Rapid Freshening of Iceland Scotland Overflow Water Driven by Entrainment of a Major Upper Ocean Salinity Anomaly

Newly available mooring observations from the Overturning in the Subpolar North Atlantic Program (OSNAP) show an abrupt decline in Iceland Scotland Overflow (ISOW) salinity from 2017 to 2018 summer. Previous declines in ISOW salinity of similar magnitude have largely been attributed to changes in convectively formed deep waters in the Nordic Seas on decadal time scales. We show that this rapid decline in salinity was driven by entrainment of a major upper ocean salinity anomaly in the Iceland Basin. This is shown by tracking the propagation of the upper ocean anomaly into ISOW using a combination of mooring and Argo observations, surface drifter trajectories, and numerical model results. A 2‐year total transit time from the upper ocean into the ISOW layer was found. The results show that entrainment allows for rapid modification of ISOW, and consequently the lower limb of Atlantic Meridional Overturning Circulation, on subdecadal timescales. Plain Language Summary New observations from the Overturning in the Subpolar North Atlantic Program (OSNAP) show a major decline of deep ocean salinity in a layer known as the Iceland Scotland Overflow (ISOW). The ISOW layer is an important component of the deep ocean circulation in the North Atlantic formed through a mixing of cold, deep water from the Nordic Seas and salty, mid‐depth water in the Atlantic. Previously recorded salinity changes of similar magnitude in the ISOW layer have occurred over timescales greater than a decade. This ISOW freshening event is traced back to a major freshening of the upper ocean that propagated into the ISOW layer through entrainment, a process of intense mixing between deep and mid ocean waters. Using a combination of numerical model output, Argo and surface drifter data, and moored observations, we show that entrainment facilitated a significant change to the ISOW layer in just 2–3 years. Key Points Significant freshening of the Iceland Scotland Overflow plume is observed in the Iceland Basin Salinity changes in the overflow plume are directly linked to changes in the upper ocean through entrainment Entrainment significantly modifies North Atlantic deep water mass properties on sub‐decadal timescales