BOCATS 2023. CTD-02 Data Report

BOCATS 2023 cruise focused on the 11th repetition of the OVIDE section. OVIDE (BOCATS) cruises have been run biennially since 2002, always in June-July, and the section is labelled as a high-resolution reference section in the international program GO-SHIP. BOCATS 2023 started in Vigo on June 8 and ended in Reykjavik on July 14 on the R/V Sarmiento de Gamboa. The 98 stations of the OVIDE section, from Portugal to the tip of Greenland, could be realized in totality. The core of the work consisted in acquiring surface-to-bottom hydrographic profiles of pressure, temperature, salinity and dissolved oxygen, and analyzing 14 physical and biogeochemical tracers from 11 920 samples drawn from the Niskin bottles of the CTD frame. We report here a focus on the acquisition and calibration steps of the physical variables measured specifically by the hydrographic CTD-O2 probe. After calibration, we find precisions for pressure, temperature, salinity and dissolved oxygen that fit the GO-SHIP international quality requirements. Pressure and temperature were calibrated at the laboratory before and after the cruise, leading to a repeatability better than 1 dbar and 0.001 °C respectively. The calibration of salinity and dissolved oxygen data is obtained by applying polynomial correcting functions that are calculated to statistically minimize the differences between the probe data and the sample data analyzed on board in the chemistry container. These chemical data are also evaluated by the comparison of replicates. After calibration, the differences in salinity and oxygen follow a zero-centered Gaussian-like distribution which standard deviation is used to evaluate the probe precision for each variable. For salinity, we find a standard deviation of 0.001, and for oxygen, 0.021 ml/l (or 1 μmol/kg). Those numbers correspond to a 68 % confidence interval and must be doubled to obtain the precision within a 95 % confidence interval.