Context

• Dry winters across Himalayan states are indicative of disastrous consequences on crop production and livelihoods of people. 
• The Western Disturbance, which is responsible for the winter rainfall in northwest India, has been weakening.

Western Disturbances

• Western Disturbances are cyclonic storms that form over land, and they occur mostly in the Mediterranean region due to a temperature gradient caused by the mixing of warm air from the tropics and cold air from the northern polar regions. 
• It is a non-monsoonal precipitation pattern driven by the westerlies.
• The moisture in these storms usually originates over the Mediterranean Sea, the Caspian Sea and the Black Sea.
• Western disturbances are more frequent and strong in the winter season.

Indian Context

• While the storm systems occur throughout the year, they travel to India mostly between December and April because the trajectory of the subtropical westerly jet stream, which transports them, shifts during the winter months to the rim of the Himalayas. 
• For the rest of the year, the jet stream travels from above the Himalayas to the Tibetan Plateau and China. 
• The trajectory of the jet stream changes as per the position of the Sun.
• The jet stream appears over northern India in October after the withdrawal of monsoon and shifts progressively southwards in the winter months. 
• It reaches its southernmost position in February and moves out of the subcontinent after May.

Factors influencing western disturbances 

• El Nino and La Nina
  • For the past three years, the world has been in a La Niña phase (cooling of ocean surface temperature in the Pacific Ocean). 
  • It weakens the temperature gradient for the formation of Western Disturbances as it reduces the temperature of the hot tropical air. 
  • Western disturbances are generally weaker during the La Niña, which produces a drier winter, while during El Niño, they are more intense.
• Northern Oscillation
  • It is a random fluctuation of air pressure over the North Atlantic Ocean due to a high-pressure region above the Azores Islands and a low-pressure region over Iceland. 
  • It is currently in a negative phase and it reduces the frequency and intensity of Western Disturbances.
• Subtropical westerly jet stream
  • It is the wind system which brings Western Disturbances to India. 
  • The jet stream is shifting northwards during winters, which reduces winter precipitation in northwest and central India, and southwards during summers, increasing the chances of extreme rainfall events.

Impact of western disturbances on India

• They bring moderate to heavy rain in low-lying areas and heavy snow to mountainous areas of the Indian Subcontinent.
• They bring pre-monsoon showers and winters in Northwest India.
• They are primarily important for the cultivation of Rabi crops.
• Western Disturbances are also the primary source of snowfall that replenishes the Himalayan glaciers during winter.
• They can also cause extreme weather events like floods, flash floods, landslides, dust storms, hail storms, and cold waves, causing damage to life and property.

Conclusion

• In the upcoming years it is essential to strengthen monitoring systems and weather forecast models.
• There is also a need to build disaster resilient infrastructure to withstand climate change calamities in the near future.