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The Indian Monsoon

Monsoon is a period of heavy rainfall, especially during the summer over South and Southeast Asia.
 

Monsoon winds strongly influence the climate in India.
 

The monsoons are experienced in the tropical area roughly between 20° N and 20° S.
 

Now let us look at the important factors that activate the monsoons:
 

(i) The differential heating and cooling of land and water

(ii) The shift of the position of Inter Tropical Convergence Zone
 

(iii) The presence of the high-pressure area east of Madagascar
 

(iv) The heating of the Tibetan plateau
 

(v) The movement of the westerly jet stream
 

(vi) The tropical easterly jet stream over the Indian peninsula
 

(i) The differential heating and cooling of land and water

The difference in temperature over land and water creates a low-pressure over the land mass in India. This low-pressure attracts winds from high pressure area.
 

(ii) The shift of the position of Inter Tropical Convergence Zone
The equatorial inter-tropical convergence zone, which is a low-pressure belt of highly unstable weather, moves northward towards India. This shift is also a cause for the on set of monsoon in India.

 

(iii) The presence of the high-pressure area east of Madagascar
The intense high –pressure area that is formed east of Madagascar, at approximately 20°S over the Indian Ocean causes the wind to flow towards the low pressure over the Indian land mass.

 

(iv) The heating of the Tibetan plateau


The Tibetan plateau which gets intensely heated during summer, results in strong vertical air currents and the formation of low pressure over the plateau at about 9 km above sea level.
 

(v) The movement of the westerly jet stream
The movement of the westerly jet stream, which lies north of the Himalayas create western cyclonic disturbances, which influences the monsoon rains

 

(vi) The tropical easterly jet stream over the Indian peninsula.


Atmospheric Pressure


Apart from the wind, its temperature and direction, atmospheric pressure plays an important role in the on-set of the Indian monsoon.

The periodic reversal change in atmospheric pressure during certain years in Indian and Pacific Ocean is known as the Southern Oscillation or SO.
 

The Southern Oscillation refers to the oscillation in air pressure between the southeastern and southwestern Pacific waters. When there is an increase in temperature in the eastern Pacific waters atmospheric pressure rises in the western Pacific and drops in the east. This pressure drop is accompanied by a weakening of the easterly Trade Winds.

This phenomenon is known as Southern Oscillation.

 


 

The strength of the Southern Oscillation is measured by the Southern Oscillation Index.
A feature connected with the SO is the El Nino, a warm ocean current that flows past the Peruvian Coast.

 

The signs of an El Niño are:
 

1. Rise in air pressure over the Indian Ocean, Indonesia, and Australia.

2. Fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean.

3. Trade winds in the south Pacific weaken or head east.

4. Warm air rises near Peru, causing rain in the deserts there.

5. Warm water spreads from the west Pacific and the Indian Ocean to the east Pacific. It takes the rain with it, causing rainfall in normally dry areas and extensive drought in eastern areas.





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