Monitoring Present-day Ocean Carbon Cycling Response to Terrestrial Biogeochemical Fluxes with the ECCO-Darwin Data-Assimilative Global Ocean Biogeochemistry Model

While the Estimating the Circulation and Climate of the Ocean-Darwin (ECCO-Darwin) ocean biogeochemistry state estimate has demonstrated outstanding capabilities in representing space-time variability in global-ocean carbon cycling, it uses a simplified implementation of terrestrial discharge that does not contain biogeochemistry (i.e., carbon and nutrient exports). Land surface or watershed models, coupled with ocean biogeochemical models, now permit quantification of the spatiotemporally-resolved ocean carbon cycle response to riverine fluxes, which can be used to force global ocean biogeochemistry models. In this study, we add lateral fluxes of carbon and nutrients to ECCO-Darwin and evaluate the response of the ocean carbon cycle during 2000–2019. We compute daily riverine biogeochemical export by combining point-source freshwater discharge from JRA55-do with the Global NEWS 2 watershed model, accounting for lateral fluxes from 5171 watersheds worldwide Globally, the role of present-day riverine exports in ECCO-Darwin results in a source of 0.03 Pg C yr^-1 from the ocean to the atmosphere; resulting from significant, compensating regional responses in ocean carbon uptake and outgassing. In carbon-dominated margins, such as the Arctic and Tropical Atlantic Oceans, rivers drive a large source of CO₂ from the ocean to the atmosphere. However, in nitrogen-dominated margins, such as South-east Asia, rivers drive a large ocean carbon sink. Furthermore, we leverage ECCO-Darwin’s physically-consistent budgets to separate and quantify the role of physical, chemical, and biological processes driven by land-to-ocean biogeochemical fluxes. Accounting for these critical land-to-ocean processes in ECCO-Darwin represents a major step forward in the development of a holistic, data-constrained modeling framework for monitoring the global carbon cycle, which can be used to support national policy decisions, inform climate resilience strategies for land and marine practices, and evaluate marine carbon dioxide removal (mCDR) strategies.