Ocean carbon sink assessment via temperature and salinity data assimilation into a global ocean biogeochemistry model

Abstract. Global ocean biogeochemistry models are frequently used to derive a comprehensive estimate of 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 and uncertainties in the initialization of model variables lead to errors in their predictions. Here, observations of ocean physics (temperature and salinity) are assimilated into the ocean biogeochemistry model FESOM-REcoM over the period 2010–2020 to study the effect on the air-sea CO₂ flux and other biogeochemical variables. While the free running model already represents temperature and salinity rather well, the assimilation further improves it and hence influences the modeled ecosystem and CO₂ fluxes. The assimilation has mainly regional effects on the air-sea CO₂ flux, with the largest imprint of assimilation in the Southern Ocean during winter. South of 50° S, winter CO₂ outgassing is reduced and thus the mean CO₂ uptake increases by 0.18 Pg C yr^-1 through the assimilation. Other particularly strong regional effects on the air-sea CO₂ flux are located in the area of the North Atlantic Current. Yet, the effect on the global ocean carbon uptake is a comparatively small increase by 0.05 Pg C yr^-1 induced by the assimilation, yielding a global mean uptake of 2.78 Pg C yr^-1 for the period 2010–2020.