Asia, Climates of Siberia, Central and East Asia

Central Asian, Siberian, and East Asian mega climatic regions, located on the world’s largest and most physically diverse continent, are experiencing a period of marked climatic change. The impact of climatic change is affecting the lives of nearly two billion people. With more climatic types than any other continent, and with one-third of the world’s land mass, any perturbation of Asian weather and climate is biophysically and socioeconomically important. Containing the highest point above sea level and the coldest inhabited place on earth, Asia’s location and geomorphology contribute much to climatic diversity. Major variations of climatic types are defined by receipt of solar energy, moisture from the Atlantic Ocean in the west and the Pacific Ocean in the east, and the powerful Siberian Winter High Pressure Cell. A decrease in temperature occurs from south to north, with an increase in continentality from west to east. Latitudinal variations of climate are significant.

Asian climates

Using the Köppen climatic classification, north and east Asia may be divided into three major climatic realms: Siberia or Northeast Asia in the north (Dfb, Dfc, Dwa, Dwb, Dwc, Dwd, and ET); Central Asia or Desert Asia in the west and center (BS and BW); and East Asia or Monsoon Impacted Southeast Asia (Cfa and Cw). Siberia or Northeast Asia is the largest. A complex set of interactive climatic controls and many multifaceted physiographic features influence local climates and give distinctive regional climatic character to places (Figure A60). One of the most impacting and heat-determining factors is the length of day and night at different latitudes, particularly in Siberia and Central Asia (Table A35).

Siberia extends from the Ural Mountains in the west to the Pacific Ocean in the east. It is bounded on the north by the cold Arctic Ocean and on the south by Central Asia and South Asia. The climate of Siberia is one of the most continental on Earth, with great seasonal changes. The average frost-free period is only 75–82 days. Average annual precipitation ranges from 250

Table A35 Maximum lengths of day and night at different latitudes (excluding twilight)

Table A36 Number of days with urban fog in selected Siberian cities

mm to over 2000 mm. During the winter Siberia is divided into two climatic parts: the northwest, dominated by frequent storms, high winds, and snow; and the southeast, dominated by clear skies and calm weather. A major part of Siberia is underlain by continuous permafrost, up to 1600 meters deep and extremely cold (−12°C). These factors impact temperature efficiency and moisture effectiveness in this region. The seasonal snow-melt layer is dynamical in space and time, impacts heat energetic conditions, and serves as diagnostic criteria for estimating global change or climate. Since the late 1960s a decrease in snow cover, a reduction in annual duration of lake and river ice cover, a temperature decrease, and significant changes in precipitation, cloud cover, temperature ranges, and drought frequency have occurred (Dando et al., 2003).

Figure A60

Asian climates (2003). Anna R. Carson 2003.

Along with a change in Siberia’s climate, and an increase in urban conglomeration, urban climates have impacted all biophysical processes. Increased atmospheric humidity, increased absolute humidity, and extensive use of firewood and low-quality coal for heating and for power have induced a phenomenon that is detrimental to all cultural and physical activities — urban fog (Table A36).

East Asia, recording atypical weather perturbations, including persistent drought in normally moist areas and devastating floods in normally dry regions, is experiencing an unusually strong manifestation of global warming. Changes in high-pressure cell positions and characteristics, shifts in low-pressure cell locations and intensities, and a variation of jet stream seasonal paths have modified weather patterns. Warming of the atmosphere has increased evaporation, increased specific humidity, intensified drought, increased the amount of moisture in air masses, and increased the amount of moisture and energy in the air for rainstorms, snowstorms, typhoons, and tornadoes. Concomitantly, extreme cold temperatures in early winter, followed by anonymously warm temperatures in late winter, have disrupted natural processes in eastern Mongolia and northeastern China. Changes in precipitation regimes in spring, summer, and autumn in China and Japan have enhanced the potential for flooding. A combination of human activities that modify climatic elements and controls and natural causes of climatic change could lead to large-scale weather-related disasters and to a redefinition of the traditional boundaries of East Asia’s climatic regions (Figure A61).

Figure A61

Trends in temperature and precipitation change: 1901–1998. Anna R. Carson 2003.

