ISOW Spreading and Mixing as Revealed by Deep‐Argo Floats Launched in the Charlie‐Gibbs Fracture Zone.

To improve our understanding of deep circulation, we deployed five Deep‐Argo floats (0–4,000 m) in the Charlie‐Gibbs Fracture Zone (CGFZ), which channels the flow of Iceland‐Scotland Overflow Water (ISOW), a dense water mass of the North Atlantic Ocean. The floats were programed to drift at 2,750 dbar in the ISOW layer. The floats mainly moved westward in the CGFZ, although some of them followed different routes for few cycles depending on northward intrusions of the North Atlantic Current over the CGFZ. One float revealed a direct route for ISOW from CGFZ to the Deep Western Boundary Current at Flemish Cap. In the CGFZ, oxygen data acquired by the floats revealed that the ISOW layer, characterized by salinity higher than 34.94 and density greater than 27.8 kg/m, was mainly composed of the highly oxygenated ISOW and the less oxygenated North East Atlantic Deep Water (NEADW), a complex water mass from the East Atlantic. In the ISOW layer, the relative contribution of ISOW was generally larger in the northern valley than in the southern valley of CGFZ. Northward intrusions of the North Atlantic Current above the CGFZ increased the relative contribution of NEADW in the northern valley and favors mixing between ISOW and NEADW. The ISOW‐NEADW signal flowing westward from the CGFZ toward the Deep Western Boundary Current was progressively diluted by Labrador Sea Water and Denmark Strait Overflow Water. Oxygen measurements from Deep‐Argo floats are essential for a better understanding and characterization of the mixing and spreading of deep water masses. Plain Language Summary: The North Atlantic Ocean contributes to the uptake in the deep ocean of the excess of heat received by Earth due to human activities. The heat redistribution toward the rest of the ocean depends on the deep circulation, which is still largely unknown. To improve our understanding of this deep circulation, we deployed in 2015 and 2017 five Deep‐Argo floats in the Charlie‐Gibbs Fracture Zone (CGFZ), a gap in the Mid‐Atlantic Ridge that constraints the pathway of deep water masses. Those autonomous platforms freely drifted at 2,750 dbar in the core of the Iceland‐Scotland Overflow Water (ISOW), a young water mass, rich in O2, originating from the Nordic Seas. One float revealed a new direct route of ISOW toward the subtropical gyre. The pathway followed by the floats west of the CGFZ depended on northward intrusions of the North Atlantic Current over the CGFZ. This interaction between the North Atlantic Current and the deep flow in the CGFZ favors the mixing of ISOW with the North East Atlantic Deep Water, an old water mass characterized by low O2. These results advocate for equipping Deep‐Argo floats with oxygen sensors to improve understanding of deep circulation and water mass mixing. Key Points: A direct route is suggested for ISOW from the Charlie‐Gibbs Fracture Zone to the Deep Western Boundary Current at Flemish CapAt the Charlie‐Gibbs Fracture Zone, oxygen measurements are key to quantifying ISOW mixing with North East Atlantic Deep WaterMixing between ISOW, North East Atlantic Deep Water, Labrador Sea Water, and Denmark Strait Overflow Water is observed in the western basin [ABSTRACT FROM AUTHOR]