Jet streams are narrow bands of high speed winds that generally blow from west to east across the globe. Jet streams are like rivers of wind flowing through the upper layers of the troposphere. Jet streams have a huge influence on climate, as they can push air masses around and affect weather patterns.
M.T.Yin had given this concept stating that the burst of monsoon depends upon the upper air circulation. Two prominent jet streams affect the monsoon winds, the sub-tropical westerly jet stream and equatorial easterly jet stream.
Sub-tropical westerly jet stream: It dominates in winter time in upper troposphere circulation of the northern latitudes and can be located over Indian sub-continent at a height of about 12 km. Himalayan mountain system splits the jet stream into two parts. The jet stream south of Himalaya tends to descend over north- western part of India resulting in atmospheric stability.
Equatorial easterly jet stream: This jet is a prominent feature of the upper air circulation during the Indian monsoon season appearing as a band of strong easterlies extending from south East Asia across the Indian Ocean and Africa to the Atlantic.
Fig4 (a): Direction of Winds in India in winter at the Height of 9-13 km
Fig4 (b) The Direction of Winds at 13 km Altitude in Summer Season
Western disturbance
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- Western disturbance (WD) is an extra-tropical storm which originates in the Mediterranean region. It flows from west to east in the extra-tropical region. Here, Disturbance means an area of “disturbed” or reduced air pressure.
- It bringsrainfall, snowfall and fog in northern India. It is a non-monsoonal precipitation pattern driven by the westerlies. It is characterised by cloudy sky, higher night temperatures and unusual rain.
- It is very important for Indian agriculture, especially for Rabi crops like wheat etc. in north and north-western India. However, Excessive precipitation can cause crop damage, landslides, floods and avalanches. Sometimes, It also bring cold wave conditions and dense fog in Northern India
Formation
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- It originates in the Mediterranean Sea as extra-tropical cyclones. Mediterranean Sea and/or the Atlantic Ocean provide western disturbances its moisture. A high-pressure area over Ukraine and neighbourhood consolidates, causing the intrusion of cold air from Polar Regions towards an area of relatively warmer air with high moisture. This generates favourable conditions for cyclogenesis in the upper atmosphere, which promotes the formation of an eastward-moving extra tropical depression. They gradually travel across the middle-east from Iran, Afghanistan and Pakistan to enter the Indian sub-continent.