Climatic changes

In the 1999–2002 period a severe drought persisted across a large part of Central Asia, portions of south and southwest Asia, including Afghanistan, Iraq, Iran, Saudi Arabia, Syria, Israel, Turkmenistan, Uzbekistan, Kazakhstan, Tajikistan, Pakistan, and parts of India (Lawton, 2002). At the time it was the largest contiguous region of severe drought in the world. The International Research Institute for Climatic Prediction considered this widespread drought a “grave humanitarian crisis”. Scientists from the Ohio State University analyzing ice cores from glaciers in Central Asia noted a warming in this region over the past 50 years. At one site the recent warming and drying trend exceeded anything observed in the past 12 000 years. Current global warming has exceeded the normal range of climatic variations during the last 5000 years (Jones, 2001). Warming trends in Central and Desert Asia are not just natural fluctuations that will reverse quickly. Concomitant with the Pacific (SO) and North Atlantic Oscillations (NAS), humans have contributed to climatic changes in Central Asia. Drying of the Aral Sea is one of the best-known environmental disasters caused by humans in the past three or four decades. There is widespread agreement in the scientific community that, with the reduction in Aral Sea level, the regional climate has changed for the worst and become more extreme.

Climatic elements

The unequal distribution of solar radiation over Asia is the primary factor in Asia’s multifaceted weather and climate. In Asia’s tropical belt the sun remains high with little seasonal variation, and this accounts for continuous warm to hot year-round temperatures. In Asia’s mid-latitudes, solar radiation receipts exhibit a strong seasonal maximum and a strong seasonal minimum which are reflected in greater seasonal variations in temperature than the tropical belt. And in Asia’s high latitudes there is a period of limited to no solar radiation received at the surface of the Earth, resulting in a season with extremely low temperatures in the winter or low sun period (Figures A62 and A63). Total solar radiation received at the Earth’s land — sea surface during the entire year in kcal/cm² per year ranges from 140 to 180 in Asia’s tropical belt, 160 to 120 in Asia’s mid-latitudes, and 100 to 60 in Asia’s high latitudes. Maximum solar radiation received at the Earth’s surface occurs in the steppe and desert regions of west-central Asia. Transformation of available solar radiation is an essential ingredient of the process that produces Asia’s climate — particularly temperature ranges (Borisov, 1965).

Figure A62

Average temperature: January. Source: Geograficheski Atlas, 1959, Moskva, p. 14; Preroda i Resoursi Zemli, 1999, Moskva and Vienna, p. 52. Anna R. Carson 2003.

Figure A63

Average temperature: July. Source: Geograficheski Atlas, 1959, Moskva, p. 14; Preroda i Resoursi Zemli, 1999, Moskva and Vienna, p. 52. Anna R. Carson 2003.

Temperature

Northern Asia, specifically Siberia, is climatically isolated from moist tropical air masses and records the greatest mean annual temperature range on earth (Table A37). At Verkhoyansk, in the valley of the Yana River, January temperatures average −49°C, July temperatures average +15°C, and the absolute temperature range is 103°C. Oymyakon, located in the same physiogeographic region, records an absolute temperature range of 104°C. Siberian winter low temperatures are proverbial, and for most of Siberia the January mean annual temperature is less than −25°C. In contrast, much of southwestern Asia is subject to moderate heat (Arakawa, 1969).

The temperatures of eastern China resemble those of the eastern United States. Winter cP and cA air masses from Siberia are more powerful and colder than those that move south from Canada. Summer mT air masses from the South

Table A37 Average a annual temperatures at selected Siberian stations (in °C)

China Sea are stronger than mT air masses from the Gulf of Mexico. China extends farther north to south than the United States and has a wider range of climates and temperature regimes. China extends from 18° 53° north latitude. This is the equivalent distance from Puerto Rico to Labrador. Concomitantly, no simple summary can give adequate insights into Japan’s temperature regimes. The Japanese islands extend for 1400 kilometers or more and are surrounded by temperature- modifying ocean and seas. Irregularities in island topography induce sharp vertical temperature contrasts. The winter monsoon typically brings cold air masses and low temperatures from Siberia, while the summer monsoon brings warm, moist maritime air masses and warm, mild temperatures to Japan. The normal January temperature gradient is slightly more than 2°C per degree of latitude; the normal July temperature gradient is less than 1/2°C per degree of latitude.

