Climatology—South Asia | Research & Encyclopedia Articles

In South Asia climate is governed by the tropical monsoon system. The monsoons occur in summer and winter and are variable over space and time.

Because of atmospheric circulation, the monsoon over South Asia alternates seasonally between equatorial westerlies in summer and trade winds (tropical easterlies) in winter, which are mostly deflected to southwesterlies and northeasterlies at ground level. Both flows vary in rain capacity. The southwesterlies are associated with moist and unstable air masses of great rain-carrying capacity; the northeasterlies are comparably dry, stable, continental airflows with little rain-carrying capacity. In most areas the summer monsoon defines the rainy season and the winter monsoon the dry season. The duration of either season varies depending on the region, as do rainfall totals. The summer monsoon is the major agricultural season; winter agriculture depends on irrigation, which is widely used throughout South Asia.

For South Asia the monsoon climate system defines on average four seasons: summer, from June through September; winter, from December through February; and two transition seasons, March through May (premonsoon) and October through November (postmonsoon). The onset of the summer monsoon may vary by several weeks or may even fail, upsetting the agricultural calendar and causing famine. It generally starts between late May in the south to early July in the north, and withdraws between mid-September and early December from north to south, varying in length, therefore, from six to three months. It affects rainfall throughout South Asia. The winter monsoon is a continental flow that establishes a dry season.

The striking seasonal nature of rainfall over South Asia is illustrated by computing the monthly distribution of rainfall through the year for 306 representative stations throughout India for a 120-year recording period (1871–1990). The monthly and seasonal rainfall totals are as follows, given in millimeters:

Dec.	Jan.	Feb.	Mar.	Apr.	May
12	11	13	15	26	52
winter		36	pre-monsoon		93
June	July	Aug.	Sept.	Oct.	Nov.
163	275	243	171	77	31
summer			852	postmonsoon	108

Seventy-eight percent of the annual total of 1,089 millimeters of rain falls during summer, and only 3 percent during winter; the transition seasons have 8.5 percent (premonsoon) and 10 percent (postmonsoon). Hence rainfall other than during the four months of summer is negligible.

In practice, however, rainfall varies to a hazardous degree over South Asia. The world's record rainfall total, at Cherrapunji (located in the Shillong Hills of Assam), averages an annual 10,798 millimeters, with the wettest year being 24,000 millimeters. In extreme contrast, desert conditions occur over most of the Indus plain and Tharr Desert (where in some locations annual rainfall is less than 100 millimeters). The overall distribution of rainfall shows the wettest parts over the western coastal lowlands on the Indian peninsula and its eastern provinces on the Deccan plateau, the Ganges plains, Brahmaputra lowlands, and surrounding parts of northeastern India and the Bengali lowlands. The driest regions are northwestern India and Pakistan. In most parts of South Asia annual rainfalls vary between 750 and 1,500 millimeters, enough to support abundant crop cultivation in the rainy season.

The high interannual variability of rainfall makes monsoon rains unreliable for agriculture. Interannual variability is largest in dry regions, with a coefficient above 30 percent, and lowest in the wettest parts. with less than 15 percent variability.

Rainfall annual totals have changed over the years on a small scale. The maximum annual rainfall increase during the past one hundred years (588 millimeters) occured at Bombay; the maximum decrease (679 millimeters) occurred at Nuwara Eliya, Sri Lanka. In South Asia low rainfall occurs during El Niño years and high rainfall during La Niña years.

Under the prevailing hot and warm conditions large amounts of water are evaporating at all times. Only during the summer monsoon is a rain surplus observed; that is, more water falls as rain than is evaporated. In all other seasons evaporation exceeds rainfall.

Temperatures also vary greatly over South Asia, though tropical temperatures principally prevail. Shown by a cross section from north to south, mean annual temperatures are high. The mean hottest (summer) month shows only slight variation across South Asia, while there is considerable difference in the temperature mean for the coldest (winter) month. The annual temperature range (the difference in temperature between the hottest and coldest month) increases with latitude. Winter and summer are therefore defined only over the northern parts of South Asia, while the southern parts have only slight annual temperature variation. Synchronously, as one moves south, the daily range of temperature also decreases, yet the daily temperature range surpasses the annual temperature range, a characteristic of tropical conditions. Temperature drops with increasing altitude, leading to freezing temperatures in the highlands (above 2,000 meters) of Sri Lanka and South India. The altitudinal decrease of temperature most strikingly affects the Himalayas and Karakoram mountains above 1,500 meters, where precipitation occurs as snow and ice. The affect of global temperature change on South Asia is discussed in the final paragraph.

Rainfall and temperature variabilities over space and time are summarized by climate diagrams in which annual variations in both elements are plotted month to month. Changes in rainfall and temperature in most cases occur synchronously, such that summer is both wet and hot, while winter is dry and cold.

South Asia suffers from floods and droughts that originate from heavy rainfalls and failure of monsoon rains, respectively. The worst hazards are tropical cyclones, which originate in the Andaman Sea, move in a western direction, and three to five times a year visit the lowlands on the east coasts of India and Bangladesh. Tropical cyclones develop extremely heavy storms moving at over 200 kilometers per hour; they are accompanied by torrential rainfalls and high sea waves, which wreak havoc on the coast and in the hinterlands.

Resulting mostly from the rainfall conditions and to a smaller extent from snow precipitation on the northern fringe of South Asia and the Himalayas, river runoff is subject to extreme variations. The maximum flow is mainly in summer, the minimum in winter. The contrasting discharges of the Himalayan rivers are illustrated by the Brahmaputra at Pandu, with a maximum flood discharge over 70,000 cubic millimeters per second and a minimum at 3,000 cubic millimeters per second. The rivers of peninsular India also show great variation, with the Mahanadi having a maximum of 46,000 cubic millimeters per second and a minimum of only 6 cubic millimeters per second. In summer the rivers regularly inundate the land along their lower stretches, particularly the deltaic regions. Control of river discharge, by dikes and dams, is a major task on all rivers.

Observations during the past one hundred years identify a warming trend over the northern, eastern, central, and southern parts of South Asia, which partly exceeds the global trend (0.55° C per 100 years), whereas a cooling trend prevails over the western and northwestern parts. Both trends show maximum values of 1.2° C. Both temperature increase and decrease are determined by the temperature trend during winter.

Müller, M. J. (1996) Handbuch ausgew hlter Klimastationen der Erde (Manual of Selected Climate Stations of the Earth). 5^th ed. Trier, Germany: Universität Trier, Forschungs-stelle Bodenerosion 5.

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