Longterm Changes In Multiyear Ice Extent In The Arctic Basin

The characteristics of multiyear ice, which include its area, thickness, concentration, and location, are all important indicators of climatic change in the Arctic Ocean. In addition, successful high-latitude passage of transport vessels and icebreakers as well as the survival of drifting research stations depend on this ice. Thus, there is clear value in studying the processes of multiyear ice formation and the variability of its boundaries. Its observations were drawn from AARI routine 10-day ice charts and provide a basis for analyzing the long-term variability of the multiyear ice boundary to the north of the Arctic Seas on the Siberian shelf for half a century.

Figure 4.17 shows changes in the locations of boundaries of prevailing old ice in February-March, close residual ice during the third 10-day period of September in the preceding year at meridians of the Laptev, East Siberian, and Chukchi Seas, and

Figure 4.18. Average location of the old ice boundary in the eastern Arctic Seas for the periods 1960-1979 (1) and 1980-2000 (2).

Figure 4.18. Average location of the old ice boundary in the eastern Arctic Seas for the periods 1960-1979 (1) and 1980-2000 (2).

the winter ice exchange of these seas with the Arctic Basin calculated using Equation 4.10 for 1954-2002. All curves in this figure include negative linear trends. Most significant are the trends in the location of old ice (their contributions to variability range from 6% to 22%); the least significant trends are the changes in the boundaries of residual ice (where the contribution to variability is 0.4% to 7%). The linear trends of the East Siberian Sea make the largest contribution to variability.

A study of the average location of the winter boundary of prevailing multiyear ice (with a concentration of more than 5 tenths) during the 20-year period from 1960 to 1979 and the subsequent 20-year period from 1980 to 2000 showed consistent southward displacement of the boundary (Figure 4.18). On average, it moved over 300 km southward over the 40-year period. This displacement usually took place when there was a small negative trend in the region's sea ice extent (see Figure 2.3). The cause of the southward displacement of close ice in this region is a noticeable weakening of the Arctic High (with an increased number of cyclones) toward the end of the twentieth century, accompanied by a diverging ice cover (see also section 4.3), and the aforementioned decrease in ice export from the Arctic Basin to the Greenland Sea during the warm periods as compared with the cold periods.

This southward displacement of the prevailing multiyear ice boundary is confirmed by Asmus et al. (2005), who noted a small positive trend, a 5% increase in the area of multiyear ice in the Arctic Basin between 40°E and 105°E, based on the analysis of ice charts constructed from radar and microwave satellite images for the period 1983-2005. Hence, there was an expansion of the multiyear ice zone during the period of climate warming not only in the eastern sector of the Eurasian part of the Arctic Basin, but also in its western sector.

The linear trends shown in Figure 4.17 are accompanied by cyclic fluctuations of varying duration. The time interval examined here includes an epoch of elevated sea ice extent from the late 1960s to the early 1980s and partly covers the epochs of decreased sea ice extent in the late 1950s and in the 1980s-1990s. The ice exchange of the seas with the Arctic Basin clearly depends upon the sea ice extent of the region and ice export to the Greenland Sea through Fram Strait. For estimating this relationship, the average values of ice exchange of the seas with the Arctic Basin were determined for the years from 1965 to 1985 ("cold" epoch) and for the combined time intervals of 1954-1964 and 1986-1995 ("warm" epochs). A comparison of these values showed average ice export from the Laptev Sea in the "cold" epoch was 42% higher than in the "warm" epoch. There was a similar, but smaller, excess of ice export from the East Siberian Sea of 25%. On the contrary, the ice export from the Chukchi Sea in the "warm" epoch was higher than in the "cold" epoch by almost 60%; This is caused by increased cyclonicity in the baric field over the Arctic Basin during the warm epoch, which leads to increased recurrence of southerly winds to the north of the Chukchi Sea.

These results are based on the reasonable assumption that the location of the old ice boundary to the north of the marginal Arctic Seas at the end of winter depends both on the intensity of ice export from these seas and on the location of the boundary of prevailing residual ice at the end of the preceding summer. However, each of these factors plays a different role in different seas, as indicated by the correlation coefficients presented in Table 4.5.

As Table 4.5 shows, the Laptev Sea exhibits the closest relationship between the location of the boundary of prevailing old ice and the location of close residual ice during the preceding summer; this relationship diminishes noticeably with increasing distance eastward from the Laptev Sea. The relationship with winter ice export to the Arctic Basin is slightly greater in the East Siberian Sea. The methodology described in Anon. (H) was used to estimate the role of both factors in the multiple regression equations connecting the boundaries of old and residual ice and the ice exchange of the seas with the Arctic Basin. Table 4.6 shows the results of this assessment.

Table 4.5. Correlation coefficients of relationships between the boundaries of (X) prevailing old ice at the end of winter, (Y) the value of the marginal seas' ice exchange with the Arctic Basin, and (Z) the boundary of close ice at the end of the preceding summer in the Laptev, East Siberian, and Chukchi Seas

Table 4.5. Correlation coefficients of relationships between the boundaries of (X) prevailing old ice at the end of winter, (Y) the value of the marginal seas' ice exchange with the Arctic Basin, and (Z) the boundary of close ice at the end of the preceding summer in the Laptev, East Siberian, and Chukchi Seas

Connection

Laptev Sea

East-Siberian Sea

Chukchi Sea

X-Z

0.94

0.45

0.36

X-Y

0.63

0.73

0.58

Y-Z

0.33

-0.27

-0.55

Table 4.6. Role of variability in the location of residual ice (Z) and ice exchange with the Arctic Basin ( Y) in the formation of the old ice boundary at meridians of the Laptev, East Siberian, and Chukchi Seas at the end of winter

Sea

Z

Y

Laptev

77

23

East Siberian

33

67

Chukchi

59

41

The cause of the regional differences is probably the difference in the prevailing geographical location of residual ice (its remoteness from the shore, shelf boundaries, landfast ice, Bering Strait, etc.), because the speed and stability of currents and action of internal forces in the ice cover depend on it. The significance of such factors is indicated by the correlation coefficients shown in the bottom line of Table 4.5. As this table shows, the further south the location of residual ice in the Chukchi Sea, the more intensive is its export from the sea. This pattern is less evident in the East Siberian Sea and changes sign in the Laptev Sea, where the ice export slightly increases with northward displacement of the residual ice boundary.

Regarding the influence of the location of the old ice boundary at the end of winter on the sea ice extent the following summer, the correlation analysis shows that the corresponding correlation coefficients (0.20-0.25) are statistically insignificant, although the negative sign of the two eastern seas' coefficients indicates a possible weak dependence: the more northern the old ice boundary, the smaller the sea ice extent.

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