Asia’s spatial temperature differences and ranges are greatest in winter, least in summer, and more extreme in all seasons within the land-locked core, rather than the south and eastern maritime periphery (Figures A62 and A63). Winter isotherms reflect the influence of ameliorating ocean currents, mountain barriers, altitude above sea level, high-pressure cells, and solar radiation. Summer isotherms reflect solar radiation and latitude, along with altitude (Global Change, A and B, 1999).

Precipitation

Precipitation receipts, in general, increase from north to south and from the southwest to the southeast. Throughout most of Siberia the annual average precipitation is scarcely 250 mm and in parts of Central Asia, the Tarim Basin, and the Gobi Desert, less than 100 mm. In east China there are regions that receive more than 2000 mm per year (Figure A64). A small part of China, north of Nepal, receives over 3000 mm annually. Seasonal distribution of precipitation becomes of greatest importance, for the summer heat and continental character of Asia affect precipitation effectiveness and thermal efficiency. In insular and island-studded southeast and east Asia, where there is limited or no frost, rainfall is relatively evenly distributed throughout the year (Cfa and Dfb). Interior and rainshadow locations in south and east Asia experience a distinct dry season in the low-sun period or winter (CW, Dwa, and Dwb). Mediterranean Sea-influenced west Asia and the dry eastern coastal region of the Caspian Sea receive most of their precipitation in winter, at least three times as much precipitation in the wettest winter month as the driest month of summer (Cs). Precipitation in the dry realms of southwest Asia, Central Asia, Tarim Basin, and the Gobi Desert is minimal and erratic, but most of the precipitation is secured in summer from violent convective showers (BW) or a combination of violent convective showers and frontal activity (BS). And in the subarctic and arctic areas of Siberia (Dwc, Dwd, and ET), summer is the season of maximum temperatures, highest specific humidity, deepest penetration of maritime air masses under the influence of the summer monsoon, and is the period of maximum precipitation (Figures A62 and A63). Asia’s central dry realm separates Asia’s eastern warm-to-hot wet belt from Asia’s north and northeastern cool-to-cold limited precipitation belt (Griffiths and Driscoll, 1982).

Figure A64

Asian precipitation. Source: Geograficheski Atlas, 1959, Moskva, p. 16; Preroda i Resoursi Zemli, 1999, Moskva and Vienna, p. 56; Oxford Economic Atlas of the World, 4th edn, 1972, p. 2. Anna R. Carson 2003.

Barnaul — an example of a precipitation regime impacted by continentality

Barnaul is located in Siberia, west of the Ural Mountains, in the extreme eastern portion of the zone of the Russian forest steppe. Barnaul’s maximum monthly precipitation falls in July that is also the warmest month of the year. Precipitation is marginal for agriculture and for supplying moisture for economic development. Droughts are common, occurring once every 4 years on the average (Figure A65). A review of the climatic records for Barnaul from 1890 to 1966 shows that droughts were recorded in 1900–1902, 1904–1905, 1909–1910, 1917, 1920, 1923, 1927, 1929, 1931–1932, 1934, 1939, 1945, 1951–1952, 1955, and 1962–1963 (Figure A65). Analysis of precipitation data for Barnaul reveals a tendency toward the repetition of very moist years when all four seasons are wet or three seasons very wet and then a dry or very dry year or two followed by an average year or two.

Figure A65

Barnaul — variation in precipitation, 1840–1960. Source: A.M. Shulyin, Izv. AN USSR, serila geogr., 1963, no. 2, p. 168. Anna R. Carson 2003.

Sukhovei — an Asian atmospheric enigma

Hot desiccating winds at times become associated with droughts, but these hot, dry winds or sukhovei can occur when there is no drought and can cause death to plants even when there are adequate reserves of moisture within the soil.

Sukhovei are hot, dry masses of air that envelop plants, increase evapotranspiration rates by factors of 2–3 to 20 times, and injure or destroy crops. Air in a sukhovey is so devoid of moisture that, in general, the relative humidity is less than 30% and, in many cases, less than 20% for an extended period of time. In a drought there are day and night variations in relative humidity, and at night dew may even form. In a sukhovey the relative humidity varies very little during the entire day/night period, and evaporation takes place continuously. The majority of cultivated plants cannot endure desiccation of their tissues; therefore the moisture lost to the atmosphere must be replaced by an inflow of moisture from the roots. Plants, unable to compensate for losses as a result of increased evaporation, lose turgescence, wilt, and die.

