Two full-depth velocity sections in the Drake Passage in 2006—Transport estimates

In January/February 2006, a hydrographic section under Jason track 104 across Drake Passage (DP) was performed twice within 3 weeks, on board of the R/V Polarstern. Two LADCPs mounted on the CTD rosette provided the most comprehensive synoptic top-to-bottom observations of directly measured velocity available in the Drake Passage to date, with an estimated error of less than 3 cm s−1. Geostrophic velocities computed from the hydrographic data were referenced to LADCP velocities. The velocity structure of the Antarctic Circumpolar Current (ACC) along the track and its evolution within 3 weeks reflect the frontal branches presented by Barré et al. (2011) and Provost et al. (2011). The repeated section offers two independent transport estimates, computed from the LADCP velocities (145±8.8 and 137.9±10.5 Sv, respectively) and from the adjusted geostrophic velocities (136.6±7 and 129±7 Sv, respectively). Error bars of the transport estimates are small. Contributions of the fronts and eddies to the total transport are modified within 3 weeks but tend to compensate each other. The total transport is reduced by about 10% (15—17 Sv) between the southward and northward journey. Total transport estimations are compared to previous results, in particular the ISOS (International Southern Ocean Study) data, from which the canonical value of 134 Sv has been estimated. Within the uncertainties due to uneven data distribution, no trend is discernible neither in the baroclinic nor total transports from 1975 to 2006.