Figure 2: World distribution of mean sea-level pressure (in millibars) for ...
Figure 3: World distribution of mean sea-level pressure (in millibars) for ...
Figure 8: Positions of jet streams in the atmosphere. Arrows indicate direc ...
Figure 7: Vertical cross sections through a wave system depicting typical d ...

The Indian monsoon

At the Equator, the area near India is unique in that dominant or frequent westerly winds occur at the surface almost constantly throughout the year; the surface easterlies reach only to 20º N in February, and even then they have a very strong northerly component. They soon retreat northward, and drastic changes take place in the upper-air circulation. This is a time of transition between the end of one monsoon and the beginning of the next. Late in March the high-sun season reaches the Equator and moves farther north. With it go atmospheric instability, convectional (rising, turbulent) clouds, and rain. The westerly subtropical jet stream still controls the flow of air across northern India and the surface winds are northeasterlies. As the high-sun season moves northward during April, India becomes particularly prone to rapid heating because the highlands to the north protect it from any incursions of cold air. In May the southwesterly monsoon is well established over Sri Lanka. There are three distinct regions of relative upper tropospheric warmth--namely (1) above the southern Bay of Bengal, (2) above the highlands of Tibet, and (3) across the still, dry trunks of the peninsulas. The relatively warm area above the southern Bay of Bengal occurs mostly at the 500-100 millibar level. It does not appear at a lower level and is probably caused by the release of condensation heat (associated with the change from water vapour to liquid water) at the top of towering cumulonimbus clouds along the advancing intertropical convergence.

In May the dry surface of Tibet (above 4,000 metres) absorbs and radiates heat that is readily transmitted to the air immediately above. At about 6,000 metres an anticyclonic cell arises, causing a strong easterly flow in the upper troposphere above northern India. The subtropical jet stream suddenly changes its course to the north of the anticyclonic ridge and the highlands, though it may occasionally reappear southward of them for very brief periods. This change of the upper tropospheric circulation above northern India from westerly jet to easterly flow coincides with a reversal of the vertical temperature and pressure gradients between 600 and 300 millibars. On many occasions the easterly aloft assumes jet force. It anticipates by a few days the "burst," or onset, of the surface southwesterly monsoon some 1,500 kilometres farther south, with a definite sequential relationship, although the exact cause is not known. Because of India's inverted triangular shape, the land is heated progressively as the Sun moves northward. This accelerated spread of heating, combined with the general direction of heat being transported by winds, results in a greater initial monsoonal activity over the Arabian Sea (at late spring time), where a real frontal situation often occurs, than over the Bay of Bengal. The relative humidity of coastal districts in the Indian region rises above 70 percent and some rain occurs. Above the heated land the air below 1,500 metres becomes unstable, but it is held down by the overriding easterly flow. This does not prevent frequent thunderstorms in late May.

During June the easterly jet becomes firmly established at 150 to 100 millibars. It reaches its greatest speed at its normal position to the south of the anticyclonic ridge, at about 15º N from China through India.

In Arabia, it decelerates and descends to the middle troposphere (3,000 metres). A stratospheric belt of very cold air, analogous to the one normally found above the intertropical convergence near the Equator, occurs above the anticyclonic ridge, across southern Asia at 30º-40º N and above the 6,000-metre (500-millibar) level. These upper air features that arise so far away from the Equator are associated with the surface monsoon and are absent when there is no monsoonal flow. The position of the easterly jet controls the location of monsoonal rains, which occur ahead and to the left of the strongest winds and behind them to the right. The surface flow, however, is a strong, southwesterly, humid, and unstable wind that brings humidities of more than 80 percent and heavy, squally showers that are the "burst" of the monsoon. The overall pattern of the advance follows a frontal alignment, but local episodes may differ considerably. The amount of rain is variable from year to year and place to place. Most spectacular clouds and rain occur against the Western Ghats, where the early monsoonal airstream piles up against the steep slopes, then recedes, and piles up again to a greater height. Each time it pushes thicker clouds upward until wind and clouds roll over the barrier and, after a few brief spells of absorption by the dry inland air, cascade toward the interior. The windward slopes receive from 2,000 to 5,000 millimetres of rain in the monsoon season. Various factors, and especially topography, combine to make up a complex regional pattern. Oceanic air flowing toward India below 6,000 metres is deflected in accordance with the Coriolis effect. The converging, moist oncoming stream becomes unstable over the hot land and is subject to convectional turmoil. Towering cumulonimbusclouds rise thousands of metres, producing violent thunderstorms and releasing latent heat in the surrounding air. As a result, the upper tropospheric warm belt migrates northwestward from the ocean to the land. The main body of air above 9,000 metres maintains a strong easterly flow.

