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Physical Geography of Northern Eurasia

Climate at Present and in the Historical Past

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Temperature

The thermal regimes of Northern Eurasia are controlled by two main geographical factors: high latitude and the landlocked position of most regions. The two main meteorological controls, responsible for the formation of thermal climates, are atmospheric circulation and the energy balance of the underlying surface. These factors are interconnected and their relative importance varies between regions and seasonally. Generally, temperatures across Northern Eurasia are low but their annual ranges are large. In winter, there is a considerable west-east temperature gradient reflecting increasing continentality of climate and weakening of the ameliorating effect of the Atlantic. In the western part of Eurasia, winter temperatures are considerably higher than in the east and their seasonal variability is reduced. Mean January temperatures vary from slightly negative along the western border of the FSU to -50°C in the Verkhoyansk-Oimyakon region of Eastern Siberia (Figure 3.5).

Fig. 3.5 Mean surface air temperature in January and absolute minimum surface air temperature. Modified from Lydolf (1977)

In the European territory, atmospheric circulation is the main control over thermal regime in winter. There are five main circulation factors affecting the thermal climate of the East European plain: frequency of depressions; depression tracks; frequency of anticyclones; source and trajectory of the air, circulating in anticyclones; and the duration of anticyclonic weather. The combination of these factors creates a strong temperature gradient which is directed from west to east.

Depression activity affects mainly the western (west of the 40-th longitude) and northern parts of the region. Temperatures, associated with cyclonic weather, vary depending on the depression tracks and the nature of advection associated with these tracks. Thus depressions, crossing the East European plain from south-west to north-east, bring about mildly negative temperatures while depressions, migrating from the Mediterranean and the Black Sea, bring about low positive temperatures which is very warm weather for this time of year. Colder advection from the Arctic is associated with depressions, travelling from west to east, over the northern East European plain. This factor explains the thermal climate in very broad terms because the same storm brings one type of weather to an area on one side of the storm and an entirely different type of weather to an area on the other side of the storm. However, in general, it is frequent cyclonic activity that is responsible for the relatively mild thermal climate in the western part of the East European plain, where January temperatures are up to 8°C higher than latitudinal averages (Figure 3.5). Thaws are frequent in winter and between 50°N and 60°N their frequency changes from 45-50 days per winter in the west to 20-25 days in the east. Another important characteristic of the thermal regime, associated with depression activity, is rapid day-to-day temperature fluctuations. On average, temperature changes between two consecutive days by 4°C. However, 10-15°C temperature fluctuations are not uncommon (Myachkova, 1983).

The eastern part of the region (east of the 40th longitude), which experiences a higher frequency of anticyclonic weather in the south and advection from the colder sectors of the Arctic in the north, has lower winter temperatures. Thus at 60°N, on average temperatures drop to -30°C once per winter in St. Petersburg while in the easternmost part of the region, such frosts are observed on 10 days. In addition to the changing circulation, warm air masses cool while moving over the snow-covered East European plain. The Atlantic air has low positive (0-2°C) daytime temperatures in St. Petersburg but reaches the east with temperatures of-2°C to -7°C (Myachkova, 1983). The decline in depression activity is particularly notable in the south-east. Depressions reach the south-east 2-3 times per month while the number of days with anticyclonic circulation increases to about 20 days per month (Klimenko, 1994a). It is the position of the extension of the Siberian high that controls temperatures in the east: a more southerly position results in the advection from Kazakhstan and temperatures of about -10°C while a more northerly position draws the air from Siberia and brings about temperatures of -20°C. The duration of anticyclonic weather is important too and after a few calm and clear days, temperatures are appreciably lower than at the onset of anticyclonic weather.

