Monsoon climates are characterised by large-scale seasonal reversals of the prevailing wind, giving two very distinct seasons. The origin of word monsoon is from the Arabic Mausim, meaning ŌseasonÕ. The name originally referred to wind reversals in the Arabian Sea, but has come to mean the whole range of phenomena associated with annual weather cycles in Tropical and Sub-tropical Asia, Australia and Africa. Here we concentrate on the South Asian monsoon, the great weather system that dominates life on the subcontinent.
Summer: in summer, moist air is carried northwards from the Indian Ocean over the Indian subcontinent, bringing rains. Winter: in winter, cool, dry air is carried southwards from subcontinent. Thus, the year is divided into wet and dry seasons. In addition, a short north-east monsoon affects the south-east coastal states of India, due to winds bringing moisture from the Bay of Bengal.
The summer winds originate in an area of high pressure in the southern Indian Ocean, and cross the equator before blowing onshore. The air thus acquires abundant moisture on its northward journey, which fuels convection and storm cloud development during the summer monsoon. The summer monsoon arrives in southern India in late May or early June, and gradually advances northwards and westwards, reaching Pakistan by early July. The monsoon begins to retreat from Pakistan by the beginning of September, and has usually withdrawn from southern India by early December. This pattern of advance and withdrawal gives the Indian subcontinent its characteristic seasonal rainfall pattern. Pakistan has a short summer rainy season, with generally light rainfall, whereas areas like the Ganges-Brahmaputra Delta have a longer, heavier monsoon. Annual rainfall totals are typically 10,000 mm (10 metres) per year in this region. The northern extent of the monsoon in India and Nepal is governed by the presence of the Himalayas. Monsoon storm systems typically rise 10km into the air, and the great Himalayan peaks rise to a substantial proportion of this (commonly 7 km). Thus, they act as a major topographic barrier, preventing the northward flow of moist air and maintaining the arid climate of Tibet. Similarly, in the winter, the Himalayas prevent the southward flow of cold air, keeping the winters of northern India milder than they would otherwise be.
Three basic mechanisms can be identified as the driving forces behind the monsoon:
Land and ocean respond differently to given inputs or outputs of energy. Sunlight falling on land surfaces results in rapid warming of the land, which in turn heats the atmosphere. Similarly, heat is rapidly lost from land masses in winter or at night, particularly in clear-sky conditions when heat (in the form of longwave radiation) is rapidly lost to space. In contrast, sunlight over the ocean can penetrate to greater depths, and heat can be mixed downwards by currents and eddies. Thus the sunlight must warm up a greater volume of material, so the temperature increase is lower compared to the land, and less heat is available at the surface to heat the atmosphere. Additionally, part of the sunÕs energy evaporates water, again meaning that less is available to heat the atmosphere. In winter, the oceans act as a great heat store, which is slowly given up to the overlying air. Thus, continents tend to heat up more in summer, and cool down more in winter, than ocean surfaces, with corresponding effects on the overlying air. In South Asia, this effect is enhanced by the presence of the Tibetan Plateau, which heats up more than the free atmosphere would at that height. The contrasting response of land and sea to heating and cooling means that in summer, the land is warmer than the oceans, whereas in winter the opposite is true. This creates differences in air pressure, consisting of (a) summer low pressure over the Indian Subcontinent, and high pressure over the southern Indian Ocean; and (b) winter high pressure over India and low pressure over the equatorial Indian Ocean. The contrast in pressure sets up winds and convection, with summer convection and uplift over India, fed by low-level moist winds blowing off the Indian Ocean.,
The exact path taken by the summer monsoon winds as they cross from the southern hemisphere to India is controlled by the rotation of the Earth. Winds will be deflected to the right in the northern hemisphere, and the left in the southern, an effect called the Coriolis effect. This means that in the southern hemisphere the winds are deflected to become south-easterlies, then curve rightwards as they cross the equator to become south-westerlies. The winter winds blowing southwards off the continent are deflected rightwards (looking in the direction of travel of the wind) and hence become north-easterlies (see map 1 above).
Energy consumed during evaporation over the oceans is transferred to the land, where it is released as heat (latent heat) when air rises and cools, and condensation occurs. This fuels much greater rates of uplift, increasing the amount of convection and storm formation.
The summer monsoon is not an unbroken deluge, but consists of alternating wet and dry events known as active and break periods. During active periods, low pressure systems bring frequent thunderstorms and heavy rain, whereas break periods are typically bright and sunny. The timing and duration of active and break periods account for much of the year-to-year variation in the monsoon, dry years coinciding with frequent, long-lived breaks.
Cyclones (known as Hurricanes in the Atlantic and Typhoons in the western Pacific) are intense rotating tropical storms, with surface winds in excess of 33 metres per second. Severe cyclones may have winds greater than 70 metres per second. Cyclones form over warm oceans, where sea-surface temperatures are greater than 27” C. These conditions are met over the northern Indian Ocean, especially the Bay of Bengal. Cyclones tend to form over the Bay of Bengal at the beginning and end of the monsoon, in April-June and October-November, when temperatures are highest. When cyclones travel onshore severe damage and loss of life can result, especially on the low-lying lands around the Ganges-Brahmaputra Delta. Cyclone damage results from two main causes:
(1) strong winds, which can devastate flimsy buildings;
(2) storm surges, or a temporary increase in the level of the sea due to wind stress (the onshore wind drives water onland) and low air pressures (which cause water to dome upwards below the centre of a cyclone).
Storm surges can flood large areas of low, coastal lands. On low-lying islands, it may be impossible to escape the rising waters. Bangladesh is particularly badly affected. The worst in recent years was in1970, when 250,000 were killed in a single cyclone. Other bad years were1965, when three cyclones killed a total of 57,000 people and 1985 (10,000). In April 1991, Bangladesh was hit by a cyclone that left 130,000 dead and over 10 million homeless. The most recent cyclone disaster affected the Indian state of Orissa, on the Bay of Bengal to the west of Bangladesh. The cyclone struck on 29th October 1999, and was responsible for the deaths of over 10,000 people. India and Bangladesh now have sophisticated cyclone predicting systems, and early warnings are given over the radio and via the Muslim churches (imams, who call people to prayer, call out warnings). The main difficulty lies in the evacuation of large populations, especially from remote rural areas. A system of storm shelters now exists in Bangladesh, but its capacity is still not enough to accommodate all of the people under threat.
Occasionally, cyclones (in weakened form) travel northward into the Himalayas, where they cause heavy snowfalls in the normally dry seasons of the year. On November 11-12, 1995, a storm originating in the Bay of Bengal delivered heavy rain to northern India, and heavy snowfall to Nepal. The snowfall and associated avalanches were responsible for a reported 61 deaths and hundreds of rescues during the weekend of Nov. 11-12. One avalanche at Phanka, to the west of Mount Everest, on Nov. 11, killed 26 people, including 13 members of a Japanese trekking expedition. The Japanese group was buried along with 13 Nepali guides and porters as they slept in a tourist lodge. Others died under a heavy six foot snowfall which toppled some houses. Survivors of the storm reported Sherpas clearing trails using their hands and plastic kerosene cans sliced in half to form shovels. Even yaks, some of the Himalayas best-adapted inhabitants, succumbed to the storm, suffocating in the deep snow.
Trail to Everest Base Camp in December 1995
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