Monsoon is actually a wind regime operating at a level of 20 km from the earth’s surface. It is characterised by seasonal reversal of wind direction at regular intervals.
Although the monsoon is a global phenomenon influenced by a variety of factors not yet completely understood, the real monsoon rains cover mainly the South Asian region, represented by India, Myanmar, Sri Lanka, Bangladesh, Bhutan and parts of South East Asia.
Besides the monsoons, the Indian climate is influenced substantially by two more factors. The Himalayas contribute a continental nature to the climate, recognised by land winds, dry air, large diurnal range and scanty rainfall. The Indian Ocean, on the other hand, contributes a tropical character to the Indian climate characterised by uniformity of temperature throughout the year, short diurnal range, damp air, and frequent rainfall.
The monsoon system of the Indian subcontinent differs considerably from that of the rest of Asia. The centres of action, air masses involved, and the mechanism of precipitation of the Indian monsoon are altogether different from other monsoon systems.
Classical Theory or Thermal Concept of Indian Monsoons:
According to this theory, the differential heating of land and sea at the time when the sun makes an apparent northward movement is the main cause of the Indian monsoonal regime.
Two factors are mainly responsible for this very strong development of monsoons:
(i) Vast size of the Indian subcontinent and adjacent seas;
(ii) Very high and extensive mountain systems of the Himalayas in the north, extending in an east-west direction, thus posing a formidable physical barrier between tropical and polar air masses.
The second factor is of great meteorological significance.
The high mountain chains of the Himalayas which border the subcontinent on three sides work as both a break and motor at the same time. During the winter season, they prevent the penetration of the cold polar air masses from Siberia into the subcontinent, while in summer, the Himalayas do not allow the equatorial maritime air masses to cross the Himalayas and force them to curve round the north-west. The mighty Himalayas produce hydro-dynamic effects that determine the type of precipitation in India.
According to the thermal concept, during the period following the Spring Equinox (March 23), the sun starts its apparent northward shift. As a result, the areas lying north of the equator (tropics and sub-tropics)- get a progressive high incidence of solar radiation.
The effect of this phenomenon ‘ on the Indian subcontinent is seen in the form of intense heating of the vast northern plains and the adjoining highlands. As a result, a massive low pressure trough is formed extending from the Punjab plains in the north-west to the Bengal delta in the east.
This low pressure zone attracts wind regimes from the adjoining areas, from short distances in the beginning. But as the level of solar incidence reaches its peak during May-June, the pressure gradient between this low pressure trough and the adjoining seas is so great that it attracts winds from as far as the south of the equator. Accompanying this process and helping this pull of wind regimes is the development of some high pressure centres—in the Indian Ocean, Arabian Sea and over Australia (it being the winter season in Australia).
The wind patterns which are prevalent south of the equator are actually the south-east trade winds which blow from the south-east towards the north-west. These winds, attracted by the low pressure trough over the Indian subcontinent, while moving north of the equator, turn in a clockwise direction (or towards the right), following Farrel’s law. This shift in direction is brought about by the earth’s rotation. Now, the originally south-east trade winds become south-west monsoons blowing towards the north-east.
At this juncture, the Inter- Tropical Convergence Zone (ITCZ) also shifts northwards. The ITCZ is the hypothetical line where the north-east trade winds from the northern hemisphere and the south-east trades from the southern hemisphere meet. The south-west wands now approaching the Indian peninsula have to travel a long distance over the Indian Ocean.
During their long journey, these winds pick up large amounts of moisture and by the time they reach India they are oversaturated. Here, they are known as the south-west monsoons which get divided into the Arabian Sea branch and the Bay of Bengal branch because of the shape of peninsular India. There moisture-laden winds cause heavy rainfall on the windward sides.
The Arabian Sea Branch:
This branch of the south-west monsoons strikes the highlands of the Western Ghats at almost right angles. The windward slopes of the Western Ghats receive heavy orogenic precipitation. Although the western currents of the monsoon penetrate further into the Indian mainland the intensity of rainfall goes on decreasing on the leeward side.
While the windward slopes of the Western Ghats are the areas receiving the highest rainfall, the leeward slopes form a well-marked rain-shadow belt which is drought- prone. For instance, the average annual rainfall at Mumbai and Pune is 188 cm and 50 cm respectively, despite the fact that they are only 160 km apart.
The most characteristic feature of the distribution of rainfall on the windward slope is that the amount of rains is heavier higher up the slopes. However, the heavy rains are concentrated in a narrow strip along the Western Ghats.
After crossing the Western Ghats, the rain- bearing air currents descend the eastern slopes where they get warmed up adiabetically. This results in a pronounced rain-shadow area. The higher the mountains-, the larger is the rain-shadow effect. Towards the north, where the Western Ghats are not very high, the difference in the amount of rainfall between the windward and leeward side is rather negligible.
Why no precipitation in Kachchh and Western Rajasthan?
There is no mountain barrier to tap the advancing winds. As the Aravallis have an almost north-south axis, they fail to block the passage of these monsoon currents (which rather blow parallel to the Aravallis) and lift them.
The monsoon currents heading towards Rajasthan are rather shallow and are superimposed by stable anti-cyclonic air.
The hot and dry continental air masses from western Pakistan (Baluchistan) are drawn towards the thermal low developed in this region. These air masses check the ascent of air and absorb its moisture.