The winter thermal climate of Western Siberia and Kazakhstan is formed under the influence of air masses arriving from the Arctic, East European plain, Eastern Siberia, and Central Asia. The northern part of the region (north of the 55th latitude) experience strong cyclonic activity with depressions arriving either from the Barents Sea or from the European territory and travelling towards the Taymyr peninsula. These depressions bring relatively warm air especially to the south-west. The southern part of the region (south of the 55th latitude) is dominated by the extension of the Siberian high. While the south-west, located at the western periphery of the high, receives relatively warm air from Central Asia (associated with daytime temperatures of about -5°C) or occasionally from Iran (low positive daytime temperatures), the eastern part receives much colder air from Mongolia in the south (daytime temperatures of about -15°C) and Eastern Siberia in the north (daytime temperatures well below -20°C). These circulation patterns create a temperature gradient directed from the south-west to the north-east (Figure 3.5). Similar to the European territory, the intense depression activity results in strong day-to-day temperature fluctuations. The average day-to-day change is 5.5°C but fluctuations of over 15°C occur especially in the north-east where the advection of relatively warm Atlantic air alternates with the extremely cold advection from the Siberian Seas or the Central Siberian tableland (Myachkova, 1983).

In contrast to the European and West Siberian sectors where winter temperatures are primarily controlled by the atmospheric circulation, east of the Yenisey winter temperatures are controlled by the radiative loss of energy. Depressions do reach Eastern Siberia (especially the Baikal region and the Arctic coast) but it is dominated by the Siberian high and its extension to the Yana-Indigirka-Kolyma watersheds on 80 per cent of all days or more. Temperatures are extremely low and rather uniform across the region, being accentuated locally by topography. The difference in mean January temperatures between 50°N and 70°N does not exceed 10°C (Figure 3.5). The area between the Yana and the Kolyma is the coldest with temperatures being about 20°C below latitudinal means. The lowest temperature in the Northern Hemisphere of-71°C has been registered here in Oimyakon, in an intermountain depression at an elevation of 13 7 m. Day-to-day temperature fluctuations are small. Infrequent and short-lived increases in temperature are associated with the arrival of fresh arctic air in the north, westerly flow in the south-eastern part of the region, and air masses from the Sea of Okhotsk in the east. Characteristic of the region are temperature inversions (see below). Temperature increases with height through the lowest 500-1000 m. Observations in the Central Siberian tableland have shown that the average vertical temperature gradient in the lowest 300 m is 5°C per 100 m while maximum temperature gradients can reach 19°C per 100 m (Konstantinov, 1995). As the Siberian high, which is responsible for clear weather, does not extend above the 850 hPa level, temperatures in the mountains of Eastern Siberia are higher than in valleys due to the ameliorating effect of zonal flow.

In contrast to the uniformly low temperatures in Eastern Siberia, extremely strong temperature gradients exist in the Far East between the Eurasian land mass and the Pacific Seas (Figure 3.5). Much of the region is affected by the winter monsoon which draws the very cold air from Siberia. Another important feature is the extension of the Aleutian low towards the Sea of Okhotsk and intense cyclonic activity associated with this extension. The cold continental air is advected along the eastern periphery of the Siberian high and the western periphery of the Aleutian low southwards, occasionally reaching as far as Malaysia (Cheang, 1977; Chiyu, 1979) and pushing the Pacific branch of the Polar front to the south-east of Japan. As a result, even the southernmost regions of the Russian Pacific experience very low winter temperatures. The western part of the Amur basin, dominated by Siberian air throughout the winter, is the coldest. In this area, temperature drops below -40°C on about 30 days per winter, while in Sakhalin and Kamchatka such temperatures occur on about 10 days (Myachkova, 1983). The Siberian high does extend towards Kamchatka and Beringia but on no more than 20 per cent of all days in winter and continental air warms while passing over the north-eastern Pacific (Mock et al, 1998). The Sea of Okhotsk is a centre of intense depression activity maintained mainly by the storms which develop on the Polar front, migrate northwards, and regenerate over the Sea of Okhotsk where the Arctic front is positioned. These depressions bring warm maritime air to the region which can penetrate as far north as the Koryak highlands and the Chukchi peninsula. In short, the winter thermal climate of the Far East is milder than that of Eastern Siberia but mild it is not. The mean January temperature in Vladivostok (43°06'N, 131°50'E) is -15°C despite its southern and coastal location.

