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During late February and early March 2018, an unusual polynya was observed off the north coast of Greenland. This period was also notable for the occurrence of a sudden stratospheric warming. Here we use satellite and in situ data, a reanalysis and an ice-ocean model to document the evolution of the polynya and its synoptic forcing. We show that its magnitude was unprecedented and that it was associated with the transient response to the sudden stratospheric warming leading to anomalous warm southerly flow in north Greenland. Indeed, regional wind speeds and temperatures were the highest during February going back to the 1960s. There is evidence that the thinning sea ice has increased its wind-driven mobility. However, we show that the polynya would have developed under thicker ice conditions representative of the late 1970s and that even with the predicted trend toward thinner sea ice, it will only open during enhanced southerly flow.

Plain Language Summary In late February 2018, satellite imagery revealed the presence of a large polynya (a region of reduced sea ice cover within the pack), in the Wandel Sea off the north coast of Greenland. Since this region is not known for the development of polynyas, this discovery generated interest among Arctic observers and in the science community, raising questions about the nature and cause of this unusual event. In this paper, we show that its opening coincided with a period of sustained and unusually warm winds from the south, with above-freezing temperatures and wind speeds in excess of 25m/s reported at local weather stations. February 2018 was also notable for a sudden stratospheric warming event, in which an abrupt warming of the atmosphere between 10- and 50-km altitudes occurred in conjunction with a reversal of the stratospheric winds. We show that this event was responsible for the polynya. We also use a computer model to confirm the dominant role of the winds in creating the polynya. Finally, we show that even with future thinning of sea ice due to climate change, extreme winds will remain necessary to create a polynya in this region over the next few decades.