Forced mechanisms of decadal variability in the ocean carbon sink

The ocean has absorbed approximately 40% of industrial-age fossil carbon emissions, and thus has substantially damped climate change. Better understanding of decadal variability in the ocean carbon sink is crucial for accurate diagnosis of the global carbon cycle, and will improve confidence in future predictions. Ensembles of hindcast ocean models and ensembles of observation-based products indicate globally-coherent changes in the sink since the 1990s. Though interannual variability of CO₂ fluxes is associated with El Nino / Southern Oscillation, the dominant pattern of decadal variability occurs outside the eastern equatorial Pacific, and is largely globally homogenous. This global pattern includes a rapid sink increase in the early 1990s, a stagnation until 2001, and then a recovery through 2017. Using a single-reservoir upper ocean diagnostic box model, we attribute these globally-coherent changes to two external forcings. The slowdown of the growth rate of atmospheric pCO₂ in the 1990s contributed to the reduced mean sink in this decade, but does not explain the intra-decadal timing. This timing is attributable to the surface ocean temperature response to the eruption of Mt. Pinatubo in 1991. This eruption caused rapid surface ocean cooling and a dramatic increase of the sink for 1992-93. Subsequently, the sink declined as the surface ocean temperature recovered from the Pinatubo effect. The ocean sink then grew from 2001 to 2017 due to an enhanced growth rate of atmospheric pCO₂. When only these two external forcings are applied to the diagnostic box model, air-sea CO₂ fluxes are highly correlated with the data-based products (r = 0.94) and the hindcast models (r=0.93). Thus, we conclude that these two external forcings have been primarily responsible for the recent decadal variability of the ocean carbon sink.