The influence of landfast ice on the navigation in the Arctic Northeast Passage

Landfast ice is one of the most important factors that affect the navigation safety of Arctic Northeast and Northwest Passages but usually is treated as drift ice in previous studies. This study focused on the situation of landfast ice in four key traits of the Arctic Northeast Passage from 2007 to 2021 and calculated the navigational windows for different ice-class vessels in the influence of landfast ice. The results show that the extent of landfast ice in these straits generally reaches its maximum from March to June, and decreases to a minimum or even disappears in July and August. The proportion of landfast ice extent in the four straits is quite different, such as Bering Strait (1.3%), De Long Strait (3.6%), Dmitry Laptev and Sannikov Strait (DLS Strait) (53.5%), and Vilkitsky Strait (27.8%). The average navigational windows range from 27 weeks (Bering Strait) to 8 weeks (Vilkitsky Strait) per year, and the Bering Strait showed the smallest increase trend (0.20 weeks per year), while the Vilkitsky Strait experienced the largest increase trend (0.62 weeks per year). The increasing navigational windows in the key straits are beneficial for future commercial and scientific expeditions of polar vessels in the Northeast Passage.


Introduction
The Arctic Passage refers to a collection of polar maritime routes that traverse the Arctic Ocean, connecting the Pacific and Atlantic Oceans [1].Generally, it includes the Central Passage, the Northwest Passage, and the Northeast Passage, among which the Northeast Passage holds immense commercial value for most Asian countries such as China.The general route of the Northeast Passage starts from Northern Europe, passes along the coast of continental Europe and northern Siberia, crosses the Bering Strait, and reaches the Pacific Ocean, or vice versa.Key straits within the Northeast Passage include the Bering Strait, the De Long Strait, the Dmitry Laptev and Sannikov Strait (DLS Strait), as well as the Vilkitsky Strait, which is shown in Figure 1 [2].Against the background of global warming, the navigation conditions of the Northeast Passage are continuously improving [3].The number of commercial vessels sailing through the Arctic region is gradually increasing [4].
According to statistics, nearly 600 commercial vessels were registered to pass through the Northeast Passage in 2013 alone.With the further opening of the Northeast Passage, commercial activities in the Arctic region through this route will become more frequent and intensive, indicating a promising future for the utilization of the Northeast Passage [5].2021) used a daily sea ice concentration dataset from 2000 to 2019 and a threshold of 40% sea ice concentration to define the navigating window for the Arctic Northeast Passage and found that the navigable time is basically maintained at 30 days [14].
The previous studies usually focused on the concentration of drift ice when studying navigational issues, while landfast ice was ignored because it was not included in the passive remote sensing dataset.Landfast ice commonly exists in the Northeast Passage.Landfast ice is attached to coastlines, ice shelves, or grounded icebergs without significant horizontal movement.Compared with drift sea ice, landfast ice undergoes smaller changes throughout the seasons, sometimes even persisting throughout the entire ice period in some high-latitude regions [15].Therefore, the distribution and evolution of landfast ice in the four key straits are crucial factors determining the navigation in the Northeast Passage.
The influence of landfast ice on navigation of the Northeast Passage was focused on in this paper, based on the landfast ice information extracted from the ice production of the National Ice Center (NIC).The data and methods are introduced in the section 2, and the results are shown in the section 3. The conclusion is presented in section 4. The results in this paper are expected to provide scientific support for future navigation in polar routes.

Landfast ice data
The landfast ice data used in this study are obtained from the sea ice analysis map, released by the National Ice Center (NIC).This dataset is a comprehensive analysis of results from various sources, including visual observations along coastal zones, ship-based observations, aerial reconnaissance, airborne radar measurements, near-real-time satellite imagery, climate data, and other information.The sea ice analysis map consists of sea ice concentration, stages of ice development, types of sea ice and other elements [16].In term of forms of ice existence "FP" denote the primary forms of sea ice.The FP field, equaling 08 is chosen as landfast ice and projects into the standard polar stereo grid of 6.25 km * 6.25 km.The landfast ice was assigned a value of 1.1.This method for extracting fastice has been verified through the paper [17].

