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

Abstract

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.

Preparing the New Phase of Argo: Scientific Achievements of the NAOS Project

“…The floats drifted in the core of ISOW at 2750 m. Based on the mean circulation scheme of Daniault et al (2016), we expected that all the floats would veer northward downstream of the CGFZ and flow cyclonically in the Irminger Sea. However, Racapé et al (2019) showed that none of the floats reached the Irminger Sea and one float even revealed a new direct route from ISOW toward the subtropical gyre without looping in the Irminger Sea. Oxygen data acquired by the floats showed that the ISOW layer was mainly composed of highly oxygenated ISOW and less oxygenated North East Atlantic Deep Water (NEADW), a complex water mass from the East Atlantic.…”

“…The deoxygenation of the ocean is a major concern in the context of climate change (Keeling et al, 2010), and the Deep-Arvor float is therefore equipped with an oxygen sensor to monitor oceanic oxygen content. In addition, dissolved oxygen concentration (referred to as O 2 in the following) is a classic parameter in oceanography, useful for gaining insight into biogeochemical (e.g., Riser and Johnson, 2008) and physical oceanic processes (Piron et al, 2016;Racapé et al, 2019). O 2 data are also important for operational oceanography that aims at monitoring and predicting the biogeochemical state of the ocean and marine ecosystems (Brasseur et al, 2009;Biogeochemical-Argo Planning Group, 2016;Le Traon et al, 2019).…”

“…For each, a calibrated ship-based reference profile was acquired at float deployment. Comparisons made to the reference profiles revealed that most floats exhibited a fresh bias at depth (see Figure 13, Le Reste et al, 2016;Racapé et al, 2019). While there is certainly a pressure effect on the sensor, the question of whether this pressure effect is well corrected is also raised.…”

“…Correction is made either by comparison with in situ reference data following Takeshita et al (2013) or with in-air measurements following Bittig and Körtzinger (2015) or Johnson K. S. et al (2015). According to a comparison with ship-based high-quality reference profiles, additional pressure correction was required on some floats (Gallian and Racapé et al, 2019). While a simple pressure-correction model helps remove this pressure-dependent bias, its origin still needs to be determined.…”

Redrawing the Iceland−Scotland Overflow Water pathways in the North Atlantic

“…The study further shows that in the model, most of the ISOW passing through the CGFZ follows this path, with a much lower fraction turning northward into the Irminger Sea. More recently, Racapé et al 19 reported that one of the five Deep-Argo floats initialized in the CGFZ followed a direct westward route from the CGFZ to the Flemish Cap, where it was able to join the Deep Western Boundary Current.…”