Summer monsoon mechanism in Indian Sub-Continent

Although precise definition of monsoon is lacking but it can be used to define a wind that blows with consistency & regularity during one part of the year & is absent or blow from another direction during the other part of the year, which is due to the heat received by the different parts of the earth.

                   In genetic sense monsoon is associated with the following features:-

1) system with seasonal marked wind shifts due to differential heating of land & sea;

2) winds that are largely confined to the tropics between 20(degrees)N to 20 (degrees)S;

3)summer monsoons may be thought of as south-east trade winds of southern hemisphere which on crossing the equator are deflected to the right by the earth's rotation & approach the land from the south-west direction.

                    However, it must be kept in mind that monsoon mechanism of other continents is not so well marked as the Indian monsoon mechanism. The earliest systematic study of this monsoon mechanism has been done by H.F.Blanford called "climates & weather of India,shri Lanka & Burma".

Main feature of this monsoon mechanism:-

1)Heat Low:-The northern hemisphere movement of the sun causes the land around the Arabian sea to be heated up,Prof. Budyoka(Russian) estimated that the heat influx rom the earth into atmosphere is 160 Watts/metre square of earth's surface, during june over the arid regions of north western india, pakistan etc.The large input of power forms a trough of low pressure from Somalia(Africa) acros Arabia to north-western India.By the end of may the heat low is established causing south westerly wind spreading north-wards over the Arabian sea, Bay of Bengal & Indian Sub-continent, this onset is sudden.

2)Near equatorial troughs & mecernes high:-Before the onset of south-west monsoon a low pressure zone is formed along the 5(degrees)N to 5(degres)S,referred to as the equatorial double high. Prior to the monsoon near equatorial trough north of 5(degrees)N weakens while the trough near the 5(Degrees)S remains active.The trough along the 5(degrees)N moves with the progress with the monsoon & merges with eat low & forms quasi stationery monsoon trough.onset of monsoon is connected with sudden acceleration of the air from the southern hemisphere towards the indian sub-continent across the equator. Southern hemisphereal circulation is dominated by an anticyclone circulation around a region of high pressure off the Madagascar coast called the Mecernes high.

3)Sub-tropical westerly & tropical easterly jet stream:-

Sub-tropical westerly jet stream which blows from west to east over the northern indian region, suddenly weakens & moves north of Himalayas by the end of May.Another jet stream which blows from east to west sets in over southern part of Indian peninsula called Easterly jet.the sub-tropical westerly jet occurs at low altitude of 300mb checking early incursion of monsoon.

4)Tibetan High:-

A high pressure zone is formed over Tibet. Hadley's & Walker's cell reveal a pattern in atmospheric response to differential heating. The ascending air above the tibet plateau spreads south to join a descending limb over north Indian ocean near Mecernes High & the south westerly wind completes the Hadley's cell. Chinese meteorologists suggest widespread thunderstorm over south-east Tibet in April-May releasing latent heat by rainfall to the amount of 142 Watt/metre square into atmosphere. These ascending air above the Tibetan plateau after descending near the mescernes high joins the south-east trade winds of oceanic origin & enter the bay of bengal after deflection by Coriolis force after crossing the equator & the large extent of warm Arabian sea provide the required moisture for rainfall-as south-west monsoons.

The east-west Walker cell located over the Indonesia on account of heavy rain & high convection, generated a heat source. The descending limb of this cell is located over the semi-aid regions of north-western india,pakistan,middle-east etc. acting as a heat sink to compensate the outgoing solar radiation. Kinetic energy of winds increases as per MONEX observations. Early June see strengthening of I.T.C.Z. to attract winds from different directions. The monsoon 'bursts' accompanied with thunder & lightning.

Short period feature of rainfall variation during south-west monsoon:-

1)Monson depressions:-The rai during the monsoons are also formed due to generation of westward passage of depressions over low pressure systems over the Bay of Bengal & 1 - 3 such systems are observed during monsoon months with usual life span of a week. A peculiar feature of these depressions are their concentration of rainfall in the south-west sector of it's track.

Nor'westers & 'Kalbaishakhis' are examples of intensive atmospheric vortices of small dimension & are associated with strong convective motion.

they appear as group of cells or a large shower cloud. They produce average daily rain of 10 cm. to 20 cm.

