Chlorofluorocarbon Constraints on North Atlantic Ventilation

Abstract

The North Atlantic Ocean vigorously ventilates the ocean interior. Thermocline and deep water masses are exposed to atmospheric contact there and are sequestered in two principal classes: Subtropical Mode Water (STMW: 26.5 ≤ σ? ≤ 26.8) and Subpolar Mode Water (SPMW: 26.9 ≤ σ? ≤ 27.8). These ventilation rates and pathways are uncertain, and a powerful way to estimate them is to monitor the penetration of chlorofluorocarbon (CFC) tracers. Here, a CFC dataset of over 44 000 observations, taken between 1982 and 1998, is combined with a non-eddy-resolving ( resolution) general circulation model of the North Atlantic Ocean. The CFC data are assimilated with the model by optimizing the uncertain air?sea CFC flux. The assimilated CFC fields are then systematically compared with the observations to identify the best fit and hence the most realistic ventilation. Three GCM experiments are performed this way to find the dependence on model thickness diffusivity. Each GCM solution is close to being statistically consistent with the CFC observations and likely sources of error. Lower diffusivity gives the best match to data although some systematic bias in sequestering tropospheric CFC remains. Lower diffusivity, around 150 m2 s?1, permits a stronger circulation with a more realistic North Atlantic Current. For this experiment, the subduction rate is around 16 Sv (Sv ≡ 106 m3 s?1) in the subtropics and eastern subpolar Atlantic (26.35 ≤ σ? ≤ 27.13) averaged over 1975?95. Around 26 Sv is formed in the Labrador and Irminger Seas (27.58 ≤ σ? ≤ 27.8). Only about 40% of the CFC carried into the subpolar interior by this flux remained there in 1998, however. The rest was returned to the subpolar mixed layer after an average period of 6?8 yr. In contrast, 70% of the CFC subducted into the subtropical interior remained there in 1998.