Sukhovei winds may reach speeds in excess of 70 miles per hour, but the usual velocity is from 7 to 11 miles per hour. Plant transpiration is increased by the most moderate wind speeds via the removal of water vapor from the apertures of the stomatal apparatus. Even wind with a velocity of 4–5 miles per hour increases plant transpiration threefold. When sukhovei winds achieve great speed they often turn into dust storms or so-called “black storms”, which remove vast amounts of unconsolidated material and even small plants over great distances. As much as 5 inches of soil have been removed from various steppe regions in one year by dust storms. Sukhovei winds may last for an hour or two, or they may last for several days, but the end result is the same: dry winds increase evaporation and disrupt the water balance (the ratio of the water inflow from roots to that transpired to the atmosphere) of the plant tissue.

The origin of sukhovei remains an enigma, but there is a general agreement that both droughts and sukhovei develop under the south-central edge of displaced anticyclones, anticyclones 10° to 20° north from their usual summer position. Displaced anticyclones with diverging, clockwise air masses are characterized by little cloud cover, low relative humidity (dropping, in some instances, to 10–17%), and the absence of rainfall. Intense solar radiation and very high temperatures create moisture deficits and desiccation of the soil.

Basic atmospheric circulation

Air masses

The basic atmospheric circulation features involved in giving regional character to Asia’s climates are the movement of air masses, transformation of air mass properties, and interactions between them along fronts. Asian air masses show great contrasts in temperature, moisture, and density. At any particular site the properties of air masses depend not only upon the nature of the source region but also the modification the air mass experienced en route from the source region. En route air mass modifications are of great importance in determining the nature of weather associated with an air mass (Lydolph, 1977; Takahashi and Arakawa, 1981). Some typical characteristics of air masses and their source regions are as follows:

• 1. Continental Arctic (winter) air masses are stable, intensely cold (−55° to −35°C), extremely dry, with inversions; cA air masses enter Siberia from the Arctic seas and at times penetrate to the Pacific Ocean; they are associated with clear skies, low temperatures, and dense air.

• 2. Continental polar (winter) air masses are very stable with strong temperature inversions and are cold (−35° to −20°C) and dry; they spread to the north and to the south of the “great ridge” of high pressure in Siberia and Outer Mongolia at approximately the 50th parallel and dominate the weather of the entire continent; generally cP air masses produce clear, cold, and cloudless weather.

• 3. Maritime tropical (winter) air masses are, for the most part, unable to penetrate eastern Asia in winter due to the intensity of the great Siberian High Pressure Cell; mT air masses that influence the weather in central Asia in winter circulate around the eastern edge of the Azores — Bermuda High Pressure Cell where upper level subsistence is strong; mT air masses are cool, dry, and stable in winter.

• 4. Maritime polar (winter) air masses have great difficulty penetrating deep into the Asian continent in winter, for the Siberian High Pressure Cell and strong outflowing winds in eastern Asia inhibit mP incursions. The mP air masses basically are conditionally stable, cool (0–10°C) and moist; they have significant influence upon the weather of maritime eastern Siberia, Manchuria, and Korea.

• 5. Continental polar (summer) air masses are, in general, stable or conditionally stable, cool (5–15°C) and somewhat moist; east and south Asia are so dominated by mT air in summer that cP air contributes little to the weather of that region.

• 6. Maritime polar (summer) air masses are a contributing factor to the summer weather of Manchuria, eastern Siberia, and Japan, and are, in most cases, stable to conditionally stable, mild (2–14°C), and humid; conflicts between mP and mT air masses in summer lead to the formation of a semistationary front in northeast Asia with associated overcast and drizzly weather.

• 7. Maritime tropical (summer) air masses initially are conditionally stable and very moist; mT air masses provide moisture for the summer monsoon and in spring invading mT air meets cP air in central and south China producing very active cyclogenesis; mT air masses dominate the weather of eastern Asia during the high-sun period.

• 8. Continental tropical (summer) air masses develop over southwest Asia in summer and are conditionally stable, dusty, very hot (30–42°C), and hold moisture; cT air flows north and northwest in summer into western Asia, advecting heat and absorbing moisture en route, plus contributing a characteristic opalescent haze to the local climates.

