Annually-resolved propagation of CFCs and SF6 in the global ocean over
eight decades
Abstract
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 presented here is a new approach to leverage
the available CFCs and SF6 observations to solve for the Green’s
functions describing the steady-state transport from the surface to the
ocean interior. From the Green’s functions, we reconstruct global tracer
concentrations (and associated uncertainties) in the ocean interior at
annual resolution (1940 to 2021). The spatial resolution includes 50
neutral density levels that span the water column along WOCE/GO-SHIP
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 NADW components, and
the spreading of CFC-rich AABW 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 (~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 time-correction method in leveraging the sparse tracer
observations.