High concentration sea ice exists even during the summer in both sides of Vilkitskiy Strait, the Long Strait and its west side. Both of these two areas receive multi-year ice from the Arctic Ice Massif sustaining local ice massifs.
Another threat for NSR navigation is the ice massifs such as the Severnaya Zemlya, Taymyr, Ion and Wrangel Ice Massifs.
The following section provides an overview of sea ice conditions in each sea area.
In the Kara Sea, freezing begins in September in the northern reaches of the sea and extends to the southern part in October. From October to May virtually the entire sea is covered with ice in various phases of growth. Along the coast a non-uniform fast ice forms, and beyond the grounded ice the flaw polynya can be seen, such as the Ob/Yenisey Polynya in the southern Kara Sea. Flaw polynya, which include open water and thin ice areas, are generally observed between the fast and drifting ice. From June to September sea ice is rare in the Kara Sea, but thick ice is scattered around the western part of the sea. In the east, the Severnaya Zemlya Ice Massif, consisting of thick first-year ice of high concentration, presses in on the NSR. The Vilkitskiy Strait between the Taymyr Peninsula and Severnaya Zemlya is among the most treacherous for the NSR, as it is wracked by violent ice movement. The sea ice decreases to the minimum extent in mid-September, and is completely absent south of 75｡?. In particularly hot summers the Kara sea is free of sea ice as far north as 80｡?.
The Laptev Sea is the sea whose fast ice extends farthest into the Arctic Ocean, from January to June. With the average temperature dropping below -30℃ in mid-winter, the fast ice reaches a thickness of 2m, and as much as 2.5m in the coldest winters. In the summer, the sea ice drifts southward in the western part of the Laptev Sea to collect in the gap between the Taymyr Peninsula and Severnaya Zemlya. In terms of navigation of the NSR, the Vilkitskiy Strait and the area off the coast of the Taymyr Peninsula may remain difficult to pass through even in the summer.
The East Siberian Sea is characterized by the shallowest continental shelf in the NSR. This shallow continental shelf keeps the fast ice highly stable; it extends 500km out to sea with thickness of 170-200cm. In the summer the prevailing winds blow from the south and carry drifting ice into the sea, creating a region of thin sea ice and flaw polynya that is easy to navigate. The East Siberian Sea has the largest presence of multi-year ice in the NSR, with multi-year ice accounting for 60% of the ice cover on the Ion Ice Massif and growing to thickness of 2.5m. In the summer the winds shift to arise from the north, supplying sea ice from the north to support the Ion Ice Massif. Freezing begins in September, and by mid-October the sea is entirely frozen over.