Asian air masses show great contrasts in temperature, moisture, and density. At any particular site, properties of air masses depend not only upon the nature of the source region but also the modification the air mass experienced en route from the source region. En route air mass modifications are of great importance in determining the nature of weather associated with an air mass (Lydolph, 1977; Takahashi and Arakawa, 1981).

Siberian high-pressure cell

General atmospheric circulation over Asia is controlled by centers or cells of high or low pressure whose axes are, in general, east-west and whose pressure centers vary drastically from winter to summer. In winter an extensive and well-developed high-pressure cell, centered over Mongolia, dominates the weather over most of east Asia (Figure A66). Triangular in shape, with the apex extending in the west to the Caspian Sea and the base anchored in the northeast near the Verkhoyansk Mountains of Siberia and in the southeast near the Chin Ling Mountains of east-central China, the Siberian High effectively blocks penetration of moisture-bearing, moderating maritime air masses in winter. Acting as a wedge forcing air masses to skirt northeasterly from the Black Sea across much of northern Siberia and to flow southerly along the Kamchatka Peninsula toward northern Japan, this intense high-pressure cell generates continental, land-trajectory, dry, cool, low-level air masses which surge from the north and northeast to the southwest across all of south and east Asia. The Siberian High, whose core and area of highest pressure is focused upon Lake Baikal, pulsates in intensity and breaks at times into smaller high-pressure cells of less intensity. February marks the height of the Siberian High’s dominance of the winter circulation over Asia. The Siberian High weakens and shifts its center westward into a position over northeastern Central Asia in April, then dissipates in May. The weather map of Asia begins to be dominated by an intensive, thermally induced low-pressure cell over southwestern Asia and focused upon the tip of the Arabian Peninsula, the Iranian Plateau, and the Thar Desert of Pakistan (Davydova et al., 1966).

Figure A66

Atmospheric pressure and winds: January. Source: Geograficheski Atlas, 1959, Moskva, p. 15; Preroda i Resoursi Zemli, 1999, Moskva and Vienna, p. 51. Anna R. Carson 2003.

Southwest Asian low

In summer, east Asia’s weather is dominated by a large, deep, thermally induced low-pressure cell that extends from the Arabian Peninsula to central China and from central India to Central Asia — centered approximately 30°N latitude and 70°E longitude (Figure A67). A complex cell, the Southwest Asian Low experiences east-west locational oscillations and occasional intense pressure deepening. This low in summer has the effect of interrupting the subtropical high-pressure system in the northern hemisphere by dividing the globe-girdling zonal band into two distinct large oceanic cells. One intense depression, the Southwest Asian Low, induces a radical change in prevailing winds and storm tracts during the high-sun period. Air masses from the stable eastern end of the Azores-Bermuda High Pressure Cell skirt this low from a north to northwesterly direction across the eastern rim of the Arabian Peninsula; less intense air masses from the northern quadrant of the Azores-Bermuda High sweep eastward across Turkey and into the northern extremities of Central Asia. Air masses and storms spawned under the western unstable quadrant of the Hawaiian High sweep from the south and southeast in a northerly trajectory across Japan, extreme eastern China, and the Russian Maritime Provinces. But the most constant and climatically significant air masses and storms are advected to this intense low-pressure cell as southwesterly trade winds spawned from a semipermanent high-pressure cell in the southern hemisphere over the Indian Ocean. In India and east Asia this modification of the general planetary wind system in summer constitutes the Asian monsoon (Tsuchija, 1964; Chang, 1967).

Figure A67

Atmospheric pressure and winds: July. Source: Geograficheski Atlas, 1959, Moskva, p. 15; Preroda i Resoursi Zemli, 1999, Moskva and Vienna, p. 51. Anna R. Carson 2003.

Frontal dynamics

Frontal zones and wind systems conform to the location and circulation of major air masses. In winter three major zones of cyclonic activity are distinguished over Asia. One zone is located along the Asiatic Arctic Front, well above the Arctic Circle in northern Siberia, extending along the shores of the continent. This front fluctuates greatly and Arctic air masses, at times, penetrate east-central Asia. The second zone is along the Southwest Asian Polar Front that develops in winter over the Mediterranean Sea and extends to the Caspian Sea. The third zone is along the East Asian Polar Front that aligns itself in a northeasterly path from extreme south Asia toward Japan. Along these winter frontal zones, and moving at various directions and speeds, depressions and anticyclones impart to the climates of Asia that special character by which one area differs from another.

