Haline Сontrol of Unusually Deep Winter Mixing in the Gulf of Maine
Investigated using a Regional Data-Assimilative Model
Abstract
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.