Variability of atmospheric circulation above Spitsbergen in the second half of 20th Century

The study presents variability of different circulation indices above the Spitsbergen for the period 1951-2000. Investigated area covers the part of the Atlantic sector of the Arctic located between 75-80°N and 0-30°E. The study based on the original calendar of circulation types (Niedźwiedź 1981, 2001), prepared with the help of the synoptic maps of Europe (Europäischer Wetterbericht, 1976-2000, Tägliche Wetterbericht 1950-1975). Twenty circulation types have been distinguished. The advection directions are marked by the capital letters while the anticyclonic situations by the subscript a and the cyclonic ones by subscript c; for example, Wa and Wc denote the anticyclonic and cyclonic situations respectively, with the air advection from the West. Thus, there are 16 circulation types with definite directions of the air masses. The other 5 situations are nonadvective: Ca - centre of anticyclone, Ka - anticyclonic wedge, Cc - centre of cyclone, Bc - cyclonic trough, and x - col and the situations which cannot be classified. This classification is similar to Lamb (1972) types and based on methods described in the most important works in synoptic climatology (Barry an Perry 1974, Yarnal 1993). The frequencies of the occurrence of all the distinguished circulation types for the 50-years period of 1951-2000 are presented on the table 4. On the average, the anticyclonic wedge (Ka ? 10.4 %) is the most frequent in the Spitsbergen. The second one is situation Ec and NE occurs during 9.9 and 8.8% of the days. The centre of high pressure over Spitsbergen (1.3 % of the days) and the NW situation (1.5 % of the days) are the least frequent. Weather and climate of Spitsbergen are modelled by the intensive cyclonic activity during 56 % of the days in a year. The largest frequency of the occurrence of low pressure systems is characteristic for the period from September to March with maximum in November (66 %), December and January (65%). The number of days with high pressure systems exceeds 50 % only in May (59 %). The variability of circulation have been obtained using the simple circulation indices: zonal westerly circulation W index, similar to P progression index, index of southerly circulation - S, and index of cyclonicity - C, as proposed by R. Murray and R. Lewis (1966) with some modifications. The author of this paper calculated these indices for each year, season and month (tables 1-3). The most characteristic for Spitsbergen is the zonal form of circulation with the eastern component (W = -147 for a year) with the great intensity in the period from October to April (March ?20.9). The minimum in the eastern air-flow can be observed in summer (July +0.7). Another characteristic feature for Spitsbergen is predomination of the cyclonic patterns (index C = 56 for a year), especially in the period from September to March (November 12.8). In May the index C is negative (-9.4), which confirms the great activity of anticyclonic pattern. Among the southerly circulation forms the northern component dominates (index S = -36 for a year). Only in July and August the opposite situation can be observed. Circulation forms over Spitsbergen have been fluctuated in the long-term period. In 1951-2000 the greatest changes have been observed in the indices C and W (fig. 3-7). Significant increasing trend was observed in annual values of C and S indices. Southerly circulation index S is well connected with North Atlantic Oscillation (NAO) index (table 6). The best correlation between the mean temperature at Hornsund and circulation indices exists for the S index (table 7)