Question

Discuss the mechanism of the Indian monsoon. Discuss the relevance of Easterly and westerly Jet Streams in monsoon.

Answer 1

Context:

The origin of monsoons is shrouded in mystery, it is a very dynamic geographical event and an important topic from the prelims as well as mains point of view.

Key points to be addressed/(Approach)
In intro you can define Monsoon and its significance with respect to India
In body part discuss Mechanism and different theories associated with it.
Discuss the relevance of Easterly and westerly Jet Streams in monsoon and how it acts as a driving mechanism of Indian monsoon.


The term monsoon has been derived from the Arabic word “mausam” meaning season or the Malayan word “monsin” meaning season. Thus, the monsoons are seasonal winds experienced in the tropical area roughly between 20⁰N and 20⁰S which completely reverse their direction of flow with the change of season. They flow from sea to land during summer in SW-NE direction called as SW monsoons and from land to sea during winter in NE-SW direction called as NE monsoons.

The origin of monsoons is still shrouded in mystery. Several attempts have been made to explain the mechanism of the monsoons but no satisfactory explanation is available till date.

Over the years many mysteries of the monsoons have been unravelled but still much remains to be done. The theories regarding the monsoons are generally divided into the following two broad categories:

1. Classical Theory, and
2. Modem Theories.

In classical theory Hailey explained the monsoon as resulting from thermal contrasts between continents and oceans due to their differential heating.However, the monsoons do not develop equally everywhere and the thermal concept of Hailey fails to explain the intricacies of the monsoons. Besides differential heating, the development of monsoon is influenced by the shape of the continents, orography, and the conditions of air circulation in the upper troposphere.

Therefore, Hailey’s theory has lost much of its significance and modern theories based on air masses and jet streams are becoming more relevant.

To understand the mechanism of the monsoons, the following facts are important:

• The differential heating and cooling of land and water creates low pressure on the landmass of India while the seas around experience comparatively high pressure.
• The shift of the position of Inter- Tropical Convergence Zone (ITCZ) in summer over the Ganga plain (this is the equatorial trough normally positioned around 20⁰N of the equator- also known as monsoon trough during the monsoon season.
• The presence of the high pressure area over Madagascar approximately at 20⁰S over the Indian Ocean. The intensity and position of this high pressure area affects the Indian monsoon.
• The Tibetan plateau gets intensely heated during summer which results in strong vertical air currents and the formation of high pressure over the plateau of about 5 km above sea level.
• The movement of the westerly Jet- stream to the north of the Himalayas and the presence of the tropical easterly jet over the Indian peninsula during summer.
• Teleconnections, the Southern Oscillation and the El Nino

Relevance of Upper Air Circulations (Easterly and westerly Jet Streams) in monsoon
The upper atmospheric conditions over Tibetan Plateau and positions and intensity of subtropical westerly jet stream and tropical easterly jet stream play a significant role in the onset, withdrawal and intensity of Indian monsoon.

• The subtropical westerly jet occupies a position over north India in winter season and the Himalayas and Tibet plateau lead to bifurcation of this jet stream into two branches.
• The northern branch occupies a position to the north of the Tibetan Plateau and the southern branch is located over north India to the south of the Himalayas.
• In the upper troposphere a ‘high’ pressure system (anticyclonic conditions with clockwise air circulation) develops towards south of southern branch of subtropical jet stream over Afghanistan and north- west Pakistan. Consequently, the winds tend to descend over the north- western parts of India, resulting into atmospheric stability and dry conditions.
• It also contributes to the flow of north east monsoon winds of winter season.
• The subtropical westerly jet streams also help western disturbances to enter the Indian subcontinent and affects its weather.
• These disturbances result in snowfall in western Himalaya and rainfall in the Great Plains and provide moisture to rabi season crops.
• During the summer season ( during the months of April, May and June) due to shift in the overhead position of sun, low pressure areas develop at the surface near Peshawar (Pakistan) and north-west India.
• The winds descending from the upper air high pressure restrict ascend of winds from the surface low pressure. This results in warm and dry weather conditions. This is why the months of April and May (hottest month) are dry in spite of high temperature and evaporation.
• Contrary to it, upper air low pressure is formed in the eastern Himalayan region due to upper air seasonal easterly jet streams. Due to these conditions the winds coming from southern Myanmar are forced to ascend and produce rainfall in Myanmar, Bangladesh and North East India.
• After the first week of June the southern branch of subtropical westerly jet steam disappears and only northern branch operates to the north of the Tibetan plateau. This results in the development of a dynamic depression over north western part of Indo-Pakistan. As this dynamic depression gets established over the thermal depression present in this area, burst of monsoon takes place.

M.T. Yin (1949), while explaining the origin of monsoon stated that the burst of monsoon depends on upper air circulation. P.Koteswaram (1952) established relationship between upper air circulation and atmospheric conditions over Tibetan plateau. He concluded that the fact that northward movement of the subtropical jet stream is the first indication of the onset of monsoon over India.