Drivers of ocean carbon sink variability across spatial scales

Since the preindustrial era, the ocean has removed roughly 40% of fossil CO₂ from the atmosphere, and it will eventually absorb at least 80% of human CO₂ emissions. While there is no doubt that the ocean is a critical player in the global carbon cycle, many uncertainties remain and the drivers and magnitude of interannual-to-decadal timescale variability remain poorly constrained. A key question is the extent to which external forcing, specifically the variability of the atmospheric pCO₂ growth rate, or internal ocean variability is the dominant mechanism of variability. We use a suite of experiments from the ECCO-Darwin data-assimilative ocean biogeochemistry model to isolate and explore the impact of these two drivers. We demonstrate that at the global scale, external and internal variability equally drive ocean sink variability. However, as the spatial scale becomes more regional, internal variability becomes increasingly dominant. To diagnose the future evolution of the global-scale ocean carbon sink in response to a changing atmospheric growth rate, both skillful observation-based products and data-assimilative models will be required.