Unraveling the Physical and Biological Controls of the Global Coastal CO₂ Sink

The drivers governing the air-sea CO₂ exchange and its variability in the coastal ocean are poorly understood. Using a global ocean biogeochemical model, this study quantifies the influences of thermal changes, oceanic transport, freshwater fluxes, and biological activity on the spatial and seasonal variability of CO₂ sources/sinks in the global coastal ocean. We identify five typical coastal behaviors (dominated by biological drawdown, vertical transport, land imprint, intracoastal alongshore currents, and weak CO₂ sources and sinks coastal regions) and propose a new processed-based delineation of the coastal ocean based on the quantification of these controlling processes. We find that the spatiotemporal variability of CO₂ sources/sinks is dominated by strong exchanges with the open ocean and intracoastal processes, while continental influences are restricted to hotspot regions. In addition, where thermal changes appear to drive the seasonal CO₂ variability, it often results from compensating effects between individual non-thermal terms, especially biological drawdown and vertical transport.