Drift ice data
The sea ice concentration dataset of drift ice consists of two products, SSMIS/Bootstrap (January 2007-June 2012) and AMSR 2/Bootstrap (July 2012-December 2021).The SSMIS/Bootstrap sea ice concentration ranges from 0% to 100% and is gridded on the SSM/I polar coordinate stereo grid (25 km * 25 km) [18].Sea ice concentration of AMSR2/Bootstrap varies from 0% to 100% and the grid resolution is 6.25 km [19].SSMIS sea ice concentration data was resampled to 6.25km the same as AMSR2.

Spatial distribution of landfast ice
The annual distributions of sea ice, including landfast ice and drift ice, in Bering Strait from 2007 to 2021 are shown in Figure 2, where the mean sea ice concentration in the strait is between 0.8-1.0 from January to March.According to the results, the sea ice concentration shows a clear difference between the north and south sides of the strait in April, May, and December.The sea ice, especially the landfast ice in the northern part of the strait is more abundant than in the southern part.There is basically no ice in the summer season, during June-November.The annual distributions of landfast ice and drift ice in De Long Strait during the period from 2007 to 2021 are shown in Figure 3, which shows that landfast ice primarily occurs along the southern coast of the strait and persists from December to July of the following year.Meanwhile, the landfast ice on the northern coast of the strait reaches its maximum extent in March and April, while it melts faster than the southern coast.

Annual and interannual variation of landfast ice
The proportions of landfast ice and drift ice in the four key straits are shown in Figure 6.Among the four key straits, the smallest proportion of landfast ice throughout the year is found in the Bering Strait.
During the ice-free period from week 25 to week 45, the sea ice coverage is always below 10%, which is shown in Figure 6a.The periods of the existence of landfast ice in the De Long Strait are from January to May, where the mean proportion of landfast ice is 6.8%, and the largest is in April at 9.3% (Figure 6b). Figure 6c shows that landfast ice in the DLS Strait started to grow from the 41st week, and reached the maximum in the 17th week of the following year.and completely melts away around the 33rd week.In the Vilkitsky Strait, as shown in Figure 6d, the largest proportion of landfast ice is 45.3% in April.From August to October, no landfast ice existed, while the drift ice maintained a proportion of 9.8%-33.3%during this period.
Both DLS Strait and Vilkitsky Strait are significantly affected by landfast ice, and the landfast ice coverage in the DLS Strait far exceeds that in the other straits.Besides, all key straits experience melting of landfast ice during the summer as shown in Figure 6, however, the proportion of drifting ice in the New Siberian Islands is higher than that in the Vilkitsky during this period.

The influence of landfast ice on navigation
The ice conditions in the waters along the route affected the navigational windows.Three different thresholds were proposed, considering the main Chinese vessels navigating in the Arctic (Table 1).The ship type refers to the polar ship system [20] and the ice class refers to the IACS polar region class [21].The ice classes of ships were usually determined by the sea ice concentration, thickness and types.However, only the threshold of sea ice concentration was considered here, because no accurate ice thickness dataset can be used in the landfast ice zone, especially in summer and the landfast ice were almost first-year ice in our study regions.
Table 1.The threshold of sea ice concentration Corresponding to the main ice-class ships in China.

