Assessing the recent ocean carbon sink with data assimilation into a global ocean biogeochemistry model

Global ocean biogeochemistry models are a key tool for estimating the global ocean carbon uptake. These models are designed to represent the most important processes of the ocean carbon cycle, but the idealized process representation, uncertainties in the initialization of model variables and in the atmospheric forcing lead to errors in their estimates. To improve the agreement with observations, we use ensemble-based data assimilation into the ocean biogeochemistry model FESOM2.1-REcoM3. In addition to the recently implemented assimilation of temperature and salinity observations, which improves the physical model state and indirectly influences biogeochemical variables, we extend the set-up further. Here, we explicitly include the assimilation of biogeochemical observations. Specifically, in-situ sea surface pCO₂ measurements, remotely sensed chlorophyll-a, and in-situ measurements of dissolved inorganic carbon, alkalinity, oxygen, and nitrate, are assimilated to reduce the uncertainty stemming from the ecosystem model. This directly affects the modelled air-sea CO₂ flux. Here, we present an updated estimate of the ocean carbon uptake for the period 2010–2020 and compare it to prior estimates.