2)Off-shore vortices:-They cause heavy rain in the west coast of india. The winds deflected by the Western Ghats forms an of-shore vortex having a linear dimension & can be detected by a weak easterly wind at the coastal stations. They are capable of generating spells lasting for 2-3 days. They have diameters of about 100 Km.

3)Mid-Tropospheric disturbances:-This feature was discovered during the International Ocean Expedition in late '60's.They appear as circular vortices between 3 & 6 Km. with maximum amplitude near 600 mb.They are confined to mid-troposphere & are not observed at the surface & brings rain over northern parts of Maharashtra,Sourashtra & Gujrat.

Regional scale feature o south-west monsoon(not a direct mechanism of monsoon precipitation)

1)Low level cross-equatorial jet stream & Somali Current:-

the major part of low level jet stream penetrates East Africa in May & traverses the northern part of Arabian Sea before reaching India in June.Since,it's path at 90 (degrees)N coincides with zone of coastal upwelling. The wide variations in sea surface temperature influence monsoon winds & it picks moisture & when it is forced to rise above western ghats it brings rainfall.

                             Somali current considered to be a estern boundary current of Indian ocean with the advent of the monsoon (summer) it reverses it's direction & flows from south to north.

2)Air sea interactions & temperature inversions :-The cold air sea surface temperature off the coastal east africa houses a pronounced temperature inversion in atmosphere. The height of inversion base proceeds eastwards & gradually becomes less marked. East of 66 (degrees)E the inhibiting effect of inversion wears off allowing rapid cloud development. These convective clouds are accompanied by heavy rain.Hence,the inversion layer thus modifies the monsoon air as it traverses Arabian sea.

3)Movement of monsoon troughs:-The axis of this trough is parallel o the Himalayan foot hills & is a non-quasi stationary system. The reverse direction flow of south-east monsoon in the lower troposphere beneath easterly jet stream which is basically a return flow in the lower lvels of meridional circulation system. It is convergent & pick moisture from warm sea & after meting the monsoon trough it rises causing rainfall. The resultant release of heat(latent) goes for augmenting the heat source over the Tibet plateau, completing atmospheric circulation the intensity of the rough rapidly increases over the foot hills of N.E. India which is called "Break" monsoon rains. another "break" is caused due to westward passage of low pressure systems across Indian peninsula 7 is closely related to heat budget & bringing flash floods. As the axis of the track oscillates the track of these depressions also vary.

                          the southwest monsoon moves in 2 branches:-

a)Arabian sea branch:-this branch brings rainfall along the west coast of Western Ghat - Maharashtra, Gujrat & parts of Madhya Pradesh. They pass over Aravallis causing scanty precipitation. It enters the Narmada-Tapti trough & brings rain to Nagpur. They enter Ganga plains & mingle with the Bay of Bengal branch.

b) Bay of Bengal Branch:-This branch is directed towards Burmese coast & south-east Bangladesh. Being deflected by the Burma's Arakan Hills,a big chunk of this branch enter Indian sub-continent, one of it's branch moves into The sub-continent. One of it's branch moves westwards along Gangetic plain as far as Punjab. The other moves north into Bramhaputra valley bringing rain in N.E.India.

                                                                                The inflow of the cold polar air from autumn onwards ceases circulation & s.w.monsoon retreats. It arrives in Kolkata on 7th June. There is weakening of monsoon trough in autumn 7 establishment of southern branch of Jet stream, withdrawal is gradual than the onset & is marked by clear skies & rise in temperature. The land is still moist, owing to high temperature 7 humidity the conditions are oppressive enough & is known as the October heat.

According to research paper forward during the Atmospheric chemistry & Physics discussions (2005) on the Indian summer monsoon rainfall-"Interplay of coupled dynamics, radiation & cloud microphysics"- human activities has influenced the earth's  climatic system which has it's manifestation in breakdown of dynamical link between Indian Summer Monsoon & El Nino Southern oscillation. Different aerosol types of continental origin can absorb or reflect the incoming solar radiation to exert large radiative cooling (upto 3 watt/meter square) & affect by reducing evaporation, weakening of pressure gradient between African high 7 Tibetan low & cross equatorial /lateral circulation of monsoon 7 inhibiting the growth of cloud droplets. Takemura et al. (2005) have shown from 1850 - 2000 there is a decrease in cloud effective radius & precipitation due to aerosols loading over Indian sub-continent, while focusing on the 2002 - 2003 anomaly of +15% it has shown the cooling anomaly over north Arabian sea as a result of aerosols.