The frequency and cause of shallow winter mixed layers in the Gulf of Maine (GOM) is investigated using hydrographic profiles, Gulf of Maine Ocean Observing System (GOMOOS) moorings, and National Center for Environmental Prediction wind data. Historical and recent observations of mixed layer depth reveal distinct spatial variability. Mooring data show that shallow mixed layers often occur and persist through the entire winter in the coastal and eastern GOM. In the interior GOM, however, cast data suggest that deep mixing is more common. Both mooring and cast data show that the presence of shallow wintertime mixed layers throughout the GOM is primarily controlled by salinity increase (not temperature decrease) with depth. Additionally, mooring data indicate that periods of increased stratification are caused by freshening at both 1 and 20 m, with more freshening occurring at the surface (1 m) than at depth (20 m). Comparison of wind stress and stratification at the GOMOOS moorings shows that winter wind events are not responsible for long‐term temporal variability in salinity‐driven stratification.
