Transit Time Distributions and Ventilation Pathways Using CFCs and Lagrangian Backtracking in the South Atlantic of an Eddying Ocean Model

Abstract

Transit time distributions (TTDs) for the Antarctic Intermediate Water (AAIW) in the South Atlantic Ocean are estimated from an eddying ocean model with a twofold scope: validation of the TTD method and identifying pathways of the AAIW. The TTDs are inferred both from Lagrangian particle backtracking and the modeled CFC-11 concentrations, under the assumption that the TTDs can be described with an inverse Gaussian function. A bimodal distribution is obtained for the Lagrangian TTDs with four major subduction regions identified: near the Agulhas retroflection, south of New Zealand, west of the Drake Passage (smallest mean age G=13 years), and in the Argentine basin (largest mean age G=25 years). With the Southern Ocean as source region, the inverse Gaussian is a reasonable representation for the TTDs in the eastern Atlantic basin (408–358S, 08–208E), whereas the fit for region west (408–358S, 608–408W) of the mid-Atlantic ridge is not as good and overestimates the TTDs for transit times < 15 years. Mean ages from the modeled CFC-11 are mostly larger (up to 12 years) in the eastern Atlantic basin, and they are mostly smaller than the Lagrangian mean ages in the west. Both methods yield mean ages smaller in the western than in the eastern Atlantic basin and an aging of AAIW from the 1990s to the 2000s that is consistent with reduced flow velocities. The Antarctic Circumpolar Current appears to be the prime determinant of the transit times. The results suggest that the inverse Gaussian, despite assuming 1D advection–diffusion with constant mean flow and diffusivity, is a surprisingly good fit.