
the reviews in [4, 11]), with particular emphasis on transport estimâtes. The jet
structure of the ACC (discussed in Sec. 2 below) is seen to extend almost to the
bottom [11]; a strong coupling in the mid-depth range has been observed in current meter data which show corrélation from the surface down to depths of 2000
m and more [12].
The first aspect of the dynamics of the PSSO studied in this work is the surface
circulation in correspondence of the ACC, investigated by means of surface drifter
data. This is done in Section 2. In Section 3 the flow at a depth of around 1000 m
is evaluated using subsurface float data. In Section 4, XBT data collected along a
section approximately coinciding with the PUS WOCE section are presented; an
estimate of the velocity field transverse to the section based on the geostrophic
method is first given using the maximum measurement depth (750 m) as the reference level. This is later tentatively refïned by forcing the flow to comply with the
velocity estimâtes derived from the surface Lagrangian data. Finally, summary
and conclusions can be found in Section 5.
2 The Lagrangian Picture of the Surface Flow
The surface drifters deployed over the years in the ACC by the Istituto
Universitario Navale in the framework of the PNRA are WOCE-TOGA holey-sock