ALBERT

Description: McMurdo Sound sea ice can generally be partitioned into two regimes: (1) a stable fast-ice cover, forming south of approximately 77.6 ∘S around March – April and then breaking out the following January – February, and (2) a more dynamic region north of 77.6 ∘S that the McMurdo Sound and Ross Sea polynyas regularly impact. In 2019, a stable fast-ice cover formed unusually late due to repeated break-out events. Here we analyse the 2019 sea-ice conditions and relate them to a modified storm index (MSI), a proxy for southerly wind events. We determined there is a strong correlation between the timing of break-out events and several unusually large MSI events and our key finding is that an increase in the frequency of intense winter storms in 2019 resulted in a delayed formation of a stable fast-ice cover. Further, recent observations (post 2019) demonstrate that fast-ice conditions in 2019 were not unique and suggest that the fate of fast ice in the sound may be a symptom of some larger change. Winter fast-ice dynamics in the sound appear to be largely driven by synoptic events as there are no identifiable trends in monthly-averages of atmospheric drivers (e.g. air temperature, mean sea level pressure and wind speed and direction) of fast-ice breakout in the period 1985 – 2022. This study offers new insights into the mechanisms behind individual break-out events and is one of a few case studies that investigate the stability of a fast-ice cover in winter.