Water masses in the Atlantic Ocean: water mass ages and ventilation

Abstract. The distribution of water masses and their characteristics, including ventilation, provides fundamental insights into large-scale oceanographic processes such as thermohaline circulation and marine biogeochemical cycles. The characteristics of main water masses in the Atlantic Ocean have been comprehensively documented in a companion study (Liu and Tanhua, 2021); this study presents quantitative assessments of water mass age characteristics and ventilation time scales through an analysis using the transient tracers chlorofluorocarbon-12 (CFC-12), sulfur hexafluoride (SF6), and argon-39 (39Ar). We use two distinct age concepts: mean-age as an integrative metric of water mass chronology, and mode-age as a proxy for advective time scales. Under our fixed transit-time distribution shape (Δ/Γ=1), the two ages are linearly related (mode-age ≈ 0.162⋅ mean-age). Here we mainly report mode-ages; the corresponding mean-ages can be obtained by dividing with 0.162. Empirical results demonstrate systematic age progression with increasing depth and along water mass trajectories. Surface layer central waters exhibit mode ages up to ∼ 30 years (mean-age ∼ 100 years). In the intermediate layer, meridional age gradients characterize the Antarctic Intermediate Water (AAIW) reaching maximum mode-age ∼ 80 years (mean-age ∼ 300 years) at 30° N, whereas zonal variations manifest in Mediterranean Water (MW) with peak mode-ages ∼ 100 years (mean-age ∼ 400 years) observed in equatorial regions. As the dominant deep water component, North Atlantic Deep Water (NADW) exhibits extreme ages in the Antarctic Circumpolar Current (ACC) region at 50° S, achieving mode-age ∼ 100 years (mean-age ∼ 600 years). Bottom layer water masses display their oldest signatures: Antarctic Bottom Water (AABW) from the Weddell Sea reaches mode-age ∼ 100 years (mean-age ∼ 600 years) at equatorial latitudes, while its extension, Northeast Atlantic Bottom Water (NEABW), attains exceptional values of mode-age ∼ 120 years (mean-age ∼ 800 years) at 50° N. The age analysis reveals significant basin-scale asymmetries, with western basins exhibiting younger ages compared to eastern counterparts. Ventilation efficiency modulates these age distributions, as evidenced by lower mode-ages and reduced apparent oxygen utilization (AOU) in better-ventilated western basins. The calculated oxygen utilization rate (OUR) demonstrates spatial concordance with dissolved oxygen (DO) concentrations, corroborating enhanced oxidative processes in high-oxygen regimes. This integrated age framework provides novel insights into water mass ventilation dynamics and their biogeochemical implications through quantitative characterization of temporal-spatial age distributions across multiple oceanographic provinces.