Along the southern border of the FSU, between the Crimea and the mountains of Central Asia, low positive winter temperatures are observed. First, there is no stable snow cover south of 43°N and here the radiation budget remains positive throughout the winter. Second, the Crimea and Transcaucasia are protected from the cold northerly flow by the mountains while Central Asia is protected from the cold easterly flow. Third, eastern Transcaucasia and Central Asia experience frequent warm advection from Asia Minor, Iran, and Afghanistan which occurs both in warm sectors of depressions, developing over the southern Caspian on the Iranian branch of the Polar front, or along the western periphery of the Siberian high. The position of the latter has a profound effect on the nature of advection: the more northerly position of the ridge results in the advection from Siberia and Kazakhstan. While in the southernmost regions of Central Asia the chill of the easterly flow is counterbalanced by the fohn effect and on the plains temperatures do not drop below -20°C, it brings temperatures of about -25°C to the Aral Sea region and of -35°C to eastern Kazakhstan (Myachkova, 1983).

While severe frosts are almost ubiquitous in winter, summer temperatures are almost universally high and reach 30°C everywhere apart from the extreme northeast. First, this means that most of Northern Eurasia experiences enormous annual temperature ranges (Figure 3.6). In the region of Oimyakon, an absolute temperature range exceeds 100°C. Second, spatial temperature contrasts are smaller in summer than in winter and their nature is different.

Fig. 3.6 Absolute annual ranges of air temperatures (°C). Modified from Lydolf (1977)

In contrast to winter, summer temperatures are primarily controlled by radiative factors. Insolation increases from north to south in the European territory, in Western Siberia, Kazakhstan, and Central Asia and so does air temperature (Figures 3.7 and 3.8).

Fig. 3.7 Insolation (kcal cm-2) in June. Modified from Myachkova (1983)

Due to a lack of moisture, a small proportion of net radiation is spent on evaporation in the south especially in Central Asia and Kazakhstan where latent heat accounts for as little as 10 per cent of net radiation. This contributes to the formation of high temperatures. The highest temperatures are observed in the deserts of the Turanian lowland where monthly means reach 30°C while absolute maxima reach 50°C. The persistent high pressure ridges, forming either as the extension of the Azores high or of the Arctic highs, or low pressure-gradient fields create ideal conditions for the gradual warming of air over the European territory and Western Siberia. Even the arctic air, which initially brings about fresher weather, warms quickly and the longer such conditions persist, the higher are the temperatures.

Over the central part of the East European plain anti-cyclonic conditions occur on nearly 80 per cent of all days in summer (Klimenko, 1994b). Although depression activity occurs across the European territory and Western Siberia in summer, it is more frequent in the Baltic region and in the north (north of the 55th latitude) where it contributes to cooler summers.

Fig. 3.8 Mean surface air temperature in July and absolute maximum surface air temperature. Modified from Lydolf (1977)

In Eastern Siberia, where low pressure becomes established and cloud cover increases, the distribution of insolation is remarkably uniform. The lack of change in the components of radiation and heat budgets and transformation of air over the vast land mass create a very uniform distribution of temperature. Generally summer temperatures are high. Advection from Mongolia and China brings about extremely hot weather with temperatures exceeding 35°C. Such events are particularly typical of southern Transbaikalia where they occur on 5-10 days each summer (Myachkova, 1983). By contrast, advection from the Arctic brings about much colder weather with daytime temperature of just 10-15°C and nocturnal frosts. The frequency of the arctic air masses is quite high, changing from 30 per cent in the north of the Central Siberian tableland to 20 per cent in the south. Large diurnal ranges of temperature and nocturnal frosts are typical.

The two main factors, controlling summer temperatures in the Far East, are the proximity to the cold seas and the summer monsoon. Frequent fogs and increased cloud cover result in insolation values that are lower than latitudinal averages, especially in the coastal regions and on the islands. The incoming solar radiation increases landwards rather than southwards (Figure 3.7). The Sea of Okhotsk has very low surface temperatures and floating ice does not melt until mid-summer. This is the main source of cold air which is transported along the periphery of a high, building over the Sea of Okhotsk in late spring and early summer, southwards to the Amur basin and the Maritime Province. Later in the summer, warm tropical air reaches the Russian Far East as the monsoon intensifies but dense clouds and fogs, forming on contact with the cold sea surface, restrict heating.

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