In addition to a "60-year" cycle, shorter cycles lasting about 20 and 10 years appear to be present in long-term changes in ice extent. We studied these cycles using a periodogram analysis of ice extent time series in the seas under consideration for 1933-2003, i.e., for the period of the routine monitoring of ice conditions in the Eurasian Seas by air reconnaissance (up to 1992) and satellite information analysis (since the mid 1960s) (Borodachev and Shilnikov, 2002).
To exclude the influence of short-period variability, all series were first subjected to smoothing by a five-year running mean procedure. The results of this process are shown in Figure 2.10. The various seas and seasons depicted in the periodograms in this figure have common features: significant increases in the amplitude of variability within period ranges of 18-22 years and 8-13 years. However, the ratios between the amplitudes of first- and second-range variability are different for different seas.
Table 2.6 provides information on the amplitudes of these and other variations and their influence on ice extent interannual changes. The variance of each wave was determined by a formula similar to Equation 2.2. Account was also taken of the fact that the five-year running smoothing data filter decreases the values of the distinguished amplitudes (by 35% for a 10-year wave and by 10% for a 20-year wave). As Table 2.6 indicates, the amplitude of 20-year variability in the western seas (from the Greenland Sea to the Kara Sea) is much higher than that of 10-year variability (both in winter and summer). This ratio decreases in general from west to east. In the eastern seas (from the Laptev Sea to the Chukchi Sea), the differences in amplitude are much less, and in the Laptev and the Chukchi Seas the amplitudes of 10-year variability are larger than those of 20-year variability. The contribution of the former to the total ice extent variance for the western seas (23%) is notably less than for the eastern seas (38%), and vice versa for the "20-year" variation: in the western seas, it comprises 13%, on average, and for the eastern seas, about 7%.
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