These conditions are unfavourable for precipitation in Kachchh and western Rajasthan where desert conditions prevail.
Some of the currents from the Arabian Sea branch manage to proceed towards Chhotanagpur plateau through the Narmada and Tapti gaps. These currents ultimately unite with the Bay of Bengal branch.
Although a few air currents from the main Arabian Sea branch are diverted northward towards Kachchh and the Thar desert, these currents continue upto Kashmir without causing rain anywhere on their way. In fact, an east-to- west line drawn near Karachi in Pakistan practically marks the limit of the monsoon rainfall.
Bay of Bengal Branch:
This branch is active in the region from Sri Lanka to Sumatra Island of the Indonesian archipelago. Like the Western Ghats of India in the case of the Arabian Sea branch, the windward slopes of the West Coast Mountains of Myanmar (Arakan and Tenasserim mountains) get heavy rainfall when the main monsoon currents of this branch strike the Myanmarese coast. Akyab on the west coast records 425 cm during the June-September period. As in case of the leeward sides of the Western Ghats in India, here too, the rain shadow effect is pronounced on the leeward side.
A northern current of this branch strikes the Khasi hills in Meghalaya and causes very heavy rains. Mawsynram (near Cherrapunji), situated on the southern slopes of Khasi hills, has the distinction of recording the highest annual average precipitation in the old.
This is because of its peculiar geographical location. Mawsynram is flanked on all sides by the Garo, Khasi and Jaintia hills except for a gap through which the rain-bearing winds enter and are forced to rise, thus yielding the heaviest rainfall. Shillong, a mere 40 km away on top of the Khasi hills, receives only about 140 cm of rainfall during June-September.
Another current of the Bay of Bengal branch takes a left turn at the eastern end of the low pressure trough (roughly the Bengal delta). From here, it blows in a south-east to north-west direction along the orientation of the Himalayas. This current causes rainfall over the northern plains.
The monsoon rainfall over the northern plains is assisted by west-moving monsoon or cyclonic depressions called ‘westerly disturbances’. These are formed in the Bay of Bengal and move along the southern fringe of the northern plains causing copious rains there which are vital for the rice crop.
The intensity of rainfall decreases from east to west and from north to south in the northern plains. The decrease westwards is attributed to the increasing distance from the source of the moisture. The decrease in rainfall intensity from north to south, on the other hand, is due to increasing distance from the mountains which are responsible for lifting the moisture-laden winds and causing orogenic rainfall in the plains, especially in the foothills.
The two main branches of the monsoon winds follow different courses:
but originally, they set out to fill the intense low pressure void created in the north-west of the subcontinent. The two branches meet at the Chhotanagpur Plateau. Of the total moisture carried by the two branches, only 20 per cent falls as precipitation. The Arabian Sea branch is more powerful of the two because of two reasons—one, the size of the Arabian Sea is bigger and two, most of the Arabian Sea branch falls over India, while most of the Bay of Bengal branch goes to Myanmar, Malaysia and Thailand.
Retreating or North-East Monsoons:
Towards the end of September, the low pressure centre in the north-west begins to disintegrate and eventually shifts to the equatorial region. The cyclonic conditions are replaced by anti-cyclonic ones. As a result, winds start blowing away from the northern region. Similar anti-cyclonic winds blow from the Tibetan highlands and beyond.
This is also the time when the sun makes an apparent movement south of the equator. The ITCZ also moves equatorwards (Fig. 13.19). Now the winds that dominate the subcontinental landscape are the ones which move from the north-east to the south-west.
These conditions continue from October till mid-December and are known as the retreating monsoons or the north-east monsoons. By December end, the monsoons have completely withdrawn from India. The retreat of the monsoons is markedly gradual in contrast to the ‘sudden burst’ of the south-west monsoons.
The retreating monsoons over the Bay of Bengal pick up moisture on their way which is dropped over eastern or coastal Orissa, Tamil Nadu and parts of Karnataka during October-November. This is the main season of rains over these areas as they almost lie in the rain-shadow area of the south-west monsoons (Fig.13.21).
During October, easterly depressions occur at the head of Bay of Bengal which move southwards and in November get sucked into Orissa and Tamil Nadu coasts causing heavy rain—sometimes with destructive cyclonic winds in coastal and interior areas. The depressions weaken southwards and towards the interiors.
The stable, dry anti-cyclonic winds prevailing over the subcontinent after the retreat of the south-west monsoons are not capable of causing precipitation because they are free of moisture. Instead, these winds produce dry and fine weather. However, certain areas in the north get winter precipitation: from sources far away.
The north-western parts of India—Punjab and Ganga plains—are invaded by shallow cyclonic disturbances moving from west to east and having their origin in the Mediterranean Sea. These are called “Westerly Disturbances’ which travel across West Asia and Afghanistan before they reach India. These disturbances come with cloudiness and rising temperature in the front and cold wind in the rear.
These disturbances cause upto 5 cm rainfall in Punjab and Kashmir and up to 2.5 cm over the Uttar Pradesh plains. These showers are very good for the rabi crop, especially wheat and gram, and are very effective because of less runoff, less evaporation (because of low winter temperatures) and the fact that moisture from these showers is confined to the root area of the crops.