Large Re-emergence 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 (re-emergence) rates of anthropogenic carbon (Cant) 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 Cant re-emergence pathways to the relatively rapid renewal timescales of mode waters through the overturning circulation. Our main finding is that for this model with 2.04 PgC/yr of uptake of Cant via gas exchange, the subduction and obduction rates across the base of the mixed layer (MLbase) are 4.96 PgC/yr and 4.50 PgC/yr, respectively, which are twice as large as the gas exchange at the surface. Given that there is net accumulation of 0.17 PgC/yr in the mixed layer itself, this implies the residual downward Cant transport of 1.40 PgC/yr across the MLbase is associated with diffusion. Importantly, the net patterns for subduction and obduction transports of Cant mirror the large-scale patterns for transport of water volume, thereby illustrating the processes controlling Cant uptake. Although the net transfer across the MLbase by compensating subduction and obduction is relatively smaller than the diffusion, localized pattern of Cant subduction and obduction implies significant regional impacts. The median timescale for re-emergence of obducting particles is short (less than 10 years), indicating that re-emergence should contribute to limiting future carbon uptake through its contribution to perturbing the Revelle factor for surface waters