Later, in June and July, the monsoon is strong and well-established to a height of 6,000 metres (less in the far north), with occasional thickening to 9,000 metres. Weather conditions are cloudy, warm, and moist all over India. Rainfall varies between 400 and 500 millimetres, but topography introduces some extraordinary differences. On the southern slopes of the Khasi Hills at only 1,300 metres, where the moist airstreams are lifted and overturned, Cherrapunji has an average rainfall of 2,730 millimetres in July, with record totals of 897 millimetres in 24 hours in July 1915, more than 9,000 millimetres in July 1861, and 16,305 millimetres in the monsoon season of 1899. Over the Ganges Valley the monsoon, deflected by the Himalayan barrier, becomes a southeasterly air flow. By then the upper tropospheric belt of warmth from condensation has moved above northern India, with an oblique bias. The lowest pressures prevail at the surface.

It is mainly in July and August that waves of low pressure appear in the body of monsoonal air. Fully developed depressions appear once or twice a month. They travel from east to west more or less concurrently with high-level easterly waves and bursts of speed in the easterly jet, causing local strengthening of the low-level monsoonal flow. The rainfall consequently increases and is much more evenly distributed than it was in June. Some of the deeper depressions become tropical cyclones before they reach the land, and these bring torrential rains and disastrous floods.

A totally different development arises when the easterly jet moves farther north than usual because the monsoonal wind rising over the southern slopes of the Himalayas brings heavy rains and local floods. The weather over the central and southern districts, however, becomes suddenly drier and remains so for as long as the abnormal shift lasts. The opposite shift is also possible, with mid-latitude upper air flowing along the south face of the Himalayas and bringing drought to the northern districts. Such dry spells are known as "breaks" of the monsoon. Those affecting the south are similar to those experienced on the Guinea coast during extreme northward shifts of the wind belts (as later discussed), whereas those affecting the north are due to an interaction of the middle and low latitudes. The southwest monsoon over the lower Indus Plain is only 500 metres thick and does not hold enough moisture to bring rain. On the other hand, the upper tropospheric easterlies become stronger and constitute a true easterly jet stream. Western Pakistan, Iran, and Arabia remain dry (probably because of divergence in this jet) and thus become the new source of surface heat.

By August the intensity and duration of sunshine have decreased, temperatures begin to fall, and the surge of southwesterly air diminishes spasmodically almost to a standstill in the northwest. Cherrapunji still receives over 2,000 millimetres of rainfall at this time, however. In September dry, cool, northerly air begins to circle the west side of the highlands and spread over northwestern India. The easterly jet weakens and the upper tropospheric easterlies move much farther south. Because the moist southwesterlies at lower levels are much weaker and variable, they are soon pushed back. The rainfall becomes extremely variable over most of the region, but showers are still frequent in the southeastern areas and over the Bay of Bengal.

By early October variable winds are very frequent everywhere. At the end of the month the entire Indian region is covered by northerly air and the winter monsoon takes shape. The surface flow is deflected by the Coriolis force and becomes a northeasterly flow. This causes an October-December rainy season for the extreme southeast of the Deccan (including the Madras coast) and eastern Sri Lanka, which cannot be explained by topography alone because it extends well out over the sea. Tropical depressions and cyclones are important contributing factors.

Most of India thus begins a sunny, dry, and dusty season. The driest period comes in November in the Punjab; December in Central India, Bengal, and Assam; January in the northern Deccan; and February in the southern Deccan. Conversely, the western slopes of the Karakoram and Himalayas are then reached by the mid-latitude frontal depressions that come from the Atlantic and the Mediterranean. The winter rains they receive, moderate as they are, place them clearly outside the monsoonal realm.

Because crops and water supplies depend entirely on monsoonal rains, it became imperative that quantitative, long-range weather forecasts be available. For a forecast to be released at the beginning of June, it is necessary to use, in April, South American pressure data and Indian upper-wind conditions (positive correlation) and, in May, rainfall in Zimbabwe and Java and easterly winds above Calcutta (negative correlation).

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