INDIAN MONSOON
- Monsoons are a complex meteorological phenomenon. Experts of meteorology have developed a number of concepts about the origin of monsoons. Some of the important concepts about the origin of monsoons have been given as under.
1. The Thermal Concept of Halley (1686):
- Halley, a noted astronomer, hypothesised that the primary cause of the annual cycle of the Indian monsoon circulation was the differential heating effects of the land and the sea. According to this concept monsoons are the extended land breeze and sea breeze on a large scale, produced by the differential heating of continents and ocean basins.During the summer season in the Northern Hemisphere, when the Sun's rays are vertical over the Tropic of Cancer, the huge landmass of Asia heats quickly and develops a strong low pressure centre near Lake Baikal (Siberia) and Peshawar (Pakistan). This thermal low extends up to 700 mb. Moreover, the pole-ward shift of the Inter-Tropical Convergence Zone (ITCZ) to a position over southern Asia reinforces the thermally induced low pressure centre. In comparison to this, the pressure over the adjacent water of the Indian and the Pacific Oceans is relatively high. Under these conditions, a sea-to-land pressure gradient develops. Consequently, the surface air flow is from the high pressure over the oceans towards the low pressure areas over the heated landmass. Under the extreme low pressure condition on land, the wind from the southern part of the Indian Ocean (south of Equator) is attracted towards the subcontinent of India. The air coming from oceans towards land is warm and moist. When land barriers like mountain ranges and plateaus come in the way of the moisture-laden winds, they ascend and result into saturation, condensation, and precipitation.
- Contrary to this, in the Northern Hemisphere during winter season, there develops high pressure areas near Baikal Lake (Siberia), and Peshawar (Pakistan). As compared to these high pressures, the Indian Ocean and the Pacific Ocean (south of Japan) remain relatively warm, having low pressure areas. Consequently, there is an outflow of air from the high pressure of the land to the low pressure areas of the oceans. The air blowing from high pressure areas of land towards the sea is cold and dry. This cold and dry air is incapable of giving precipitation unless it comes into contact with some water body (ocean/sea).
- The thermal concept about the origin of monsoon has, however, not been accepted universally as it fails to explain the intricacies of monsoon. Besides differential heating, the origin and development of monsoon are also influenced by the shape of the continents, orography, and the conditions of air circulation in the upper troposphere.
The Halley's concept has been criticised on more than one count as follows:
I. The low pressure areas that develop over the continents during the summer season in the Northern Hemisphere are not stationary. These low pressure areas change their position (location) suddenly. This sudden change in the low pressure areas are not exclusively related to low thermal conditions. The low pressure areas stabilises in June in the north-eastern parts of the subcontinent. In fact, they represent the cyclonic lows associated with the dynamic factors, and therefore, these low pressure areas cannot be termed as only thermally induced.
2. Had the monsoon been thermally induced, there would be anti-monsoon circulation in the upper air of the troposphere, which is lacking.
3. Although high temperature and the consequent low pressure takes the north-west in its grip from the middle of April, no rain starts in northern India till the middle of June.
4. The modern researches in meteorology have shown that the monsoon rainfall is not wholly orographic. They are an amalgamation of convectional, orographic and cyclonic rainfall.
5. Instead of two broad seasons (winter and summer) the monsoon climate has more seasons (four in India), due to the highly variable characteristics of temperature and precipitation
6. Halley did not take into consideration the Coriolis effect of rotation of the earth on its axis. On a rotating earth, the wind has a tendency to move towards its right in the Northern Hemisphere and towards its left in the Southern Hemisphere.
7. The role of latent heat passing into the atmosphere through water vapour was also not considered by Halley; water vapour also plays an important role in the origin and development of monsoons.
2.The Dynamic Concept by Flohn :
- The dynamic concept about the origin of monsoons was put forward by Flohn in 1951. According to this concept, monsoon is the result of seasonal migration of planetary winds and pressure belts. The Inter-Tropical Convergence Zone (ITCZ) is formed due to the convergence of north-east and south-east trade winds near the equator. The northern and the southern boundaries of the ITCZ are called NITC (Northern Inter-Tropical Convergence) and SITC (Southern Inter-Tropical Convergence) respectively. There is a belt of doldrums within the Inter-Tropical Convergence, characterised by equatorial westerlies. At the time of the summer solstice (21' June), when the Sun's rays are vertical over the Tropic of Cancer, the NITC is extended up to 30° N latitude,covering South and south-east Asia. Thus, equatorial westerlies are established over these areas. The equatorial westerlies become south-west or summer monsoons. On a rotating earth, the trade winds of the Southern Hemisphere after crossing the equator turn towards their right (Coriolis effect).
