| Summary: | In winter and spring, ice-coast interactions driven by winds and ocean currents cause sea ice fractures (leads) to form repeatedly along Arctic coastlines. These events are often associated with rapid and expansive changes in sea ice drift and state that are challenging to predict and represent in models. We investigate the mechanisms behind lead formation from Point Barrow, a prominent Alaskan headland that divides the Beaufort and Chukchi Seas. We use Moderate Resolution Imaging Spectroradiometer (MODIS) thermal infrared imagery to identify 135 leads that originate from Point Barrow during low-cloud conditions in January-April of 2000-2020. ERA5 reanalysis atmospheric conditions, Polar Pathfinder sea ice motion, and MODIS-derived lead position are averaged across events to generate an ensemble lead formation sequence. In the ensemble average sequence, a high pressure weather system travels eastward across the sea ice cover, strengthening winds along the Alaskan coast. Drift speeds increase in the Chukchi Sea but remain low in the Beaufort Sea where ice motion is restricted by the coast. The difference in drift speed across Point Barrow peaks on the day of lead formation then erodes in subsequent days. Across individual events, lead position varies with the structure of the weather pattern. Orientations of the leads relative to winds and ice drift are used to interpret modes of lead failure. More than half of the leads form under winds from the east, usually opening in combined shear and tensile failure. Nearly a third of the leads form under winds from the north and are dominated by shear. Just over 10% of the leads form under winds from the south, opening in tension. Fewer than 5% of the leads form under winds from the west. The results of this analysis suggest that accurate representation of coastal geometry, sea ice state, and synoptic scale wind fields are each important for accurate simulation of these episodic fracturing events.
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