Near sea ice-free conditions in the northern route of the Northwest Passage at the end of the 2024 melt season

Abstract. The Northwest Passage through the Canadian Arctic Archipelago (CAA) provides a shorter transit route connecting the Atlantic Ocean to the Pacific Ocean but ever-present sea ice has prevented its practical navigation. Sea ice area in the northern route of the Northwest Passage on 30 September 2024 fell to a minimum of 4×103 km2 or ∼3 % of its total area the lowest ice area observed since 1960. Here, we investigate the processes responsible for the record low sea ice area in 2024 and show it was driven by a perfect sequence of thermodynamic and dynamic forcing events acting on an increasingly less resilient ice cover. Specifically, multi-year ice (MYI) only made up ∼10 % of total sea ice area at the start of the melt season that was characterized by an atmospheric circulation pattern that brought warm southerly air directly into the middle of CAA. This resulted in a record summer air temperature anomaly of 2.1 °C that drove rapid melt and limited the import of ice from higher latitude regions to 50 % of the 2016–2024 mean. Finally, positive air temperature anomalies upwards of 12 °C persisted into October, extending the melt season further and delaying freeze-up by 1 month, compared to the 1991–2020 baseline. Overall, a series of specific and cascading thermodynamic and dynamic processes is required to melt all ice in the northern route of Northwest Passage as it did in 2024, therefore ice conditions along this route will likely continue to remain highly variable during the transition to a summertime sea ice free Arctic.