Home General Knowledge GK SPECIAL TOPICS : MONSOON
GK SPECIAL TOPICS : MONSOON
Saturday, 12 July 2014 10:31

 

 


MONSOON

Monsoon

Monsoon is the seasonal reversal of wind system accompanied by seasonal changes in atmospheric circulation and precipitation.Monsoons cause excessive rainfall in many parts of the world including Asia, Australia, North America, South America, and Africa. The primary mechanism behind monsoon is a shift in global wind patterns.

During most of the year, winds blow from land to ocean making the air dry. Winds originating from land are called continental. During certain months of the year, the winds begin to blow from the ocean to the land making the air moist. Winds originating over a body of water are called maritime. This moist ocean air is what causes monsoonal rains over many countries.

Why Do Wind Patterns Shift in a Monsoon?

Differential heating occurs when the sun heats the land and oceans. Incoming solar radiation heats landmasses faster than large bodies of water. As the land gets heated throughout the summer, a large low pressure system builds over the land. The heat from the sun also warms the surrounding ocean waters, but the effect happens much more slowly due to the high heat capacity of water. Therefore, the oceans as well as the layer of air above the oceans stay cooler. The cooler air above the oceans is moist and denser creating a high pressure zone relative to the pressure above the landmass.

Winds flow from high pressure areas to low pressure areas due to the pressure gradient. Once the temperature conditions on the land and oceans change, the resultant pressure changes cause the winds to change from a land-to-ocean direction to an ocean-to-land direction. Monsoon season does not end as abruptly as it begins. While it takes time for the land to heat up, it also takes time for that land to cool in the fall. This makes monsoon season a time of rainfall that diminishes rather than ends.

The southwest monsoons occur from June to September. The Thar Desert as well as adjoining areas of the northern and central Indian subcontinent heats up considerably during the hot summers, which causes a low pressure area over the northern and central India. To fill this void, the moisture-laden winds from the Indian Ocean rush into the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas. The moisture-laden winds on reaching the southernmost point of the Indian Peninsula, due to its topology, become divided into two parts: the Arabian Sea Branch and the Bay of Bengal Branch.

The Arabian Sea Branch of the Southwest Monsoon first hits the Western coasts of Kerala, thus making it the first state in India to receive rain from the Southwest Monsoon. The monsoon winds originating over the Arabian Sea further split into three branches:

(i) The first branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats and soon, they become cool, and as a result, the windward side of the Sahyadris and Western Coastal Plain receive very heavy rainfall ranging between 250 cm and 400 cm. After crossing the Western Ghats, these winds descend and get heated up. This reduces humidity in the winds. As a result, these winds cause little rainfall east of the Western Ghats. This region of low rainfall is known as the rain-shadow area.

(ii) Another branch of the Arabian Sea monsoon strikes the coast north of Mumbai. Moving along the Narmada and Tapi valleys, these winds cause rainfall in extensive areas of central India. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch.

(iii) A third branch of this monsoon wind strikes the Saurashtra Peninsula and the Kuchchh. It then passes over west Rajasthan and along the Aravallis, causing only a scanty rainfall due to the parallel orientation of the Aravallis. In Punjab and Haryana, it too joins the Bay of Bengal branch. These two branches, reinforced by each other, cause rains in the western Himalayas.

The Bay of Bengal Branch of Southwest Monsoon flows over the Bay of Bengal heading towards North-East India and Bengal, picking up more moisture from the Bay of Bengal. The Bay of Bengal branch strikes the coast of Myanmar and part of southeast Bangladesh. But the Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent. The monsoon, therefore, enters West Bengal and Bangladesh from south and southeast instead of from the south-westerly direction. From here, this branch splits into two under the influence of the Himalayas and the thermal low is northwest India.

One of its branches moves westward along the Ganga plains reaching as far as the Punjab plains. The other branch moves up the Brahmaputra valley in the north and the northeast, causing widespread rains. Its sub-branch strikes the Garo and Khasi hills of Meghalaya. Mawsynram, located on the crest of Khasi hills, receives the highest average annual rainfall in the world.

Northeast Monsoon takes place from December to early March when the surface high-pressure system is strongest. Around September, with the sun fast retreating south, the northern land mass of the Indian subcontinent begins to cool off rapidly. With this, air pressure begins to build over northern India; the Indian Ocean still holds its heat. This causes the cold wind to sweep down from the Himalayas and Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsula. This is known as the Northeast Monsoon or Retreating Monsoon.

