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Thursday, 3 December 2015

#Western #Disturbances#Affects and#Importance to #Indian #Subcontinent



Western Disturbance is the term used to describe an extra-tropical storm that brings sudden winter rain and snow to the northwestern parts of the Indian subcontinent. This is a non-monsoonal precipitation pattern driven by the Westerlies.
Origination:
1. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean.
2. They carry moisture usually in the upper atmosphere (unlike tropical storms where it is carried in the lower atmosphere).
3. In India, moisture is sometimes shed as rain when the storm system encounters with the Himalayas.
4. In the Himalayan region of India, monsoon current progresses from east to west. But the WDs move across north India from west to east, with consequent rise in pressure and cold pool of air in the rear.

Affects and Importance

1. Western Disturbances are important to the development of the Rabi crop in the northern subcontinent, which includes the locally important staple wheat. 
2. Their effect sometime extends up to Gangetic plains and Northeast India, also.But in the past few years western disturbances have been linked to disasters. 
Last year hailstorm and rains in Gujarat, MP, UP and Rajasthan has caused severe damage to Cotton, Mango, Wheat and gram crops. It has weakened economic conditions of farmers in these states.
So, though beneficial, western disturbances are causing problems in agriculture output of India, due to climate change all over world.
Mechanism

---WDs originate in the Caspian Sea or the Mediterranean Sea as extra-tropical cyclones. They gradually travel across the middle-east from Iran, Afghanistan and Pakistan to enter the Indian sub-continent.
---Though WDs move across the Indian region throughout the year, they are in their peak during winter months of January and February. Their effect is minimal during the monsoon months in India.
Effect on Economy
---WD is the principle rain producing systems during non-monsoonal months over Northwest India. They are also responsible for bringing snowfall in the higher reaches of Jammu & Kashmir, Himachal Pradesh and Uttarakhand.

---WD brings winter and pre-monsoon rain and is important for the development of the Rabi crop in the Northern subcontinent. Considering that wheat is one of the most important Rabi crops, which is the staple diet of people in this region, winter showers contribute to meet India’s food security.
---But if extended over a longer time, such erratic disturbances can lead to the failure of the group, as was seen in the present year, which makes the govt compensate the farmers for their loss, and leads to further unplanned expenditure.
---Such losses from disasters cost India about 2% if its GDP, and this can further increase if steps are not taken to mitigate the negative impact of such erratic weather behaviour, along with other disasters.
---Lack of timely aid from the govt, worsens the condition of the farmers even further, which demotivates them from continuing in the sector, which can eventually increase to a drastic scale if the present condition prevails.
Negative affect 
However due to various reasons like Tibetan plateau heating, Pacific decadal oscillations and low pressure jet streams, the pattern of Western Disturbances has been altered.Winds normally blow from west to east, however, due to the low pressure the winds dip to the low pressure region to equalize the air pressure. As a result, a U effect is created. This rigmarole of winds, laden with moisture from the Mediterranean travels over Iran, Afganistan, Pakistan and reaches north west India, where it brings rain, hail, snow and dense fog and sometimes catastrophes like the cloud burst in Leh in 2010, the floods and landslide in Uttarakhand in 2013 and the excessive rain in Jammu and Kashmir in 2014 and 15. This year too as per the India Meteorological Department (IMD), the average rain received between March 1 and March 18 was 197 per cent above normal. This caused severe damage to crops in over eleven million hectare in fourteen states of the country. To mitigate the losses of the farmers and in the wake of rising number of suicides by the farmers the government has doled out relief packages, quality norms for purchase of wheat (the worst affected crop) has also been relaxed in some states. However, a loss of 4-5% is expected in wheat crop this year. Tractor and motorcycle sales have declines to 30% a direct sign of declining rural economy.
Following views are significant
1. According to IMD, the severe rain this year is the result of the confluence of western disturbance and easterly wave from the Bay of Bengal. Easterly wave, or Easterlies, blow throughout the year from east to west. The confluence of the two winds happens throughout the year, but the results vary. They generally bring rain only to the northern part of the country but this year states in central and south India also received rain. Western parts of Madhya Pradesh, for instance, received over 2,025 times more than usual rainfall during March 1-18, while the rainfall in central Maharashtra was 3,671 times above normal(IMD data).
2. A phenomenon called Pacific Decadal Oscillation (PDO) is said to have contributed to the severity of this year’s rainfall. PDO is the name given to long-term fluctuations in the surface temperature of the Pacific Ocean.
3. Widely used weather models, such as the Global Forecast System, are consistently showing the movement of new upper air troughs into India. Such troughs in the jet streams (narrow bands of strong winds flowing in the upper troposphere) could be affecting the western disturbances which, IMD says, are present in the lower and middle troposphere. One such trough started forming in the upper troposphere over Iran, Afghanistan and Pakistan on February 26 and intensified and moved towards north-western parts of India on February 28. This led to the formation of a low-pressure region in the lower troposphere over northwest India, causing an incursion of moisture from Arabian Sea, and produced heavy rains. This shows how problematic the combination of western disturbances and upper air troughs can be for India.
4. Climate change induced: A study by the Indian Institute of Tropical Meteorology (IITM), Pune, has directly linked western disturbances to global warming. Researchers say global warming is impacting air currents and causing freak weather events. Pronounced warming over the Tibetan plateau in recent decades has increased the instability of the Westerlies and this has increased the variability of the western disturbances. According to the study, the western Himalayan region has seen a significant rise in surface temperatures since the 1950s. Observations from the area show a significant increase in precipitation in recent decades.

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