The Gulf of St. Lawrence has been nearly free of sea ice five times in its recorded history, three of which have occurred since 2010. This study examines the inter-annual variability of sea ice cover characteristics (1969-2023) and winter mixed layer heat content (1996-2023), their sensitivity to fall oceanic conditions (since fall of 1995) and to winter air temperatures. The study finds no relationship between the first occurrence of sea ice, maximum seasonal volume or winter mixed layer heat content and fall oceanic conditions as determined by the heat content of the water column in early fall. However, it shows that the first occurrence of sea ice in the northwestern Gulf is related to the timing of sea surface temperature crossing the 0C threshold with a lag time of about 3 weeks, and with air temperature dropping below -1.8C with a lag of roughly 40 days. The average air temperature over the Gulf between December and February or March is highly correlated to seasonal maximum sea ice area and volume, as well as ice season duration. This is likely through a link with sensible heat flux. The five nearly ice-free winters correspond to the warmest December to February (or December to March) average air temperatures over the Gulf. From this is inferred that a warming of 2.2 to 2.4C above the 1991-2020 climatology leads to nearly ice-free conditions in the Gulf of St. Lawrence. This finding is consistent with numerical simulation studies.

The St. Lawrence Estuary connects the Great Lakes with the Atlantic Ocean. During summer, the Estuary’s surface layer receives its nutrient supply from vertical mixing processes caused by the estuarine circulation and tidal-upwelling at the Head of the Laurentian Channel (HLC). There has been few oceanographic process studies during winter when ice forms and flows on the surface. Winter monitoring is typically confined to vertical profiles of salinity and temperature and near-surface water samples collected from a helicopter. In 2018, however, the Canadian Coast Guard approved a science team to sample in tandem with its icebreaking and ship escorting operations. This opportunistic sampling provided the first winter turbulence observations, which covered the largest spatial extent ever measured during any season within the St. Lawrence Estuary and Gulf. The nitrate enrichment from tidal mixing resulted in an upward nitrate flux of about 30 nmol m$^{-2}$s$^{-1}$, comparable to summer values obtained at the same tidal phase. Further downstream, deep nutrient-rich water from the Gulf was mixed into the subsurface nutrient-poor layer at a rate more than an order of magnitude smaller than at the HLC. These fluxes were compared to the nutrient load of the upstream St. Lawrence River. Contrary to previous assumptions, fluvial nitrate inputs are the most significant source of nitrate in the Estuary. Nitrate loads from vertical mixing processes would only exceed those from fluvial sources at the end of summer when fluvial inputs reach their annual minimum.