Annually Resolved Propagation of CFCs and SF6 in the Global Ocean Over Eight Decades

Oceanic transient tracers, such as chlorofluorocarbons (CFCs) and sulfur-hexafluoride (SF6), trace the propagation of intermediate-to-abyssal water masses in the ocean interior. Their temporal and spatial sparsity, however, has limited their utility in quantifying the global ocean circulation and its decadal variability. The Time-Correction Method (TCM) presented here is a new approach to leverage the available CFCs and SF6 observations to solve for the Green's functions (GFs) describing the steady-state transport from the surface to the ocean interior. From the GFs, we reconstruct global tracer concentrations (and associated uncertainties) in the ocean interior at annual resolution (1940-2021). The spatial resolution includes 50 neutral density levels that span the water column along World Ocean Circulation Experiment/Global Ocean Ship-Based Hydrographic Investigations Program lines. The reconstructed tracer concentrations return a global view of CFCs and SF6 spreading into new regions of the interior ocean, such as the deep north-western Pacific. For example, they capture the southward spreading and equatorial recirculation of distinct North Atlantic Deep Water components, and the spreading of CFC-rich Antarctic Bottom Water out of the Southern Ocean and into the North Pacific, East Indian, and West Atlantic. The reconstructed tracer concentrations fit the data in most locations (similar to 75%), indicating that a steady-state circulation holds for the most part. Discrepancies between the reconstructed and observed concentrations offer insight into ventilation rate changes on decadal timescales. As an example, we infer decadal changes in Subantartic Mode Water (SAMW) and find an increase in SAMW ventilation from 1992 to 2014, highlighting the skill of the TCM in leveraging the sparse tracer observations. Plain Language Summary The penetration of chlorofluorocarbons (CFCs) and sulfur-hexaflouride (SF6) into the oceans represents an opportunity to estimate the ventilation rate of the global ocean more directly than other seawater properties. Properties like temperature and salinity are nearly in balance with the ocean circulation and thus are challenging for finding rates of motion, but CFCs and SF6 are transiently evolving due to a great increase in atmospheric concentrations since 1940. However, the analysis of CFCs and SF6 also poses challenges, as they are observed infrequently and they have not had time to permeate the entire global ocean. Here, we analyze over similar to 10(6) CFC and SF6 observations that have been taken over four decades, permitting the largest fraction to date of the global ocean to be analyzed using these tracers. A Time-Correction Method is developed to address the temporal sparsity of the observations and an eight-decade annually resolved picture of CFCs and SF6 evolution is produced. Roughly 75% of the observations can be explained by the large-scale, statistically steady ocean circulation acting on the anthropogenically driven time varying atmospheric concentration.