Air-sea CO2 exchange in the Eastern Atlantic and the Mediterranean Sea based on autonomous surface measurements

Riccardo Martellucci^1*Carlotta Dentico^1,2Laurent Coppola³Ingunn Skjelvan⁴Michele Giani¹Sara Pensieri⁵Carolina Cantoni⁶Vanessa Rossana Cardin¹Marine Fourrier³Roberto Bozzano⁵Melf Paulsen⁷Elena Mauri¹

• ¹National Institute of Oceanography and Applied Geophysics (Italy), Trieste, Italy
• ²Ca' Foscari University of Venice, Venice, Veneto, Italy
• ³UMR7093 Laboratoire d'océanographie de Villefranche (LOV), Villefranche-sur-Mer, Provence-Alpes-Côte d'Azur, France
• ⁴Norwegian Research Institute (NORCE), Bergen, Hordaland, Norway
• ⁵Institute for the Study of Anthropogenic Impacts and Sustainability in Marine Environment, Department of Earth System Sciences and Technologies for the Environment, National Research Council (CNR), Rome, Lazio, Italy
• ⁶CNR-ISMAR, Trieste, Italy
• ⁷GEOMAR Helmholtz Center for Ocean Research Kiel, Helmholtz Association of German Research Centres (HZ), Kiel, Schleswig-Holstein, Germany

The ATL2MED mission conducted between October 2019 and July 2020 explored the variability of air-sea CO₂ exchange in the Eastern Atlantic and the Mediterranean Sea by integrating high-resolution autonomous measurements from Saildrone Unmanned Surface Vehicles (USVs), fixed ocean stations, gliders, and research vessels. The primary aim was to assess spatial and temporal variability of the seawater partial pressure of CO₂ (pCO2sw), identify its driving physical and biogeochemical processes, estimate the CO2 fluxes between the sea and the atmosphere, and evaluate the performance of neural network-based predictions (CANYON-MED) in diverse oceanographic regions. This study reveals pronounced regional differences in pCO2sw and CO2 fluxes, controlled by thermal effects, biological activity, and physical mixing processes. In the Eastern Atlantic, upwelling systems off northwest Africa resulted in strong outgassing, while the western Mediterranean acted as a CO₂ sink due to enhanced biological uptake during the spring bloom. In contrast, the Adriatic Sea, particularly its southern and northern basins, showed strong CO₂ outgassing episodes linked to thermal stratification, river plumes, and coastal upwelling. The Saildrone (SD) measurements were useful in resolving sub-mesoscale processes and capturing variability generally not detected by fixed platforms. Despite sensor drift challenges caused by biofouling, the autonomous platforms provided high-quality datasets, supported by cross-validation with fixed stations and gliders. Comparisons with neural network-based estimates demonstrated good agreement in most offshore regions. Overall, this study highlights the potential of SDs to enhance ocean CO₂ observations, particularly in under-sampled or logistically constrained regions. The results underscore the importance of high-resolution, multi-platform approaches for accurately quantifying CO₂ fluxes and improving predictive capabilities in a changing ocean.