The fast ice that grows in the winter generally thaws in the summer, causing its extent to shrink. Figure 4.2-5 shows the summertime changes in the extent of open water in each region. The open water area continues to grow throughout the summer. Unfortunately, none of the regions becomes completely free of ice, shattering expectations of easy NSR operations during the summer. The extent of open water varies widely from sea to sea. In particular, the northeast Kara Sea and the East Siberian Sea have no open water at the end of June and remain extensively covered by ice even at the end of September. By contrast, however, the southwest Kara Sea and southwest Chukchi Sea, adjoining those harsh regions on the east and west sides respectively, display a marked increase in open water during the summer, so that scarcely any ice remains at the end of the summer.
The presence of ice massifs, together with the differences in climate and the effects of inflows of warm currents from the Atlantic and Pacific Oceans as described earlier, result in marked variation in ice coverage among the various seas. As discussed in Chapter 3, ice massifs are found in certain regions even in the summer thawing period. A comparison of the ice massifs map in Section 3.2.3 with Figure 4.2-5 reveals that the areas of little open water-the eastern Kara Sea, western Laptev Sea and East Siberian Sea-are those possessing ice massifs (respectively, the Severnaya Zemlya, Taymyr and Ion Ice Massifs). As the following section explains, NSR navigation is charted so as to avoid these ice massifs.
Figure 4.2-5 Expansion of open water in summer
(3) Route selection
Clearly, as the foregoing discussion illustrates, the route through which the NSR passes is strongly affected by its natural conditions. Of all of the natural conditions with impact on the selection of the routes, the depth of the water along the routes is the primary factor. Because the NSR consists entirely of continental shelf, a major portion of the passage and especially its straits are extremely shallow. Shoals no deeper than 20m are by no means rare, raising the danger that deep-draft ships may run aground on them. The second key factor in selection of a route is the condition of the ice. The sea ice in the NSR is grueling, with large ice fields often remaining even in summer. Under these extreme conditions, at times the ice may prevent passage of ships or cause damage to hulls and propulsion systems. For this reason, NSR ships are often forced to detour to avoid the harsh ice. To ensure safe passage through the severe Arctic ice, ships traveling through the NSR should request instructions for their routes from an agency named Marine Operations Headquaters (MOH).
