First direct estimates of volume and water mass transports across the Reykjanes Ridge

• Published in: Journal of Geophysical Research: Oceans
• Main Authors: Petit, Tillys, Mercier, Herle, Thierry, Virginie
• Format: Article in Journal/Newspaper
• Language: English
• Published: Amer Geophysical Union 2018
• Subjects: North Atlantic subpolar gyre; bathymetry; water masses; circulation; observations; Arctic; Bight Fracture Zone; Mid-Atlantic Ridge; Reykjanes; Iceland; Labrador Sea; north atlantic current; North Atlantic
• Online Access: https://archimer.ifremer.fr/doc/00454/56604/58296.pdf; https://doi.org/10.1029/2018JC013999; https://archimer.ifremer.fr/doc/00454/56604/

The Reykjanes Ridge is a major topographic feature located south of Iceland in the North‐Atlantic Ocean that strongly influences the subpolar gyre (SPG) circulation. Based on velocity and hydrographic measurements carried out along the crest of the Reykjanes Ridge from the Icelandic continental shelf to 50°N during the RREX cruise in June – July 2015, we derived the first direct estimates of volume and water mass transports over the Reykjanes Ridge. North of 53.15°N, circulation was mainly westward; south of this latitude it was mainly eastward. The westward transport was estimated at 21.9 ± 2.5 Sv (Sv = 106 m3 s‐1) and represents the SPG intensity. The westward flows followed two main pathways at 57°N near the Bight Fracture Zone and at 59 – 62°N. We argue that those pathways were connected to the northern branch of the North Atlantic Current and to the Sub‐Arctic Front respectively, which were both intersected by the southern part of the section. In addition to this horizontal circulation, mixing and bathymetry shaped the water mass distribution. Water mass transformations in the Iceland Basin lead to the formation of weakly stratified SubPolar Mode Water (SPMW). We explain why SPMW, the main water mass contributing to the westward flow, was denser at 57°N than at 59 – 62°N. At higher densities, both Intermediate Water and Icelandic Slope Water contributed more to the westward transport across the Reykjanes Ridge than the sum of Labrador Sea Water and Iceland‐Scotland Overflow Water. Plain Language Summary The Reykjanes Ridge, the northern section of the Mid‐Atlantic Ridge, strongly influences the cyclonic circulation of the North‐Atlantic subpolar gyre, a major component of the climate system. Up to now, no dedicated dataset was available to describe the circulation across this ridge. To fill this gap, surface‐to‐bottom measurements of flow velocity and water mass properties were carried out along the crest of the ridge, from Iceland to 50{degree sign}N, in 2015. North of 53.15{degree sign}N, the flow was mainly westward. It defines the westward branch of the subpolar gyre and our study provides the first direct estimate of its intensity. The westward flow followed two main pathways related to specific bathymetry features: at the Bight Fracture Zone (57{degree sign}N), which is a deep opening in the ridge, and at 59 ‐ 62{degree sign}N where the bathymetry rapidly deepens southward. The horizontal circulation of the Iceland Basin connects these pathways to the North‐Atlantic Current flowing eastward south of 53.15{degree sign}N. Knowledge of the westward cross‐ridge flows is a prerequisite for understanding the northward evolution of the Irminger Current, a major conduit for the subtropical waters towards the deep convection regions in the Irminger and Labrador Seas.