The oceanic physical injection pump of organic carbon

The contribution of the ocean biological carbon pump to the export of organic carbon at depth has predominantly been assessed by considering sinking particulate matter and vertically migrating organisms. Despite growing recognition of the importance of dynamical pathways that export carbon through upper-ocean mixing and advection, observation-based estimates of their global impact are still lacking. Here, we quantify the values and uncertainties of the export driven by the physical injection pump (PIP) and its interannual variability by leveraging a 4D data-driven time series (1997-2018) of particulate organic carbon concentration (POC) and ocean circulation, as well as 3D fields of climatological dissolved organic carbon (DOC). Vertical diffusion dominates our POC export estimates, but remains the most uncertain process. Assuming maximal diffusivity estimates that are consistent with observations, POC and DOC export amount to 0.37 Pg C yr⁻¹ and 0.48 Pg C yr⁻¹, respectively. The contribution from entrainment and advection is strongly modulated by the POC annual cycle, revealing the critical coupling between biological production and upper-layer mixing in driving the net annual export. Observed interannual signals correlate with a linear combination of El Niño-Southern Oscillation and Southern Annular Mode indices, suggesting that the PIP is connected to intermediate- and mode-water formation dynamics in the Southern Ocean.