Establishing a Timeline of Upper to Deep AMOC Connections in the North Atlantic Subpolar Gyre Through Observations of Dramatic Freshening in the Iceland Scotland Overflow Plume

Mooring based hydrographic observations from the Overturning in the Subpolar North Atlantic Program (OSNAP) show a strong upper ocean freshening event in the Iceland Basin sector of the North Atlantic Subpolar Gyre beginning in mid 2015. By early 2017, freshening is observed in the abyssal Iceland Scotland Overflow (ISOW) layer of the basin. We utilize the salinity anomaly as a pseudo-tracer to show that the observed abyssal freshening is driven by entrainment of North Atlantic upper ocean waters. This is done by establishing the characteristic timescales and pathways of the Iceland Basin’s upper to deep ocean connections. A combination of surface drifter trajectories and ARGO based hydrography are used to establish transit times of the salinity anomaly through the Iceland Basin to its entrainment site at the Faroe Bank Channel, as well as to deep water formation regions in the Nordic Seas. The dominant upper ocean pathway is via the North Atlantic Current in the central Iceland Basin, with a 1-year transit time from OSNAP array at 58°N to the Faroe Bank entrainment area. Notably faster advection also occurs via the Hatton Bank and Rockall Bank jets (3-5 months). The anomaly is also shown entering the Norwegian Sea, a deep water formation site, two years after its arrival in the Iceland Basin. Model derived particle trajectories in conjunction with deep mooring observations imply a 1.5-2 year abyssal transit time from the entrainment region back to the OSNAP array via the Iceland-Scotland Overflow plume. The results indicate that entrainment is the dominant and most rapid mechanism for North Atlantic upper ocean changes to propagate into abyssal flows. This work aids in characterizing the timescales and pathways of the Atlantic Meridional Overturning Circulation’s upper to deep ocean connections.