Quantifying the Present-day Ocean Carbon Cycle Response to Terrestrial Biogeochemical Fluxes with the ECCO-Darwin Data-Assimilative Global-ocean Biogeochemistry Model

Resolving riverine biogeochemical fluxes in ocean biogeochemistry models is pivotal for capturing the spatiotemporal variability of carbon sinks/sources in the global ocean. Here we add present-day lateral fluxes of carbon, nitrogen, and silica to the ECCO-Darwin ocean biogeochemistry model to quantify the impact of land-to-ocean exports on the ocean carbon cycle. We generate a daily export product by combining point-source freshwater discharge from JRA55-do with the Global NEWS 2 watershed model, accounting for lateral fluxes from 5171 watersheds worldwide. On regional scales, exports to the Tropical Atlantic and Arctic Ocean are dominated by organic carbon, which originates from terrestrial vegetation and peats and increases CO2 outgassing (+10 and +20%, respectively). In contrast, Southeast Asia is dominated by nitrogen from anthropogenic sources, such as agriculture and pollution, leading to increased CO2 uptake (+7%). While the addition of carbon and nutrients along with freshwater improves the solution in river plume and coastal-ocean, the open-ocean response may be overestimated due to an excess of carbon and nutrients advected offshore. Based on our results, we recommend that future model versions include: improved parameterizations of sediment-water exchange, explicit partitioning and more-realistic remineralization rates of semi-labile and refractory of organic matter, and better representation of estuarine processes. In summary, our results provide a major step forward in the development of a data-constrained modeling framework for estimating the present-day global carbon cycle, river loop, and natural and anthropogenic land-to-ocean fluxes.