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Physical Geography of Northern Eurasia

Climate at Present and in the Historical Past

<<< Evaporation | Physical Geography Index | Snow cover >>>

Droughts and Associated Climatic Hazards

Droughts are perhaps the most serious and damaging of all climatic hazards. Droughts affect mainly the southern part of Northern Eurasia, extending northwards in the Yana and Lena basins (Figure 3.11).

Frequency of droughts (% of all years) between the middle of the 19th and 20th centuries

Fig. 3.11 Frequency of droughts (% of all years) between the middle of the 19th and 20th centuries. Modified from Rudenko (1958)

It is the south, the forest-steppe and steppe zones, that are the main regions of arable agriculture and, consequently, droughts have profound implications for food production and human welfare. Associated with droughts are such phenomena as sukhovey (dry, hot wind) and dust storms which further exacerbate the impacts of hot, dry weather and often destroy crops. An enormous effort has been devoted to the study of droughts and associated hazards, atmospheric processes causing droughts, their prediction and methods used to minimize the effects of droughts on crops. Studies of droughts date back to the middle of the 18th century. Systematic research began in the second half of the 19th century and was prompted further by the succession of droughts that affected the central and southern regions of the European territory in 1890, 1891, and 1892. The drought of 1891 lasted from May to August and spread as far north as the middle course of the Kama and the Vyatka, affecting a population of more than 30 million (Buchinsky, 1976).

Two types of droughts are distinguished: soil drought and atmospheric drought. (This discussion does not include physiological droughts, that is, those conditions when moisture is available but plants are incapable of using it. Physiological droughts affect healthy plants under the low temperatures and are most widespread in the Arctic and in the high mountains.) Atmospheric droughts occur when temperatures remain high for a considerable period of time predetermining high potential evaporation while precipitation is low. Two additional factors control soil droughts: precipitation during the cold part of the year and the amount of snow and ground water availability. Due to the stable snow cover, a watertable that is close to the surface and, in the case of the Asiatic part of the region, permafrost, severe deficits of soil moisture do not affect the northern part of Eurasia (Figure 3.12a).

Frequency of soil droughts and distribution of sukhoveys (dry, hot winds) in European territory

Fig. 3.12 Frequency of soil droughts and distribution of sukhoveys (dry, hot winds) in European territory. Modified from Buchinsky (1976)

The early students of droughts attributed atmospheric droughts exclusively to the advection of hot, dry air from the Caspian region and Central Asia. However, the main factor in the formation of droughts is the prolonged persistence of high pressure, which disrupts zonal flow, and local transformation of air. Discussing the origin of droughts in the context of the East European plain, Buchinsky (1976) points out that it is not the tropical air but the Arctic anticyclones, which advance from the Barents or even from the Kara Sea and become stationary over the plain, that provide ideal conditions for the development of droughts. Over 70 per cent of all droughts in the central part of European Russia and the Volga region are associated with these high pressure systems (Klimenko, 1996). Similar to all other blocking highs, the Arctic anticyclones prevent the normal eastwards propagation of moisture-bearing frontal systems and curtail rainfall. In addition to this common factor, the Arctic anticyclones draw in air from the Arctic Seas. In contrast to the air advected along the periphery of the Azores high, the arctic air is initially cold and, consequently, has a low moisture content. Should high pressure persist over the continent for a long time and a soil moisture deficit develop, moisture input through evaporation will be reduced. The fresh arctic air, which brings in little moisture, enhances dry conditions over the central East European plain. The Azores high, which extends to the East European plain in summer, plays an important role in the development of droughts in Moldova and the Ukraine (Volevaha and Koshelenko, 1969).

The severity of droughts depends on the duration of anticyclonic weather. One of the worst modern droughts occurred in the summer of 1972 (Fedorov, 1973; Buchinsky, 1976). An anticyclone, centred over Moscow, established over the East European plain in May and persisted throughout the summer, blocking the passage of Atlantic depressions. The drought, which started in the south-eastern Ukraine and in the lower courses of the Don and the Volga, had spread as far as Karelia by the middle of July. The prolonged transformation of air, aided by the advection from Caspian deserts which became established with the expansion of the blocking high, resulted in extremely dry, hot weather. The southeastern Ukraine, central Russia, and the Volga region received 20-30 per cent of the average summer rainfall while monthly temperatures exceeded the norms by 3-7°C. Daytime air temperatures were above 30-35°C while relative humidity dropped to 10-20 per cent. In addition to the widespread loss of crops, the extension of the drought far north resulted in extensive forest and peat fires. Droughts as severe as this occur on average once in a hundred years.

The sukhovey, a dry, hot wind, is an important phenomenon in the drier regions of Eurasia and is a typical attribute of prolonged periods of dry weather. Although in the European territory, sukhoveys are usually south-easterly winds, their origin is more complex than simple advection from the Caspian and Central Asian deserts. Trajectory analysis has shown that over 50 per cent of air flows that produce sukhoveys originate over the Arctic seas and have an original temperature of about 10°C and a relative humidity of about 60 per cent (Volevaha and Romushkevich, 1971). As the air, circulating around the periphery of a high pressure system, passes over the continent it warms at a rate of 2-5°C day-1, while relative humidity drops at a rate of 5-10 per cent day-1. By the time the wind changes its direction from northerly to south-easterly over the Caspian deserts, it becomes hot and dry, these characteristics being enhanced by the influence of the deserts. Adiabatic descent in high pressure systems contributes to the formation of sukhoveys. As little as 10 per cent begins in Central Asia and Iran. Figure 3.12b gives frequency of sukhoveys, defined as wind with a speed exceeding 5 m s-1, a daytime temperature above 25°C, and a relative humidity below 30 per cent, for the East European plain. Sukhoveys occur between April and September, peaking in high summer but being particularly damaging to young crops in April-June. They usually last for 2-4 days but the extreme events span two to three weeks (Buchinsky, 1976).

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