CROSSROAD 2024. CTD-O2 Data report

A key vector for climate signal propagation is the North Atlantic Deep Water (NADW), an ocean water mass that connects with the atmosphere at high northern latitudes before sinking and spreading southward. The rate and extent of NADW spreading and the modification of its properties are determined by a complex dynamical system at two topographic subpolar-subtropical choke points: Flemish Cap and the Grand Banks of Newfoundland. Existing observations, however, only enables a partial description of typical circulation and mixing patterns and cannot support a comprehensive mechanistic understanding NADW dynamics across this western subpolar-subtropical boundary. The goal of the CROSSROAD project is to use novel observational and numerical tools to provide this process-oriented assessment of NADW motions and transformation across these choke points and to study their impacts on climate-relevant metrics. As part of a series of three cruises during 2023-2025 partly supported by the European project EPOC (Explaining and Predicting the Ocean Conveyor), an hydrographic cruise on R/V Thalassa started in St. John’s (Newfoundland) the 2nd of July and ended the 28th of August in the same place. The core of the work was carried out above the continental slope of Flemish Cap and the tail of the Grand Banks and consisted in full-depth CTD-O2 and LADCP profiles (including several tow-yos), VMP profiles, deployment of an autonomous moored VMP (MicroRiyo), acoustic telemetry on bottom pressure recorders, and deployments of Deep Argo floats. 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. Pressure and temperature were adjusted at Seabird and calibrated to the laboratory 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 0.002, and for oxygen, 0.045 ml/l (or 2 μmol/kg). Those numbers correspond to a 68 % confidence interval and must be doubled to obtain the precision within a 95 % confidence interval.