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* 1951-1980

* 1951-1980

Analysis of the data in Table 5.6 reveals that the behaviour of the calculated variability parameters of seasonal and annual values is similar. They differ mostly in terms of their values. Average deviation is characterised by the lowest values and average year-to-year variability is characterised by the highest values. These differences are contingent on the degree of sensitivity of a given statistical parameter to extreme values. The higher the sensitivity, the higher are the values calculated. For the stations analysed, it is evident that, out of the three variability parameters shown, the strongest connection obtains between standard deviation and average year-to-year variability. Because of the similarity of the results obtained, it has been decided to discuss in detail the variability of T values using the most commonly employed and recommended measure of dispersion, namely cr (Gregory 1976; Jokiel & Kostrubiec 1981).

Figure 5.8. The spatial distribution of the standard deviations of winter, summer, and annual Tt over the period 1951-1990 in the Arctic.

Significantly, the highest values of cr were observed for mean T. for the winter (Table 5.6, Figure 5.8). Three areas of the highest variability of this parameter can be identified in the Arctic in this season: 1. the central and eastern parts of ATLR, 2. the strip of land encompassing the central part of BAFR and the south-eastern fragment of CANSRs, and 3. the eastern part of PACR (the area of Alaska). The reason for the highest day-today variability of values in the Arctic in this season is undoubtedly the intensively active atmospheric circulation in the winter (see Przybylak 1992a and subsection 5.3 of the present publication). This is manifested mostly in the frequent intrusions of violent cyclones, bringing in warm air masses from the south. As mentioned above, the process pertains mostly to the inflow of air masses from the area of the North Atlantic and, to a lesser degree, the Pacific. The areas characterised by the highest variability of 7] values are those which are relatively often reached from one direction by the warm air masses mentioned above and, from the other, by the cold air masses of the Arctic or polar-continental air. The remaining areas, which are usually influenced throughout the year by either cold air masses or warm ones (e.g. sea areas of the Arctic adjacent to the sea areas of the moderate latitudes), are characterised by a lower a. Thus, the significant year-to-year variability of mean winter values in the Arctic will be determined mostly by the variability in the intensity of the atmospheric circulation affecting this area in consecutive years. In general, these results are similar to those obtained by Craddock (1964). A detailed comparison, however, is impossible, because that author presented the distribution of a for the Northern Hemisphere (this is why the isarithms are more generalised) and for particular months. Moreover, the data used in his analysis came from a much earlier period and from a different set of stations. It is worth observing that the regions of the Arctic furthest from the Atlantic and Pacific oceans - i.e., SIBR (except for its western part) and the north-eastern part of CANSRn (stations Alert and Eureka) -are characterised by the highest a in one of the transient seasons. In the spring and autumn, the spatial distribution of in the Arctic is, in general, similar to that for the winter. There is a marked difference, however, in the values of which at that time are much lower and most often oscillate within the range of 1.0°C to 2.5°C. For the greater part of the Arctic, the year-to-year variability of spring T. values is stronger than that for the autumn (Table 5.6). The parts of the Arctic in question are, most importantly, CANR, BAFR, and the southern fragments of ATLR and SIBR.

The variability of summer T. values in the period 1951-1990 is markedly lower. Standard deviations rarely exceed 1.5°C. This happens exclusively in some areas of the continental Eurasian Arctic (Table 5.6, Figure 5.8). T. values are characterised by the greatest stability in the northern sub-region of the Atlantic region (ATLSRn), where for its central part a < 0.5oC. Significant areas of CANSRn, PACR, the southern parts of SIBR, the south-eastern part of the southern sub-region of the Atlantic region (ATLSRs), and the southern part of ATLSRe are also characterised by a relatively high dispersion of T. (ct = 1.0°C - 1.50C). The low dispersion of summer T. values around its long-term mean can be accounted for by the fact that the atmospheric circulation in this season is weakened. Although the intruding cyclones are not especially less frequent than in the winter (Przybylak 1992a), they are weak centres and, consequently, they bring much less warmth to the Arctic. What is more, as demonstrated Przybylak (1992a), the thermal differentiation of the inflowing air masses is much smaller in the summer than in the rest of the year. The fact which has a stabilising effect on T values in the Arctic in this season is that the inflowing warmth, coming both directly from the sun and indirectly (mostly through the advection from the south mentioned above), is used up in the process of melting snow, sea-ice, and continental ice.

The year-to-year variability of mean annual T. values is determined mostly by the behaviour of T. in the cold half-year, most importantly in the winter. This is why the spatial distributions of of winter and annual values in the Arctic are similar. The a of its annual values are lower by approximately 1°C than they are for the winter values (Figure 5.8). This is well illustrated by the course of isarithms 1.0°C (for the year) and 2.0°C (for the winter), which exhibit very small differences. The greatest standard deviations of annual mean T. values are in the region between Spitsbergen, Zemlya Frantza Josifa, and Novaya Zemlya. Another area characterised by increased values of standard deviations stretches, just like in the winter, from the midpoint of the western shore of Greenland to the southern part of the Baffin Island region and further to the west into the southern part of Hudson Bay. The most stable conditions (a < 1.0°C) occur throughout most of the SIBR area, in the north of CANSRn, and probably in the centre of the Arctic. The results given above correspond with the mean values of a 7 for particular zones of geographical latitudes, calculated on the basis of the data from the period 19511980 by Subbotin (Aleksandrov et al. 1986).

