Delayed-Mode Calibration of Hydrographic Data Obtained from Animal-Borne Satellite Relay Data Loggers

International audienceA delayed-mode calibration procedure is presented to improve the quality of hydrographic data from CTD-Satellite Relay Data Loggers (CTD-SRDL) deployed on elephant seals. This procedure is applied on a dataset obtained with 10 CTD-SRDLs deployed at Kerguelen Islands in 2007. A comparison of CTD-SRDLs with a ship-based CTD system is first presented. A pressure-effect correction, linear with pressure, is deduced for both temperature and salinity measurements. An external field effect on the conductivity sensor is also detected, inducing an additional salinity offset. The salinity offset cannot be estimated directly from the ship-based CTD comparisons, because the attachment of the CTD-SRDL on the seal head modifies the magnitude of the external field effect. Two methods are proposed for estimating a posteriori the salinity offset. The first method uses the stable salinity maximum characterizing the Lower Circumpolar Deep Water (LCDW), sampled by seals foraging south of the Southern Antarctic Circumpolar Current Front. Where this approach is not possible, a statistical method of cross-comparison of CTD-SRDLs surface salinity measurements is used over the sluggish Northern Kerguelen Plateau. Accuracies are respectively estimated as +/- 0.02 degrees C for temperature and +/- 0.1 for derived salinity without corrections. The delayed-mode calibration significantly improves the CTD-SRDL data, improving accuracies to +/- 0.01 degrees C and +/- 0.03, respectively. A better salinity accuracy of +/- 0.02 is achieved when the LCDW method can be used. For CTD-SRDLs where ship-based CTD comparisons are not available, the expected accuracy would be +/- 0.02 degrees C for temperature and +/- 0.04 for the derived salinity