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 sub-decadal timescales.