Context - Night temperatures are below normal over East India but have looked up over the rest of the country to the West (North-West India) as a causative western disturbance pulled in farther away beyond the border and locked in over North-East Afghanistan.
What are western disturbances?
- These are the extra-tropical cyclones.
- They originate in the Mediterranean region and travel eastward to northern Bangladesh, south-eastern Nepal and Northern India.
- The sub- tropical westerly jet streams helps western disturbances to enter the Indian sub- continent and affect its weather.
- The winds while moving also takes up the moisture from the Mediterranean Sea, Black Sea and Caspian Sea
- These moisture laden western disturbances eventually come up against the Himalayas and get blocked, as a consequence, the moisture gets trapped, and precipitation is shared.
- On an average 4-6 disturbances (temperate cyclones) per month pass over northern India between November to April.
- It is a non-monsoonal precipitation pattern that induces low pressure systems in the lower layers of the atmosphere.
- This, ultimately, causes rainfall in the plains and snowfall in the mountains.
- These disturbances result into snow fall in western Himalaya and rainfall in the Great Plains and provide moisture to Rabi season crops.
Recent changes in the behaviour of western disturbances
- For at least three years now, western disturbances have displayed unusual behaviour in India.
- The precipitation has reduced and winters have remained largely dry across India.
Causes
- Arctic warming makes the Polar Jetstream wavier and increases the chance of merging with Sub tropical westerly Jetstream in winters. This causes the Jetstream to shift Northwards and reduce precipitation.
- North Atlantic oscillation which is a random fluctuation of air pressure over the North Atlantic Ocean due to a High pressure region above the Azores island and low pressure region over Iceland. It is currently in a negative phase and it reduces the frequency and intensity of the Western Disturbances.
- La-Nina phases in the previous years reduced the temperature of hot tropical air that is crucial for the temperature gradient for the formation of the Western Disturbances.
Impacts of this change:
- Alteration in temperature and precipitation patterns across India.
- The lack of snowfall in the mountains affects the yield of Rabi crops in some regions.
- Rivers also get less water and hence affect the availability of the freshwater resources.
- Farm income is greatly impacted due to the disastrous impacts on Apple production.
- Air quality: The associated winds and precipitation can help in dispersing pollutants, leading to temporary improvements in air quality, especially in urban areas. Fluctuations in western disturbance can thus effect the air quality.
- Disaster frequency increases as reduction in W.D intensity can lead to droughts and increase in intensity creates floods.