The warming of the Arctic and the Mediterranean Sea mean that Western Disturbances are striking India less in winter and more in summer; with catastrophic consequences
India has not experienced a normal winter in three years. The second wettest season in the country after the monsoons 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 83 per cent rainfall deficit.
Then, after a near-normal January, February broke all records to become the hottest since 1901. The northwest region saw 76 per cent deficit rainfall.
The reason for the abnormal winter seasons since 2020-21 lies in the changing character of the Western Disturbances, a series of cyclonic storms that originate in the Mediterranean region, and travel over 9,000 km to bring winter rains to northwest India.
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. Strong Western Disturbances reach the central and eastern Himalayas and cause rain and snow in Nepal and northeast India.
The last time the storm systems visited the country in all their glory was in 2019. Since then, their arrival 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, as per the Western Disturbances: A Review, published in Reviews of Geophysics in April 2015.
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.
Clouds formed by the Western Disturbances have a moderating effect on the maximum temperatures during the winter season, reads Western Disturbances—An Indian Meteorological Perspective, a book published in 2016 by Springer.
As they were missing this winter season, the north Indian plains experienced severe cold waves and cold days in December and most of January due to the cold northern winds flowing down from the Himalayas.
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, says Akshay Deoras, research scientist at the National Centre for Atmospheric Science and Department of Meteorology, University of Reading, UK.
On February 20, the maximum temperature in some parts of Gujarat and Rajasthan reached 39°C, forcing IMD to issue a special press release for farmers.
“This higher day temperature might lead to adverse effect on wheat approaching reproductive growth period, which is sensitive to temperature. High temperature during flowering and maturing period leads to loss in yield,” reads the IMD release.
In a way, IMD warned that the country will most likely see a repeat of last year, when high temperatures in March due to weak Western Disturbances damaged 30-40 per cent wheat crop in Punjab, Haryana, Himachal Pradesh, Uttar Pradesh and Madhya Pradesh.
This had a rippling effect. The domestic wheat prices skyrocketed and forced the Centre to take several difficult decisions, from banning wheat exports to selling its wheat reserves at low prices.
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.
“They are crucial for the water security of the region. As permafrost melts because of the warming, replenishment of glaciers will not happen,” says A P Dimri, director, Indian Institute of Geomagnetism, Mumbai.
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.
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.
They occasionally form as far as in Alaska or the Arctic region, according to a research paper published in the Quarterly Journal of the Royal Meteorological Society in 2018.
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).
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 southern-most position in February and moves out of the subcontinent after May,” says V S Prasad, head, National Centre for Medium Range Weather Forecasting under the Union Ministry of Earth Sciences.
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,” says Raghu Murtugudde, climate scientist at the University of Maryland, US, and the Indian Institute of Technology, Mumbai.
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, according to a research paper published in the journal Climate Dynamics in August 2022.
Western Disturbances are growing elusive, a trend that is likely to worsen in the future. “Western Disturbances are typically weaker and therefore associated with less precipitation. When they are associated with extreme precipitation, the extremes are getting much worse,” says Kieran Hunt, Research Fellow in Tropical and Himalayan Meteorology, University of Reading, UK.
Deoras says Western Disturbances in December were weak in five of the past eight years, barring 2017 and 2019.
The main reason for this is the northward shift of the subtropical westerly jet stream in December.
“They are moving a little bit further away from the Arabian Sea, so they have a little bit less access to that moisture channel,” says Hunt.
Such a shift not only reduces the chance of Western Disturbances striking India but also increases the chance of them affecting higher latitudes such as the Tibetan Plateau or even as far up as China and Russia.
This could indirectly affect the southwest monsoon, which accounts for 80 per cent of India’s annual rainfall.
Snow reflects most of the Sun’s rays falling on it and stops the land from heating up. This phenomenon is called the albedo effect. If there is excessive snowfall over the Tibetan plateau then it would not get as warm as it should, and this would hamper the incoming monsoon winds in June.
The upward movement of the subtropical westerly jet stream in winter season happens when it merges with the polar front jet, as per a research paper co-authored by Dimri in April 2015. The chances of such a merger increase due to the Arctic warming that makes the polar front jet wavier.
While Western Disturbances are avoiding the winters, they have started visiting India more frequently during the summers. “Owing to the warming in the Arctic region, we are observing that the subtropical westerly jet stream is moving downward in the summer season,” says Dimri.
Western Disturbances during summer, monsoon, and post-monsoon periods increase the chances of them interacting with the southwest monsoon and other associated local convection systems such as tropical depressions that travel northward from either the Bay of Bengal or the Arabian Sea. Such interactions can cause catastrophic weather disasters.
“When tropical depressions hit land, they start to run out of the fuel as they need warm surface temperatures and shear to maintain themselves. Western Disturbances help them last longer and cause heavy rainfall over parts of India they normally do not travel,” says Hunt.
Such an interaction triggered the Uttarakhand floods in June 2013, which killed over 6,000 people, and caused damages worth US $1.1 billion.
The floods were triggered after a tropical depression associated with the southwest monsoon transferred moisture to a Western Disturbance.
In May 2021, a remnant of the extremely severe cyclone Tauktae, which made landfall along the Gujarat coast, travelled all the way to Delhi and interacted with a Western Disturbance to cause heavy rainfall in Delhi and its vicinity. It caused the maximum temperature of the national capital to drop by 16oC.
The weather phenomenon will grow more unpredictable in a warming world, but the exact impact, say experts, remains to be studied.
“This is still an area of active research. Due to the warming of the Arctic region, the frequency of Western Disturbance occurrence over India should increase. Warmer sea surface temperatures in the western Indian Ocean can lead to increased evaporation, which can increase the amount of moisture in the atmosphere,” says Prasad.
At the same time, global warming could reduce the temperature gradient in the Mediterranean that is crucial for the formation of Western Disturbances.
“We expect the tropical Pacific to experience lesser warming in the east as has been happening in the past two decades. This could be bad news for the Himalayan glaciers. But some parts of the Himalayas will get more snow due to the warmer air holding more moisture. We also have to worry about warmer air producing more rain instead of snow, which can be bad news for the glaciers,” says Murtugudde.
This was first published in the 16-31 March, 2023 print edition of Down To Earth
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