New Delhi: The unusually heavy hailstorm that lashed National Capital Region (NCR), creating snowfall-like conditions on 7 February isn’t the freak weather that many think it to be: senior government scientists say it fit into a climate change pattern. And they warn of more such extreme weather in the future.
“It was one of the most intense hailstorms to hit the plains," said K.J. Ramesh, director general, India Meteorological Department (IMD). “The activity was intense for at least 14 hours and widespread, showing its impact right from Jammu and Kashmir up to Patna in Bihar. It was unusual. What we witnessed was an extreme nature of the western disturbance events," he said.
A western disturbance is a storm that originates in the Mediterranean, moves eastwards and causes precipitation when it hits the western Himalayas. It is the main weather system bringing rain and snow during winter and spring over north India.
Scientists are concerned that long-term trends point toward an increase in the intensity of these western disturbances over the last few decades.
“We studied the data sets of the last 40 years and found that the amplitude of these western disturbances is increasing. So, when the amplitude of these westerlytroughs is high, the intensity and scale is high and it affects a larger area," said R. Krishnan, senior scientist at the Indian Institute of Tropical Meteorology (IITM), Pune.
Using high resolution climate models, the scientists at IITM performed climate experiments and found that the extreme precipitation events are increasing during winter because of the western disturbances and it is largely because of climate change. According to the study, strong surface warming over the eastern Tibetan plateau relative to the western side is favouring western disturbances.
As concerns grow around occurrence of extreme weather, Ramesh said, “Cold waves and below normal minimum temperature days are characteristic of winter, but we are witnessing more number of such days. These are winter extremes and intensity of such extreme weather events is bound to increase. All these are manifestations of climate change."
On an average, five to six western disturbances hit the Himalayas every month from December to February (peak season), usually moving across the Kashmir region. However, this year, their impact was seen much more in the plains, particularly in Punjab, Haryana, and the NCR.
D.S. Pai, senior scientist, IMD, Pune, put down the hailstorm to a weakening of mid-latitude westerlies.
“It is not unexpected. It can happen. There is year-to-year variation due to several favourable factors. However, this year is a special case, somewhat anomalous because of the weakening of mid-latitude westerlies, which is leading to lot of trough (low pressure systems) activity across the globe," Pai said.
Westerlies are winds blowing from the west to the east and their strength can vary from year to year. These are generally formed because of the pressure gradient between the mid-latitude and polar regions.
The higher the pressure gradient, the stronger are the westerlies. However, the recent warming trends in the polar regions have led to a general weakening of the westerlies globally.
“In the US, this is linked to the formation of the polar vortex. In India, it manifests in the form of deep troughs, which are basically western disturbances," said Pai.
The trough formed this time was so deep that the effect of the cold wave in the northern states was even felt in parts of Maharashtra, where the minimum temperatures plummeted during the day and winds were cooler. High altitude areas in Jammu and Kashmir, Uttarakhand, and Himachal Pradesh witnessed record snowfall.
Gusts of strong wind in the upper troposphere dragged the super-cooled clouds to farther distances, resulting in the formation of large-sized hailstones.