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

This study presents the first continuous observations of Iceland Scotland Overflow Water (ISOW) passing through the Bight Fracture Zone (BFZ), the northernmost deep bathymetric channel across the Reykjanes Ridge between the Iceland and Irminger Basins in the subpolar North Atlantic. Data from two 2‐year moorings, measuring temperature, salinity, and current velocity from 2015 to 2017, along with a set of deep ISOW‐embedded RAFOS floats, are used to investigate ISOW transport and water property variability through the BFZ, as well as advective pathways between the Iceland and Irminger Basins. The mooring‐derived record‐mean ISOW transport through the BFZ was −0.59 ± 0.27 × 1e6 m3/s (westward) and varied seasonally with weaker transport in winter and stronger transport in summer. Flow direction of ISOW through the BFZ was consistently westward except in winter, when week‐long flow reversals were frequently observed. The previously reported subpolar North Atlantic freshening event of the 2010s is evident in the BFZ mooring records beginning about January 2017. About one‐quarter of floats deployed in ISOW at 1800‐m depth upstream in the Iceland Basin show a direct advective pathway into the BFZ that appears to be primarily determined by bathymetry. Another quarter of the floats crossed over the ridge to the Irminger Sea through other gaps prior to reaching the Charlie‐Gibbs Fracture Zone. Plain Language Summary: The deep currents in the North Atlantic Ocean transport cold, dense waters from high latitudes southward as part of the climate‐relevant overturning circulation. Understanding of the pathways and transports of these currents is improving, but some knowledge gaps persist. This paper describes the first sustained measurements of the deep current transporting cold, dense water through a major gap in the shallow Mid‐Atlantic Ridge, called the Bight Fracture Zone (BFZ), during 2015–2017. The resulting estimate of the mean volume transport through BFZ (about 600,000 m3/s westward) helps to quantify the total amount of this dense water mass passing from the eastern to western North Atlantic and provides a benchmark for numerical simulations of the deep ocean circulation in this region. The measurements also revealed an apparent seasonal cycle in volume transport, and unexpected flow reversals, mostly in winter. In early 2017, a significant decrease in salinity was observed, indicating the arrival in the BFZ of a known large‐scale low‐salinity anomaly that has been circulating around the northern North Atlantic since about 2010. Taken together, the results add several pieces to the puzzle that is the complex deep ocean circulation of the North Atlantic. Key Points: First time series of Iceland Scotland Overflow Water (ISOW) transport through the Bight Fracture Zone (BFZ) indicates mean of −0.6 ± 0.3 Sv (westward)These year‐round observations of ISOW transport through the BFZ reveal, for the first time, wintertime flow reversals and an annual cycleDeep floats show a direct advective pathway into the BFZ that appears most influenced by local bathymetry [ABSTRACT FROM AUTHOR]