Abstract Arctic sea ice plays a critical role in Earth’s climate system, and as it continues to thin and retreat, understanding the processes driving its variability is increasingly important. Using satellite data and a coupled ocean–sea ice model, we examined how freshwater from the Mackenzie River influences fall sea ice formation in the Beaufort Sea. An “ice bridge” between the coast and offshore ice edge consistently forms over the river’s freshwater plume, with its location and extent varying interannually with freshwater distribution. Regions influenced by the plume experienced sea ice onset an average of 3 weeks earlier than adjacent, saltier waters. Earlier ice formation was associated with enhanced stratification, shallower mixed layers, and reduced upper ocean heat content, all of which promotes faster surface cooling. Our findings highlight the importance of river discharge in shaping sea ice formation and suggest continued Arctic freshening will impact future sea ice timing and extent.

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