High-resolution air–sea CO₂ observations during the ATL2MED mission: data correction and process variability across the Eastern Atlantic Ocean and the Mediterranean Sea

The ATL2MED mission (October 2019–July 2020) investigated air–sea CO₂ exchange across the Eastern Atlantic Ocean and the Mediterranean Sea using high-resolution measurements from Saildrone autonomous surface vehicles (SDs), complemented by fixed stations, gliders, and research vessels. Operating under diverse environmental conditions, the SDs provided detailed observations of seawater CO₂ and hydrographic parameters, although sensor drift and biofouling affected data quality during the long deployment. Dedicated data correction and validation procedures were applied: salinity was corrected using model products and validated against independent observations. Dissolved oxygen was adjusted using the Argo oxygen correction. These efforts compensated for limited discrete sampling during COVID-19 restrictions. The corrected data revealed strong regional contrasts in CO₂ dynamics driven by physical and biogeochemical processes. Intense outgassing occurred in the upwelling regions off northwest Africa, while the western Mediterranean Sea acted as a CO₂ sink during the spring bloom. The Adriatic Sea showed recurrent outgassing episodes linked to stratification, river plumes, and coastal upwelling. The SDs captured sub-mesoscale and short-term variability often missed by traditional platforms and model simulations. The study highlights the importance of high-frequency, multi-platform measurements to resolve the highly variable air–sea CO₂ fluxes occurring at short temporal scales.