The North Atlantic Ocean is a major driver of the European climate at seasonal to multi-decadal timescales. It also contributes to damp the impact of human activity on climate by storing part of the anthropogenic CO₂ and heat excess related to the strengthening of the greenhouse effect. In this project, we focus on the subpolar North Atlantic for which the interannual to decadal variability is much more important than the climatic trends associated to the anthropogenic perturbations. After a relatively warm and salty decade in the 2000s, the subpolar North Atlantic shifted to a cold and fresh state in 2012 that presents some similarities with that observed at the beginning of the 1990s, but in a context of warming in all the other oceans of the planet. This change is also characterized up to now by a deeper convection and an increase in the amplitude of the Meridional Overturning Circulation (MOC). In this context, the project aims to quantify the variability of the horizontal circulation, the MOC, the heat budget, the carbon budget and the water mass properties by repeating the OVIDE-A25 section in June-July 2018, from Portugal to Greenland. The section is composed of 98 stations that combine hydrographic, biogeochemical and current measurements down to the bottom.

Data of the CTDO₂ probe are complemented with measurements of pH, alkalinity and nutrients to determine the concentration in dissolved inorganic carbon and anthropogenic carbon. Independent measurements of the isotopic fractions of carbon and oxygen in certain areas also help to follow the anthropogenic signal and the sources of fresh water respectively. In order to determine the age and the origin of the water masses, four anthropic tracers are measured from the sampled sea water: chlorofluorocarbon (CFC), Sulfur hexafluoride (SF₆), iodine and uranium isotopes. Currents are determined by Acoustic Doppler Current Profilers (mounted on the ship and on the rosette), that are combined to the currents deduced from density gradients to obtained absolute geostrophic velocities and transports across the section. The deployment of deep-Argo floats in specific circulation features identified in real time brings additional information on the deep circulation that suffers from a lack of observations, as revealed by our previous studies. The oceanic currents around the Reykjanes Ridge is studied with the renewal of 2 ASFAR moorings (Autonomous System for Argo floats Release) that release an ARGO float at pre-determined dates. The data analysis of the survey is performed in synergy with the Argo and OSNAP programs, the satellite altimetry, the oceanic re-analysis (ISAS, EN4, ORAS4, GLORYS) and the output of physical-biogeochemical coupled models.