Sea Ice Interannual Variability and Sensitivity to Fall Oceanic Conditions and Winter Air Temperature in the Gulf of St. Lawrence, Canada.

The Gulf of St. Lawrence has been nearly free of sea ice in the winter six times in its recorded history, four of which have occurred since 2010. This study examines the inter‐annual variability of sea ice cover characteristics (1969–2024) and winter mixed layer heat content (1996–2024), their sensitivity to fall oceanic conditions (since fall of 1995) and to winter air temperatures. The study finds no relationship between fall oceanic conditions with either the first occurrence of sea ice, maximum seasonal estimated volume or winter mixed layer heat content. 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 1°C threshold with a lag time of 30–37 days, and with air temperature dropping below −2°C with a lag of 37–44 days; longer lags have weak correlations. The seasonal maximum conditions in area or estimated volume can be estimated by the preceding measurements of the same metrics with a lead time of only 29 days for volume and 36 days for area. The average air temperature over the Gulf between December and February or March is highly correlated to seasonal maximum sea ice area and estimated volume, as well as ice season duration. The six nearly ice‐free winters correspond to the warmest December to February (or December to March) average air temperatures over the Gulf. A warming of >1.9°C–2.4°C (DJFM) or >2.2°C–2.9°C (DJF) above the 1991–2020 climatology leads to nearly ice‐free conditions. Plain Language Summary: The first occurrence and seasonal severity of sea ice in the Gulf of St Lawrence are shown to not be related to early fall ocean conditions. Atmospheric conditions prevalent when water temperature approaches the freezing point are drivers of first occurrence and can be used to predict onset of sea ice in the northwest Gulf with about 37–44 days of lead time. Seasonal sea ice severity metrics (duration, maximum area and estimated volume) are shown to be related to winter air temperatures. The seasonal maximum conditions in area or estimated volume can be estimated by the preceding measurements of the same metrics with a lead time of only 29 days for volume and 36 days for area. Six nearly ice‐free winters correspond to the warmest December to February (or December to March) average air temperatures over the Gulf, providing the thresholds required for future climate warming to result in a nearly ice‐free Gulf of St. Lawrence. Key Points: Seasonal maximum sea ice area and first occurrence are predictable with at most 36–44 days of lead time in the Gulf of St LawrenceSeasonal sea ice severity metrics (duration, maximum area and estimated volume) are shown to be related to winter air temperaturesThe only six nearly ice‐free winters correspond to the warmest winter air temperatures over the Gulf [ABSTRACT FROM AUTHOR]