The variability of sea level and surface geostrophic currents in the Southern Ocean is investigated from the first 26 months of unclassified Geosat altimeter data (November 1986 to December 1988). Because of problems unique to Geosat, it has been necessary to develop new techniques for analyzing the height data. These techniques are presented here, and the processed Geosat data are used to examine the relation between mesoscale variability and the mean circulation (as determined from historical hydrographic data). The two are shown to be significantly correlated, implicating the importance of hydrodynamic instabilities in the Antarctic Circumpolar Current. The geographical patterns of both the mean flow and the mesoscale variability are shown to be controlled by the bathymetry. An efficient objective analysis algorithm is introduced for generating smoothed fields from observations randomly distributed in time and two space dimensions. The algorithm is applied to the 26 months of Geosat data, and the smoothed fields are used to investigate the large‐scale, low‐frequency variability of sea level and surface geostrophic velocity in the Southern Ocean. Approximately 33% of the variance is accounted for by the first three empirical orthogonal functions (EOFs) of sea level variability. These three modes describe variability over seasonal time scales and separate into an annual cycle (mode 1), a semiannual cycle (mode 2) and a mode which describes year‐to‐year variability in the seasonal cycles for 1987 and 1988 (mode 3). The complexity of the spatial patterns of the second‐ and higher‐order modes and the small percentages of variance accounted for by the first three modes (15%, 10% and 8%, respectively) reflect the generally regional, as opposed to coherent circumpolar, nature of sea level variability in the Southern Ocean. The inherent weak zonal coherence of the variability is further emphasized by EOF analysis separately within each basin of the Southern Ocean.

A Geosat altimeter wind speed algorithm is derived by cross‐calibrating Geosat and Seasat altimeter estimates of the normalized radar cross section σ0 and modifying an existing Seasat altimeter wind speed model function to obtain a model function appropriate for Geosat observations. It is argued that the σ0 distribution measured by an altimeter is relatively stable over a sufficiently large geographical region and a long enough time period. Systematic differences between σ0 estimates from two altimeters can therefore be identified based on comparisons of their σ0 histograms. Any such systematic differences can then be corrected using independent σ0 estimates. When this method is applied to the Geosat and Seasat altimeters, a systematic difference between the two σ0 histograms is shown to be consistent with differences between Seasat altimeter and nadir Seasat scatterometer estimates of σ0 deduced independently by a previous study. This supports the conclusions that (1) the σ0 distribution is stable, and (2) the Seasat altimeter estimates of σ0 were miscalibrated. After modifying the existing Seasat altimeter wind speed algorithm to account for this apparent σ0 error, the resulting Geosat estimates of wind speed agree with high‐quality buoy observations to within an rms difference of less than 2 m/s.

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Motivated by satellite altimeter observations of enhanced sea level variability near steep topographic slopes in the Southern Ocean, effects of topography on the spatial distribution of mesoscale eddies and on eddy-mean flow interaction are investigated using a two-layer, wind-forced, quasigeostrophic channel model. The principal topography, a zonal ridge with a zonal modulation of ridge height and width, is an idealized version of a segment of the Southeast Indian Ridge along the path of the Antarctic Circumpolar Current. Geosat altimeter observations in this region suggest that spatial variations of eddy energy are related to alongstream modulations of ridge morphology. The time-mean flow and distribution of time-dependent eddies in the model are sensitive to relatively subtle alongstream variations of topography. Topographic steering leads to alongstream variations of time-mean baroclinic and barotropic shear and to alongstream variations in the meridional position of the jet relative to the crest of the zonal ridge. Linear stability analysis demonstrates that zonal variations of flow stability are strongly coupled to the topography. Unstable mode growth rates are largest where topographic steering forces the jet into regions of reduced ambient potential vorticity gradient. Growth rates are lower where topography steers the jet into regions of higher ambient potential vorticity gradient. As a result, the largest eddy energies occur downstream of zonal modulations of ridge height or width. Unlike flows over flat-bottom topography, the zonal distribution of unstable mode growth rate is negatively correlated with velocity shear. Analysis of area-averaged mean-to-eddy energy conversions shows that zonal modulations of topography modify the regime of flow instability. Baroclinic instability and recycling of eddy energy in the upper part of the water column occur in cases with zonally uniform topography. Mixed baroclinic-barotropic instability and strong downward transfers of eddy energy occur in cases with zonal modulations of topography. Local vorticity analyses demonstrate that alongstream variations of topography produce strong zonal modulations of flow dynamics. Zonal variations of topography shift the region of eddy-influenced dynamics within the model domain and modify the relative contributions of the mean and eddy field to the time-mean vorticity balance. When interpreted in the context of Southern Ocean dynamics, these results suggest that eddy-active regions near steep topographic slopes may contribute disproportionately to the dynamics of the Antarctic Circumpolar Current.