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Biomes and Regions of Northern Eurasia

The Mountains of Central Asia and Kazakhstan

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Contemporary Climate

The mountains of Central Asia and Kazakhstan are located in the subtropical zone and at the very south of the temperate zone close to the centre of the Eurasian continent. They receive more insolation than any other region in the FSU but very little precipitation and have an arid continental climate. Altitude and complexity of orography are other important controls which force strong horizontal and vertical climatic variations across relatively short distances. Climatic conditions, therefore, are discussed in general terms only because climates of slopes with different aspects, locked-in basins, and exposed plateaux can be vastly different. Due to their enormous height and extent, together with the Tibetan plateau, the Pamir and Tien-Shan are themselves important controls over large-scale atmospheric circulation (see above).

Across the region, there are strong seasonal variations in temperature. Winters are relatively mild in the west and become more severe landwards (Figure 16.5). Thermal conditions are also a function of altitude and exposure. In winter, the northern foothills are affected by continental polar air masses, forming over Siberia and the plains of Kazakhstan. However, usually the advection of cold air does not reach above 500 m and typical of the foothills and lower mountains are strong temperature inversions, which persist to an altitude of approximately 1500 m, and often make the foothills warmer than the surrounding plains. The shielded locations, such as the Fergana valley, have mild winters. Above 1500 m a normal vertical temperature distribution is restored. At an altitude of approximately 2000 m winter temperatures are negative and in the enclosed valleys the mean January temperature can be below -15°C. Exceptionally mild for its location is the Issyk-Kul depression, whose climate is moderated by Lake Issyk-Kul which remains unfrozen in winter. The mean January temperature of Przhevalsk, which is located at 1774 m is the same as in Alma-Ata at 848 m (Figure 16.5). In summer continental tropical air masses (which form over the Iranian plateau and the Turanian plain) dominate, producing hot and dry weather in the low mountains. Generally, seasonal thermal contrasts become less marked with altitude as air becomes more humid, cloudiness increases, and precipitation becomes more abundant. However, on the highly elevated plateaux the annual temperature range can be even higher than at the same longitude in the foothills. Thus in Khorog, which is situated at a height of 2100 m, the annual temperature variation exceeds 30°C.

Moisture is delivered to the mountains of Central Asia and Kazakhstan mainly by the westerly flow but the moisture content of the Atlantic air masses is depleted. Although the Caspian supplies as little as 5 per cent of the moisture received by the mountains, it influences precipitation through the combined effect of additional water vapour and conditional instability in the lower atmosphere. Evaporation from the Caspian accounts for about 40 mm of precipitation received by the northern slopes and for 10-20 mm received by the interior mountains (Lydolph, 1977). A precipitation maximum occurs in March-April when the Iranian branch of the Polar front passes over the mountains migrating further north in May (Figure 16.5). In the eastern Pamir, which is affected by the Asian monsoon, a summer precipitation maximum is recorded.

Fig. 16.5 Mean monthly temperatures (°C) and precipitation (mm) in the Central Asian mountains

The belt of maximum precipitation is located between 2000 m and 4000 m, although the vertical pluvial gradients differ between slopes (Table 16.2).

Table 16.2 Vertical pluvial gradient in the Central Asian mountains

Local topography has a very strong control over the spatial distribution of precipitation through the orographic uplift and blocking. Heavy precipitation is associated with the development of a synoptic situation known as an orographic occlusion of a weather front (Figure 16.6).

Fig. 16.6 Schematic representation of orographic occlusion in the Pamir-Alay. Modified from Lydolf (1977)

The predominant latitudinal orientation of ridges and their fan-shaped divergence to the west allows the westerly flow to penetrate deep into the valleys. The flow is channelled through the valleys and condensation of moisture takes place on the slopes in their western sections. Thus, while the Fergana valley has an arid climate, the upwind slope of the Fergana Ridge receives about 800 mm of precipitation at an altitude of 1200 m and the Gissar Ridge receives over 1500 mm. The western slopes of the north-south orientated ridges benefit most from the Atlantic moisture. Where such ridges attain a considerable height, the development of extensive permanent snow fields and glaciers takes place such as, for example, on the western slope of the Academy of Sciences Ridge. The north-south orientated ridges intercept most of the moisture brought from the west and the interior mountains are dry despite their enormous height. While the western Pamir receives 300-600 mm of precipitation, the eastern Pamir is one of the driest regions in the FSU. In the basin of Lake Karakul and the Murgab valley, mean annual precipitation does not exceed 100 mm. Large lakes further complicate the spatial distribution of precipitation. Thus, the western part of the Issuk-Kul valley receives about 100 mm of precipitation per annum while the eastern sector receives about 400 mm due to the moderating effect of the lake.

Controlled by orography is the local wind regime and both mountain-valley winds and fohns are typical, extending to an altitude of 2-2.5 km. Fohns are most frequently observed between November and April when they are forced by cyclonic activity. Associated with southerly and south-easterly winds forming ahead of the depressions advancing from the south-west, winter fohns are particularly common on the northern slopes, where their velocity often exceeds 20 m s^-1. In the regions of Tashkent and Ashgabat, during fohn events air temperature rises above +22°C in December (Borisov, 1965; Lydolf, 1977). The fohn often drives snow away (evaporation of snow is not unusual since the relative humidity can drop from 70-90 per cent to 10-20 per cent) and maintains warm weather for a few consecutive days even in the middle of the winter, which allows mountainous pastures to be used throughout the year. During the warm part of the year between March and October, fohns can damage crops although more frequently in summer the descending air is usually not much warmer than the air in the foothills. Locally, the strong fohns are known as harmsil, hot and dry wind. A variety of local circulations develop in the mountains of Central Asia and Kazakhstan, particularly in the Fergana and Issyk-Kul valleys and in the north-eastern foothills of the Dzungarsky Alatau (Lydolf, 1977, provides a detailed review).

The proximity of high mountains and deserts accentuates the role of local circulations in the transportation of dust. The afganets, a characteristic post cold-frontal wind usually of westerly or south-westerly direction, brings dust storms after passing over the southern Karakum into the upper reaches of the Amudarya and in southern Tajikistan, occasionally reaching into the Vaksh valley. The aeolian transfer of solid matter from the mountains to the plains accounts for 6 x 108 tonne a^-1 while 1 x 1013 tonne a^-1 is transported in the opposite direction (Alibekov, 1997). Currently, an increase in the aeolian transfer of dust and salts to the mountains is observed mainly due to the regression of the Aral Sea (see below). Annually, about 43 million tonnes of dust is transported from the exposed bottom of the Aral Sea and partially deposited in the mountains (Rubanov and Bogdanov, 1987).

The relatively mild climate in the low and middle mountains (Table 16.3) provide conditions for farming where water is available.

Table 16.3 Annual accumulated temperatures at various altitudes (using daily mean of 10°C as a reference temperature)

The altitude of the upper boundary of arable agriculture varies across the region in line with the availability of thermal and moisture resources, being on average 700-1000 m lower in the Tien-Shan than in the Pamir-Alay where it extends to 3500-3800 m (Chelpanova, 1963). Thus, the upper boundary of cotton growing extends to 850-1000 m in the western Tien-Shan and the Fergana valley, rising to 1000-1200 m in the Pamir-Alay and Kopetdagh (Table 16.4).

Table 16.4 Altitude (m) of the upper boundary of cultivation of various crops in the Tien-Shan and Pamir-Alay

In Central Asia, arable farming extends higher into the mountains than anywhere else in the FSU. For example, in the Caucasus its upper limit does not exceed 1800-2000 m.

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