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Thursday, 19 May 2016

Monsoon mechanics: The 'How' and 'Why' of Southwest rainfall

Monsoon in South Asia can be categorized into two based on their spread over the subcontinent – Arabian Sea Branch and the Bay of Bengal Branch. Alternatively, it can be categorized into Southwest and Northwest monsoon based on the direction of rain-bearing winds.

Southwest monsoon hits the Kerala coast every year precisely on June 1. Monsoon does not come gradually, it “bursts” suddenly on a single day. The most powerful supercomputers are used to analyze monsoon, its aspects and its exact date of hitting land. 

The summertime heating of the plateau of Tibet is the most important factor in the causation and maintenance of monsoonal circulation. 

Sub-Tropical Jet stream (STJ) is a narrow band of fast moving air flowing from west to east [Westerlies].STJ in northern hemisphere flows between 25° to 35° N in the upper troposphere at a height of about 12-14 km p.h. 

Westerly jet stream blows at a very high speed during winter over the sub-tropical zone. This jet stream is bifurcated by the Himalayan ranges and Tibetan Plateau. The two branches reunite off the east coast of China. With the beginning of summer in March, the STJ start their northward march due to the intense heating and the low pressure created over the sub-continent. 

The southerly branch of STJ remains positioned south of Tibet, although weakening in intensity. The Tibet Plateau is 600 kilometers wide in the west and 1,000 kilometers wide in the east. Its length is about 2,000 km. The average height of the plateau is about 4,000 meters. Thus, it is an enormous block of high ground acting as a formidable barrier. The summer-time heating of Tibetan Plateau makes it a heat source. 

A warm-core anticyclone is formed over this plateau during the summer monsoon period. It is the result of a process called anticyclogenesis. 

However, on the southern side of this upper-air anticyclone, the direction of air flow is from east to west. An easterly jet emerges over peninsular India with the northward migration of STJ. 

In fact, these easterly winds blowing in the mid-troposphere are known as the Tropical Easterly Jet. This jet extends far to the south of Tibet and the air flow is roughly along the Calcutta-Bangalore axis.

These upper-air easterlies descend in the permanent high pressure area formed over the south Indian Ocean (Mascarene high). This naturally intensifies the high pressure already present there and it is from this high pressure cell that the onshore winds start blowing towards the thermally-induced low pressure area developed in the northwestern part of the Indian sub-continent due to intense heating in the summer months. 

It is clear that the strength of the easterly jet is directly related to the intensification of the permanent 'high' formed over the south Indian Ocean. Since this higher pressure makes the pressure gradient steeper, so it is the main causative factor for determining the vigor of the summer monsoon. 

At the beginning of June, the STJ disappears from the southern side of the plateau. In other words, the jet disappears completely over northern India accentuated by the anticyclogenesis created by the Tibetan plateau. This sudden shift is the cause of the “bursting” of the monsoon. An immediate phenomenon ! 

The easterly winds become very active in the upper troposphere ,hence the south west monsoon. A timely northward shift of the subtropical westerly jet at the beginning of summer is critical to the onset of the southwest monsoon over India. 

If the shift is delayed, so is the southwest monsoon. An early shift results in an early monsoon. After crossing the equator, such winds become southwesterly (due to coriolis effect) and are known as the Southwesterly Summer Monsoon. 

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