Circulation in Drake Passage revisited using new current time series and satellite altimetry: 2. The Ona Basin

Abstract

Time series of horizontal velocities were obtained at four mooring sites across the Ona Basin, southern Drake Passage, during 26 months (February 2006 toApril 2008). The moorings were located under the Jason satellite ground-track #104, allowing precise comparisons with various other altimetry products. Velocities as high as 0.5 ms(-1) at 500 m depth were observed during current pulses. Mean velocity amplitudes at 500 m reached 0.22 ms(-1) at 58.5 degrees S and 0.15 ms(-1) at 60 degrees S, but were smaller at 59 degrees S and 60.5 degrees S. Mean velocities at 2500 m depth varied between 0.05 and 0.10 ms(-1) and were westward on two of the moorings, suggesting recirculation in the center of the basin. Mean velocities were consistent with a general cyclonic circulation in the Ona Basin. The mean velocity vectors were observed to rotate with depth, the sense of rotation depending upon mooring sites. Standard deviation ellipses were close to circular except on the continental slope (60.5 degrees S) where they were stretched in the direction of isobaths. The southernmost mooring was under sea ice in winter, and velocity variations were reduced in amplitude during that period. The horizontal velocities were highly coherent in the vertical. Altimetrically derived surface geostrophic velocities compared well with the in situ velocities and were used to investigate further the flow over the West Scotia Ridge and in the center of the Ona Basin to the East of the highest part of the Shackleton Ridge, which provides some shelter from the eastward flowing Antarctic Circumpolar Current.