Anthropogenic carbon in the ocean - surface to interior connections

Quantifying the surface to interior transport of anthropogenic carbon (C_A) is critical for projecting future carbon uptake and for improved understanding of the role of the oceans in the global carbon cycle. Here we develop and apply a diagnostic tool that provides a volumetric stream function in (C_A,σ₀) coordinates to calculate the total diapycnal C_A transport in the ocean, where σ₀ is the surface referenced potential density anomaly. We combine this with air-sea fluxes of C_A to infer the internal ocean mixing of C_A to obtain a closed globally integrated budget analyses of the ocean's C_A transport. This diagnostic separates the contribution from the mean flow, seasonal cycles, trend, surface fluxes, and mixing in the distribution and the accumulation of C_A in the ocean. We find that the redistribution of C_A from the surface to the interior of the ocean is due to an interplay between circulation and mixing. The circulation component is dominated by the mean flow; however, effects due to seasonal cycles are significant for the C_A redistribution. The two most important pathways for C_A subduction are through the transformation of thermocline water (TW) into subantarctic mode water and by transformation of Circumpolar Deep Water (CDW) into lighter Antarctic Intermediate Water. The results suggest that an accurate representation of intermediate and mode water formation, deep water formation, and spatial and temporal distribution of ocean mixing in ocean models is essential to simulate and project the oceanic uptake of C_A.