VENTILATION OF THE ATLANTIC-OCEAN EQUATORIAL THERMOCLINE

The ventilation of the lower equatorial Atlantic thermocline (100 m to 400 m) is investigated from the distributions of salinity, oxygen, th e chlorofluoromethanes F-11 and F-12, tritium, and the excess helium 3 ; 1983 data suggest a correlation on isopycnal surfaces between oxygen , chlorofluoromethanes and to a lesser extent, with the apparent ages derived from F-11/F-12 and helium 3/tritium. The apparent ages increas e with depth from 10 years on sigma(THETA)=26.5 to over 15 years near 400 m. The tracers are largely undersaturated with respect to the surf ace concentrations corresponding to the apparent ages. We investigate the ventilation processes responsible for these distributions with sim ple circulation models, where water from the subtropical gyres reaches isopycnally the equatorial thermocline. The few model parameters are determined by fitting the simulations to the chlorofluoromethane distr ibutions in 1983. The sensitivity of these parameters to expected erro rs on the past surface concentrations and to vertical mixing is compar able and can result in an error of more than 20% in the model paramete rs. In 1983, model [H-3] and [He-3] are too large, in particular when no diapycnal mixing is modeled, but the differences could have arisen from errors on past surface concentrations. During the 1980s, die mod el overestimates the increase in chlorofluoromethanes, in particular F -12. The fitted model inflow from the southern subtropical gyre betwee n 110 m and 400 m is 14 x 10(6) m3 s-1, in the range of expected trans ports by the western boundary current near Brazil. The model oxygen ut ilization rate is on the order of 12 mmol kg-1 yr-1 at 100 m and 5 mmo l kg-1 yr-1 at 400 m (below sigma(theta)=27.1).