A BRIEF OVERVIEW OF THE CLASS (05:12 PM)

MODERN THEORY OF MONSOON (05:24 PM)

• Winter season (November- February)
• Low temperature and high-pressure conditions are prevailing over the northern plains. 
• It causes Anti-cyclonic circulations causing winds to blow outward from the land over which air is subsiding. 
• The sky is clear and dry. 
• The subtropical westerly Jetstream is strong and well-established. It is bifurcated into two branches i.e. towards the north and south of the Tibetan plateau. This southern branch is stronger and is well-established over the northern Ganga plains. 
• This intensifies surface Anti-cyclones and causes winds to blow from land to sea in a North-east direction. 
• North-east winds after crossing the Bay of Bengal pick up moisture and causes precipitation along the Coromandel coast. 
• Spring season (March-May)
• The temperature has started to rise and pressure is falling 
• The weather is extremely hot and dry 
• The subtropical westerly Jet stream begins to weaken and the southern Branch eventually moves entirely to the North of the Tibetan Plateau
• This triggers convection and precipitation in some regions. They are called Pre-monsoon showers. 
• Mango showers in Kerala- It helps in the ripening of mangoes
• Blossom showers in Karnataka- It helps in coffee flowering
• Norwesters or Kal Baishaki. In Assam, it is called as Bardoli Chheerha. It helps in Jute and rice cultivation
• Loo in northern plains from Punjab to Bihar. 
• Summer season (May-August) (06:26 PM)
• The weather is extremely hot and dry. High temperature causes the complete development of low-pressure cells over deserts and the Northern Plains
• ITCZ gradually shifts toward the North and attracts the southern trade winds toward the north of the equator.
• These southern Trade winds after crossing the equator turn right under the influence of Coriolis force and starts to blow as southwest Monsoon winds. 
• Temperature stratification in the air will not allow large-scale convection preventing major storms. 
• The tropical easterly jetstreams and depressions in the Bay of Bengal help in disturbing the stratified air and triggers large-scale precipitation. 
• The tropical easterly Jet stream is a low-level Jet stream existing over South Asia and Africa in Summer. It causes a change of divergence to convergence on the surface. 
• The sudden onset of moisture-laden winds associated with violent Thunder and lightning is called as Bursting of the Monsoon.
• The Somali jetstream is a local jetstream off the coast of East Africa. It strengthens High pressure near Madagascar. The High-pressure cell causes a faster flow of winds toward India. 
• In some regions due to local stability conditions, temperature stratification reappears or the wind starts blowing parallel to the Topography resulting in Break in the Monsoon. 
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• Autumn season (September- October) 
• ITCZ or Monsoon trough starts to move back gradually towards the south. 
• This also brings back the maximum extent up to which southwest winds blow which is called the retreat of Monsoon. 
• The southwest winds slowly get replaced by North-east winds. 
• The high temperature with dry conditions existing over the Northern plains during October is called October Heat. 
• The subtropical westerly Jet streams reappear to the south of the Tibetan Plateau creating dry conditions. 

CHARACTERISTICS OF INDIAN MONSOON (07:08 PM)

• The onset of monsoon is gradual and withdrawal is more gradual. 
• As the wind moves from sea to land, the amount of precipitation gradually decreases. 
• The duration of the monsoon also decreases from sea to land. 
• Temporal variation- Variation of rainfall with time. 
• Spatial variation- Variation of rainfall from one location to another. 
• Distribution of Monsoon rainfall 
• The monsoon winds get divided into two branches- The Arabian Sea branch and the Bay of Bengal branch
• Bay of Bengal Branch 
• It moves parallel to the Coromandel coast and causes no precipitation. From the K-G delta, rainfall starts to increase towards West Bengal. 
• Along the Ganga delta, the Bay of Bengal branch is divided into eastern and western by Himalayan Foothills.
• The Eastern branch hits Meghalaya Plateau perpendicularly causing very heavy precipitation. 
• The Western Branch moves across the Ganga plains. 
• The precipitation decreases from east to west along the Ganga plains  
• Precipitation increases towards the Himalayas from South to North. 
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• Arabian Sea Branch 
• It hits the Western Ghat at the right angle causing heavy precipitation along the western side. 
• Along the Eastern side, the descending winds result in a rainshadow effect creating dry conditions in Maharashtra and Karnataka. 
• The Arabian Sea Branch advances towards Gujarat and blows parallel to Aravallis resulting in very less precipitation in Rajasthan. 
• The Arabian Sea Branch meets the Bay of Bengal Branch near Agra and the combined branch causes precipitation in the regions to the North. 

WESTERN DISTURBANCES (07:46 PM)

• Western Disturbance is active between November to April. 
• During winters Northern India experiences cold and dry conditions along with strong westerly Jet streams. 
• Mediterranean region receives rainfall due to onshore westerlies and temperate cyclones. 
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• The subtropical westerly Jetstreams along the Mediterranean region pick up the moisture content and low-pressure disturbance and are brought to India. 
• When these winds accumulate near North-western Himalayas it causes precipitation. 
• The precipitation due to western disturbance decreases from west to east along the Ganga plains
• Significance of Western disturbances
• It causes an abrupt decrease in temperature and snowfall along the northern hills 
• It may result in cloudbursts and flash floods. example- 2010 Leh Cloudburst
• It is good for winter crops of wheat and mustard. 
• Impact of El-Nino, La-Nina, Indian Ocean Dipole, and Maden Julian Oscillation on Indian Monsoon.
• Refer to oceanography. 

CLIMATIC REGIONS OF INDIA (08:04 PM)

• Refer to PPT in the class
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The topic for the next class:- Soils.