The delayed freeze is the result of an unprecedented heatwave in Siberia in the first half of the year. The record high temperatures, in addition to fuelling an outbreak of Arctic wildfires that have burned an area the size of Greece, have warmed ocean waters by more than 5°C.
https://www.economist.com/graphic-detail/2020/10/28/more-bad-news-for-the-arctic-the-laptev-sea-hasnt-frozen
https://www.economist.com/graphic-detail/2020/10/28/more-bad-news-for-the-arctic-the-laptev-sea-hasnt-frozen