We can monitor the ACC transport and its variability using a combination of altimetry and in-situ hydrographic data from CTDs and the SURVOSTRAL XBTs. Topex/Poseidon altimetry data have been interpolated onto the mean WOCE/SR3 and SURVOSTRAL lines, and the hydrographic data are then used to determine the depth-integrated transport (Rintoul and Sokolov, 2001; Rintoul et al., 2002). A strong correlation was found between the steric height variations and the barotropic transport from the hydrographic data. This regression is then applied to the altimetry data to infer a continuous time series of barotropic transport between 44 S and 49 S (Figure - Rintoul et al., 2002). Their method allows us to determine the seasonal and interannual transport variations, especially in winter when there are very few in-situ data. This transport calculation represents a spatially integrated value, which is less affected by individual measurement errors or mesoscale eddies and meanders.
The baroclinic transport (from the surface to the bottom) of the circumpolar current, determined from CTD data is estimated at 147 ± 10 Sv across the SR3 section. The variability of this baroclinic transport (relative to 2500 m) is estimated at ~8 Sv from the SURVOSTRAL data. This variability corresponds to only 7% of the total transport (Rintoul et al., 1999). In the deeper layers along SR3, the transport variability is even smaller.