| Summary: | 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.
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