Assessment of long-term stability of inductive conductivity sensors at Argo floats

This study analyses accuracy and stability of salinity measurements collected by four Argo autonomous drifters with RBR Ltd. inductive conductivity sensors operating in the Pacific Ocean during the recent 2-4 years. Inductive sensors have advantages over traditionally used electrode-type cells due to their better resistance to surface contamination and low power requirements, resulting in more robust and accurate measurements and extended float lifetimes.  Proper assessment of the quality of the data collected by autonomous drifters is challenging due to lack of reference information. An important part of Argo program is the Delayed-Mode Quality Control process including salinity drift analysis and correction using the ‘Owens-Wong Calibration’ (OWC) method based on objective mapping of available reference data. This method, however, can misinterpret imperfect reference data as sensor drift. In this study, analyzing OWC output we introduce a combination of visualization methods focused on the locations where reference data can be treated as problematic. These methods include the analysis of spatial locations of the ‘profile correction factor’ along the float trajectory, comparing reference salinity fields calculated by the OWC method to additional reference sources (climatologies) and comparative analysis of different floats operating in the same area using the same reference datasets. The results demonstrate high level of stability of inductive conductivity cells on Argo floats, making them promising alternative for traditionally used Argo float CTDs equipped with electrode-type conductivity sensors.