During summer, three major zones of pronounced cyclonic activity can be identified. The first is the southward-displaced Asiatic Arctic Front that at times extends east-west across northwestern Siberia along the 70°N parallel. A second zone of pronounced cyclonic activity is the Asian Polar Front that normally extends from the eastern tip of Lake Balkash in Central Asia, over Mongolia, to the northernmost bend of the Amur River along the 50°N latitude. This oscillating and at times southward-dipping Asiatic Polar Front zone has been called the “barometric backbone of Asia” in summer or the “great ridge”, for a large number of anticyclones is observed each year between 50° and 55°N latitude. The third major zone is along the South Asian Intertropical Front. Extending over China at approximately 25°N latitude, the South Asian Intertropical Front is well defined in some locations, but in others it is weak or absent. In this belt, convergence of surface winds result in large-scale lifting of warm, humid, relatively unstable air, producing numerous weak, rain-generating disturbances (Trewartha, 1981). Along major frontal zones are found pronounced horizontal variations in temperature, humidity, and stability — usually jet streams. Strong, narrow jet stream currents, thousands of kilometers long, hundreds of kilometers wide, and several kilometers deep, are concentrated along a nearly horizontal axis in the upper troposphere or stratosphere, producing strong vertical and lateral shearing action.

Jet streams

Three distinct jet systems, the Polar Front Jet, the Subtropical Jet, and the Tropical Easterly Jet, have a major impact upon weather and climate in Asia. Latitudinal location of these jet systems, especially the Polar Jet, shifts considerably from day to day and from season to season, often following a meandering course. But in general, the Polar Front Jet gives rise to storms and cyclones in the middle latitudes of Asia, the Subtropical Jet noted for a predominant subsidence motion gives rise to fair weather, and the Tropical Easterly Jet is closely associated with the Indian monsoon. Over most of Asia the Polar Front Jet and the Subtropical Jet are most intense on the eastern margin of the continent and are best developed during winter and early spring. Jet stream wind speeds up to 500 km/h have been encountered over Japan. The Subtropical Jet, at times in winter, forms three planetary waves with ridges over eastern China and Japan. It frequently merges with the Polar Front Jet producing excessively strong jet wind speeds. During summer the Subtropical Jet loses its intensity and is mapped only occasionally.

East Asian monsoon

The East Asian monsoon is a gigantic multifactored, multifaceted, complex weather system, composed of diverse heat and moisture cycles intimately related to local topography, modified air masses, quasistationary troughs and ridges, and jet streams. Weather over most of eastern Asia during winter is dominated by the Siberian High-Pressure Cell and outflowing continental air masses. Low-level air flow is mainly from the north and is cold, dry, and stable. Successive waves of cold northeasterlies commence in late September and early October, progressing farther and farther southward, reaching the south China coast by late November or early December. In the higher midlatitudes of Asia during the low-sun period, a steep north-south pressure gradient persists. During March and April the Siberian High gradually weakens, and incursions of moist maritime tropical air from the south and east replace the cold-to-cool northeasterlies, producing widespread stratus clouds, fog, and drizzle that may persist for days (Das, 1968).

In summer a combination of the deep and elongated Southwest Asian Low-Pressure Cell extending from the Arabian Peninsula to China, and the South Asian Intertropical Front, reaching its maximum poleward displacement, sets the stage for a marked seasonal reversal of air flow. Warm, moist, and conditionally unstable southwesterly maritime air masses from relatively cooler oceanic source regions eventually overcome blocking atmospheric conditions, and flow northwestward over China and Japan. Considerable convective activity develops over land, and heavy showers and thunderstorms contribute largely to the summer rainfall maximum of the region. Characteristics and attributes and onset and duration of the monsoon are site-specific. Duration of the summer monsoon in China varies between north and south. Dates of monsoon onset and percent of annual average rainfall received have been a focus of numerous studies in the past century. A synthesis of these studies is given in Table A38.

In all cases low-level wind patterns and resultant weather in summer are complicated by topography. Distance from air mass source regions, moisture content of air masses, and orographic barriers and atmospheric disturbances associated with cyclonic or inter-air mass convective activity determine distribution and quantity of precipitation. There is a pronounced difference between the East Asian and South Asian monsoonal weather. The East Asian winter monsoon is much stronger than the South Asian winter monsoon, and the East Asian summer

Table A38 Dates, duration and percent of rainfall by region

monsoon is much weaker than the South Asian summer monsoon (Trewartha and Horn, 1980).

