An unusually large positive salinity anomaly was observed across the eastern Gulf of Maine (GoM) in winter 2017-2018. Buoy measurements in Jordan Basin found this anomaly extended down to at least 100 m, the deepest mixing observed in the past 19 years. Similarly, this is the strongest positive regional salt anomaly in sea surface salinity (SSS) satellite observations. To determine the source waters driving this event and to diagnose the relative importance of forcing processes, passive tracer adjoint sensitivity experiments are performed using a data assimilating version of the Regional Ocean Modeling System. The model shows that northeastward Scotian Shelf wind anomalies cause a dramatic decrease in freshwater transport to Jordan Basin, which leads to an early winter upper water column salinity surplus. This salinity change weakens the normally haline-controlled vertical stratification across the eastern Gulf. Modeled upper ocean density and vertical diffusivity from 2007-2021 both show a maximum in January 2018. Winter 2017-2018 is the only period where the enhanced winter mixing extends below 100 m. Thus, anomalous vertical entrainment of saltier subsurface Gulf water is the major factor driving the extreme positive satellite-observed SSS anomaly across the eastern Gulf including Jordan Basin. Other factors, including a modest increase in wind-forced slope water transport, and positive fall 2017 salinity anomalies on the Scotian Shelf and Slope Sea appear to play lesser roles in the observed salinification. The adjoint sensitivity analysis demonstrates its utility for back tracing transport pathways for periods of several months.