Keywords:Western Boundary Currents, Air-Sea Interaction, Radiocarbon, Thermocline
The western boundary currents and their extension regions have long been expected to play an important role in the ocean’s carbon cycle. However, it is still uncertain how western boundary currents impact the ocean uptake pathways of anthropogenic carbon (Cant), and if they sustain a new transfer of Cantfrom thermocline to subpolar water masses. Here we consider water mass transformation diagnostics applied to a forward ocean circulation model in conjunction with an analysis of bomb-DI14C (bomb-radiocarbon) as a proxy for a “pulsed” atmospheric source function of Cantover the period 1955-1963. Our main result is that the extension regions of the western boundary currents play an important role in sustaining rapid (32±3 years) renewal timescales for the full ocean volume above the density horizon of the base of the thermocline. This rapid renewal timescale sustains a vigorous exchange of the waters with maximum bomb-DI14C content. In 1995 in the model (mid-WOCE) 66% of the global cumulative air-sea flux of bomb-14CO2enters the ocean directly into thermocline densities, but with only 42% of the inventory in the thermocline. Approximately 70% of the net transfer of bomb-DI14C from subtropical to subpolar waters in 1995 is shown to occur south of 30°S, sustained largely through the densification of near-surface waters through the release of heat to the atmosphere within the western boundary currents and their extension regions. The results may help to understand pathways of Cantwithin the ocean under a future transition to zero or negative emissions.