Jet streams are fast flowing, narrow air currents found in the upper atmosphere. It is a river of wind that blows horizontally through the upper layers of the troposphere, generally from west to east, at an altitude of 6 to 11 km. This rapid current is typically thousands of kilometers long, a few hundred kilometers wide, and only a few kilometers thick. The main jet streams are located near the tropopause, the transition between the troposphere (where temperature decreases with height) and the stratosphere (where temperature increases with height).

Jet stream develops where air masses of differing temperatures meet.The position of this upper-level jet stream denotes the location of the strongest surface temperature contrast. Thus, surface temperatures determine where the jet stream will form. The greater the temperature difference, the faster is the wind velocity inside the jet stream. During the winter months, Arctic and tropical air masses create a stronger surface temperature contrast resulting in a strong jet stream. However, during the summer months, when the surface temperature variation is less dramatic, the winds of the jet are weaker.

Inter Tropical Convergence Zone (ITCZ)

The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator, where trade winds converge, and so, it is a zone where air tends to ascend. In July, the ITCZ is located around 20°N-25°N latitudes over the Gangetic plain. This trough encourages the development of thermal low over north and northwest India. Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40° and 70°E longitudes and start blowing from southwest to northeast due to the Coriolis force. In winter, the ITCZ moves southward, and so the reversal of winds from northeast to south and southwest, takes place.

Onset/Burst of the Monsoon

The shift in the position of the ITCZ is related to the phenomenon of the withdrawal of the westerly jet stream from its position over the north Indian plain, south of the Himalayas. The easterly jet stream sets in along 15°N latitude only after the western jet stream has withdrawn itself from the region. This easterly jet stream is held responsible for the burst of the monsoon in India.

The southwest monsoon sets in over the Kerala coast by 1st June and moves swiftly to reach Mumbai and Kolkata between 10th and 13th June. By mid- July, southwest monsoon engulfs the entire Subcontinent.

EI-Nino

EI-Nino is a complex weather system that involves oceanic and atmospheric phenomena with the appearance of warm currents off the coast of Peru in the Eastern Pacific and affects weather in many places including India. EI-Nino is merely an extension of the warm equatorial current which replaces temporarily the cold Peruvian current or Humboldt Current. This current increases the temperature of water on the Peruvian coast by 10°C which results in:

(i) The distortion of equatorial atmospheric circulation;

(ii) irregularities in the evaporation of sea water;

(iii) Reduction in the amount of planktons thus reducing the number of fishes in the sea.

The word EI-Nino means ‘Child Christ’ because this current appears around Christmas in December. December is a summer month in Peru (Southern Hemisphere). EI-Nino is used in India for forecasting long range monsoon rainfall.

Break in the Monsoon

During the south-west monsoon period after having rains for a few days, if rain fails to occur for one or more weeks, it is known as break in the monsoon. These dry spells are quite common during the rainy season. These breaks in the different regions are due to different reasons:

(i) In northern India rains are likely to fail if the rain-bearing storms are not very frequent along the monsoon trough or the ITCZ over this region.

(ii) Over the west coast the dry spells are associated with days when winds blow parallel to the coast.

Characteristics of Monsoonal Rainfall

(i) Monsoon rainfall is seasonal in character, which occurs between June and September.

(ii) It is largely governed by relief or topography. For instance the windward side of the Western Ghats registers a rainfall of over 250 cm. Again, the conical hills around Mawsynram make it the wettest place on earth.

(iii)The monsoon rainfall decreases with increasing distance from the sea. Kolkata receives 119 cm during the southwest monsoon period, Patna 105 cm, Allahabad 76 cm and Delhi 56 cm.

(iv) The wet spells in the monsoon rains are interspersed with rainless interval known as ‘breaks’. These breaks in rainfall are related to the cyclonic depressions mainly formed at the head of the Bay of Bengal.

(v) The summer rainfall comes in a heavy downpour leading to considerable run off and soil erosion.

(vi) Over three-fourths of the total rain in the country is received during the southwest monsoon. Thus it has a strong bearing on our agrarian economy.

(vii) Its spatial distribution is also uneven which ranges from 12 cm to even 400 cm.

