Variability in ocean ventilation from three decades of transient tracer measurements.

Measurements of chlorofluorocarbons (CFCs) and other transient tracers provide information on the rates and pathways of ocean transport. Here we use measurements of CFCs, SF6, tritium and helium made over the last three decades, as part of WOCE, Repeat Hydrography, GO-SHIP and other programs, to constrain the time scales of ocean ventilation. Simultaneous measurements of transient tracers with different time dependencies are used to constrain the mean time since water was last at the surface (mean age) as well as other ventilation time scales, while repeat sampling of CFCs between the late 1980s and recent years late 2000s are used to infer changes in the ventilation. Measurements along multiple transects in the southern oceans (including the most recent sampling in the Pacific) show large-scale coherent changes in ventilation of southern oceans over the last three decades, with a decrease in the age of subtropical SubAntarctic Mode Waters and an increase in the age of Circumpolar Deep Waters. This is contrasted with variability of ventilation times in the North Atlantic and Pacific oceans.