The circulation in the subtropical South Indian Ocean derived from Argo floats

K. Getzlaff, H. Bryden, E. McDonagh, B. King,

This study uses CTD profiles from Argo floats between 2001 and 2004 to compute the circulation in the subtropical South Indian Ocean. All float data have passed the Argo real-time tests and have been visually checked on the temperature-salinity relationship and changes of salinity anomaly on isotherms. Relative geostrophic transports are computed using geopotential anomalies with a zero-velocity surface (ZVS) at 2000dbar. Weighted 5degree-bin averaging represents the structure of the subtropical gyre reasonably well and the estimated strength of the relative transport is 54 +/- 12 Sv. Prelimnary results indicate interannual oscillation in the gyre strength at 32S over the period 2002 to 2004. Additionally, mean velocity fields computed at the floats parking depth from float trajectories using a linear regression highlight the strong zonal currents associated with the Subtropical Front (STF) and the Subantarctic Front (SAF) south of 38S, the western boundary current near the south-eastern tip of Africa as well as a slower northwestward return flow of the subtropical gyre.
More appropriate mapping techniques using generalized distance weighting on varying length-scales and the combination of the mean velocity fields from the floats drift at parking depth will reduce the errors and uncertainties in the velocity field and enable us to compare results from different relative geostrophic transport calculations.