(viii) There can be delays in the beginning of the rains over the whole or a part of the country. Sometimes the rains end considerably earlier than usual, causing great damage to standing crops and making the sowing of winter crops difficult.

Distribution of Rainfall

The average annual rainfall in India is about 125 cm, but it has great spatial variations

Areas of High Rainfall: The Western Ghats, the sub-Himalayan areas is the northeast and the hills of Meghalaya are the regions where the rainfall exceeds 200 cm. In some parts of Khasi and Jaintia hills, the rainfall exceeds 1,000 cm. In the Brahmaputra valley and the adjoining hills, the rainfall is more than 200 cm.

Areas of Medium Rainfall : Southern parts of Gujarat, east Tamil Nadu, northeastern Peninsula covering Orissa, Jharkhand, Bihar, eastern Madhya Pradesh, northern Ganga plain along the sub-Himalayas, the Cachar Valley and Manipur experience rainfall between 100 and 200 cm.

Areas of Low Rainfall: Western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat and Deccan Plateau receive rainfall between 50 and 100 cm.

Areas of Inadequate Rainfall: Parts of the Peninsula, especially in Andhra Pradesh, Karnataka and Maharashtra, Ladakh and most of western Rajasthan receive rainfall below 50 cm. Snowfall is restricted to the Himalayan region.

Variability of Rainfall

A characteristic feature of rainfall in India is its variability. The variability of rainfall is computed with the help of the following formula:

The values of coefficient of variation show the change from the mean values of rainfall. The values of coefficient of variation show variability of rainfall in India. A variability of less than 25 per cent exists on the western coasts, Western Ghats, northeastern Peninsula, eastern plains of the Ganga, northeastern India, Uttaranchal and Himachal Pradesh and south-western part of Jammu and Kashmir. These areas have an annual rainfall of over 100 cm. A variability of over 50 per cent exists in the western part of Rajasthan, northern part of Jammu and Kashmir and interior parts of the Deccan plateau. These areas have an annual rainfall of less than 50 cm. Rest of India have a variability of 25-50 per cent and these areas receive an annual rainfall between 50 -100 cm. Thus, in general, higher the rainfall, lower is the variability.

The monsoon trough is that portion of the Intertropical Convergence Zone (ITCZ) which is depicted by a line in the weather map showing the locations of relatively minimum sea level pressure in a monsoon region. Thus it is a convergence zone between the wind patterns of the southern and northern hemispheres. Westerly monsoon winds lie in its equatorward portion while easterly trade winds lie poleward of the trough. Right along its axis, heavy rains can be found. Depressions and tropical cyclones often form in the vicinity of the monsoon trough, with each capable of producing heavy rainfall in a relatively short time frame.

Monsoon Depression

It is a depression that forms within the monsoon trough. It denotes weak cyclonic disturbances that form over the Bay of Bengal and generally track northwestward towards the Indian subcontinent. These may intensify into tropical cyclones if they remain over warm ocean water long enough. Most monsoon depressions that develop in the western North Pacific eventually acquire persistent central convection and accelerated core winds, marking their transitions into conventional tropical cyclones.

Mango showers are the pre-monsoon showers in the Indian states of Karnataka and Kerala that help in the ripening of mangoes. Also known as ‘April rains’ or ‘Summer showers’, they are a result of thunderstorms over the Bay of Bengal.

Kalbaishakhi

During summer the eastern and north eastern states of the country like West Bengal, Assam, Bangladesh and parts of Orissa and Bihar experience dramatic appearance of a special type of violent thunderstorm known as Norwesters. It is locally known as ‘Kalbaishakhi’ or calamity of the month of Baisakh.Originating from the Chhotanagpur plateau and influenced by the westerlies, this warm wind moves eastwards and is responsible for heavy rain and hailstones in West Bengal, Assam and Orissa. These winds many a time cause destruction of life and property. Apart from its destructive effects it is somewhat helpful for the pre-kharif crops like jute, Aus paddy, summer til and a large number of vegetables and fruits and a sudden drop in temperature gives relief after the mid-day heat.

Cherry Blossom is a local wind that blows over the interior Karnataka during the summer season and is extremely good for coffee cultivation.

While travelling towards the Indian Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and pours it over peninsular India and parts of Sri Lanka. Cities like Chennai, which get less rain from the Southwest Monsoon, receive rain from this Monsoon. About 50% to 60% of the rain received by the state of Tamil Nadu is from the Northeast Monsoon.