Large reemergence of anthropogenic carbon into the ocean's surface mixed layer sustained by the ocean's overturning circulation

We evaluate the output from a widely used ocean carbon cycle model to identify the subduction and obduction (reemergence) rates of anthropogenic carbon (C_ant) for climatological conditions during the World Ocean Circulation Experiment (WOCE) era in 1995 using a new set of Lagrangian diagnostic tools. The principal scientific value of the Lagrangian diagnostics is in providing a new means to connect C_ant reemergence pathways to the relatively rapid renewal time scales of mode waters through the overturning circulation. Our main finding is that for this model with 2.04 PgC yr^-1 of uptake of C_ant via gas exchange, the subduction and obduction rates across the base of the mixed layer (ML_base) are 4.96 and 4.50 PgC yr^-1, respectively, which are twice as large as the gas exchange at the surface. Given that there is net accumulation of 0.17 PgC yr^-1 in the mixed layer itself, this implies the residual downward C_ant transport of 1.40 PgC yr^-1 across the ML_base is associated with diffusion. Importantly, the net patterns for subduction and obduction transports of C_ant mirror the large-scale patterns for transport of water volume, thereby illustrating the processes controlling C_ant uptake. Although the net transfer across the ML_base by compensating subduction and obduction is relatively smaller than the diffusion, the localized pattern of C_ant subduction and obduction implies significant regional impacts. The median time scale for reemergence of obducting particles is short (< 10 yr), indicating that reemergence should contribute to limiting future carbon uptake through its contribution to perturbing the Revelle factor for surface waters.