2. Results

Figure 1

[8]  Contrary to the distinct seasonal reversals in the lower atmosphere circulation for summer and winter, the direction of the prevailing zonal wind in the lower thermosphere climatology remains eastward in winter and in summer as well. As for equinoxes, the wind direction depends on the longitude (region). The eastward wind decreases, and sometimes (e.g., in central Europe) even a wind reversal occurs. Figure 1 demonstrates the multiyear averaged seasonal variations in the zonal prevailing wind V_ox at three midlatitude (52^o N) observatories: Irkutsk (East Siberia), Collm (central Europe), and Saskatoon (Canada). The 10-day running smoothing was used. This seasonal pattern based on the daily data has a high statistical significance. Also, it is evident that the seasonal variation in the zonal circulation really depends on the longitude: The autumn minimum over Siberia occurs earlier than over Europe, and the spring minimum is accompanied by a reversal of the wind only over Europe and Canada but not over Siberia. During winter the averaged wind over Siberia and over Canada is about twice stronger than over central Europe.

Figure 2

[9]  However, if we consider the day-to-day wind variations without statistical averaging and smoothing, we can obtain an extremely high wind variability especially for the periods of equinoctial circulation rearrangements. It is known that in the winter months the wind reversals in the lower thermosphere can be associated with sudden stratospheric warmings and the spring circulation rearrangements there often coincide in time with so-called "final" stratospheric warmings [e.g., Kazimirovsky et al., 2003]. Principally, strong geomagnetic storms can be also associated with the lower thermosphere wind reversals [e.g., Kazimirovsky et al., 1999]. The variations in daily values of the prevailing zonal wind velocity ( V_ox ) are shown in Figure 2 for two observatories and 10-11 years of observations.

[10]  We define "summer" and "winter" periods of circulation as periods with rather stable eastward winds and periods of rearrangement in the circulation as periods of high instability of the wind direction. It is clear that accepting these definitions, the duration of the rearrangement period can vary from some weeks up to some months. The boundaries between the seasonal regimes (dates) were determined as days of an abrupt change of the wind direction or as the days of a significant decrease of the wind with stable direction. Of course, in some situations, there is some arbitrariness in the determination within 2-5 days. The regional and specific features in the character of V_ox variations can be seen in Figure 2. For Irkutsk especially, the "nonstandard" year was 1988. In the first half of this year a strong eastward wind was observed. In the middle of July a reversal occurred, and a westward wind was observed during almost 4 months. The winter eastward wind was established only for a short period at the end of November.

Figure 3

[11]  Using the aforementioned definitions and criteria for the beginning and end of the circulation rearrangements, it is possible to follow how these dates vary between one year and another. Figure 3 demonstrates these variations for spring and autumn. At Collm the duration of the autumn rearrangement remains almost constant from year to year, but the duration of the spring rearrangement increases systematically and so summer duration decreases. At Irkutsk, after 1986 the autumn rearrangement begins earlier and its duration increases. The spring rearrangement here during 1981-1992 begins and ends later and later. So, at Irkutsk the duration of the spring rearrangement varies within the 58-20 days interval, and the duration of the autumn rearrangement varies within the 99-12 days interval. At Collm the tendency to increasing of the duration of spring rearrangement is observed. It was minimal (40 days) and maximal (~100 days) in 1985-1986 and 1993-1995, respectively. It is worth mentioning that the duration of the spring rearrangement at Collm varies within a wider interval than at Irkutsk (36-105 days) but the duration of the autumn rearrangement at Collm varies within a narrower interval than at Irkutsk (13-43 days). Therefore, for both regions the summer circulation period (the interval between the end of the spring rearrangement and the beginning of the autumn rearrangement) demonstrates the tendency to a systematic decrease.

Figure 4

[12]  The variations of the winter (Figure 4a) and summer (Figure 4b) are presented for both observatories. At Irkutsk the winter duration vary from 141 days in 1983 up to 183 days (  6 months) in 1987 what is longer than at Collm (122-169 days). Also, contrary to Irkutsk, at Collm the winter circulation duration decreases from 169 days in 1984 down to 122 days in 1996. The summer circulation duration at Irkutsk was maximal in 1982 (179 days) and minimal in 1987 (84 days). At Collm a systematic decrease of the summer circulation duration from 1984 to 1996 (145 days and 97 days, respectively) is pronounced more clearly than at Irkutsk.