- The NITC is associated with numerous atmospheric storms (cyclones) which yield heavy rainfall during wet monsoon months (July to September). Similarly, the north-east or winter monsoon does not originate only due to low pressure in the Southern Hemisphere during winter solstice (when the Sun's rays are vertical over the Tropic of Capricorn). In fact, the north-east monsoons are north-east trade winds which are re-established over south-east Asia due to southward shifting of pressure and wind belts. It is obvious that due to southward movement of the Sun at the time of winter solstice, the NITC is withdrawn from over south and south-east Asia, and north-east trade winds occupy their normal position. These north-east trades, thus, become winter monsoons. The north-east monsoons having their origin on land are generally dry and devoid of rains
- In brief, according to Flohn, the existence of monsoons in Asia, especially in the subcontinent of India, is not due to temperature contrasts between land and sea, but mainly due to the annual migration of thermally produced planetary winds and pressure belts. Despite the relative shifting of Inter-Tropical Convergence (thermal equator) and pressure belts, Flohn seems to have ignored the upper atmospheric circulation (jet streams) and the southern oscillation, which make the Asi-atic monsoon a fairly complex system. He also could not explain the causes of early arrival of Indian monsoons in the states of north-east India. The dynamic concept, therefore, was also not taken as the sole explanation of the origin of monsoons.
3. Aerological Concept-
Given by a German MeterologistR. Seherhag (1948)
According to him the changes inthe direction of winds at all levels in the atmosphereare directly related to the temperature changes in theair above the friction layer.
4. Recent Concepts
a. Jet Streams:
Jet streams are high altitudegeostrophic winds (i.e.. Blowing parallel toequator) blowing between middle latitude at highspeed in a meandering course. During winterseason the upper air westerly jet streams arepositioned in Asia. These are bifurcated in twobranches due to Tibet Himalayan obstruction.North branch blows north of Himalayas and theTibetan Plateau. Southern branch blows south ofthe mighty mountains The southern branchinscribes an anticyclone (Clockwise) arc acrossAfghanistan followed by a cyclonic (Anticlockwise)are along the southern flank of the Himalayas. Ahigh pressure system gets formed south of the jetstream over Afghanistan and NW Pakistan whereair tends to subside leading to atmosphericstability and dry conditions there by causing NEwinter monsoons. The jet stream helpsdisturbances in the NW of the subcontinent, whichtend to follow paths immediately beneath the jetstream. These disturbances move long theeastern Mediterranean and into NW Indiaappearing here as waves rather then as welldeveloped frontal cyclones. During summerseason as sun falls vertically over the Tropic ofCancer the polar surface high pressure isweakened and upper air circum polar whirl shiftnorthward as a result of which the upper airwesterly jet are also withdrawn from southernslopes of the Himalayas. The removal of jetstream to north of the Tibetan plateau results inreversal of the curvature of How of free air to thenorth and north west of the subcontinent. Thisevent may well be the trigger that sets off the‘burst’ of the monsoon.
b. Tibet Plateau:
In 1973, the Monsoon Expedition(MONEX) was organized under the joint auspicesof the erstwhile Soviet Union and India.Experiments concluded that summer time heatingof Tibetan Highland plays a dominant role in theorigin of Monsoon circulation. Due to itsprotected height Tibetan plateau receives 23° Cmore insolation than the neighboring areas. Theplateau affects the atmosphere in two ways
I. as a mechanical barrier
II. as a high level heat sources.
Infact theplateau accentuates the northlanddisplacement of the jet stream. The summertime heating of the Tibetan Plateau makes ita high level heat source, which producesthermal anticyclone over this region, therebyweakening the western subtropical jet streamsouth of the Himalayas and intensifying themove of S. W monsoon.
Effect because of ocean:
El Nino a warmocean current appears along the Peru coast inDecember. It replaces the Peru or HumboldtCold Ocean current flowing over this regionduring normal years. Under normal times thelayer over the eastern Pacific is cool andshallow, while over the western Pacific it iswarm and deep. Such conditions are helpfulfor strong southwest monsoons. Theappearance of El Nino reverses the conditions(warm condition over eastern Pacific and coldin western Pacific). Since El Nino representslarge atmospheric perturbations to which theocean responds with warm of colder surfacetemperature, it lands to extreme events, suchas drought, flood and poor monsoons. TheSouthern Oscillation is the name ascribed to aseesaw pattern of meteorological changesthat are often observed between the Pacific,the pressures over the Indian Ocean tend tobe low, and vice versa. The oscillation wasdiscovered by Sir Gilber Walker and istherefore also known as “Walker circulation”The oscillation has a period varying from 27years. The intensity of the SouthernOscillation is measured by the difference insea level pressures of Tahiti and Port DarwinEl Nino Southern OscillationsIV. The Somali Current: It is one of the fewcurrents, which reverse its direction with theoverlying wind. Summer Flows northwardWinters Flows southward
