Air–sea CO₂ exchange in the Southern Adriatic Sea: assessing its role as a moderate carbon sink over the last decade

Abstract. Coastal waters contribute significantly to the total oceanic carbon uptake. In this context, the cumulative influence exerted by marginal seas may be conspicuous. However sparse and unevenly distributed observations in such regions pose a serious limit to an accurate, experimentally based quantification of carbon dynamics. The Southern Adriatic (SAd) is one of the key sites of the Mediterranean Sea where open-ocean deep water formation occurs, a process recognized as a major driver of carbon sequestration. However, only a limited number of studies have investigated surface carbon dynamics and quantified air-sea carbon flux in the SAd. In this study, a newly validated, decade-long (2015–2024) high-resolution time series of CO₂ and hydrographic measurements collected at the EMSO-E2M3A South Adriatic observatory, located at the center of the Southern Adriatic Pit, has been analysed. The results showed that seasonal temperature variability and winter convection were the main processes driving the dynamics of surface partial pressure of CO₂ (pCO_2sw). Air–sea CO₂ flux (FCO₂), derived from in situ observations, indicated a clear seasonal pattern, with the SAd acting as a CO₂ sink during winter and as a source during summer. Importantly, the results revealed that the SAd acted as a moderate carbon sink over the last decade. Uncertainty analysis in the flux estimates were also evaluated, revealing that wind speed input data used for FCO₂ calculations influenced the magnitude of FCO₂. Finally, the results presented here showed how time series such as the SAd dataset can serve as critical assets for validating operational ocean models, such as the EU Copernicus Marine Service for the Mediterranean, by helping to identify discrepancies in the simulation of key processes.