Upwelling velocity and ventilation in the western South China Sea deduced from CFC-12 and SF6 observations

Abstract

This study presents observations of the transient tracers CFC-12 and SF6 in the western South China Sea during the fall of 2015. A CFC-12 maximum was discovered in the western South China Sea at the subsurface layer (150–200 m), which could be traced back to the North Pacific Tropical Water. The transit time distribution approach was used to estimate the ventilation time in this area. The constrained Δ /Γ ratio of 0.5 was obtained using CFC-12/SF6 tracer pair. This ratio is lower than the empirical unit ratio of one as used for previous estimates. Waters in the northern region of the western South China Sea appear younger than waters in the southern region. The water mass corresponding to the salinity minimum has a mean age of ∼67 ± 16 years along the 15º N line (marked by the red dashed rectangle in Fig. 1), which increases to ∼76 ± 18 years along the 10º N line (blue dashed rectangle, Fig. 1). The higher mean ages indicate that the intermediate water was ventilated from the North Pacific, which is far distant from the South China Sea. The column inventory of Cant is ∼31.3 mol C m–2. Upwelling velocities of up to ∼55 × 10–5 m s–1 was computed using the tracer data, indicating that tracer-free water as yet not influenced by human perturbation could be carried to the upper layer by upwelling. Using the transit time distribution derived mean age with transient tracers provides a possible way to determine the ventilation time scale for the study area