Context

• India has not experienced a normal winter in three years. It has remained unusually dry and hot.
• In this winter, for instance, the country experienced its hottest ever December, as per the India Meteorological Department (IMD). The northwest region, which receives almost 30 per cent of its annual rainfall in the season, saw an 83 per cent rainfall deficit. 
• Then, after a near-normal January, February broke all records to become the hottest since 1901. 
• The reason for the abnormal winter seasons since 2020-21 lies in the changing character of the Western Disturbances.

Western Disturbances

• They are a series of cyclonic storms that originate in the Mediterranean region, and travel over 9,000 km to bring winter rains to northwest India.
• A Western Disturbance is in the shape of a spiral with a narrow mouth at the bottom (formed at a height of about 5,500 metres above sea level) and a wide mouth at the top (formed at a height of more than 9,000 metres above sea level). 
• The low-pressure storm systems help farmers in India grow their rabi crop, bring snow to the Himalayas and maintain the flow of the northern rivers. 
• They reach the country riding on a wind system called the subtropical westerly jet stream that circles the Earth throughout the year. During its journey, a Western Disturbance collects moisture from the Mediterranean Sea, Black Sea and Caspian Sea and traverses over Iran and Afghanistan before hitting the western Himalayas. 

Impacts

• Strong Western Disturbances reach the central and eastern Himalayas and cause rain and snow in Nepal and northeast India. 
• Since 2019, the arrival of the storm systems has either been delayed or weakened. 
• On an average, India receives four to six intense Western Disturbances a month between December and March, or 16 to 24 such events in the entire period. This winter season, the country has received only three intense Western Disturbances: two in January and one in March. December and February passed without a single intense Western Disturbance. 
• A migrating Western Disturbance is preceded by warm, moist air, and is followed by cold, dry air. This keeps the temperatures warm in the peak winter months of December and January and stops the temperature from rising in February and March. 
• One of the reasons for the abnormally hot February was the formation of a high pressure area near the land surface, which caused the air to descend, compress and heat up. A strong Western Disturbance would have dissipated the high pressure.

Other Impacts

• Western Disturbances are also the primary source of snowfall that replenishes the Himalayan glaciers during winter. These glaciers feed major Himalayan rivers like the Ganga, Indus and Yamuna as well as myriad mountain springs and rivulets.
• However, not everything that the Western Disturbances bring is good. They are responsible for hailstorms that damage standing crops, fog events that interrupt air, rail and road services and cloud bursts that result in flash floods. 

Reasons for weakening of Western Disturbances 

• For the past three years, the world has been in a La Niña phase, which refers to the 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. 
• They are generally weaker during the La Niña phase of the El Niño-Southern Oscillation, which produces a drier winter. During El Niño, they are more intense. 
• Western Disturbances are also influenced by the North Atlantic Oscillation, a random fluctuation of air pressure over the North Atlantic Ocean due to a high pressure region above the Azores Islands in the central North Atlantic and a low pressure region over Iceland. 

The weather system is currently in a negative phase, as both low and high pressure systems are weak, and it makes Western Disturbances 20 per cent less frequent and 7 per cent less intense than a positive phase.