The imprint of the Amazon River on the air-sea CO2 flux in the Tropical North Atlantic Ocean: key drivers and spatiotemporal variability

The Amazon River greatly modifies the physical and chemical properties of the ocean up to 3000 km away from its mouth, reversing the typical CO₂ outgassing of the Tropical North Atlantic Ocean (TNA). The exact quantification of this plume-related carbon sink is still disputed. This disagreement highlights both the difficulty to capture the extreme hydrographic and environmental variability of the region and gaps in our comprehension of the underlying processes responsible for these pCO₂ anomalies. To disentangle these processes, we ran simulations with an eddy-resolving configuration of a regional model (ROMS) coupled to a full biogeochemical/ecological module that includes a new phytoplankton functional group representing a symbiotic diatom-diazotrophic assemblage. In the areas close to the mouth of the river, the CO₂ enrichment of the river water and the remineralization of DOC cause a net outgassing of 0.5 Tg C yr^-1, with a seasonal maximum in June. The rest of the plume acts as a sink of 8 Tg C yr^-1. This uptake is driven primarily by biology (~70%) but also by the physical dilution of inorganic carbon and alkalinity by river water (~30%). The high biological productivity driving the CO₂ uptake is sustained by the river-derived nutrients, and amplified by the diatom-diazotroph assemblage that adds a substantial amount of additional nitrogen to the TNA by nitrogen fixation. The imprint of the Amazon River persists beyond the physical structure of its plume and leads to a river-induced total CO₂ uptake of more than 14 Tg C yr^-1 in the TNA, inferred from contrasting the standard simulation with one where the Amazon River is turned off. This river-induced CO₂ sink in the TNA is strongly modulated by the Pacific El Nino teleconnection that leads to anomalously warm sea surface temperatures, a change in the plume pathways and a reduced river discharge.