Ship Ship Type Ice-class Threshold Value
Barrier-free in drift ice The navigation windows for different straits are shown in Figure 8 and Figure 9.For the different classes of vessels, the navigable time is the longest in the Bering Strait, where No ice-class vessels can have a navigable time of up to 24 weeks.The navigational conditions are the poorest in the DLS Strait and Vilkitsky Strait.Even for vessels with certain ice-breaking capabilities, their navigable time is only around 13 weeks.The trends in navigable time for the different ice-class vessels in the four straits are all increasing (Figure 10).For no ice-class vessels, the Vilkitsky Strait has shown the fastest growth at 0.55 weeks per year, while the Bering Strait has exhibited the slowest growth at 0.22 weeks per year.For PC7class vessels, the Vilkitsky Strait has also experienced the fastest growth at 0.77 weeks per year, while the Bering Strait has had the slowest growth at 0.16 weeks per year.Similarly, for PC6-class vessels, the Vilkitsky Strait has shown the fastest growth at 0.53 weeks per year, while the Bering Strait has demonstrated the slowest growth at 0.23 weeks per year.
Among the three straits, the Bering Strait has the smallest increase in navigable time for all three categories of vessels, about 0.20 weeks per year.The DLS Strait showed the second smallest increase of 0.25 weeks per year, followed by the De Long Strait, with an average increase of 0.42 weeks per year.The Vilkitsky Strait showed the largest growth in navigable time, about 0.62 weeks per year.In all four straits, the maximum coverage of landfast ice occurred in April and May.The sea ice distribution in the DLS Strait was greatly affected by landfast ice, and the mean proportion of landfast ice was 47.0% to 62.0% for the different years and even landfast ice covered the entire region in the winter in the DLS Strait.The Vilykitsky Strait is severely affected by wind and iceberg drift, where high concentrations of sea ice occur frequently [22].The interannual variation of landfast ice coverage was large and complicated in the Vilykitsky Strait, varying from 16.4% to 44.1%.
The spatial distribution of sea ice each year is largely influenced by changes in Arctic temperature, pressure distribution, surface runoff flowing into the marginal seas, and sea ice drift, resulting in significant fluctuations in the navigable time of different straits.The navigable time of three types of ships in the Bering Strait is the largest, up to 25 weeks, but its increasing trend is the slowest (0.20 weeks per year).The ice situation in the Vilykitsky Strait is the most severe, and the navigable time is about 13 weeks for ships with strong ice resistance.
The statistical data from the Northern Sea Route Information Office (NSRIO) of Russia showed that the monthly voyages passing through the Northeast Passage from 2016 to 2021 experienced a gradual increase and most voyages were accomplished from July to October (exceeds 50% of the total, Figure 11).Nearly 20% of voyages occurred in September when the navigation windows are optimal for all the ice-class vessels, as shown in Figure 9.The sea ice thickness and types should be considered when we analyse the influence of landfast ice on the navigation windows, because the ice classes of ships were determined by those parameters.Sea ice thickness can be obtained by the satellite remote sensing, but has large uncertainties and only available in winter seasons.Numerical model can be used to simulate the landfast ice thickness in the future studies.
Under the background of global warming and the development and utilization of the Northeast Passage, the navigational windows serve as crucial safety information for Arctic ships.They play an important role in route planning in ice-covered areas of the Northeast Passage.This study is of significant importance in understanding the landfast ice conditions and navigational situations in key straits of the Northeast Passage.The results can provide a more scientific reference for future Arctic route planning, and navigation in ice-covered areas.With the increasing availability of multi-source remote sensing data and numerical models, the influence of landfast ice will be better studied in further Arctic shipping navigation and risk assessments.