The fluctuations of the variability of winter, summer, and annual T. in the Arctic, calculated with the use of in running decades for the last 40-70 years, are shown in Figure 5.9. This method allows us to trace the changes of T and to mark off the periods of its highest and lowest values. As may be seen from the figure, change their values quite significantly in this period, with the ct of winter T. undergoing the greatest changes. This can be particularly well observed in the regions with intense atmospheric circulation (e.g. in Jan Mayen, Ostrov Vize, Ostrov Dikson, Mys Shmidta). Dispersion of mean summer T. is characterised by significantly lower variability. In the Arctic, it is highest in the areas where the continental climate is expressed to the highest degree (e.g. in Ostrov Kotelny, Coral Harbour A, and Resolute A). The changes in ct ofT. in some of its parts clearly manifest a cyclic nature, especially in the case of mean winter values (e.g. in Jan Mayen, Ostrov Vize, or Ostrov Dikson).

Comparing the dispersion of T. from the last 10 or 20 years of the period of observation to previous periods, it may be seen that throughout the Arctic - except for the regions represented by the stations Ostrov Vize and Clyde A - the variability of T. does not appear to increase. On the contrary, it exhibits a downward trend (e.g. in Ostrov Dikson, Mys Shmidta, Coral Harbour A, and Resolute A). There are also regions in which a of T. in the last decades of the period of observation did not undergo significant changes (e.g. Jan Mayen and Ostrov Kotelny). Moreover, in ATLSRn (Ostrov Vize), where an increase in variability did occur, it did not exceed the value recorded in the 1950s. Only in Clyde A did the maximal 10-year value of a occur in the decade 1980-1989 (in the case of winter and annual T.). In the majority of the remaining area of the Arctic, its maximal values for annual T. were observed in the 1950s or 1970s.

tft] Danmarkshavn alt] Shmidta tft] Danmarkshavn alt] Shmidta

| | | | | | | —- DJF JJA -Year

CM n I/Î S K ffl cn> oi o> o> ci cn o>

Figure 5.9. The standard deviations of mean winter, summer, and annual T. in running decades in selected Arctic stations.

Changes in the values of dispersion of seasonal and annual T. (Figure 5.9) are quite consistent in some areas of the Arctic (Ostrov Vize, Coral Harbour A), whereas in other areas they differ, exhibiting (e.g. in Resolute A) a markedly contradictory tendency in the case of winter and summer T.. The regions with the most intense cyclonic circulation are characterised by the most convergent patterns of a of winter and annual T. (Jan Mayen, Ostrov Vize, Clyde A) whereas in the areas where anticyclonic systems prevail, the greatest convergence obtains for the summer and the year as a whole (Coral Harbour A, Resolute A). It is worth adding, however, that not everywhere does one of these two above relationships obtain (Ostrov Dikson, Ostrov Kotelny). A certain common rhythm of the changes (and particularly long-term changes) of of annual can be observed in the following pairs of stations: Danmarkshavn and Jan Mayen, Ostrov Dikson and Ostrov Kotelny, Coral Harbour A and Resolute A.

This shows that a of T. were subject to significant changes in the last decades. It is worth determining whether those changes were statistically significant. To do this, we may use the formula for the standard error of the difference of (Gregory 1976):

ct;, a, - the highest and the lowest standard deviations respectively, the size of samples, in the present case

The calculations made for the extreme values of a showed that in the regions of the Arctic characterised by the greatest changes of the variability of mean annual T. (ATLSRn, ATLSRe, and CANSRs) differences higher than 0.82-0.92°C are statistically significant at the level of 0.05. In the remaining area, even smaller differences, exceeding 0.6-0.7°C, are significant. Differences of this order occurred in the area of the Arctic in the last decades of the period of observation, except for SIBR and PACR (Ostrov Kotelny, Mys Shmidta).

The analysis appears interesting in the case of a of winter and summer The dispersion of summer is much smaller than that for the winter (Figure 5.9); however, its changes in time, as the calculations show, are statistically significant in a much greater area of the Arctic than in the winter. Nonsignificant differences of of summer occurred exclusively in Danmarks-havn, while in winter these differences were noted in as many as four stations: Danmarkshavn, Ostrov Kotelny, Resolute A, and Clyde A.

The spatial and temporal distributions of <7 of the extreme temperatures in the Arctic is similar to those discussed above for T. (cf. Tables 5.6 and 5.7). The calculated values of a display minor differences. T . values are charac-

1 J mm terised by somewhat lower deviations than while values for have some where:

bs - the standard error of the difference of a.

what higher standard deviations. It is worth noting that examining the day-today variability of T in Hornsund (Spitsbergen) Przybylak (1992a) observed an inverse relationship: was characterised by the greatest variability. This demonstrates that in order to obtain a full picture of the variability of T, it must be studied in various time-scales. Certain small differences can also be observed in the course of the year. In most of the Arctic, mean winter T . and

J mm

Tmax values were characterised by the highest year-to-year variability.

Table 5.7. Values of standard deviations of the mean seasonal and annual T and T . (in °C)

in selected Arctic stations from 1951 to 1990

Table 5.7. Values of standard deviations of the mean seasonal and annual T and T . (in °C)

in selected Arctic stations from 1951 to 1990

Station

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