Storms

Local winds

Owing to the great variety in relief and exposure, Asia is subject to numerous local winds and their associated weather. Ubiquitous in Asia are the warm, dry, gusty, downslope foehn winds generated, in most cases, by passing atmospheric disturbances in highland or mountainous areas. For example, in Central Asia and Siberia, foehn-like winds experienced along the Caspian Sea are locally called germich; in Uzbekistan, Afghanets; in Tadzhikistan, harmsil; in and near the great Fergana Valley, ursatevskiy and kastek; and along the Kazakh-Chinese border at the Dzhungarian Gate east of Lake Balkhash, evgey. Winds are given a medley of names, and the largest and most pronounced are a major contributing factor responsible for the broad aspects of Asia’s climate (Critchfield, 1983).

For each season of the weather year, representative winds of local significance have been identified and named:

• 1. Summer — A hot, strong, and constant northerly summer wind carrying considerable dust and obscuring the atmosphere is called karaburan in the Tarim Basin of Sinkiang and northwestern China and chang in Turkmenia. At times, on the southern edge of a modified Arctic air mass that advects into southern Siberia and Central Asia, hot, dry moisture-absorbing sukhovei winds eliminate local cloud cover, permit intense solar radiation to strike the Earth and reduce relative humidity at the surface to a very low value both day and night.

• 2. Fall — Cold and often very dry northerly or northeasterly winds, preceded by a cold front, often blow with great strength and violent gusts down from the mountains and high plateaus of northern Siberia and the Caucasus. Bora is the term identifying this type of wind in the region between the Black and Caspian seas, and sarma and kharanka are bora-type winds in the Lake Baikal region of southern Siberia.

• 3. Winter — Very cold northerly or northeasterly gale-force winds, often blowing at temperatures below −20°C and accompanied by falling or drifting snow, are generated from the back side of winter depressions in Siberia and Central Asia. The sensation of cold temperatures is increased by the low wind-chill factor associated with a buran or a purga, and the break in the comparative calm associated with the Siberian High-Pressure Cell.

• 4. Spring — Continental depressions passing eastward over central China and the Yellow Sea in spring and early summer bring heavy overcasts, high humidity, and rain to China and Japan. Bai-u, mai-yu, or plum rains, as they are called, are an extended period of unstable weather caused by stagnation of the polar front.

Thunderstorms and tornadoes

Thunderstorms reach their maximum development in Asia over lowlands in summer. Most thunderstorms are ordinary convective cumulonimbus clouds within maritime tropical air masses that produce localized precipitation. Air mass thunderstorms, randomly scattered, are initiated primarily by daytime solar heating of land surfaces. Frontal and orographic thunderstorms have distinct patterns and movements, for they are triggered in a place or zone where unstable air is forced upward. Severe thunderstorms may produce hail, strong surface winds, and tornadoes. Convective thunderstorms develop more frequently here because of strong insolation and low wind velocities. Clouds of convective thunderstorms reach elevations of 12 000 meters or more, and rainfall associated with these cells is short in duration, intense, and localized. Very few thunderstorms are observed over the tundra regions of Siberia — less than five per year. Thunderstorm activity is a summer phenomenon in China, much of Central Asia, Singkiang, Mongolia, the Maritime Provinces of the Far East, and Siberia. They commence during the fall transitional period, are more prevalent in winter, and decline in number during the spring transitional period.

The most destructive spin-off of a thunderstorm is the tornado, an extremely violent rotating column of air that descends from a thunderstorm’s cloud base and can cause great destruction along a narrow track. An Asian tornado travels in a 150–500 meter wide, approximately several kilometers long, straight track, at speeds ranging between 50 and 100 kph. Although one of the least extensive, it is the most violent of all Asian storms. As the whirling mass of unstable air gains force, a rotating column of white condensation is formed at the base of the cloud. Dirt and debris sucked into this whirlwind darken the column as the column of air reaches the ground. A notable feature of the climate of Asia is the relative infrequency of tornadoes, particularly as compared to central and eastern North America at similar latitudes. Although records are inadequate, there are sufficient data to conclude that tornadoes occur on the order of once every 3–5 years in the northern Caspian Sea area and Central Asia during May, June, and July and two to five or more annually in China and Japan during August and September.