Figure 1 .
Figure 1.Schematic diagram of key Straits in the Arctic Northeast Passage.The sea ice is the key factor influencing the navigation and utilization of the Arctic shipping routes, which is also influenced by various factors such as temperature, air pressure, wind fields, ocean currents, and surface runoff [6-10].Thus, the significant variability in the annual melting and freezing of sea ice leads to substantial uncertainty in the navigational period of the Arctic shipping routes each year.Numerous scholars have conducted relevant research on the impact of sea ice on navigation.For example, Shibata et al. (2011) evaluated long-term sea ice concentrations along Arctic Passages by using microwave products from the SMMR and SSM/I sensors and analyzed the number of navigable days by using the dataset from the AMSR-E passive microwave sensor [11].Pang et al. (2022) analyzed the sea ice conditions and the navigability of the Arctic Northeast Passage during the 2002-2021 summer seasons based on the multi-source remote-sensing data with inter-sensor calibration processing and the ship-based observation from R/V Xuelong and M/V Yongsheng [12].Yu et al. (2021) used the remote-sensing dataset from 1979 to 2019 to analyze sea ice conditions and navigability along four typical routes of the Northeast Passage, determined the influence of air temperature and surface wind on the sea ice concentration and the navigability of the route [13].Ji et al. (2021) used a daily sea ice concentration dataset from 2000 to 2019 and a threshold of 40% sea ice concentration to define the navigating window for the Arctic Northeast Passage and found that the navigable time is basically maintained at 30 days[14].The previous studies usually focused on the concentration of drift ice when studying navigational issues, while landfast ice was ignored because it was not included in the passive remote sensing dataset.Landfast ice commonly exists in the Northeast Passage.Landfast ice is attached to coastlines, ice shelves, or grounded icebergs without significant horizontal movement.Compared with drift sea ice, landfast ice undergoes smaller changes throughout the seasons, sometimes even persisting throughout the entire ice period in some high-latitude regions[15].Therefore, the distribution and evolution of landfast ice in the four key straits are crucial factors determining the navigation in the Northeast Passage.The influence of landfast ice on navigation of the Northeast Passage was focused on in this paper, based on the landfast ice information extracted from the ice production of the National Ice Center (NIC).The data and methods are introduced in the section 2, and the results are shown in the section 3. The conclusion is presented in section 4. The results in this paper are expected to provide scientific support for future navigation in polar routes.

Figure 2 .
Figure 2. Annual mean distribution of landfast and drift ice in Bering Strait during 2007-2021, where dark red (1.1) represents landfast ice and 0-1 represents the concentration of drift ice.

Figure 3 .
Figure 3. Annual mean distribution of landfast and drift ice in De Long Strait during 2007-2021, where dark red (1.1) represents landfast ice and 0-1 represents the concentration of drift ice.

Figure 4
Figure 4 shows the annual distribution of landfast ice and drift ice in the DLS Strait from 2007 to 2021, where both straits are predominantly covered with landfast ice.The sea ice begins to melt in July and completely disappears by September.Compared to the Laptev Strait, the Sannikov Strait is more susceptible to the impact of landfast ice.

Figure 4 .
Figure 4. Annual mean distribution of landfast and drift ice in DLS Strait during 2007-2021, where dark red (1.1) represents landfast ice and 0-1 represents the concentration of drift ice.

Figure 5
Figure 5 shows the annual distribution of landfast ice and drift ice in Vilkitsky Strait from 2007 to 2021, where the distribution and changes of sea ice are complicated.Landfast ice in this strait begins to appear in November, and the coverage area continues to increase until March of the following year.The landfast ice coverage continued to decrease during the period from April and totally disappeared in August when the maximum sea ice concentration was below 0.4.

Figure 5 .
Figure 5. Annual mean distribution of landfast and drift ice in Vilkitsky Strait during 2007-2021, where dark red (1.1) represents landfast ice and 0-1 represents the concentration of drift ice.

Figure 6 .
Figure 6.The multi-year mean proportion of landfast ice and drift ice in the four key straits averaged during 2007-2021.

Figure 7 .
Figure 7.The proportion of the mean of landfast ice extent in four key Straits from 2007 to 2021.

Figure 8 .
Figure 8. Navigable time statistics of four key straits.

Figure 9 .
Figure 9. Navigable windows of four key Straits.

Figure 10 .
Figure 10.The trend of navigable time for the different ice-class ships in the four straits

Figure 11 .
Figure 11.Monthly distribution of the number of ships passing through the Northeast Passage of the Arctic from 2016 to 2021 (Data source: NSRIO, https://arctic-lio.com).