Observations of the Dynamics of the Antarctic Circumpolar Current
49
5 Summary and Conclusions
The circulation in the PSSO, and in particular in correspondence of the Antarctic
Circumpolar Current, has been investigated by means of surface drifters and subsurface floats. At the surface, the velocity présents a gross zonal flow pattern,
directed eastwards. The field is characterized by strong méridional shear and by
a complex multiple jet structure, suggested in particular by the observations in
the vicinity of the Pacific-Antarctic Ridge. The interaction with the bottom
topography is seen to play a major rôle in shaping both the mean and the eddy
fields, with a characteristic space lag which implies the presence of eddy-generating mechanisms in correspondence of major topographie features. This is the
case for the Pacific-Antarctic Ridge as well as for the Macquarie Ridge and for the
area of the Drake Passage. Two surface drifters deployed in the ACC hâve been
caught in the South Pacific Current, sampling a field which appears less influenced by the above effects.
Our subsurface float observations dénoté a strong vertical coupling in the
velocity field. Velocities around 1000-m depth resemble those at the surface, with
a scale factor of about 1/4 in magnitude. This vertical cohérence is consistent
with previous hydrographie analyses and with preliminary potential vorticity
estimâtes drawn from our surface drifters.
An attempt has been made to connect surface and subsurface Lagrangian
observations by means of évaluation of the vertical and horizontal distribution of
the velocity from an XBT section along 175°E. However, the deepest reference
level utilizable coincides with the maximum instrument measurement depth, i.e.
only 750 m. The resulting flow picture is obviously affected by this: it is qualitatively analogous to the corresponding section presented in the literature, but the
eastward velocities are systematically too low, being computed on the basis of a
too shallow level of no motion. In the future it is planned to utilize deeper-reaching XBTs, so as to be able to correctly evaluate the total transport along the section, estimating the depth of the reference level from the indications coming
from the Lagrangian measurements.
Acknowledgements. This work was funded by the Italian National Programme for
Antarctic Research (PNRA: Programma Nazionale di Ricerche in Antartide). The
authors wish to thank Elio Paschini for useful discussions and Giannetta Fusco
for her valuable help in the processing and in the preliminary analysis of XBT
data. Figures 1-5 were produced utilizing the GMT System [33] .
References
1. CLIMA Project Group (1997) Climatic Long-term Interaction for the Mass balance in Antarctica.
Int WOCE News 27:34-35
2. Deacon GER (1937) A general account of the hydrology of the Southern Océan. Discovery Reps
15:1-124
3. Deacon GER (1963) The Southern Océan. In: Hill MN (ed) The sea. Interscience Publishers, New
York, vol 2, pp 281-296
4. Nowlin WD Jr, Klinck JM (1986) The physics of the Antarctic Circumpolar Current. Rev Geophys
24:469-491
49
5 Summary and Conclusions
The circulation in the PSSO, and in particular in correspondence of the Antarctic
Circumpolar Current, has been investigated by means of surface drifters and subsurface floats. At the surface, the velocity présents a gross zonal flow pattern,
directed eastwards. The field is characterized by strong méridional shear and by
a complex multiple jet structure, suggested in particular by the observations in
the vicinity of the Pacific-Antarctic Ridge. The interaction with the bottom
topography is seen to play a major rôle in shaping both the mean and the eddy
fields, with a characteristic space lag which implies the presence of eddy-generating mechanisms in correspondence of major topographie features. This is the
case for the Pacific-Antarctic Ridge as well as for the Macquarie Ridge and for the
area of the Drake Passage. Two surface drifters deployed in the ACC hâve been
caught in the South Pacific Current, sampling a field which appears less influenced by the above effects.
Our subsurface float observations dénoté a strong vertical coupling in the
velocity field. Velocities around 1000-m depth resemble those at the surface, with
a scale factor of about 1/4 in magnitude. This vertical cohérence is consistent
with previous hydrographie analyses and with preliminary potential vorticity
estimâtes drawn from our surface drifters.
An attempt has been made to connect surface and subsurface Lagrangian
observations by means of évaluation of the vertical and horizontal distribution of
the velocity from an XBT section along 175°E. However, the deepest reference
level utilizable coincides with the maximum instrument measurement depth, i.e.
only 750 m. The resulting flow picture is obviously affected by this: it is qualitatively analogous to the corresponding section presented in the literature, but the
eastward velocities are systematically too low, being computed on the basis of a
too shallow level of no motion. In the future it is planned to utilize deeper-reaching XBTs, so as to be able to correctly evaluate the total transport along the section, estimating the depth of the reference level from the indications coming
from the Lagrangian measurements.
Acknowledgements. This work was funded by the Italian National Programme for
Antarctic Research (PNRA: Programma Nazionale di Ricerche in Antartide). The
authors wish to thank Elio Paschini for useful discussions and Giannetta Fusco
for her valuable help in the processing and in the preliminary analysis of XBT
data. Figures 1-5 were produced utilizing the GMT System [33] .
References
1. CLIMA Project Group (1997) Climatic Long-term Interaction for the Mass balance in Antarctica.
Int WOCE News 27:34-35
2. Deacon GER (1937) A general account of the hydrology of the Southern Océan. Discovery Reps
15:1-124
3. Deacon GER (1963) The Southern Océan. In: Hill MN (ed) The sea. Interscience Publishers, New
York, vol 2, pp 281-296
4. Nowlin WD Jr, Klinck JM (1986) The physics of the Antarctic Circumpolar Current. Rev Geophys
24:469-491