Tropical cyclones

One of the most powerful and destructive types of cyclonic storms is the tropical cyclone (Hsu, 1982). Referred loosely to any pressure depression (near 1000 mb) originating above warm oceans in tropical regions, tropical cyclones form an important feature of the weather and climate of south and east Asia — particularly from July to October. Different terms are used worldwide to describe this tropical storm: typhoon in the western Pacific; baquios or baruio in the Philippines; tropical cyclone in the Indian Ocean; willey-willeys in Australia; hurricane in the eastern Pacific and Atlantic; cordanazo in Mexico; and taino in Haiti.

Organization and development of tropical cyclones are not fully understood, and they are under intensive study. Formation of this type of storm is associated with warm ocean surfaces not less than 27°C, located between 5° and 10°N latitude, light to calm initial winds, and waves or troughs of low pressure deeply embedded in easterly wind streams converging into an unstable atmospheric zone. Large quantities of latent heat released through condensation are converged and transferred to higher levels, deepening the pressure center and intensifying the storm. An almost circular storm of extremely low pressure, into which winds spiral with great speed, is formed. Asian tropical cyclones travel slowly at speeds of 16°48 kph, cut a destructive storm path 80°160 km wide, and winds in the wall cloud area achieve speeds in excess of 200 kph. Passage of a tropical cyclone over water and land is associated with strong winds and heavy rainfall. Storm tracks vary annually and no two recorded tracks have been exactly the same. Despite all irregularities, most tropical cyclones have a tendency to move westward, then poleward, finally turning eastward, toward higher latitudes under the influence of both internal circulation and external steering currents, penetrating into the belt of westerly winds. This awesome tropical storm contributes between 25% and 50% of the annual precipitation received in many tropical weather stations. Flooding, destructive wind force, and storm surge are responsible for much property damage and for human casualties.

Occurrence of tropical cyclones is restricted to specific seasons depending upon the geographical location of the stormaffected region. The Observatory of Hong Kong reports that 83% of the annual total recorded in Hong Kong occur between June and November. Approximately 50% of these severe tropical storms attain typhoon intensity. From mid-November to April, very few tropical storms pass over the coasts of China and Korea, but in the warm July through October period numerous tropical depressions, tropical storms, severe tropical storms, and tropical cyclones (typhoons) are experienced. In the 1884–1955 period at least 438 tropical cyclones crossed various sections of the Chinese and Korean coasts. Japan’s tropical cyclone season begins in June and ends in November, reaching its peak occurrence in September. In the 1918–1947 period 85 typhoons were reported in Japan. The tropical cyclone season is slightly longer in the Philippines, extending from June to December. Almost 90% of all tropical cyclones in the 1948–1962 period were noted in the summer and fall seasons. Tropical cyclones usually weaken over land, and few penetrate and persist more than 500 km inland. The rise in sea level, when combined with high tides, accounts for more damage and loss of life in Asia than violent wind (Riehl, 1979).

Prospectus

Continentality, as a climatic control or factor, is best manifested in the extremes found in the climates of Central Asia, Siberia, and East Asia. The land-dominated interior climates present striking contrasts to the maritime-impacted coastal and island areas. Winters are much colder than corresponding latitudes in western Europe and North America; also, those regions where continentality is a climatic factor are the regions where climatic change is most noticeable. Vegetal, agricultural, and biotic zones have advanced 10 km northward in some climatic regions. This is best revealed in satellite remote sensing images of Central Asia and Siberia. In the past 20 years Asian temperatures have remained above the long-term averages (Figures 3 and 4). An annual change in temperature of only a few degrees centigrade affects the climates of Asia sufficiently to render marginal agricultural regions unacceptable for food production and to wreak havoc on local food supplies (Bryson and Murry, 1977). Moreover, a combination of human activities that modify climatic elements with natural causes of climatic change could lead to more frequent or more severe changes in Asia’s climatic regions (Christianson, 1999). For those struggling to secure a meager existence from a hectare or two of land in Asia, and for the urban dweller seeking water to maintain bodily functions, any climatic change disrupts lifestyles — because humans and human institutions are adjusted to precisely the climate and weather that prevail (Budyko, 1977; Dando, 1980; Martens, 1999).