Implementing riverine biogeochemical inputs in ECCO-Darwin: a sensitivity analysis of terrestrial fluxes in a data-assimilative global ocean biogeochemistry model

Short Summary: Accounting for carbon and nutrients in rivers is essential for resolving carbon dioxide (CO₂) exchanges between the ocean and the atmosphere. In this study, we add the effect of present-day rivers to a pioneering global-ocean biogeochemistry model. This study highlights the challenge for global ocean numerical models to cover the complexity of the flow of water and carbon across the Land-to-Ocean Aquatic Continuum.

Abstract: Terrestrial sources of carbon and nutrients drive biogeochemical cycles in coastal regions and in the global ocean. Quantifying their impact on the spatiotemporal variability of the ocean carbon cycle is pivotal to understanding the distinctive characteristics of ocean basins dominated by riverine inflow. ECCO-Darwin is a data-constrained, global-ocean biogeochemistry model that has heretofore lacked lateral inputs of carbon and nutrients. The objective of this study is to add this new capability to ECCO-Darwin and to carry out a suite of sensitivity experiments in order to quantify the impact of these lateral fluxes on coastal- and open-ocean biogeochemistry. 

In this work, we use an optimized version of the data-assimilative global-ocean biogeochemistry ECCO-Darwin model to perform a sensitivity analysis of the ocean to lateral inputs of carbon and nutrients. We generate riverine inputs by combining daily point-source freshwater discharge from JRA55-do with the Global NEWS 2 watershed model, accounting for lateral inputs from 5171 watersheds worldwide. The addition of riverine inputs drives a small CO₂ outgassing (+0.02 Pg C yr⁻¹) due to compensating processes at regional scales. In basins dominated by carbon runoff, such as the Tropical Atlantic and Arctic Oceans, the addition of riverine inputs increases CO₂ outgassing (+13 % and +9 %, respectively). In contrast, runoff in nutrient-dominated Southeast Asia leads to increased CO₂ uptake (+9 %). 

This new riverine biogeochemical input capability will enable future ECCO-Darwin solutions to better capture key processes that occur along coastal margins in global oceans.