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Monsoon and its mechanism


The great climate event of the year, ‘the bursting of the monsoon" arrives in early June.  The actual date of onset of the monsoon varies from year to year.  The average date of the burst of the monsoon varies from June 3rd in Kerala to about the 30th at Delhi; but its arrival may be delayed as much as three weeks.

The monsoon air-flow, coming from the south west, divides into two branches, an Arabian Sea branch and a Bay of Bengal branch. The goal of each these branches  is to occupy  the low pressure belt of the north-western part of India, which is created due to heat of the summer season.

The rains spread over the whole of India within three weeks of their appearance off the west coast. But the retreat is a much slower process and occupied about three months. Towards the end of September the rains slacken and cease in the northern plains. But in the tip of the peninsula they linger until the middle of December. The northerly winds extending slowly southwards drive back the southerly rain-bearing monsoon.

The burst of the monsoon marks the end of the Hot Season and the arrival of Wet Season. Likewise slow withdrawal of the monsoon  marks the transition from the Wet to the Cold Season. The advance of the monsoon is northwards whereas its retreat is towards the south. Consequently the length of the of the rainy seasons decreases away from the equator just as in normal tropical climates.

The areas of heavy rainfall are to the windward side of the Western Ghats. The rainfall in these areas is mainly orographic. Rainfall varies from about 1080 cm at Cherranpunji in the Khasi Hills of Assam to the north-east of the Bay of Bengal, and from about 890 cm in certain portions of the Western Ghats to as little as 10 cm or less per annum in the desert areas of Rajasthan in the north-west.

The Mechanism of Monsoon

The word monsoon, as is well known, has been derived from an Arabic work ‘mansim’ which literally means season.  The word monsoon, therefore, denotes a season in which the wind regime is completely reversed.  The moist monsoon winds, after crossing the equator in the Indian ocean, acquire southwesterly direction as they are attracted towards the lowpressure area in Northwest India and Central Myanmar.  The dry and hot land-bearing trades are thus completely replaced by sea-bearing winds full of moisture.  Based on the difference between tropical continental air and equatorial maritime air, the meteorologists’ definition of the monsoons is very simple.  According to them it is a complete replacement of the dry hot air by the equatorial maritime air up to an altitude of three to five kilometers over the land and water surface.

The phenomenon of the monsoons is certainly very old, but its exact nature and cause are being discovered only recently.  The real breakthrough has come when it was studied at the global rather than regional level.  By and large this phenomenon is confined to tropical lands lying between 20o N and 20oS.  But in the Indian subcontinent it is greatly influenced by the Himalayan ranges bringing the whole subcontinent under the sway of the  moist equatorial wind for a season ranging between two to five months.  It accounts for 75 to 90 per cent of the annual rainfall just from June to September.The nature and mechanism of the monsoons is understood with the help of meteorological data which are collected from stations on land, ships in the ocean and from upper air.  It was originally through that monsoon was a phenomenon of surface winds.  It is now known that upper air currents called Jet streams also play an important role in the mechanism of the monsoon.

It has also been found that the intensity of monsoon can be broadly predicated by measuring the different in pressure between Tahiti (roughly 18”S and 149”W) in the Northern Territory of Australia in the Indian Ocean southeast of Indonesia.

Advancing Monsoon

The rain-bearing monsoon winds are strong. They blow at an average speed of 30 Km per hour. Barring the extreme north-west they overrun the country in a month’s time. The sudden approach of the moisture –laden winds is associated with violent thunder and lighting. This is known as “break” or “burst” of the monsoons.

It is of interest to note that these monsoon winds follow a south-westerly direction. But as they approach the land their direction is modified by the relief and thermal low pressure belt over north-west India. To begin with, the India Peninsula divides the monsoon into two branches. They are the Arabian Sea branch and the Bay of Bengal branch.

The Arabian Sea branch of the monsoon is obstructed by the Western Ghats. The windward side of the Sahyadris receives very heavy rains. Crossing the Ghats they overrun the Deccan plateau and Madhya Pradesh causing fair amount of rainfall. Another part the Arabian Sea branch strikes the Saurashtra peninsula and the Kutch. It then passes over the west Rajasthan and along the Aravallis, causing only a scanty rainfall as direction of Aravali is parallel to the direction of the Monsoon winds. In Punjab and Haryana, it too joins reinforced by the Bay of Bengal branch .These two branches, reinforced by each other cause rains in the Western Himalayas.

The Bay of Bengal branch is naturally directed towards the coast of Maynmar and of the south-east Bangaldesh. But the Arakan Hills along the coast of Myanmar are good enough to deflect a big chunk of this branch, enabling it to enter the Indian subcontinent. The monsoons, therefore, enter West Bengal and Bangladesh from south and south-east instead of the south westerly direction. Thereafter this branch splits into two under the influence of the mighty Himalayas and the thermal low in Northwest India. One branch moves westwards along the Ganga plains reaching as far as the Punjab plains. The other branches moves up the Brahamputra valley in the north and northeast causing widespread rains in the Northeastern India. Its sub-branch strikes the Garo and Khasi  hills of Meghalaya.  Mawsynram, located on the crest of the southern range of Khasi Hills, receives the highest average annual rainfall in the world.

The monsoons are known for their vagaries and uncertainties.  The alternation of dry and wet spells keeps on varying in  intensity, frequency and in duration.  On the one hand if they cause heavy floods in one part, they may be respobsible for droughts in the other.  They are often found  irregular and unpunctual in their arrival as well as retreat,  dislocating the entire farming schedule of the millions and millions of farmers.

Retreating Monsoon

The months of October and November are known for the retreating monsoons.  During this season, the monsoon through of low pressure becomes weaker and is gradually replaced by high pressure.  This results in the retreat of the monsoon.  The out-reach of the monsoons begins to become unsustainable.  Their sway over the  Indian land-mass begins to shrink.  By the beginning of October they withdraw from the Northern Plains.

The month of October-November form a period of transition from a hot rainy season to dry winter conditions.  The retreat of the monsoons is marked by clear skies and rise in temperature.   The land is still moist.  Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive.  This is commonly known as ‘October heat’.   In the second half of October the mercury begins to fall rapidly particularly in northern India.

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