Chlorofluoromethanes in South Atlantic Antartic intermediate water

Abstract

Distribution of the dissolved atmospheric chlorofluoromethanes (CFMs) F-11 and F-12 in the South Atlantic Ocean are used to study the ventilation and circulation of Antarctic Intermediate Water (AAIW). CFM distributions on an isopycnal surface representative of AAIW are consistent with recently ventilated water entering the subtropical gyre in the southwestern Atlantic and then being advected anticyclonically around this gyre. The westward-flowing northern limb of the gyre apparently divides near the coast of South America with some water flowing southward to recirculate in the gyre, and the balance flowing northward along the coast of Brazil. At the equator this northward current divides again with one branch going eastward along the equator and the other continuing into the Northern Hemisphere. In the eastern tropical Atlantic, the CFM concentrations on this isopycnal surface in the cyclonic gyre are extremely low between the subtropical gyre and the equatorial tongue. Along the prime meridian, the F-11 and F-12 concentrations on the 27.2 θσ isopycnal surface between the mixed layer outcrop and the northern edge of the subtropical gyre are fitted to a one-dimensional advection-diffusion model. This model assumes that the CFMs enter the tubtropical gyre solely by northward diffusion from the mixed layer outcrop to the southern edge of the subtropical gyre, and that their distributions within the gyre are controlled by both advective and diffusive processes. Velocity and eddy diffusion coefficients are calculated from a least-squares fit to the data. These values are then used to calculate a mean oxygen consumption rate which is consistent with rates calculated using models of other time-dependent geochemical tracers.