Figure 5.10. Relative frequency (in %) of the occurrence of mean winter, summer, and annual T. in selected stations representing climatic sub-regions and regions of the Arctic over the period 1951-1990.

In CANR the variability of seasonal and annual T according to intervals is greatest in the south. The distributions of winter T in CANSRs are similar to the normal distribution whereas in CANSRn they have a bimodal character for T. and Tmax. The main maximum frequency of T. in this area ranged from -34°C to -32°C (40%) while the secondary maximum ranged from -30°C to -28°C (30%). The histograms for the frequency of summer and annual T are more similar, except that the most distinct maximums occur in the north of CANR and, for instance, in the station Resolute A, the annual T. occurred with 70% probability in the range between -18°C and -16°C (Figure 5.10).

BAFR, over which warm air masses from the North Atlantic inflow quite often in the winter, is characterised by a smaller range of changes in

T than the adjacent CANSRs. The significant influence of the atmospheric circulation on the thermal conditions in this season causes irregularity in the frequency histograms and their distributions are highly uniform (Figure 5.10). In the case of T., none of the intervals occurred with a frequency higher than 30%. Values for summer T., determined to a large degree by the solar factor, are more stable and clearly manifest one maximum. They occurred with the greatest frequency (72%) in the range from 2°C to 4°C.

The histograms of the frequency of occurrence of T averaged for particular climatic regions and for the Arctic as a whole (Figure 5.11) confirm the validity of the results arrived at on the basis of the data from the stations analysed above. Climatic regions characterised by the most intense circulation in the winter (ATLR, PACR and BAFR) have the greatest variability of T, manifested, for instance, in a greater span and in a more uniform frequency of the occurrence of particular intervals. In the summer, out of the three regions mentioned, only in the ATLR does circulation appear to have quite a strong influence on climate. The span of T is not large here because the thermal differentiation of the air masses inflowing from various directions is lowest at this time (Przybylak 1992a). As can be seen from Figure 5.11, as many as 68% of summer T. values oscillated between 3°C and 4°C, and the changes fell within the 2-5°C range.

This analysis shows that the distributions of frequency of T in the Arctic approximate the normal distribution and, consequently, the statistical calculations employed in this work are fully credible. The main factor responsible for the disturbance of the regular distribution of the frequency of T in the Arctic is the variability of the atmospheric circulation.

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