CO₂ and hydrography acquired by Autonomous Surface Vehicles from the Atlantic Ocean to the Mediterranean Sea: data correction and validation

Abstract. The ATL2MED demonstration experiment involved two autonomous surface vehicles provided by Saildrone Inc. (SD) along a route from the tropical eastern North Atlantic to the Adriatic Sea between October 2019 and July 2020. This nine-month long experiment located in a transition zone between the temperate and tropical belts represents a major challenge in the use of SD. The sensors on board were subjected, to varying degradation degrees depending on the geographical area and the season, to biofouling with consequent deterioration of the acquired measurements. As a result, several maintenance along the mission's course were necessary. We address the difficulty of correcting the data during a period of COVID-19 restrictions, which significantly reduced the number of discrete samples planned for SD salinity and dissolved oxygen validation. This article details alternative correction methods for salinity and dissolved oxygen. Due to the lack of in situ data, model products have been used to correct the salinity data acquired by the SDs, and then the resulting corrected salinity was validated with data from fixed ocean stations, gliders, and Argo floats. In addition, dissolved oxygen data acquired from SDs after correction using air oxygen measurements were tested and found to be in line with the oxygen values expected from temperature and chlorophyll-a data. The correction methods are relevant and useful in situations where validation capabilities are lacking, which was the case during the ATL2MED demonstration experiment. For future experiments, it is recommended that validation samples are collected more frequently. An overview over data availability is found in Section 5, Table 5.