Main Reasons for Drought

The main reason for drought formation in arid regions of Russia and other CIS regions is the penetration of anticyclonic air masses from the Arctic (Selyaninov, 1928, 1958, 1966). These air masses have low temperature and low water content and are mostly transparent for solar radiation under clear weather conditions. When these air masses move south, their temperature rises quickly, which increases the water vapor deficit in the atmosphere without changing its absolute water content. As a result, the relative humidity falls to about 10% or lower, causing droughts. When a system of arctic anticyclones moves to the west, it acts in parallel with the

Figure 15.2 The variation in winter and spring wheat yields in Novoakhtubinsk area (Volgograd region, Russia) for the 1970-2000 period.

ring of subtropical anticyclones at the southern (40-50°) and high («75°) latitudes. Normal air circulation in the Northern Hemisphere is disturbed, and dry weather conditions in the former USSR European territory, West Siberia, and Kazakhstan are formed. As the Atlantic air masses in subtropical anticyclones move away from the ocean, they are heated more slowly as compared to polar intrusions. However, having passed a longer way over the continent (from the Bay of Biscay to Dnieper), water vapor in the air is exhausted, and drought phenomena become more pronounced to the west of Dnieper.

Most often, the arctic and the Azores intrusions are combined in the Lower Volga and the Lower Yuzhy Ural, and pronounced drought conditions occur in the former USSR regions. At times, the geographical position of subtropical and the Azores anticyclones vary from year to year and even from month to month. As a result, the geographical distribution of droughts varies on a temporal scale. The Azores anticyclone may move to 55° latitude, bringing drought conditions to the whole southern part of the former USSR. In the case of the Meridional polar intrusions, not only does air dry up quickly and intensely, but also the movement of moist Atlantic air masses to the east is retarded, and droughts occur during the whole vegetation period.

Khomyakova and Zoidze (2001) determined the probabilities of drought occurrence of various regions. The degree of drought intensity (DDI) was obtained using nine indices (Zoidze and Ovcharenko, 2000).

The study of severe and extremely severe droughts in the primary arid Russian Federation regions has shown that there is no certain periodicity in droughts (Khomyakova and Zoidze, 2001). Droughts may occur during some years in succession as well as with a 1-, 2-, 3-, and even 16-year interval. Therefore, the prediction of this phenomenon becomes much more complicated, and drought monitoring at regular basis is more important.

0 0

Post a comment