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Trends for the entire period of investigation (1939-1990) were analogous to the aforementioned 30-year trends in the case of types IX-XIII and XVI, whereas types I, II, IV and VI were accompanied by trends with slopes bearing inverse signs (±). A survey of fluctuations of the frequency of occurrence of circulation types according to 10-year running means allows us to conclude that fluctuations were more often than not irregular and did not exhibit clear long-term cyclic variations for most of the types. The largest differences of frequencies (from 5-7%) were exhibited by circulation types II, X, and XIII, and the smallest (to about 2-3%) by types III-IX and XIV.

Figures 4.3 and 4.4 present the variability of the frequency of occurrence of 6 groups and 3 macrotypes of circulation according to seasonal and annual means (a) and 10-year running means (c), and the course of their trends in the period 1939-1990 (b) and 1961-1990 (d). A high compatibility of 52-and 30-year trends of annual frequency was noted for circulation groups B, D, and G, and the lowest compatibility for groups W and K. Circulation groups A and B are characterised by weakly marked trends (slightly ascending for A and slightly descending for B). The clearest positive trends belong to groups G and D, and the clearest negative trends to groups K and W, the last group lacking any trend for the last 30 years. Trends of annual frequencies were compared to the seasonal trends and they turned out to be quite similar in the case of circulation groups G, D, and K. The biggest discrepancies occur for groups A and B, where an ascending or descending character of the annual trend of frequency may be determined by the trend of only one season. According to annual means, the biggest differences of 10-year running means of frequencies were exhibited by circulation groups A and K, and the smallest by groups B and G (Figure 4.3).

Figure 4.2. Arrangement of groups of circulation in the Arctic (A, B, W, G, D, K) and macrotypes of circulation (W, C, E) according to frequency of occurrence in particular seasons of the year for the period 1939-1990.

Figure 4.3a. Course of the annual frequency (in %) of occurrence of days with particular circulation groups (A, B, W, G, D, K) in the Arctic and their trends over the period 1939-1990. Key: a - year-to-year course, b - linear trend over the period 1939-1990, c - 10-year running mean, d - linear trend over the period 1961-1990.

Figure 4.3a. Course of the annual frequency (in %) of occurrence of days with particular circulation groups (A, B, W, G, D, K) in the Arctic and their trends over the period 1939-1990. Key: a - year-to-year course, b - linear trend over the period 1939-1990, c - 10-year running mean, d - linear trend over the period 1961-1990.

Figure 4.3b. Course of the winter frequency (in %) of occurrence of days with particular circulation groups (A, B, W, G, D, K) in the Arctic and their trends over the period 1939-1990. Key as in Figure 4.3a.
Figure 4.3c, Course of the spring frequency (in %) of occurrence of days with particular circulation groups (A, B, W, G, D, K) in the Arctic and their trends over the period 1939-1990. Key as in Figure 4.3a.
Figure 4.3d. Course of the summer frequency (in %) of occurrence of days with particular circulation groups (A, B, W, G, D, K) in the Arctic and their trends over the period 1939-1990. Key as in Figure 4.3a.
Figure 4.3e. Course of the autumn frequency (in %) of occurrence of days with particular circulation groups (A, B, W, G, D, K) in the Arctic and their trends over the period 1939-1990. Key as in Figure 4.3a.
Figure 4.4a. Course of annual and seasonal frequency (in %) of the occurrence of circulation macrotype W and its trends over the period 1939-1990. Key as in Figure 4.3a.
Figure 4.4b. Course of annual and seasonal frequency (in %) of the occurrence of circulation macrotype C and its trends over the period 1939-1990. Key as in Figure 4.3a.

194(1 1950 1960 1970 1980 1990

194(1 1950 1960 1970 1980 1990

Figure 4.4c. Course of annual and seasonal frequency (in %) of the occurrence of circulation macrotype E and its trends over the period 1939-1990. Key as in Figure 4.3a.

The frequency of occurrence of macrotype W was decreasing until the mid-1970s, and then subsequently increased rapidly; this can be observed in the courses of frequencies of annual and seasonal means (Figure 4.4a). In spite of the descending tendency during the period 1939-1990, the 30-year trend is now positive. The increase in the frequency of occurrence of this macrotype in the last decade is especially distinct in winter and spring. Macrotype C exhibits the highest compatibility of frequency in the annual cycle (Figure 4.4b). The negative trend is clearly visible both for each individual season and for the whole year. Except for the 30-year trend for winter, the 30- and 52-year trends of the frequency of occurrence of macrotype E are positive. However, when analysing 10-year running means it is worth observing that in the middle of the 1970s a change occurred in the tendency and the frequency of this macrotype has been decreasing from then on (Figure 4.4c).

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