Turnover times and transport rates in the warm water sphere of the South Atlantic derived from tracer data

Tritium and CFC-11 data obtained from the programmes WOCE (1990-1995) and SAVE (1987-1989, CFC-11 data only) are used for an analysis of the upper level circulation in the South Atlantic Ocean (density range sigma-0 = 26.0-27.1 kg/m**3). A new bomb tritium input function for the South Atlantic is presented. The atmospheric deposition is based on previous work by Doney et al. [1992]. The new input function also accounts for tritium deposition due to river runoff and for tritium exchange with other ocean basins. The calculated total tritium input for 1960-1995 between 0°S-50°S and the observed tritium inventory in 1995 agree well (within 1%). This agreement supports a value of 0.70 for the liquid/vapor isotopic fractionation factor of tritium, disproving GCM model calculations by Koster et al. [1989] that yielded a fractionation factor of 0.35. Tritium surface water concentrations are inferred from a simple mixed layer model driven by atmospheric tritium fluxes and calibrated using GEOSECS and WOCE data. The surface water concentrations of the South Atlantic are lower than in the North Atlantic. The highest surface values in the 1990´s are found in the subtropics where the surface water concentrations are still slowly increasing. As boundary condition for CFC-11 it is assumed that the sea surface is in a solubility equilibrium with the atmosphere. For quasi synoptic analyses the tritium and CFC-11 data are projected to the year 1993 using a 1D advection-diffusion model. Meridional sections show tracer concentrations decreasing with depth. Maximum penetration of tritium and CFC-11 is found in the subtropical region. North of about 20°S there is a concentration decline in both tracers indicating upwelling processes. Distributions on isopycnal surfaces show high tritium values in the western South Atlantic which may be due to the formation of Subtropical and Subantarctic Mode Waters