Deep through-flow in the Bight Fracture Zone

Abstract

Abstract. Iceland–Scotland Overflow Water (ISOW) is exported from the Nordic Seas into the Iceland Basin to feed the lower limb of the Meridional Overturning Circulation. The Bight Fracture Zone (BFZ) is known to be a major route for ISOW toward the Irminger Sea, but the role of this gateway in the evolution of ISOW properties over the subpolar gyre is unclear. A combination of ship-based and Deep-Argo data gathered between 2015 and 2018 allows us to investigate the pathways and hydrographic evolution of ISOW as it flows through the BFZ, as well as its influence on the North Atlantic Deep Water (NADW) properties in the Irminger Sea. The ISOW flow through the BFZ amounts to 0.8 ± 0.2 Sv and is mainly fed by the lighter part of the ISOW layer flowing west of 29–30∘ W as part of the East Reykjanes Ridge Current in the Iceland Basin. In the rift valley of the BFZ, between an eastern and a western sill, the bathymetry of the BFZ shapes a cyclonic circulation along which the ISOW layer is homogenized. The largest changes in ISOW properties are however observed downstream of the western sill, at the exit of the BFZ. There, ISOW is mixed isopycnally with comparatively fresher NADW circulating in the Irminger Sea. Hence, our analysis reveals the key role of the BFZ through-flow in the salinification of the NADW in the Irminger Current.

Observing the full ocean volume using Deep Argo floats

“…Small clusters of Deep Argo floats have demonstrated their ability to detect the reversal of a long-term freshening trend in the Australian Antarctic Basin, at higher temporal and spatial resolution than historical shipboard CTD occupations (Thomas et al, 2020;Foppert et al, 2021). By providing simultaneous temperature and salinity profiles over broad swaths of ocean basins, Deep Argo has proven effective at mapping large-scale circulation pathways of dense water masses formed near Antarctica (Thomas et al, 2020;Foppert et al, 2021), in the subpolar North Atlantic Ocean (Racapéet al, 2019;Petit et al, 2022), and in the Southwest Pacific Ocean (Zilberman et al, 2020; Figures 4A, B) with unprecedented detail. Seasonal fluctuations in the recirculation of the Deep Western Boundary Current of the Southwest Pacific Basin, and associated forcing mechanisms, were detected for the first time (Zilberman et al, 2020;Figures 4C), as was a slowdown of flow of AABW into the Argentine Basin (Johnson, 2022).…”

Variability of Iceland Scotland Overflow Water Across the Reykjanes Ridge: 2‐Years of Moored Observations in the Bight Fracture Zone

“…ISOW continues southwestward as a deep boundary current along the eastern flank of the RR and is partially siphoned off into the Irminger Sea to the west through gaps in the ridge (e.g., Petit et al., 2022). The first major gap in the RR with sill depth deeper than 2,000 m is the Bight Fracture Zone (BFZ) at ∼56.7°N (Figure 1).…”

“…At the BFZ (57°N), Xu et al found the mean transport to be 0.4 Sv, and mean velocities to be ∼2-5 cm/s westward. Synoptic observational transport estimates across the BFZ range from 1.3 ± 0.1 and 0.3 ± 0.1 Sv (from Lowered Acoustic Doppler Current Profiler (LADCP) sections; Petit et al, 2022) and 0.8 ± 0.8 Sv (from geostrophic estimates; Petit et al, 2018). Using Deep-Argo profiling floats set to drift at 1,900-2,100 dbar, Petit et al (2022) showed the westward pathway of ISOW through the BFZ.…”

“…Synoptic observational transport estimates across the BFZ range from 1.3 ± 0.1 and 0.3 ± 0.1 Sv (from Lowered Acoustic Doppler Current Profiler (LADCP) sections; Petit et al, 2022) and 0.8 ± 0.8 Sv (from geostrophic estimates; Petit et al, 2018). Using Deep-Argo profiling floats set to drift at 1,900-2,100 dbar, Petit et al (2022) showed the westward pathway of ISOW through the BFZ. The remaining ISOW has been shown to leave the Iceland Basin westward via the Charlie-Gibbs Fracture Zone (CGFZ) near 53°N Saunders, 1994;Xu et al, 2018;Zou et al, 2020) or southward into the West European Basin (e.g., Xu et al, 2010;Zou et al, 2017).…”

“…Using Deep‐Argo profiling floats set to drift at 1,900–2,100 dbar, Petit et al. (2022) showed the westward pathway of ISOW through the BFZ. The remaining ISOW has been shown to leave the Iceland Basin westward via the Charlie‐Gibbs Fracture Zone (CGFZ) near 53°N (Bower & Furey, 2017; Saunders, 1994; Xu et al., 2018; Zou et al., 2020) or southward into the West European Basin (e.g., Xu et al., 2010; Zou et al., 2017).…”