The considerable variability of atmospheric circulation in the Arctic is confirmed by the data related to the mean continuous duration of groups and macrotypes of circulation (Table 4.3, Figure 4.5). According to annual means for the period 1939-1990, the most stable circulation group is group W (5.9 days), then B (5.7 days) and A (5.6 days). Group W is particularly stable in winter (6.9 days) and autumn (6.0 days), and clearly least stable in summer (4.8 days). Considerable annual variability is also exhibited by group B, which reaches its maximum in the summer months (on average 6.7 days), and its minimum in autumn (4.5 days). We should also pay attention to group D, whose duration is on average longer in summer (5.2 days) than in autumn and winter by 1.3-1.4 days, and longer than in spring by 0.8 days. The least stable group of circulation is group G, whose mean continuous annual duration amounts to only 4.2 days. The duration is longest in summer (4.7 days), and shortest in winter (3.7 days).
In the annual course (Table 4.3 and Figure 4.5) the most stable macrotype of circulation is macrotype E (the annual mean is 9.5 days). It reaches its maximum in winter (10.7 days) and its minimum in autumn (8.7 days). The durations of macrotypes W and C are similar and their annual means amount to 6.1 and 6.0 respectively. Macrotype W lasts longer than C in cooler seasons of the year, while macrotype C lasts longer in the warmer seasons (Figure 4.5).
When analysing changes in the duration of circulation groups in particular decades (Table 4.3) it is worth observing that their annual means in warmer periods (1941-1960 and 1981-1990) are higher than those in the cooler period (1961-1980), except for group B, which in this respect is characterised by considerable temporal stability. In particular seasons this tendency remains stable for most of the circulation groups, but it is not as clear as for the annual means.
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