Observations of the dynamics of the Antarctic Circumpolar Current in the Pacific sector of the Southern Ocean

This paper describes long term measurements of the Antarctic Circumpolar Current carried out by means of surface and sub-surface drifters deployed as of the austral summer 1993-94, associated with XBT sections. At the surface, the velocity presents a gross zonal flow pattern, directed eastwards. The field is characterized by strong meridional shear and by a complex multiple jet structure, suggested in particular by the observations in the vicinity of the Antarctic-Pacific Ridge. The interaction with the bottom topography is seen to play a major role 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 topographic features. Our subsurface float observations denote 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 coherence is consistent with previous hydrographic analyses and with preliminary potential vorticity estimates drawn from our surface drifters data.