The India Meteorological Department (IMD) has predicted rains in the national capital region and north-western parts of India this week, thanks to a western disturbance in the region, bringing relief from a spell of warm weather sweeping the region.
Delhi and adjoining areas may experience dust storms and rain and thunderstorms are anticipated on Thursday and Friday evening.
A warning for heavy snowfall on Wednesday and Thursday has been issued in Jammu and Kashmir.
Western Disturbance refers to a system of low pressure that moves from west to east, bringing moisture from Eurasian water bodies, and is responsible for winter rain in north-western India and snowfall in the Himalayan tracts.
The north-west region, including Punjab, Haryana and Chandigarh and Delhi NCR can expect bouts of rain from Wednesday evening to Friday. Uttar Pradesh and northern Rajasthan will see light showers or thundershowers during the same period.
Himachal Pradesh and Uttarakhand may see spurts of heavy rainfall and snowfall on Thursday and hail and squall may also hit the two states on Thursday and Friday.
The western disturbance is likely to bring temporary relief to India’s northwest region as the long-term forecast from IMD shows that temperatures over the next three months from March to May will be at least a degree above normal.
These rains are important for rabi crops such as wheat.
March 1 usually marks the beginning of the pre-monsoon season in India which lasts until May. However, the winter season in the past couple of years has not been receiving adequate rainfall.
A new study centred on the impact of western disturbances on India’s weather found that the El Nino, or the unusual warming of the Pacific Ocean, may have indirectly contributed to the low rainfall during winter months by deflecting western disturbances away from India.
“In winter, the precipitation over north-west India mainly occurs due to western disturbance activity over that region,” Soumik Basu, lead author of the study and research fellow at the University of Alaska, Fairbanks, said. “In our recent research, we have shown that in 2015 November-December, north-west India received remarkably low precipitation.”
These low-pressure systems are carried by a narrow band of strong winds called the jet stream that is known to affect weather patterns. If the position of the jet stream changes, the path of the western disturbance also changes.
“It is like a boat travelling in a river,” Akshay Deoras, explained one of the co-authors of the paper.
The hypothesis put forward by the researchers to explain the low rainfall is that the warming of the tropical Pacific translated into warming of the Arabian Sea, creating an area of high pressure over western India.
During normal conditions, a high-pressure zone develops near the Yemen-Oman-Somalia region. Due to this, the jet stream and western disturbance penetrate into Jammu and Kashmir and north India, according to Deoras.
In 2015, due to this shifting of high pressure towards India, the path of the jet stream was deflected. This caused western disturbances to be more active, causing ample rainfall in Pakistan, Afghanistan and surrounding regions. Some western parts of Jammu and Kashmir also received rain due to the same effect but they were weaker than normal over most of northern India.
In all the subdivisions of north-western India, except Jammu and Kashmir, there was a fall in the average rainfall in November and December 2015 compared to the same period the year before.
In 2016 too there was hardly any rainfall in November and December that can be attributed to western disturbances.
“The relation between El Nino and Indian summer monsoon is well known but the connection of El Nino events with the winter precipitation over north-west India due to western disturbances has not been fully explored,” Basu said.
To examine whether lower precipitation from western disturbances is a new normal will require a study of long-term trends.
What is known is that dry winter months can destroy rabi crops. Even a change in the timing of the rains can prove harmful to crops.
Lack of winter rains in this region also aggravates the problem of ambient air pollution. Without rain to settle the pollutants, the pollution levels peak during the winter months.
“This was seen in the winter of 2015 in Delhi,” Deoras said.